Forward Rapidity Region Research Articles

Cold nuclear matter effects on [formula omitted] production: Intrinsic and extrinsic transverse momentum effects

Cold nuclear matter effects on J/ψ production in proton–nucleus and nucleus–nucleus collisions are evaluated taking into account the specific J/ψ-production kinematics at the partonic level, the shadowing of the initial parton distributions and the absorption in the nuclear matter. We consider two different parton processes for the cc¯-pair production: one with collinear gluons and a recoiling gluon in the final state and the other with initial gluons carrying intrinsic transverse momentum. Our results are compared to RHIC observables. The smaller values of the nuclear modification factor RAA in the forward rapidity region (with respect to the mid rapidity region) are partially explained, therefore potentially reducing the need for recombination effects.

Test of chemical freeze-out at the RHIC

We present the results of a systematic test applying statistical thermal model fits in a consistent way for different particle ratios and different system sizes using the various particle yields measured in the STAR experiment. Comparison between central and peripheral Au+Au and Cu+Cu collisions with data from p+p collisions provides an interesting tool to verify the dependence on the system size. We also present a study of the rapidity dependence of the thermal fit parameters using available data from the RHIC in the forward rapidity regions and also using different parameterizations for the rapidity distribution of different particles.

Rapidity and energy dependence of the electric charge correlations inA+Acollisions from20Ato158AGeV

Results from electric charge correlations studied with the Balance Function method in A+A collisions from 20\emph{A} to 158\emph{A} GeV are presented in two different rapidity intervals: In the mid-rapidity region we observe a decrease of the width of the Balance Function distribution with increasing centrality of the collision, whereas this effect vanishes in the forward rapidity region. Results from the energy dependence study in central Pb+Pb collisions show that the narrowing of the Balance Function expressed by the normalised width parameter \textit{W} increases with energy towards the highest SPS and RHIC energies. Finally we compare our experimental data points with predictions of several models. The hadronic string models UrQMD and HIJING do not reproduce the observed narrowing of the Balance Function. However, AMPT which contains a quark-parton transport phase before hadronization can reproduce the narrowing of the BF's width with centrality. This confirms the proposed sensitivity of the Balance Function analysis to the time of hadronization.

Centrality and system size dependence of multiplicity fluctuations in nuclear collisions at158AGeV

The centrality and system size dependence of multiplicity fluctuations of charged particles produced in nuclear collisions at $158 A$ GeV was studied by the NA49 collaboration. Centrality selected Pb+Pb collisions, semi-central C+C and Si+Si collisions as well as inelastic p+p interactions were analyzed. The number of projectile participants determined on an event-by-event basis was used to characterize the collision centrality. The scaled variance of the multiplicity distribution obtained in the forward rapidity region ($1.1 <y_{c.m.} <2.6$) shows a significant increase towards peripheral collisions. The results are similar for negatively and positively charged particles and about 50% larger for all charged particles. String-hadronic models of nuclear reactions without the fusion process do not reproduce the rise of fluctuations from central towards peripheral collisions. The measured centrality dependence can be reproduced in superposition models with the assumption of contributions from target participants to particle production in the forward hemisphere or in string models with fusion.

PION AND KAON PRODUCTION IN NUCLEON–NUCLEON COLLISIONS

Inclusive cross section for pion production in proton - proton collisions are calculated based on unintegrated parton distribution functions (uPDFs). In addition to purely gluonic terms the present approach includes also quark degrees of freedom. Phenomenological fragmentation functions from the literature are used. The new mechanisms are responsible for π+ - π- asymmetry. In contrast to standard collinear approach, application of 2 → 1 kt - factorization approach can be extended towards much lower transverse momenta, both at mid and forward rapidity region. The results of the calculation are compared with SPS and RHIC data.

Electroweak gauge boson production at hadron colliders throughO(αs2)

We describe a calculation of the O({alpha}{sub s}{sup 2}) QCD corrections to the fully differential cross section for W and Z boson production in hadronic collisions. The result is fully realistic in that it includes spin correlations, finite width effects, {gamma}-Z interference and allows for the application of arbitrary cuts on the leptonic decay products of the W and Z. We have implemented this calculation into a numerical program. We demonstrate the use of this code by presenting phenomenological results for several future LHC analyses and recent Tevatron measurements, including the W cross section in the forward rapidity region and the central over forward cross section ratio.

