Search for Physics Beyond the Standard Model


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Events with Isolated Leptons and Missing Transverse Momentum

    In the positron-proton data, 6 events were found with a hadronic final state and an isolated lepton which do not show the typical configuration of deep inelastic scattering processes. In 1 event an electron is found, the other 5 events have a muon.  
All six events show missing transverse momentum and therefore the signature of possible neutrino production. In the figure the transverse momentum P_t^X of the hadronic final state is shown depending on the transverse mass M_T^l-nu of the lepton-neutrino system. The measured events are shown together with the uncertainty in the two observables. 
    Also shown are standard model expectations which correspond to 500 times the measured integrated luminosity. The circles represent the predictions of W-boson production. For the muon decay channel also inelastic muon production from two-photon processes is shown (triangle symbols). A direct comparison of the measured events with the expectations from the different production mechanisms of the standard model shows that the electron event can be explained by W-boson production. However, for the five muon events only 0.8+-0.2 are expected. Two of the muon events are in a region of the phase space in which W-boson production is expected. The observables of the other 3 muon events are in a phase space region that is atypical for W-boson production. The possibilities to explain these events in the frame work of the standard model are therefore limited. 

High Q2 Events

    In the analysis of the data between 1994-96 an excess of neutral current events at very high Q2 was found. This excess was 3 standard deviations above the expectation of the standard model calculation. With the data of 1997 the total integrated luminosity was doubled. 
Also in the new data a tendency for more events at very high Q2 is found. The magnitude of the excess is not confirmed by 1997 data. 

Leptoquark Particles

    In lepton-proton collisions, a natural area of searches for physics beyond the Standard Model is the search for leptoquarks, that are bound states between a lepton and a quark. No sign for the production of such particles was found in the data. 
The figure shows limits for the production of leptoquarks from positron-d-quark and positrons-u-quark processes. The limits reach mass values of the leptoquarks up to about m_LQ = 250 GeV. The reduced strength of the limits at mass values slightly above m_LQ = 200 GeV reflects the excess events that were found in neutral current events at high Q2. At small values of the branching ratio beta(LQ -> positron quark) for the decay of the leptoquark, the H1 data are significantly more sensitive compared to the data of other accelerators. This demonstrates the discovery potential for new physics at HERA. 

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last updated by
H1 webmaster on 19/05/98