Parton Scatterings Research Articles

Heavy-quark production in ultrarelativistic heavy-ion collisions within a partonic transport model

The production and space-time evolution of charm and bottom quarks in nucleus-nucleus collisions at RHIC and LHC are investigated with the partonic transport model BAMPS (Boltzmann Approach of MultiParton Scatterings). Heavy quarks, produced in primary hard parton scatterings during nucleon-nucleon collisions, are sampled using the Monte Carlo event generator PYTHIA or the leading order mini-jet model in conjunction with the Glauber model, revealing a strong sensitivity on the parton distribution functions, scales, and heavy quark mass. In a comprehensive study exploring different charm masses, K factors, and possible initial gluon conditions, secondary production and the evolution of heavy quarks are examined within a fully dynamic BAMPS simulation for central heavy ion collisions at RHIC and LHC. Although charm production in the quark-gluon plasma can be neglected at RHIC, it is significant at LHC but very sensitive to the initial conditions and the charm mass. Bottom production in the quark-gluon plasma, however, is negligible both at RHIC and LHC.

Differential elliptic flow of identified hadrons and constituent quark number scaling at the GSI Facility for Antiproton and Ion Research (FAIR)

Differential elliptic flow $v_2(p_{T})$ for identified hadrons has been investigated in the FAIR energy regime, employing a hadronic-string transport model (UrQMD) as well as a partonic transport model (AMPT). It has been observed that both the models show a mass ordering of $v_2$ at low $p_{T}$ and a switch over resulting a baryon-meson crossing at intermediate $p_{T}$. AMPT generates higher $v_2$ values compared to UrQMD. In addition, constituent quark number scaling behavior of elliptic flow has also been addressed. Scaling behavior in terms of the transverse momentum $p_T$ is found to absent for both the partonic as well as the hadronic model. However UrQMD and AMPT with string melting scenario do exhibit a NCQ scaling of $v_2$ at varying degree, with respect to transverse kinetic energy $KE_T$. But default AMPT, where partonic scatterings are not included, does not show any considerable scaling behavior. A variable $\alpha$ is defined to quantify the degree of $KE_T$ scaling. We found that UrQMD gives better scaling than AMPT at FAIR.

Double parton distributions incorporating perturbative QCD evolution and momentum and quark number sum rules

It is anticipated that hard double parton scatterings will occur frequently in the collisions of the LHC, producing interesting signals and significant backgrounds to certain single scattering processes. For double scattering processes in which the same hard scale t = ln(Q^2) is involved in both collisions, we require the double parton distributions (dPDFs) D_h^{j_1j_2}(x_1,x_2;t) in order to make theoretical predictions of their rates and properties. We describe the development of a new set of leading order dPDFs that represents an improvement on approaches used previously. First, we derive momentum and number sum rules that the dPDFs must satisfy. The fact that these must be obeyed at any scale is used to construct improved dPDFs at the input scale Q_0, for a particular choice of input scale (Q_0^2 = 1 GeV^2) and corresponding single PDFs (the MSTW2008LO set). We then describe a novel program which uses a direct x-space method to numerically integrate the LO DGLAP equation for the dPDFs, and which may be used to evolve the input dPDFs to any other scale. This program has been used along with the improved input dPDFs to produce a set of publicly available dPDF grids covering the ranges 10^{-6}<x_1<1, 10^{-6}<x_2<1, and 1<Q^2<10^9 GeV^2.

A general derivation of differential cross section in quark-quark and quark-gluon scatterings at fixed impact parameter

We propose a general derivation of differential cross section in quark-quark and quark-gluon scatterings at fixed impact parameters. The derivation is well defined and free of ambiguity in the conventional one. The approach can be applied to a variety of partonic and hadronic scatterings in low- or high-energy particle collisions.

A Double Parton Scattering Background to Associate WH and ZH Production at the LHC

Higgs boson production in association with W and Z bosons at high luminosity CERN Large Hadron Collider (LHC, s = 14 TeV ), is one of the most promising discovery channel for a SM Higgs particle with a mass below 135 GeV, where the Higgs decays into b b ¯ final states is dominant. The experimental capability of recognizing the presence of b quarks in a complex hadronic final state has brought attention towards the final states with pairs for observing the production of the Higgs at the LHC. We point out that double parton scattering processes are going to represent a sizable background to the process.

FORWARD- AND MID-RAPIDITY JET-LIKE CORRELATIONS

Mid-rapidity azimuthal correlations probe di-jets originating mainly from gluon-gluon hard-scattering. Measurements of such correlations have revealed significant (gluon-)jet modification in central Au + Au collisions. Azimuthal correlations of hadrons at forward rapidity with a mid-rapidity high-p⊥ hadron, on the other hand, probe asymmetric partonic scatterings involving large- x quarks and small- x gluons. We present preliminary results from STAR on correlations of charged hadrons at forward rapidity in the forward TPCs (2.7 &lt; |η| &lt; 3.9, p⊥ &lt; 2 GeV /c) with high-p⊥ charged hadrons at mid-rapidity from the main TPC (|η| &lt; 1, p⊥ &gt; 3 GeV /c) in pp , d + Au , and Au + Au collisions at [Formula: see text]. The implications of the results for small- x gluon distributions (Color Glass Condensate formation) and the energy loss of quark jets at forward rapidity in nuclear medium are discussed.

