Longitudinal Single-spin Asymmetry Research Articles

Perturbative QCD contribution to transverse single spin asymmetries in the Drell-Yan process and SIDIS

In a previous publication [Benić , ], we have computed the perturbative QCD contribution to transverse single spin asymmetries (SSAs) in semi-inclusive deep inelastic scattering (SIDIS) involving the gT(x) distribution. In this paper, we first present a more efficient derivation of the asymmetries which is applicable to both transverse and longitudinal SSAs and correct some inconsistencies in our previous calculation. We then adapt the method to compute transverse SSAs in the Drell-Yan process proportional to gT(x) and its gluonic counterpart and discuss the crossing symmetry between the results for SIDIS and the Drell-Yan process. Finally, we present numerical results for various asymmetries measurable at the EIC, RHIC, COMPASS, and Fermilab (SpinQuest), including also part of the genuine twist-three corrections to gT(x) from a recent global analysis. We find that the asymmetries can reach percent-level magnitude, if the kinematics predominantly probes the large-x (valence) region of the polarized proton, but remain at subpercent levels otherwise. Published by the American Physical Society 2024

W-boson production in polarized proton-proton collisions at RHIC through next-to-next-to-leading order in perturbative QCD

We perform a study of W-boson production in polarized proton-proton collisions through next-to-next-to-leading order (NNLO) in perturbative QCD. This calculation is required to extend the extraction of polarized parton distribution functions to NNLO accuracy. We present differential distributions at s=510 GeV, relevant for comparison to measurements from the Relativistic Heavy Ion Collider (RHIC). The NNLO QCD corrections significantly reduce the scale dependence of the cross section. We compare the longitudinal single-spin asymmetries as a function of lepton pseudorapidity to RHIC data. The asymmetries exhibit excellent stability under perturbative QCD corrections.

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 protonion collisions in the years beyond 2021 at STAR.

Nucleon spin structure and measurements on sea quark polarization at RHIC

The nucleon spin structure in terms of its parton constituents is a fundamental question in QCD. The sea quark contribution to the nucleon spin is an important piece for a complete understanding of the nucleon spin structure. The production of W bosons in longitudinally polarized proton-proton collisions at RHIC provides a unique probe for the sea quark polarization, through the parity-violating single-spin asymmetry A L = ( σ + − σ − ) / ( σ + + σ − ) . At RHIC, W ’s can be detected via the leptonic channel W → e ν with the kinematics of the decay lepton alone. Longitudinally polarized proton-proton collision events at s =500 GeV were recorded at RHIC through the years 2009, 2011, 2012 and 2013. Both STAR and PHENIX experiments completed their measurements on longitudinal single spin asymmetries for W bosons as a function of decay lepton rapidity. These results provide new constraints on sea quark polarizations through the QCD global analyses, which clearly indicate the existence of a flavor asymmetry in the polarization of the light-quark sea, Δ u ¯ > Δ d ¯ , in the proton for parton momentum fraction range of 0.05 x d ¯ > u ¯ over a wide x range has been observed. On the other hand, the phenomenological studies showed that the spin transfer to Λ ¯ anti-hyperons provides a natural connection to helicity or transversity distributions of strange and anti-strange quarks in the proton. STAR experiment made the first and an improved measurement on the longitudinal spin transfer D LL to Λ and Λ ¯ hyperons in longitudinally polarized proton-proton collisions at 200 GeV. STAR also completed the first measurement on transverse spin transfer D LL to Λ and Λ ¯ hyperons in transversely polarized proton-proton collisions at 200 GeV. These data and future measurements with higher precision and larger momentum fraction in the forward region at RHIC will shed new lights on the polarized parton distributions for strange and anti-strange quark in the nucleon.

