abstract

The significant increase of the centre-of-mass energy of the Large Hadron Collider (LHC) from 8 to 13 TeV has allowed the LHC experiments to explore previously inaccessible kinematic regimes in their search for phenomena beyond the Standard Model (BSM). Many BSM theories predict new phenomena accessible by the LHC. Searches for new physics models are performed using the ATLAS experiment at the LHC. The results reported here use the proton–proton (\(pp\)) collision data sample collected in 2015 and 2016 by the ATLAS detector at the LHC with a centre-of-mass energy of 13 TeV with a total integrated luminosity of 45 fb\(^{-1}\).

abstract

The H1 and ZEUS collaborations continue to produce final precision measurements of QCD physics. To achieve high-precision measurements, data from both experiments have been combined leading to significantly reduced experimental uncertainties. The two collaborations recently obtained combined results and a QCD analysis of beauty and charm production cross-section measurements in deep inelastic \(ep\) scattering. New results on the first determination of \(\alpha _{\rm s}\) from jet data from deep inelastic scattering at NNLO from the H1 Collaboration and studies of prompt photon production from the ZEUS Collaboration are also presented.

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Is the three-quark confinement potential better described by the Y-string or the \(\Delta \)-string form? We have re-analysed the recent lattice QCD calculations and the older results using hyper-spherical three-body variables. The presently available lattice data do not give a conclusive answer to the above question. We briefly discuss sources of uncertainties in these calculations, the ways to reduce them, and how to avoid them.

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Employing the Polyakov extended Nambu–Jona-Lasinio model, we determine the net-baryon number fluctuations of magnetized three-flavor quark matter. We show that the magnetic field changes the nature of the strange quark transition from crossover to first-order at low temperatures. In fact, the strange quark undergoes multiple first-order phase transitions and several critical end points emerge in the phase diagram.

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The search for multi-quark states beyond the constituent quark model (CQM) has resulted in the discovery of many new exotic states, starting with the observation of the \(X(3872)\), discovered by Belle in 2003. Also in the sector of charm–strange physics, the CQM does not seem to describe properly all spectrum, despite of theoretical expectations. These new forms of quark bounds clearly show that mesons and baryons are not the only possibilities to be considered. We shortly report in this paper on selected recent results of searching for such states at Belle, with the perspectives in the hadron physics program at the Belle II experiment.

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Half a century of work on the light scalar mesons \(f_0(500)\), \(f_0(980)\), \(K_0^\star (700)\), and \(a_0(980)\) is briefly reviewed. After summarising all light scalar candidates in the Review of Particle Physics since 1963, a selection of different theoretical and phenomenological descriptions is presented, including pure meson–meson models, a tetraquark construction, unitarised quark–meson models, unitarised effective chiral approaches, and a very recent lattice-QCD simulation.

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With about 12 fb\(^{-1}\) collected \(XYZ\) data sets, BESIII continues the exploration of the exotic charmonium-like states. In this paper, recent results on the measurements of the spin-parity determination of \(Z_{c}(3900)\), as well as on line-shapes of \(e^+e^- \rightarrow J/\psi \,\pi \pi , h_{c}\pi \pi , \psi (2S)\,\pi ^0\pi ^0/\pi ^+\pi ^-\), and \(\pi ^+ D^0 D^{*-}\) from open charm are discussed. Also, the recent observation of \(e^+ e^- \rightarrow \phi \chi _{c1/2}\) at \(\sqrt {(s)}=4.6\) GeV is reported.

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We discuss a calculation of large-\(N_{\rm c}\) masses of light scalar mesons from the QCD sum rules. Two methods based on the use of linear radial Regge trajectories are presented. We put a special emphasis on the appearance of pole near 0.5 GeV in the scalar–isoscalar channel which emerges in both methods and presumably corresponds to the scalar sigma meson.

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In this paper, we briefly review the theory of resonances dynamically generated from hadron–hadron scattering, sometimes referred to as “molecules”. We give some classical examples of meson–meson and meson–baryon systems, as well as a few examples of different approaches to describe the interaction between hadrons. To conclude, we comment on a few recent works that suggest that some of the five new narrow \({\mit \Omega }_c\) states, recently discovered by the LHCb Collaboration, can be interpreted as meson–baryon molecules.

