Longitudinal double-spin asymmetry for inclusive jet production in p[over -->] + p[over -->] collisions at sqrt[s]=200 GeV

We report a new STAR measurement of the longitudinal double-spin asymmetry A(LL) for inclusive jet production at midrapidity in polarized p + p collisions at a center-of-mass energy of sqrt[s]=200 GeV. The data, which cover jet transverse momenta 5相似文献   

Measurement of the spin asymmetry in the photoproduction of pairs of high- p(T) hadrons at HERMES

We present a measurement of the longitudinal spin asymmetry A(||) in photoproduction of pairs of hadrons with high transverse momentum p(T). Data were accumulated by the HERMES experiment using a 27.5 GeV polarized positron beam and a polarized hydrogen target internal to the HERA storage ring. For h(+)h(-) pairs with p(h(1))(T)>1.5 GeV/c and p(h(2))(T)>1.0 GeV/c, the measured asymmetry is A(||) = -0. 28+/-0.12(stat)+/-0.02(syst). This negative value is in contrast to the positive asymmetries typically measured in deep inelastic scattering from protons, and is interpreted to arise from a positive gluon polarization.     

\Lambda, \bar{\Lambda} polarization and spin transfer in lepton deep-inelastic scattering

The flavor and helicity distributions of the and hyperons for both valence and sea quarks are calculated in a perturbative QCD (pQCD) based model. We relate these quark distributions to the fragmentation functions of and , and calculate the z-dependence of the longitudinal spin transfer to and in lepton deep-inelastic scattering (DIS). It is shown that the spin transfer to the is compatible with the first HERMES results at DESY, and further tests are suggested. We also make predictions for the z-dependence of the and longitudinal polarizations in neutrino (antineutrino) DIS processes. We investigate the sea contribution to the fragmentation functions, and we test a possible scenario where the sea quarks in (or the sea antiquarks in ) are negatively polarized, whereas the sea antiquarks in the (or the sea quarks in ) are positively polarized. The asymmetry of the polarized fragmentation functions of the sea quarks and antiquarks to and provides a way to understand the different behavior between the and spin transfers observed in the recent E665 experiment at FNAL. Received: 5 April 2000 / Published online: 26 July 2000     

Measurement of direct photon production in p+p collisions at sqrt[s] = 200 GeV

Cross sections for midrapidity production of direct photons in p+p collisions at the Relativistic Heavy Ion Collider (RHIC) are reported for transverse momenta of 3 < pT < 16 GeV/c. Next-to-leading order perturbative QCD (pQCD) describes the data well for pT >5 GeV/c, where the uncertainties of the measurement and theory are comparable. We also report on the effect of requiring the photons to be isolated from parton jet energy. The observed fraction of isolated photons is well described by pQCD for pT >7 GeV/c.     

First measurement of the double spin asymmetry in (-->)e(-->)p-->e(prime)pi(+)n in the resonance region

The double spin asymmetry in the (-->)e(-->)p --> e(prime)pi(+)n reaction has been measured for the first time in the resonance region for four-momentum transfer Q2 = 0.35-1.5 GeV(2). Data were taken at Jefferson Lab with the CLAS detector using a 2.6 GeV polarized electron beam incident on a polarized solid NH3 target. Comparison with predictions of phenomenological models shows strong sensitivity to resonance contributions. Helicity-1/2 transitions are found to be dominant in the second and third resonance regions. The measured asymmetry is consistent with a faster rise with Q(2) of the helicity asymmetry A1 for the F(15)(1680) resonance than expected from the analysis of the unpolarized data.     

Longitudinal double-spin asymmetry and cross section for inclusive jet production in polarized proton collisions at square root of s = 200 GeV

We report a measurement of the longitudinal double-spin asymmetry A(LL) and the differential cross section for inclusive midrapidity jet production in polarized proton collisions at square root of 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 A(LL) data cover 5 < pT < 17 GeV/c and disfavor at 98% C.L. maximal positive gluon polarization in the polarized nucleon.     

Regge description of spin-spin asymmetry in photon diffractive dissociation

We explore the possibility to extract Δ(G(x) from a comparison of experimental data on the longitudinal spin-spin asymmetry A _LL in diffractive scattering of the heavy photon on the proton with calculations performed in perturbative QCD (pQCD). The data could be obtained at the HERA collider in scattering of polarized leptons off polarized protons. We have found a kinematic region where contributions to A _LL from soft processes are suppressed to guarantee an applicability of pQCD, hence the pQCD prediction can be reliably compared with data in this kinematic region.     

