Hadronic Rescattering Research Articles

Signature of intermittency in hybrid UrQMD-hydro data at 10 AGeV Au$$+$$Au collisions

An attempt has been made, in the light of scaled factorial moment (SFM) analysis, to investigate hybrid UrQMD-hydro generated events of Au+Au collisions at 10 AGeV to realize the role of hydrodynamic evolution on observed intermittency, if any. $$ln\langle F_{q}\rangle $$ values for $$q=2$$ –6 are found to increase with increasing values of $$lnM^{2}$$ indicating unambiguously the presence of intermittency in our data sample generated with both chiral and hadronic equations of state (EoS). Although various late processes like meson-meson (MM) and meson-baryon (MB) hadronic re-scattering and/or resonance decays are found to influence the intermittency index significantly, these process could not be held responsible for the observed intermittency in hybrid UrQMD-hydro data. Moreover, the signature of intermittency is also found to exists in different sets of data sample generated with a change in initial conditions such as the start time ( $$t_{start}$$ ) and transition energy density (TED) of the UrQMD-hydro model confirming the robustness of the observed power law behavior $$F_{q} \propto (M^{2})^{\alpha _{q}}$$ in our various generated sets of hydro data.

Pion–kaon femtoscopy and the lifetime of the hadronic phase in Pb−Pb collisions at [formula omitted] TeV

In this paper, the first femtoscopic analysis of pion–kaon correlations at the LHC is reported. The analysis was performed on the Pb–Pb collision data at sNN=2.76 TeV recorded with the ALICE detector. The non-identical particle correlations probe the spatio-temporal separation between sources of different particle species as well as the average source size of the emitting system. The sizes of the pion and kaon sources increase with centrality, and pions are emitted closer to the centre of the system and/or later than kaons. This is naturally expected in a system with strong radial flow and is qualitatively reproduced by hydrodynamic models. ALICE data on pion–kaon emission asymmetry are consistent with (3+1)-dimensional viscous hydrodynamics coupled to a statistical hadronisation model, resonance propagation, and decay code THERMINATOR 2 calculation, with an additional time delay between 1 and 2 fm/c for kaons. The delay can be interpreted as evidence for a significant hadronic rescattering phase in heavy-ion collisions at the LHC.

Latest results on the production of hadronic resonances in ALICE at the LHC

Measurement of short-lived hadronic resonances are used to study different aspects of particle production and collision dynamics in pp, p–A and relativistic heavy-ion collisions. The yields of resonances are sensitive to the competing processes of hadron rescattering and regeneration, thus making these particles unique probes of the properties of the late hadronic phase. Measurements of resonances with different masses and quantum numbers also provide insight into strangeness production and processes that determine the shapes of particle momentum spectra at intermediate transverse momenta, as well as the species dependence of hadron suppression at high momentum. We present the comprehensive set of results in the ALICE experiment with unprecedented precision for ρ(770)0, K*(892), ϕ(1020), Σ(1385)±, Λ(1520), and Ξ(1530)0 production in pp, p–Pb, Xe–Xe and Pb–Pb collisions in the energy range sNN=2.76-13TeV, including the latest measurements from LHC Run 2. The obtained results are used to study the system-size and collision-energy evolution of transverse momentum spectra, particle ratios and nuclear modification factors and to search for the onset of collectivity in small collision systems. We compare these results to lower energy measurements and model calculations where available.

