Laser-plasma accelerator group

Abstract:

We present measurements of the structure function F2 in e⁺p scattering at HERA in the range 3.5 GeV² < Q² < 5000 GeV². A new reconstruction method has allowed a significant improvement in the resolution of the kinematic variables and an extension of the kinematic region covered by the experiment. At Q² < 35 GeV² the range in x now spans 6.3 · 10^-5 < x < 0.08 providing overlap with measurements from fixed target experiments. At values of Q² above 1000 GeV² the x range extends to 0.5. Systematic errors below 5% have been achieved for most of the kinematic region. The structure function rises as x decreases; the rise becomes more pronounced as Q² increases. The behaviour of the structure function data is well described by next-to-leading order perturbative QCD as implemented in the DGLAP evolution equations. © Springer-Verlag 1996.

Abstract:

Jet photoproduction, where the two highest transverse energy (ETjet) jets have ETjet above 6 GeV and a jet-jet invariant mass above 23 GeV, has been studied with the ZEUS detector at the HERA ep collider. Resolved and direct photoproduction samples have been separated. The cross section as a function of the angle between the jet-jet axis and the beam direction in the dijet rest frame has been measured for the two samples. The measured angular distributions differ markedly from each other. They agree with the predictions of QCD calculations, where the different angular distributions reflect the different spins of the quark and gluon exchanged in the hard subprocess.

Abstract:

In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe, with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of Bjorken x and Q² in the range 3 · 10^-4 < xBJ < 6 · 10^-3 and 10 < Q² < 100 GeV².

Abstract:

The cross sections for the deep inelastic charged-current interactions e+p → ν̄X and e-p → νX have been measured in the kinematic region x > 0.006 and Q2 > 200GeV2, using the e+p and e-p data collected with the ZEUS detector during 1993 and 1994. For the integrated cross sections, σe+→ν̄X(Q2>200 GeV2) = 30.3- 4.2+ 5.5 [stat] -2.6+1.6 [syst] pb σe-p→νX(Q2>200 GeV2) = 54.7-9.8+15.9 [stat] -3.4+2.8 [syst] pb are obtained, in good agreement with the Standard Model expectations of 32.3pb and 65.8pb. The differential cross sections have been measured as functions of Q2, x and y. The differences between e+p and e-p scattering, which are expected from the V-A helicity structure of the weak interaction and from the quark content of the proton, are clearly observed. No significant deviation from the Standard Model predictions is found. The measured Q2 dependence of the differential cross sections for e+ and c- induced CC DIS reactions is used to determine the W mass. The measured value of MW is 79 -7 -4+8 +4 GeV, in good agreement with direct measurements performed at pp̄ colliders. A jet analysis has been performed and CC events with multiple jets have been observed. The distributions of the jet transverse momentum and rapidity as well as transverse energy flow jet profiles have been measured and are compared to the script O sign(αs) predictions of the ARIADNE and MEPS Monte Carlo models. Good agreement is observed in both the jet profiles and the jet rates. A search for charged-current events with a large rapidity gap between the observed hadronic system and the outgoing proton system yielded one candidate event in the e+p sample, corresponding to a cross section of σe+p→ν̄X(Q2 > 200 GeV2; ηmax < 2.5) = 0.8 -0.7+1.8 [stat] ±0.1 [syst] pb. © Springer-Verlag 1996.