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ALICE is the LHC experiment dedicated to the study of heavy-ion collisions. Its features also make it an ideal detector for QCD studies in \(pp\) collisions. Thanks to its excellent particle identification capabilities and low material budget, ALICE can measure hadron and lepton production over a wide momentum range both in \(pp\) and in Pb–Pb collisions. In this paper, we review recent QCD results, focusing, in particular, on charged-particle multiplicity density, strange particle production, particle ratios, identified particle spectra and heavy flavours in \(pp\) collisions at \(\sqrt {s} = 0.9\), 2.76 and 7 TeV. Results on these observables and on elliptic flow will also be presented for Pb–Pb collisions at \(\sqrt {s_{NN}}=2.76\) TeV.

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We review the most recent results, derived within the combined framework of the pinch technique and the background field method, describing certain QCD nonperturbative properties.

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Chiral symmetry breaking in Quantum Chromodynamics is associated with the low lying spectral modes of the Dirac operator according to the Banks–Casher relation. Here we study how removal of a variable number of low lying modes from the valence quark sector affects the masses of the ground states and first excited states of baryons and mesons in two flavor lattice QCD.

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We briefly review the general status of meson spectroscopy, especially in light of the often made claim that there are too many observed resonances to be accounted for as \(q\bar {q}\) states. Also, the adequacy of the usual Coulomb-plus-linear, alias “funnel”, confining potential for reproducing the experimental spectra of light, heavy–light, and heavy mesons is critically analysed. Thus, many serious discrepancies are observed and discussed. As possible causes, we suggest the neglect of unitarisation and other coupled-channel effects, as well as the deficiency of the funnel potential itself. In order to illustrate our alternative, “unquenched” approach, we present some recent examples of successfully described puzzling mesonic enhancements and resonances, such as the charmonium states \(X(4260)\) and \(X(3872)\), as well as the axial-vector charmed mesons \(D_{1}(2420)\), \(D_{1}(2430)\), \(D_{s1}(2536)\), and \(D_{s1}(2460)\).

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Because of the controversial \(X(3872)\) meson’s very close proximity to the \(D^0\overline {D}^{*0}\) threshold, this charmonium-like resonance is often considered a meson–meson molecule. However, a molecular wave function must be essentially of a meson–meson type, viz. \(D^0\overline {D}^{*0}\) in this case, with no other significant components. We address this issue by employing a simple two-channel Schrödinger model, in which the \(J^{\rm PC}=1^{++}\) \(c\bar {c}\) and \(D^0\overline {D}^{*0}\) channels can communicate via the \(^{3\!}P_0\) mechanism, mimicked by string breaking at a sharp distance \(a\). Thus, wave functions and their probabilities are computed, for different bound-state pole positions approaching the \(D^0\overline {D}^{*0}\) threshold from below. We conclude that at the PDG \(X(3872)\) mass and for reasonable values of \(a\), viz. \(2.0\)–\(3.0\) GeV\(^{-1}\), the \(c\bar {c}\) component remains quite substantial and certainly not negligible, despite accounting for only about 6–10% of the total wave-function probability, owing to the naturally long tail of the \(D^0\overline {D}^{*0}\) component.

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In this contribution we explain the so-called unitarity method for the calculation of Next-to-Leading Order perturbative corrections with high-multiplicity final states.

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Using CUDA as programming language, we create a code named CuBA which is based on the CPU code “Boltzmann Approach for Many Parton Scattering (BAMPS)” developed in Frankfurt in order to study a system of many colliding particles resulting from heavy ion collisions. Furthermore, we benchmark our code with the Riemann Problem and compare the results with BAMPS. They demonstrate an improvement of the computational runtime, by one order of magnitude.

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We present a Monte Carlo simulation of an effective theory for local Polyakov loops at finite temperature and density. The sign problem is overcome by mapping the partition sum to a flux representation. We determine the phase diagram of the model as a function of the temperature and the chemical potential.

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We study the Landau gauge gluon propagator at zero and finite temperature using lattice simulations. Particular attention is given to the finite size effects and to the infrared behaviour.

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The thermodynamics of the \(O(N)\) model in 1+1 dimensions is studied applying the CJT formalism and the auxiliary field method as well as fully nonperturbative finite temperature lattice simulations. The numerical results for the renormalized mass of the scalar particles, the pressure and the trace anomaly are presented and compared with the results from lattice simulation of the model. We find that when going to the two-loop order we observe a good correspondence between the CJT formalism and the lattice study.

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Within the context of the gauge/gravity duality, we discuss the framework to compute the spectral functions governing the generalized Langevin dynamics of a heavy quark propagating through a strongly coupled, non-conformal large-\(N\) plasma. Particular attention is focused on the definition of a spectral function that has the correct large-frequency fall-off and satisfies appropriate dispersion relations.

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Simulations of the viscous hydrodynamic model for relativistic heavy-ion collisions at RHIC and LHC energies are presented. Results for spectra, femtoscopy radii, and transverse momentum fluctuations are favorably compared to the experimental data. Effects of the local charge conservation on correlation observables are also studied.