Production and reconstruction of short-lived ρ(770)0, K*(892)0, ϕ(1020) and Λ(1520) resonances as a function of centrality and energy in Au+Au collisions at NICA

The short-lived resonances have proved to be very useful in the study of heavy-ion collisions at SPS, RHIC, and the LHC. Properties of these particles measured in dominant hadronic decay channels carry a wealth of information about the hadron chemistry and reaction dynamics. Resonances containing one or two strange quarks contribute to the study of the strangeness enhancement phenomenon predicted as a signature of the phase transition to quark-gluon plasma in heavy-ion collisions. Resonance integrated and differential yields are sensitive to the hadron re-scattering and regeneration in the hadronic phase. The resonance production has only scarcely been studied in heavy-ion collisions at NICA energies. These proceedings are devoted to the review of the expected properties of the resonances and their sensitivity to different stages of Au+Au collisions at = 11 GeV. Results of feasibility studies for reconstruction of ρ(770)0, K*(892)0, ϕ(1020) and Λ(1520) resonances in the MPD experimental setup as a function of collision energy and centrality are presented.

Identical pion and kaon femtoscopy in EPOS 3 with and without the hadronic afterburner UrQMD

Femtoscopy is an effective instrument to study the size and dynamics of the system created in heavy-ion collisions. In particular, a decreasing dependence of system size on increasing pair momentum and particle mass could be interpreted as an evidence of the strong collective flow. Such phenomena are naturally modeled by hydrodynamics. The large data sample collected in Pb–Pb collisions at by the ALICE collaboration enables femtoscopic measurements for pions and kaons. In this paper, the ALICE data are compared with calculations within the EPOS 3 hadronic interaction model, which is based on a (3+1)D viscous hydrodynamical evolution and employs the UrQMD cascade to describe the hadronic phase. The femtoscopic radii for pions and kaons are considered as a function of pair transverse momentum/mass and collision centrality. The obtained results show the importance of the hadronic rescattering phase at LHC energies.

A framework for hadronic rescattering in pp collisions

In this article, a framework for hadronic rescattering in the general-purpose Pythia event generator is introduced. The starting point is the recently presented space–time picture of the hadronization process. It is now extended with a tracing of the subsequent motion of the primary hadrons, including both subsequent scattering processes among them and decays of them. The major new component is cross-section parameterizations for a range of possible hadron–hadron combinations, applicable from threshold energies upwards. The production dynamics in these collisions has also been extended to cope with different kinds of low-energy processes. The properties of the model are studied, and some first comparisons with LHC mathrm {p}mathrm {p} data are presented. Whereas it turns out that approximately half of all final particles participated in rescatterings, the net effects in mathrm {p}mathrm {p} events are still rather limited, and only striking in a few distributions. The new code opens up for several future studies, however, such as effects in mathrm {p}A and AA collisions.

Initial nuclear effects in the improved HIJING code and the production of charged pions, kaons and (anti)protons at STAR BES

The improved Heavy Ion Jet Interaction Generator (HIJING) code is introduced to explore possible initial nuclear effects on the bulk observables of charged pions, kaons and (anti)protons observed in Au+Au collisions at sNN=7.7, 11.5, 19.6, 27 and 39 GeV. The standard HIJING code is extended by (i) different baryon production mechanisms, and (ii) Regge collective cascade recipe. We find that the centrality dependence of the identified particle multiplicity dN/dy at mid-rapidity is quite sensitive to Regge cascading. The overall contribution of baryon production mechanism is shown to enhance the centrality dependence of the mixed particle ratios, K±/π± and p±/π± at <Npart><200 (which corresponds to 20%−100% centrality), especially at higher collision energy. It turns out that the bulk data are shown to be more consistent with HIJING (plus a suitable hadron rescattering code) employing both an advanced popcorn baryon production mechanism and Regge initial scatterings.

Hadron production in elementary nucleon–nucleon reactions from low to ultra-relativistic energies

