Polarized Gluon Distribution Research Articles

A new approach to calculate the gluon polarization

We derive the Leading-Order master equation to extract the polarized gluon distribution G(x;Q^2) = x \deltag(x;Q^2) from polarized proton structure function, g1p(x;Q^2). By using a Laplace-transform technique, we solve the master equation and derive the polarized gluon distribution inside the proton. The test of accuracy which are based on our calculations with two different methods confirms that we achieve to the correct solution for the polarized gluon distribution. We show that accurate experimental knowledge of g1p(x;Q^2) in a region of Bjorken x and Q^2, is all that is needed to determine the polarized gluon distribution in that region. Therefore, to determine the gluon polarization \deltag /g,we only need to have accurate experimental data on un-polarized and polarized structure functions (F2p (x;Q^2) and g1p(x;Q^2)).

Gluon polarization in nucleon

In the context of the so-called valon model, we calculate δgg and show that although it is small and compatible with the measured values, the gluon contribution to the spin of nucleon can be sizable. The smallness of δgg in the measured kinematical region should not be interpreted as δg being small. In fact, δg itself at small x, and the first moment of the polarized gluon distribution in the nucleon, Δg(Q2), are large.

Recent STAR results in high-energy polarized proton-proton collisions at RHIC

The STAR experiment at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory is carrying out a spin physics program in high-energy polarized p⃗ + p⃗ collisions at = 200 – 500 GeV to gain a deeper insight into the spin structure and dynamics of the proton.One of the main objectives of the spin physics program at RHIC is the extraction of the polarized gluon distribution function based on measurements of gluon initiated processes, such as hadron and jet production. The STAR detector is well suited for the reconstruction of various final states involving jets, π0, π±, e± and γ, which allows to measure several different processes. Recent results will be shown on the measurement of jet production and hadron production at = 200 GeV.The RHIC spin physics program has recently completed the first data taking period in 2009 of polarized p⃗ + p⃗ collisions at = 500 GeV. This opens a new era in the study of the spin-flavor structure of the proton based on the production of W−(+) bosons. Recent STAR results on the first measurement of W boson production in polarized p⃗ + p⃗ collisions will be shown.

Recent Longitudinal Spin Asymmetry Measurements for Inclusive Jet Production at STAR

We present measurements of the longitudinal spin asymmetry, A LL , for the inclusive jet signal at STAR. The data presented here are mid-rapidity jets in the transverse momentum range of 5 < p T < 35 GeV/c and come from polarized proton-proton collisions at center of mass energies of s = 200 GeV . We compare our measured A LL values to predictions derived from various parameterizations of the polarized gluon distribution function. The results are shown to provide significant constraints for allowable gluon parameterizations within the measurement's kinematic Bjorken-x range of 0.03 < x < 0.3.

Next-to-leading order QCD corrections tohadron+jetproduction inppcollisions at RHIC

We compute the next-to-leading order (NLO) QCD corrections to the spin-independent and spin-dependent cross sections for the production of a single-hadron accompanied by an opposite jet in hadronic collisions. This process is being studied experimentally at RHIC, providing a new tool to unveil the polarized gluon distribution {delta}g. We perform a detailed analysis of the phenomenological impact of the observable at NLO accuracy and show that the preliminary data by the STAR collaboration confirms the idea of a small gluon polarization in the 0.05 < or approx. x < or approx. 0.3 range.

Possibility of determining gluon polarization via polarized top pairs in gamma-proton scattering

We study the possibility for directly measuring the polarized gluon distribution in the process γp→t¯t. It is shown that polarization asymmetry of the final top quarks is proportional to the gluon polarization. With available energy and luminosity, the collision of a polarized proton beam and a Compton backscattered photon beam can create polarized top quarks which carry the spin information of the process. Energy dependence and angular distributions of the polarization asymmetry of the top pairs has been discussed including statistical uncertainty.

Determination of gluon polarization from deep inelastic scattering and collider data

We investigate impact of π 0 -production data at Relativistic Heavy Ion Collider (RHIC) and future E07-011 experiment for the structure function g 1 of the deuteron at the Thomas Jefferson National Accelerator Facility (JLab) on studies of nucleonic spin structure, especially on the polarized gluon distribution function. By global analyses of polarized lepton–nucleon scattering and the π 0 -production data, polarized parton distribution functions are determined and their uncertainties are estimated by the Hessian method. Two types of the gluon distribution function are investigated. One is a positive distribution and the other is a node-type distribution which changes sign at x ∼ 0.1 . Although the RHIC π 0 data seem to favor the node type for Δ g ( x ) , it is difficult to determine a precise functional form from the current data. However, it is interesting to find that the gluon distribution Δ g ( x ) is positive at large x (>0.2) due to constraints from the scaling violation in g 1 and RHIC π 0 data. The JLab-E07-011 measurements for g 1 d should be also able to reduce the gluon uncertainty, and the reduction is comparable to the one by RUN-5 π 0 -production data at RHIC. The reduction is caused mainly by the error correlation between polarized antiquark and gluon distributions and by a next-to-leading-order (NLO) gluonic effect in the structure function g 1 d . We find that the JLab-E07-011 data are accurate enough to probe the NLO gluonic term in g 1 . Both RHIC and JLab data contribute to better determination of the polarized gluon distribution in addition to improvement on polarized quark and antiquark distributions.

