Quark-gluon Plasma Formation Research Articles

Probing confinement by direct photons and dileptons

The intensive synchrotron radiation resulting from quarks interacting with the collective confining color field in relativistic heavy ion collisions is discussed. The spectrum of photons with large transverse momentum is calculated and compared with the experimental data to demonstrate the feasibility of this type of radiation. A study of the earlier predicted azimuthal anisotropy in the angular distribution of dileptons with respect to the three-momentum of the pair is performed as well. This boundary-induced mechanism of lepton pair production is shown to possess the features that are distinctly different from the standard mechanisms and can potentially provide an efficient probe of quark-gluon plasma formation.

Centrality and transverse momentum dependent suppression of Upsilon (1S) and Upsilon (2S) in p\u2013Pb and Pb\u2013Pb collisions at the CERN Large Hadron Collider

Deconfined QCD matter in heavy-ion collisions has been a topic of paramount interest for many years. Quarkonia suppression in heavy-ion collisions at the relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) experiments indicate the quark-gluon plasma (QGP) formation in such collisions. Recent experiments at LHC have given indications of hot matter effect in asymmetric p–Pb nuclear collisions. Here, we employ a theoretical model to investigate the bottomonium suppression in Pb–Pb at sqrt{s_{NN}}=2.76, 5.02 TeV, and in p–Pb at sqrt{s_{NN}}=5.02 TeV center-of-mass energies under a QGP formation scenario. Our present formulation is based on an unified model consisting of suppression due to color screening, gluonic dissociation along with the collisional damping. Regeneration due to correlated Qbar{Q} pairs has also been taken into account in the current work. We obtain here the net bottomonium suppression in terms of survival probability under the combined effect of suppression plus regeneration in the deconfined QGP medium. We mainly concentrate here on the centrality, N_text {part} and transverse momentum, p_{T} dependence of Upsilon (1S) and Upsilon (2S) states suppression in Pb–Pb and p–Pb collisions at mid-rapidity. We compare our model predictions for Upsilon (1S) and Upsilon (2S) suppression with the corresponding experimental data obtained at the LHC energies. We find that the experimental observations on p_t and N_text {part} dependent suppression agree reasonably well with our model predictions.

Evaluation study of hypernuclei production at NICA/MPD

The main goal of the NICA/MPD facility is a study of the properties of hot and dense baryonic matter. One of the possible ways of doing this is to study strangeness production in nuclear collisions. The enhanced production of strange particles in heavy-ion collisions is considered to be a signal for the quark-gluon plasma formation. In this paper the MPD detector abilities to measure different hypernuclei species in Au+Au collisions at NICA energy region are presented.

Nuclear modification factors of KS and ω mesons in Cu+Au collisions at p

Jet quenching is an important evidence of the quark-gluon plasma (QGP) formation. The effect of jet quenching is emerged in ultra-relativistic heavy-ion collisions (A+A) by suppressed production of hadron yields at high transverse momenta (pT > 5 GeV/c) when compared to one measured in elementary proton-proton collisions (p+p). Cu+Au collisions are characterised by the unique asymmetric nuclei overlap geometry different from one presented in symmetric collisions (Au+Au, Cu+Cu). It makes the Cu+Au collision system especially interesting for jet quenching studies. In this paper we present results of the KS meson production measurement in Cu+Au collisions at with respect to the meson pT and collision centrality obtained using the electromagnetic calorimeter of PHENIX experiment. KS mesons are reconstructed via the KS →π0(→ γγ) π0(→ γγ) decay channel. Obtained KSmeson nuclear modification factors are consistent with ones measured for π0 and η mesons and reconstructed jets in the same collision system and ones measured in Cu+Cu and Au+Au collisions at the same collision energy.

Justification of Landau Hydrodynamic-Tube model in central relativistic heavy-ion collisions

The statistical event-by-event analysis of inelastic interactions of 16O and 32S nuclei in emulsion at 60 A GeV/c and 200 A GeV/c reveals the existence of groups of high multiplicity events belonging to very central nuclear interactions with Gaussian pseudorapidity distributions for produced particles as suggested by the original hydrodynamic-tube model. Characteristics of these events are presented. The experimental observations are interpreted as a result of quark-gluon plasma formation in the course of central nuclear interactions.