We study the hadron production in $$p+p$$ , $$p+n$$ and $$n+n$$ reactions within the microscopic Parton–Hadron-Dynamics (PHSD) transport approach in comparison to PYTHIA 8.2. We discuss the details of the “PHSD tune” of the Lund string model (realized by event generators FRITIOF and PYTHIA) in the vacuum (as in $$N+N$$ collisions) as well as its in-medium modifications relevant for heavy-ion collisions where a hot and dense matter is produced. We compare the results of PHSD and PYTHIA 8.2 (default version) for the excitation function of hadron multiplicities as well as differential rapidity y, transverse momentum $$p_T$$ and $$x_F$$ distributions in $$p+p$$ , $$p+n$$ and $$n+n$$ reactions with the existing experimental data in the energy range $$\sqrt{s_{NN}} = 2.7 - 7000$$ GeV. We discuss the production mechanisms of hadrons and the role of final state interactions (FSI) due to the hadronic rescattering. We also show the influence of the possible quark–gluon plasma (QGP) formation on hadronic observables in $$p+p$$ collisions at LHC energies. We stress the importance of developing a reliable event generator for elementary reactions from low to ultra-relativistic energies in view of actual and upcoming heavy-ion experiments.

CP asymmetry from hadronic charm rescattering in B± → π−π+π± decays at the high mass region

A model for the B±→π−π+π± decay amplitude is proposed to study the large CP violation observed at the high mass region of the Dalitz plane. A short distance b→u amplitude with the weak phase γ is considered together with the contribution of a hadronic charm loop and a s-wave DD¯→ππ rescattering. In the model, the χc0 appears as a narrow resonant state of the DD¯ system below threshold. It is introduced in an unitary two channel S-matrix model of the coupled DD¯ and ππ channels, where the χc0 complex pole in DD¯ channel shows its signature in the off-diagonal matrix element and in the associated DD¯→ππ transition amplitude. The strong phase of the resulting decay amplitude has a sharp sign change at the DD¯ threshold, changing the sign of the CP asymmetry, as it is observed in the data. We conclude that the hadronic charm loop and rescattering mechanism are relevant to the broadening of the CP asymmetry around the χc0 resonance in the ππ channel. This novel mechanism provides a possible interpretation for the CP asymmetry challenging experimental result presented by the LHCb collaboration for the B±→π−π+π± decay in the high mass region.

Effects of resonance weak decay and hadronic rescattering on the proton number fluctuations in Au+Au collisions at sNN=5GeV from a microscopic hadronic transport (JAM) model

Proton number fluctuation is sensitive observable to search for the QCD critical point in heavy-ion collisions. In this paper, we studied rapidity acceptance dependence of the proton cumulants and correlation functions in most central Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 5$ GeV from a microscopic hadronic transport model (JAM). At mid-rapidity, we found the effects of resonance weak decays and hadronic re-scattering on the proton cumulants and correlation functions are small, but those effects get larger when further increasing the rapidity acceptance. On the other hand, we found the baryon number conservation is a dominant background effect on the rapidity acceptance dependence of proton number fluctuations. It leads to a strong suppression of cumulants and cumulant ratios, as well as the negative proton correlation functions. We also studied those two effects on the energy dependence of cumulant ratios of net-proton distributions in most central Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 5-200$ GeV from JAM model. This work can serve as a non-critical baseline for future QCD critical point search in heavy-ion collisions at high baryon density region.

Clustering structure effect on Hanbury-Brown–Twiss correlation in $$^{12}\hbox {C} {+^{197}}\hbox {Au}$$ collisions at 200 GeV

Through $$^{12}\hbox {C} {+^{197}}\hbox {Au}$$ collisions at $$\sqrt{s_{NN}} = 200\,\hbox {GeV}$$ using a multiphase transport (AMPT) model, the azimuthal angle dependences of the Hanbury Brown–Twiss (HBT) radii relative to the second- and third-order participant plane from $$\pi $$–$$\pi $$ correlations are discussed. Three initial geometric configurations of $$^{12}\hbox {C}$$, namely three-$$\alpha $$-cluster triangle, three-$$\alpha $$-cluster chain and Woods–Saxon distribution of nucleons, are taken into account, and their effects on the correlations are investigated. The ratio of the third- to the second-order HBT radii $$R_{o(s),3}^2/R_{o(s),2}^2$$ is shown to be a clear probe for three configurations. In addition, this work presents the hadronic rescattering time evolution of the azimuthally dependent HBT radii. From the present study, one can learn that the HBT correlation from identical particles at freeze-out is able to provide the information of different initial configurations as collective flow proposed before.