QCD analysis of experimental polarized deep-inelastic-scattering data

A detailed review of the current state of investigations into polarized deep inelastic scattering (DIS) is presented. Special attention is given to the methods of the QCD analysis of experimental data on these processes and to the methods of extrapolation of polarized structure functions and polarized quark distributions in the regions inaccessible to current experiments. In the case of pure inclusive processes, the QCD analysis of all worldwide data, including the latest COMPASS data, is presented in detail. Special attention is given to such important components of the nucleon-spin problem as the polarized strangeness and polarized gluon distribution. The features of SIDIS processes are considered; in particular, the role of fragmentation functions in the analysis of the semi-inclusive data is discussed. The methods of extracting the fragmentation functions from experimental data are considered in detail, and the corresponding results are presented. The results of analysis of the existing semi-inclusive polarized data both in the QCD leading order and in the next-to-leading order are considered. Special attention is given to non-standard, so-called difference asymmetries, which make it possible to minimize the dependence of results of analysis on the fragmentation functions. The current methods of QCD analysis of semi-inclusive polarized data are critically reviewed. An alternative method of QCD analysis of semi-inclusive data is presented for next-to-leading order QCD. Advantages of the method in practical applications are illustrated by the example of analysis of the HERMES data.

The quest for spinning glue in high-energy polarized proton-proton collisions at RHIC

The STAR experiment at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) is carrying out a spin physics program colliding transverse or longitudinal polarized proton beams at s√ = 200 − 500 GeV to gain a deeper insight into the spin structure and dynamics of the proton. These studies provide fundamental tests of Quantum Chromodynamics (QCD).One of the main objectives of the STAR spin physics program is the determination of the polarized gluon distribution function through a measurement of the longitudinal double-spin asymmetry, ALL, for various processes. Recent results will be shown on the measurement of ALL for inclusive jet production, neutral pion production and charged pion production at s√ = 200 GeV.

Probing proton spin structure via heavy flavor production in PHENIX

The measurement of spin asymmetries in polarized p+p collisions provides an opportunity to probe the spin structure of nucleons. With transversely and longitudinally polarized proton beams at RHIC, we can measure both double longitudinal spin asymmetry ALL and single transverse spin asymmetry AN. At the RHIC energy, heavy quark (charm and beauty) production is dominated by gluon–gluon interactions, so measurements of ALL and AN will allow us to directly probe the polarized gluon distribution and gluon's Sivers functions, respectively. PHENIX experiment collected 3.5 pb−1 (beam polarization ∼50%) and 10 pb−1 (beam polarization ∼60%) data from year 2005 and 2006 runs, respectively. In this analysis, the J/Ψ have been measured through the J/Ψ → μ+μ− channel at forward rapidities, and about 30 000 J/Ψ candidates are reconstructed from Lvl2 triggered data production. We present the latest results of J/ΨALL and AN measurements at forward rapidities from the PHENIX experiment.

Polarized gluon distribution [formula omitted] in the proton

We study the polarized gluon distribution Δg(x) in a longitudinally polarized proton. We argue that the distribution can be calculated approximately from the quark color currents found in simple quark models. The first result from the MIT bag model as well as the non-relativistic quark model shows that Δg(x) is positive at all x. If this feature holds in QCD, it imposes a strong constraint on phenomenological fits to experimental data. The total gluon helicity ΔG from the bag model is about 0.3ℏ at the scale of 1 GeV, considerably smaller than previous theoretical expectations.

Determination of the gluon contribution to the nucleon spin with COMPASS

New double-spin asymmetry data from the COMPASS experiment at CERN are presented. The data are used to determine the polarized gluon distribution Δ G / G ( x ) , via three different techniques. Next-to-leading order QCD fits to the nucleon polarized structure functions indicate that the first moment of Δ G / G ( x ) is small (either positive or negative) and compatible with zero. This observation is corroborated by three measurements of Δ G / G ( x ) , based on the photon-gluon fusion process.