Exploring potential signatures of QGP in UHECR ground profiles

In this work we explore the possibility that the formation of Quark Gluon Plasma (QGP) during the first interactions of Ultra High Energy Cosmic Rays (UHECRs) may result in observable signatures in ground profile and shower particle composition that could conceivably be detectable by an air shower array experiment such as the Pierre Auger Observatory. Knowledge of whether QGP formation affects the properties of UHECR development will further the understanding of both UHECR behavior and high energy hadronic interaction behavior. We find that for the vast majority of showers signals of QGP do not manifest themselves in ways that are observable, but on rare occasion, such as within deeply penetrating showers, observable signals can be seen. Results show potential for QGP detection at 100 PeV initial energy at an initial interaction event height of 12 km through a μ± excess at 10 GeV between 100–300 m from the shower core favoring QGP forming events. In contrast, higher initial interaction heights of 24 and 36 km at 100 PeV initial energy show no significant potential for QGP detection. Unfortunately at present, the 12 km observable signals cannot be seen with current detectors, such as the Pierre Auger Observatory [1]; however, there may be potential for detection in future experiments.

Quarkonium in A—A collisions with the ALICE experiment

Quarkonia, i.e. bound states of heavy quarks (charm/bottom), provide remarkable probes of the hot and strongly-interacting medium, the quark gluon plasma (QGP), which is created during heavy-ion collisions. Heavy quarks are produced during initial hard-scattering processes prior to the QGP formation and their number is conserved throughout the partonic and hadronic phases of the collision. At LHC, experimental observations of quarkonium states in A-A collisions are reproduced through two antagonist mechanisms: the sequential suppression and quarkonium production by (re)combination of deconfined quarks. Recent measurements of a significant J/ψ elliptic flow (v2) in Pb-Pb collisions at TeV suggest a participation of the charm quarks to the collectivity of the medium. However, quarkonium interactions in the medium are not yet fully understood and several points should still be clarified. The ALICE experiment at the LHC measures quarkonia at mid-rapidity in the dielectron decay channel and at forward rapidity in the dimuon one, both down to zero transverse momentum. Latest measurements of quarkonium nuclear modification factor and elliptic flow in Pb-Pb collisions will be presented and compared to lower energy results and theoretical predictions.

Light Meson Production in Heavy Ion Collisions in the PHENIX Experiment

Production of a strongly interacting quark−gluon plasma (QGP) has been established in central heavy ion collisions during the studies carried out at the RHIC collider. The experiments on interactions of asymmetric nuclei (Cu + Au, U + U) provide the means to study systematically a jet quenching effect, which is a signature of QGP formation. In this paper, the factors of nuclear modification of π0 and η mesons in Cu + Au and U + U collisions at 200 GeV measured in the PHENIX experiment at RHIC are presented.

Experimental research of π0 meson production in U+U collision at 192 GeV

To study properties of quark-gluon plasma(QGP) is one of the main tasks of modern High Energy Physics. One of the signatures of QGP formation is jet quenching, which was observed as a suppression of particle yields (compared to the yield of the same particles in proton-proton collisions) at high transverse momentum region in central collisions of ultra-relativistic heavy nuclei. Jet quenching is the effect of the final state of heavy nuclei collision which is usually explained through parton energy loss in hot and dense medium. Experimentally the jet quenching is studied with nuclear modification factor. This paper presents invariant transverse momentum spectra and nuclear modification factors for π0 mesons in different classes of centrality in U+U collisions at √SNN=192GeV. Spectra and factors are measured in a wide pT range up to 18 GeV/c for different classes of centrality. In central U+U collisions suppression of neutral π-mesons is the same as the suppression of neutral π-mesons in Au+Au collisions at the similar collision energy and similar numbers of participants.

Enhancement in p–Pb collision as an indication of quark–gluon plasma formation at the Large Hadron Collider

Proton–nucleus collisions serve as an important baseline for the understanding and interpretation of the nucleus–nucleus collisions. These collisions have been employed to characterize the cold nuclear matter effects at SPS and Relativistic Heavy-Ion Collider energies for the past several years, as it was thought that quark–gluon plasma (QGP) is not formed in such collisions. However, at the Large Hadron Collider (LHC), there seems a possibility that QGP is formed during proton–lead (p–Pb) collisions. In this work, we have derived an expression for gluon induced excitation of to , using pNRQCD, and show that the relative enhancement of vis-à-vis , especially at high pT, gives further indication that the QGP is indeed formed in p–Pb collisions at the most central collisions at LHC energy. and suppression effects seen at ALICE are also qualitatively explained.

