Light Hadron Production Research Articles

Particle production in Au+Au collisions at beam energy scan II energies at STAR

We report on measurements of light charged hadron production in Au+Au collisions at √SNN = 14.6, 19.6, 27, and 54.4 GeV from the second phase of the beam energy scan (BES-II) program at RHIC. Transverse mass spectra and rapidity density distributions of π±, K±, p, and ¯p are measured as a function of collision energy, centrality, and rapidity. This work extends the previously published BES-I mid-rapidity (|y| < 0.1) measurement to a rapidity of 0.9. This new rapidity dependence highlights the effects of baryon stopping for proton yields and the different production mechanisms of π± and K±. The relative abundance of these light hadrons indicates different thermodynamic environments at different energies, centralities, and rapidities. The temperature (T) and chemical potentials (μB and μS) at chemical freeze-out are reported with their dependence on rapidity to further understand the QCD phase diagram.

Radiative Decay Width of J/ψ→γη(2) from Nf=2 Lattice QCD

The large radiative production rate for pseudoscalar mesons in the J/ψ radiative decay remains elusive. We present the first lattice QCD calculation of partial decay width of J/ψ radiatively decaying into η_{(2)}, the SU(2) flavor singlet pseudoscalar meson, which confirms QCD U_{A}(1) anomaly enhancement to the coupling of gluons with flavor singlet pseudoscalar mesons. The lattice simulation is carried out using N_{f}=2 lattice QCD gauge configurations at the pion mass m_{π}≈350 MeV. In particular, the distillation method has been utilized to calculate light quark loops. The results are reported here with the mass m_{η_{(2)}}=718(8) MeV and the decay width Γ(J/ψ→γη_{(2)})=0.385(45) keV. By assuming the dominance of U_{A}(1) anomaly and flavor singlet-octet mixing angle θ=-24.5°, the production rates for the physical η and η^{'} in J/ψ radiative decay are predicted to be 1.15(14)×10^{-3} and 4.49(53)×10^{-3}, respectively, which agree well with the experimental measurement data. Our study manifests the potential of lattice QCD studies on the light hadron production in J/ψ radiative decays.

The ϕ meson production in small collision systems observed by PHENIX

The measurements of light hadron production in small collision systems (such as p+Al, p+Au, d+Au, 3He+Au) may allow to explore the quarkgluon plasma formation and to determine the main hadronization mechanism in the considered collisions. Such research has become particularly crucial with the observation of the light hadrons collective behavior in p/d/3He+Au collisions at √SNN = 200 GeV and in p+Al collisions at the same energy at forward and backward rapidities. Among the large variety of light hadrons, ϕ meson is of particular interest since its production is sensitive to the presence of the quark-gluon plasma. The paper presents the comparison of the obtained experimental results on ϕ meson production to different light hadron production in p+Al and 3He+Au at √SNN = 200 GeV at midrapidity. The comparisons of ϕ meson production in p+Al, p+Au, d+Au, and 3He+Au collisions at √SNN = 200 GeV at midrapidity to theoretical models predictions (PYTHIA model and default and string melting versions of the AMPT model) are also provided. The results suggest that the QGP can be formed in p/d/3He+Au collisions, but the volume and lifetime of the produced medium might be insufficient for observation of strangeness enhancement effect. Conceivably, the main hadronization mechanism of ϕ meson production in p+Al collisions is fragmentation, while in p/d/3He+Au collisions this process occurs via coalescence.

Impact of fully coherent energy loss on heavy meson production in pA collisions

Hadron production in proton-nucleus (pA) collisions was previously shown to be suppressed by medium-induced fully coherent energy loss (FCEL). We show that the quenching of D and B mesons in pPb collisions at the LHC due solely to FCEL is, at least, on par with other nuclear effects such as gluon shadowing or saturation. This is consistent with previous findings for both quarkonium and light hadron production in pA collisions, emphasising that FCEL effects need to be included for a reliable understanding of hadron production measurements in pA collisions.

Updates on Flavor Production from STAR

In these proceedings, some of the latest results from the STAR experiment on heavy, strange and light flavor hadron production and their flow are summarized. Over the years STAR has collected heavy ion collision data over a wide range of collision energies from √SNN = 200 GeV down to √SNN = 3 GeV. In high energy heavy ion collisions, heavy flavor hadrons are a valuable probe to study properties of the Quark Gluon Plasma (QGP) created in these collisions. Measurements of strange and light flavor hadrons at lower collision energies help understand the properties of the QCD matter at finite baryon densities and the phase structure of the QCD matter. Furthermore, the high statistics data collected at √SNN = 3 GeV allows measurements of hypernuclei production and flow, which can provide insights into the hyperon - nucleon interactions and hyperon contribution to the nuclear equation of state.