PHENIX results on elliptic and triangular flow from the small-system geometry scan at 200 GeV

Using the extraordinary versatility of RHIC in selecting different colliding species, the PHENIX experiment has collected data in p+Al, p+Au, d+Au, and 3He+Au collisions at 200 GeV center-of-mass energy and conducted a comprehensive set of anisotropic flow measurements. These geometry-controlled experiments provide a unique testing ground for theoretical models that produce azimuthal particle correlations based on initial- and/or final-state effects.A complete set of triangular anisotropies of inclusive charged particles and final results on identified pion and proton v2(pT) are shown. The mass-ordered splitting in v2(pT) provides information about the role of early-stage collective flow and late-stage hadronic rescattering. Detailed model comparisons with all observables are discussed.

The space\u2013time structure of hadronization in the Lund model

The assumption of linear confinement leads to a proportionality of the energy–momentum and space–time pictures of fragmentation for a simple mathrm{q}bar{mathrm{q}} system in the Lund string model. The hadronization of more complicated systems is more difficult to describe, and in the past only the energy–momentum picture has been implemented. In this article also the space–time picture is worked out, for open and closed multiparton topologies, for junction systems, and for massive quarks. Some first results are presented, for toy systems but in particular for LHC events. The density of hadron production is quantified under different conditions. The (not unexpected) conclusion is that this density can become quite high, and thereby motivate the observed collective behaviour in high-multiplicity mathrm{p}mathrm{p} collisions. The new framework, made available as part of the Pythia event generator, offers a starting point for future model building in a number of respects, such as hadronic rescattering.

Discrepancy in low transverse momentum dileptons from relativistic heavy-ion collisions

The dilepton transverse momentum spectra and invariant mass spectra for low $p_T <0.15$~GeV/c in Au+Au collisions of different centralities at $\sqrt{s_{NN}}$ = 200 GeV are studied within the parton-hadron-string dynamics (PHSD) transport approach. The PHSD describes the whole evolution of the system on a microscopic basis, incorporates hadronic and partonic degrees-of-freedom, the dynamical hadronization of partons and hadronic rescattering. For dilepton production in p+p, p+A and A+A reactions the PHSD incorporates the leading hadronic and partonic channels (also for heavy flavors) and includes in-medium effects such as a broadening of the vector meson spectral functions in hadronic matter and a modification of initial heavy-flavor correlations by interactions with the partonic and hadronic medium. The transport calculations reproduce well the momentum integrated invariant mass spectra from the STAR Collaboration for minimum bias Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, while the description of the STAR data - when gating on low $p_T < 0.15$ GeV/c - is getting worse when going from central to peripheral collisions. An analysis of the transverse momentum spectra shows that the data for peripheral (60-80\%) collisions are well reproduced for $p_T>0.2$ GeV/c while the strong peak at low $p_T < 0.15$ GeV/c, that shows up in the experimental data for the mass bins ($0.4 < M < 0.7$ GeV and $1.2 < M < 2.6$ GeV), is fully missed by the PHSD and cannot be explained by the standard in-medium effects. This provides a new puzzle for microscopic descriptions of low $p_T$ dilepton data from the STAR Collaboration.

