Particle Physics seminar archive - Department of Theoretical Physics

Weekly seminars of the Department of Theoretical Physics

Location: 2nd floor, 2.54, Novobatzky room

Time: Wednesdays at 14:15


  • 10 September 2014, Mate Lencses (Eotvos University, Hungary)

    Excited state TBA and renormalized TCSA in the scaling Potts model slides

    We consider the field theory describing the scaling limit of the Potts quantum spin chain using a combination of two approaches. The first is the renormalized truncated conformal space approach (TCSA), while the second one is a new thermodynamic Bethe Ansatz (TBA) system for the excited state spectrum in finite volume. For the TCSA we investigate and clarify several aspects of the renormalization procedure and counter term construction. The TBA system is first verified by comparing its ultraviolet limit to conformal field theory and the infrared limit to exact S-matrix predictions. We then show that the TBA and the renormalized TCSA match each other to a very high precision for a large range of the volume parameter, providing both a further verification of the TBA system and a demonstration of the efficiency of the TCSA renormalization procedure. We also discuss the lessons learned from our results concerning recent developments regarding the low-energy scattering of quasi-particles in the quantum Potts spin chain. See arXiv:1405.3157

  • 17 September 2014, Marton Kormos (BME, Hungary)

    Correlations after quantum quenches in the XXZ spin chain: Failure of the Generalized Gibbs Ensemble slides

    The topic of thermalization of isolated quantum systems has enjoyed a lot of attention lately, partially due to the fast evolving experimental techniques in cold atom systems. Until recently, it was generally accepted that even completely isolated many-body systems self-thermalize to a certain extent: their evolution from a non-equilibrium initial state leads to a local thermal equilibrium (Gibbs ensemble) or to the Generalized Gibbs Ensemble (GGE) for integrable systems that have extra conserved quantities. We studied the non-equilibrium time evolution of the integrable spin-1/2 anisotropic Heisenberg (XXZ) spin chain, and we found that various short-ranged spin correlators in the long-time limit deviate significantly from predictions based on the GGE hypothesis. By computing the asymptotic spin correlators within the recently proposed quench action formalism, however, we find excellent agreement with the numerical data. We therefore conclude that the GGE cannot give a complete description even of local observables. This surprising result reopens the quest for the correct statistical description of the equilibrium state of integrable systems.

  • 24 September 2014, Attila Pasztor (Eotvos University, Hungary)

    Heavy quarkonium at finite temperature from 2+1 flavour lattice QCD slides

    Heavy quarkonium states are of great interest as a possible thermometer of hot QCD matter. For this idea to work, the determination of the dissociation temperatures of the different states is necessary. In this talk I will review some recent results from lattice QCD with 2+1 dynamical quarks that gives input to this long-standing problem. In the first part of the talk I will discuss the direct Maximum Entropy determination of spectral functions and to how much extent the lattice data depend on the heavy quark diffusion coefficient. In the the second part I will present continuum results on the static quark-antiquark pair free energies and the electric and magnetic screening masses in the QGP.

  • 1 October 2014, Guido Franchetti (Heriot-Watt University, UK)

    Geometric models of matter slides

    Geometric models of matter is a framework introduced by Atiyah, Manton and Schroers with the aim of modelling particles by means of 4-dimensional Riemannian manifolds. In this talk I will review the original proposal as well as more recent work about ALF gravitational instantons as models for multi-particle systems. Harmonic forms on these spaces have interesting properties which will be also discussed.

  • 8 October 2014, Gergo Zarand (BME, Hungary)

    Mott skyrmions: stabilizing the false vacuum slides

    Topological excitations keep fascinating physicists since many decades. While individual vortices and solitons emerge and have been observed in many areas of physics, their most intriguing higher dimensional topological relatives, skyrmions and magnetic monopoles remained mostly elusive. Here we propose that loading a three-component nematic superfluid such as 23Na into a deep optical lattice and thereby creating an insulating core, one can create topologically stable skyrmion textures and investigate their properties in detail. We show furthermore that the spectrum of the excitations of the superfluid and their quantum numbers change dramatically in the presence of the skyrmion, and they reflect the presence of a trapped monopole, as imposed by the skyrmion's topology. Signatures of the presence of the skyrmion in time of flight experiments as well as experimental protocols to create it shall also be discussed.

  • 15 October 2014, Istvan Nandori (ATOMKI, Hungary)

    Renormalization scheme-dependence in the FRG Method slides

    The Functional Renormalization Group (FRG) method has been constructed to perform the renormalization non-perturbatively and it has been successfully applied in many cases over the last four decades. The modern form of FRG is the Wetterich equation which can be related to the previously introduced Wegner-Houghton and Polchinski equations by the appropriate choice of the so called regulator function, i.e. the renormalization scheme. FRG equations are integro-differential equations for functionals thus their solutions require approximations. However, the approximated FRG equations depend on the choice of the regulator function thus the predictions for physical quantities could become scheme-dependent too. I discuss this renormalization scheme-dependence of the FRG method in the context of the so called compactly supported smooth (CSS) regulator function introduced recently.

  • 22 October 2014, Gabor Somogyi (CERN, Switzerland)

    Colorful NNLO -- Higher order QCD corrections via local subtraction slides

    At the LHC very high energy proton-proton collisions are measured at a level of precision which demands that the theory predictions be computed at higher orders in perturbation theory. In recent years the precision frontier for QCD calculations has moved beyond next-to-leading order and today fully differential results at next-to-next-to-leading order accuracy are available for a handful of processes. One serious bottleneck which hampers the straightforward evaluation of cross sections at higher perturbative orders is the presence of infrared singularities in intermediate stages of the computation. One possible way of dealing consistently with infrared singularities is through a so-called subtraction scheme. In this talk I will describe the Colorful NNLO algorithm, a completely local subtraction scheme for fully differential predicitions at NNLO in QCD. I will give an overview of the main conceptual issues that need to be addressed as well as present some technical aspects and a first application of the method.

  • 29 October 2014, Janos Polonyi (University of Strasbourg, France)

    Classical and quantum effective theories slides

    A generalization of the action principle of classical mechanics, motivated by the Closed Time Path (CTP) scheme of quantum field theory, is presented to deal with initial condition problems and dissipative forces. The similarities of the classical and the quantum cases are underlined. In particular, effective interactions which describe classical dissipative forces represent the system-environment entanglement. The relation between the traditional effective theories and their CTP extension is briefly discussed and few qualitative examples are mentioned.