V2 and RAA of non-photonic single electrons: which restriction do the current measurements pose on the production of charm, beauty?

In relativistic heavy ion collisions, heavy flavor is expected to result predominately from initial hard parton–parton scatterings. Hence, in the absence of later stage effects, the production of heavy flavor in A+A collisions can be viewed as a superposition of N+N collisions. Measurements of v2 or RAA of heavy flavor (or their decay electrons) in A+A collisions violate the simple superposition picture and therefore present themselves as probes of the medium formed in such collisions. On the basis of the measured v2 and RAA of non-photonic single electrons in A+A collisions at RHIC, we will investigate the interplay between these observables as well as the restrictions they pose for charm and bottom production.

Classical and non-classical ADD-phenomenology with high-E⊥jet observables at collider experiments

We use the results from a recent investigation of hard parton-parton gravitational scattering in the ADD scenario to make semi-quantitative predictions for a few standard high-ET jet observables at the LHC. By implementing these gravitational scattering results in the Pythia event generator and combining it with the Charybdis generator for black holes, we investigate the effects of large extra dimensions and find that, depending on the width of the brane, the relative importance of gravitational scattering and black hole production may change significantly. For the cases where gravitational scatterings are important we discuss how to distinguish gravitational scattering from standard QCD partonic scatterings. In particular we point out that the universal colorlessness of elastic gravitational scattering implies fewer particles between the hard jets, and that this can be used in order to distinguish an increased jet activity induced by gravitational scattering from an increased jet activity induced by eg. super-symmetric extensions where the interaction is colorful.

Multiphase transport model for relativistic heavy ion collisions

We describe in detail how the different components of a multiphase transport (AMPT) model that uses the heavy ion jet interaction generator (HIJING) for generating the initial conditions, Zhang's parton cascade (ZPC) for modeling partonic scatterings, the Lund string fragmentation model or a quark coalescence model for hadronization, and a relativistic transport (ART) model for treating hadronic scatterings are improved and combined to give a coherent description of the dynamics of relativistic heavy ion collisions. We also explain the way parameters in the model are determined and discuss the sensitivity of predicted results to physical input in the model. Comparisons of these results to experimental data, mainly from heavy ion collisions at the BNL Relativistic Heavy Ion Collider, are then made in order to extract information on the properties of the hot dense matter formed in these collisions.

“Naked” Cronin effect in A + A collisions from SPS to RHIC

Baseline computations of the Cronin effect in nuclear collisions at energies spanning the SPS and the RHIC accelerators are performed in the Glauber-Eikonal model, which ascribes the effect to initial-state incoherent multiple parton scatterings. The model accounts very well for the mid-rapidity Cronin effect in hadron-nucleus collisions in the 27-200 GeV center of mass energy range, and will be extended to nucleus-nucleus collisions. The computations are performed under the assumption that the partons do not interact with the medium produced in the collision. Therefore, medium effects such as energy loss in a Quark-Gluon Plasma may be detected and measured as deviations from the presented baseline computation of the "naked" Cronin effect.

Production of K0s, ϕ, Λ and Ξ from 200 GeV d+Au collisions at RHIC

We present the preliminary measurement of K0s, ϕ, Λ and Ξ transverse momentum (pt) distribution from GeV d+Au collisions at RHIC. The measured spectra can be described by using a double exponential fit. The centrality dependence of the particle yield with respect to the number of binary collision scaling (RCP) will be presented for a pt up to 6 GeV/c. The particle dependence of the RCP among K0s, ϕ, Λ and Ξ is consistent with expectations from parton recombination models for hadron formation. The Cronin effect and its particle dependence appear not due to initial parton scatterings alone as previously assumed in models.

High-pt hadron production and di-hadron azimuthal correlations from the STAR experiment

High-pt hadrons are produced in large-momentum transfer parton scatterings in p + p and Au + Au collisions at RHIC. In Au + Au collisions, the production is modified compared to p + p collisions by interactions of the hard-scattered partons with the hot and dense medium. Relevant results on high-pt hadron spectra, elliptic flow and azimuthal di-hadron correlations in Au + Au collisions at √sNN = 62.4 GeV are presented and compared to available results at 200 GeV. Qualitatively similar suppression patterns are observed at both energies.

