Polarized Proton-proton Collisions Research Articles

Simultaneous reweighting of Transverse Momentum Dependent distributions

The Bayesian reweighting procedure is extended to the case of multiple independent extractions of transverse momentum dependent parton distributions (TMDs). By exploiting the data on transverse single spin asymmetries, AN, for inclusive pion production in polarized proton-proton collisions measured at RHIC, we perform a simultaneous reweighting of the quark Sivers, transversity and Collins TMD functions extracted from semi-inclusive deep inelastic scattering (SIDIS) and e+e− annihilation into hadron pairs. The impact of the implementation of the Soffer bound, as well as the differences between older and newer AN data, are investigated. The agreement with AN data at large-xF values, a kinematical region complementary to those explored in SIDIS measurements, is enhanced, improving the knowledge of the polarized quark TMDs in the large-x region.

Measurement of Transverse Spin Dependent Azimuthal Correlations of Charged Pion(s) in $p^{\uparrow} p$ Collisions at $\sqrt s = 200$ GeV at STAR

At the leading twist, the transversity distribution function, h^{q}_{1}(x)h1q(x), where xx is the longitudinal momentum fraction of the proton carried by quark qq, encodes the transverse spin structure of the nucleon. Extraction of it is difficult because of its chiral-odd nature. In transversely polarized proton-proton collisions (p^\uparrow pp↑p), h_{1}^{q}(x)h1q(x) can be coupled with another chiral-odd partner, a spin-dependent fragmentation function (FF). The resulting asymmetries in hadron(s) azimuthal correlations directly probe h_{1}^{q}(x)h1q(x). We report the measurement of correlation asymmetries for charged pion(s) in p^\uparrow pp↑p, through the Collins and the Interference FF channel.

Longitudinal double-spin asymmetry for inclusive jet and dijet production in polarized proton collisions at s=510 GeV

We report measurements of the longitudinal double-spin asymmetry, ALL, for inclusive jet and dijet production in polarized proton-proton collisions at midrapidity and center-of-mass energy s=510 GeV, using the high luminosity data sample collected by the STAR experiment in 2013. These measurements complement and improve the precision of previous STAR measurements at the same center-of-mass energy that probe the polarized gluon distribution function at partonic momentum fraction 0.015≲x≲0.25. The dijet asymmetries are separated into four jet-pair topologies, which provide further constraints on the x dependence of the polarized gluon distribution function. These measurements are in agreement with previous STAR measurements and with predictions from current next-to-leading-order global analyses. They provide more precise data at low dijet invariant mass that will better constrain the shape of the polarized gluon distribution function of the proton.Received 21 October 2021Accepted 4 April 2022DOI:https://doi.org/10.1103/PhysRevD.105.092011Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuark & gluon jetsPhysical SystemsGluonsPropertiesSpinTechniquesHadron collidersParticles & FieldsNuclear Physics

Scale evolution of T-odd gluon TMDs at small x

We study the energy or TMD evolution of the three leading twist dipole type T-odd gluon TMDs inside a transversely polarized nucleon, all of which at small $x$ dynamically originate from the spin dependent odderon. Their energy dependence presents a unique opportunity to study the polarization dependent TMD evolution in the small-$x$ region, where the distributions are identical up to a normalization constant at tree level. We further propose to study their evolution via azimuthal asymmetries in virtual photon-jet production in polarized proton-proton collisions at RHIC. We present model predictions for the asymmetries as functions of the large jet or photon transverse momentum and $Q^2$ which set the hard scales in this process.

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.

Investigating the Transverse Momentum Dependent Gluon Sivers Function in Quarkonium Production at pp Colliders

In this contribution, we will present a short overview of the transverse momentum dependent (TMD) approach as a tool for studying the 3-dimensional structure of hadrons in high-energy (un)polarized hadron collisions. We will then summarize the present status of a running research programme that aims at constraining the poorly known transverse momentum dependent gluon Sivers function, through the study of single spin asymmetries in quarkonium (mainly $J/\psi$), pion, and $D$-meson production in polarized proton-proton collisions at RHIC. Finally, we will shortly discuss perspectives for this field of research, emphasizing in particular its role in the physics programme of LHC in the fixed-target setup and NICA.

