Leading and next-to-leading order gluon polarization in the nucleon and longitudinal double spin asymmetries from open charm muoproduction

Abstract

The gluon polarization in the nucleon was measured using open charm production by scattering 160 GeV=c polarized muons off longitudinally polarized protons or deuterons. transverse momentum and energy. At leading order QCD accuracy, the average gluon polarization is determined as hÁg=gi LO ¼ À0:06 AE 0:21ðstat:Þ AE 0:08ðsyst:Þ at the scale h 2 i % 13 ðGeV=cÞ 2 and an average gluon momentum fraction hxi % 0:11. The average gluon polarization is also obtained at next-to-leading order QCD accuracy as hÁg=gi NLO ¼ À0:13 AE 0:15ðstat:Þ AE 0:15ðsyst:Þ at the scale h 2 i % 13 ðGeV=cÞ 2 and hxi % 0:20.

Proton structure, its spin and tensor gluons

“…This value is unexpectedly small, compared with the naive expectation of ΔΣ S U (6) 1 and of the Ellis-Jaffe sum rule ΔΣ EJ 0.65 [13]. By now it is experimentally well established that indeed the matrix element a 0 of the flavour-singlet axial-vector current is small and only of the order of 0.2 − 0.3.…”

“…This corresponds to a scaling contribution α(Q 2 )ΔG(Q 2 ) = const to the first moment I ep,en (6) of g 1 (see formulas (41) and (42)). This suggestion sparked a program to measure ΔG which finally provides the first evidence for nonzero gluon polarisation in the proton [6,12,15].…”

“…One of its most significant manifestations is the so-called spin crisis, or spin puzzle, related to the surprisingly small fraction of proton polarisation carried by the quarks [3][4][5][6][7][8][9][10]. The contribution from the quarks spin is found out to be small, approximately 25% of the total proton spin, and it is expected that the rest should come from the spin of gluons, quark sea polarisation and the orbital angular momentum of quarks and gluons.…”

“…One of the most interesting suggestions to come out of the spin crisis was that gluon polarisation G may contribute significantly to the nucleon spin [20][21][22][23]. The amount of gluon polarisation from the experiment is approximately 20% [6,11,12,15,16] and it is not by itself sufficient to resolve the problem. It remains to identify the rest (50% ) of the nucleon spin.…”

Deep Inelastic Scattering with the SPS Muon Beam

“…Later results from open charm 64 and events with high-p T hadron pairs at Q 2 > 1 GeV 2 followed. 65 The LO results of such determinations are summarised in Fig.…”