Isolated Leptons and Missing at HERA
The search for events containing isolated leptons (electrons or muons) and missing transverse momentum produced in collisions is performed individually and in a common phase space with the H1 and ZEUS detectors at HERA in the period 1994–2007. The analysed H1+ZEUS data sample corresponds to an integrated luminosity of 0.97 fb, and composes of the complete high energy data from the HERA programme. A total of 87 events are observed in the data, compared to a Standard Model (SM) prediction of 92.7 11.2. At large hadronic transverse momentum 25 GeV in the data, luminosity 0.58 fb, 23 data events are observed compared to a SM prediction of 14.6 1.9. Production cross section measurements of events containing isolated leptons and missing transverse momentum and single production, as well as polarisation fractions performed by the H1 Collaboration are also presented. The H1 isolated lepton events are also examined in the context of a search for single top production.
Events containing a high isolated electron or muon and associated with missing transverse momentum have been observed at HERA [1, 2, 3, 4]. An excess of HERA I data events (1994–2000, mostly in collisions) compared to the SM prediction at large hadronic transverse momentum was reported by the H1 Collaboration , which was not confirmed by the ZEUS Collaboration, although the analysis was performed in a more restricted phase space .
The main SM contribution to the signal topology is the production
of real bosons via photoproduction with subsequent leptonic decay
(), where the hadronic
system is typically of low .
The equivalent charged current process
() also contributes to the
total signal rate, although only at a level of around 7%.
The production of bosons with subsequent decay to neutrinos
() results in a
further minor contribution
The event selection employed by the H1  and ZEUS  analyses are very similar and may be summarised as follows: The identified lepton should have high transverse momentum 10 GeV, be observed in the central region of the detector and be isolated with respect to jets and other tracks in the event. The event should also contain a large transverse momentum imbalance, 12 GeV. Further cuts are then applied, which are designed to reduce SM background, whilst preserving a high level of signal purity. Event quantities sensitive to the presence of high energy undetected particles in the event are employed such as the azimuthal balance of the event, the difference in azimuthal angle between the lepton and the hadronic system and the longitudinal momentum imbalance. To ensure that the two lepton channels are exclusive and may therefore be combined, electron events must contain no isolated muons.
2 Results from the H1 and ZEUS Analyses
Both H1 and ZEUS have recently performed the analysis of the electron and
3 A Combined H1 and ZEUS Analysis
A study of the selection efficiency for signal process using the event generator EPVEC  found the H1 and ZEUS analyses to be compatible in the kinematic region where they are directly comparable [9, 10]. The majority of the data events observed by H1 at 25 GeV are also found to fall into the region of overlap of the two analyses. Nevertheless, in order to coherently combine the results from the two experiments, a common phase space has been established. The common selection is based on the H1 event selection [2, 5], but over a more restricted lepton polar angle range of 15 120 , as employed in the ZEUS analysis . The signal expectation rates of the H1 and ZEUS analyses using the common selection are found to be comparable, taking into account the respective luminosities of the data sets and the signal processes included. More details on the combination of the H1 and ZEUS analyses can be found in .
The results of the combined H1+ZEUS analysis are summarised in table 1. The signal contribution, dominated by real production, is seen to dominate the total SM expectation in all data samples. At large hadronic transverse momentum 25 GeV a total of 29 events are observed in the H1+ZEUS data compared to a SM prediction of 25.3 3.2. In the data alone, 23 events are observed with 25 GeV compared to a SM prediction of 14.6 1.9, equivalent to an excess of data over the SM prediction of 1.8. Seventeen of the 23 data events are observed in the H1 data compared to a SM expectation of 7.1 0.9, equivalent to an excess of data over the SM prediction of 2.9. Figure 1 shows the transverse mass, and distributions of the H1+ZEUS data for the combined electron and muon channels.
4 Cross Sections and Polarisation Fractions
The H1 selection results described in section 2 are used to calculate production cross sections for events with an energetic isolated lepton and missing transverse momentum () and for single boson production (, for which the branching ratio for leptonic decay is taken into account . The results are shown below with statistical (stat) and systematic (sys) uncertainties compared to the SM, quoted with a theoretical systematic error (th.sys) of 15%.
|H1||HERA I+II Data||SM|
|0.24||0.05||0.05 (sys)||0.26||0.04 (th.sys)|
|1.23||0.25||0.22 (sys)||1.31||0.20 (th.sys)|
5 Search for Single Top Quark Production
A search for single top quark production at HERA is performed by H1
as an extension of the search for isolated lepton events,
using the full HERA I+II data [13, 14].
The investigated model considers anomalous production of top quarks in a
Flavour Changing Neutral Current process involving the coupling
A multivariate analysis is performed to discriminate top from SM
background (dominated by real production).
No evidence for single top production is observed.
An upper limit on the anomalous top production cross section of
pb is established at 95% CL.
The corresponding H1 limit on the coupling is
shown in figure 2 (right) and is currently the best limit compared to
those from other colliders
|+ events at||obs./exp.||obs./exp.||obs./exp.|
|HERA I+II||(Signal contribution)||(Signal contribution)||(Signal contribution)|
|1994-2007||Full Sample||39 / 41.3 5.0 (70%)||18 / 11.8 1.6 (85%)||57 / 53.1 6.4 (73%)|
|0.58 fb||GeV||12 / 7.4 1.0 (78%)||11 / 7.2 1.0 (85%)||23 / 14.6 1.9 (81%)|
|1998-2006||Full Sample||25 / 31.6 4.1 (63%)||5 / 8.0 1.1 (86%)||30 / 39.6 5.0 (68%)|
|0.39 fb||GeV||4 / 6.0 0.8 (67%)||2 / 4.8 0.7 (87%)||6 / 10.6 1.4 (76%)|
|1994-2007||Full Sample||64 / 72.9 8.9 (67%)||23 / 19.9 2.6 (85%)||87 / 92.7 11.2 (71%)|
|0.97 fb||GeV||16 / 13.3 1.7 (73%)||13 / 12.0 1.6 (86%)||29 / 25.3 3.2 (79%)|
- This process is not included in the present ZEUS analysis.
- The H1 Collaboration have also performed the analysis of the tau decay channel using the full HERA I+II data and the hadronic 1–prong tau decay mode . In this search, where the signal purity is much lower at around 14%, 20 events are observed in the data compared to a SM prediction of 19.5 3.2.
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