Hadron Yields Research Articles

Investigating the microscopic properties of strongly interacting matter with HADES

Abstract In the energy domain of 1–2 GeV kinetic energy per nucleon, HADES has measured rare and penetrating probes in elementary and heavy ion collisions. Our results demonstrate that electron pair emission in C+C collisions can essentially be explained as a superposition of independent N+N collisions. HADES results on e+e− production in Ar+KCl collisions, however, show a strong enhancement of the dilepton yield relative to a reference spectrum obtained from elementary nucleon-nucleon reactions, signal the onset of medium effects beyond the superposition of individual N+N collisions. Intriguing results where also obtained from the reconstruction of hadrons with open and hidden strangeness. Analyses of the experimentally obtained hadronic yields measured in Ar+KCl allows to extract the chemical freeze-out conditions in the T -µB phase diagram of strongly interacting matter. While the measured abundance of all reconstructed particles are well described assuming thermalization, the also reconstructed double strange baryon ≡− appears about ten times more abundant than expected. This result will be discussed in the context of the exploration of the nuclear matter phase diagram in the region of finite density. Further investigations to search for significant medium effects, will be followed over the coming years with an upgraded HADES detector.

Hadron yield correlation in quark combination models in high-energyAAcollisions

We study the hadron yield correlation in the combination models in high energy heavy ion collisions. We derive the relationship between the average yields of different hadrons produced in the combination of a system consisting of equal number of quarks and antiquarks. We present the results for the directly produced hadrons as well as those for the final hadrons including the strong and electromagnetic decay contributions. We also study the net quark influence by considering the case when the number of quarks is larger than that of the antiquarks. We make comparison with the data wherever possible.

Suppression of the High-pTCharged-HadronRAAat the LHC

We present a parameter-free postdiction of the high-${p}_{T}$ charged-hadron nuclear modification factor (${R}_{AA}$) in two centralities, measured by the CMS Collaboration in Pb-Pb collisions at the LHC. The evolution of the bulk medium is modeled using viscous fluid dynamics, with parameters adjusted to describe the soft hadron yields and elliptic flow. Assuming the dominance of radiative energy loss, we compute the medium modification of the ${R}_{AA}$ using a perturbative QCD-based formalism, the higher twist scheme. The transverse momentum diffusion coefficient $\stackrel{^}{q}$ is assumed to scale with the entropy density and is normalized by fitting the ${R}_{AA}$ in the most central Au-Au collisions at the Relativistic Heavy-Ion Collider. This setup is validated in noncentral Au-Au collisions at the Relativistic Heavy-Ion Collider and then extrapolated to Pb-Pb collisions at the LHC, keeping the relation between $\stackrel{^}{q}$ and entropy density unchanged. We obtain a satisfactory description of the CMS ${R}_{AA}$ over the ${p}_{T}$ range from 10 to 100 GeV.

Comparison of results from a (2+1)-D relativistic viscous hydrodynamic model to elliptic and hexadecapole flow of charged hadrons measured in Au-Au collisions atsNN=200GeV

Simulated results from a (2+1)-D relativistic viscous hydrodynamic model have been compared to the experimental data on the centrality dependence of invariant yield, elliptic flow (${v}_{2}$), and hexadecapole flow (${v}_{4}$) as a function of transverse momentum (${p}_{T}$) of charged hadrons in Au-Au collisions at $\sqrt{{s}_{NN}}=200$ GeV. Results from two types of initial transverse energy density profile, one based on the Glauber model and other based on the color glass condensate (CGC) model, are presented. We observe no difference in the simulated results on the invariant yield of charged hadrons for the calculations with different initial conditions. The comparison to the experimental data on invariant yield of charged hadrons supports a shear-viscosity to entropy-density ratio ($\ensuremath{\eta}/s$) between 0 and 0.12 for the 0$%$--10$%$ to 40$%$--50$%$ collision centralities. The simulated ${v}_{2}({p}_{T})$ is found to be higher for a fluid with CGC based initial condition compared to Glauber based initial condition for a given collision centrality. Consequently the Glauber based calculations when compared to the experimental data require a lower value of $\ensuremath{\eta}/s$ relative to CGC based calculations. In addition, a centrality dependence of the estimated $\ensuremath{\eta}/s$ is observed from the ${v}_{2}({p}_{T})$ study. The ${v}_{4}({p}_{T})$ for the collision centralities 0$%$--10$%$ to 40$%$--50$%$ supports a $\ensuremath{\eta}/s$ value between 0 and 0.08 for a CGC based initial condition. Simulated results using the Glauber based initial condition for the ideal fluid evolution underestimate the ${v}_{4}({p}_{T})$ for collision centralities 0$%$--10$%$ to 30$%$--40$%$.

