NA49 Collaboration Research Articles

Deconfinement of Quarks and Gluons in Nucleus-nucleus Collisions

The energy dependence of hadron production in relativistic nucleus-nucleus collisions reveals the anomalies. They were predicted as the signals of the deconfinement phase transition and observed by NA49 collaboration in Pb + Pb collisions at the CERN SPS. This indicates the onset of the deconfinement in central nucleus-nucleus collisions at about 30 AGeV.

Dependence of related parameters on centrality and mass in a new treatment for transverse momentum spectra in high energy collisions

We collected the experimental data of transverse momentum spectra of identified particles produced in proton-proton ($p$-$p$), deuteron-gold ($d$-Au or $d$-$A$), gold-gold (Au-Au or $A$-$A$), proton-lead ($p$-Pb or $p$-$A$), and lead-lead (Pb-Pb or $A$-$A$) collisions measured by the ALICE, CMS, LHCb, NA49, NA61/SHINE, PHENIX, and STAR collaborations at different center-of mass energies. The multisource thermal model at the quark level or the participant quark model is used to describe the experimental data. The free parameters, the effective temperature $T$, entropy index-related $n$, and revised index $a_{0}$, in the revised Tsallis--Pareto-type function are extracted at the quark level. In most cases, $T$ and $n$ in central collisions are larger than those in peripheral collisions, and $a_0$ does not change in different centrality classes. With the increase in the mass of produced particle or participant quark, $T$ and $a_0$ increase, and $n$ does not change significantly. The behaviors of related parameters from $p$-$p$, $p(d)$-$A$, and $A$-$A$ collisions are similar.

Search for an exotic S=−2 , Q=−2 baryon resonance in proton-proton interactions at sNN=17.3 GeV

Pentaquark states have been extensively investigated theoretically in the context of the constituent quark model. In this paper results of an experimental search for pentaquarks in the Ξ− π−, Ξ− π+, Ξ¯+ π− and Ξ¯+ π+ invariant mass spectra in proton-proton interactions at s=17.3 GeV are presented. Previous possible evidence from the NA49 collaboration of the existence of a narrow Ξ− π− baryon resonance in p+p interactions is not confirmed with almost 10 times greater event statistics. The search was performed using the NA61/SHINE detector which reuses the main components of the NA49 apparatus. No signal was observed with either the selection cuts of NA49 or newly optimized cuts.Received 8 January 2020Accepted 26 February 2020DOI:https://doi.org/10.1103/PhysRevD.101.051101Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNuclear reactionsNuclear Physics

Transverse momentum spectra of J/ψ produced in collisions over an energy range from 17.4 GeV to 13 TeV

Transverse momentum spectra of inclusive J/ψ mesons produced in particle–particle, particle–nucleus and nucleus–nucleus collisions over a center-of-mass energy range from 17.4 GeV to 13 TeV are analyzed by a multi-component Schwinger mechanism. The model results are found to fit the experimental data measured by the ALICE, CDF, D0, HERA-B, E789, NA50, E705, LHCb and PHENIX Collaborations. We extract the distribution of minimum distance between the partons which form the charmonium . Moreover, the energy and rapidity dependent free parameters in Schwinger mechanism are obtained. The parameter related to string tension is found to increase with the increase of energy and to decrease with the increase of rapidity and centrality.

Transverse Energy Density in High Energy Heavy Ion Collisions

A phenomenological model describing the transverse energy distribution (ET) of nuclear collisions is first studied in detail by fitting it on ET data for O-Pb collisions at √sNN = 200 GeV per nucleon obtained from the NA35 collaboration. Next, the model is used to fit the ET data for Pb-Pb collisions at LHC energies of √sNN = 2.76 TeV per nucleon obtained from the ATLAS collaboration. From the fits, we determine an upper bound for the energy density for Pb-Pb collisions at LHC energies of √sNN = 2.76 TeV per nucleon.

Probing QCD critical fluctuations from the yield ratio of strange hadrons in relativistic heavy-ion collisions

By analyzing the available data on strange hadrons in central Pb+Pb collisions from the NA49 Collaboration at the Super Proton Synchrotron (SPS) and in central Au+Au collisions from the STAR Collaboration at the Relativistic Heavy-Ion Collider (RHIC) in a wide collision energy range from sNN=6.3 GeV to 200 GeV, we find a possible non-monotonic behavior in the ratio OK-Ξ-ϕ-Λ=N(K+)N(Ξ−)N(ϕ)N(Λ) of K+, Ξ−, ϕ, and Λ yields as a function of sNN. Based on the quark coalescence model, which can take into account the effect of quark density fluctuations on hadron production, a possible non-monotonic behavior in the dependence of the strange quark density fluctuation on sNN is obtained. This is in contrast to the coalescence model that does not include quark density fluctuations and also to the statistical hadronization model as both fail to describe even qualitatively the collision energy dependence of the ratio OK-Ξ-ϕ-Λ. Our findings thus suggest that the signal and location of a possible critical endpoint in the QCD phase diagram, which is expected to result in large quark density fluctuations, can be found in the on-going Bean Energy Scan program at RHIC.

