arXiv subjects

Qing-hua Xu

Publications and source records attributed to Qing-hua Xu.

15 recordsLinked to original sources

arXiv2020

Recent results on cold-QCD from RHIC

Polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) provide unique opportunities to study the spin structure of the nucleon. We will highlight recent results on the nucleon spin structure from the STAR and PHENIX experiments at RHIC: (1) A sizable gluon polarization in the proton is measured with longitudinal double spin asymmetries of jet and hadron production; (2) Longitudinal single spin asymmetries in W boson production improve constraints on the sea quark polarization. The new spin asymmetry results for W boson confirmed the SU(2) flavor asymmetry of the light sea quark polarization in the proton; (3) Transverse spin effects in hadronic systems offer new implications on parton distribution functions in the collinear and transverse momentum dependent frameworks. We will also discuss near term plans for the STAR forward detector upgrade and prospects for proton-proton and proton-ion collisions in the years beyond 2021 at STAR.

nucl-ex↗

arXiv2010

Longitudinal spin transfer of Lambda and anti-Lambda in polarized pp collisions at \sqrt s=200 GeV at STAR

We report our measurement on longitudinal spin transfer, D_LL, from high energy polarized protons to $Λ$ and $\barΛ$ hyperons in proton-proton collisions at $\sqrt{s} = 200 GeV$ with the STAR detector at RHIC. The current measurements cover $Λ$, $\barΛ$ pseudorapidity $|η| < 1.2$ and transverse momenta $p_T$ up to $4GeV/c$ using the data taken in 2005. The longitudinal spin transfer is found to be D_LL= -0.03\pm 0.13 (stat) \pm 0.04(syst)$ for inclusive $Λ$ and $D_{LL} = -0.12 \pm 0.08(stat) \pm 0.03(syst)$ for inclusive $\barΛ$ hyperons with $ = 0.5$ and $ = 3.7 GeV/c$. The prospects with 2009 data and the future measurements are also given.

hep-ex↗

arXiv2010

The longitudinal polarization of hyperons in the forward region in polarized $pp$ collisions

We study the longitudinal polarization of hyperons and anti-hyperons at forward pseudorapidity, $2.5<η<4$, in singly polarized $pp$ collisions at RHIC energies by using different parameterizations of the polarized parton densities and different models for the polarized fragmentation functions. The results show that the $Σ^+$ polarization is able to distinguish different pictures on spin transfer in high energy fragmentation processes; and the polarization of $Λ$ and $\barΛ$ hyperons can provide sensitivity to the helicity distribution of strange sea quarks. The influence from beam remnant to hyperon polarization in the forward region is also discussed.

hep-ph↗

arXiv2009

Longitudinal Spin Transfer to $Λ$ and $\barΛ$ in Polarized Proton-Proton Collisions at $\sqrt{s} = 200 Gev at Star

We report our measurement on longitudinal spin transfer, $D_{LL}$, from high energy polarized protons to $Λ$ and $\barΛ$ hyperons in proton-proton collisions at $\sqrt{s} = 200GeV$ with the STAR detector at RHIC. The measurements cover $Λ$, $\barΛ$ pseudorapidity $|η| < 1.2$ and transverse momenta $p_T$ up to $4GeV/c$. The longitudinal spin transfer is found to be $D_{LL}= -0.03\pm 0.13(stat) \pm 0.04(syst)$ for inclusive $Λ$ and $D_{LL} = -0.12 \pm 0.08(stat) \pm 0.03(syst)$ for inclusive $\barΛ$ hyperons with $<η> = 0.5$ and $ = 3.7GeV/c$. The prospect on transverse spin transfer measurement is also given.

hep-ex↗

arXiv2009

Longitudinal polarization of hyperon and anti-hyperon in semi-inclusive deep-inelastic scattering

We make a detailed study of the longitudinal polarization of hyperons and anti-hyperons in semi-inclusive deep-inelastic lepton-nucleon scattering. We present the numerical results for spin transfer in quark fragmentation processes, analyze the possible origins for a difference between the polarization for hyperon and that for the corresponding anti-hyperon. We present the results obtained in the case that there is no asymmetry between sea and anti-sea distribution in nucleon as well as those obtained when such an asymmetry is taken into account. We compare the results with the available data such as those from COMPASS and make predictions for future experiments including those at even higher energies such as at eRHIC.

hep-ph↗

arXiv2008

Anti-Hyperon polarization in high energy pp collisions with polarized beams

We study the longitudinal polarization of the Sigma_bar and Xi_bar anti-hyperons in polarized high energy pp collisions at large transverse momenta, extending a recent study for the Lambda_bar anti-hyperon. We make predictions by using different parametrizations of the polarized parton densities and models for the polarized fragmentation functions. Similar to the Lambda_bar polarization, the Xi_bar0 and Xi_bar+ polarizations are found to be sensitive to the polarized anti-strange sea in the nucleon. The Sigma_bar- and Sigma_bar+ polarizations show sensitivity to the light sea quark polarizations, Δ\bar u(x) and Δ\bar d(x), and their asymmetry.

hep-ph↗

arXiv2006

Anti-Lambda polarization in high energy pp collisions with polarized beam

We study the polarization of the anti-Lambda particle in polarized high energy pp collisions at large transverse momenta. The anti-Lambda polarization is found to be sensitive to the polarization of the anti-strange sea of the nucleon. We make predictions using different parameterizations of the polarized quark distribution functions. The results show that the measurement of longitudinal anti-Lambda polarization can distinguish different parameterizations, and that similar measurements in the transversely polarized case can give some insights into the transversity distribution of the anti-strange sea of nucleon.

