abstract

The ATLAS experiment is measuring both \(pp\) and PbPb collisions at the largest energies available in laboratory delivered by the Large Hadron Collider. Tests of the Standard Model and searches for exotic phenomena require precise information on QCD processes, which are responsible for the background present in \(pp\) interactions. In the heavy ion collisions, the QCD processes are determining the properties of the quark-gluon plasma which is created in the conditions of the extreme energy density. In this contribution, selected recent \(pp\) results from the ATLAS experiment are presented, including studies of jet properties and correlations in the multi-particle production. The effects of strong interactions in the dense matter created in PbPb collisions observed as the jet suppression and the collective flow are discussed in more detail.

abstract

In this paper, we review our recent developments on the understanding of the nature of the \(f_0(500)\) resonance — or \(\sigma \) meson — coming from the \(N_c\) expansion, dispersion relations with Chiral Perturbation Theory, as well as finite energy sum rules and semi-local duality.

abstract

Once subtracted dispersion relations with imposed crossing symmetry condition for the \(\pi \pi \) \(S\)-, \(P\)-, \(D\)- and \(F\)-wave scattering amplitudes were recently derived. Together with the well known Roy equations with two subtractions, they allowed e.g. for unambiguous and very precise determination of parameters of the \(f_0(500)\) and \(f_0(980)\) resonances in the \(S\)-wave. Analytic continuation of the amplitude to the complex energy plane led to finding the poles related with these resonances at \((457^{+14}_{-13}-i\,279^{+11}_{-7})\) MeV and at \((996 \pm 7-i\,25^{+10}_{-6})\) MeV respectively. These results led to significant changes in section of Particle Data Tables 2012 for light scalar mesons in comparison with previous editions. In this short paper, general mathematical structure of these dispersion relations is presented. It is shown that they produce output amplitudes with very small errors what significantly increases the accuracy of determined amplitudes. It can be very beneficial in practical applications e.g. in description of final state interactions in two-pion photo-production processes.

abstract

The recent results concerning the \(\eta \to 3\pi \) decay, the study of the box anomaly in \(\eta \to \pi ^+\pi ^-\gamma \), the search for signals of dark photons in the decay \(\phi \to \eta e^+ e^-\), and the \(\eta \)-meson production in \(\gamma \gamma \) collisions are presented.

abstract

We call attention to a class of current-quark mass dependent multi-quark interaction terms which break explicitly the chiral SU(3)\(_{\rm L}\times {\rm SU}(3)_{\rm R}\) and U\((1)_A\) symmetries. They complete the set of effective quark interactions that contribute at the same order in \(N_c\) as the ’t Hooft flavor determinant interaction and the eight quark interactions in the phase of spontaneously broken chiral symmetry. The \(N_c\) classification scheme matches the counting rules based on arguments set by the scale of spontaneous chiral symmetry breaking. Together with the leading in \(N_c\) four quark Nambu–Jona-Lasinio Lagrangian and current quark mass matrix, the model is apt to account for the correct empirical ordering and magnitude of the splitting of states in the low lying mass spectra of spin zero mesons. The new terms turn out to be essential for the ordering \(m_K \lt m_\eta \) in the pseudoscalar sector and \(m_{\kappa _0} \lt m_{a_0}\sim m_{f0}\) for the scalars.

abstract

Here, we present the cuLGT (www.cuLGT.com) code for gauge fixing in lattice gauge field theories with graphic processing units (GPUs). Implementations for SU(3) Coulomb, Landau and maximally Abelian gauge fixing are available and the overrelaxation, stochastic relaxation and simulated annealing algorithms are supported. Performance results for single and multi-GPUs are given.

abstract

Meson, being the simplest hadronic bound system, is the ideal “laboratory” to study the interaction between quarks, to understand the role of the gluons inside hadrons, and to investigate the origin of color confinement. To perform such studies, it is important to measure the meson spectrum with precise determination of resonance masses and properties, looking for rare \(q\overline {q}\) states and for unconventional mesons with exotic quantum numbers. With the imminent advent of the 12 GeV upgrade of the Jefferson Lab, a new generation of meson spectroscopy experiments will start. The “Meson-Ex” experiment in Hall B will use quasi-real photo-production to explore the spectrum of mesons in the light-quark sector, in the energy range of few GeVs.