Rapidity distributions of pions in p+p and Pb+Pb collisions at energies available at the CERN Super Proton Synchrotron

The centrality dependence of rapidity distributions of pions in Pb+Pb reactions can be understood by imposing local energy-momentum conservation in the longitudinal "fire-streaks" of excited matter. With no tuning nor adjustment to the experimental data, the rapidity distribution of pions produced by the fire-streak which we obtained from Pb+Pb collisions reproduces the shape of the experimental pion rapidity distribution in p+p interactions, measured by the NA49 Collaboration at the same energy. The observed difference in the absolute normalization of this distribution can be explained by the difference in the overall energy balance, induced by baryon stopping and strangeness enhancement phenomena occurring in heavy ion collisions. We estimate the latter effects using a collection of SPS experimental data on $\pi^\pm$, $K^\pm$, net $p$, and $n$ production in p+p and Pb+Pb reactions. Implications of the above findings are discussed.

Recent results of strong interaction program from NA61/SHINE experiment at CERN SPS

The NA61/SHINE experiment at the CERN SPS pursues a rich program of strong interactions. The main physics goals of this program are the study of properties of the onset of deconfinement and search for the signatures of the critical point of strongly interacting matter by performing the two-dimensional scan in a broad region of energy (beam momentum 13A-158A GeV/c) and system size (p+p, Be+Be, Ar+Sc and Xe+La). Recent NA61/SHINE results on particle spectra and event-by-event fluctuations in p+p, Be+Be and Ar+Sc collisions are shown with emphasis on the measurements of particle ratios, namely of pion a strangeness production, multiplicity fluctuations versus energy and the system size of colliding nuclei. It will be shown that the hadron production properties in heavy ion collisions which change rapidly in the low SPS energy domain and are interpreted as the beginning of quark-gluon plasma production – onset of deconfinement could be also the case in inelastic p+p interactions and probably in Be+Be collisions. The paper presents a selection of NA61/SHINE results on particle production properties discussed together with existing data from the NA49 collaboration. The evolution of non-monotonic structures in the pion and strangeness production as a function of the system size and energy is addressed. The rapid change of hadron production properties from p+p and Be+Be up to Ar+Sc and Pb+Pb collisions can be interpreted as the beginning of the large clusters formation of strongly interacting matter - the onset of fireball. The NA61/SHINE strong interaction programme is presented including the recent status on proton intermittency analysis and strongly intensive fluctuation observables of particle multiplicity and transverse momentum.

Ψ(2S) production in p + A collisions

We have analyzed the data of ψ(2S) production in proton-nucleus (p + A) collisions, available from the NA50 Collaboration in the SPS energy domain. The investigated data sets include the absolute production cross-sections as well as ψ(2S)-to-Drell-Yan (DY) cross-section ratios. An adapted version of the two-component QVZ model has been employed to calculate ψ(2S) production cross-sections. For both ψ(2S) and DY production, nuclear modifications to the free nucleon structure functions are taken into account. For ψ(2S), the final state interaction of the produced pairs with the nuclear medium is also taken into account, in accordance with the previously analyzed data. A satisfactory description of the data in p + A collisions is obtained. Model calculations are extrapolated to predict the ψ(2S) suppression in proton-induced collisions at near-threshold energies.

Light fragment production at CERN Super Proton Synchrotron

Recent data on the deutron and $^3$He production in central Pb+Pb collisions at the CERN Super Proton Synchrotron (SPS) energies measured by the NA49 collaboration are analyzed within the model of the three-fluid dynamics (3FD) complemented by the coalescence model for the light-fragment production. The simulations are performed with different equations of state---with and without deconfinement transition. It is found that scenarios with the deconfinement transition are preferable for reproduction rapidity distributions of deuterons and $^3$He, the corresponding results well agree with the experimental data. At the same time the calculated transverse-mass spectra of $^3$He at midrapidity do not that nice agree with the experimental data. The latter apparently indicates that coalescence coefficients should be temperature and/or momentum dependent.

