Measurement of Nucleon Structure Function in Muon Scattering at Highq2

Abstract

The nucleon structure function, F 2 , has been measured up to q 2x 120 (GeV/c) 2 and for 40 < W 2 < 300 GeV 2 . The data exhibit a significant pattern of scaling violation. Compared to lower-energy data, F 2 shows an observable increase of ~15% at high q 2 for x< 0.4. The pattern of the increase may accommodate a threshold in W or new theoretical parametrizations.Previous experiments 1 " 3 on deep-inelastic muon and electron scattering have established significant deviations from Bjorken scaling 4 up to q 2 = 50 (GeV/c) 2 . The nucleon structure function, Fz( x 9 tf 2 )> can n0 l°ng er be expressed as a function of a single scaling variable, say x (= q 2 /ZMv, where cp is the square of the four-momentum transfer of the scattered muon, M is the nucleon mass, and v = E 0 -E r is the difference in energy of incident and scattered JLA). Theoretical interpretations 5 of nonscaling behavior include field-theoretic arguments, composite constituents of the nucleon, gauge theory of strong interactions with color degree of freedom [quantum chromodynamics (QCD)], and new hadronic degrees of freedom in deep inelastic processes.The present experiment, carried out at Fermilab, increases significantly the statistical certainty and the kinematical range over which deepinelastic muon scattering {i±N-+\±X) has thus far been explored. For 2 x 10 10 incident muons with an energy of 270 GeV (M + and JU"), 10 6 deep inelastic events above # 2 = 5 (GeV/c) 2 are recorded,, Results from a large fraction of the [i + exposure are reported., The target consisted of a 7.4-mlong (4260-g/cm 2 ) iron-scintillator calorimeter which also measured the final-state hadron energy. Following the target was a 745-g/cm 2 -thick steel hadron shield and a spectrometer 6 consisting of eight toroidal magnets (4973 g/cm 2 thick and about 90 cm in radius). Both the hadron shield and spectrometer were interleaved with wire spark chambers. In addition, three vertical and horizontal trigger bands of scintillation counters were positioned within the spectrometer. Three scintillation counters (15.9 cm radius) centered on the beam axis formed a veto which eliminated events with a penetrating particle at a small angle. Proportional chambers and beam halo veto (scintillation) counters defined the incident muon.The momentum of the scattered muon is determined from its trajectory through the magnetic spectrometer. Track-finding efficiencies vary from 80% at low q 2 to 90% at high q 2 (<r 50 GeV 2 / c 2 ). The resolution of the spectrometer was about 9%, with its central value calibrated to about 0.7%. The scattering angle is known to 0.4 rnrad. The energy of the incident muon is determined to within 0.75%. The apparatus acceptance was relatively large (>50%) for events having a scattered muon energy E f > 50 GeV and an angle 20<#<80 mrad. Figure 1 shows curves of equal apparatus acceptance in the qp-v plane. Also shown are lines of constant W 2 (= 2Mv ~(f + M 2 ), the square of the center-of-mass energy of the virtual-photon-nucleon system. To reduce acceptance uncerta...

Experimental Evidence on QCD

“…Scale-breaking effects have also been observed in other muon scattering experiments at FNAL and CERN (Ball et al 1979, Bollini et al 1980. In particular, the Bolog n a�CERN-Dubna-Miinchen-Saclay collabora tion (Bollini et al 1980) has collected data with very high statistics at large Q 2 ; a preliminary analysis has shown only a very weak Q 2 depen dence of F2• The EMC group in addition to fl-Fe scattering has also measured liP scattering at high Q 2 , with preliminary results on A being identical to those obtained with the Fe target (Aubert et al 1980).…”