abstract

The FAIR facility in Darmstadt has a broad program in the field of hadron and nuclear physics utilizing ion beams with unprecedented intensity and accuracy. The PANDA experiment, which is integrated in the HESR storage ring for antiprotons, is at the center of the hadron physics program. One of the main topics is hadron spectroscopy in the charmonium mass region where completely new phenomena in terms of unexpected resonances have recently opened a window to gain a deeper understanding of QCD.

abstract

We summarise recent results for the quark loop part of the light-by-light scattering contribution to the muons anomalous magnetic moment. In particular, we focus on the impact of a momentum dependent quark and quark-photon vertex. We compare the Dyson–Schwinger description with that of the extended Nambu–Jona-Lasinio model (ENJL) and find important quantitative differences. In particular, the transverse parts of the quark-photon vertex, which serve as a dynamical extension of simple vector meson dominance models, do not yield the large suppression as found in the ENJL model.

abstract

We present the predictions of a hydrodynamic model for the flow observables recently measured in the highest-multiplicity \(p\)+Pb collisions at the LHC. We focus on the “ridge” phenomenon, which provides an important probe of the long-range dynamics and may be used to support the collective interpretation of the \(p\)+\(A\) data.

abstract

Quantum fluctuations induce interesting structures in the initial state profiles for hydrodynamic calculations of the hot and dense quark-gluon plasma phase of heavy ion collisions. Especially, after the discovery of non-zero odd higher flow coefficients, there has been a lot of theoretical effort to quantify the correlation lengths of energy density fluctuations. A new method to characterize initial state profiles based on a 2D Fourier analysis is presented and an outlook on how this analysis can lead to a better quantitative understanding of the properties of hot and dense nuclear matter is given.

abstract

The ALICE experiment at the LHC has as its main purpose the study of the properties of the Quark-Gluon Plasma (QGP) produced in ultra-relativistic heavy ion collisions. To reach this goal ALICE has excellent vertexing, tracking and particle identification capabilities on a wide rapidity and transverse momentum range that make it perfect not only for the study of Pb–Pb collisions but also of \(pp\) and \(p\)–Pb interactions. In this paper, some of the latest ALICE results in Pb–Pb collisions will be shown together with the first results in \(p\)–Pb interactions.

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We analyze geometrical scaling (GS) in Deep Inelstic Scattering at HERA and in \(pp\) collisions at the LHC energies and in NA61/SHINE experiment. We argue that GS is working up to relatively large Bjorken \(x \sim 0.1\). This allows to study GS in negative pion multiplicity \(p_{\mathrm T}\) distributions at NA61/SHINE energies where clear sign of scaling violations is seen with growing rapidity when one of the colliding partons has Bjorken \(x \ge 0.1\).

abstract

Particle production has been studied in \(ep\) collisions with the H1 and ZEUS detectors at HERA. Charged particle spectra were measured in deep inelastic scattering. Predictions from Monte Carlo generators with different models of parton evolution were compared to the data. The production of the neutral strange hadrons \(K^0_{\rm S}\) and \({\mit \Lambda }^0\) was measured and was used to test various fragmentation models. Next-to-leading-order QCD calculations with fragmentation functions fitted to previous data as well as Monte Carlo generators were compared to the data. Spectroscopy of excited charm mesons with \(L=1\) was performed. In addition, fragmentation fractions of charm quarks hadronising into these states were extracted.

abstract

Potential-NRQCD offers an effective-theory based approach to heavy-quark physics. While meson \(Q\bar {Q}\) computations are tractable in pure \(\alpha _{\rm s}\)-perturbation theory, more complex many-body quark systems transcend it. A possibility inherited from nuclear physics is to employ the perturbative static potentials in a numerical diagonalization, eventually obtaining the exact lowest eigenvalue in each channel for a given-order perturbative potential. The power counting is manifest in the potential instead of the spectrum. The NNLO-potential for the 3-body problem is already available, so we have addressed triply-heavy baryons in this initial work with a computer-aided 2-parameter variational treatment.

abstract

Although the CMS is an LHC experiment designed to study \(pp\) collisions it is well suited to study heavy-ion collisions too. In this paper, we present the results of anisotropic particle emission and jet production in PbPb collisions at the CMS. The anisotropy is measured using four methods each of which has different sensitivities to the non-flow correlations and initial geometrical fluctuations. Additionally, the two-dimensional two-particle correlations are also used to extract Fourier coefficients which quantify the magnitude of the anisotropy. The strong CMS magnet allowed to measure accurately particle anisotropy up to 60 GeV/\(c\). Besides transverse momentum dependence, the results are presented versus centrality too.