Light and strange hadron production and anisotropic flow measurement in Au+Au collisions at √SNN = 3 GeV from STAR

In this proceeding, we present on our first measurements of identified particle (π, K, p, Ks0, Λ, ϕ, Ξ−) production and anisotropic flow (υ1, υ2) in Au+Au collisions at √SNN = 3 GeV. Various models including thermal and transport model calculations are compared to data, these results imply that the matter produced in the 3 GeV Au+Au collisions is considerably different from that at higher energies.

Production of K(892)*0 mesons in 3He+Au collisions at

Collisions of small systems such as p/d/3He+Au are important for the investigation of quark-gluon plasma (QGP) in relativistic heavy ion collisions. These experiments distinguish the effects of the initial state of cold nuclear and the final state of hot matter. The research of light hadron production is one of the leading directions in the effects of the cold and hot nuclear matter studies. Due to a short lifetime and strange quark content, the K *0-meson is sensitive to the properties of the hot dense matter and strangeness production from an early partonic phase (i.e. QGP). This report presents invariant transverse momentum spectra and nuclear modification factors (RHeAu ) of K *0-meson as a function of pT measured in 3He+Au collisions at by the PHENIX experiment. In results, the nuclear modification factors for K *0-meson in d+Au and 3He+Au collisions are in a good agreement. Values of RHeAu for K *0, φ, and π 0 mesons are equal within uncertainties in all centrality bins in the whole pT range. In conclusion, the obtained results might indicate that CNM effects are not responsible for the differences of K *0 and φ to π 0 suppression level seen in heavy ion collisions.

Fully coherent energy loss effects on light hadron production in pA collisions

We single out the role of fully coherent induced gluon radiation on light hadron production in pA collisions. The effect has the same general features as for quarkonium production, however with a richer color structure as the induced radiation depends on the global color charge of the partonic subprocess final state. Baseline predictions for light hadron nuclear suppression in pPb collisions at the LHC are provided, taking into account only the effect of fully coherent energy loss, which proves to be of the same order of magnitude as gluon shadowing or saturation. This underlines the need to include fully coherent energy loss in phenomenological studies of hadron production in pA collisions.

Quenching of Light Hadron Spectra in p-A Collisions from Fully Coherent Energy Loss.

We single out the role of fully coherent induced gluon radiation on light hadron production in p-A collisions. The effect has an interesting color structure, as the induced radiation depends on the global color charge of the partonic subprocess final state. Baseline predictions for light hadron nuclear suppression in p-Pb collisions at the LHC are provided, taking into account only the effect of fully coherent energy loss, which proves of the same order of magnitude as gluon shadowing or saturation. This underlines the need to include fully coherent energy loss in phenomenological studies of hadron production in p-A collisions.

Update of inclusive cross sections of single and pairs of identified light charged hadrons

We report new measurements of the production cross sections of pairs of charged pions and kaons as a function of their fractional energies using various fractional-energy definitions. Two different fractional-energy definitions were used and compared to the conventional fractional-energy definition reported previously. The new variables aim at either identifying dihadron cross sections in terms of single-hadron fragmentation functions, or to provide a means of characterizing the transverse momentum created in the fragmentation process. The results were obtained applying the updated initial-state radiation correction used in other recent Belle publications on light-hadron production cross sections. In addition, production cross sections of single charged pions, kaons, and protons were also updated using this initial-state radiation correction. The cross sections are obtained from a $558\,{\rm fb}^{-1}$ data sample collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider.

Overview of ALICE results on light flavour hadron production

Light flavour hadrons are copiously produced in hadronic and heavy-ion interactions and bring a wealth of information about properties of the produced medium and reaction dynamics. Having different masses, quark content and lifetimes, light flavour hadrons do not only serve as general observables in the soft sector, but also play an important role as high transverse momentum probes and signatures of the onset of collectivity in high-multiplicity collisions of small and large systems. In this manuscript, we review the most recent ALICE results on the production of different light-flavour hadrons, including transverse momentum spectra, yields, nuclear modification factors and particle ratios in pp, p-Pb, Xe-Xe and Pb-Pb collisions at LHC energies.

Recent results from PHENIX at RHIC

The PHENIX experiment at the relativistic heavy ion collider (RHIC) finished data taking in 2016. However, large datasets collected in different collision systems (p+p, p+A and A+A) at different energies (√sNN = 19-500 GeV) during the last years of the detector operation are actively analysed by the collaboration and bring a wealth of new experimental results. This paper reviews the most recent PHENIX results on the light flavour hadron production, yields and angular correlations of the direct photons in heavy-ion collisions as well as on the search for the onset of collectivity in high multiplicity p+p and p+A collisions.

Thermal model description of p\u2013Pb collisions at sqrt{s_{NN}} = 5.02\xa0TeV

The ALICE data on light flavor hadron production obtained in p–Pb collisions at sqrt{s_{NN}} = 5.02 TeV are studied in the thermal model using the canonical approach with exact strangeness conservation. The chemical freeze-out temperature is independent of centrality except for the lowest multiplicity bin, with values close to 160 MeV but consistent with those obtained in Pb–Pb collisions at sqrt{s_{NN}} = 2.76 TeV. The value of the strangeness non-equilibrium factor gamma _s is slowly increasing with multiplicity from 0.9 to 0.96, i.e. it is always very close to full chemical equilibrium.