Beam energy dependence of the squeeze-out effect on the directed and elliptic flow in Au + Au collisions in the high baryon density region

We present a detailed analysis of the beam energy dependence of the mechanisms for the generation of directed and elliptic flows in Au+Au collisions focusing on the role of hadronic rescattering and spectator shadowing within a microscopic transport model JAM with different equation of state. A systematic study of the beam energy dependence is performed for Au+Au collisions at $\sqrt{s_{NN}} =2.3 - 62.4$ GeV. The transition of the dynamical origin of the directed flow is observed. We find that the initial Glauber type nucleon-nucleon collisions generate negative $v_1$ for nucleons at midrapidity due to the presence of spectator matter, and this negative nucleon $v_1$ is turned to be positive by the meson-baryon interactions at the beam energy region of $\sqrt{s_{NN}} < 30$ GeV. In contrast, above 30 GeV there is no spectator shadowing at midrapidity, and initial nucleon-nucleon collisions do not generate directed flow, but subsequent rescatterings among produced particles generate negative $v_1$ for nucleons. It is demonstrated that negative pion-directed flows are mostly generated by the interaction with the spectator matter. It is also shown that the squeeze-out effect is largely suppressed in the case of softening, which leads to the enhancement of elliptic flow around $\sqrt{s_{NN}}=5-7$ GeV. The elliptic flow at midrapidity above 10 GeV is not influenced by the squeeze-out due to spectator matter, while its effect is seen at the forward rapidity range of $y/y_\mathrm{c.m.}>0.5$, which decreases as beam energy increases.

Femtoscopy with ALICE at the LHC

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Effects of bulk viscosity and hadronic rescattering in heavy ion collisions at energies available at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider

We describe ultra-relativistic heavy ion collisions at RHIC and the LHC with a hybrid model using the IP-Glasma model for the earliest stage and viscous hydrodynamics and microscopic transport for the later stages of the collision. We demonstrate that within this framework the bulk viscosity of the plasma plays an important role in describing the experimentally observed radial flow and azimuthal anisotropy simultaneously. We further investigate the dependence of observables on the temperature below which we employ the microscopic transport description.

Minimal conditions for collectivity in e+e− and p+p collisions

Signatures of collective behavior have been measured in highly relativistic p+p collisions, as well as in p+A, d+A, and 3He+A collisions. Numerous particle correlation measurements in these systems have been successfully described by calculations based on viscous hydrodynamic and transport models. These observations raise the question of the minimum necessary conditions for a system to exhibit collectivity. Recently, numerous scientists have raised the question of whether the quarks and gluons generated in e+e- collisions may satisfy these minimum conditions. In this paper we explore possible signatures of collectivity, or lack thereof, in e+e- collisions utilizing A Multi-Phase Transport (AMPT) framework which comprises melted color strings, parton scattering, hadronization, and hadron re-scattering.

Final state hadronic rescattering with UrQMD

In this talk we discuss the effects of the hadronic rescattering on final state observables in high energy nuclear collisions. We do so by employing the UrQMD transport model for a realistic description of the hadronic decoupling process. The rescattering of hadrons modifies every hadronic bulk observable. For example apparent multiplicity of resonances is suppressed as compared to a chemical equilibrium freeze-out model. Stable and unstable particles change their momentum distribution by more than 30% through rescattering. The hadronic rescattering also leads to a substantial decorrelation of the conserved charge distributions. These findings show that it is all but trivial to conclude from the final state observables on the properties of the system at an earlier time where it may have been in or close to local equilibrium.

Anisotropic (v1 and v2) Flow in Relativistic Heavy-Ion Collisions at Energies between 4 GeV and 200 GeV

Basic features of directed and elliptic flows of identified hadrons in heavy-ion collisions at intermediate and high energies are considered within two transport string models, UrQMD and QGSM. Both models indicate changing of the sign of proton directed flow at midrapidity from antiflow to normal flow with decreasing energy of collisions. The origin of this effect is traced to hadron rescattering in baryon-rich remnants of the colliding nuclei. To distinguish the effect of rescattering from the flow softening caused by creation of quark-gluon plasma one has to compare heavy-ion and light-ion collisions at the same energy. Both directed and elliptic flows at midrapidity are formed within t = 10-12 fm/c. The differences in the development of elliptic flows of mesons and baryons are found at high energies. These differences can be explained by dissimilar freeze-out conditions, thus suggesting simultaneous study of particle collective flow and freeze-out.