abstract

For the \(\pi ^+\pi ^-\) effective masses of 1.0–1.5 GeV, the \(\gamma p\to \pi ^+\pi ^- p\) reaction is influenced by the \(f_2(1270)\) photoproduction. We present introductory results on the model describing the \(f_2(1270)\) photoproduction as a final state \(\pi \pi \) interaction effect. We have calculated Born cross sections for \(f_2(1270)\) helicities \(M=-1, 0, +1\) and found that our model is in agreement with experimental observation that the cross section is dominated by the partial wave corresponding to \(M=0\). The projection of \(\pi \pi \) angular momentum on the spin quantisation axis is entirely determined by Born amplitudes.

abstract

The existence of glueballs, bound states of gluons, is one of the basic predictions of QCD; the lightest state is expected to be a scalar. The experimental situation, however, is still ambiguous. The existence of \(f_0(1370)\) would point to a supernumerous state within the nonet classification of scalars and would, therefore, provide a hint towards a glueball. In this paper, we summarise some arguments in favour and against the existence of \(f_0(1370)\) and discuss schemes with and without this state included.

abstract

We study light scalar mesons with particular focus on the \(a_0(980)\) using lattice QCD with 2+1+1 dynamical quark flavors. To investigate the structure of these scalar mesons and to identify, whether a sizeable tetraquark component is present, we use a large set of operators, including diquark–antidiquark, mesonic molecule and two-meson operators. We find that the low-lying states overlap essentially exclusively with two-meson states. This indicates that in the channels investigated no tightly bound four-quark states of either molecular or diquark–antidiquark type exist.

abstract

Zero temperature properties of an (axial-) vector meson extended linear \(\sigma \)-model are discussed, concerning the possible different realizations of the axial anomaly term. The different anomaly terms are compared with each other on the basis of a \(\chi ^2\) minimalization process. It is found that there is no essential difference among the different realizations. This means that any of them can be equally used from phenomenological point of view.

abstract

The Schwinger–Dyson Bethe–Salpeter approach to the bound state problem is applied to the spin zero glueball spectrum. Quark and ghost propagators are obtained from the lattice gluonic two-point function and used as an input to the glueball bound state problem. Although reasonably good results are obtained for all quantities, inconsistencies in the gauge couplings point to a moderate truncation sensitivity.

abstract

The strength of the kaon–antikaon interaction is a crucial quantity for many physics topics. It is, for example, an important parameter in the discussion on the nature of the scalar resonances \(a_{0}(980)\) and \(f_{0}(980)\), in particular, for their interpretation as \(K\bar {K}\) molecules. So far, one of the few possibilities to study this interaction is the kaon pair production in multi-particle exit channels such as \(pp \to ppK^+K^-\). In this article, we present the latest results of the \(K^+K^-\) interaction preformed based on near threshold data gathered at the Cooler Synchrotron COSY. We discuss also shortly perspectives for a new measurement of the kaon–antikaon scattering length in the \(e^+e^-\) collisions.

abstract

Results for the excited meson and baryon spectrum with two flavors of Chirally Improved sea quarks are presented. We simulate several ensembles with pion masses ranging from 250 to 600 MeV and extrapolate to the physical pion mass. Strange quarks are treated within the partially quenched approximation. Using the variational method, we investigate the content of the states. Among others, we discuss the flavor singlet/octet content of Lambda states. In general, our results compare well with experiment, in particular, we get very good agreement with the \({\mit \Lambda }(1405)\) and confirm its flavor singlet nature.

abstract

We discuss how masses and widths of hadron resonances are extracted from lattice QCD. Recent lattice results on the light, strange and charm meson resonances are reviewed. Their properties are revealed by simulating the corresponding scattering channels \(\pi \pi \), \(K\pi \) and \(D\pi \) on the lattice and extracting the scattering phase shifts. In particular, we address the resonances \(\rho \), \(D_0^*(2400)\), \(D_1(2430)\), \(K^*\), \(\kappa \) and \(K_0^*(1430)\).

