Short Introduction to
  High Q² Events of H1  

Scientific Potential of HERA

The electron proton collider HERA at DESY, Hamburg, Germany offers the unique possibility to probe the proton at very small distances of about 10^-16 cm via the exchange of highly virtual gauge bosons. It moreover allows a search for resonant production of new particles which couple to lepton-parton pairs with masses up to the kinematic limit of about 300 GeV.

The H1 Analysis

The H1 experiment at HERA has now completed an analysis in a new (i.e. yet unexplored) kinematic domain corresponding to high momentum transfer squared Q² or large masses M available in the hard interaction. The analysis profits from an integrated luminosity of 14.19 pb^-1, an increase by a factor 5 compared to that used in previously published data.

The complete results were published on the 24 of February 1997 in a paper "Oberservation of Events at very High Q² in ep Collisions at HERA" (DESY red report 97-024). Meanwhile the paper has been accepted for publication in Zeitschrift fü Physik C. The results were first presented in public in a special seminar during then 42nd meeting of the Physics Research Committee at DESY.

Comparison with DIS

At low Q² or M and y, the data is found to be in agreement with standard model expectation for Deep Inelastic Scattering (DIS), At high Q² or M and y, and excess of events is observed. Already in 1994, the H1 Collaboration had observed events in the region of Q² > 15000 GeV² and M > 180 GeV. With the increased statistics, H1 observes N_obs =12 neutral current candidate events for Q² > 15000 GeV² where the expectation is N_DIS = 4.71 +/- 0.76 events. In the same Q² range, N_obs =4 charged current candidates are observed where the expectation is N_DIS = 1.77 +/- 0.87 events. The probability P(N>=N_DIS) that the DIS model signal N fluctuates to N>=N_DIS in a random set of experiments is 0.006 for neutral current and 0.14 for charged current.

The H1 analysis in the mass (M) and y plane shows that the difference in the observed and expected number of neutral current events is mostly due to events at large masses M = sqrt(xs) in which the positron is backscattered at large y = Q²/M². For the H1 analysis, 7 events are observed where 0.95 +/- 0.18 events are expected in a mass window of 25 GeV width with a central value of 200 GeV and for y>0.4. Individually, the events appear to have properties of standard DIS events.

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 the order of 1%.

The theoretical expectation from Standard Model DIS processes is known with confidence and there appears to be no known mechanism which would lead to an enhancement of the cross-section at high Q² or high M and y as observed. Within the standard DIS model the only explanation of the observed excess is a statistical fluctuation.

A similar measurement was performed for a Neutral Current sample of events by the ZEUS experiment at HERA which also observes an excess of events at high Q^2 and high M ( DESY-97-025, acceped for publ. in Z. Phys. C).

Beyond the Standard Model ?

Beyond the Standard Model, the excess at high Q² and/or high M and y could be explained by the resonant production of new particles in the positron-parton system. Prominent examples for new particles with couplings to positron-parton pairs are leptoquarks, leptogluons and squarks in R-parity violating versions of Supersymmetry. An enhancement of the cross-section at high Q² could also possibly be induced by contact interactions originating from a substructure of fermions.

More data will be needed to decide whether the deviations are due to statistical fluctuations or due to new physics. The next period of data taking will start beginning of march 1997.

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