Double Longitudinal Spin Asymmetry Research Articles

Recent results from the STAR spin program at RHIC

The STAR experiment uses polarized p+p collisions at RHIC to determine the contributions to the spin of the proton from gluon spin and from orbital angular momentum of the quarks and gluons. Selective STAR measurements of the longitudinal double spin asymmetry for inclusive jet and inclusive hadron production are presented here. In addition, we report measurements of the transverse spin asymmetry for di-jet production at mid-rapidity and the transverse single-spin asymmetry for forward π° production.

Longitudinal Double-Spin Asymmetry and Cross Section for Inclusive Jet Production in Polarized Proton Collisions ats=200 GeV

We report a measurement of the longitudinal double-spin asymmetry ALL and the differential cross section for inclusive midrapidity jet production in polarized proton collisions at s=200 GeV. The cross section data cover transverse momenta 5<pT<50 GeV/c and agree with next-to-leading order perturbative QCD evaluations. The ALL data cover 5<pT<17 GeV/c and disfavor at 98% C.L. maximal positive gluon polarization in the polarized nucleon.Received 10 August 2006DOI:https://doi.org/10.1103/PhysRevLett.97.252001©2006 American Physical Society

Transverse momentum dependence of the quark helicity distributions and the Cahn effect in double-spin asymmetryALLin semiinclusive DIS

Within the LO QCD parton model of Semi-Inclusive Deep Inelastic Scattering, $\ensuremath{\ell}N\ensuremath{\rightarrow}\ensuremath{\ell}hX$, with unintegrated quark distribution and fragmentation functions, we study the ${\mathbit{P}}_{hT}$ dependence of the double longitudinal-spin asymmetry ${A}_{LL}$. We include $1/Q$ kinematic corrections, which induce an azimuthal modulation of the asymmetry, analogous to the Cahn effect in unpolarized SIDIS. We show that a study of ${A}_{LL}$ and of the weighted DSA ${A}_{LL}^{\mathrm{cos}{\ensuremath{\phi}}_{h}}$ allows to extract the transverse momentum dependence of the unintegrated helicity distribution function ${g}_{1L}^{q}(x,{k}_{\ensuremath{\perp}})$ [or $\ensuremath{\Delta}q(x,{k}_{\ensuremath{\perp}})]$. Predictions, based on some models for the unknown functions, are given for ongoing COMPASS, HERMES and JLab experiments.

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.

PHENIX spin program, recent results

Acceleration of polarized protons in Relativistic Heavy Ion Collider (RHIC) provides unique tool to study the spin structure of the nucleon. We give a brief overview of the PHENIX program to investigate poorly known gluon and flavor decomposed see quark polarization in the proton, utilizing polarized proton collisions at RHIC. We report PHENIX first results on transverse single-spin asymmetry inπ 0 and charged hadron production and longitudinal double-spin asymmetry inπ 0 production at mid-rapidity.

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.

Nuclear polarization of molecular hydrogen recombined on a non-metallic surface

The nuclear polarization of $\mathrm{H}_2$ molecules formed by recombination of nuclear polarized H atoms on the surface of a storage cell initially coated with a silicon-based polymer has been measured by using the longitudinal double-spin asymmetry in deep-inelastic positron-proton scattering. The molecules are found to have a substantial nuclear polarization, which is evidence that initially polarized atoms retain their nuclear polarization when absorbed on this type of surface

QCD hard scattering and the sign of the spin asymmetry A(pi)LL.

Recent preliminary PHENIX data are consistent with a negative and sizable longitudinal double-spin asymmetry A(pi)(LL) for pi(0) production at moderate transverse momentum p( perpendicular ) approximately 1-4 GeV and central rapidity. By means of a systematic investigation of the relevant degrees of freedom, we show that the perturbative QCD framework at leading power in p( perpendicular ) produces at best a very small negative asymmetry in this kinematic range.

NNLO corrections to massive lepton-pair production in longitudinally polarized proton–proton collisions

We present the full next-to-next-to-leading order (NNLO) coefficient functions for the polarized cross section dΔ σ/ dQ for the Drell–Yan process p+ p→ l + l −+′ X′. Here ′ X′ denotes any inclusive hadronic state and Q represents the invariant mass of the lepton pair. All QCD partonic subprocesses have been included provided the lepton pair is created by a virtual photon, which is a valid approximation for Q<50 GeV. Unlike the differential distribution w.r.t. transverse momentum the dominant subprocess for the integrated cross section is given by q+ q ̄ →γ ∗+′X′ and its higher order corrections so that massive lepton pair production provides us with an excellent tool to measure the polarized anti-quark densities. Our calculations are carried out using the method of n-dimensional regularization by making a special choice for the γ 5 matrix. We give predictions for double longitudinal spin asymmetry measurements at the RHIC.