Measurement of single- and double-spin asymmetries in deep inelastic pion electroproduction with a longitudinally polarized target

We report the first measurement of the transverse momentum dependence of double-spin asymmetries in semi-inclusive production of pions in deep-inelastic scattering off the longitudinally polarized proton. Data have been obtained using a polarized electron beam of 5.7 GeV with the CLAS detector at the Jefferson Lab (JLab). Modulations of single spin asymmetries over the azimuthal angle between lepton scattering and hadron production planes ? have been measured over a wide kinematic range in Bjorken x and virtual photon squared four-momentum Q2. A significant nonzero sin2? single spin asymmetry was observed for the first time indicating strong spin-orbit correlations for transversely polarized quarks in the longitudinally polarized proton.     

On the T-old quark fragmentation function and on transversal handedness

The first probe of the correlation of the T-odd one-particle fragmentation function responsible for the left-right asymmetry of fragmentation of a transversely polarized quark is done by using the 1991–95 DELPHI data for Z→2-jet decay. Integrated over the fraction of longitudinal and transversal momenta, this correlation is of 1.5% order, which means order of 13% for the analyzing power.A rather large (≈ 10%) handedness transversal to the production plane was observed in the diffractive production of (π^?+π⁺π^?) triples from nuclei by the 40 GeV/c π^?-beam. It was shown that the phenomenon has a clear dynamic origin and resembles the single spin asymmetry behavior. All this makes us hope to use this effects in polarized DIS experiments for transversity measurement.     

RHIC polarized proton status and plan

The Relativistic Heavy Ion Collider (RHIC) as the first high energy polarized proton collider has been providing collisions at a beam energy of 100 GeV since 2001. Equipped with two full Siberian snakes in each ring, polarization is preserved during the acceleration from injection to 100 GeV with careful control of the betatron tunes and the vertical orbit distortions. In the latest RHIC polarized proton run in 2006, a peak luminosity of 28 × 10³⁰cm⁻² s⁻¹ with 60% average polarization at store was achieved. During the run, RHIC also demonstrated its capability in providing a combination of polarized proton collisions with longitudinal polarization and radial polarization were provided to the STAR experiment and PHENIX experiment with the local spin rotators installed on either side of the STAR detector and PHENIX detector. Polarized protons were also first accelerated to 250 GeV at the end of RHIC 2006 run with a 46% polarization measured at this new store energy in one of the RHIC accelerators. Currently, the luminosity in RHIC is limited by the beam-beam effect. The plan is to triple the luminosity. Plans to achieve polarized proton collision at 250 GeV are also reported.     

Scaling of single photon production in hadronic collisions

Scaling of single photon production in pp and collisions is studied. It is empirically observed that the available data scales for and for larger . The NLO pQCD predictions for pp collisions at an of 200 and 5500 GeV, relevant for RHIC and LHC energies are seen to closely follow this scaling behavior. Implications for single photon production in heavy ion collisions are discussed. Received: 2 February 2001 / Revised version: 28 May 2001 / Published online: 5 October 2001     

QCD analysis of DIS and SIDIS data with two alternative methods

The global fit analysis of all published data on DIS and semi-inclusive DIS (SIDIS) asymmetries is performed in the next to leading (NLO) QCD order. The respective parameterization on polarized PDFs is constructed. The especial attention is paid to the impact of novel SIDIS data on the polarized distributions of light sea and strange quarks as well as on the polarized gluon distributions. The first moments of these distributions entering the nucleon spin are found to be surprisingly small quantities. The alternative direct (free of any fitting procedure) method of NLO QCD analysis is elaborated. Method is especially important for analysis of SIDIS data because it allows to avoid the problems arising in the conventional fitting procedure: functional arbitrariness at initial scale and ambiguities in the error band calculation. Within the alternative method the central values and uncertainties of the measured asymmetries directly propagate to the central values and uncertainties of the polarized PDFs we are interested in. The method is applied to all existing SIDIS data on pion production for an estimation in NLO QCD of the valence and sea quark contributions to the proton spin. As a result one arrives at the conclusion that, contrary to the valence contributions, the sea contributions to the proton spin are compatible with zero within the errors.     