abstract

Two-point correlators represented by either perturbative or non-perturbative integral equations in Euclidean space are considered. In general, it is difficult to determine the analytic structure of arbitrary correlators analytically. When relying on numerical methods to evaluate the analytic structure, exact predictions of, e.g. , branch point locations (i.e. , the multi-particle threshold) provide a useful check. These branch point locations can be derived by Cutkosky’s cut rules. However, originally they were derived in Minkowski space for propagators with real masses and are thus not directly applicable in Euclidean space and for propagators of a more general form. Following similar considerations that led Karplus et al. , Landau and Cutkosky more than 50 years ago to the mass summation formula that became known as Cutkosky’s cut rules, we show how the position of branch points can be derived analytically in Euclidean space from propagators of very general form.

abstract

We study basic properties of scalar hadronic resonances within a quantum field theoretical toy model. In particular, we focus on the spectral function, the mass and the decay width of the resonance \(f_{0}(500)\). In this work, this meson is understood as a seed state in an effective Lagrangian which couples to pions. With such a setup, we use the position of the pole on the second Riemann sheet in order to obtain its spectral function. We confirm that \(f_{0}(500)\) cannot be described by an ordinary Breit–Wigner function, and that a more complicated structure is needed.

abstract

Using the U(3)\(_{\rm R}\times {\rm U(3)}_{\rm L}\) extended Linear Sigma Model with the ordinary (pseudo)scalar and (axial)vector mesons as well as a scalar glueball, we study the vacuum phenomenology of the scalar–isoscalar resonances \(f_{0}(1370), f_{0}(1500)\) and \(f_{0}(1710)\). We present here a solution, based only on the masses and not yet on decays, in which the resonances \(f_{0}(1370)\) and \(f_{0}(1500)\) are predominantly nonstrange and strange \(\bar {q}q\) states respectively, and the resonance \(f_{0}(1710)\) is predominantly a scalar glueball.

abstract

We use \(O(4) \simeq O(3) \times O(3)\) algebraic methods to calculate the energy-splitting pattern of the \(K\!=\!2\), 3 excited states of the Y-string in two dimensions. To this purpose we use the dynamical O(2) symmetry of the Y-string in the shape space of triangles and compare our results with known results in three dimensions and find qualitative agreement.

abstract

We present the surface worm algorithm (SWA) which is a generalization of the Prokof’ev–Svistunov worm algorithm to perform the simulation of the dual representation (surfaces and loops) of Abelian gauge-Higgs models on a lattice. We compare the SWA to a local Metropolis update in the dual representation and show that the SWA outperforms the local update for a wide range of parameters.

abstract

The phase transitions characterized by deconfinement and restoration of chiral symmetry as well as the restoration of axial symmetry, at finite temperature, are investigated in the framework of SU(2) Polyakov–Nambu–Jona-Lasinio (PNJL) models with the U\(_A\)(1) anomaly. The thermodynamics of the phase transitions, the topological susceptibility, the meson spectrum, and, in particular, the convergence of axial and chiral partners are analysed, in the framework of the ordinary PNJL model and its extension, the entangled Polyakov–Nambu–Jona-Lasinio (EPNJL) model. The latter incorporates entanglement between restoration of chiral symmetry and deconfinement.

abstract

Jets production in ultra-relativistic heavy ion collisions provides understanding of the mechanisms responsible for the hard scattered partons energy loss while crossing the hot and dense medium. The large acceptance and high granularity of the ATLAS Detector is well suited to study the phenomenon of jet suppression, namely its dependence on the jet transverse momentum and size, as well as the internal structure modification. Measurements of these observables provided by the Pb+Pb collision data collected during the 2010 and 2011 LHC runs, at the nucleon–nucleon center-of-mass energy of 2.76 TeV, are presented.

abstract

Recently, the quark-gluon vertex has been investigated in Landau gauge using a combined Dyson–Schwinger and nPI effective action approach. We present here a numerical analysis of a simpler system where the quarks have been replaced by charged scalar fields. We solve the coupled system of the Dyson–Schwinger equations for the scalar propagator, the scalar-gluon vertex and the Yang–Mills propagators in a truncation related to earlier studies. The calculations have been performed for scalars both in the fundamental and the adjoint representation. A clear suppression of the Abelian diagram is found in both cases. Thus, within the used truncation the suppression of the Abelian diagram predominantly happens dynamically and is to a high degree independent of the colour structure. The numerical techniques developed here can directly be applied to the fermionic case.