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We perform a semi-perturbative calculation of the quark–gluon vertex inspired by the three-loop expanded 3PI effective action and investigate the relative strengths of the chirally symmetric/broken tensor structures below and above the crossover.

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In the past few years, cosmic-rays beyond the GZK cut-off (\(E \gt 5 \times 10^{19}\) eV) have been detected by leading collaborations such as the Pierre Auger Observatory. Such observations raise many questions as to how such energies can be reached and what source can possibly produce them. Although at lower energies, mechanisms such as Fermi acceleration in supernovae front shocks seem to be favored, top–down scenarios have been proposed to explain the existence of ultra-high energy cosmic-rays: the decay of super-massive long-lived particles produced in the early Universe may yield to a flux of ultra-high energy photons. Such photons might be presently generating so-called super-preshowers, an extended cosmic-ray shower with a spatial distribution that can be as wide as the Earth diameter. The Cosmic Ray Extremely Distributed Observatory (CREDO) mission is to find such events by means of a network of detectors spread around the globe. CREDO’s strategy is to connect existing detectors and create a worldwide network of cosmic-ray observatories. Moreover, citizen science constitutes an important pillar of our approach. By helping our algorithms to recognize detection patterns and by using smartphones as individual cosmic-ray detectors, non-scientists can participate in scientific discoveries and help unravel some of the deepest mysteries in physics.

abstract

Dynamical bosonisation within the functional renormalisation group is used to describe mesons as quark–antiquark bound states. Employing for simplicity the two-flavour quark–meson model, it is exemplified how the kinetic terms for pseudoscalar and scalar mesons are generated from the quark kinetic term upon lowering the renormalisation group scale. Relating this method to the Dyson–Schwinger–Bethe–Salpether approach, one can identify the momentum-dependent Yukawa three-point function in the limit of vanishing renormalisation group scale with the Bethe–Salpeter amplitude. This, in turn, might allow for a systematic comparison of the impact of truncations in the Dyson–Schwinger–Bethe–Salpether approach on the one hand and the functional renormalisation group with dynamical bosonisation on the other hand. This paper is concluded by an outlook on a respective on-going investigation.

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Understanding the energy loss of partons traversing the strongly interacting matter created in heavy-ion collisions is one of key goals of the heavy-ion physics program. We present results of phenomenological analyses of various recent jet quenching data and data on charmonia suppression. The core of the model used in these analyses is based on the shift formalism which allows for an extraction of the magnitude of parton energy loss from the data with minimal assumptions on the underlying physics mechanisms.

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We briefly review the basic features of a new framework for relativistic perfect fluid hydrodynamics of polarized systems consisting of particles with spin one half. Using this approach, we numerically study the stability of a stationary vortex-like solution, representing global equilibrium of a rotating medium.

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We discuss possible interpretation of five excited \({\mit \Omega }^0_c\) states within the Chiral Quark–Soliton Model. We show that it is not possible to interpret all five \({\mit \Omega }^0_c\)s as parity minus excitations and argue that two narrowest states are pentaquarks belonging to the SU(3) representation \(\overline {15}\).

abstract

Starting from the symmetric \(2\times 2\) mass matrix in the \(\omega _8, \omega _0\) space and, subsequently, by its diagonalisation into physical vector meson states \(\omega (782)\), \(\phi (1020)\) by means of the orthogonal matrix in the most general form, all possible forms of the \(\omega \)–\(\phi \) mixing are found. Taking into account the quark structure of the \(\omega _8\), \(\omega _0\) states and exploiting the ideal mixing angle value \(\theta =35.3^\circ \), it is demonstrated that only four of the found mixing forms are physically acceptable, as only they are in conformity with experimentally observed decays of the \(\omega (782)\) and \(\phi (1020)\) vector mesons.