  • 5 November 2014, Zoltan Bajnok (Wigner Center, Hungary)

    Integrability for 2 and 3pt functions in AdS5/CFT4 slides

    In my talk I review the strategy of the integrable bootstrap method. I explain how it determines the spectrum of string states on AdS5 x S5, i.e. the scaling dimensions of single trace operators in planar N=4 SYM. I emphasize its non-perturbative nature and that it starts at infinite volume and systematically calculates the finite size corrections based on the infinite volume scattering data. In the second part of the talk I outline a similar program to determine the 3pt functions via the bootstrap method.

  • 12 November 2014, Gergely Marko (Eotvos University, Hungary)

    Bose-Einstein condensation and Silver Blaze in the 2-loop Phi-derivable approximation slides

    We numerically study the chemical potential-temperature phase diagram of the O(2) model within the 2-loop truncation of the Phi-derivable formalism. We find that different parameters allow for a phase transition of either SSB or BEC type. In the BEC case we observe the Silver Blaze phenomenon, which we understand and generalize using analyticity arguments. We compare our results both with the Hartree-Fock approximation and with lattice simulations. We discuss the loss of solution strongly connected with the Goldstone-theorem.

  • 19 November 2014, Gabor Etesi (BME, Hungary)

    Gravity as a four dimensional algebraic quantum field theory slides

    Based on an indefinite unitary representation of the diffeomorphism group of an oriented 4-manifold an algebraic quantum field theory formulation of gravity is exhibited. More precisely the representation space is a Krein space therefore as a vector space it admits a family of direct sum decompositions into orthogonal pairs of maximal definite Hilbert subspaces coming from the Krein space structure. It is observed that the C*-algebra of bounded linear operators associated to this representation space contains algebraic curvature tensors. Classical vacuum gravitational fields i.e., Einstein manifolds correspond to quantum observables obeying at least one of the above decompositions of the space. In this way classical general relativity exactly in 4 dimensions naturally embeds into an algebraic quantum field theory whose net of local C*-algebras is generated by local algebraic curvature tensors and vector fields. This theory is constructed out of the structures provided by an oriented 4-manifold only hence possesses a diffeomorphism group symmetry. Motivated by the Gelfand--Naimark--Segal construction and the Dougan--Mason construction of quasi-local energy-momentum we construct certain representations of the limiting global C*-algebra what we call the ``positive mass representations''. Finally we observe that the bunch of these representations give rise to a 2 dimensional conformal field theory in the sense of G. Segal.

  • 26 November 2014, Janos Balog (Wigner Center, Hungary)

    Walking in a 3-dimensional scalar toy model slides

    The O(N) scalar field theory in the large N limit is an ideal toy model exhibiting "walking" behavior, expected in some SU(N) gauge theories with a large number of fermion flavors. We study the model using lattice regularization and show that when the ratio of the particle mass to an effective 4-point coupling (with dimension mass) is small, the beta function associated to the running 4-point coupling is ?walking?. We show the robustness of the walking phenomenon by showing that it can also be observed by studying physical observables such as the scattering phase shifts and the mass gap in finite volume.

  • 17 December 2014, Kim Splittorff (Niels Bohr Institute, Copenhagen, Denmark)

    Complex Langevin in QCD with chemical potential larger than half the pion mass

    The complexification of gauge fields in the complex Langevin approach to QCD at non zero chemical potential drastically affects the Dirac spectrum. In particular, we show that the eigenvalues the Dirac operator in complex Langevin simulations of QCD at non-zero chemical potential must accumulate at the origin in order to support spontaneous breaking of chiral symmetry. This mechanism is exemplified through application of the complex Langevin method in chiral random matrix theory.

  • 28 January 2015, Chaiho Rim (Sogang University, South Korea)

    Some applications of random matrix models slides

    Random matrix models are used to describe (1) statistical variance of multivariables using Gaussian pontential (2) 2d quantum gravity using polynomial potential (3) regular and irregular conformal block using Penner-type (logarithmic) potential.

  • 11 February 2015, Fidel I Schaposnik Massolo (La Plata, Argentina)

    D5-brane boundary reflection factors slides

    a We compute the strong coupling limit of the boundary reflection factor for excitations on open strings attached to various kinds of D5-branes that probe AdS(5)xS(5). We study the crossing equation, which constrains the boundary reflection factor, and propose that some solutions will give the boundary reflection factors for all values of the coupling. Our proposal passes various checks in the strong coupling limit by comparison with diverse explicit string theory computations. In some of the cases we consider, the D5-branes correspond to Wilson loops in the k-th rank antisymmetric representation of the dual field theory. In the other cases they correspond in the dual field theory to the addition of a fundamental hypermultiplet in a defect.

  • 18 February 2015, Yunfeng Jiang (Saclay, France)

    Integrability in AdS/CFT : Three-Point Functions in N=4 SYM theory

    The discovery of integrability in the planar N=4 Super-Yang-Mills theory and its AdS dual has triggered a lot of progress in recent years. Equipped with powerful integrability based techniques, it is now possible to compute many interesting quantities such as the spectrum, Wilson loop and scattering amplitudes of the theory at any value of the coupling constant. In this talk, I will explain how to compute three-point functions in the planar N=4 SYM theory at tree level and one loop in perturbation theory using Bethe ansatz. I will also discuss the semi-classical limit and the comparison with the computation at strong coupling.

  • 25 February 2015, Robert Wald (Enrico Fermi Institute, Chicago)

    Dynamic and Thermodynamic Stability of Black Holes and Black Branes slides

    I describe work with with Stefan Hollands that establishes a new criterion for the dynamical stability of black holes in $D \geq 4$ spacetime dimensions in general relativity with respect to axisymmetric perturbations: Positivity of the canonical energy, $\mathcal E$, on a subspace of linearized solutions that have vanishing linearized ADM mass, momentum, and angular momentum at infinity and satisfy certain gauge conditions at the horizon implies mode stability. Conversely, failure of positivity of $\mathcal E$ on this subspace implies the existence of perturbations that cannot asymptotically approach a stationary perturbation. We further show that $\mathcal E$ is related to the second order variations of mass, angular momentum, and horizon area by $\mathcal E = \delta^2 M - \sum_i \Omega_i \delta^2 J_i - (\kappa/8\pi) \delta^2 A$. This establishes that dynamic stability of a black hole is equivalent to its thermodynamic stability (i.e., its area, $A$, being a maximum at fixed ``state parameters'' $M$, $J_i$). For a black brane, we prove that a sufficient condition for instability is the failure of the Hessian of $A$ with respect to $M$, $J_i$ to be negative, thus proving the Gubser-Mitra conjecture. We also prove that positivity of $\mathcal E$ is equivalent to the satisfaction of a ``local Penrose inequality,'' thus showing that satisfaction of this local Penrose inequality is necessary and sufficient for dynamical stability.

