Hadron Transverse Momentum Research Articles

PACIAE 2.1: An updated issue of the parton and hadron cascade model PACIAE 2.0

We have updated the parton and hadron cascade model PACIAE 2.0 for relativistic nuclear collisions (cf. Comput. Phys. Comm. 183(2012)333) to the new issue of PACIAE 2.1. The PACIAE model is based on PYTHIA. In the PYTHIA model, once the generated particle/parton transverse momentum $p_T$ is randomly selected, its component of $[p_x,p_y]$ was originally put on the circle with radius $p_T$ randomly. It is now replaced by put on the circumference of ellipse with half major and minor axes of $p_T(1+\delta_p)$ and $p_T(1-\delta_p)$, respectively, in order to investigate better the transverse momentum anisotropy in final hadronic state.

Study of the Helix Structure of QCD String

The properties of a helixlike shaped QCD string are studied in the context of the Lund fragmentation model, where the concept of a three-dimensional structure of the gluon field offers an alternative approach to the modeling of the transverse momentum of hadrons. The paper is focused on the phenomenology of the model, which introduces correlations between transverse and longitudinal components of hadrons, as well as azimuthal ordering of hadrons along the string. The similarities between 2-particle correlations stemming from the helix-string structure and those commonly attributed to the Bose-Einstein interference are pointed out. It is shown that the charged asymmetry observed in hadron production of close hadron pairs can be associated with fluctuations in the space-time history of the string breakup and with the presence of resonant hadronic states.

Leading order determination of the gluon polarisation from DIS events with high-[formula omitted] hadron pairs

We present a determination of the gluon polarisation Δg/g in the nucleon, based on the longitudinal double-spin asymmetry of DIS events with Q2>1 (GeV/c)2 including a pair of large transverse-momentum hadrons in the final state. The data were obtained by the COMPASS experiment at CERN using a 160 GeV/c polarised muon beam scattering off a polarised LiD6 target. The gluon polarisation is evaluated by a Neural Network approach for three intervals of the gluon momentum fraction xg covering the range 0.04<xg<0.27. The values obtained at leading order in QCD do not show any significant dependence on xg. Their average is Δg/g=0.125±0.060 (stat.)±0.063 (syst.) at average xg=0.09 and a scale of μ2=3 (GeV/c)2.

I – Experimental investigation of transverse spin asymmetries in μ-p SIDIS processes: Collins asymmetries

The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV μ+ beam and a transversely polarised NH3 target. The Collins asymmetry of the proton was extracted in the Bjorken x range 0.003<x<0.7. These new precise measurements confirm previous measurements from the COMPASS and HERMES Collaborations, which exhibit a definite effect in the valence quark region. The asymmetries for negative and positive hadrons are similar in magnitude and opposite in sign. They are compatible with model calculations in which the u-quark transversity is opposite in sign and somewhat larger than the d-quark transversity distribution function. The asymmetry is extracted as a function of Bjorken x, the relative hadron energy z and the hadron transverse momentum pTh. The high statistics and quality of the data also allow for more detailed investigations of the dependence on the kinematic variables. These studies confirm the leading-twist nature of the Collins asymmetry.