abstract

The inverse relations of the four independent couples of physically acceptable \(\omega \)–\(\phi \) mixing forms give expressions for \(\omega _8\) and \(\omega _0\) as functions of the unknown mixing angle \(\theta \) and physical states \(\omega \) and \(\phi \). Substituting for expressions obtained in such a way for \(\omega _8\) repeatedly into Gell-Mann–Okubo quadratic mass relation, which yields quadratic mass of \(m^2_{\omega _8}\) as a combination of quadratic masses of \(K^*(980)\) and \(\rho ^0(770)\) vector mesons, always determines the same value of mixing angle \(\theta \). Next, the same result is obtained also by using all physically non-acceptable \(\omega \)–\(\phi \) mixing forms.

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We study the excitation function of the low-lying charmonium state: \({\mit \Psi }\)(3686) in \(\bar p\)Au collisions taking into account their in-medium propagation. The time evolution of the spectral functions of the charmonium state is studied with a BUU-type transport model. We calculated the excitation function of \({\mit \Psi }\)(3686) production and show that it is strongly effected by the medium. The energy regime will be available for the PANDA experiment.

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We present progress on the study of decay-channel effects in the properties of hadrons using covariant Bethe–Salpeter equations (BSEs). The main goal will be to develop BSE kernels that contain explicit decay mechanisms. This will be first explored in the meson sector where, for example, a kernel suitable to study the rho meson should contain a virtual \(\rho \rightarrow \pi + \pi \) decay mechanism. This will be tackled by including explicit pion degrees of freedom in addition to quarks and gluons.

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We present a phase diagram of effective Polyakov line actions, derived from the SU(3) lattice gauge theory with 695 MeV dynamical staggered quarks. We find a phase-transition line in the temperature–density plane. The derivation is via the method of relative weights and the effective theories are solved at finite chemical potential by mean-field theory.

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We study the structure of the hybrid static potential flux tube in the \({\mit \Pi }_u\) sector in SU(2) lattice Yang–Mills theory. To this end, we compute the squares of the chromoelectric and chromomagnetic field strengths in the presence of a static quark–antiquark pair. We show clear evidence that the gluon distribution is significantly different compared to that of the ordinary static potential with quantum numbers \({\mit \Sigma }_g^+\).

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Observed dependence of flow symmetry plane in ultra-relativistic heavy-ion collisions on transverse momentum (\(p_{\rm T}\)) and pseudorapidity \(\eta \) is attributed to lumpy hot-spots raised by the fluctuations of the initial states. Studying different orthogonal modes of the same flow harmonic has been suggested as a promising way to explore this phenomena. Prediction of leading and sub-leading modes for elliptic and triangular flow for charged pions for PbPb collisions at the center-of-mass energy per nucleon pair of 2.76 TeV from HYDJET++ model are presented. Calculations are done by applying principal component analysis technique (PCA) on a long-range two-particle azimuthal correlations, requesting \(|\Delta \eta |\gt 2\) gap in order to avoid non-flow effects. The results are shown as a function of transverse momentum in a range of \(0.3\lt p_{\rm T}\lt 3.0\) GeV/\(c\), pseudorapidity range of \(|\eta |\lt 2.4\), and in a various centrality classes, from ultra-central events (0–0.2%) up to rather peripheral ones (50–60%). Obtained values are compared with data measurement from the CMS experiment. Rather good agreement between the model and the experimental data is a step in a better understanding of the initial-state fluctuations and dynamics of QGP expansion.

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Fluctuations and correlations of conserved quantities in the confined phase of QCD are a viable way to characterize the existence of exotic and missing states with given quantum numbers in the hadronic spectrum. We study a realization of the Hadron Resonance Gas model in the light quark (uds) flavor sector of QCD to study the fluctuations and static correlators of electric charge, baryon number and strangeness. It is also conjectured an interesting duality between the correlators at zero temperature and the fluctuations of integrated quantities at low temperatures, leading to the appearance of a dual Hagedorn distance for the former.

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In ultrarelativistic collisions of heavy ions, we have seen behaviour which can be interpreted as a formation of locally thermalised system expanding as a fluid. I discuss the use of hydrodynamics to model the expansion of the collision system and what such a modelling has taught us about the properties of QCD matter.