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In an absorptive plasma, damping of radiation mechanisms can influence the bremsstrahlung formation in the case of large radiation formation lengths. We study qualitatively the influence of this effect on the gluon bremsstrahlung spectrum off heavy quarks in the quark-gluon plasma. Independent of the heavy-quark mass, the spectrum is found to be strongly suppressed in an intermediate gluon energy region which grows with increasing gluon damping rate and increasing energy of the heavy quark. Thus, just as polarization effects in the plasma render the bremsstrahlung spectra independent of the quark mass in the soft gluon regime, damping effects tend to have a similar impact for larger gluon energies.

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In the framework of the U(2)\(_{\rm R}\times {\rm U}(2)_{\rm L}\) symmetric linear sigma model with (axial)vector mesons generalized by including a dilaton field, we study the phenomenology of the scalar–isoscalar resonances below \(2\) GeV. It turns out that in our favoured scenario, the resonance \(f_{0}(1370)\) is predominantly a \(\bar {q}q\) state and \(f_{0}(1500)\) is predominantly a glueball state. Additionally, we are able to calculate the value of the gluon condensate, which is in agreement with lattice QCD results.

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We present the vector spectral function for the \(\tau \) lepton calculated in the framework of the extended Linear Sigma Model. The \(\tau \) decays weakly with intermediate \(\rho \) and \(a_1\) meson states. Electroweak interaction was introduced into the model by requiring invariance under U\((1)_{\rm Y} \times {\rm SU}(2)_{\rm L}\) gauge transformations. We determine the mass of the \(\rho \) meson and consequently the decay width that yield the best description of the data and obtain slightly smaller values with respect to the values given by the Particle Data Group.

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We investigate the phase diagram of the so-called Polyakov–Nambu–Jona-Lasinio (PNJL) model at finite temperature and nonzero chemical potential. The calculations are performed in the light and strange quark sectors (\(u\), \(d\), \(s\)) which includes the ’t Hooft instanton induced interaction term that breaks the axial symmetry, and the quarks are coupled to the (spatially constant) temporal background gauge field. On one hand, a special attention is paid to the critical end point (CEP). The strength of the flavor-mixing interaction alters the CEP location, since when it becomes weaker the CEP moves to low temperatures and can even disappear. On the other hand, we also explore the connection between QCD, a nonlocal Nambu–Jona-Lasinio type model and the Landau gauge gluon propagator. Possible links between the quenched gluon propagator and low energy hadronic phenomenology are investigated.

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We examine the phase diagram of hadronic matter when the number of colours \(N_c\), as well as temperature and density, are varied. We show that in this regime a new percolation phase transition is possible, and examine the implications of this transition for extrapolations to physical QCD of the large-\(N_c\) limit.

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Recent large-volume lattice simulations have established that, in the Landau gauge, the gluon propagator is infrared-finite. The most natural way to explain this observed finiteness is the generation of a nonperturbative, momentum-dependent gluon mass. Such a mass may be generated gauge-invariantly by employing the Schwinger mechanism, whose main assumption is the dynamical formation of massless bound-state excitations. In this work, we demonstrate that this key assumption is indeed realized by the QCD dynamics. Specifically, the Bethe–Salpeter equation describing the aforementioned massless excitations is derived and solved under certain approximations, and non-trivial solutions are obtained.

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We study the interactions of the pseudoscalar glueball with scalar and pseudoscalar quark–antiquark meson fields and with the nucleon and its chiral partner. In both cases, we introduce the corresponding chiral Lagrangian and discuss its properties. We calculate the mesonic and baryonic decays of a pseudoscalar glueball with mass of about 2.6 GeV, as predicted by lattice simulations.

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We discuss the phenomenology of the axial-vector mesons within a three-flavour Linear Sigma Model containing scalar, pseudoscalar, vector and axial-vector degrees of freedom.

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The PHENIX experiment has produced an extensive array of measurements in heavy ion collisions in order to study the created strongly interacting medium. This medium is seen to modify the properties of many global and high-momentum observables; a strong suppression relative to that expected from \(p+p\) collisions is observed. Initially scattered partons traverse the formed medium prior to fragmentation. A comparison of the final measured jet (a stream of particles with large transverse momenta in a localized region of phase space) in heavy ion and \(p+p\) collisions reflects the energy loss of the partons within the medium. In order to systematically study the parton-medium interactions, PHENIX has used a suite of analysis methods. In these paper, I will present our latest QCD results, focusing on single-particle spectra, inclusive jet production and two-particle correlations in heavy ion collisions.

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We present the new results of the Wuppertal–Budapest Lattice QCD Collaboration on flavor diagonal and non-diagonal quark number susceptibilities with \(2+1\) staggered quark flavors, in a temperature regime between 125 and 400 MeV. A Symanzik improved gauge and a stout-link improved staggered fermion action is utilized; the light and strange quark masses are set to their physical values. Lattices with \(N_t=6,8,10,12,16\) are used. We perform a continuum extrapolation of all observables under study. Preliminary results for charm quark susceptibilities are also presented, with the charm quark treated at the partially quenched level.