Collins fragmentation function within the NJL-jet model

The NJL-jet model is extended to accommodate hadronization of a transversely polarized quark in order to explore the Collins effect within a multihadron emission framework. This is accomplished by calculating the polarized quark spin flip probabilities after a pseudoscalar hadron emission and the elementary Collins functions. The model is used to calculate the number densities of the hadrons produced in the polarized quark's decay chain. The full Collins fragmentation function is extracted from the sine modulation of the polarized number densities with respect to the polar angle between the initial quark's spin and hadron's transverse momentum. Two cases are studied here. First, a toy model for elementary Collins function is used to study the features of the transversely polarized quark-jet model. Second, a full model calculation of transverse momentum dependent pion and kaon Collins functions is presented. The remarkable feature of our model is that the 1/2 moments of the favored Collins fragmentation functions are positive and peak at large values of z but decrease and oscillate at small values of z. The 1/2 moments of the unfavored Collins functions have comparable magnitude and opposite sign to the favored functions, vanish at large z and peak at small values of z. This feature is observed for both the toy and full models and can be attributed to the quark-jet picture of hadronization. Moreover, the transverse momentum dependencies of the model Collins functions differ significantly from the Gaussian form widely used in the empirical parametrizations. Finally, a naive interpretation of the Schafer-Teryaev sum rule is proven not to hold in our model, where the transverse momentum conservation is explicitly enforced. This is attributed to the sizable average transverse momentum of the remnant quark that needs to be accounted for to satisfy the transverse momentum sum-rule.

Study of the helix structure of QCD string

The properties of a helixlike shaped QCD string are studied in the context of the Lund fragmentation model, where the concept of a three-dimensional structure of the gluon field offers an alternative approach to the modeling of the transverse momentum of hadrons. The paper is focused on the phenomenology of the model, which introduces correlations between transverse and longitudinal components of hadrons, as well as azimuthal ordering of hadrons along the string. The similarities between 2-particle correlations stemming from the helix-string structure and those commonly attributed to the Bose-Einstein interference are pointed out. It is shown that the charged asymmetry observed in hadron production of close hadron pairs can be associated with fluctuations in the space-time history of the string breakup and with the presence of resonant hadronic states.

Clustering of color sources and the shear viscosity of the QGP in heavy ion collisions at RHIC and LHC energies

The Color String Percolation Model (CSPM) is used to determine the shear viscosity to entropy ratio ($\eta/s$) of the Quark-Gluon Plasma (QGP) produced in Au-Au collisions at $\sqrt{s_{NN}}$ = 200 GeV at RHIC and Pb-Pb at $\sqrt{s_{NN}}$ = 2.76 TeV at LHC. The relativistic kinetic theory relation for $\eta/s$ is evaluated using CSPM values for the temperature and the mean free path of the QGP constituents. The experimental charged hadron transverse momentum spectrum is used to determine the percolation density parameter $\xi$ in Au-Au collisions (STAR). For Pb-Pb at $\sqrt{s_{NN}}$ = 2.76 TeV $\xi$ values are obtained from the extrapolation at RHIC energy. The value of $\eta/s$ is 0.204$\pm$0.020 and 0.262$\pm$0.026 at the CSPM initial temperatures of 193.6$\pm$3 MeV (RHIC) and 262.2 $\pm$13 MeV (LHC) respectively. These values are 2.5 and 3.3 times the AdS/CFT conjectured lower bound $1/4\pi$. We compare the CSPM $\eta/s$ analytic expression with weak coupling (wQGP) and strong coupling (sQGP) calculations. This indicates that the QGP is a strongly coupled fluid in the phase transition region.

Properties of inclusive hadron production in deep inelastic scattering on heavy nuclei at lowx

In this paper we present a comprehensive study of inclusive hadron production in DIS at low $x$. Properties of the hadron spectrum are different in different kinematic regions formed by three relevant momentum scales: photon virtuality $Q^2$, hadron transverse momentum $k_T$ and the saturation momentum $Q_s(x)$. We investigate each kinematic region and derive the corresponding asymptotic formulas for the cross section at the leading logarithmic order. We also analyze the next-leading-order (NLO) corrections to the BFKL kernel that are responsible for the momentum conservation. In particular, we establish the asymptotic behavior of the forward elastic dipole--nucleus scattering amplitude at high energies deeply in the saturation regime and a modification of the pomeron intercept. We study the nuclear effect on the inclusive cross section using the nuclear modification factor and its logarithmic derivative. We argue that the later is proportional to the difference between the anomalous dimension of the gluon distribution in nucleus and in proton and thus is a direct measure of the coherence effects. To augment our arguments and present quantitative results we performed numerical calculations in the kinematic region that may be accessible by the future DIS experiments.