Unintegrated parton distributions and pion production in pp collisions at RHIC energies

We compare the results of the 2 → 1 kt-factorization approach (often called BFKL multi-Regge factorization) with Kwieciński unintegrated parton distribution functions (UPDF) and the standard collinear-factorization approach at RHIC and slightly smaller energies. Our approach contains only one free parameter responsible for internal parton motion in nucleons. In contrast to recent works in the literature, our kt-factorization approach also includes quark degrees of freedom in addition to purely gluonic terms. Both mid- and forward-rapidity regions are considered. We discuss uncertainties due to fragmentation functions. In general, the kt-factorization approach gives a much better description of the pt∼ 1–4 GeV region than the leading order collinear approach both at mid- and forward-rapidity regions. Our approach leads to asymmetry in the production of π+ and π−, very similar to that observed very recently by the BRAHMS Collaboration.

The Color Glass Condensate and particle production in the forward rapidity region

Hadron production in the forward rapidity region is considered in the Color Glass Condensate approach to high energy deuteron-nucleus collisions. The projectile deuteron is treated via collinear factorization and pQCD formalism while the nucleus is described as a Color Glass Condensate. It is shown that both DGLAP evolution of the projectile wave function and JIMWLK evolution of the target are essential for a proper understanding the RHIC data in the forward rapidity region.

What can we learn from hydrodynamic analysis of elliptic flow?

We can establish a new picture, the perfect fluid sQGP core and the dissipative hadronic corona, of the space-time evolution of produced matter in relativistic heavy ion collisions at RHIC. It is also shown that the picture works well also in the forward rapidity region through an analysis based on a new class of the hydro-kinetic model and is a manifestation of deconfinement.

What can we learn from hydrodynamic analysis at RHIC?

We can establish a new picture, the perfect fluid sQGP core and the dissipative hadronic corona, of the space-time evolution of produced matter in relativistic heavy-ion collisions at RHIC. It is also shown that the picture works well also in the forward rapidity region through an analysis based on a new class of the hydro-kinetic model and that this is a manifestation of the rapid increase of the entropy density in the vicinity of QCD critical temperature, namely, deconfinement.

The color glass condensate and hadron production in the forward region

We consider one loop corrections to single inclusive particle production in parton–nucleus scattering at high energies, treating the target nucleus as a color glass condensate. We prove by explicit computation that in the leading log Q 2 approximation, these corrections lead to collinear factorization and DGLAP evolution of the projectile parton distribution and hadron fragmentation functions. In single-inclusive cross sections, only two-point functions of Wilson lines in the adjoint and fundamental representations (Mueller's dipoles) arise, which can be obtained from the solution of the JIMWLK equations. The application of our results to forward-rapidity production shows that, in general, recoil effects are large. Hence, the forward rapidity region at RHIC is rather different from the central region at LHC, despite comparable gluon densities in the target. We show that both the quantum x-evolution of the high-density target as well as the DGLAP Q 2 -evolution of the parton distribution and fragmentation functions are clearly seen in the BRAHMS data. This provides additional strong evidence for the color glass condensate at RHIC.

Multiplicity and transverse energy of produced gluon in relativistic heavy ion collision

We present a simple gluon production picture which is based on the McLerran-Venugopalan model and gluon BFKL evolution in relativistic heavy ion collision. Results for the multiplicity and transverse energy distribution in both the central and forward rapidity regions for gluon production in early stages of heavy ion collisions at the LHC are given. Finally, we provide a general qualitative discussion of the consequences of the forward rapidity behavior of produced gluons.

HEAVY-QUARK PRODUCTION IN PROTON-NUCLEUS COLLISIONS AT THE LHC

One may expect a large rate of events, in hadron-nucleus collisions at the LHC, with several pairs of heavy-quarks produced contemporarily by multiple parton interactions. We estimate the inclusive cross sections of [Formula: see text], [Formula: see text] and [Formula: see text] production in p–A collisions in the central and in the forward rapidity regions.

Saturation effects in $pA \rightarrow$ dilepton and photon production

I discuss the inclusive dilepton/photon cross section for proton (quark)-nucleus collisions at high energies in the very forward rapidity region, predicting leading twist shadowing together with anomalous scaling behavior.