  • 25 February 2015, George Sparling (Pittsburgh)

    Searching for structure: A tribute to Doctor Zoltan Perjes - the universe, large and small.

    One hundred years after the final formulation of general relativity by Albert Einstein, the quest to comprehend his theory continues. Doctor Zoltan Perjes was a master of the theory, constantly stretching its boundaries. He would work in cosmology, the study of the whole universe, always being concerned with relating the abstract theory to measurable quantities, such as the Sachs-Wolfe effect. He would also work locally dealing with isolated bodies, such as stars, testing their stability and trying to construct realistic models. He had his own speciality the theory of space times with a timelike symmetry, for which he developed his beautiful triad formalism and the sub-theory of conformastat space-times.

    In this talk I will present two recent results of mine and my colleague Jonathan Holland, which I believe would have been dear to Zoltan's heart: First an analysis of Bach flat metrics of a split form: in particular it is proved that all such have a symmetry. Second a discussion of the new approach to cosmology that we have initiated in the last two years: in our basic model a key role is played by a timelike conformal symmetry. This new theory leads to a remarkable new picture of the relationship between local and global cosmology and brings to bear for the first time major modern mathematical tools: the theory of Fano manifolds.

  • 4 March 2015, Gabor Takacs (BME, Hungary)

    Form faktorok a lepcsohazbol slides

    Alyosha Zamolodchikov a sinh-Gordon modell termodinamikai Bethe Ansatz egyenletenek analitikus elfolytatasaval lepcsos renormalasi csoport folyamot konstrualt, amivel sikerult alternativ leirast adnia a konform minimalis modellek (multikritikus Ising modellek) kozotti cross-over folyamokra. Otletet a c-tetelen keresztul megfogalmazva eljarast adunk a cross-overt leiro terelmeletek egzakt form faktorainak konstrukciojara. A feltarulo struktura felveti annak lehetoseget, hogy szamos integralhato terelmelet egzakt form faktorait a bootstrap jelentette igen bonyolult matrix Riemann-Hilbert problema helyett egy, a TBA univerzalis megfogalmazasabol kapcsolodo diagrammatikus konstrukciobol megkonstrualjuk.

  • 11 March 2015, Tamas Kovacs (ATOMKI, Debrecen, Hungary)

    The chiral and the Anderson transition in QCD slides

    Recently we have been studying an Anderson-type localization-delocalization transition in the spectrum of the Dirac operator describing quarks in the quark-gluon plasma. In the talk I summarize the latest results about the properties of this transition and its possible implications on the thermodynamics of QCD and other QCD-like theories.

  • 18 March 2015, Peter Mati (BME, Hungary)

    The Vanishing Beta Function Curves from the Functional Renormalisation Group

    We will discuss the derivation of the so called Vanishing Beta Function curves which can be used to explore the fixed point structure of the theory under consideration. This can be applied to the O(N) symmetric theories essentially for arbitrary dimensions (D) and field component (N). We will show the restoration of the Mermin-Wagner theorem for theories defined in D <= 2 and the presence of the Wilson Fisher fixed point in 2<D<4. Triviality is found in D>4. Interestingly, one needs to make an excursion to the complex plane to see the triviality of the four dimensional O(N) theories. The large N analysis shows a new fixed point candidate in 4<D<6 dimensions which turns out to define an unbounded fixed point potential supporting the recent results by R. Percacci and G. P. Vacca. Corresponding paper: http://arxiv.org/abs/1501.00211

  • 1 April 2015, Peter Gnadig (Eotvos)

    Alkalmazhato-e a Biot--Savart-torveny nem zarodo "aramkorokre"?

  • 8 April 2015, Richard Szabo (Heriot-Watt University, UK)

    Nonassociative Quantum Mechanics slides

    We briefly review recent developments from string theory which suggest that non-geometric string backgrounds experience a nonassociative deformation of spacetime geometry. A geometrisation of this frame leads to a sigma-model for closed strings propagating in an effective target space that is the phase space of the original compactification manifold. Quantization of the sigma-model produces an explicit nonassociative star-product algebra on functions on phase space, which is related to the quantization of Nambu-Poisson structures. We use this formalism to develop a phase space formulation of nonassociative quantum mechanics, and demonstrate that, against all odds, a consistent formulation seems indeed possible. Our approach is completely quantitative and adds to previous qualitative discussions of nonassociativity in quantum mechanics, and it moreover avoids previous no-go theorems.

  • 22 April 2015, Peter Bantay (Eotvos)

    Replication identities in 2D Conformal Field Theory slides

    We discuss an approach to replication identities satisfied by conformal characters of a 2D CFT that links them with symmetries of symmetric products, providing a physics interpretation of the Hauptmodul property of Monstruous Moonshine.

  • 29 April 2015, Marietta Homor (Eotvos)

    Transport Coefficients and Thermalisation in Classical Field Theories slides

    Our first objective is to show a practical method for the determination of viscosity in classical, relativistic field theories. We will discuss the relevant physical quantities in thermalised systems by the example of Phi^4 theory. Furthermore, we recently observed that the thermalisation in itself has interesting properties e.g. the distributions thermalize in a much shorter scale than the temperature. Inspired by this, our second objective is to examine the thermalisation properties of Phi^4 theory from various initial conditions. We propose, that the first correction to the Maxwell-Boltzmann distribution is the Tsallis distribution.

  • 20 May 2015, Andreas Ringwald (DESY Hamburg)

    Ultralight Axion-Like Particles from Strings slides

    There are various puzzling observations in sastrophysics which may be explained by the existence of light axion-like particles (ALPs) with a tiny coupling to the known particles of the Standard Model. In this talk, we will discuss how such ALPs and their couplings to the Standard Model may arise naturally from ultraviolet extensions inspired by string theory.