Cross section and longitudinal single-spin asymmetry AL for forward W±→μ±ν production in polarized p+p collisions at s=510 GeV

We have measured the cross section and single spin asymmetries from forward $W^{\pm}\rightarrow\mu^{\pm}\nu$ production in longitudinally polarized $p$$+$$p$ collisions at $\sqrt{s}=510$ GeV using the PHENIX detector at the Relativistic Heavy Ion Collider. The cross sections are consistent with previous measurements at this collision energy, while the most forward and backward longitudinal single spin asymmetries provide new insights into the sea quark helicities in the proton. The charge of the W bosons provides a natural flavor separation of the participating partons.

Gluon orbital angular momentum at small x

We present a general analysis of the orbital angular momentum (OAM) distribution of gluons ${L}_{g}(x)$ inside the nucleon with particular emphasis on the small-$x$ region. We derive a novel operator representation of ${L}_{g}(x)$ in terms of Wilson lines and argue that it is approximately proportional to the gluon helicity distribution ${L}_{g}(x)\ensuremath{\approx}\ensuremath{-}2\mathrm{\ensuremath{\Delta}}G(x)$ at small $x$. We also compute longitudinal single-spin asymmetry in exclusive diffractive dijet production in lepton-nucleon scattering in the next-to-eikonal approximation and show that the asymmetry is a direct probe of the gluon helicity/OAM distribution as well as the QCD odderon exchange.

Recent results of gluon and sea quark polarization measurements in polarized proton-proton collisions at STAR

The STAR experiment at RHIC is carrying out a comprehensive high-energy spin physics program to understand the internal structure and dynamics of the proton in polarized proton-proton collisions at = 200 GeV and = 500/510 GeV. STAR has the capability, with nearly full azimuthal coverage, to reconstruct leptons, hadrons and jets in the mid-rapidity region (|η| < 1). The results for inclusive jet longitudinal double spin asymmetries taken during the 2009 RHIC run indicate the first non-zero gluon contribution (Δg(x,Q2)/g(x,Q2)) to the proton spin for 0.05 < x < 1 (Bjorken-x: momentum fraction of partons). Recent longitudinal single-spin asymmetry measurements of W+/− bosons at = 500/510 GeV in polarized proton-proton collisions provide a direct probe of the polarized anti-u and anti-d quark distributions (Δū(x, Q2), Δd̄(x, Q2)). These results better constrain the polarized gluon and sea quark distributions of the proton in the RHIC sensitive kinematic region. Future measurements with continuing high energy polarized proton-proton run at RHIC and detector upgrade will explore the gluonic contribution to the proton spin in extended range.

Measurement of longitudinal single spin asymmetry of W ± → μ± at forward/backward rapidities with PHENIX at √s = 500–510 GeV polarized p + p collisions

The PHENIX experiment has collected about 25 pb−1 longitudinally polarized p + p collisions at √s = 500 GeV in 2011 run for probing antiquark polarization. In this talk the first preliminary result of ALμ± is presented.

Recent STAR results on W production and asymmetry in polarized $$\vec p + \vec p$$ collisions at √r = 500 GeV

The production of W± bosons in longitudinally polarized proton + proton collisions at RHIC is a unique way to study the flavor-dependent spin structure of the proton, since at leading order it couples directly and exclusively to specific quark flavor pairings: $$u + \bar d \to W^ +$$ and $$\bar u + d \to W^ -$$ . Measurement of the longitudinal single-spin asymmetry of the electron decay daughters in W production is sensitive to the polarizations of the quarks involved. Results from the first proton-proton collisions at √r = 500 GeV in 2009 are presented, along with a discussion of the forward-rapidity tracking upgrade.

Probing the sea contribution to the protons' spin

Longitudinal single-spin asymmetries (AL) of parity-violating W bosons are measured with PHENIX via the decay daughter muons in = 500 GeV p + p collisions at RHIC. These proceedings report on the latest W±→μ±ν asymmetry results obtained during the 2011 RHIC run using the forward muon-arm detector in PHENIX.

