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Recent results of searches for production of isolated leptons in events w ith large missing transverse momentum in e±p collisions at HERA are reviewed. The H1 collaboration observes an exce ss of these events over the, W-production dominated, Standard Model (SM) predictions in e+ p collisions, this excess is not confirmed by ZEUS. In e−p collisions the rate of these events observed by the H1 collaboration is consistent with th e Standard Model predictions. A potential new-physics source of isolated leptons in events with large missing transve rse momentum at HERA is single top production via flavour changing neutral currents (FCNC). The most rece nt results of searches for this process from the H1 collaboration are presented including limits on the FCNC couplings. The HERA limits remain the strongest on photonic FCNC. International Europhysics Conference on High Energy Physics 2005 Lisboa, Portugal Table 1: The numbers of observed events in different PX T ranges in the H1 isolated high pT lepton search compared to the SM expectations. The fraction of singleW production included in the expectation is indicated in pare nth ses. The statistical and systematic uncertainties, added in quadra ture, are also indicated. Electron Muon Combined H1 Preliminary obs./exp. ( W ) obs./exp. ( W ) obs./exp. ( W ) 1994-2004 Full Sample 19/14.6±2.0(70%) 9/3.9±0.6(84%) 28/18.5±2.7(73%) 158 pb−1 e+ p pT > 25 GeV 9/2.3±0.4(80%) 6/2.3±0.4(84%) 15/4.6±0.8(82%) 1998-2005 Full Sample 6/5.8±0.9(62%) 0/1.5±0.5(76%) 6/7.3±1.4(65%) 53 pb−1 e−p pT > 25 GeV 2/0.9±0.2(71%) 0/0.9±0.2(73%) 2/1.8±0.3(72%) 1994-2005 Full Sample 25/20.4±2.9(68%) 9/5.4±1.1(82%) 34/25.7±4.0(71%) 211 pb−1 e±p pT > 25 GeV 11/3.2±0.6(77%) 6/3.2±0.5(81%) 17/6.4±1.1(79%) Table 2: The numbers of observed events in the ZEUS isolated l epton search, compared to the SM expectations. The fraction of singleW production included in the expectation is indicated in pare nth ses. The statistical and systematic uncertainties added in quadrature are also indicated. Isolatede candidates 12< PX T < 25 GeV P X T > 25 GeV ZEUS (prel.) 99-00 (66 pb −1) 1/1.04±0.11(57%) 1/0.92±0.09(79%) ZEUS (prel.) 03-04 (40 pb −1) 0/0.46±0.10(64%) 0/0.58+0.08 −0.09(76%) 1 Isolated leptons in events with missing transverse momentum The observation of anomalous events with an isolated high en ergy lepton and missing transverse momentum in ep collisions at HERA was reported by the H1 collaboration at the end o f the 1994-2000 running period [1]. Production of single W bosons with subsequent leptonic decay is the only process wi th a measurable cross section at HERA which leads to this type of events. The dominant leading order Feynman diag ram for W production is shown in Fig.1(a); the total cross section is about 1 pb, evaluated including O(ααs) QCD corrections [2]. New results in searches for isolated electrons and muons fro m H1 for the 1994-2005 running period are summarised in Table 1 [3]. An excess of observed data over the SM expectat ions can be seen in the H1 results, this is illustrated in Fig.1(b). This excess is actually observed only in the e+ p data (see Table 1), it is not confirmed in a similar search by th e ZEUS collaboration [4], which is summarised in Table 2. The e xcess over the SM prediction at high values of hadronic PT is suggestive of a heavy particle decay process. More specia lis d searches have been applied to look for high pT leptons arising from the leptonic decay channel of single-top produ ction, as discussed in section 2. A slight excess in theτ channel was reported by ZEUS for 130 pb −1 of data from the 1994-2000 running period [5]. The results of the latest H1 search for isolated τ leptons using 108 pb −1 of data from the 1996-2000 running period [6] are given in Table 3. No excess over the SM is observed.

Isolated Leptons with missing transverse momentum and Search for single top production