Transverse single spin asymmetry for very forward π0 production in polarized proton-proton collisions at √s = 510 GeV

Transverse single spin asymmetry, AN, of very forward π0 production from polarized p + p collisions provides new information toward an understanding of its production mechanism. AN of forward π0 in the pseudorapidity region of 3 < η < 4 has been described by the partonic structure of the proton in the perturbative QCD framework. However, recent data indicates a potential contribution from not only partonic but also diffractive interactions. In order to provide a new insight on the origin of the AN, we measured the very forward π0 production in the pseudorapidity region of 6 < η from √s = 510 GeV polarized p + p collisions at RHIC in 2017. We report our measurement of the very forward π0 over the transverse momentum range of 0 < pT < 1 GeV/c and the preliminary result.

Double longitudinal-spin asymmetries in J/\u03c8 production at RHIC

The double longitudinal-spin asymmetry, ALL, of the J/ψ production in polarized proton-proton collisions is presented in this paper at QCD next-to-leading order. It is found that the obtained values of ALL are in general consistent with the PHENIX measurements. Various sets of the long-distance matrix elements (LDMEs) are employed in our calculation to study the possible theoretical uncertainties. It is found that, for pt < 5 GeV, all these LDMEs lead to almost the same results, which are within the tolerance of the experimental data uncertainties.

5a-B-11 偏極陽子-陽子反応におけるスピン非対称

5a-B-11 偏極陽子-陽子反応におけるスピン非対称

A possible layout of the Spin Physics Detector with toroid magnet.

The Spin Physics Detector project for carrying out experiments at the 2-nd interaction point of the NICA collider is under preparation. The design of the collider allows reaching collision energy in the c.m.s. as high as √s = 26 GeV for polarized proton-proton collisions and √s = 12 GeV for polarized deuteron-deuteron collisons with a luminosity of up to 1032 cm2 s-1 (for protons) and 1031cm2s-1 for deuterons. Such a high luminosity of polarized beams interactions opens unique possibilities to investigate a variety of polarization phenomena including those related to the nucleon spin structure. A proposal for the experimental set-up based on a toroid type magnet is presented.

χ cJ polarization in polarized proton–proton collisions at RHIC

We study inclusive $\chi_{cJ}$ production with definite polarizations in polarized proton-proton collisions at $\sqrt{s}$ = 200 GeV and 500 GeV at RHIC by using non-relativistic QCD (NRQCD) color-octet mechanism. We present results of rapidity distribution of $\chi_{c0}$, $\chi_{c1}$ and $\chi_{c2}$ production with specific polarizations in polarized p-p collisions at RHIC within the PHENIX detector acceptance range. We also present the corresponding results for the spin asymmetries.

Spin asymmetries for vector boson production in polarizedp+pcollisions

We study the cross section for vector boson ($W^{\pm}/Z^0/\gamma^*$) production in polarized nucleon-nucleon collisions for low transverse momentum of the observed vector boson. For the case where one measures the transverse momentum and azimuthal angle of the vector bosons, we present the cross sections and the associated spin asymmetries in terms of transverse momentum dependent parton distribution functions (TMDs) at tree level within the TMD factorization formalism. To assess the feasibility of experimental measurements, we estimate the spin asymmetries for $W^{\pm}/Z^0$ boson production in polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) by using current knowledge of the relevant TMDs. We find that some of these asymmetries can be sizable if the suppression effect from TMD evolution is not too strong. The $W$ program at RHIC can, thus, test and constrain spin theory by providing unique information on the universality properties of TMDs, TMD evolution, and the nucleon structure. For example, the single transverse spin asymmetries could be used to probe the well-known Sivers function $f_{1T}^{\perp q}$, as well as the transversal helicity distribution $g_{1T}^{q}$ via the parity-violating nature of $W$ production.

Achievements and Open Issues in the Determination of Polarized Parton Distribution Functions

I review the current status of the determination of helicity-dependent, or polarized, parton distribution functions from a comprehensive analysis of experimental data in perturbative quantum chromodynamics. I illustrate the latest achievements driven by new measurements in polarized proton-proton collisions at the Relativistic Heavy Ion Collider, namely the first evidence of a sizable polarized light sea quark asymmetry and of a positive polarized gluon distribution in the proton. I discuss which are the open issues in the determination of polarized distributions, and how these may be addressed in the future by ongoing, planned and proposed experimental programs.

