Single Spin Asymmetries Research Articles

Azimuthal transverse single-spin asymmetries of inclusive jets and identified hadrons within jets from polarized pp collisions at s=200 GeV

The STAR Collaboration reports measurements of the transverse single-spin asymmetries, $A_N$, for inclusive jets and identified `hadrons within jets' production at midrapidity from transversely polarized $pp$ collisions at $\sqrt{s}$ = 200 GeV, based on data recorded in 2012 and 2015. The inclusive jet asymmetry measurements include $A_N$ for inclusive jets and $A_N$ for jets containing a charged pion carrying a momentum fraction $z>0.3$ of the jet momentum. The identified hadron within jet asymmetry measurements include the Collins effect for charged pions, kaons and protons, and the Collins-like effect for charged pions. The measured asymmetries are determined for several distinct kinematic regions, characterized by the jet transverse momentum $p_{T}$ and pseudorapidity $\eta$, as well as the hadron momentum fraction $z$ and momentum transverse to the jet axis $j_{T}$. These results probe higher momentum scales ($Q^{2}$ up to $\sim$\,900 GeV$^{2}$) than current, semi-inclusive deep inelastic scattering measurements, and they provide new constraints on quark transversity in the proton and enable tests of evolution, universality and factorization breaking in the transverse-momentum-dependent formalism.

Triple Regge exchange and transverse single-spin asymmetries of the very forward neutral pion production in polarized p+p collisions

Recently, the RHICf Collaboration measured the transverse single-spin asymmetries of the very forward neutral pion in polarized $p+p$ collisions at $\sqrt{s}=510$ GeV, produced at large pseudorapidity ($\eta\gtrsim 6$). The data show large asymmetries both in longitudinal momentum fraction $x_F$ and transverse momentum $p_T$ at $p_T<1\,\mathrm{GeV}/c$. Employing baryonic triple Regge exchanges, we describe the complete RHICf data for the first time and show that the neutral pion production at low $p_T$ can be interpreted as a diffractive one.

Probing the dark photon via polarized DIS scattering at the HERA and EIC

The dark photon is widely predicted in many new physics beyond the Standard Model. In this work, we propose to utilize the polarized lepton cross section and single-spin asymmetry in neutral current DIS process at the HERA and upcoming Electron-Ion Collider (EIC) to probe the properties of the dark photon. It shows that we can constrain the mixing parameter ϵ<0.02 when the dark photon mass mAD<10GeV at the HERA with polarized lepton beam and this bound is comparable to the limit from the unpolarized data of HERA I and II. With the help of high integrated luminosity and high electron beam polarization of the EIC, the upper limit of the ϵ could be further improved. Depending on the assumption of the systematic error of the cross section measurements, we obtain ϵ<0.01∼0.02 for the mass region mAD<10GeV under the integrated luminosity of 300fb−1. A similar and robust upper limit for the ϵ could be obtained from the single-spin asymmetry measurement, which is not depending on the assumption of the systematic errors.

T -odd proton-helicity asymmetry in semi-inclusive DIS in perturbative QCD

We compute the single-spin asymmetry ${A}_{\mathrm{UL}}$ in semi-inclusive deep-inelastic scattering of unpolarized leptons and longitudinally polarized protons at a large transverse momentum of the produced hadron. Our calculation is performed in collinear factorization at the lowest order of QCD perturbation theory. For photon exchange, the asymmetry is $T$-odd and receives contributions from the interference of the tree level and one-loop absorptive amplitudes. We consider the behavior of the spin asymmetry at low transverse momentum where contact to the formalism based on transverse momentum dependent distribution functions can be made. We also present some phenomenological results relevant for the COMPASS and HERMES experiments and the future Electron-Ion Collider.

Jet charge: A new tool to probe the anomalous [formula omitted] couplings at the EIC

We propose to probe the Zbb¯ interactions at the Electron-Ion Collider (EIC) by utilizing the average jet charge weighted single-spin asymmetry AebQ, which is induced by different cross sections of a left-handed and right-handed electron beam scattering off an unpolarized proton beam in the neutral current deeply-inelastic scattering processes with one observed b-tagged jet. This novel observable at the EIC is sensitive to the axial-vector component of the Zbb¯ coupling, providing similar information as the gg→Zh cross section measurement at the high-luminosity Large Hadron Collider, and is complementary to the single-spin asymmetry measurement at the EIC which is sensitive to the vector component of the Zbb¯ coupling. We show that the apparent degeneracy of the allowed Zbb¯ coupling in its vector and axial-vector components, implied by the Large Electron-Positron Collider (LEP) and the Stanford Linear Collider (SLC) precision electroweak data, can be broken by the AebQ measurement at the EIC. With a large enough integrated luminosity collected at the EIC, the measurement of AebQ could also resolve the long-standing discrepancy between bottom quark forward-backward asymmetry AFBb at the LEP and the Standard Model prediction, with a strong dependence on the b-tagging efficiency.