hep-ph↗

arXiv2004

Probing gluon helicity distribution and quark transversity through hyperon polarization in singly polarized pp collisions

We study the polarization of hyperon in different processes in singly polarized $pp$ collisions, in particular its relation to the polarized parton distributions. We show that by measuring hyperon polarization in particularly chosen processes, one can extract useful information on these parton distributions. We show in particular that, by measuring the $Σ^+$ polarization in high $p_T$ direct photon production process, one can extract information on the gluon helicity distribution; and by measuring the transverse polarization of hyeprons with high $p_T$ in singly polarized reactions, one can obtain useful information on the transversity distribution. We present the numerical results obtained for those hyperon polarizations using different models for parton distribution function and those for the spin transfer in fragmentation processes.

hep-ph↗

arXiv2003

Spin alignment of vector mesons in unpolarized hadron-hadron collisions at high energies

We argue that spin alignment of the vector mesons observed in unpolarized hadron-hadron collisions is closely related to the single spin left-right asymmetry observed in transversely polarized hadron-hadron collisions. We present the numerical results obtained from the type of spin-correlation imposed by the existence of the single-spin left-right asymmetries. We compare the results with the available data and make predictions for future experiments.

hep-ph↗

arXiv2003

Spin alignment of the high $p_T$ vector mesons in polarized pp collisions at high energies

Using the same method as that used in $e^+e^-$ annihilation and lepton-nucleon deep-inelastic scattering, we calculate the spin alignment of vector mesons with high transverse momenta in high energy $pp$ collisions with one longitudinally polarized beam. We present the results obtained at RHIC energies. We also study the spin alignment of such vector mesons in the case of a transversely polarized proton beam and present its relation to the transversity distribution in the nucleon. Numerical results obtained using simple models for the transversity distributions are also given. These results show that there exist significant spin alignment in both cases, and they can be measured in experiments such as those at RHIC.

hep-ph↗

arXiv2002

Spin alignments of vector mesons in deeply inelastic lepton-nucleon scattering

We extend the calculations of the spin alignments of vector mesons in $e^+e^-$ annihilation in a recent Rapid Communication to deeply inelastic lepton-nucleon scatterings. We present the results for different mesons in the current fragmentation regions of $μ^- N$$ \to$$ μ^- VX$ at high energies and $ν_μN $$\to$$ μ^- VX$ at both high and low energies. We also present the predictions for $ν_μN$$ \to$$ μ^- VX$ at NOMAD energies in the target fragmentation region using a valence quark model.

hep-ph↗

arXiv2002

Hyperon polarization in e^-p --> e^-HK with polarized electron beams

We apply the picture proposed in a recent Letter for transverse hyperon polarization in unpolarized hadron-hadron collisions to the exclusive process e^-p --> e^-HK such as e^-p-->e^-ΛK^+, e^-p --> e^-Σ^+ K^0, or e^-p--> e^-Σ^0 K^+, or the similar process e^-p\to e^-nπ^+ with longitudinally polarized electron beams. We present the predictions for the longitudinal polarizations of the hyperons or neutron in these reactions, which can be used as further tests of the picture.

hep-ph↗

arXiv2002

Longitudinal polarization of hyperons in high $p_\perp$ jets in singly polarized pp collisions at high energies

We calculate the longitudinal polarizations of hyperons in high $p_\perp$ jets in $pp$ collisions in which one of the protons is longitudinally polarized at RHIC energies using different models for the spin transfer in fragmentation process. The results show that the measurements of these polarizations can be used to study the spin transfer in high energy fragmentation processes in general and to test the different models in particular. Our results show especially that the magnitude of the polarization of $Λ$ is rather small whereas that of $Σ^+$ is considerably larger in the large rapidity region. The differences between the results from different pictures for $Σ^+$ polarizations is also much larger. Hence, the measurement of $Σ^+$ polarization should be more effective to distinguish between different models especially the SU(6) or the DIS picture for spin transfer in fragmentation processes.

hep-ph↗

arXiv2001

Hyperon polarization in semi-inclusive deeply inelastic lepton-nucleon scattering at high energy

We calculate the polarizations for different octet hyperons produced in the current fragmentation regions of the deeply inelastic lepton-nucleon scatterings $μ^-N \to μ^- HX$ and $ν_μ N \to μ^- HX$ at high energy using different models for spin transfer in fragmentation processes. The results show that measurements of those hyperon polarizations should provide useful information to distinguish between different models in particular the SU(6) and the DIS pictures used frequently in the literature. We found, in particular, that measuring the polarization of $Σ^+$ produced in these processes can give a better test to the validity of the different spin transfer models.

hep-ph↗

arXiv2001

Spin alignment of vector meson in e+e- annihilation at Z0 pole

We calculate the spin density matrix of the vector meson produced in e+e- annihilation at Z^0 pole. We show that the data imply a significant polarization for the antiquark which is created in the fragmentation process of the polarized initial quark and combines with the fragmenting quark to form the vector meson. The direction of polarization is opposite to that of the fragmenting quark and the magnitude is of the order of 0.5. A qualitative explanation of this result based on the LUND string fragmentation model is given.

hep-ph↗