LHCb VErtex LOcator (VELO) upgrade

The VErtex LOcator (VELO) is a vital piece of apparatus providing tracking, triggering and vertexing to the LHCb experiment. Its superb impact parameter resolution and high efficiency enable LHCb to make precision measurements and searches for New Physics in the flavour sector. These proceedings focus on the R&D for the future LHCb VELO detector to be installed in the upgraded LHCb experiment. In order to increase hadronic yields and to be able to run at higher luminosities LHCb plans to upgrade the front end electronics, currently limited by a first level hardware trigger. For the silicon vertex detector this implies a complete rebuild of the modules and electronics, with improved cooling capability to cope with the high voltages and currents expected after significant radiation damage has been accrued. This paper presents the design considerations for the vertex detector upgrade together with the two proposed options for the sensor layout, based on strips or pixels. As part of the R&D programme a pixel based tracking telescope has been developed for use in testbeams, to gain experience with pixel tracking and to evaluate the technologies under consideration. This paper presents results from the last years running of the TimePix telescope and describes the integration of various test devices.

Net charge probability distributions in heavy ion collisions at chemical freeze-out

We explore net charge probability distributions in heavy ion collisions within the hadron resonance gas model. The distributions for strangeness, electric charge and baryon number are derived. We show that, within this model, net charge probability distributions and the resulting fluctuations can be computed directly from the measured yields of charged and multi-charged hadrons. The influence of multi-charged particles and quantum statistics on the shape of the distribution is examined. We discuss the properties of the net proton distribution along the chemical freeze-out line. The model results presented here can be compared with data at RHIC energies and at the LHC to possibly search for the relation between chemical freeze-out and QCD cross-over lines in heavy ion collisions.

Identified Hadron Compositions inp+pandAu+AuCollisions at High Transverse Momenta atsNN=200 GeV

We report transverse momentum ($p_{T} \leq15$ GeV/$c$) spectra of $\pi^{\pm}$, $K^{\pm}$, $p$, $\bar{p}$, $K_{S}^{0}$, and $\rho^{0}$ at mid-rapidity in p+p and Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. Perturbative QCD calculations are consistent with $\pi^{\pm}$ spectra in p+p collisions but do not reproduce $K$ and $p(\bar{p})$ spectra. The observed decreasing antiparticle-to-particle ratios with increasing $p_T$ provide experimental evidence for varying quark and gluon jet contributions to high-$p_T$ hadron yields. The relative hadron abundances in Au+Au at $p_{T}{}^{>}_{\sim}8$ GeV/$c$ are measured to be similar to the p+p results, despite the expected Casimir effect for parton energy loss.

Suppression of high [formula omitted] hadrons in Pb + Pb collisions at LHC

Hadron production and their suppression in Pb+Pb collisions at LHC at a center-of-mass energy of sNN=2.76 TeV are studied within a multiphase transport (AMPT) model whose initial conditions are obtained from the recently updated HIJING 2.0 model. The centrality dependence of charged hadron multiplicity dNch/dη at midrapidity was found quite sensitive to the largely uncertain gluon shadowing parameter sg that determines the nuclear modification of the gluon distribution. We find final-state parton scatterings reduce considerably hadron yield at midrapidity and enforces a smaller gluon shadowing to be consistent with dNch/dη data at LHC. With such a constrained parton shadowing, charged hadron and neutral pion production over a wide transverse momenta range are investigated in AMPT. Relative to nucleon–nucleon collisions, the particle yield in central heavy ion collisions is suppressed due to parton energy loss. While the calculated magnitude and pattern of suppression is found consistent with that measured in Au+Au collisions at sNN=0.2 TeV at RHIC, at the LHC energy the suppression is overpredicted which may imply the medium formed at LHC is less opaque than expected from simple RHIC extrapolations. Reduction of the QCD coupling constant αs by ∼30% in the higher temperature plasma formed at LHC as compared to that at RHIC was found to reproduce the measured suppression at LHC.