Production of forward rapidity photons in high energy heavy ion collisions

We consider production of prompt photons in high energy gold–gold and deuteron–gold collisions in the forward rapidity region of RHIC ( y ∼ 3.8 ). In this kinematics, the projectile partons typically have large x b j while the target partons are mostly at very small x b j , so that the primary partonic collisions involve valence quarks from the projectile and gluons from the target. We take the target nucleus to be a color glass condensate while the projectile deuteron or nucleus is treated as a dilute system of partons. We show that the photon production cross section can be written as a convolution of a quark–nucleus scattering cross section, involving a quark–antiquark dipole, with the leading order quark–photon fragmentation function. We consider different models of the quark–antiquark dipole and show that measurement of photons in the forward rapidity region at RHIC can distinguish between different parameterizations of the dipole cross section as well as help clarify the role of parton coalescence models in hadron production at RHIC.

Collective behavior in nuclear interactions and shower development

The mechanism of hadronic interactions at very high energies is still unclear. Available accelerator data constrain weakly the forward rapidity region which determines the development of atmospheric showers. This ignorance is one of the main sources of uncertainty in the determination of the energy and composition of the primary in hadron-induced atmospheric showers. In this paper we examine the effect on the shower development of two kinds of collective effects in high-energy hadronic interactions which modify the production of secondary particles. The first mechanism, modeled as string fusion, affects strongly the central rapidity region but only slightly the forward region and is shown to have very little effect on the shower development. The second mechanism implies a very strong stopping; it affects modestly the profile of shower maximum but broadens considerably the number distribution of muons at ground. For the latter mechanism, the development of air showers is faster mimicking a heavier projectile. On the other hand, the number of muons at ground is lowered, resembling a shower generated by a lighter primary.

Forward rapidity hadron production in deuteron–gold collisions from valence quarks

We consider hadron production in deuteron–gold collisions at RHIC in the forward rapidity region. Treating the target nucleus as a color glass condensate and the projectile deuteron as a dilute system of valence quarks, we obtain good agreement with the BRAHMS minimum bias data on charged hadron production in the forward rapidity ( y = 3.2 ) and low p t region. We provide predictions for neutral pion production in minimum bias deuteron–gold collisions in the forward rapidity region, y = 3.8 , measured by the STAR Collaboration at RHIC.

Inside-out tracking at CDF

The Central Outer Tracker (COT) at the Collider Detector at Fermilab (CDF) is used to reconstruct charged particles in the central rapidity region. The silicon tracker is used to improve the central track measurement and to reconstruct charged particles in the forward rapidity region. We describe the inside-out tracking algorithm, which improves the track measurement in the intermediate rapidity region by attaching COT hits to reconstructed silicon tracks.

Hydrodynamic afterburner for the color glass condensate and the parton energy loss

We take hydrodynamic initial conditions in relativistic heavy ion collisions from the Color Glass Condensate (CGC) picture through the k T factorization formula. Gluon distributions produced from the CGC are found to provide good initial conditions for the hydrodynamic simulations in Au + Au collisions at Relativistic Heavy Ion Collider (RHIC) energies. We reproduce the centrality, rapidity, and energy dependences of multiplicity within this approach. We also investigate the energy loss of high p T partons in the dense thermalized medium created from colliding two CGCs. We find that our results on the centrality dependence of nuclear modification factors for pions and back-to-back correlation for charged hadrons at midrapidity are consistent with the RHIC data up to semicentral events. Whereas, our approach in which jets are calculated from perturbative QCD 2 → 2 processes predicts less suppression at forward rapidity region compared to the BRAHMS data in Au + Au collisions at RHIC.

Limiting fragmentation from the color glass condensate

We show how the limiting fragmentation phenomenon can arise from the Color Glass Condensate model of high energy QCD. We consider the very forward rapidity region in relativistic heavy ion collisions and argue that in this region, nucleus-nucleus collisions are similar to proton-nucleus collisions (up to shadowing corrections). We then use the known results for proton-nucleus cross sections to show that it leads to the phenomenon of limiting fragmentation in the very forward region of heavy ion collisions as observed at RHIC.