A new method to extract the valence transversity distributions

A new method is suggested for the extraction of uu-quark and dd-quark transversity distributions, using single spin asymmetry (SSA) data in semi-inclusive deep inelastic scattering (SIDIS) processes, where they couple to the Collins or the di-hadron fragmentation functions. We discuss a recent suggestion to extract the transversity distribution using the concept of difference asymmetries and their ratios, which avoids the requirement of Collins function. We suggest new measurements, involving ratios of polarized cross-sections, that would directly probe the ratio h_1^{d_v}/h_1^{u_v}h1dv/h1uv. We also show some numerical estimates.

Hard exclusive $\pi^{+}n$ electro production beam spin asymmetries off the proton in the GPD and TDA regimes

Hard exclusive \pi^{+}π+n electro-production can be used to gain access to the 3D nucleon structure. The QCD factorisation mechanism in the “nearly forward region” (t/Q^{2}t/Q2 small) allows a description by Generalized Parton Distributions (GPDs), while for the “nearly backward” kinematic region (u/Q^{2}u/Q2 small) a description based on nucleon to pion transitions (TDAs) is available. The paper presents a measurement of single beam spin asymmetries to extract the A^{\sin\phi}_{LU}ALUsinϕ moments and the related cross section ratio \sigma_{LT^\prime}/\sigma_{0}σLT′/σ0 from the hard exclusive \pi^{+}π+ channel off the unpolarized hydrogen target in a wide range of kinematics based on data taken with the CLAS spectrometer at Jefferson Lab. In addition, under forward angles a detailed multidimensional study will be presented based on CLAS12 data and compared to theoretical predictions.

Non-zero transverse single spin asymmetry of very forward $\pi^0$ in polarized $p + p$ collisions at $\sqrt{s} = 510$ GeV

The RHICf experiment measured transverse single spin asymmetry of very forward (\eta &gt; 6η&gt;6) \pi^0π0 from polarized p + pp+p collisions at \sqrt{s} = 510s=510 GeV. In order to measure it precisely, we installed a new electromagnetic calorimeter at zero-degree area of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) and measured the \pi^0π0s over the kinematic range of x_F &gt; 0.25xF&gt;0.25 and 0 &lt; p_T &lt; 10&lt;pT&lt;1 GeV/cc in June 2017. A clear non-zero asymmetry was observed even in low p_T &lt; 1pT&lt;1 GeV/cc showing a similar x_FxF dependence with that of the forward (2 &lt; \eta &lt; 42&lt;η&lt;4) \pi^0π0. A possible diffractive contribution may need to be taken into account to explain the very forward \pi^0π0 asymmetry. RHICf-STAR combined analysis and follow-up experiment will give a clue to understand it qualitatively.

Azimuthal single- and double-spin asymmetries in semi-inclusive deep-inelastic lepton scattering by transversely polarized protons

A comprehensive set of azimuthal single-spin and double-spin asymmetries in semi-inclusive leptoproduction of pions, charged kaons, protons, and antiprotons from transversely polarized protons is presented. These asymmetries include the previously published HERMES results on Collins and Sivers asymmetries, the analysis of which has been extended to include protons and antiprotons and also to an extraction in a three-dimensional kinematic binning and enlarged phase space. They are complemented by corresponding results for the remaining single-spin and double-spin asymmetries for transverse target-polarization orientation.

Transverse Single Spin Asymmetries of Heavy Flavor Electrons and Charged Pions in 200 GeV $p+p^{\uparrow}$ Collisions at Midrapidity

Transverse single spin asymmetries of particles produced in p+p^{\uparrow}p+p↑ collisions provide insight on the partonic spin and momentum structure of hadrons; heavy flavor electrons provide access to initial state spin-momentum correlations of gluons in the proton, while charged pions provide access to initial and final state transverse spin effects. Charged particles are measured at midrapidity at PHENIX using the silicon vertex detector and central arm spectrometer, made of an electromagnetic calorimeter, a ring-imaging Cherenkov detector, and drift and pad chambers. Recent results for both electron and charged pion measurements from the 2015 running period will be presented.