  • 16 September 2015, Anna Zsigmond (MTA-Wigner)

    Studies of nuclear parton distribution functions in proton-lead collisions with the CMS experiment slides

    The purpose of the study of nuclear parton distribution functions (nPDFs) is two-fold. On one hand, it provides a handle on the initial state cold nuclear effects that can be separated from the final state effects of the hot and dense medium produced in nucleus-nucleus collisions at the LHC and RHIC. On the other hand, precise knowledge of nPDFs is required for understanding the mechanisms associated with nuclear binding from a QCD improved parton model perspective. The proton-lead collision data recorded by CMS in 2013 provides the best sample to constrain nuclear parton distribution functions in the previously unexplored high Q^2 and low Bjorken x region. Results from the CMS experiment on electroweak boson, charged hadron and jet production in proton-lead collisions will be presented.

  • 30 September 2015, Andras Laszlo (MTA-Wigner)

    A natural extension of the conformal Lorentz group in a field theory context slides

    In this talk a finite dimensional unital associative algebra is presented, and its group of algebra automorphisms is detailed. The studied algebra can physically be understood as the creation operator algebra in a formal quantum field theory at fixed momentum for a spin 1/2 particle along with its antiparticle. It shall be shown that the essential part of the corresponding automorphism group can naturally be related to the conformal Lorentz group. In addition, the non-semisimple part of the automorphism group can be understood as ``dressing'' of the pure one-particle states. The studied mathematical structure may help in constructing quantum field theories in a non-perturbative manner. In addition, it provides a simple example of circumventing Coleman-Mandula theorem using non-semisimple groups, without SUSY. preprint

  • 7 October 2015, Zoltan Bajnok (MTA-Wigner)

    Form factor approach to the string vertex slides

  • 14 October 2015, Andrey Slavnov (Steklov Mathematical Institute, Moscow)

    Soliton solutions of classical equations of motion in the modified Yang-Mills theory slides

    It is shown that classical solutions with finite energy exist in the modified formulation of Yang-Mills theory.

  • 21 October 2015, Yoichi Kazama

    AdS/CFT and integrability: Cognate structure at weak and strong coupling for three-point functions

  • 4 November 2015, Sona Pochybova (MTA-Wigner)

    High Momentum Particle identification with the HMPID detector at the CERN's ALICE Experiment slides

    The High Momentum Particle IDentification (HMPID) detector is a Ring Imaging CHerenkov (RICH) radiation detector within the ALICE experiment used to identify pions, Kaons and protons between 1.0 < pT < 5.0 GeV/c. It is used by the collaboration in several physics analyses as a supporting tool for the tracking detectors in the ALICE central barrel. In the first part of my talk I will show how the track-by-track particle identification is utilized to extend the pion identification range by 1 GeV/c in momentum. Further, I will show how this extension helps in the spectra and correlation analyses to cross-check the results. Such and independent cross-check not only strengthens the robustness of the experimental claims, moreover, it has the potential of decreasing the systematic errors connected to these analyses. In the second part of my talk I will discuss the physics implications of the results I obtained in the analysis of p/pi ratio, where protons and pions are identified using the HMPID. This ratio is extracted from the spectra analysis and the origins of it's value are studied in the correlation analysis.

  • 11 November 2015, Tamas Gombor (ELTE)

    Algebraic Bethe Ansatz for O(2N) sigma models with integrable diagonal boundaries slides

    The finite volume problem of the O(2N) sigma models with integrable diagonal boundaries on a finite interval is investigated. The double row transfer matrix is diagonalized by Algebraic Bethe Ansatz. The boundary Bethe Yang equations for the particle rapidities and the accompanying Bethe Ansatz equations are derived.

  • 18 November 2015, Robert Vertesi (MTA-Wigner)

    Recent results from the STAR experiment slides

    In ultrarelativistic heavy ion collisions, a phase transition occurs from hadronic matter into a state of deconfined quarks and gluons. Properties of this latter state of matter, dubbed as the strongly interacting Quark Gluon Plasma (sQGP) has been a subject of extensive measurements at the Relativistic Heavy Ion Collider (RHIC), which collides several types of heavy nuclei (eg. d+Au, Cu+Cu, Au+Au, U+U) with collision energies that range from \sqrt{s_{NN}}=7.7 to 200 GeV. The Solenoidal Tracker at RHIC (STAR) identifies particles and measures their properties in the full azimuth angle and in a wide rapidity range. STAR's beam energy scan (BES) program aims to understand the QCD phase diagram and search for the critical point, and includes a new fixed target program that extends the lower energy limit down to \sqrt{s_{NN}}=3.3 GeV. Heavy and light flavor probes, on the other hand, are used to understand the thermal and dynamical properties of the sQGP. Some of the most interesting recent heavy-ion results from STAR will be overviewed in this talk.

  • 25 November 2015, Miklos Horvath (MTA-Wigner)

    Liquid-gas crossover within a quasi-particles picture slides

    We consider an effective field theory description of quasi-particle excitations aiming to associate the transport properties of the system with the spectral density of states. Tuning various properties of the many-particle correlations, we investigate how robust microscopic features are translated into the macroscopic observables like shear viscosity and entropy density. The liquid-gas crossover is discussed using several counterexamples. We sketch the adaptability of the present examination to much more intricate questions, like transport in (continuum extrapolated) lattice QCD or in correlated solid state systems with condensate.

  • 2 December 2015, Gabor Sarosi (BME)

    Warped Weyl fermion partition functions

    Warped conformal field theories (WCFTs) are a novel class of non-relativistic theories. A simple, yet non-trivial, example of such theory is a massive Weyl fermion in (1+1)-dimensions, which we study in detail. We derive general properties of the spectrum and modular properties of partition functions of WCFTs. The periodic (Ramond) sector of this fermionic system is non-trivial, and we build two novel partition functions for this sector which have no counterpart in a two dimensional CFT. The thermodynamical properties of WCFTs are revisited in the canonical and micro-canonical ensemble.