BESSEL-WEIGHTED ASYMMETRIES AND TRANSVERSE SPIN EFFECTS

We consider the cross section for semi-inclusive deep inelastic scattering in Fourier space, conjugate to the outgoing hadron's transverse momentum, where convolutions of transverse momentum dependent parton distribution functions and fragmentation functions become simple products. Individual asymmetric terms in the cross section can be projected out by means of a generalized set of weights involving Bessel functions. Advantages of employing these Bessel weights are that they suppress (divergent) contributions from high transverse momentum and that soft factors cancel in (Bessel-) weighted asymmetries. Also, the resulting compact expressions immediately connect to previous work on evolution equations for transverse momentum dependent parton distribution and fragmentation functions and to quantities accessible in lattice QCD. Bessel-weighted asymmetries are thus model independent observables that augment the description and our understanding of correlations of spin and momentum in nucleon structure.

Formation time of hadronic resonances

In heavy-ion collisions, formation time of hadrons of high transverse momentum can play a pivotal role in determining the perturbative dynamics of the final-state parton and particle system. We present methods to evaluate the formation times of light hadrons, hadronic resonances, open heavy flavor and quarkonia. Experimental implications of the short formation times of heavy particles are discussed in light of recent RHIC and LHC data.

Bessel-weighted asymmetries in semi-inclusive deep inelastic scattering

The concept of weighted asymmetries is revisited for semi-inclusive deep inelastic scattering. We consider the cross section in Fourier space, conjugate to the outgoing hadron's transverse momentum, where convolutions of transverse momentum dependent parton distribution functions and fragmentation functions become simple products. Individual asymmetric terms in the cross section can be projected out by means of a generalized set of weights involving Bessel functions. Advantages of employing these Bessel weights are that they suppress (divergent) contributions from high transverse momentum and that soft factors cancel in (Bessel-) weighted asymmetries. Also, the resulting compact expressions immediately connect to previous work on evolution equations for transverse momentum dependent parton distribution and fragmentation functions and to quantities accessible in lattice QCD. Bessel weighted asymmetries are thus model independent observables that augment the description and our understanding of correlations of spin and momentum in nucleon structure.

Longitudinal spin physics results from COMPASS

The COMPASS experiment at the CERN SPS has a broad physics program focused on the nucleon spin structure and hadron spectroscopy, using muon and hadron beams, respectively. Results from the COMPASS on the inclusive longitudinal double spin asymmetries A1d,p measured in deep-inelastic polarized muon scattering off the polarized 6LiD and NH3 targets are presented. The new proton data cover the range Q2>1(GeV2/c), 0.004<x<0.7 and improve the statistical precision of g1p(x) by factor of two at x<0.02 compared to previous measurements. The spin-dependent non-singlet function g1NS(x,Q2) extracted from the data allows to evaluate the isovector quark density Δq3(x,Q2) and check the Bjorken sum rule prediction. The helicity distributions for the three lightest quark and antiquark flavours extracted from the measured double spin asymmetries for production of charged pions and kaons in semi-inclusive deep-inelastic scattering are also presented and discussed. The gluon polarization in the nucleon was determined using open charm production or high transverse-momentum hadron pairs production in polarized muon scattering off longitudinally polarized target.

Azimuthal asymmetries of charged hadrons produced muons off longitudinally polarised deuterons at COMPASS