Probing QCD critical fluctuations from light nuclei production in relativistic heavy-ion collisions

Based on the coalescence model for light nuclei production, we show that the yield ratio Op-d-t=NH3Np/Nd2 of p, d, and 3H in heavy-ion collisions is sensitive to the neutron relative density fluctuation Δn=〈(δn)2〉/〈n〉2 at kinetic freeze-out. From recent experimental data in central Pb+Pb collisions at sNN=6.3 GeV, 7.6 GeV, 8.8 GeV, 12.3 GeV and 17.3 GeV measured by the NA49 Collaboration at the CERN Super Proton Synchrotron (SPS), we find a possible non-monotonic behavior of Δn as a function of the collision energy with a peak at sNN=8.8 GeV, indicating that the density fluctuations become the largest in collisions at this energy. With the known chemical freeze-out conditions determined from the statistical model fit to experimental data, we obtain a chemical freeze-out temperature of ∼144 MeV and baryon chemical potential of ∼385 MeV at this collision energy, which are close to the critical endpoint in the QCD phase diagram predicted by various theoretical studies. Our results thus suggest the potential usefulness of the yield ratio of light nuclei in relativistic heavy-ion collisions as a direct probe of the large density fluctuations associated with the QCD critical phenomena.

Analysis of hadron yield data within hadron resonance gas model with multi-component eigenvolume corrections

We analyze the sensitivity of thermal fits to heavy-ion hadron yield data of ALICE and NA49 collaborations to the systematic uncertainties in the hadron resonance gas (HRG) model related to the modeling of the eigenvolume interactions. We find a surprisingly large sensitivity in extraction of chemical freeze-out parameters to the assumptions regarding eigenvolumes of different hadrons. We additionally study the effect of including yields of light nuclei into the thermal fits to LHC data and find even larger sensitivity to the modeling of their eigenvolumes. The inclusion of light nuclei yields, thus, may lead to further destabilization of thermal fits. Our results show that modeling of eigenvolume interactions plays a crucial role in thermodynamics of HRG and that conclusions based on a non-interacting HRG are inconclusive.

Solving the problem of anomalous J/ $\psi$ suppression by the MPD experiment on the NICA collider

The meassurements of charmonium states production via their decay on lepton pairs by the MPD experiment on the NICA collider at the energies $ \sqrt{s_{NN}}$ = 4-11 GeV per nucleon could provide important data for solving the problem of anomalous J/ $ \psi$ suppression first observed in central Pb-Pb collisions by the NA50 Collaboration at 158 GeV/nucleon. The anomalous J/ $ \psi$ suppression could be due to the formation of the QGP in the central heavy-ion collisions. However, this effect could be also interpreted as the result of the comover interactions in nuclear matter. The recent experiments at the SPS, at the RHIC, and the LHC reviewed in this article indicate a more complicated picture of the J/ $ \psi$ production including the recombination, medium effects, parton shadowing, and the coherent energy loss mechanism. A more simple production mechanism could be expected at low colliding energies. However, no data were obtained at energies below $ \sqrt{s_{NN}}=17$ GeV for heavy-ion collisions. After the short review of the whole set of the data of charmonium states observation the estimation of the production rate for the MPD/NICA is made.

Net-Baryon Production in Nucleus-Nucleus Collisions and Rare QGP Events

The experimental data on net proton and net Λ-hyperon spectra obtained by the NA35 Collaboration, as well as the inclusive densities of Λ and obtained by NA49, NA57, and STAR collaborations, are compared with the predictions of the Quark-Gluon String Model. The contributions of String Junction diffusion, interactions with nuclear clusters, and percolation corrections are accounted for. The level of numerical agreement of the calculations with the experimental data is of about 20-30%. We also consider the experimental ratios of multistrange to strange antibaryon production. The significant differences between the experimental and, especially, values and the QGSM predictions can be interpreted as the signal of the existence of an additional source of multistrange antibaryon production.

In-medium Spectral Functions in a Coarse-Graining Approach

We use a coarse-graining approach to extract local thermodynamic properties from simulations with a microscopic transport model by averaging over a large ensemble of events. Setting up a grid of small space-time cells and going into each cell's rest frame allows to determine baryon and energy density. With help of an equation of state we get the corresponding temperature T and baryon-chemical potential μB. These results are used for the calculation of the thermal dilepton yield. We apply and compare two different spectral functions for the ρ meson, firstly a calculation from hadronic many-body theory and secondly a calculation from experimental scattering amplitudes. The results obtained with our approach are compared to measurements of the NA60 Collaboration. A relatively good description of the data is achieved with both spectral functions. However, the hadronic many-body calculation is found to be closer to the experimental data with regard to the in-medium broadening of the spectral shape.