  • 9 December 2015, Judit Kovacs (Eotvos)

    Doublet-singlet model and unitarity

    Simplified models are useful tools to study the low energy extensions of the Standard Model. Here, we study a doublet-singlet fermionic extension, where new vector-like fermions couple to the weak gauge bosons and the Higgs via new Yukawa couplings. These allow for nontrivial mixing in the new sector, providing a stable, neutral dark matter candidate. We calculate the two particle scattering amplitudes in the model, impose the perturbative unitarity constraints and establish bounds on the Yukawa couplings.

  • 16 December 2015, Istvan Kaposvari (Eotvos)

    Scalar mass stability bound in a simple Yukawa-theory from renormalisation group equations slides

    Functional Renormalisation Group equations are constructed for a simple fermion-scalar Yukawa-model with discrete chiral symmetry, including also the effect of a nonzero composite fermion background beyond the conventional scalar condensate. Two approximate versions consistent with the scale dependent equations of motion are solved, taking into account also field renormalisation. The lower bound for the mass of the scalar field is determined requiring the stability of effective potential in the full momentum range, from the cutoff down to vanishing momentum. Close agreement is demonstrated with the results of previous studies done exclusively in presence of scalar condensate. A semiquantitative explanation is provided both for the negligible effect of the wave-function renormalisation and the narrow dispersion in the scalar mass bounds found from different approximation schemes.

  • 10 February 2016, Diego Correa (La Plata, Argentina)

    1-loop exponentiation for large rank Wilson loops slides

    In this talk I will describe Wilson loops in N=4 super Yang-Mills, for external particles in k-rank totally symmetric representations. I will argue that for large k the perturbative resummation of diagrams is dominated by ladder diagrams and simply given by the exponential of the 1-loop result. This will be verified -for a generic family of Wilson loops- in the strong coupling limit performing a classical D-brane computation and using the AdS/CFT correspondence.

  • 17 February 2016, Attila Pasztor (Wuppertal)

    QCD at small chemical potentials: the equation of state and fluctuations

    I will present some recent continuum extrapolated lattice results on QCD in the grand canonical ensemble. The chemical potentials and temperatures considered are relevant for the RHIC beam energy scan. The methods used are Taylor expansion near zero chemical potential and analytic continuation from an imaginary chemical potential. I will present comparisons with the hadron resonance gas model at low and with resummed perturbation theory at high temperatures.

  • 24 February 2016, Marton Lajer (Eotvos)

    Truncated Hilbert Space Approach for the 1+1D phi^4 Theory slides

    We used the massive analogue of the truncated conformal space approach to study the broken phase of the 1+1 dimensional scalar phi^4 model in finite volume, similarly to the work by S. Rychkov and L. Vitale. In our work, the finite size spectrum was determined numerically using an effective eigensolver routine, which was followed by a simple extrapolation in the cutoff energy. We analyzed both the periodic and antiperiodic sectors. The results were compared with semiclassical and Bethe-Yang results as well as perturbation theory. We obtained the coupling dependence of the infinite volume breather and kink masses for moderate couplings. The results fit well with semiclassics and perturbative estimations, and confirm the conjecture of Mussardo that at most two neutral excitations can exist in the spectrum. We believe that improving our method with the renormalization procedure of Rychkov et al. enables to measure further interesting quantities such as decay rates and the inelastic part of scattering matrices.

  • 2 March 2016, Gabor Cynolter MTA-ELTE)

    Diphoton excess, gauge invariance and perturbative unitarity

    I discuss some aspects of the recently reported excess in the diphoton resonance search by ATLAS and CMS. Assuming that the resonance is due to a new singlet (pseudo)scalar, 5-dimensional effective interactions are postulated. SU(2)xU(1) gauge invariance implies additional dim-5 interactions with the weak bosons leading to nontrivial constraints on the validity of the effective theory. The effective interactions can be generated at 1-loop by additional coloured and charged vector-like quarks, these favour narrow-width and are further constrained.

  • 9 March 2016, Gabor Takacs (BME)

    Particle Formation and Ordering in Strongly Correlated Fermionic Systems: Solving a Model of Quantum Chromodynamics slides

    We study a (1+1)-dimensional version of the famous Nambu-Jona-Lasinio model of Quantum Chromodynamics (QCD2) both at zero and finite matter density using non-perturbative techniques (non-Abelian bosonization and Truncated Conformal Space Approach). At zero density we describe a formation of fermion three-quark (nucleons and Delta-baryons) and boson (two-quark mesons, six-quark deuterons) bound states and also a formation of a topologically nontrivial phase. At finite matter density, the model has a rich phase diagram which includes phases with density wave and superfluid quasi-long-range (QLR) order and also a phase of a baryon Tomonaga-Luttinger liquid (strange metal). The QLR order results as a condensation of scalar mesons (the density wave) or six-quark bound states (deuterons).

  • 16 March 2016, Laszlo Hollo (Ecole Normale Superieure)

    On form factors of boundary changing operators slides

    We develop a form factor bootstrap program to determine the matrix elements of local, boundary condition changing operators. We propose axioms for these form factors and determine their solutions in the free boson and Lee-Yang models. The sudden change in the boundary condition, caused by an operator insertion, can be interpreted as a local quench and the form factors provide the overlap of any state before the quench with any outgoing state after the quench. http://arxiv.org/abs/1510.08232

  • 23 March 2016, Gabriella Pasztor (MTA-ELTE)

    Search for a New Heavy Boson in the di-photon channel (and elsewhere) at the LHC slides

    The ATLAS and CMS experiments observe a moderate excess of events in their search for new heavy bosons in the di-photon final state in the 2015 LHC data collected at a centre-of-mass energy of 13 TeV corresponding to about 3 fb^{-1} integrated luminosity per experiment. Is this a statistical fluctuation in the limited amount of 2015 data or maybe the first sign of New Physics? I will review the updated results on the di-photon search as well as the newly emerging results from other final states such a new particle could decay into.

