Single Spin Asymmetries Research Articles

Spin asymmetries for C -even quarkonium production as a probe of gluon distributions

Within the framework of transverse momentum dependent factorization in combination with nonrelativistic quantum chromodynamics (QCD), we study charmonium and bottomonium production in hadronic collisions. We focus on quarkonium states with even charge conjugation, for which the color-singlet production mechanism is expected to be also dominant in the small transverse momentum region, qT2≪4Mc,b2. It is shown that the distributions of linearly polarized gluons inside unpolarized, longitudinally, and transversely polarized protons contribute to the cross sections for scalar and pseudoscalar quarkonia in a very distinctive, parity-dependent way, whereas their effects on higher angular momentum states are strongly suppressed. We derive analytical expressions for single and double spin asymmetries, which would allow for the direct extraction of the gluon transverse momentum dependent distributions, mirroring the phenomenological studies of the Drell-Yan processes aimed at the extraction of their quark counterparts. By adopting Gaussian models for the gluon transverse momentum dependent distributions, which fulfill without saturating everywhere their positivity bounds, we provide numerical predictions for the transverse single-spin asymmetries. These observables could be measured at LHCSpin, the fixed target experiment planned at the LHC. Published by the American Physical Society 2024

Transverse distortion and single-spin asymmetries for low-lying octet baryons

Transverse distortion and single-spin asymmetries for low-lying octet baryons

Single spin asymmetry AULsin(2ϕh) in dihadron production in SIDIS

The paper calculates the helicity-dependent dihadron fragmentation function by extending the dihadron spectator model and examine the single longitudinal spin asymmetry AULsin(2ϕh) from dihadron in semi-inclusive inelastic scattering. This function elucidates the relationship between the longitudinal polarization of the fragmented quark and the transverse momentum of the resulting hadron pairs. A study by the COMPASS collaboration detected a minimal signal in their experimental search for this azimuthal asymmetry in semi-inclusive inelastic scattering. Here, we use the spectator model to calculate the unknown T-odd dihadron fragmentation function H1⊥. Adopting collinear factorization to describe the data, avoiding the transverse momentum dependent factorization and the associated resummation effects, helping us understand the asymmetry and explaining why the signal is so weak. We involve the approach of transverse momentum dependence in the model calculations, in order to formulate the differential cross sections and the spin asymmetries in terms of the collinear parton distributions and the collinear dihadron fragmentation functions. A transverse momentum factor analysis method was used, in which the transverse momentum of the final hadron pairs was not integrated. The asymmetry of sin(2ϕh) in COMPASS kinematics was calculated and compared with experimental data. In addition, predictions for the same asymmetry are also presented for HERMES and the Electron Ion Collider. Published by the American Physical Society 2024

Measurements of the Z0/γ⁎ cross section and transverse single spin asymmetry in 510 GeV p + p collisions

The differential cross section for Z0 production, measured as a function of the boson's transverse momentum (pT), provides important constraints on the evolution of the transverse momentum dependent parton distribution functions (TMDs). The transverse single spin asymmetry (TSSA) of the Z0 is sensitive to one of the polarized TMDs, the Sivers function, which is predicted to have the opposite sign in p+p→W/Z+X from that which enters in semi-inclusive deep inelastic scattering. In this Letter, the STAR Collaboration reports the first measurement of the Z0/γ⁎ differential cross section as a function of its pT in p+p collisions at a center-of-mass energy of 510 GeV, together with the Z0/γ⁎ total cross section. We also report the measurement of Z0/γ⁎ TSSA in transversely polarized p+p collisions at 510 GeV.

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.

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

Updated numerical study of transverse single-spin asymmetries in single-inclusive pion production from lepton-nucleon collisions

We revisit the analysis of transverse single-spin asymmetries AN in lepton-nucleon scattering where only a single pion is detected in the final state, ℓN↑→hX. This observable is the Electron-Ion Collider (EIC) analogue to AN in proton-proton collisions, p↑p→hX, that has been studied intensely for decades, especially at the Relativistic Heavy Ion Collider (RHIC). We incorporate new theoretical developments in the collinear twist-3 framework and utilize recent extractions of (Sivers-like and Collins-like) quark-gluon-quark correlators in the numerical computations. We compare our calculations to HERMES measurements as well as make predictions for Jefferson Lab, COMPASS, and EIC kinematics. We further explore the role of next-to-leading order (NLO) corrections to the (twist-2) unpolarized cross section (denominator of AN) and consider what can be deduced empirically about the potential numerical significance of the full NLO calculation of AN in this process. We consider sources of theoretical uncertainty in our predictions, which present potential opportunities then for future measurements to improve our understanding of AN and multi-parton correlations in hadrons.