Azimuthal asymmetries in semi-inclusive production of positive (h+) and negative hadrons (h−) have been measured by scattering of 160 GeV muons off longitudinally polarised deuterons at CERN. The asymmetries were decomposed in several terms, according to their expected modulation as a function of the outgoing hadron azimuthal angle ϕ. Each term receives contributions from one or several spin and transverse-momentum-dependent parton distribution and fragmentation functions. The amplitudes of all ϕ-modulation terms of the hadron asymmetries integrated over the kinematic variables are found to be consistent with zero within statistical errors, while the constant terms are nonzero and equal for h+ and h−within the statistical errors. The dependencies of the ϕ-modulated terms versus the Bjorken momentum fraction x, the hadron fractional momentum z, and the hadron transverse momentum pTh were studied. The x dependence of the constant terms for both positive and negative hadrons is in agreement with the longitudinal double-spin hadron asymmetries, measured in semi-inclusive deep-inelastic scattering. The x dependence of the sin ϕ-modulation term is less pronounced than that in the corresponding HERMES data. All other dependencies of the ϕ-modulation amplitudes are consistent with zero within the statistical errors.

Two-particle angular correlations in pp and heavy-ion collisions

This paper presents a selected review of recent experimental results on two-particle angular correlations in AuAu and pp collisions. Two-particle correlations in AuAu collisions exhibit a rich structure as a function of hadron transverse momentum, carrying information about the expansion of the medium produced in nuclear collisions, the energy loss of high- p T partons in the medium and the medium response to these high- p T partons or other initial state inhomogeneities. Evidence on the interplay between initial state fluctuations and the final state correlation structure is discussed.

Measurement of neutral mesons inp+pcollisions ats=200 GeVand scaling properties of hadron production

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the invariant differential cross section for production of K^0_S , \omega, \eta prime, and \phi mesons in p + p collisions at = 200 GeV. Measurements \omega and \phi production in different decay channels give consistent results. New results for the \phi are in agreement with previously published data and extend the measured pT coverage. The spectral shapes of all hadron transverse momentum distributions measured by PHENIX are well described by a Tsallis distribution functional form with only two parameters, n and T, determining the high-pT and characterizing the low-pT regions of the spectra, respectively. The values of these parameters are very similar for all analyzed meson spectra, but with a lower parameter T extracted for protons. The integrated invariant cross sections calculated from the fitted distributions are found to be consistent with existing measurements and with statistical model predictions.

Azimuthal asymmetries of charged hadrons produced by high-energy muons scattered off longitudinally polarised deuterons

Azimuthal asymmetries in semi-inclusive production of positive (h^+) and negative hadrons (h^-) have been measured by scattering 160 GeV muons off longitudinally polarised deuterons at CERN. The asymmetries were decomposed in several terms according to their expected modulation in the azimuthal angle phi of the outgoing hadron. Each term receives contributions from one or several spin and transverse-momentum-dependent parton distribution and fragmentation functions. The amplitudes of all phi-modulation terms of the hadron asymmetries integrated over the kinematic variables are found to be consistent with zero within statistical errors, while the constant terms are nonzero and equal for h^+ and h^- within the statistical errors. The dependencies of the phi-modulated terms versus the Bjorken momentum fraction x, the hadron fractional momentum z, and the hadron transverse momentum p_h^T were studied. The x dependence of the constant terms for both positive and negative hadrons is in agreement with the longitudinal double-spin hadron asymmetries, measured in semi-inclusive deep-inelastic scattering. The x dependence of the sin phi-modulation term is less pronounced than that in the corresponding HERMES data. All other dependencies of the phi-modulation amplitudes are consistent with zero within the statistical errors.

Identified hadrons and jet chemistry for p+p and Au+Au collisions at RHIC

The study of hadron spectra at high pT in p+p collisions provides a good test of perturbative quantum chromo-dynamic calculations (pQCD) and a baseline for measurements of nuclear modification factors in Au+Au collisions. Using events triggered with electro-magnetic calorimetery, identified charged hadron transverse momentum (pT) spectra are measured up to 15 GeV/c at mid-rapidity (|y| < 0.5) and neutral kaon pT spectra up to 12 GeV/c in p+p collisions at = 200 GeV in the STAR experiment (Ackermann K H et al 2003 Nucl. Instrum. Methods A 499 624. The particle ratios of p/π+, and K±, 0/π± in p+p collisions are shown and compared with next-to-leading-order pQCD calculations. In central Au+Au collisions, we report nuclear modification factors (RAA) for pion, kaon, proton and ρ, and discuss several model calculations: color-charge dependence of jet quenching and jet conversion. Finally, the centrality dependence of RAA at high pT (> 5.5 GeV/c) for kaons is compared with that of pions in Au+Au collisions at 200 GeV.