  • 30 March 2016, Gyula Bencedi (MTA-Wigner)

    Multiplicity dependence of charged pion, kaon, and (anti)proton production at large transverse momentum in p-Pb collisions at 5.02 ATeV

    The discovery of fluid-like behaviour in small collision systems (like pp, pA) is one of the recent discoveries at the Large Hadron Collider (LHC). To understand the origin of the phenomenon several observables are being studied as a function of the event multiplicity. In particular, the measurement of identified particle production in a broad transverse momentum (pT) range provides valuable information since it is sensitive to flow-like effects and new hadronization mechanisms like recombination and jet quenching. In this talk I am going to present the recent results, arXiv:1601.03658v1 [nucl-ex], on the multiplicity dependence of charged pion, kaon and (anti)proton production in p-Pb collisions at 5.02 ATeV measured by the ALICE Collaboration at the LHC. The detailed discussion of the analysis to measure the pT spectra will be covered. Comparisons among pp, p-Pb and Pb-Pb collisions will be shown, in addition the nuclear modification factor for identified particles in non-single diffractive p--Pb collisions will be discussed as well.

  • 6 April 2016, Janos Balog (MTA-Wigner)

    Mass-coupling relation in quantum integrable models slides

    We determine exactly the mass-coupling relation in the simplest multi-scale quantum integrable model, the homogenous sine-Gordon model with two independent mass scales. We find a generalization of the ? sum rule Ward identity which enables one to derive a differential equation for the mass-coupling relation. The solution can be expressed in terms of hypergeometric functions.

  • 13 April 2016, Falk Bruckmann (Regensburg)

    Phase diagram of an asymptotically free sigma model through lattice dualization slides

    The 2d O(3) sigma model shares asymptotic freedom, mass generation and other nonperturbative features with 4d QCD. Likewise, a sign problem at nonzero chemical potential hampers numerical simulations of its thermodynamics. I demonstrate how mapping the corresponding lattice fields to dual degrees of freedom solves the sign problem. At finite volume and low temperatures this can be used to extract the phase shifts governing the particles' interaction. Moreover, I discuss the phase diagram of this model including a quantum phase transition and a comparison to an integrable model.

  • 20 April 2016, Zoltan Keresztes (Szeged)

    Gravitational, shear and matter waves in Kantowski-Sachs cosmologies slides

    A general treatment of vorticity-free, perfect fluid perturbations of Kantowski-Sachs models with a positive cosmological constant are considered within the framework of the 1+1+2 covariant decomposition of spacetime. The dynamics is encompassed in six evolution equations for six harmonic coefficients, describing gravito-magnetic, kinematic and matter perturbations, while a set of algebraic expressions determine the rest of the variables. The six equations further decouple into a set of four equations sourced by the perfect fluid, representing forced oscillations and two uncoupled damped oscillator equations. The two gravitational degrees of freedom are represented by pairs of gravito-magnetic perturbations. In contrast with the Friedmann case one of them is coupled to the matter density perturbations, becoming decoupled only in the geometrical optics limit. In this approximation, the even and odd tensorial perturbations of the Weyl tensor evolve as gravitational waves on the anisotropic Kantowski-Sachs background, while the modes describing the shear and the matter density gradient are out of phase dephased by Pi/2 and share the same speed of sound.

  • 27 April 2016, Nicolas Filipovic (Eotvos)

    Latest results on Quarkonium production in nuclear matter at the LHC slides

    In this talk I will review the latest experimental results on quarkonium production and nuclear modification at the LHC. I will introduce the basics of quarkonium modification in the Quark-Gluon Plasma (QGP), and present the current state-of-the-art measurements on charmonia and bottomonia. The emphasis will be put on the pp, pPb and PbPb data recorded by the CMS Collaboration during Run 1, with a look at the results from other experiments at LHC and RHIC.

  • 4 May 2016, Denes Sexty (Wuppertal)

    The complex Langevin equation and the sign problem in lattice QCD slides

    The Complex Langevin method is a proposal to solve sign problems by generating a positive measure ensemble on the complexified field manifold using analytical continuation. I report on recent developments such as gauge cooling and show simulation results in full QCD, and point out open questions and pitfalls of the method.

  • 11 May 2016, Antal Jakovac (Eotvos)

    Functional Renormalization Group at finite chemical potential slides

    In the talk I give a short overview about the Functional Renormalization Group (FRG) technique, the usually applied approximations and solution methods. Then I will discuss our recently developed approach to treat fermionic systems at small (zero) temperature and finite chemical potential on the example of a simple Yukawa-model. We determine the phase diagram on the coupling constant space, and discuss the role of the different approximation schemes (mean field, one-loop, FRG LPA). Finally I will mention some possible applications.

  • 18 May 2016, Szilveszter Harangozo (MTA-Wigner)

    A new Heavy-Ion Jet Interaction Generator, model updates and first results

    The popular HIJING event generator is redesigned to match the compatibility with ALIROOT and is rewritten to C++. We review here the design of the C++ interface and the connections to the PYTHIA event generator. With the development, new physics is introduced into the code, like the inclusion of new particle distribution functions, and the DGLAP evolution of the shadowing effect. We examine the first results of this updated model and discuss further update possibilities.

  • 25 May 2016, Walter Winter (DESY)

    High-energetic cosmic neutrinos and the test of fundamental physics slides

    The very recent discovery of a diffuse flux of cosmic neutrinos has introduced a new messenger of the high-energy universe in addition to photons at multiple wavelengths, cosmic rays, and now also gravitational waves. We discuss the possible origin of these neutrinos, and we illustrate how they could be used for tests of fundamental particle physics.

  • 15 June 2016, Balint Erdi (Eotvos)

    A sarkany-konfiguracio -- egy megoldas csodai slides

    A centralis konfiguraciok nagy szerepet jatszanak az n-test problemakkal kapcsolatos kutatasokban. Centralis konfiguraciok eseten az egyes tesekre hato eredo ero a rendszer tomegkozeppontjan megy at, es a testek onhasonlo konfiguracioju mozgast vegeznek. A haromtest-problema centralis konfiguracioi az Euler-Langrange-megoldasok. n>3-ra egyenlo tomegu szimmetrikus elrendezesek ismertek. Az eloadas roviden attekinti az eddigi legfontosabb eredmenyeket, es bemutat egy kozelmultban kapott analitikus megoldast a negytest-problema egy tengelyszimmetrikus esetere.

