Stopping In Heavy-ion Collisions Research Articles

Diquark break-up jet effects on particle production in central Ar+Sc and Pb+Pb collisions at CERN SPS energies

We investigate the rapidity spectra of protons in central Ar+Sc and Pb+Pb collisions at Conseil Européen pour la Recherche Nucléaire (CERN) super proton synchrotron (SPS) energies, utilizing the HIJING code integrated with the advanced popcorn (AP) mechanism from PYTHIA6.4. We find that the standard HIJING/AP model is inadequate for fully capturing the proton rapidity distributions observed in these collisions. To overcome this, we focus on the leading diquark (LD) break-up component, distinguishing between diquark preserving and diquark breaking (DB) behaviors. Our analysis reveals that the suppressed DB component is essential for accurately describing the proton and neutron x F spectra, particularly noting an excess of baryon number at lower x F values consistent with experimental observations. By integrating the suppressed LD break-up mechanism, informed by pp and Be+Be collision data, we successfully replicate the proton rapidity spectra and also effectively capture the rapidity spectra of negative pions and charged kaons for the studied reactions. These outcomes underscore the critical role of the suppressed LD break-up component in elucidating the mechanisms of baryon production and stopping in heavy-ion collisions at CERN SPS energies.

Correlations of baryon and charge stopping in heavy ion collisions* *Supported in part by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB34030000) and the National Key Research and Development Program of China (2022YFA1604900), the U.S. DOE Office of Science365 under contract (DE-SC0012704, DE-FG02-10ER41666, DE-AC02-98CH10886)

Baryon numbers are theorized to be carried by valence quarks in the standard QCD picture of the baryon structure. Another theory proposed an alternative baryon number carrier, a non-perturbative Y-shaped configuration of the gluon field, called the baryon junction in the 1970s. However, neither of these theories has been verified experimentally. Recently, searching for the baryon junction by investigating the correlation of net-charge and net-baryon yields at midrapidity in heavy-ion collisions has been suggested. This paper presents studies of such correlations in collisions of various heavy ions from oxygen to uranium with the UrQMD Monte Carlo model. The UrQMD model implements valence quark transport as the primary means of charge and baryon stopping at midrapidity. Detailed studies are also conducted for isobaric + and + collisions. We found a universal trend of charge stopping with respect to baryon stopping and discovered that the charge stopping is always greater than the baryon stopping. This study provides a model baseline in valence quark transport for what is expected in net-charge and net-baryon yields at the midrapidity of relativistic heavy-ion collisions.

Bremsstrahlung as a probe of baryon stopping in heavy-ion collisions

In collisions between heavy ions at ultra-relativistic energies the participating protons lose energy, which is converted into new particles. As the protons slow down, they emit bremsstrahlung radiation. The yield and angular distribution of the emitted radiation are sensitive probes of how much energy the incoming protons have lost. In this paper, the spectrum of bremsstrahlung radiation is calculated for different stopping scenarios, and the results are compared with the expected yield of photons from hadronic interactions.

Bremsstrahlung photons from stopping in heavy-ion collisions

We examine the spectrum of bremsstrahlung photons that results from the stopping of the initial net charge distributions in ultrarelativistic nucleus-nucleus collisions at the CERN Large Hadron Collier (LHC). This effect has escaped detection so far since it becomes sizable only at very low transverse momentum and at sufficiently forward rapidity. We argue that it may be within reach of the next-generation LHC heavy-ion detector ALICE-3 that is currently under study, and we comment on the physics motivation for measuring it.

Collective flow and nuclear stopping in heavy ion collisions in Fermi energy domain

The effects of the in-medium nucleon–nucleon (NN) elastic cross section on the observables in heavy ion collisions in the Fermi energy domain are investigated within the framework of the ultrarelativistic quantum molecular dynamics model. The results simulated using medium correction factors of $${\mathcal{F}}=\sigma ^{\mathrm{in-medium}}_\mathrm{NN}/ \sigma ^{\mathrm{free}}_\mathrm{NN}=0.2,~0.3,~0.5,$$ and the density- and momentum-dependent factor obtained from the FU3FP1 parametrization are compared with the FOPI and INDRA experimental data. It is found that the calculations using the correction factors $${\mathcal{F}} =$$ 0.2 and 0.5 reproduce the experimental data (i.e., collective flow and nuclear stopping) at 40 and 150 MeV/nucleon, respectively. Calculations with the FU3FP1 parametrization can best fit these experimental data. These conclusions can be confirmed in both $$^{197}\text{Au}+^{197}\text{Au}$$ and $$^{129}\text{Xe}+^{120}\text{Sn}$$ .