Recent PHENIX results on high-pT light hadron production

Production of Quark Gluon Plasma (QGP) has been established in central heavy ion collisions (Au+Au, Cu+Cu) at RHIC energies. Observation of strong suppression of hadron yields at high transverse momentum served as one of the most important evidences in favor of production of a new state of matter in such collisions. Recent RHIC run with asymmetric collision system (Cu+Au) provides the means to systematically study suppression pattern of hadrons in different nuclear overlap geometry needed to improve theoretical description of parton energy loss in QGP. Non-zero elliptic flow and a hint of suppression of high pT hadrons suggests that mini-QGP can be formed in collisions of light and heavy nuclei characterized by high charged particle multiplicities. To address the question of collective behavior in small systems RHIC provided series of geometry controlled experiments with highly asymmetric systems (p+Al, p+Au, 3He+Au). The recent results from the PHENIX experiment at RHIC on π0 and η mesons production in asymmetric systems will be presented and discussed.

Heavy and light hadron production and D-hadron correlation in relativistic heavy-ion collisions

We establish a linear Boltzmann transport (LBT) model coupled to hydrodynamical background to study hard parton evolution in heavy-ion collisions. Both elastic and inelastic scatterings are included in our calculations; and heavy and light flavor partons are treated on the same footing. Within this LBT model, we provide good descriptions of heavy and light hadron suppression and anisotropic flow in heavy-ion collisions. Angular correlation functions between heavy and light flavor hadrons are studied for the first time and shown able to quantify not only the amount of heavy quark energy loss, but also how the parton energy is re-distributed in parton showers.

Single spin asymmetry in forward pA collisions. II. Fragmentation contribution

We compute the twist-three fragmentation contribution to the transverse single spin asymmetry (SSA) in light hadron production $p^\uparrow p\to hX$ and $p^\uparrow A\to hX$ including the gluon saturation effect in the unpolarized nucleon/nucleus. Together with the results in our previous paper, this completes the full evaluation of the SSA in this process in the "hybrid" formalism. We argue that the dependence of SSAs on the atomic mass number in the forward region can elucidate the relative importance of the soft gluon pole contribution from the twist-three quark-gluon-quark correlation in the polarized nucleon and the twist-three fragmentation contribution from the final state hadron.

Single spin asymmetry in forwardpAcollisions

We compute the transverse single spin asymmetry in light hadron production $p^\uparrow p\to hX$ and $p^\uparrow A\to hX$ including the gluon saturation effect in the unpolarized nucleon/nucleus. In the forward (large-$x_F$) region, the dominant contribution comes from the so-called derivative term associated with the soft gluonic pole. This leads to the cancellation of nuclear effects in $A_N$ which can be tested at RHIC. We also show that the soft fermionic pole disappears in the saturation environment.

Coherent energy loss and the production of light hadrons in p–A collisions

A model based on coherent energy loss in nuclear matter, initially proposed for heavy-quarkonium production in p–A collisions, is extended to the case of light hadron production. Preliminary results at the LHC are discussed.

Twist-3 effect from the longitudinally polarized proton for A in hadron production from pp collisions

We compute the contribution from the longitudinally polarized proton to the twist-3 double-spin asymmetry $A_{LT}$ in inclusive (light) hadron production from proton-proton collisions,i.e., $p^\uparrow \vec{p}\to h\,X$. We show that using the relevant QCD equation-of-motion relation and Lorentz invariance relation allows one to eliminate the twist-3 quark-gluon correlator (associated with the longitudinally polarized proton) in favor of one-variable twist-3 quark distributions and the (twist-2) transversity parton density. Including this result with the twist-3 pieces associated with the transversely polarized proton and unpolarized final-state hadron (which have already been calculated in the literature), we now have the complete leading-order cross section for this process.

Jet fragmentation functions in proton-proton collisions using soft-collinear effective theory

The jet fragmentation function describes the longitudinal momentum distribution of hadrons inside a reconstructed jet. We study the jet fragmentation function in proton-proton collisions in the framework of soft-collinear effective theory (SCET). We find that, up to power corrections, the jet fragmentation function can be expressed as the ratio of the fragmenting jet function and the unmeasured jet function. Using renormalization group techniques, we are able to resum large logarithms of jet radii R in the perturbative expansion of the cross section. We use our theoretical formalism to describe the jet fragmentation functions for light hadron and heavy meson production measured at the Large Hadron Collider (LHC). Our calculations agree very well with the experimental data for the light hadron production. On the other hand, although our calculations for the heavy meson production inside jets are consistent with the PYTHIA simulation, they fail to describe the LHC data. We find that the jet fragmentation function for heavy meson production is very sensitive to the gluon-to-heavy-meson fragmentation function.