Reweighting the quark Sivers function with STAR jet data

The Bayesian reweighting procedure is applied for the first time to a TMD distribution, the quark Sivers function extracted from SIDIS data. By exploiting the recent published single spin asymmetry data for the inclusive jet production in p^\uparrow pp↑p collisions from the STAR collaboration at RHIC, we show how such a procedure allows to incorporate the information contained in the new data set, without the need of re-fitting, and to explore a much wider xx region compared to SIDIS measurements. The reweighting method is also extended to the case of asymmetric errors, and the results show a significant improvement on the knowledge of the quark Sivers function.

Transverse Single-Spin Asymmetries of Midrapidity Direct Photons and Neutral Mesons at PHENIX

Results are presented for the transverse single-spin asymmetries of direct photons, neutral pions, and eta mesons for |\eta|&lt;0.35|η|&lt;0.35 from p^\uparrow + pp↑+p collisions with \sqrt{s} = 200s=200 GeV at PHENIX. As hadrons, \pi^0π0 and \etaη mesons are sensitive to both initial- and final-state effects and at midrapidity probe the dynamics of gluons along with a mix of quark flavors. Because direct photon production does not include hadronization, the direct photon TSSA is only sensitive to initial-state effects and at midrapidity provides a clean probe of the gluon dynamics in transversely polarized protons. All three of these results will help constrain the collinear twist-3 trigluon correlation function as well as the gluon Sivers function, improving our knowledge of spin-dependent gluon dynamics in QCD.

Predictions for the Sivers single-spin asymmetry from holographic QCD

A new approach to nonperturbative QCD, holographic light-front QCD, provides a comprehensive model for hadron dynamics and spectroscopy, incorporating color confinement, a universal hadron mass scale, the prediction of a massless pion in the chiral limit, and connections between the spectroscopy of mesons, baryons and tetraquarks across the full hadron spectrum. In this article we present predictions for the Sivers asymmetry and related transverse momentum distributions for the proton based on the light-front wavefunctions of the baryon eigenstate predicted by holographic QCD.

New Measurements of the Beam-Normal Single Spin Asymmetry in Elastic Electron Scattering over a Range of Spin-0 Nuclei.

We report precision determinations of the beam-normal single spin asymmetries (A_{n}) in the elastic scattering of 0.95 and 2.18GeV electrons off ^{12}C, ^{40}Ca, ^{48}Ca, and ^{208}Pb at very forward angles where the most detailed theoretical calculations have been performed. The first measurements of A_{n} for ^{40}Ca and ^{48}Ca are found to be similar to that of ^{12}C, consistent with expectations and thus demonstrating the validity of theoretical calculations for nuclei with Z≤20. We also report A_{n} for ^{208}Pb at two new momentum transfers (Q^{2}) extending the previous measurement. Our new data confirm the surprising result previously reported, with all three data points showing significant disagreement with the results from the Z≤20 nuclei. These data confirm our basic understanding of the underlying dynamics that govern A_{n} for nuclei containing ≲50 nucleons, but point to the need for further investigation to understand the unusual A_{n} behavior discovered for scattering off ^{208}Pb.

Probing the [formula omitted] anomalous couplings via exclusive Z boson decay

We propose to utilize the exclusive Z-boson rare decays Z→ϒ(ns)+γ to constrain the Zbb¯ couplings at the HL-LHC and 100 TeV proton-proton collider. We demonstrate that the event yield of the proposed processes is sensitive to the axial-vector component of the Zbb¯ coupling and can provide complementary information to the jet-charge weighted single-spin asymmetry measurement at the EIC and the gg→Zh production rate measurement at the LHC. By applying the NRQCD factorization formalism, we calculate the partial decay width of Z→ϒ(ns)+γ to the NLO accuracy in strong interaction, which is found to agree with those obtained from the light-cone distribution amplitude approach. We show that the HL-LHC can break the degeneracy of the Zbb¯ couplings, as implied by the precision electroweak data at LEP and SLC, if the signal efficiency can be improved by a factor of 1.7 from the present ATLAS analysis at the 13 TeV LHC with an integrated luminosity of 36.1fb−1.

Two photon exchange with nuclei from e^+/e^- elastic cross section ratios

Hadronic box diagrams pose a significant source of radiative corrections to lepton scattering and beta -decay measurements, yet their calculation remains highly model-dependent. Two-photon exchange is a box diagram generally relevant to most lepton scattering experiments, and is directly detectable via multiple observables. Single-spin asymmetries (SSA) are sensitive to the imaginary part of the TPE amplitude, while the ratio of e^+/e^- elastic scattering cross sections is sensitive to the real part of the TPE amplitude. While SSA measurements have been reported for several complex nuclei, measurements of the e^+/e^- ratio exist only for the proton. Proposed here is a measurement of the e^+/e^- ratio on ^4He, a feasible nuclear target for which the ratio’s deviation from unity due to TPE is expected to be on order of 1–2%. Choosing a low Z nucleus minimizes Coulomb distortions, which could greatly overwhelm TPE for heavier nuclei. The proposed 19-day program, including measurements at three kinematic settings between 0.3< Q^2 < 0.4 GeV^2, would be made possible by the addition of a positron source at Jefferson Lab.

