Study of Neutral Current DIS events at very high x and Q2 with the ZEUS detector at HERA. Observation of Events at Very High Q2 in ep Collisions at HERA with the H1 detector

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by Ulrich KATZ / UniversitŽ de Bonn Study of Neutral Current DIS events at very high x and Q2 with the ZEUS detector at HERA. Using 20.1 pb-1 of e+p data collected with the ZEUS detector at HERA, we have studied deep- inelastic scattering e+p® e+x for Q2>5000GeV2. Two events are observed for Q2>35000GeV2, where 0.145 ± 0.013 events are expected (Poisson probability 0.0096). Four events are observed for x>0.55 and y>0.25, where 0.91 ± 0.08 events are expected (Poisson probability 0.014). The selected five events at high x and high Q2 are particularly interesting because they occur in an unexplored kinematic region. and Yves SIROIS / Ecole Polytechnique, Palaiseau, CNRS-IN2P3 Observation of Events at Very High Q2 in ep Collisions at HERA with the H1 detector. Recent H1 Measurements of ep scattering with Q2 up to 35000 GeV2 are compared with the expectation of the standard deep-inelastic model of lepton-nucleon scattering (DIS). For Q2 > 15000 GeV2, Nobs = 12 neutral current candidate events are observed where the expectation is NDIS = 4.71 ± 0.76 events. In the same Q2 range, Nobs = 4 charged current candidates are observed where the expectation is Ndis = 1.77 ± 0.87 events. The probability P (N ³ Nobs) that the DIS model signal N fluctuates to N ³ Nobs in a random set of experiments is 6 x 10-3 for neutral current and 0.14 for charged current. The difference in the observed and expected number of Neutral Current events is mostly due to events at large masses M = Ö xs in which the positron is backscattered at large y = Q2/M2. In a mass window of 25 GeV width with a central value of 200 GeV and for y > 0.4, 7 events are observed where 0.95 ± 0.18 events are expected. For this and other choices of mass windows and y thresholds, the probability to observe an excess as large as the measured one anywhere in the mass range investigated is of order 1.

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