Quarkonium production in Pb-Pb collisions at √SNN = 5.02 TeV with ALICE

Ultra-relativistic heavy-ion collisions at the Large Hadron Collider provide a unique opportunity to study the properties of matter at extreme energy densities where a phase transition from the hadronic matter to a deconfined medium of quarks and gluons, the Quark-Gluon Plasma (QGP) is predicted. Among the prominent probes of the QGP, heavy quarks play a crucial role since they are created during the initial stages of the collision, before the QGP formation, and their number is conserved throughout the partonic and hadronic phases of the collision. The azimuthal anisotropy of charmonium production, quantified using the second harmonic Fourier coefficient (referred to as elliptic flow), provides important information on the magnitude and dynamics of charmonium production. Measurements of the quarkonium nuclear modification factor at forward rapidity and J/ψ elliptic flow in Pb-Pb collisions as a function of centrality, transverse momentum and rapidity will be presented and compared to different collision energy results and available theoretical calculations.

Holography for Heavy-Ion Collisions at LHC and NICA. Results of the last two years

In the previous Quarks 2016 conference I have presented a concise review of description of quark-gluon plasma (QGP) formation in heavy-ion collisions (HIC) within the holographic approach. In particular, I have discussed how to get the total multiplicity and time formation of QGP in HIC that fit the recent experimental data. For this purpose we had to use an anisotropic holographic model. There are also experimental indications that QGP formed in HIC is anisotropic. In this talk I discuss static properties of anisotropic QGP, in particular, phase transition and diffusion coefficients.

Strange hadron production in pp, pPb, and PbPb collisions at LHC energies

Identified particle spectra provide an important tool for understanding the particle production mechanism and the dynamical evolution of the medium created in relativistic heavy ion collisions. Studies involving strange and multi-strange hadrons, such as K0S, Λ, and Ξ−, carry additional information since there is no net strangeness content in the initial colliding system. Strangeness enhancement in AA collisions with respect to pp and pA collisions has long been considered as one of the signatures for quark-gluon plasma (QGP) formation. Recent observations of collective effects in high-multiplicity pp and pA collisions raise the question of whether QGP can also be formed in the smaller systems. Systematic studies of strange particle abundance, particle ratios, and nuclear modification factors can shed light on this issue. The CMS experiment has excellent strange-particle reconstruction capabilities over a broad kinematic range, and dedicated high-multiplicity triggers in pp and pPb collisions. The spectra of K0S, Λ, and Ξ− hadrons have been measured in various multiplicity and rapidity regions as a function of pT in pp, pPb, and PbPb collisions for several collision energies. The spectral shapes and particle ratios are compared in the different collision systems for events that have the same multiplicity and interpreted in the context of hydrodynamics models.

The meaning behind observed pT regions at the LHC energies

We argue that [Formula: see text] distribution data from the LHC on the invariant differential yield of the charged primary particles in [Formula: see text] collisions at [Formula: see text] and in Pb–Pb collisions at [Formula: see text]TeV with six centrality bins contains several [Formula: see text] regions with special properties. These distributions were analyzed by fitting the data with exponential functions. We conclude that the regions reflect features of fragmentation and hadronization of partons through the string dynamics. The nuclear transparency results in negligible influence of the medium in the III region ([Formula: see text]), which has highest [Formula: see text] values. The effects and changes by the medium start to appear weakly in the II region ([Formula: see text]) and become stronger in the I region ([Formula: see text]). It seems that the II region has highest number of strings. The increase in string density in this region could lead to fusion of strings, appearance of a new string and collective behavior of the partons in the most central collisions. These phenomena can explain anomalous behavior of the Nuclear Modification Factor in the II region. We propose the II region as a possible area of Quark Gluon Plasma formation through string fusion. The first [Formula: see text] regions are the ones with the maximum number of hadrons and minimum number of strings due to direct hadronization of the low energy strings into two quark systems–mesons.

Collective Phenomena in Heavy Ion Collisions

To create conditions which ruled one billionth of a second after the Big Bang, it is necessary to heat and compact the nuclear matter. During the first microseconds after the Big Bang the universe went through such a phase transition at very high temperatures but very low net baryon density. At very high temperatures or densities, the hadrons melt and their constituents, the quarks and gluons, form a new phase of matter, the so called quark-gluon plasma. Relativistic heavy ion collisions aim to create a quark gluon plasma where quarks and gluons can move freely over volumes that are large in comparison to the typical size of a hadron. When the particles collide at high energies, it leads to the conversion of particle collision participants in a much heavier particle. If the energy density is large enough, after a collision occurs the formation of quark-gluon plasma. In the dense nuclear medium, it comes to collective phenomena such as increased production of strangeness, damping charmonium and collective motion of particles. In nuclear medium, it comes to individual collision of quarks, which also hadronize. Using simulation package Pythia, we analyzed the reaction system that results in individual collisions of quarks and antiquarks, and emergence of collective phenomena.