Effects of the in-medium nucleon-nucleon cross section on collective flow and nuclear stopping in heavy-ion collisions in the Fermi-energy domain

With the newly updated version of the ultrarelativistic quantum molecular dynamics (UrQMD) model, a systematic investigation of the effects of in-medium nucleon-nucleon ($\mathit{NN}$) elastic cross section on the collective flow and the stopping observables in $^{197}\mathrm{Au}+^{197}\mathrm{Au}$ collisions at beam energies from 40 to 150 MeV/nucleon is performed. Simulations with the medium correction factors $\mathcal{F}={\ensuremath{\sigma}}_{NN}^{\mathrm{in}\text{\ensuremath{-}}\mathrm{medium}}/{\ensuremath{\sigma}}_{NN}^{\mathrm{free}}=0.2,0.3,0.5$ and the one obtained with the FU3FP1 parametrization which depends on both the density and the momentum are compared to the FOPI and INDRA experimental data. It is found that, to best fit the experimental data of the slope of the directed flow and the elliptic flow at midrapidity as well as the nuclear stopping, the correction factors of $\mathcal{F}=0.2$ and 0.5 are required for reactions at beam energies of 40 and 150 MeV/nucleon, respectively. Whereas calculations with the FU3FP1 parametrization can simultaneously reproduce these experimental data reasonably well. And, the observed increasing nuclear stopping with increasing beam energy in experimental data can also be reproduced by using the FU3FP1 parametrization, whereas the calculated stopping power in Au + Au collisions with beam energies from 40 to $150\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/\mathrm{nucleon}$ almost remains constant when taking $\mathcal{F}$ equal to a fixed value.

Baryon stopping in heavy-ion collisions at Elab = 2A-200A GeV

It is argued that an irregularity in the baryon stopping is a natural consequence of the onset of deconfinement occurring in the compression stage of a nuclear collision. It is an effect of the softest point inherent in an equation of state (EoS) with a deconfinement transition. In order to illustrate this effect, calculations within the three-fluid model were performed with three different EoSs: a purely hadronic EoS, an EoS with a first-order phase transition and a third one with a smooth crossover transition. It is demonstrated that this irregularity is a very robust signal of the first-order phase transition that survives under acceptance conditions of the NICA MPD experiment.

Investigation of collective radial expansion and stopping in heavy ion collisions at Fermi energies

We present an analysis of multifragmentation events observed in central $\text{Xe}+\text{Sn}$ reactions at Fermi energies. Performing a comparison between the predictions of the stochastic mean-field (SMF) transport model and experimental data, we investigate the impact of the compression-expansion dynamics on the properties of the final reaction products. We show that the amount of radial collective expansion, which characterizes the dynamical stage of the reaction, influences directly the onset of multifragmentation and the kinematic properties of multifragmentation events. For the same set of events, we also undertake a shape analysis in momentum space, looking at the degree of stopping reached in the collision, as proposed in recent experimental studies. We show that full stopping is achieved for the most central collisions at Fermi energies. However, considering the same central event selection as in the experimental data, we observe a similar behavior of the stopping power with the beam energy, which can be associated with a change of the fragmentation mechanism, from statistical to prompt fragment emission.

Probing the equation of state of asymmetric nuclear matter with isospin diffusion and stopping in heavy-ion collisions

Results from the study of isospin asymmetric Sn+Sn collisions at E/A=35 MeV and 50 MeV, collected with the LASSA and the Chimera detectors at the NSCL of MSU, respectively, are shown. The diffusion of neutrons and protons between projectiles and targets with different N/Z-asymmetries is studied by means of imbalance ratios measurements. The obtained results and their comparison to transport model simulations provide constraints on the density dependence of the symmetry energy. The systematic study of these phenomena at different impact parameters also allows us to explore isospin transparency and stopping phenomena in more central collisions, with important implications about the attainment of chemical equilibrium in central collisions at 35 MeV/nucleon.

Stability of the fragments and thermalization at the peak centre-of-mass energy

We simulated the central reactions of nearly symmetric and asymmetric systems, for energies at which maximum production of intermediate mass fragments (IMFs) occurred ( $E_{\rm c.m.}^{\rm peak}$ ). This study was carried out using hard EOS along with Cugnon cross-section employing MSTB method for clusterization. We studied the various properties of fragments. The stability of fragments was checked through persistence coefficient and gain term. The information about the thermalization and stopping in heavy-ion collisions was obtained via relative momentum, anisotropy ratio and rapidity distribution. We found that for a complete stopping of incoming nuclei very heavy systems are required. The mass dependence of various quantities (such as average and maximum central density, collision dynamics as well as the time zone for hot and dense nuclear matter) was also presented. In all cases (i.e., average and maximum central density, collision dynamics as well as the time zone for hot and dense nuclear matter) a power-law dependence was obtained.

