Determination of αs and the gluon distribution using direct photon production in and pp collisions

Abstract

Direct photon cross sections obtained in pp and pp collisions at p s = 2 4 : 3 GeV at the CERN SPS, are used in a next-to-leading order QCD analysis. From the cross-section dierence (p p ! X) (pp ! X), and quark distributions measured in deep inelastic scattering, a determination of the strong coupling constant, s , is performed via a measurement o f 4 MS. This rst measurement o f 4 MS using direct photon data yields a value 4 MS = 235 79 (stat:

A critical phenomenological study of inclusive photon production in hadronic collisions

“…Both sets of structure functions (CTEQ4M and MRS-98-2) lead essentially to the same conclusions: no agreement with R806 but satisfactory agreement with the other experiments. More precisely, the UA6pp data are in excellent agreement with both sets of structure functions, independently of the scale choice which makes them a very good channel to extract the value of Λ M S [35]. The UA6 pp data coincide very well with both sets of predictions based on the scale choice p T /3 while clearly the other experiments prefer the larger scale p T /2 or even a larger scale (alternatively, a lower Λ M S value would fit the WA70, R110 and R807 data very well as can be seen from fig.…”

Constraints on the proton's gluon distribution from prompt photon production

“…We then use the complete set of parton distributions obtained from this F p 2 fit for a fixed gluon shape to determine the χ 2 dir. γ for the prompt photon data of WA70, R806/7, UA2 and CDF [1,14,15,18], already discussed in section 3, supplemented by the pp inclusive results of NA24 [8] and UA6 [16], both at √ s = 24 GeV. Due to the very small point-to-point errors of the CDF data [18] their overall normalization uncertainty is rather important.…”

“…5 displays the results of three of our fits, again compared to the data of [1,14,15,18]. For clarity, the additional large-x T data sets of [8,16] are not shown in the figure.…”

“…Experimentally, much progress has been made during the last years. Some of the direct photon data sets mentioned above have been superseded by improved analyses [14,15,16], usually providing smaller statistical and systematic errors. Even more importantly, the accessible range of fractional gluon momenta x has been considerably enlarged by the recent, partly very precise measurements at the Fermilab Tevatron [17,18,19].…”

The Strong Coupling Constant

“…only the well known valence quark distribution is needed as external input for an α s -determination. The UA6 collaboration [50] finds Λ (4) MS = 235 ± 106(exp) +146 −9 (theo) MeV. Translated into α s at the typical scale µ = 4 GeV of the measurement one finds α s (µ) = 0.206 +0.042 −0.033 , which corresponds to α s (M Z ) = 0.112 +0.012 −0.010 .…”