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We analyze the topological susceptibility in SU(3) and SU(2) gauge theories, using Padé approximation and Refined Gribov–Zwanziger gluon propagator.

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Describing the non-triviality of the QCD vacuum by closed and quantized color magnetic flux lines with some average thickness, the center vortex model can explain the non-perturbative phenomena of QCD, especially confinement and chiral symmetry breaking. There are various methods to identify vortices in lattice calculations, in particular direct and indirect maximal center gauge and adjoint Laplacian gauge. It turns out that the vortex identification suffers from a Gribov copy problem. The non-Abelian Stokes theorem may give a hint how to overcome this problem by investigating the color homogeneity of lattice configurations, especially its behavior for smooth field configurations. We discuss that the homogeneity of the color direction of the gauge field may facilitate detecting regions of center flux.

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The Landau–Khalatnikov–Fradkin transformations (LKFTs) allow to interpolate \(n\)-point functions between different gauges. In this work, we offer a derivation for both Abelian and non-Abelian LKFT using gauge-invariant fields. Secondly, this subject is studied using a direct path integral formalism, finding full consistency.

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In this short report, we provide an overview of selected new results from the heavy-ion physics program of the ATLAS experiment at the LHC with the emphasis on jet quenching, quarkonia suppression and long-range azimuthal correlations.

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NA61/SHINE is a fixed target experiment at the CERN Super-Proton-Synchrotron. The main goals of the experiment are to discover the critical point of strongly interacting matter and to study the properties of the onset of deconfinement. In order to reach these goals, a study of hadron production properties is performed in nucleus–nucleus, proton–proton and proton–nucleus interactions as a function of collision energy and size of the colliding nuclei. In this paper, I will review recent results on strangeness production in \(p\)+\(p\), Be+Be and Ar+Sc collisions in the SPS energy range. Kinematic spectra and mean multiplicities of kaons obtained with various analysis methods will be shown. An overview of strangeness production and its dependence on system size in the vicinity of the phase transition will be presented as well.

abstract

The AMADEUS experiment at the DA\(\Phi \)NE collider of LNF-INFN deals with the investigation of the at-rest, or low-momentum, \(K^-\) interactions in light nuclear targets, with the aim to constrain the low energy QCD models in the strangeness sector. The 0 step of the experiment consisted in the reanalysis of the 2004/2005 KLOE data, exploiting \(K^-\) absorptions in H, \({}^4\)He, \({}^9\)Be and \({}^{12}\)C, leading to the first invariant mass spectroscopic study with very low momentum (about 100 MeV) in-flight \(K^-\) captures. With AMADEUS step 1, a dedicated pure carbon target was implemented in the central region of the KLOE detector, providing a high statistic sample of pure at-rest \(K^-\) nuclear interaction. The first measurement of the non-resonant transition amplitude \(|A_{K^-n \rightarrow {\mit \Lambda } \pi ^-}|\) at \(\sqrt {s}=33\) MeV below the \(\bar {K}N\) threshold is presented, in relation with the \({\mit \Lambda }(1405)\) properties studies.

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The NA48/2 experiment at CERN collected a very large sample of charged kaon decays into multiple final states. These data allow measurements related to QCD. We obtained our final measurement of the charged kaon semileptonic decays form factors based on 4.28 million \(Ke3\) and 2.91 million \(K\mu 3\) selected decays, with the smallest uncertainty for \(Ke3\) and a competitive result for \(K\mu 3\) and leading to the most precise combined \(Kl3\) result coming from the kaon sector that reduces the form factor uncertainty of \(|V_{us}|\). The NA62 experiment at CERN SPS is designed to measure the branching ratio of the \(K^+\to \pi ^+\nu \bar {\nu }\) decay with 10% precision. \(K\to \pi \nu \bar {\nu }\) is one of the theoretically cleanest meson decay where to look for indirect effects of new physics complementary to LHC searches. NA62 took data in 2015–2017; the analysis of a partial data set allows to reach the Standard Model sensitivity. The status of the experiments will be presented.