Single Transverse Spin Asymmetry as a New Probe of Standard-Model-Effective-Field-Theory Dipole Operators.

Electroweak dipole operators in the standard-model-effective-field theory (SMEFT) are important indirect probes of quantum effects of new physics beyond the standard model (SM), yet they remain poorly constrained by current experimental analyses for lack of interference with the SM amplitudes in constructing cross section observables. In this Letter, we point out that dipole operators flip fermion helicities and so are ideally studied through single transverse spin asymmetries. We illustrate this at a future electron-positron collider with transversely polarized beams, where such an effect exhibits as azimuthal cosϕ and sinϕ distributions which originate from the interference of the electron dipole operators with the SM and are linearly dependent on their Wilson coefficients. This new method can improve the current constraints on the electron dipole couplings by 1-2 orders of magnitude, without depending on other new physics operators, and can also simultaneously constrain both their real and imaginary parts, offering a new opportunity for probing potential CP-violating effects.

Normal single-spin asymmetries in electron-proton scattering: Two-photon exchange with intermediate-state resonances

Normal single-spin asymmetries in electron-proton scattering: Two-photon exchange with intermediate-state resonances

Single-Spin Asymmetry of Neutrons in Polarized pA Collisions

Absorptive corrections, which are known to suppress proton-neutron transitions with a large fractional momentum z→1 in pp collisions, become dramatically strong on a nuclear target, and they push the partial cross sections of leading neutron production to the very periphery of the nucleus. The mechanism of the pion π and axial vector meson a1 interference, which successfully explains the observed single-spin asymmetry in a polarized pp→nX, is extended to the collisions of polarized protons with nuclei. When corrected for nuclear effects, it explains the observed single-spin azimuthal asymmetry of neutrons that is produced in inelastic events, which is where the nucleus violently breaks up. This single-spin asymmetry is found to be negative and nearly atomic mass number A-independent.

Target normal single-spin asymmetry in inclusive electron-nucleon scattering in the 1/Nc expansion

The target normal single-spin asymmetry in inclusive electron-nucleon scattering is studied in the low-energy regime that includes the $\mathrm{\ensuremath{\Delta}}$ resonance. The particular interest in the asymmetry resides in that it is driven by two-photon exchange effects. It probes the spin-dependent absorptive part of the two-photon exchange amplitude, which is free of infrared and collinear singularities and represents the most pristine expression of two-photon exchange dynamics. The study presented here uses the $1/{N}_{c}$ expansion of QCD, which combines the $N$ and $\mathrm{\ensuremath{\Delta}}$ through the emergent $SU(4)$ spin-flavor symmetry in the baryon sector and allows for a systematic construction of the transition electromagnetic (EM) currents. The analysis includes the first subleading corrections in the $1/{N}_{c}$ expansion and presents results for elastic and inelastic final states. The asymmetry is found to be in the range ${10}^{\ensuremath{-}3}\ensuremath{-}{10}^{\ensuremath{-}2}$. The $\mathrm{\ensuremath{\Delta}}$ resonance plays an important role as an intermediate state in the elastic asymmetry and as a final state in the inclusive asymmetry.

Improving constraints on gluon spin-momentum correlations in transversely polarized protons via midrapidity open-heavy-flavor electrons in p↑+p collisions at s=200 GeV

Polarized proton-proton collisions provide leading-order access to gluons, presenting an opportunity to constrain gluon spin-momentum correlations within transversely polarized protons and enhance our understanding of the three-dimensional structure of the proton. Midrapidity open-heavy-flavor production at s=200 GeV is dominated by gluon-gluon fusion, providing heightened sensitivity to gluon dynamics relative to other production channels. Transverse single-spin asymmetries of positrons and electrons from heavy-flavor hadron decays are measured at midrapidity using the PHENIX detector at the Relativistic Heavy Ion Collider. These charge-separated measurements are sensitive to gluon correlators that can in principle be related to gluon orbital angular momentum via model calculations. Explicit constraints on gluon correlators are extracted for two separate models, one of which had not been constrained previously.Received 28 April 2022Revised 10 September 2022Accepted 6 March 2023DOI:https://doi.org/10.1103/PhysRevD.107.052012Published 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 AreasBeam polarizationHadron-hadron interactionsPolarization phenomenaRelativistic heavy-ion collisionsPhysical SystemsElectronsPositronsPropertiesSpinParticles & Fields

SIDIS-RC EvGen: A Monte-Carlo event generator of semi-inclusive deep inelastic scattering with the lowest-order QED radiative corrections

