Background information to H1 physics topics :

HERA Events with Leptons and Missing P_T

Within the standard Charged Current analysis (the main crititerium is a calorimetrically imbalanced event, P_T_calo > 25 GeV) the H1 Collaboration found in autumn 1994 an event MUON-1 with an unexpected topology. The event consists of a high P_T hadronic shower and a penetrating muon of high P_T in phi nearly opposite to the shower. The overall event was found to be imbalanced in P_T and also E - P_Z. The event shows a very small transverse lepton-neutrino mass, which makes the interpretation of a W decaying leptonically unlikely. A detailed discussion can be found in the DESY red report DESY-94-248.

Still in 1994 a second event ELECTRON-1 with an isolated e- was found in the same Charged Current analysis of the e+ p data. This event shows all characteristics expected for W production and decay.

Whereas no more events showed up in the data taking periods of 1995 and 1996, the high luminosity accumulated in 1997 resulted in 4 more events all showing an isolated muon, a hadronic jet and significant overall missing P_T. Event MUON-2 and MUON-4 show both a hadronic jet of very high P_T (> 60 GeV) very unlikely for SM - W - production. On the other hand events MUON-3 and MUON-5 show jet transverse momenta and transverse masses as expected for the W - process. Event MUON-3 is especially interesting since it exhibits an additional e+ which allows to constrain the kinematics resulting in a muon-neutrino mass in perfect agreement with the W mass. First preliminary results were presented at the 1997 EPS conference in Jerusalem. The final results for the 94 - 97 e+ p running (37 pb-1) have been published in 1998.

In 1998 and 1999 Hera collided e- with protons. In this data taking period (14 pb-1) no events with leptons and missing P_T have been observed applying the standard selection. However one interesting event showed up in a period where not yet all tracking chambers were operational. The results have been submitted to the EPS conference 1999 in Tampere.

What about ZEUS findings in this area ?
Applying similar selection criteria as in the H1 analysis ZEUS finds in the 94-97 e+ p data sample (46 pb-1) three events. All three events show an e+ , two with a hadronic P_T around 20 GeV and the third with very small P_T_had. The overall rate and event characteristics is found to be compatible with the expectation for SM W-production and decay. The ZEUS results have been published in 1999.

There have been various attempts to search for interpretations of the 'untypical' H1 events beyond the SM :

FCNC, FCNC-Leptoquarks : The kinematics of the events is incompatible with two-body-kinematics.
Single TOP production :
--- Harald Fritzsch, Dirk Holtmannspoetter : The Production of Single t-Quarks at LEP and HERA
Such a mechanism would naturally explain the high recoil P_T of the W (t--> b W). The two events with the highest P_T (MUON2 and MUON4) both show an invariant W-jet mass compatible with the top-mass.
Production of STOP :
---T. Kon et al : Is a high $P_T$ muon of the $e^+p \to \mu^+ X$ event observed at HERA a signature of the stop?
---T. Kon et al : Possible Excess in Charged Current Events with High-Q^2 at HERA from Stop and Sbottom Production
The scenario e d --> stop --> W sbottom (sbottom -->d neutrino) has been investigated by H1 for M_stop = 205, M_sbottom = 100 GeV. The observed kinematics of the muon-events can be accomodated in such a scenario (W --> mu nu). However the rate found in the corresponding analysis for the W--> jet jet channel does not favour such an interpretation.

This physics topic has been presented in various conference talks and proceedings :

J. Meyer : Proceedings of Ringberg workshop 1999 : New Trends in HERA Physics 1999.
D. Boscherini : EPS-99 Conference Tampere (transparencies)
G. Cozzika: Proceedings of TOPICAL CONFERENCE ON HADRON COLLIDER PHYSICS, 1999, Mumbai, India

Last updated on Oct. 15, 1999
Author of this summary: Joachim Meyer