Transversity parton distribution function of the nucleon using the pseudodistribution approach

We present a determination of the non-singlet transversity parton distribution function (PDF) of the nucleon, normalized with respect to the tensor charge at $\mu^2=2$ GeV$^2$ from lattice quantum chromodynamics. We apply the pseudo-distribution approach, using a gauge ensemble with a lattice spacing of 0.094 fm and the light quark mass tuned to a pion mass of 358 MeV. We extract the transversity PDF from the analysis of the short-distance behavior of the Ioffe-time pseudo-distribution using the leading-twist next-to-leading order (NLO) matching coefficients calculated for transversity. We reconstruct the $x$-dependence of the transversity PDF through an expansion in a basis of Jacobi polynomials in order to reduce the PDF ansatz dependence. Within the limitations imposed by a heavier-than-physical pion mass and a fixed lattice spacing, we present a comparison of our estimate for the valence transversity PDF with the recent global fit results based on single transverse spin asymmetry. We find the intrinsic nucleon sea to be isospin symmetric with respect to transversity.

Transverse single spin asymmetries of forward neutrons in p+p , p+Al , and p+Au collisions at sNN=200 GeV as a function of transverse and longitudinal momenta

In 2015 the PHENIX collaboration at the Relativistic Heavy Ion Collider recorded $p$$+$$p$, $p$$+$Al, and $p$$+$Au collision data at center of mass energies of $\sqrt{s_{_{NN}}}=200$ GeV with the proton beam(s) transversely polarized. At very forward rapidities $\eta>6.8$ relative to the polarized proton beam, neutrons were detected either inclusively or in (anti)correlation with detector activity related to hard collisions. The resulting single spin asymmetries, that were previously reported, have now been extracted as a function of the transverse momentum of the neutron as well as its longitudinal momentum fraction $x_F$. The explicit kinematic dependence, combined with the correlation information allows for a closer look at the interplay of different mechanisms suggested to describe these asymmetries, such as hadronic interactions or electromagnetic interactions in ultra-peripheral collisions, UPC. Events that are correlated with a hard collision indeed display a mostly negative asymmetry that increases in magnitude as a function of transverse momentum with only little dependence on $x_F$. In contrast, events that are not likely to have emerged from a hard collision display positive asymmetries for the nuclear collisions with a kinematic dependence that resembles that of a UPC based model. Because the UPC interaction depends strongly on the charge of the nucleus, those effects are very small for $p$$+$$p$ collisions, moderate for $p$$+$Al collisions, and large for $p$$+$Au collisions.

Multidimensional, High Precision Measurements of Beam Single Spin Asymmetries in Semi-inclusive π^{+} Electroproduction off Protons in the Valence Region.

High precision measurements of the polarized electron beam-spin asymmetry in semi-inclusive deep inelastic scattering (SIDIS) from the proton have been performed using a 10.6GeV incident electron beam and the CLAS12 spectrometer at Jefferson Lab. We report here a high precision multidimensional study of single π^{+} SIDIS data over a large kinematic range in Bjorken x, fractional energy, and transverse momentum of the hadron as well as photon virtualities Q^{2} ranging from 1-7 GeV^{2}. In particular, the structure function ratio F_{LU}^{sinϕ}/F_{UU} has been determined, where F_{LU}^{sinϕ} is a twist-3 quantity that can reveal novel aspects of emergent hadron mass and quark-gluon correlations within the nucleon. The data's impact on the evolving understanding of the underlying reaction mechanisms and their kinematic variation is explored using theoretical models for the different contributing transverse momentum dependent parton distribution functions.

Transverse-single-spin asymmetries of charged pions at midrapidity in transversely polarized p+p collisions at s=200 GeV

In 2015, the PHENIX collaboration has measured single-spin asymmetries for charged pions in transversely polarized proton-proton collisions at the center of mass energy of $\sqrt{s}=200$ GeV. The pions were detected at central rapidities of $|\eta|<0.35$. The single-spin asymmetries are consistent with zero for each charge individually, as well as consistent with the previously published neutral-pion asymmetries in the same rapidity range. However, they show a slight indication of charge-dependent differences which may suggest a flavor dependence in the underlying mechanisms that create these asymmetries.