SIDIS-RC EvGen is a C++ standalone Monte-Carlo event generator for studies of semi-inclusive deep inelastic scattering (SIDIS) processes at medium to high lepton beam energies. In particular, the generator contains binary and library components for generating SIDIS events and calculating cross sections for unpolarized or longitudinally polarized beam and unpolarized, longitudinally or transversely polarized target. The structure of the generator incorporates transverse momentum-dependent parton distribution and fragmentation functions, whereby we obtain multi-dimensional binned simulation results, which will facilitate the extraction of important information about the three-dimensional nucleon structure from SIDIS measurements. In order to build this software, we have used recent elaborate QED calculations of the lowest-order radiative effects, applied to the leading order Born cross section in SIDIS. In this paper, we provide details on the theoretical formalism as well as the construction and operation of SIDIS-RC EvGen, e.g., how we handle the event generation process and perform multi-dimensional integration. We also provide example programs, flowcharts, and numerical results on azimuthal transverse single-spin asymmetries. Program summaryProgram title: SIDIS-RC EvGenCPC Library link to program files:https://doi.org/10.17632/thrkn96ydd.1Licensing provisions: GNU General Public License Version 3Developer's repository link:https://github.com/duanebyer/sidisProgramming language: C++, PythonExternal packages: FOAM, ROOT, GSL, VEGAS, Cubature, Cog, WW-SIDIS, MSTWPDFNature of problem: The task is to first create a code for calculations of the leading order Born cross section as well as radiative corrections (RCs) at the next-to-leading order (NLO) of the cross section of lepton-hadron semi-inclusive deep inelastic scattering (SIDIS) at medium to high beam energies with incident unpolarized or longitudinally polarized lepton beam and unpolarized, longitudinally or transversely polarized target, enabling to compute azimuthal single-target and double-beam-target spin asymmetries. Afterwards, a Monte-Carlo event generator based upon this code is developed, where in the coding and simulation processes multi-dimensional integrals need to be calculated precisely to obtain the exact NLO RCs to the SIDIS cross section with high precision beyond ultra-relativistic limit, which means that the lepton mass is taken into account.Solution method: The project for building the SIDIS-RC EvGen software is divided into a library component called libsidis, for the purpose of calculating the inelastic tail to the SIDIS total cross section, and a binary component called sidisgen, for generating events. The Monte-Carlo event generation is performed through the usage of the FOAM library, by applying a spatial partitioning method with hyper-cubical “foam of cells”. The multi-dimensional numerical integration is carried out by the GSL, VEGAS, and Cubature packages. The SIDIS structure functions that describe the SIDIS scattering process, and which are given in terms of transverse momentum-dependent parton distribution and fragmentation functions, are calculated in Gaussian and Wandzura–Wilczek-type approximations and implemented in the generator accordingly.Additional comments including restrictions and unusual features: The restrictions depend on the complexity of problems, limited by CPU time. As a consequence, the program running time is of order of several minutes to hours. SIDIS-RC EvGen does not comprise yet the contribution coming from the radiative tail of exclusive lepton-hadron reactions, which is a separate contribution involving exclusive structure functions that are currently not well known.Developer's repository maintained by Duane Byer; Department of Physics, Duke University.

Study of the worm-gear-T function g1T with semi-inclusive DIS data

The worm-gear-T function parametrizes the probability of finding a longitudinally polarized quark inside a transversely polarized hadron. We extract it from data on polarized semi-inclusive deep-inelastic scattering measured at COMPASS and HERMES. As a theoretical model, we use a Wandzura-Wilczek-type approximation at next-to-leading order. We find that at present, the data quality is insufficient to determine the worm-gear-T function faithfully. We also provide predictions for the transverse single-spin asymmetry associated with the worm-gear-T function, which could be measured in weak-boson production at STAR.

ECCE sensitivity studies for single hadron transverse single spin asymmetry measurements

We performed feasibility studies for various single transverse spin measurements that are related to the Sivers effect, transversity and the tensor charge, and the Collins fragmentation function. The processes studied include semi-inclusive deep inelastic scattering (SIDIS) where single hadrons (pions and kaons) were detected in addition to the scattered DIS lepton. The data were obtained in pythia6 and geant4 simulated e+p collisions at 18 GeV on 275 GeV, 18 on 100, 10 on 100, and 5 on 41 that use the ECCE detector configuration. Typical DIS kinematics were selected, most notably Q2>1GeV2, and cover the x range from 10−4 to 1. The single spin asymmetries were extracted as a function of x and Q2, as well as the semi-inclusive variables z, which corresponds to the momentum fraction the detected hadron carries relative to the struck parton, and PT, which corresponds to the transverse momentum of the detected hadron relative to the virtual photon. They are obtained in azimuthal moments in combinations of the azimuthal angles of the hadron transverse momentum and transverse spin of the nucleon relative to the lepton scattering plane. In order to extract asymmetries, the initially unpolarized MonteCarlo was re-weighted in the true kinematic variables, hadron types and parton flavors based on global fits of fixed target SIDIS experiments and e+e− annihilation data. The expected statistical precision of such measurements is extrapolated to 10 fb−1 and potential systematic uncertainties are approximated given the deviations between true and reconstructed yields. Similar neutron information is obtained by comparing the ECCE e+p pseudo-data with the same from the EIC Yellow Report and scaling the corresponding Yellow Report e+3He pseudo-data uncertainties accordingly. The impact on the knowledge of the Sivers functions, transversity and tensor charges, and the Collins function has then been evaluated in the same phenomenological extractions as in the Yellow Report. The impact is found to be comparable to that obtained with the parametrized Yellow Report detector and shows that the ECCE detector configuration can fulfill the physics goals on these quantities.