Effect of uu diquark suppression in proton splitting in Monte Carlo event generators

Monte Carlo event generators assume that protons split into (uu)-diquarks and d-quarks with a probability of 1/3 in strong interactions. It is shown in this paper that using a value of 1/6 for the probability allows one to describe at a semi-quantitative level the NA49 Collaboration data for $p+p\rightarrow p+X$ reactions at 158 GeV/c. The Fritiof (FTF) model of Geant4 was used to simulate the reactions. The reduced weight of the (uu)-diquarks in protons is expected in the instanton model.

Quarkonium Production and Proposal of the New Experiments on Fixed Target at the LHC

The brief review of the experimental data on quarkonium productions measured at the CERN SPS, at the Brookhaven Collider RHIC, and at the LHC is presented. The dissociation of quarkonium resonances produced in heavy ion collisions was suggested as a possible signal of the Quark-Gluon Plasma formation. At the CERN SPS, the anomalous suppression of theJ/ψproduction was observed in central Pb-Pb collisions by the NA50 collaboration. However, the effects ofJ/ψsuppression on cold nuclear matter, feed-down production from higher charmonium states, and regeneration processes should be taken into account. If proton and ion beams at the LHC will be used with fixed targets, the energy interval between the SPS energy and the nominal RHIC energy (200 GeV) could be investigated. The high statistics data on quarkonium productions at these energies will give the possibility of clarifying the mechanism of charmonium productions to investigate the importance of the recombination process, since the probability of recombination decreases with decreasing the energy of collisions.

Vector Meson Spectral Functions in a Coarse-Graining Approach

Dilepton production in heavy-ion collisions at top SPS energy is investigated within a coarse-graining approach that combines an underlying microscopic evolution of the nuclear reaction with the application of medium-modified spectral functions. Extracting local energy and baryon density for a grid of small space-time cells and going to each cell's rest frame enables to determine local temperature and chemical potential by application of an equation of state. This allows for the calculation of thermal dilepton emission. We apply and compare two different spectral functions for the $\rho$: A hadronic many-body calculation and an approach that uses empirical scattering amplitudes. Quantitatively good agreement of the model calculations with the data from the NA60 collaboration is achieved for both spectral functions, but in detail the hadronic many-body approach leads to a better description, especially of the broadening around the pole mass of the $\rho$ and for the low-mass excess. We further show that the presence of a pion chemical potential significantly influences the dilepton yield.

New evaluation of the antiproton production cross section for cosmic ray studies

Theoretical predictions for the cosmic antiproton spectrum currently fall short of the corresponding experimental level of accuracy. Among the main sources of uncertainty are the antiproton production cross sections in cosmic ray inelastic interactions. We analyze existing data on antiproton production in $pp$ scattering, including for the first time the measurements performed by the NA49 Collaboration. We compute the antiproton spectrum finding that in the energy range where data are available (antiproton energies of about 4--550 GeV) different approaches lead to almost equivalent results, with an uncertainty of 10%--20%. Extrapolations outside this region lead to different estimates, with the uncertainties reaching the 50% level around 1 TeV, degrading the diagnostic power of the antiproton channel at those energies. We also comment on the uncertainties in the antiproton production source term coming from nuclei heavier than protons and from neutrons produced in $pp$ scatterings, and point out the need for dedicated experimental campaigns for all processes involving antiproton production in collisions of light nuclei.

System-size and energy dependence of particle momentum spectra: The UrQMD analysis ofp+pand Pb + Pb collisions

The UrQMD transport model is used to study a system-size and energy dependence of the pion production in high energy collisions. New data of the NA61/SHINE Collaboration on spectra of negatively charged pions in proton-proton interactions at SPS energies are considered. These results are compared with the corresponding data of the NA49 Collaboration in central Pb+Pb collisions at the same collision energies per nucleon. Mean pion multiplicity per participant nucleon, inverse slope parameter of the transverse momentum spectra, and width of rapidity distribution are investigated. A role of isospin effects is discussed. We find that the UrQMD model predicts a non-monotonous behavior of mean transverse mass with collision energy for positively charged pions at the mid-rapidity in inelastic proton-proton interactions. This will be checked soon experimentally by NA61/SHINE Collaboration.