Isolated electrons and muons in events with missing transverse momentum at HERA

TL;DR

This paper reports a search for high-energy isolated electrons or muons with missing transverse momentum at HERA, finding overall agreement with the Standard Model but noting an excess of high transverse momentum events.

Contribution

It provides the first detailed analysis of isolated leptons with missing momentum at HERA, including measurements of cross sections and an observation of excess events at high transverse momentum.

Findings

Overall event counts agree with Standard Model predictions.

An excess of events with transverse momentum >25 GeV was observed.

Cross sections for isolated lepton events were measured.

Abstract

A search for events with a high energy isolated electron or muon and missing transverse momentum has been performed at the electron-proton collider HERA using an integrated luminosity of 13.6 pb-1 in e-p scattering and 104.7 pb-1 in e+p scattering. Within the Standard Model such events are expected to be mainly due to W boson production with subsequent leptonic decay. In e-p interactions one event is observed in the electron channel and none in the muon channel, consistent with the expectation of the Standard Model. In the e+p data a total of 18 events are seen in the electron and muon channels compared to an expectation of 12.4 \pm 1.7 dominated by W production (9.4 \pm 1.6). Whilst the overall observed number of events is broadly in agreement with the number predicted by the Standard Model, there is an excess of events with transverse momentum of the hadronic system greater than 25 GeV with 10 events found compared to 2.9 \pm 0.5 expected. The results are used to determine the cross section for events with an isolated electron or muon and missing transverse momentum.