Low-mass dilepton production in a relativistic Hagedorn-resonance gas model

Dilepton production in the low-invariant-mass region in $p+p$ and Au + Au collisions at $\sqrt{{s}_{NN}}=200$ GeV and in Pb + Au collisions at 158 $A$GeV is studied in the relativistic hadron resonance gas model with Hagedorn mass spectrum that grows exponentially. The model provides good agreement with the observed yield and spectra of stable hadrons and resonances produced in both $p+p$ and nucleus-nucleus collisions. While the measured dilepton yields in $p+p$ collisions are well understood in the model with Dalitz and direct decays of conventional neutral meson sources, a sizable amount of the mesons produced from the Hagedorn states still cannot explain the large enhancement observed in the invariant mass region 200--700 MeV/c${}^{2}$ in Pb + Au and especially in Au + Au collisions. In a multiphase transport (AMPT) model, the dilepton produced from the standard hadronic sources in absence of Hagedorn states underpredicts the data to a larger extent. The study suggests that Hagedorn states in conjugation with in-medium modification of the properties of $\ensuremath{\rho}$ mesons may be essential for the enhanced dilepton production.

Jet–hadron correlations in STAR

Advancements in full jet reconstruction have made it possible to use jets as triggers in azimuthal angular correlations to study the modification of hard-scattered partons in the medium created in ultrarelativistic heavy-ion collisions. This increases the range of parton energies accessible in these analyses and improves the signal-to-background ratio compared to dihadron correlations. Results of a systematic study of jet–hadron correlations in central Au–Au collisions at GeV are indicative of a broadening and softening of jets which interact with the medium. Furthermore, jet–hadron correlations suggest that the suppression of the associated hadron yield at high-pT is balanced in large part by low-pT enhancement.

Jet studies in STAR via 2+1 correlations

This paper report results from an analysis based on a new multi-hadron correlation technique. Jet-medium interactions and di-jet surface emission bias are explored at RHIC, via events tagged by back-to-back high transverse momentum hadrons pairs as proxy of di-jets, then studying the associated hadron distributions. In contrast with traditional two- and three-particle correlations with a single trigger particle, the associated hadron distribution at both trigger sides reveals no evident modification in either relative pseudo-rapidity, Δη or relative azimuthal angle, Δϕ, from d+Au to central Au+Au collisions. This observation stands for either back-to-back triggers of similar kinematic selections (symmetric, of 5< pT <10GeV/c and 4< pT <10GeV/c) or of highly different kinematic selections (asymmetric, of 10< ET <15GeV/c and 4< pT <10GeV/c). The associated hadron yields and spectra are also measured on each trigger side, to gain additional insights of medium properties.

Gluon correlations from a glasma flux-tube model compared to measured hadron correlations on transverse momentum(pt,pt)and angular differences(ηΔ,φΔ)

A Glasma flux-tube model has been proposed to explain strong elongation on pseudorapidity $\eta$ of the same-side 2D peak in minimum-bias angular correlations from $\sqrt{s_{NN}} = 200$ GeV \auau collisions. The same-side peak or "soft ridge" is said to arise from coupling of flux tubes to radial flow. Gluons radiated transversely from flux tubes are boosted by radial flow to form a narrow structure or ridge on azimuth. In this study we test the conjecture by comparing predictions for particle production, spectra and correlations from the Glasma model and conventional fragmentation processes with measurements. We conclude that the Glasma model is contradicted by measured hadron yields, spectra and correlations, whereas a two-component model of hadron production, including minimum-bias parton fragmentation, provides a quantitative description of most data, although $\eta$ elongation remains unexplained.

Momentum space saturation model for deep inelastic scattering and single inclusive hadron production

We show how the Santana Amaral-Gay Ducati-Betemps-Soyez (AGBS) model, originally developed for deep inelastic scattering applied to HERA data on the proton structure function, can also describe the RHIC data on single inclusive hadron yield for $d+Au$ and $p+p$ collisions through a new simultaneous fit. The single inclusive hadron production is modeled through the color glass condensate, which uses the quark (and gluon) condensate amplitudes in momentum space. The AGBS model is also a momentum space model based on the asymptotic solutions of the Balitsky-Kovchegov equation, although a different definition of the Fourier transform is used. This aspect is overcome, and a description entirely in transverse momentum of both processes arises for the first time. The small difference between the simultaneous fit and the one for HERA data alone suggests that the AGBS model describes very well both kinds of processes and thus emerges as a good tool to investigate the inclusive hadron production data. We use this model for predictions at LHC energies, which agrees very well with available experimental data.