Pseudorapidity asymmetry and centrality dependence of charged hadron spectra in d+Au collisions at sNN=200GeV

The pseudorapidity asymmetry and centrality dependence of charged hadron spectra in d+Au collisions at $\sqrt{s_{NN}}=200$ GeV are presented. The charged particle density at mid-rapidity, its pseudorapidity asymmetry and centrality dependence are reasonably reproduced by a Multi-Phase Transport model, by HIJING, and by the latest calculations in a saturation model. Ratios of transverse momentum spectra between backward and forward pseudorapidity are above unity for \pT below 5 GeV/$c$. The ratio of central to peripheral spectra in d+Au collisions shows enhancement at 2 $<$ \pT $<$ 6 GeV/$c$, with a larger effect at backward rapidity than forward rapidity. Our measurements are in qualitative agreement with gluon saturation and in contrast to calculations based on incoherent multiple partonic scatterings.

Photon Interferometry ofAu+AuCollisions at the BNL Relativistic Heavy-Ion Collider

We calculate the two-body correlation function of direct photons produced in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at the Relativistic Heavy-Ion Collider. Our calculation includes contributions from the early preequilibrium phase in which photons are produced via hard parton scatterings as well as radiation of photons from a thermalized quark-gluon plasma and the subsequent expanding hadron gas. We find that high energy photon interferometry provides a faithful probe of the details of the space-time evolution and of the early reaction stages of the system.

Heavy-quark production in proton-nucleus collisions at the CERN LHC

A sizable rate of events, with several pairs of $b$-quarks produced contemporarily by multiple parton interactions, may be expected at very high energies as a consequence of the large parton luminosities. The production rates are further enhanced in hadron-nucleus reactions, which may represent a convenient tool to study the phenomenon. We compare the different contributions to $b{\bar b}b{\bar b}$ production, due to single and double parton scatterings, in collisions of protons with nuclei at the CERN-LHC.

$b{\bar b}b{\bar b}$ production in proton-proton and proton-nucleus collisions at the CERN LHC

Given the large parton luminosities, sizable rate of events, with several pairs of b-quarks produced contemporarily by multiple parton interactions, are foreseen in hadronic and nuclear collisions at very high energies. We compare the different contributions to \( b\bar bb\bar b \) production, due to single and double parton scatterings, in pp and in p A collisions at the CERN LHC within the acceptance of the ALICE and of the LHCb detectors.

An Interpretation of Saturation Phenomena as Glauber–Gribov Multiple Parton Scatterings

We compare two formalism that describe minijet production in pA and AA collisions: pQCD supplemented by Glauber-Gribov multiple semihard parton scatterings (pQCD+Glauber), and the Colour Glass Condensate (CGC). We argue that in a suitable limit they are equivalent to each other, the PQCD+Glauber model being more accurate from a numerical point of view. Finally, we analyze RHIC data on Au-Au integrated charged multiplicities in the pQCD+Glauber framework, and conclude that at least at central rapidity there is no sign of gluon saturation.

Charmonium dissociation cross sections and p– A collisions

Charmonium dissociations by nucleons are first studied from a quark–quark potential in combination with quark exchange mechanism. Wave functions of nucleons and charmed baryons involved in dissociation reactions result from a fit to mass splittings of baryons with spins 1 2 and 3 2 . Transverse momentum and x F distributions of prompt J/ ψ produced in 800 GeV/ c p–Be collisions are calculated by using NRQCD dσ/d t ̂ for parton–parton scatterings and taking into account initial transverse-momentum distributions of partons inside nucleons. We emphasize that the new dissociation cross sections of J/ ψ, ψ′ and χ cJ give rise to remarkable differences of J/ ψ, ψ′ and χ c suppressions at large negative x F in p–W collisions at s NN =38.8 and 41.6 GeV and p–Au collisions at s NN =130 GeV .

Double parton scatterings inb-quark pair production at the CERN LHC

A sizable rate of events where two pairs of b quarks are produced contemporarily is foreseen at the CERN LHC, as a consequence of the large parton luminosity. At very high energies both single and double parton scatterings contribute to the process, the latter mechanisms, although power suppressed, giving the dominant contribution to the integrated cross section.

Multiple parton scattering in nuclei: twist-four nuclear matrix elements and off-forward parton distributions

Multiple parton scatterings inside a large nucleus generally involve higher-twist nuclear parton matrix elements. The gluon bremsstrahlung induced by multiple scattering depends not only on direct parton matrix elements but also on momentum-crossed ones, due to the Landau–Pomeranchuk–Migdal interference effect. We show that both types of twist-four nuclear parton matrix elements can be factorized approximately into the product of twist-two nucleon matrix elements in the limit of extremely large nuclei, A→∞, as assumed in previous studies. Due to the correlative nature of the twist-four matrix elements under consideration, it is actually the off-forward parton distributions that appear naturally in this decomposition, rather than the ordinary diagonal distributions probed in deeply-inelastic scattering. However, we argue that the difference between these two distribution classes is small in certain kinematic regimes. In these regions, the twist-four nuclear parton matrix elements are evaluated numerically and compared to the factorized form for different nuclear sizes within a schematic model of the two-nucleon correlation function. The nuclear size dependence is found to be A 4/3 in the limit of large A, as expected. We find that the factorization is reasonably good when the momentum fraction carried by the gluon field is moderate. The deviation can be more than a factor of 2, however, for small gluon momentum fractions, where the gluon distribution is very large.