  • 22 June 2016, Attila Krasznahorkay (ATOMKI)

    On the track of the dark forces slides

    Electron-positron angular correlations were measured for the magnetic dipole 17.6 MeV and the 18.15 MeV transitions in 8Be. Significant enhancement relative to the internal pair creation was observed at large angles in the angular correlation for the 18.15 MeV transition with a confidence level of > 5sigma. This observation might indicate that, in an intermediate step, a neutral particle with a mass of 16.70 MeV/c^2 was created. In January we reported the above anomaly in Physical Review Letters, but at first, few took notice. That changed in April with a paper by Jonathan Feng, a theoretical particle physicist, who presented their work with the title of: ``Evidence for a Protophobic Fifth Force from 8Be Nuclear Transitions'', which was followed by an article in Nature with a title of: ``Has a Hungarian physics lab found a fifth force of nature?'' Such an article produced a boom in the media. The proposed boson has become lunch-table talk in physics departments far and wide, and plans are afoot for testing the idea. If the particle is confirmed, that would completely upend our understanding of the universe. I am going to show the reliability of the data, place such a particle into context with other experimental results and discuss the implications of the results.

  • 7 September 2016, Daniel Barna (Wigner)

    Magyar reszvetel a jovo 50-50 TeV-es gyorsitojanak fejleszteseben

    Az eloadasban szo lesz az FCC-rol (Future Circular Collider), a CERN kovetkezo generacios gyorsito terverol; arrol, hogy hogyan lehet megszabadulni az egyenkent 8.4 GJ energiat tarolo nyalaboktol (ami 23 darab 200 m hosszu, 150 km/h sebessegu TGV vonat mozgasi energiajanak felel meg); az LHC es az FCC nyalabtemetojerol; szupravezetokrol, szupravezeto magnesekrol es szupravezeto arnyekolasrol; specialis kicsatolo magnesekrol. Minderrol annak a Budapest-CERN kozos projektnek a kereteben fogok beszelni, ami iden indult, es a fenti problemakra keres megoldast, es fejleszt prototipusokat.

  • 21 September 2016, Matteo Giordano (Eotvos)

    Landau levels in QCD in an external magnetic field slides

    The effect of an external magnetic field on strongly interacting matter is relevant for important problems in particle physics and cosmology, such as heavy ion collisions, neutron stars, and the evolution of the early universe. In this talk I will first briefly review the current knowledge on this topic, as obtained by means of first-principles lattice numerical calculations. I will then discuss in detail the issue of Landau levels in QCD, showing preliminary numerical results obtained on the lattice. In particular, I will show that, after turning on the strong interactions, only the lowest Landau level remains of the whole Landau level structure. I will then discuss how much of the observed effects of a magnetic field can be explained in terms of the lowest Landau level. Our results can be used to test the validity of low-energy models of QCD that make use of the lowest-Landau-level approximation.

  • 28 September 2016, Gergely Fejos (Osaka University)

    Thermal properties and evolution of the axial anomaly for 2+1 flavors slides

    I will be talking about the thermal evolution of the axial anomaly of QCD in terms of effective theories with 2+1 flavors. It will be shown that mesonic fluctuations are of great importance from the point of view of the thermal behavior of the 't Hooft determinant term. Results indicate that fluctuations strengthen the axial anomaly at finite temperature and it does not vanish at the critical point. The phenomenon has been found to have significance in the thermal properties of the mesonic spectrum, especially concerning the eta - eta' system. Analysis of the spectrum and the anomaly in nuclear medium will also be discussed.

  • 5 October 2016, Sandor Katz (Eotvos)

    Axion cosmology from lattice QCD slides

    The strong CP problem of QCD can be solved via the Peccei-Quinn mechanism. The resulting pseudo-Goldstone boson, the axion is a natural candidate for dark matter. In order to quantitatively understand axion dark matter production two important QCD inputs are required: the equation of state and the topological susceptibility at high temperatures. We determine these quantities and use them to constrain the axion mass in different axion production scenarios.

  • 12 October 2016, Zoltan Kokenyesi (Eotvos)

    Chiral expansion and Macdonald deformation of two-dimensional Yang-Mills theory slides

    There is a conjecture that relates the partition function of a four-dimensional BPS black hole in Type IIA string theory compactified on a Calabi-Yau threefold with the A-model topological string amplitudes on the Calabi-Yau. The topological string amplitudes on special geometries reduce to a q-deformed version of two-dimensional U(N) Yang-Mills theory. We study the refined version of this duality which leads to a refinement of the two-dimensional Yang-Mills theory.

    We derive the analog of the large N Gross-Taylor holomorphic string expansion for the refinement ofq-deformed U(N) Yang-Mills theory on a compact oriented Riemann surface.The derivation combines Schur-Weyl duality for quantum groups with the Etingof-Kirillov theory of generalized quantum characters which are related to Macdonald polynomials. In the unrefined limit we reproduce the chiral expansion of q-deformed Yang-Mills theory derived by de Haro, Ramgoolam and Torrielli. In the classical limit q=1, the expansion defines a new beta-deformation of Hurwitz theory wherein the refined partition function is a generating function for certain parameterized Euler characters, which reduce in the unrefined limit to the orbifold Euler characteristics of Hurwitz spaces of holomorphic maps. We discuss the geometrical meaning of our expansions in relation to quantum spectral curves and beta-ensembles of matrix models arising in refined topological string theory.

  • 26 October 2016, Janos Polonyi (Strasbourg)