Erratum: Transport model study of nuclear stopping in heavy-ion collisions over the energy range from0.09Ato160AGeV [Phys. Rev. C81, 034913 (2010)

Erratum: Transport model study of nuclear stopping in heavy-ion collisions over the energy range from<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>0.09</mml:mn><mml:mi>A</mml:mi></mml:mrow></mml:math>to<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>160</mml:mn><mml:mi>A</mml:mi></mml:mrow></mml:math>GeV [Phys. Rev. C<b>81</b>, 034913 (2010)

Baryon stopping in heavy-ion collisions at [formula omitted

It is argued that the experimentally observed baryon stopping may indicate (within the present experimental uncertainties) a non-monotonous behaviour as a function of the incident energy of colliding nuclei. This can be quantified by a midrapidity reduced curvature of the net-proton rapidity spectrum. The above non-monotonous behaviour reveals itself as a “zig-zag” irregularity in the excitation function of this curvature. The three-fluid dynamic calculations with a hadronic equation of state (EoS) fail to reproduce this irregularity. At the same time, the same calculations with an EoS involving a first-order phase transition into the quark–gluon phase do reproduce this “zig-zag” behaviour, however only qualitatively.

Transport model study of nuclear stopping in heavy-ion collisions over the energy range from0.09Ato160A GeV

Nuclear stopping in heavy-ion collisions over a beam energy range from SIS and AGS up to SPS is studied in the framework of the modified Ultrarelativistic Quantum Molecular Dynamics transport model, in which mean field potentials of both formed and ``preformed'' hadrons (from string fragmentation) and medium-modified nucleon-nucleon elastic cross sections are considered. It is found that nuclear stopping is influenced by both the stiffness of the equation of state and medium modifications of nucleon-nucleon cross sections at SIS energies. At high SPS energies, a two-bump structure is shown in the experimental rapidity distribution of free protons, which can be understood by considering the preformed hadron potentials.

From fusion to total disassembly: Global stopping in heavy-ion collisions

Using the quantum molecular dynamics model, we aim to investigate the emis- sion of light complex particles, and degree of stopping reached in heavy-ion colli- sions. We took incident energies between 50 and 1000 MeV/nucleon. In addition, central and peripheral collisions and different masses are also considered. We ob- serve that the light complex particles act in almost similar manner as anisotropic ratio. In other words, multiplicity of light complex particles is an indicator of global stopping in heavy-ion collisions. We see that maximum light complex particles and stopping is obtained for heavier masses in central collisions.

Can one understand stopping in heavy ion collisions without an anomalous mechanism?

We show that the baryon stopping observed in heavy ion collisions both at CERN-SPS and at RHIC can be understood using the same mechanism as in proton–proton collisions. No increase in the size of the baryon junction component is required between small size ( pp) and large size ( AA) systems.

Novel mechanism of nucleon stopping in heavy ion collisions

When a diquark does not fragment directly but breaks in such a way that only one of its quarks gets into the produced baryon, the latter is produced closer to mid rapidities. The relative size of this diquark breaking component increases quite fast with increasing energy. We show that a given energy it also increases with the atomic mass number and with the centrality of the collision and that it allows to explain the rapidity distribution of the net baryon number ( p- p ) in SS central collisions. Predictions for PbPb collisions are presented.

Nuclear stopping in heavy-ion collisions at 100 MeV/nucleon from inclusive and exclusive neutral pion measurements.

Inclusive and exclusive measurements of neutral pions in heavy-ion collisions around 100 MeV/nucleon, carried out in a near 4{pi} geometry, have been analyzed to obtain information on the nuclear stopping of the projectile. Stopping of the projectile has been investigated by the analysis of the source velocity, of the distribution of the energetic products of the collisions, and of the associated rapidity distribution of the baryon matter. Collisions were classified according to their centrality by the charged particle multiplicity. Clear evidence for this phenomenon has been obtained by the study of different observables. Both stopping and reabsorption effects play an essential role in the interpretation of the results. {copyright} {ital 1996 The American Physical Society.}

Evidence for stopping in heavy-ion collisions from a study of hard-photon source velocities.

We present data on hard-photon production in [sup 36]Ar+[sup 197]Au and [sup 36]Ar+[sup 12]C collisions at 95 MeV/nucleon. The photon source velocity [beta][sub [ital S]] is investigated as a function of system size, photon energy, and impact parameter. In the heavy system, [beta][sub [ital S]] is found to be sizably smaller than [beta][sub beam]/2, except for very peripheral reactions. This unambiguously demonstrates that at intermediate bombarding energies photon emission is not solely dominated by first-chance [ital p]-[ital n] collisions, but carries also information on the later, stopping phase of the reaction.