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We utilize Polyakov loop correlations to study (3+1)D compact U(1) flux tubes and the static electron–positron potential in lattice gauge theory. By using field operators it is possible in U(1) lattice gauge theory to probe directly the electric and magnetic fields. In order to improve the signal-to-noise ratio in the confinement phase, we apply the Lüscher–Weiss multilevel algorithm. Our code is written in CUDA, and we run it in NVIDIA FERMI generation GPUs, in order to achieve the necessary performance for our computations.

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We explore the performance of CUDA in performing Landau gauge fixing in Lattice QCD, using the steepest descent method with Fourier acceleration. The code performance was tested in a Tesla C2070, Fermi architecture. We also present a study of the string tension at finite temperature in the confined phase. The string tension is extracted from the colour averaged free energy and from the colour singlet using Landau gauge fixing.

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We investigate in a microscopical transport model the evolution of conical structures originating from the supersonic projectile moving through the matter of ultrarelativistic particles. Using different scenarios for the interaction between projectile and matter, and different transport properties of the matter, we study the formation and structure of Mach cones. Furthermore, the two-particle correlations for different viscosities are extracted from the numerical calculations and we compare them to an analytical approximation. In addition, by adjusting the cross section we investigate the influence of the viscosity to the structure of Mach cones.

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In this work, we compute the color fields in the mediator plane between a static quark and a static antiquark using quenched lattice QCD. In particular, we see the effect of the quark–antiquark distance on the flux tube. To obtain this results, an improved multihit technique is developed and an extend smearing technique is used. Then, we also discuss the flux-tubes in a system composed of two quarks and two antiquarks. The ground and first excited states fields are studied for different dispositions of the system.

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We give an elementary discussion of parton saturation and its description by the effective theory of the Color Glass Condensate. We report on progress in calculating multi-gluon correlators. The latter are necessary for many phenomenological applications, upon some of which we briefly touch.

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The Parton Distribution Functions (PDFs) and the spin structure of the nucleon are important topics studied by the COMPASS experiment. The Drell–Yan (DY) process will be used in the future COMPASS-II measurements to access the Transverse Momentum Dependent PDFs (TMD PDFs). Studying the angular distributions of dimuons from the DY reactions with a negative pion beam with \(190\,{\rm GeV}/c\) momentum and a transversely polarised proton target, we will be able to extract the azimuthal spin asymmetries and to access the various TMD PDFs, such as Sivers and Boer–Mulders functions. The start of the COMPASS DY experiment is scheduled for 2014. Three beam tests have been already performed, one of them in 2009 using a prototype hadron absorber downstream of the target, to understand the background reduction factors and the spectrometer response, and also to verify our results from Monte Carlo simulations. COMPASS aims at performing the first DY experiment with a transversely polarised target.

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Several relevant thermodynamic observables obtained within the (2+1) flavor and spin zero NJL and PNJL models with inclusion of the ’t Hooft determinant and \(8q\) interactions are compared with lattice-QCD (lQCD) results. In the case that a small ratio \(R=\frac {\mu _B}{T_{\mathrm c}}\sim 3\) at the critical end point (CEP) associated with the hadron gas to quark-gluon plasma transition is considered, combined with fits to the lQCD data of the trace anomaly, subtracted light quark condensate and continuum extrapolated data of the light quark chiral condensate, a reasonable description for the PNJL model is obtained with a strength \(g_1\sim 5\dots 6 \times 10^3\) GeV\(^{-8}\) of the \(8q\) interactions. The dependence on the further model parameters is discussed.

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In this presentation, we investigate the heavy quark sector of Coulomb gauge QCD using a leading-order heavy quark mass expansion of the QCD action adapted from M. Neubert, Phys. Rep. 245, 259(1994). In the limit where the Yang–Mills sector is truncated to only include dressed two-point functions, we show that the rainbow-ladder approximation to the gap and Bethe–Salpeter equations is exact, and provide a direct connection between the physical string tension and the temporal gluon propagator. Furthermore, we derive an exact solution for the four-point quark Green’s function and show that a natural separation between the physical and unphysical poles arises.

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We discuss the temperature dependence of the scaled jet quenching parameter of hot gluonic matter within a quasiparticle approach. A pronounced maximum in the vicinity of the transition temperature is observed, where the ratio of the scaled jet quenching parameter and the inverse specific shear viscosity increases above typical values for weakly coupled systems.

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We study the evolution of the sigma field fluctuations in a scenario featuring a critical point and a first order phase transition using the model of nonequilibrium chiral fluid dynamics (N\(\chi \)FD).

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The PANDA experiment is one of the major projects at the upcoming FAIR facility in Darmstadt, Germany. It will study interactions between antiprotons and protons or nuclei in the momentum range of 1.5 GeV/\(c\) to 15 GeV/\(c\) with a \(4\pi \) state-of-the-art detector. The purpose is to learn about fundamental aspects of the strong interaction in the transition region between perturbative QCD and nuclear phenomena. This paper reviews some of the main physics topics together with a presentation of the detector.