[Paper Review] Valence Quarks Polarization from COMPASS

A. Korzenev|arXiv (Cornell University)|Apr 26, 2007

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents the first leading-order evaluation of the polarized valence quark distribution Δuᵥ + Δdᵥ from COMPASS using semi-inclusive deep inelastic scattering (SIDIS) data with a 160 GeV polarized muon beam and a polarized ⁶LiD target. The analysis employs the difference asymmetry A^{h⁺−h⁻} to isolate valence quark polarization, yielding a first moment of 0.40 ± 0.07 (stat.) ± 0.05 (syst.) at Q² = 10 GeV², favoring an asymmetric light sea quark distribution with Δū = −Δd̄ over symmetric sea assumptions.

ABSTRACT

A first evaluation of the polarized valence quark distribution $Δu_v(x)+Δd_v(x)$ from the COMPASS experiment (CERN/SPS) is presented. The data were collected by COMPASS in the years 2002--2004 using a 160 GeV polarized muon beam scattered off a large polarized $^6$LiD target and cover the range $1< Q^2 < 100$ GeV$^2$ and $0.006

Motivation & Objective

• To extract the polarized valence quark distribution Δuᵥ + Δdᵥ from COMPASS SIDIS data using a novel asymmetry approach.
• To reduce theoretical uncertainties from fragmentation functions by employing the difference asymmetry A^{h⁺−h⁻}, which cancels leading-twist fragmentation function dependence in LO QCD.
• To determine the first moment of the valence quark polarization and assess the symmetry of the light antiquark sea distribution.
• To compare the COMPASS result with prior SMC and HERMES measurements to test the consistency of nucleon spin structure models.

Proposed method

• Utilizes the difference asymmetry A^{h⁺−h⁻} defined as the ratio of the difference in cross-sections for positive and negative hadrons in transverse and longitudinal beam-target polarization states.
• Applies the relation A^{h⁺−h⁻} = (1/(1−r)) × (A^{h⁺} − rA^{h⁻}) where r is the ratio of cross-sections for negative to positive hadrons, estimated from hadron yield ratios corrected for detector acceptance.
• Evolved the extracted Δuᵥ + Δdᵥ distribution to Q² = 10 GeV² using the LO DNS parametrization based on global QCD fits of DIS and SIDIS data.
• Corrected for deuteron D-state admixture and longitudinal structure function effects (R = σ_L/σ_T ≠ 0) via the factor (1 + R(x,Q²))(1 − 1.5ω_D) in the extraction formula.
• Used the MRST 2004 unpolarized parton distribution and the KKP fragmentation functions in the extraction process.
• Estimated systematic uncertainties from beam/target polarization, dilution, depolarization, and false asymmetry due to detector instabilities, with false asymmetry bounded at <0.5σ_stat.

Experimental results

Research questions

• RQ1 What is the first moment of the polarized valence quark distribution Δuᵥ + Δdᵥ in the kinematic range 0.006 < x < 0.7 and 1 < Q² < 100 GeV²?
• RQ2 How does the COMPASS result for Δuᵥ + Δdᵥ compare with previous measurements from SMC and HERMES in terms of magnitude and consistency?
• RQ3 Does the measured value of Δuᵥ + Δdᵥ support a symmetric sea quark distribution (Δū = Δd̄ = Δs = Δs̄) or an asymmetric one (Δū = −Δd̄)?
• RQ4 To what extent do radiative corrections and higher-order QCD effects influence the difference asymmetry A^{h⁺−h⁻}?
• RQ5 What is the contribution of the low-x region (x < 0.006) to the first moment of Δuᵥ + Δdᵥ, and can it be neglected?

Key findings

• The first moment of the polarized valence quark distribution Δuᵥ + Δdᵥ was measured as 0.40 ± 0.07 (stat.) ± 0.05 (syst.) at Q² = 10 GeV².
• This result is 2σ below the value expected for a flavor-symmetric sea quark distribution (Δū = Δd̄ = Δs = Δs̄), indicating a preference for an asymmetric sea.
• The value is consistent with the scenario where Δū = −Δd̄, as predicted by the relation in Eq. (4) when combined with axial charges a₀ and a₈.
• The integral of Δuᵥ + Δdᵥ is nearly constant at low x, with a negligible contribution (0.004) for x > 0.7, supporting the validity of extrapolation down to x = 0.006.
• The COMPASS result shows good agreement with the DNS LO parametrization and previous SMC and HERMES measurements, confirming consistency across experiments.
• Systematic uncertainties are dominated by polarization measurements and detector effects, with false asymmetry from instrumental instabilities estimated below half the statistical error.