abstract

Recent RHIC results on \(\eta '\) multiplicity in heavy-ion collisions are of great importance because they clearly signal a partial restoration of U\(_A(1)\) symmetry at high temperatures \(T\), and thus provide an unambiguous signature of the formation of a new state of matter. Prompted by these experimental results of STAR and PHENIX collaborations, we discuss and propose the minimal generalization of the Witten–Veneziano relation to finite \(T\).

abstract

I review the current status of the phase diagram of QCD-like theories as a function of temperature and/or chemical potential. The main focus is on two-color QCD with two flavors of fundamental quarks where most lattice data and model calculations exist. However, I also discuss theories with a different number of flavors, different representation of quarks, or even different gauge group to start with.

abstract

We provide a novel calculation of the Mott effect in nonlocal PNJL models. We find that the “deconfinement” transition temperature in these models is lower than the Mott temperature. Furthermore, the mass and the width of the \(\sigma \) and the \(\pi \) meson modes is calculated with the result that the width in nonlocal models is, in general, reduced as compared to local models. Difficulties encountered while attempting to “Wick rotate” covariant models are carefully discussed.

abstract

We show how excited states in QCD can be profitably used to build up the Polyakov loop in the fundamental representation at temperatures below the hadron–quark-gluon crossover. The conditions under which a Hagedorn temperature for the Polyakov loop can be defined are analyzed.

abstract

The BaBar Collaboration studied with very high precision the LO hadronic contribution to the muon magnetic anomaly. We present recent results for \(\pi ^+ \pi ^-\), \(K^+ K^-\) and \(\pi ^+ \pi ^- \pi ^+ \pi ^-\) final states produced in ISR events. We also show the study of “exotic” states in the \(\gamma \gamma \) annihilation and ISR processes.

abstract

We study the pion–nucleon system in s-wave in the framework of lattice QCD in order to gain new information on the nucleon excited states. We perform simulations for \(n_f =2\) mass degenerate light quarks at a pion mass of 266 MeV. The results show that including the two-particle states drastically changes the energy levels. The variational analysis and the distillation approach play an important role in the extraction of the energy levels. The phase shift analysis allows to extract information on the resonance nature of the observed states.

abstract

We study baryon resonances with heavy flavor in a molecular approach, thus as dynamically generated by baryon–meson scattering. This is accomplished by using a unitary coupled-channel model taking, as bare interaction, the extension of the Weinberg–Tomozawa SU(3) Lagrangian. A special attention is payed to the inclusion of heavy-quark spin symmetry and the study of the generated baryon resonances that complete the heavy-quark spin multiplets. Our model reproduces the \({\mit \Lambda }_b(5912)\) and \({\mit \Lambda }_b(5920)\) baryons, which were recently observed by the LHCb Collaboration. According to our analysis, these two states are heavy-quark spin symmetric partners. We also make predictions for few \({\mit \Xi }_b\) baryon resonances, which belong to the same SU\((3)\times \)HQSS multiplets as the \({\mit \Lambda }_b\) particles.

abstract

In this work, we briefly review the lattice results for the two quark and two antiquarks system in the static limit, in particular the flux-tube recombination. Then, we first review the results obtained for a simple model developed to described tetraquarks. A flip–flop potential which takes into account the color structure is then developed. With this model, we study meson–meson scattering, for a system of two equal quarks and two equal antiquarks. By integrating out the internal degrees of freedom, we arrive at a coupled channel Schrödinger equation, from which we find bound states and resonances, corresponding to tetraquark states.

abstract

A brief overview is given on most recent results obtained by the HERA experiments, H1 and ZEUS, on the measurements providing information on parton density functions (PDFs) in the proton and on the strong coupling constant \(\alpha _{\rm s} (M_Z)\). These measurements have very high experimental accuracy and can provide means for high precision tests of QCD.

abstract

We present preliminary results of an ongoing lattice QCD computation of the spectrum of \(D\) mesons and \(D_s\) mesons and of charmonium using \(2+1+1\) flavors of twisted mass sea and valence quarks.

abstract

We present an effective Lagrangian model calculation of the process \(\pi N \rightarrow Ne^+e^-\). We discuss in some detail the description of electromagnetic interaction of hadrons using a variant of the vector meson dominance model, and the problem of gauge-invariance preserving form factors for Born contributions.