Measurement of inclusive spin structure functions of the deuteron

We report the results of a new measurement of spin structure functions of the deuteron in the region of moderate momentum transfer ($Q^2$ = 0.27 -- 1.3 (GeV/c)$^2$) and final hadronic state mass in the nucleon resonance region ($W$ = 1.08 -- 2.0 GeV). We scattered a 2.5 GeV polarized continuous electron beam at Jefferson Lab off a dynamically polarized cryogenic solid state target ($^{15}$ND$_3$) and detected the scattered electrons with the CEBAF Large Acceptance Spectrometer (CLAS). From our data, we extract the longitudinal double spin asymmetry $A_{||}$ and the spin structure function $g_1^d$. Our data are generally in reasonable agreement with existing data from SLAC where they overlap, and they represent a substantial improvement in statistical precision. We compare our results with expectations for resonance asymmetries and extrapolated deep inelastic scaling results. Finally, we evaluate the first moment of the structure function $g_1^d$ and study its approach to both the deep inelastic limit at large $Q^2$ and to the Gerasimov-Drell-Hearn sum rule at the real photon limit ($Q^2 \to 0$). We find that the first moment varies rapidly in the $Q^2$ range of our experiment and crosses zero at $Q^2$ between 0.5 and 0.8 (GeV/c)$^2$, indicating the importance of the $\Delta$ resonance at these momentum transfers.

Spin-independent and double-spin cosφ asymmetries in semi-inclusive pion electroproduction

We consider the cosφ dependence of the longitudinal double-spin asymmetry for charged pion electroproduction in semi-inclusive deep inelastic scattering, emphasizing intrinsic transverse momentum effects. This azimuthal asymmetry allows to measure the cosφ moments of the unpolarized and double-spin cross-section, simultaneously. The size of the asymmetry, in the approximation where all twist-3 interaction-dependent distribution and fragmentation functions are set to zero, is estimated for HERMES kinematics; both the spin-independent and the double-spin cosφ moments are predicted to be sizable and negative.

Momentum-space Faddeev calculations for confining potentials

We report the results of a new measurement of spin structure functions of the deuteron in the region of moderate momentum transfer [Q(2)=0.27-1.3 (GeV/c)(2)] and final hadronic state mass in the nucleon resonance region (W=1.08-2.0 GeV). We scattered a 2.5 GeV polarized continuous electron beam at Jefferson Lab off a dynamically polarized cryogenic solid state target ((ND3)-N-15) and detected the scattered electrons with the CEBAF large acceptance spectrometer. From our data, we extract the longitudinal double spin asymmetry A(parallel toparallel to) and the spin structure function g(1)(d). Our data are generally in reasonable agreement with existing data from SLAC where they overlap, and they represent a substantial improvement in statistical precision. We compare our results with expectations for resonance asymmetries and extrapolated deep inelastic scaling results. Finally, we evaluate the first moment of the structure function g(1)(d) and study its approach to both the deep inelastic limit at large Q(2) and to the Gerasimov-Drell-Hearn sum rule at the real photon limit (Q(2)-->0). We find that the first moment varies rapidly in the Q(2) range of our experiment and crosses zero at Q(2) between 0.5 and 0.8 (GeV/c)(2), indicating the importance of the Delta resonance at these momentum transfers.

NLO differential distributions of massive lepton-pair production in longitudinally polarized proton–proton collisions

We present the full next-to-leading order (NLO) corrected inclusive cross section d 3 Δσ/ dQ 2/ dy/ dp T for massive lepton pair production in longitudinally polarized proton–proton collisions p+p→l +l −+ ′X′ . Here ′X′ denotes any inclusive hadronic state and Q represents the invariant mass of the lepton pair which has transverse momentum p T and rapidity y. All QCD partonic subprocesses have been included provided the lepton pair is created by a virtual photon, which is a valid approximation for Q<50 GeV. Like in unpolarized proton–proton scattering the dominant subprocess is given by q( q ̄ )+g→γ ∗+ ′X′ so that massive lepton pair production provides us with an excellent method to measure the spin density of the gluon. Our calculations are carried out using the method of n-dimensional regularization by making a special choice for the γ 5-matrix. Like in the case of many other prescriptions evanescent counter terms appear. They are determined by computing the NLO coefficient functions for dΔσ/ dQ 2 and the polarized cross section for Higgs production using both n-dimensional regularization and a four-dimensional regularization technique in which the γ 5-matrix is uniquely defined. Our calculations reveal that the non-singlet polarized coefficient function equals the unpolarized one up to a minus sign. We give predictions for double longitudinal spin asymmetry measurements at the RHIC.

Double-spin asymmetry in the cross section for exclusive ρ0 production in lepton–proton scattering

Evidence for a positive longitudinal double-spin asymmetry <A_1^rho> = 0.24 +-0.11 (stat) +-0.02 (syst) in the cross section for exclusive diffractive rho^0(770) vector meson production in polarised lepton-proton scattering was observed by the HERMES experiment. The longitudinally polarised 27.56 GeV HERA positron beam was scattered off a longitudinally polarised pure hydrogen gas target. The average invariant mass of the photon-proton system has a value of <W> = 4.9 GeV, while the average negative squared four-momentum of the virtual photon is <Q^2> = 1.7 GeV^2. The ratio of the present result to the corresponding spin asymmetry in inclusive deep-inelastic scattering is in agreement with an early theoretical prediction based on the generalised vector meson dominance model.

Polarized gluon distribution function of nucleon in diffractive lepto-production of charmonium

We investigate the longitudinal double spin asymmetry in the diffractive lepto-production of charmonium off the proton, and study its relation to the polarized gluon distribution Δ G of the proton. It is found that the transverse momenta of the exchanged gluons and Fermi motion of charm quarks give a finite spin asymmetry. Our calculation for the photo-production indicates the asymmetry is small and of O(10 −3) ∼ O(10 −2) at HERA collider energy. On the other hand, at fixed-target energy (HERMES), the asymmetry is of O(10 −2) for J/ψ production. The asymmetry for the ψ′ is even large, of O(10 −1), due to the large Fermi motion of c c in ψ′.