    Time scales of diffusion and decoherence slides

    A disszipativ es a dekoherenciahoz vezeto folyamatokat altalaban az utkozesek kovetesevel irjuk le (E. Joos, H. Zeh, Z. Phys. 59 223 (1985), B. Vacchini, K. Hornberg Phs. Rep. 478 71 (2009)). Ennel megbizhatobb, egy szisztematikus kozelitesen alapulo modszerrol, a tesztreszecske Schwinger--Keldysh effektiv Lagrange-fuggvenyenek kiszamolasarol lesz szo az eloadasban (Polonyi, arxiv.org/abs/1605.00834). Harom kis parameterben valo kifejtesre alapul a levezetes, melyek a kovetkezoek: a tesztreszecske-gaz kolcsonhatas erossege, a kolcsonhatas altal indukalt koordinata megvaltozas nagysaga es annak sebessege. Az utkozesi modell alapjan a dekoherenciat sok nagysagrenddel gyorsabbnak kepzelik mint a disszipacios folyamatokat. Ugyan ennek az elvarasnak az alapjaul szolgalo egyenletekhez hasonlora vezet az uj modszer, azonban az is kiderul, hogy az emlitett kovetkezmenyre az egyenleteknek alkalmazhatosagi tartomanyan kivuli hasznalata vezet el. Ez az utkozesi modellben a tobbszoros utkozesek tartomanyanak felel meg. Az egyenletek figyelmesebb hasznalata alapjan a dekoherencia nem adodik gyorsabbnak a disszipacional, inkabb egymashoz kozel marad a ket idoskala. Harom kulonbozo dekoherencia fogalmat lehet az uj levezetes gondolatmenete alapjan megkulonboztetni es az egzaktul megoldhato harmonikus rendszerekben a "valodi" dekoherencia az ido dupla exponencialis fuggese alapjan all be. Ez a Schwinger--Keldysh-leiras egy figyelmre melto tulajdonsaganak egyszeru kovetkezmenye, nevezetesen annak, hogy nyilt, disszipativ rendszerekben az ido ellentetes iranyban folyik a megfigyelheto mennyisegekben mint a kvantum fluktuaciokban. Tehat vegul is nagyon gyorsan all be a dekoherencia, sokkal gyorsabban mint barmely mas fizikai folyamat csak epp nem vilagos az aszimptotikus allapot eleresenek a modja a kolcsonhato rendszerekben, mert annak leirasara az alkalmazott kifejtesek kereten belul egyenlore nem latszik lehetoseg.

  • 2 November 2016, Istvan Szecsenyi (Durham, BME)

    TTbar-deformed 2D Quantum Field Theories slides

    It was noticed many years ago, in the framework of massless RG flows, that the irrelevant composite operator TTbar, built with the components of the energy-momentum tensor, enjoys very special properties in 2D quantum field theories, and can be regarded as a peculiar kind of integrable perturbation. Novel interesting features of this operator have recently emerged from the study of effective string theory models. In this talk, we focus on further properties of this distinguished perturbation. We discuss how it affects the energy levels and one-point functions of a general 2D QFT in finite volume through a surprising relation with a simple hydrodynamic equation. We argue that, at the classical level, the deformation naturally maps the action of N massless free bosons into the Nambu-Goto action in static gauge, in N+2 target space dimensions, and we briefly discuss a possible interpretation of this result in the context of effective string models.

  • 16 November 2016, Arpad Hegedus (Wigner)

    Numerical solution and strong coupling results in the spectral problem of planar AdS/CFT correspondence slides

    Maldacena's famous AdS/CFT correspondence states that type IIB string theory on AdS5 x S5 background is equivalent to maximally supersymmetric Yang-Mills theory with SU(N) gauge group. In the large N (planar) limit integrability was discovered on both sides of the correspondence, which makes it possible to get exact results in the planar limit. In this talk we use integrability to compute numerically the anomalous dimensions for twist-2 operators in the super-Yang-Mills theory. Numerical data allowed us to get the coefficients of the strong coupling series expansion of the anomalous dimensions with high precision and to compare them with earlier proposals.

  • 23 November 2016, Monika Kofarago (Wigner)

    Characterization of the near-side jet-peak in Pb- Pb collisions at sqrt(s_NN) = 2.76 TeV at ALICE slides

    By studying jets which propagate through the medium produced in heavy-ion collisions, one can get insight to the properties of the quark-gluon plasma. At low p_T the reconstruction of jets is difficult over the large fluctuating background, and instead two-particle angular correlation measurements can be used to study the  interplay of jets with the produced medium. In these measurements jets manifest themselves as a near-side peak, and the interactions would result in a centrality dependent modification of this peak. I will present the results obtained by the ALICE detector in pp and Pb--Pb collisions at 2.76 TeV, which show an asymmetric broadening of the near-side peak in Pb--Pb collisions towards central events at low p_T. The broadening is more pronounced in the Delta eta direction, and a novel feature is also observed at low p_T in central collisions: the peak shape depart from a Gaussian and a depletion around (Delta phi,Delta eta) = (0,0) develops. The data is compared to AMPT simulations to study the influence of longitudinal, radial and elliptic flow, and the studies suggest that the interplay of jets with radial and longitudinal flow play a key role in the development of the observed effects.

  • 30 November 2016, Bence Kocsis (Eotvos)

    The dynamical origin of black hole mergers

    With the detections of the gravitational waves emitted during black hole mergers, LIGO has recently opened the field of gravitational wave astrophysics. In this talk I will discuss some of the astrophysical processes that may be responsible for the observed events. Although less than 0.5% of the stellar mass is in dense stellar systems, I will argue that a large fraction of the black hole mergers may originate in these environments due to gravitational bremsstrahlung and multibody interactions. Accretion disks in active galactic nuclei may also facilitate black hole mergers. Finally, if primordial black holes constitute a fraction of dark matter, gravitational waves may shed light on its composition. I will discuss predictions on the event rate distribution that may be used to disentangle these astrophysical processes to understand the origin of the observed gravitational wave events.

  • 7 December 2016, Kentaroh Yoshida (Kyoto University)

    Recent progress on Yang-Baxter deformations of type IIB superstring slides

    It is well-known that type IIB superstring theory on the AdS_5 x S^5 background is classically integrable. Recently, integrable deformations of this system have been studied very actively by adopting a systematic way called the Yang-Baxter deformation. With this method, integrable deformations are specified by classical r-matrices satisfying the classical Yang-Baxter equations. In this talk, I will give a review on recent progress on this issue. Classical r-matrices in a certain class lead to well-known examples of type IIB supergravity solutions including the Lunin-Maldacena backgrounds (dual for beta deformations of the N=4 super Yang-Mills theory), the Maldacena-Russo backgrounds (dual for non-commutative gauge theories) and Schroedinger spacetimes (dual for non-relativistic conformal field theories). In general, however, the resulting deformed backgrounds are not solutions of the standard type IIB supergravity, but of the generalized one. Finally, I will describe the relation between Yang-Baxter deformations and non-abelian T-dualities.

  • 14 December 2016, Csaba Torok (Eotvos)

    The sign problem in the O(3) nonlinear sigma model at finite chemical potential

    I am going to talk about the 1+1 dimensional nonlinear O(3) model at finite chemical potential and compare different methods to overcome the sign problem: the complex Langevin algorithm, the density of states method and the worm algorithm. In the latter, the sign problem is totally eliminated. We determine the range of parameters, where complex Langevin produces correct results and study whether taking the continuum limit allows the exploration of the full phase diagram of the model.