Central Exclusive Production in Proton-Proton Collisions with the STAR Experiment at RHIC

We shall describe the physics program with tagged forward protons, focusing on Central Exclusive Production in polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC), with the STAR detector at √ s = 200 GeV. Preliminary results in CEP of two oppositely charged pions and kaons produced in the processes pp → ppπ + π − and pp → ppK + K − shall be presented. Those Double Pomeron Exchange (DPE) processes, allow the final states to be dominated by gluonic exchanges. Silicon strip detectors placed in Roman Pots were used for measuring forward protons. The preliminary results are based on the measurement of the recoil system of charged particles in the STAR experiment’s Time Projection Chamber (TPC). Ionization energy loss, dE/dx, of charged particles was used for particle identification (PID).

Unveiling the proton spin decomposition at a future electron-ion collider

We present a detailed assessment of how well a future Electron-Ion Collider could constrain helicity parton distributions in the nucleon and, therefore, unveil the role of the intrinsic spin of quarks and gluons in the proton's spin budget. Any remaining deficit in this decomposition will provide the best indirect constraint on the contribution due to the total orbital angular momenta of quarks and gluons. Specifically, all our studies are performed in the context of global QCD analyses based on realistic pseudo-data and in the light of the most recent data obtained from polarized proton-proton collisions at BNL-RHIC that have provided evidence for a significant gluon polarization in the accessible, albeit limited range of momentum fractions. We also present projections on what can be achieved on the gluon's helicity distribution by the end of BNL-RHIC operations. All estimates of current and projected uncertainties are performed with the robust Lagrange multiplier technique.

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 spin asymmetries for weak boson production in polarized proton-proton collisions at RHIC.

We report measurements of single- and double-spin asymmetries for W^{±} and Z/γ^{*} boson production in longitudinally polarized p+p collisions at sqrt[s]=510 GeV by the STAR experiment at RHIC. The asymmetries for W^{±} were measured as a function of the decay lepton pseudorapidity, which provides a theoretically clean probe of the proton's polarized quark distributions at the scale of the W mass. The results are compared to theoretical predictions, constrained by polarized deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range 0.05<x<0.2.

Collins and sivers effects in p↑p → jet πX: Universality and process dependence

In this paper we briefly review the transverse momentum dependent generalized parton model and its application to the study of azimuthal asymmetries in the distribution of leading hadrons (mainly pions) inside large transverse momentum jets inclusively produced in polarized proton-proton collisions. We put particular emphasis on the phenomenological interest of these observables, in combination with similar asymmetries measured in semi-inclusive deeply inelastic scattering, Drell-Yan processes and e+e- collisions, for the study of the universality properties of the transverse momentum dependent parton distribution and fragmentation functions. We present results for RHIC kinematics at center-of-mass energies sqrt{s} = 200 and 500 GeV, for central and mainly forward jet rapidities, in particular for the Sivers distribution and the Collins fragmentation function, that are believed to be responsible of many of the largest asymmetries measured in the last years. We also briefly discuss the case of inclusive jet production and recent phenomenological applications of other theoretical approaches, like the colour gauge invariant generalized parton model and the collinear twist-three approach, aiming at clarifying the issues of the universality and process dependence of transverse momentum dependent functions.

Neutral pion cross section and spin asymmetries at intermediate pseudorapidity in polarized proton collisions ats=200 GeV

The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8<η<2.0 in polarized proton-proton collisions at s=200 GeV are presented. Neutral pions were detected using the end cap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5<pT<16 GeV/c and is found to agree with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry ALL is measured in the same pseudorapidity range and spans a range of Bjorken-x down to x≈0.01. The measured ALL is consistent with model predictions for varying degrees of gluon polarization. The parity-violating asymmetry AL is also measured and found to be consistent with zero. The transverse single-spin asymmetry AN is measured over a previously unexplored kinematic range in Feynman-x and pT. Such measurements may aid our understanding of the onset and kinematic dependence of the large asymmetries observed at more forward pseudorapidity (η≈3) and their underlying mechanisms. The AN results presented are consistent with a twist-3 model prediction of a small asymmetry over the present kinematic range.2 MoreReceived 6 September 2013DOI:https://doi.org/10.1103/PhysRevD.89.012001© 2014 American Physical Society

Longitudinal spin transfer to Λ and $$\bar \Lambda$$ in polarized proton-proton collisions at √s = 200 GeV at STAR

The longitudinal spin transfer DLL of Λ and \(\bar \Lambda\) hyperons is expected to be sensitive to the helicity distribution function of strange (anti-)quarks, and to the polarized fragmentation functions. We report new results on DLL, extracted from √s = 200 GeV polarized pp collisions recorded at the STAR detector during the 2009 RHIC run. These new results cover mid-rapidity |η| < 1.2, with extended pT up to 6 GeV/c.