Cross Section and Parity-Violating Spin Asymmetries ofW±Boson Production in Polarizedp+pCollisions ats=500 GeV

Large parity-violating longitudinal single-spin asymmetries A(L)(e+) = -0.86(-0.14) (+0.30) and A(L)(e-) = 0.88(-0.71) (+0.12) are observed for inclusive high transverse momentum electrons and positrons in polarized p+p collisions at a center-of-mass energy of sqrt[s] = 500 GeV with the PHENIX detector at RHIC. These e± come mainly from the decay of W± and Z0 bosons, and their asymmetries directly demonstrate parity violation in the couplings of the W± to the light quarks. The observed electron and positron yields were used to estimate W± boson production cross sections for the e± channels of σ(pp → W+ X) × BR(W+ → e+ ν(e)) = 144.1 ± 21.2(stat)(-10.3) (+3.4) (syst) ± 21.6(norm) pb, and σ(pp → W- X) × BR(W- → e- ν[over ¯](e)) = 31.7 ± 12.1(stat)(-8.2) (+10.1) (syst) ± 4.8(norm) pb.

Gaugino-pair production in polarized and unpolarized hadron collisions

We present an exploratory study of gaugino-pair production in polarized and unpolarized hadron collisions, focusing on the correlation of beam polarization and gaugino/Higgsino mixing in the general minimal supersymmetric standard model. Helicity-dependent cross sections induced by neutral and charged electroweak currents and squark exchanges are computed analytically in terms of generalized charges, defined similarly for chargino-pair, neutralino-chargino associated, and neutralino-pair production. Our results confirm and extend those obtained previously for negligible Yukawa couplings and nonmixing squarks. Assuming that the lightest chargino mass is known, we show numerically that measurements of the longitudinal single-spin asymmetry at the existing polarized $pp$ collider RHIC and at possible polarization upgrades of the Tevatron or the LHC would allow for a determination of the gaugino/Higgsino fractions of charginos and neutralinos. The theoretical uncertainty coming from factorization scale and squark mass variations and the expected experimental error on the lightest chargino mass is generally smaller than the one induced by the polarized parton densities, so that more information on the latter would considerably improve on the analysis.

The Mainz A4 parity violation experiment (single spin asymmetries in elastic scattering)

The A4 experiment at the MAMI facility in Mainz measures a parity violating single spin asymmetry in the scattering of longitudinally polarized electrons off unpolarized protons. The longitudinal single spin asymmetry is parity violating and arises from the interference of the weak and electromagnetic one-boson exchange amplitude. It is sensitive to the strangeness contribution to the electromagnetic form factors of the nucleon. Recently the transverse single spin asymmetry has gained much interest. It is purely electromagnetic in origin and arises from the interference of the one-photon with the two-photon exchange amplitude. It is a time-odd quantity and sensitive to the imaginary part of the two-photon amplitude only. The A4 collaboration has measured both asymmetries at two different momentum transfer values. The physical background, the experimental apparatus, the experimental results and the implications are discussed.

Slepton production in polarized hadron collisions

We calculate cross sections and asymmetries for slepton pair production through neutral and charged electroweak currents in polarized hadron collisions for general slepton masses and including mixing of the left- and right-handed interaction eigenstates relevant for third generation sleptons. Our analytical results confirm and extend a previous calculation. Numerically, we show that measurements of the longitudinal single-spin asymmetry at the existing polarized pp collider RHIC and at possible polarization upgrades of the Tevatron or the LHC would allow for a determination of the tau slepton mixing angle and/or the associated supersymmetry breaking parameters Λ for gauge mediation and A0 for minimal supergravity. Furthermore, the Standard Model background from tau pair production can be clearly distinguished due to the opposite sign of the associated asymmetry.

New observables in longitudinal single-spin asymmetries in semi-inclusive DIS

We analyze longitudinal beam and target single-spin asymmetries in semi-inclusive deep inelastic scattering and in jet deep inelastic scattering, including all possible twist-3 contributions as well as quark mass corrections. We take into account the path-ordered exponential in the soft correlators and show that it leads to the introduction of a new distribution and a new fragmentation function contributing to the asymmetries.