New high resolution measurements of open and hidden charm production in proton-nucleus collisions at [formula omitted] with LHCb

Open and hidden charm production in nucleus-nucleus collisions is considered as a key probe of Quark Gluon Plasma (QGP) formation. In the search of specific QGP effects, proton-nucleus collisions are used as the reference as they account for the corresponding Cold Nuclear Matter (CNM) effects. The LHCb experiment, thanks to its System for Measuring Overlap with Gas (SMOG) can be operated in a fixed target mode with the LHC beams, at an intermediate center-of-mass energy between nominal SPS and RHIC energies. In 2015, for the first time, reactions of incident LHC proton beams on noble gas targets have been recorded by the LHCb experiment at a center-of-mass energy of 110 GeV and within the center-of-mass rapidity range −2.77<y⁎<−0.2. The first high resolution measurements on open and hidden charm production obtained under these conditions are presented.

Measurement of charmonium production in heavy-ion collisions with the ATLAS detector

The suppression of charmonium states in heavy-ion collisions is a phenomenon understood as a consequence of quark gluon plasma formation in the hot, dense system formed in heavy ion collisions at the LHC. In addition to hot matter effects in heavy-ion collisions, so-called cold nuclear effects that modify charmonium production are also present. Therefore, a full assessment requires detailed studies in both A+A and p+A collisions. Based on p+Pb data collected in 2013 and pp and Pb+Pb data collected in 2015 at the LHC, the ATLAS experiment has studied prompt and non-prompt J/ψ and ψ(2S) production via the dimuon decay final states. The production yields and excited-to-ground state yield ratios, measured in p+Pb and Pb+Pb with respect to that measured in pp collision data, are presented in intervals of transverse momentum, rapidity and centrality.

Review of bottomonium measurements from CMS

We review the results on the bottomonium system from the CMS experiment at the Large Hadron Collider. Measurements have been carried out at different center-of-mass energies in proton collisions and in collisions involving heavy ions. These include precision measurements of cross sections and polarizations, shedding light on hadroproduction mechanisms, and the observation of quarkonium sequential suppression, a notable indication of quark–gluon plasma formation. The observation of the production of bottomonium pairs is also reported along with searches for new states. We close with a brief outlook of the future physics program.

Early hydrodynamisation, energy loss and small systems in holographic heavy ion collisions

There has been much recent progress in describing heavy ion collisions using strongly coupled methods. We start by introducing a few caveats in applying holography to QCD, after which we will highlight recent progress on the initial stage, by simulating shock collisions in the dual gravitational theory. Secondly, we describe an attempt at describing jet evolution through strongly coupled plasma, where we specifically also included the width of the jet, and the interplay of this width with the energy lost as the jet propagates through the plasma. Lastly, there has been much recent discussion about the formation of quark-gluon plasma in small systems at low multiplicies, such as produced in proton-nucleus or even proton-proton collisions. At infinitely strong coupling it is possible to estimate that this may be possible, provided that the particle multiplicity satisfies dN tot /dy ≳ 4.

Holography for Heavy Ions Collisions at LHC and NICA

This is a contribution for the Proceedings of 5th International Conference on New Frontiers in Physics (ICNFP 2016), held at Crete, 6-14 July 2016. Our goal is to obtain phenomenologically reliable insights for the physics of the quark-gluon plasma (QGP) from the holography. I briefly review how in the holographic setup one can describe the QGP formation in heavy ion collisions and how to get quantitatively the main characteristics of the QGP formation -- the total multiplicity and the thermalization time. To fit the experimental form of dependence of total multiplicity on energy, obtained at LHC, we have to deal with a special anisotropic holographic model, related with the Lifshitz-type background. Our conjecture is that this Lifshitz-type background with non-zero chemical potential can be used to describe future data expected from NICA. In particular, we present the results of calculations of the holographic confinement/deconfinement phase transition in the $(\mu,T)$ (chemical potential, temperature) plane in this anisotropic background and show the dependence of the transition line on the orientation of the quark pair. This dependence leads to a non-sharp character of physical confinement/deconfinement phase in the $(\mu,T)$-plane. We use the bottom-up soft wall approach incorporating quark confinement deforming factor and vector field providing the non-zero chemical potential. In this model we also estimate the holographic photon production.