Experimental studies of di-jet survival and surface emission bias inAu+Aucollisions via angular correlations with respect to back-to-back leading hadrons

We report first results from an analysis based on a new multi-hadron correlation technique, exploring jet-medium interactions and di-jet surface emission bias at RHIC. Pairs of back-to-back high transverse momentum hadrons are used for triggers to study associated hadron distributions. In contrast with two- and three-particle correlations with a single trigger with similar kinematic selections, the associated hadron distribution of both trigger sides reveals no modification in either relative pseudo-rapidity or relative azimuthal angle from d+Au to central Au+Au collisions. We determine associated hadron yields and spectra as well as production rates for such correlated back-to-back triggers to gain additional insights on medium properties.

Identifying Multiquark Hadrons from Heavy Ion Collisions

Identifying hadronic molecular states and/or hadrons with multiquark components either with or without exotic quantum numbers is a long-standing challenge in hadronic physics. We suggest that studying the production of these hadrons in relativistic heavy ion collisions offers a promising resolution to this problem as yields of exotic hadrons are expected to be strongly affected by their structures. Using the coalescence model for hadron production, we find that, compared to the case of a nonexotic hadron with normal quark numbers, the yield of an exotic hadron is typically an order of magnitude smaller when it is a compact multiquark state and a factor of 2 or more larger when it is a loosely bound hadronic molecule. We further find that some of the newly proposed heavy exotic states could be produced and realistically measured in these experiments.

Fragmentation functions at Belle

Fragmentation functions (FFs) describe the formation of final state hadrons from final state quarks or gluons. Precise knowledge of these functions is a key ingredient in extracting spin-dependent parton distribution functions from spin asymmetries observed in polarized Semi-Inclusive Deep Inelastic Scattering (SIDIS) and in inclusive hadron production in polarized proton-proton collisions. However, fragmentation functions can not be determined from first principles Quantum Chromodynamics (QCD) and have to be extracted from experimental data. The Belle experiment at KEK in Japan has acquired large data samples from hadron production in e+e− annihilation. Precision measurements of identified hadron yields from Belle can be used to improve the precision of the presently available hadron fragmentation data. In this talk Belle results on spin-dependent fragmentation functions will be summarized and the status of the spin-averaged hadron yield analysis will be presented.

Parton fragment yields derived from minimum-bias jet angular correlations

Spectrum hard components from 200 GeV \auau collisions are accurately described by pQCD parton fragment distributions, indicating that a substantial population of parton fragments is present in hadron spectra at low $p_t$. Minimum-bias angular correlations contain jet-like correlation structure with most-probable hadron momentum 1 GeV/c. In this study we convert minimum-bias jet-like angular correlations to single-particle yields and compare them with spectrum hard components. We find that jet-like correlations in central 200 GeV \auau collisions correspond quantitatively to pQCD predictions, and the jet-correlated hadron yield comprises one third of the final state.

Jet Fragmentation in Vacuum and Medium with γ-hadron Correlations in PHENIX

Jet fragmentation in p+p and Au+Au collisions is studied via back-to-back correlations of direct photons and charged hadrons. The direct photon correlations are obtained by statical subtraction of the background from decay photons. Results on the nuclear modification to the associated charged hadron yields are reviewed. Further studies of jet fragmentation in p+p using isolated direct photons are also presented. A kT -smeared LO pQCD calculation is used to interpret the data. The sensitivity of the data to the underlying fragmentation function is tested and the results are found to be compatible with expectations of a sample dominated by quark jet fragmentation.

Charged Hadron Spectra Measurement with the CMS detector in pp collisions at [formula omitted], 2.36, and 7 TeV

The CMS measurement of the charged hadron pseudorapidity and transverse momentum distributions in minimum bias pp collisions at s = 0.9 , 2.36 and 7 TeV, is presented. The charged hadron yield results were obtained using the inner silicon tracker, by means of 3 different methods: counting the number of reconstructed hits, the number of hit pairs (tracklet-based algorithm), and the number of fully reconstructed particle tracks. The obtained data is compared with other experiment results, several Monte-Carlo models and tunes, as well as some current theoretical models.

Single and double inclusive particle production in d + Au collisions at RHIC, leading twist and beyond

We discuss the evidence for the presence of QCD saturation effects in the data collected in d + Au collisions at RHIC. In particular we focus our analysis on forward hadron yields and azimuthal correlations. Approaches alternative to the CGC description of these two observables are discussed in parallel.