On saturation of charged hadron production in pp collisions at LHC

First results on charged hadron transverse momentum spectra in pp collisions obtained by the CMS Collaboration at the LHC are analyzed in the z-scaling approach. These LHC data confirm z-scaling. The saturation regime of the scaling function ψ(z) observed in pp and interactions at lower energies GeV is verified. The saturation of ψ(z) for charged hadrons is found down to a value of z ≃ 0.05 at the highest energy GeV currently reached by the colliders. A microscopic scenario of hadron production is discussed in connection with the search for new signatures of phase transitions in hadron matter. Constituent energy loss and its dependences on the transverse momentum and collision energy are estimated. We suggest that the beam energy scan program in the saturation region at the LHC could be of interest.

Bulk matter evolution and extraction of jet transport parameters in heavy-ion collisions at energies available at the BNL Relativistic Heavy Ion Collider (RHIC)

Within the picture of jet quenching induced by multiple parton scattering and gluon bremsstrahlung, medium modification of parton fragmentation functions and therefore the suppression of large transverse-momentum hadron spectra are controlled by both the value and the space-time profile of the jet transport parameter along the jet propagation path. Experimental data on single-hadron suppression in high-energy heavy-ion collisions at the Relativistic Heavy Ion Collider energy are analyzed within the higher-twist (HT) approach to the medium-modified fragmentation functions and the next-to-leading order perturbative QCD parton model. Assuming that the jet transport parameter $\mathrm{q\ifmmode \hat{}\else \^{}\fi{}}$ is proportional to the particle number density in both quark gluon plasma (QGP) and hadronic phase, experimental data on jet quenching in deeply inelastic scattering off nuclear targets can provide guidance on ${\mathrm{q\ifmmode \hat{}\else \^{}\fi{}}}_{h}$ in the hot hadronic matter. One can then study the dependence of the extracted initial value of jet-quenching parameter ${\mathrm{q\ifmmode \hat{}\else \^{}\fi{}}}_{0}$ at initial time ${\ensuremath{\tau}}_{0}$ on the bulk medium evolution. Effects of transverse expansion, radial flow, phase transition, and nonequilibrium evolution are examined. The extracted values are found to vary from ${\mathrm{q\ifmmode \hat{}\else \^{}\fi{}}}_{0}{\ensuremath{\tau}}_{0}=0.54$ GeV${}^{2}$ in the ($1+3$)d ideal hydrodynamic model to 0.96 GeV${}^{2}$ in a cascade model, with the main differences coming from the initial nonequilibrium evolution and the later hadronic evolution. The overall contribution to jet quenching from the hadronic phase, about $22%--44$$%$, is found to be significant. Therefore, a realistic description of the early nonequilibrium parton evolution and later hadronic interaction will be critical for accurate extraction of the jet transport parameter in the strongly interacting QGP phase in high-energy heavy-ion collisions.

Events with an isolated lepton and missing transverse momentum and measurement of W production at HERA

A search for events containing an isolated electron or muon and missing transverse momentum produced in ep collisions is performed with the H1 and ZEUS detectors at HERA. The data were taken in the period 1994-2007 and correspond to an integrated luminosity of 0.98 fb^-1. The observed event yields are in good overall agreement with the Standard Model prediction, which is dominated by single W production. In the e+p data, at large hadronic transverse momentum PtX >25 GeV, a total of 23 events are observed compared to a prediction of 14.0 \pm 1.9. The total single W boson production cross section is measured as 1.06 \pm 0.16 (stat.) \pm 0.07 (sys) pb, in agreement with an SM expectation of 1.26 \pm 0.19 pb.