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Register a new userProbing nucleons spin structures with polarized Drell-Yan in the Fermilab SpinQuest experiment
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Although the proton was discovered about 100 years ago, its spin structure still remains a mystery. Recent studies suggest that the orbital angular momentum of sea quarks could significantly contribute to the protons spin. The SeaQuest experiment, which recently completed data collection, probed the unpolarized light quark sea distributions of the proton using the Drell-Yan process. Its successor, the SpinQuest (E1039), will access the $bar{u}$ and $bar{d}$ Sivers functions using polarized NH$_3$ and ND$_3$ targets. A non-zero Sivers asymmetry, observed in SpinQuest, would be a strong indication of non-zero sea-quark orbital angular momentum. The SpinQuest experiment can also probe the sea quarks transversity distribution, which is relevant for the determination of protons tensor charge. Recent study suggests that sea-quarks might contribute significantly to deuterons tensor polarized structure functions. This can be further probed in SpinQuest using tensor polarized ND$_3$ target. The current status and future plan of the experiment are presented.
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The Drell-Yan process provides important information on the internal structure of hadrons including transverse momentum dependent parton distribution functions (TMDs). In this work we present calculations for all leading twist structure functions describing the pion induced Drell-Yan process. The non-perturbative input for the TMDs is taken from the light-front constituent quark model, the spectator model, and available parametrizations of TMDs extracted from the experimental data. TMD evolution is implemented at Next-to-Leading Logarithmic precision for the first time for all asymmetries. Our results are compatible with the first experimental information, help to interpret the data from ongoing experiments, and will allow one to quantitatively assess the models in future when more precise data will become available.
I summarize recent results and discuss upcoming and planned experiments that attempt to elucidate how the structure of nucleons might be modified by nuclear binding.
We present a new analysis of the E772 and E866 experiments on the nuclear dependence of Drell-Yan (DY) lepton pair production resulting from the bombardment of $^2H$, Be, C, Ca, Fe, and W targets by 800 GeV/c protons at Fermilab. We employ a light-cone formulation of the DY reaction in the rest frame of the nucleus, where the dimuons detected at small values of Bjorken x_2 << 1 may be considered to originate from the decay of a heavy photon radiated from an incident quark in a bremsstrahlung process. We infer the energy loss of the quark by examining the suppression of the nuclear-dependent DY ratios seen as a function of projectile momentum fraction x_1 and dimuon mass M. Shadowing, which also leads to nuclear suppression of dimuons, is calculated within the same approach employing the results of phenomenological fits to deep inelastic scattering data from HERA. The analysis yields -dE/dz =2.73 +/- 0.37 +/- 0.5 GeV/fm for the rate of quark energy loss per unit path length, a value consistent with theoretical expectations including the effects of the inelastic interaction of the incident proton at the surface of the nucleus. This is the first observation of a nonzero energy loss effect in such experiments.
We present the full next-to-next-to-leading order (NNLO) corrections to the coefficient function for the polarized cross section $d Deltasigma/d Q$ of the Drell-Yan process. We study the effect of these corrections on the process $p+pto l^+l^-+`X$ at an C.M. energy $sqrt{S}=200 GeV$. All QCD partonic subprocesses have been included provided the lepton pair is created by a virtual photon, which is a valid approximation for a lepton pair invariant mass $Q<50 GeV$. For this reaction the dominant subprocess is given by $q+bar qto gamma^*+`X$ and its higher order corrections so that it provides us with an excellent tool to measure the polarized sea-quark densities.
We show that for Drell-Yan events by unpolarized hadronic projectiles and nuclear targets, azimuthal asymmetries can arise from the nuclear distortion of the hadronic projectile wave function, typically a spin-orbit effect occurring on the nuclear surface. The asymmetry depends on quantities that enter also the spin asymmetry in the corresponding Drell-Yan event on polarized free nucleonic targets. Hence, this study can be of help in exploring the spin structure of the nucleon, in particular the transverse spin distribution of partons inside the proton. All arguments can be extended also to antinucleon projectiles and, consequently, apply to possible future measurements involving nuclear targets at the foreseen HESR ring at GSI.
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