Polarization and asymmetry of protons in an annihilation process e− + e+ → p + ¯p in the presence of weak neutral currents

The differential and total cross sections for annihilation of longitudinally polarized and transversely polarized electrons and positrons into pairs of longitudinally polarized protons and antiprotons are calculated in the center-of-mass system. The expressions obtained for the cross sections have electromagnetic, weak, and interference terms. It is shown that the existence of a weak current-current interaction (e) (¯pp) must lead for protons and antiprotons, emitted at an angle ®=90° to a nonzero degree of longitudinal polarization P(E) =1.3 and 36% for E=2.5 and 10 GeV and G=G_F=10^–5 M^–2,to the presence of forward-backward asymmetry in the angular distribution of the protons A(E)=1 and 12% for E=2.5 and 10 GeV (the asymmetry exists even when all the particles are unpolarized), and to other effects which are absent in the case of a pure electromagnetic interaction. It is shown that in experiments with polarized particles the contributions of different form factors can be distinguished and studied separately.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 93–100, January, 1974.We thank the participants in the seminars of Professor A. A. Sokolov and Professor Ya. P. Terletskii for discussions of the work.     

How important are next-to-leading order models in predicting strange particle spectra in p+p collisions at STAR?

STAR has measured a variety of strange particle species in p+p collisions at  =200 GeV. These high statistics data are ideal for comparing to existing leading- and next-to-leading order perturbative QCD (pQCD) models. Next-to-leading (NLO) models have been successful in describing inclusive hadron production using parameterized fragmentation functions (FF) for quarks and gluons. However, in order to describe identified strange particle spectra at NLO, knowledge of flavor separated FF is essential. Such FF have recently been parameterized using data by the OPAL experiment and allow for the first time to perform NLO calculation for strange baryons. In fact, comparing the STAR Λ data with these calculations allow to put a constraint on the gluon fragmentation function. We show that the leading-order (LO) event generator PYTHIA has to be tuned significantly to reproduce the STAR identified strange particle data. In particular, it fails to describe the observed enhancement of baryon-to-meson ratio at intermediate p_T (2–6 GeV/c). In heavy-ion (HI) collisions this observable has been extensively compared with models and shows a strong dependency on collision centrality or parton density. In the HI context the observed enhancement has been explained by recent approaches in terms of parton coalescense and recombination models. PACS 25.75.Dw; 25.40.Ep; 24.10.Lx     

B→πη('), η(')η(') decays and NLO contributions in the pQCD approach

By employing the perturbative QCD (pQCD) factorization approach, we calculate the full leading and the partial next-to-leading order (NLO) contributions to the seven B→πη(') and η(')η(') decays. For B+B→π+η(') decays, the pQCD predictions for their decay rates agree very well with the data after the inclusion of the small NLO contributions. For neutral decays, the pQCD predictions are also consistent with the experimental upper limits and can be tested by the LHC experiments. The measured value of ACPdir(π±η)=-19±7% can also be accommodated by the pQCD approach.     

Spin dependence of the inclusive reaction p + p (polarized) → π0 + X at 24 GeV/c for high-pTπ0 produced in the central region

The spin dependence of π⁰ inclusive production by 24 GeV/c protons has been measured using a polarized target for Fhe Feynman x near 0 in the transverse momentum range 1.0 < p_T < 2.5 GeV/c. The results indicate a negative updown asymmetry growing strongly with p_T and greater than 50% in absolute value for p_T greater than 2 GeV/c.     

Direct photon measurement at RHIC-PHENIX

Results on direct photon measurements from the PHENIX experiment at RHIC are presented. The direct photon yields for P _T >6GeV/c as a function of centrality in Au+Au collisions at $\sqrt {s_{NN} } = 200$ GeV are found to be consistent with NLO pQCD calculation scaled by the number of binary collisions. The results suggest that the photons observed are emitted from the initial stage of hard scattering. Comparisons with several theoretical calculations are also presented.     

Spin-density matrix elements for γp→K*0Σ+ at Eγ=1.85-3.0 GeV with evidence for the κ(800) meson exchange

The exclusive reaction γp→K(+)π(-)Σ(+) was measured for the first time using linearly polarized photons at beam energies from 1.85 to 2.96 GeV. Angular distributions in the rest frame of the K(+)π(-) system were fitted to extract spin-density matrix elements of the K(*0) decay. The measured parity spin asymmetry shows that natural-parity exchange is dominant in this reaction. This result clearly indicates the need for t-channel exchange of the κ(800) scalar meson.