[Paper Review] Spin Physics with COMPASS

Gerhard K. Mallot|ArXiv.org|Nov 20, 1996

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper proposes the COMPASS experiment at CERN to measure the gluon polarization in the proton with a precision of Δ(Δg/g) = 0.1. It employs open charm muoproduction to tag the photon-gluon fusion process, enabling indirect extraction of gluon spin contributions through high-energy muon scattering on a polarized nucleon target.

ABSTRACT

The recently proposed COMPASS experiment at CERN attempts a measurement of the gluon polarisation with a precision of delta(Delta g/g) = 0.1. The experiment uses open charm muoproduction to tag the photon-gluon fusion process.

Motivation & Objective

• To measure the gluon polarization in the proton with high precision, targeting a resolution of Δ(Δg/g) = 0.1.
• To address the long-standing question of the proton spin crisis by determining the contribution of gluons to the proton's spin.
• To utilize the photon-gluon fusion process as a probe for gluon spin structure via high-energy muon scattering.
• To develop a method for tagging gluon polarization using open charm production in deep-inelastic scattering.
• To provide a phenomenologically robust framework for future spin physics experiments at high-energy colliders.

Proposed method

• Use high-energy muon beams to induce deep-inelastic scattering on a polarized nucleon target.
• Detect open charm final states (D*± mesons) to tag events where a virtual photon fused with a gluon.
• Apply kinematic reconstruction to isolate the photon-gluon fusion process from background processes.
• Utilize the high statistics and clean signature of charmed hadron production to enhance signal-to-background ratio.
• Apply theoretical modeling of the photon-gluon fusion cross section to extract the gluon polarization fraction.
• Leverage the high luminosity and polarized beam/target capabilities of the CERN SPS to achieve the required precision.

Experimental results

Research questions

• RQ1 Can the gluon polarization in the proton be measured with a precision of Δ(Δg/g) = 0.1 using open charm muoproduction?
• RQ2 To what extent does the photon-gluon fusion process in muon-charm production serve as a clean probe for gluon spin?
• RQ3 How can the signal from photon-gluon fusion be isolated and distinguished from other QCD processes in muon scattering?
• RQ4 What is the expected sensitivity of the COMPASS experiment to the gluon spin distribution at moderate x values?
• RQ5 Can the measured charm production rates be used to extract the polarized gluon distribution function Δg(x) with high confidence?

Key findings

• The COMPASS experiment is designed to achieve a precision of Δ(Δg/g) = 0.1 in measuring the gluon polarization in the proton.
• Open charm muoproduction provides a clean experimental signature for tagging the photon-gluon fusion process.
• The method relies on the high-mass, high-multiplicity final state of D*± mesons to isolate the signal from background processes.
• Theoretical modeling supports the feasibility of extracting Δg/g from the measured charm production asymmetries.
• The experiment is expected to provide critical data to resolve the proton spin puzzle by quantifying the gluon contribution.
• The proposed method offers a viable pathway to measure gluon polarization at moderate values of Bjorken-x, where other methods are less sensitive.