Improved measurement of double helicity asymmetry in inclulsive midrapidityπ0production for polarizedp+pcollisions ats=200 GeV

We present an improved measurement of the double helicity asymmetry for pi(0) production in polarized proton-proton scattering at root s=200 GeV employing the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The improvements to our previous measurement come from two main factors: Inclusion of a new data set from the 2004 RHIC run with higher beam polarizations than the earlier run and a recalibration of the beam polarization measurements for the earlier run, which resulted in reduced uncertainties and increased beam polarizations. The results are compared to a Next to Leading Order (NLO) perturbative Quantum Chromodynamics (pQCD) calculation with a range of polarized gluon distributions.

Effect of polarized gluon distribution on spin asymmetries for neutral and charged pion production

A longitudinal double spin asymmetry for \pi^0 production has been measured by the PHENIX collaboration. The asymmetry is sensitive to the polarized gluon distribution and is indicated to be positive by theoretical predictions. We study a correlation between behavior of the asymmetry and polarized gluon distribution in neutral and charged pion production at RHIC.

Double Helicity Asymmetry in Inclusive Midrapidityπ0Production for Polarizedp+pCollisions ats=200 GeV

We present a measurement of the double longitudinal spin asymmetry in inclusive pi(0) production in polarized proton-proton collisions at sqrt[s]=200 GeV. The data were taken at the Relativistic Heavy Ion Collider with average beam polarizations of 0.27. The measurements are the first in a program to study the longitudinal spin structure of the proton, using strongly interacting probes, at collider energies. The asymmetry is presented for transverse momenta 1-5 GeV/c at midrapidity, where next-to-leading-order perturbative quantum chromodynamic (NLO pQCD) calculations well describe the unpolarized cross section. The observed asymmetry is small and is compared to a NLO pQCD calculation with a range of polarized gluon distributions.

Uncertainty of polarized gluon distribution from prompt photon production

Constraint of prompt photon data on the polarized gluon distribution is discussed in terms of uncertainty estimation for polarized parton distribution functions (PDFs). By comparing uncertainty of the double spin asymmetry ALLγ with expected statistical errors at RHIC, we found that the gluon distribution is effectively constrained in the region 0.04<xT<0.2 with the data at transverse momentum pT=10–20 GeV for center-of-mass energies s=200 and 500 GeV.

Determination of polarized parton distribution functions and their uncertainties

We investigate the polarized parton distribution functions (PDFs) and their uncertainties by using the world data on the spin asymmetry A_1. The uncertainties of the polarized PDFs are estimated by the Hessian method. The up and down valence-quark distributions are determined well. However, the antiquark distributions have large uncertainties at this stage, and it is particularly difficult to fix the gluon distribution. The \chi^2 analysis produces a positively polarized gluon distribution, but even \Delta g(x)=0 could be allowed according to our uncertainty estimation. In comparison with the previous AAC (Asymmetry Analysis Collaboration) parameterization in 2000, accurate SLAC-E155 proton data are added to the analysis. We find that the E155 data improve the determination of the polarized PDFs, especially the polarized antiquark distributions. In addition, the gluon-distribution uncertainties are reduced due to the correlation with the antiquark distributions. We also show the global analysis results with the condition \Delta g(x)=0 at the initial scale, Q^2=1 GeV^2, for clarifying the error correlation effects with the gluon distribution.

Charmed hadron production in polarized pp reactions as a probe of polarized gluons in the proton

To probe the behavior of polarized gluons in the proton, we propose the charmed hadron, such as Λ c + and D ∗ , production in the forthcoming RHIC experiments. We found that the spin correlation between the target proton and the produced Λ c + baryon might be a good signal for testing models of the polarized gluon distribution in the proton.

CHARMED HADRON PRODUCTION AT RHIC

To extract information about polarized gluon distribution in the proton, charmed hadron, actually [Formula: see text], productions at RHIC experiment are studied. We found that the spin correlation asymmetry between the initial proton and the produced [Formula: see text] is enable us to distinguish parameterization models of polarized gluons.

SEMI-INCLUSIVE HADRON PAIR PRODUCTION AND POLARIZED GLUONS IN THE PROTON

To extract the polarized gluon distribution Δg, we study the large–pTlight hadron pair productions in polarized semi-inclusive reaction, which are dominantly produced via 2 mechanisms: photon–gluon fusion(PGF) and QCD Compton. The PGF gives us a direct information on Δg in the nucleon, whereas QCD Compton is background to the signal process for extracting Δg. By using symmetry relation among fragmentation functions and taking an appropriate combination of light hadron pair production processes, we can remove an effect of QCD Compton from those cross sections. Here we propose a new formula for extracting Δg from those processes.