SpinQuest

SpinQuest
Name	SpinQuest
Location	Fermilab
Type	Fixed-target Drell–Yan
Status	Active
Start	2013
Beam	120 GeV proton beam
Detector	Spectrometer with tracking, calorimetry, muon identification

SpinQuest is a fixed-target particle-physics experiment at Fermilab designed to probe the transverse-spin structure of the nucleon using the Drell–Yan process. The experiment employs a 120 GeV proton beam extracted from the Main Injector striking transversely polarized targets to measure azimuthal asymmetries and spin-dependent parton distributions. SpinQuest builds on prior measurements from experiments such as COMPASS, HERMES, and RHIC while leveraging accelerator and detector infrastructure developed at Fermilab and collaborations with international laboratories.

Overview

SpinQuest investigates transverse-momentum-dependent and collinear parton distribution functions by observing lepton pairs produced via the Drell–Yan process in proton–nucleon collisions. The experiment focuses on flavor-separated effects in the sea-quark sector, using polarized targets to access phenomena related to the Sivers function, Boer–Mulders function, and transversity in the nucleon. The experimental program complements measurements from COMPASS at the CERN SPS, PHENIX and STAR at Brookhaven National Laboratory, and semi-inclusive deep-inelastic scattering results from HERMES at DESY.

History

SpinQuest evolved from proposals motivated by the discovery of large single-spin asymmetries in hadronic collisions and the theoretical development of transverse-momentum-dependent factorization. The impetus traces to theoretical work by groups associated with JLab and Brookhaven National Laboratory that emphasized Drell–Yan as a clean probe of sign-change predictions for the Sivers function. Early conceptual design borrowed detector components and expertise from the SeaQuest experiment at Fermilab, and formal approval and construction proceeded with funding rounds involving national agencies and university partners. Commissioning phases aligned with upgrades to the Main Injector and with polarized-target developments tested at facilities such as TRIUMF and Oxford University laboratories.

Experimental Setup and Methods

SpinQuest uses a high-intensity 120 GeV proton beam from the Main Injector directed onto transversely polarized targets composed of solid-state materials polarized via dynamic nuclear polarization methods developed in collaboration with groups from BNL and MIT. The spectrometer repurposes elements from the SeaQuest apparatus, including front-end tracking chambers, toroidal magnets, electromagnetic calorimeters, and muon detectors adapted for optimized acceptance of Drell–Yan dimuons. Data acquisition and trigger systems integrate hardware developed at FNAL and firmware contributions from university groups at University of Michigan, University of Illinois Urbana-Champaign, and Virginia Tech.

Experimental methods emphasize careful control of target polarization, spin-flip cycles, and luminosity monitoring using beam instrumentation interfaced with personnel from SLAC and CERN technical teams. Background suppression leverages Monte Carlo simulations using event generators benchmarked against results from CDF and DØ, while analysis benefits from theoretical calculations from collaborations with researchers at MIT, University of Massachusetts Amherst, and Jefferson Lab.

Physics Goals and Measurements

Primary goals include determining the sign and magnitude of the Sivers function for anti-up and anti-down sea quarks, measuring azimuthal modulations sensitive to the Boer–Mulders function, and constraining sea-quark transversity and tensor charges. These objectives test nonperturbative QCD frameworks developed by theorists associated with CERN Theory Division, Institute for Nuclear Theory, and groups led at INFN and IPN Orsay. Measurements aim to verify the predicted process-dependence (sign change) of the Sivers function between semi-inclusive deep-inelastic scattering and Drell–Yan, a fundamental prediction tied to gauge-link structures in QCD elaborated by authors at Stony Brook University and Regensburg University.

SpinQuest also seeks to quantify flavor asymmetries in the nucleon sea comparable to observations made by NMC and E866/NuSea, and to provide inputs for global fits performed by collaborations at CTEQ, NNPDF, and DSSV groups. Results bear on interpretations of anomalous transverse-single-spin effects reported by STAR and PHENIX and on the modeling of nucleon structure used in interpretations at LHCb and CMS.

Results and Findings

SpinQuest has reported measurements indicating nonzero transverse single-spin asymmetries in Drell–Yan muon-pair production, with sensitivity to sea-quark Sivers distributions and hints of flavor dependence consistent with some theoretical predictions. Comparisons with semi-inclusive deep-inelastic scattering results from HERMES and COMPASS provide tests of the Sivers sign-change hypothesis; preliminary data show trends that stimulate ongoing global analyses by groups at Jefferson Lab and EPFL. SpinQuest data have also constrained Boer–Mulders-related modulations and provided inputs for extraction of sea-quark transversity and tensor charges that feed into lattice-QCD comparisons from collaborations at RBC-UKQCD and MILC.

Analyses quantify systematic uncertainties associated with target polarization, acceptance corrections, and background subtraction by cross-checking with control measurements and simulation frameworks developed by teams at University of Washington and University of Pennsylvania.

Collaborations and Funding

The SpinQuest collaboration comprises universities and national laboratories across the United States and internationally, including groups from Fermilab, Argonne National Laboratory, Brookhaven National Laboratory, MIT, University of Michigan, Rutgers University, and European partners from INFN and DESY. Funding has been provided by agencies including the U.S. Department of Energy, the National Science Foundation, and partner national research councils in collaborating countries. Institutional contributions include detector components, polarized-target expertise, computing resources from NERSC, and theoretical support from institutes such as INT.

Impact and Future Directions

SpinQuest outcomes influence theoretical developments in transverse-momentum-dependent factorization and nonperturbative QCD, shaping future experimental programs at facilities like Jefferson Lab 12 GeV upgrade, proposed polarized Drell–Yan projects at CERN, and polarized-proton programs at Brookhaven National Laboratory. Planned analyses and potential detector upgrades aim to improve statistical precision, extend kinematic reach to lower Bjorken-x, and enable comparisons with lattice QCD and global-fit results from CTEQ and NNPDF. The experiment's legacy includes training of early-career scientists and transfer of polarized-target and detector technologies to future projects.

Category:Particle physics experiments