Transverse single-spin asymmetry of very forward neutral pion

We present in this talk a recent work on the transverse single-spin asymmetry of the very forward neutral pion in polarized p+p collisions at √S = 510 GeV. The triple-Regge formalism remarkably well desribes the RHICf data at pT < 1 GeV. We found that the neutral pion production at low pT is interpreted as a diffractive one.

Odderon mechanism for transverse single spin asymmetry in the Wandzura-Wilczek approximation

We compute the transverse single spin asymmetry in forward ${p}^{\ensuremath{\uparrow}}p\ensuremath{\rightarrow}hX$ and ${p}^{\ensuremath{\uparrow}}A\ensuremath{\rightarrow}hX$ collisions from the odderon mechanism originally suggested by Kovchegov and Sievert [Phys. Rev. D 86, 034028 (2012)]. Working in the hybrid approach of the color glass condensate effective theory, we first identify the relevant collinear parton distribution function (PDF) of the transversely polarized proton ${p}^{\ensuremath{\uparrow}}$ as the intrinsic twist-3 ${g}_{T}(x)$ distribution. We further argue that the complete polarized cross section also contains contributions from the kinematical and the dynamical twist-3 PDFs, in addition to the intrinsic twist-3 PDF. By restricting to the Wandzura-Wilczek approximation, where the dynamical twist-3 PDFs are dropped, we find that the odderon contribution to the polarized cross section for inclusive hadron production is exactly zero at the next-to-leading order in the strong coupling.

Breakup corrections to the He3 beam polarization measurements at the future BNL Electron Ion Collider

Requirements for hadron polarimetry at the future Electron-Ion Collider (EIC) include measurements of absolute ${}^3\text{He} (h)$ beam polarization with systematic uncertainties better than $\sigma^\text{syst}_P/P\lesssim1\%$. Due to the successful use, since 2005, of the polarized hydrogen jet target polarimeter (HJET) to measure proton beam polarization at the Relativistic Heavy Ion Collider (RHIC), the HJET technique promises to be suitable for the ${}^3\text{He}$ polarimetry at EIC. For that, however, one needs to know the ratio of the $h^\uparrow{p}$ and $p^\uparrow{h}$ analyzing powers. For elastic scattering, this ratio can be evaluated with sufficient accuracy if $p^\uparrow{p}$ analyzing power is precisely known. In this paper, the deuteron beam data acquired at HJET in RHIC Run 16 was used to evaluate corrections to the measured helion beam polarization due to the ${}^3\text{He}$ breakup. The breakup effect was found to be negligible if the ratio of the beam and target (jet) single spin asymmetries are concurrently measured to determine the ${}^3\text{He}$ beam polarization.

Target normal single-spin asymmetry in inclusive electron-nucleon scattering with two-photon exchange: Analysis using 1/Nc expansion

We calculate the target normal single-spin asymmetry caused by two-photon exchange in inclusive electron-nucleon scattering in the resonance region. Our analysis uses the 1/Nc expansion of low-energy QCD and combines N and Δ intermediate and final states using the contracted SU(4) spin-flavor symmetry. The normal spin asymmetry obtained in leading-order accuracy in 1/Nc has magnitude ∼10−2 and different sign in ep and en scattering. It can be measured in electron scattering at lab energies ∼0.5–1.5 GeV and provides a clean probe of two-photon exchange dynamics.

Archetypal factorization and gluon poles in semi-exclusive reactions

Abstract Within the archetypal factorization procedure, we study the gluon pole contributions manifesting in the nucleon–lepton hard processes of hadron production. We prove the dominant role of gluon pole contributions in such processes. We analyze the different sources of complexity associated with the gluon pole functions. As a practical application, we derive a new single spin asymmetry directly related to the gluon poles that can be studied experimentally.