Search for low-mass leptoquarks using the llqq final states in Pb-Pb ultra-peripheral collisions

After a review over past experiments and theoretical requirements from low-energy flavour physics, we argue that the possibility of low-mass leptoquarks (LQ) cannot be fully excluded due to the assumptions made in the measurements. Therefore we propose to search for the pair production of low-mass leptoquarks in Pb-Pb ultra-peripheral collisions with the least model dependence (assuming a small coupling constant ≲ 0.1). There are a couple of advantages: 1) high photon flux provides high production rate for low mass LQs; 2) the background contamination is much lower than that in p-p collisions. The analysis strategy permits a leptoquark to decay to all possible lepton-plus-quark modes. Taking the scalar LQ, S3, with an electric charge $$ \left|q\right|=\frac{4}{3}e $$ q = 4 3 e , as example, the mass point of 100 GeV can be excluded at the 95% confidence level using a dataset of 4 pb−1 Pb-Pb ultra-peripheral collisions at $$ \sqrt{s} $$ s = 5.02 TeV and the performance of the ATLAS detector in Run 2. The proposed method also applies to searching for high-mass LQs in p-p collisions as long as the LQ pair production mechanism dominates.

Study of Dependence of Kinetic Freezeout Temperature on the Production Cross-Section of Particles in Various Centrality Intervals in Au–Au and Pb–Pb Collisions at High Energies

Transverse momentum spectra of π+, p, Λ, Ξ or Ξ¯+, Ω or Ω¯+ and deuteron (d) in different centrality intervals in nucleus–nucleus collisions at the center of mass energy are analyzed by the blast wave model with Boltzmann Gibbs statistics. We extracted the kinetic freezeout temperature, transverse flow velocity and kinetic freezeout volume from the transverse momentum spectra of the particles. It is observed that the non-strange and strange (multi-strange) particles freezeout separately due to different reaction cross-sections. While the freezeout volume and transverse flow velocity are mass dependent, they decrease with the resting mass of the particles. The present work reveals the scenario of a double kinetic freezeout in nucleus–nucleus collisions. Furthermore, the kinetic freezeout temperature and freezeout volume are larger in central collisions than peripheral collisions. However, the transverse flow velocity remains almost unchanged from central to peripheral collisions.

Study of Dependence of Kinetic Freezeout Temperature on Production Cross-Section of the Particles in Various Centrality Intervals in Au-Au and Pb-Pb Collisions at High Energies

Transverse momentum spectra of +, p, Λ, Ξ or ¯Ξ+, Ω or ¯Ω+ and deuteron (d) in different centrality intervals in Gold-Gold (Au-Au) and Lead-Lead (Pb-Pb) collisions at 62.4 GeV and 2.76 TeV respectively, are analyzed by the blast wave model with Boltzmann Gibbs statistics. We extracted Kinetic freeze out temperature,transverse flow velocity and kinetic freezeout volume from the transverse momentum spectra of the particles. It is observed that the non-strange and strange (multi-strange) particles freezeout separately due to different reaction cross-sections. While the freezeout volume and transverse flow velocity are mass dependent which decrease with the rest mass of the particles. The present work reveals the scenario of double kinetic freezeout in collisions at STAR and ALICE Collaborations. Furthermore, the kinetic freezeout temperature and freezeout volume are larger in central collisions than the peripheral collisions and they decrease from central to periphery. However the transverse flow velocity remains almost unchanged from central to peripheral collisions.

Study of Proton, Deuteron, and Triton at 54.4 GeV

Transverse momentum spectra of proton, deuteron, and triton in gold-gold (Au-Au) collisions at 54.4 GeV are analyzed in different centrality bins by the blast wave model with Tsallis statistics. The model results are approximately in agreement with the experimental data measured by STAR Collaboration in special transverse momentum ranges. We extracted the kinetic freeze-out temperature, transverse flow velocity, and freeze-out volume from the transverse momentum spectra of the particles. It is observed that the kinetic freeze-out temperature is increasing from the central to peripheral collisions. However, the transverse flow velocity and freeze-out volume decrease from the central to peripheral collisions. The present work reveals the mass dependent kinetic freeze-out scenario and volume differential freeze-out scenario in collisions at STAR Collaboration. In addition, parameter q characterizes the degree of nonequilibrium of the produced system, and it increases from the central to peripheral collisions and increases with mass .

Microscopic dynamical description of multinucleon transfer in 40Ar induced peripheral collisions at 15 MeV/nucleon

This paper deals with heavy-ion peripheral reactions in the Fermi energy region for the production of neutron-rich isotopes. Experimental data of projectile fragments from the reactions of an 40Ar beam at 15 MeV/nucleon with 64Ni and 58Ni targets, collected with the MARS spectrometer at the Cyclotron Institute of Texas A&M University, are considered. Momentum distributions, which provide valuable information on the reaction mechanisms, are extracted and compared with two types of calculations: These are, the Deep Inelastic Transfer (DIT) model and the microscopic Constrained Molecular Dynamics model (CoMD). For the latter, the parameters of the original code were systematically varied in order to achieve an overall satisfactory description of the experimental data. Our results will be discussed.

Constraining the Chiral Magnetic Effect with charge-dependent azimuthal correlations in Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 2.76 and 5.02 TeV

Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity (η) and transverse momentum (pT) ranges |η| < 0.8 and 0.2 < pT< 5 GeV/c. A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators γ1, 1 = 〈cos(φα + φβ − 2Ψ2)〉 and γ1, − 3 = 〈cos(φα − 3φβ + 2Ψ2)〉. The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured γ1,1. Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form γ1, 2 = 〈cos(φα + 2φβ − 3Ψ3)〉, which are insensitive to the CME signal, verify again that background contributions dominate the measurement of γ1,1.

Holography of low-centrality heavy ion collisions

Large vorticities in the Quark–Gluon Plasma produced in peripheral collisions studied by the STAR collaboration at the RHIC facility have been deduced from observations of polarizations of [Formula: see text] and [Formula: see text] hyperons. Recently, the STAR collaboration has reported on the dependence of these polarizations on centrality, at impact energy 200 GeV and relatively large centralities [Formula: see text]. The polarizations increase slowly with centrality, up to perhaps [Formula: see text]. Here we use a holographic model of the vortical QGP to make predictions regarding these polarizations for smaller centralities, ranging from 5–20%. The model predicts that as one moves downwards from 20%, [Formula: see text] polarizations at first decrease but then increase sharply, in a characteristic pattern which should be readily detected if collisions can be studied at impact energies below 200 GeV and centrality as low as 5–10%. The effect should be most evident at moderate impact energies below 200 GeV, so we give predictions for impact energy 27 GeV.