Events with high energy isolated electrons, muons or tau leptons and missing transverse momentum are studied using the full e^\pm p data sample collected by the H1 experiment at HERA, corresponding to an integrated luminosity of 474 pb^{-1}. Within the Standard Model, events with isolated leptons and missing transverse momentum mainly originate from the production of single W bosons. The total single W boson production cross section is measured as 1.14 \pm 0.25 (stat.) \pm 0.14 (sys.) pb, in agreement with the Standard Model expectation.
The data are also used to establish limits on the WWγgauge couplings and for a measurement of the W boson polarisation.
Recent results of searches for events containing isolated leptons and missing transverse momentum at HERA are presented. Searches in this final state have yielded notable excesses over Standard Model expectations in the past. Searches for isolated leptons in channels corresponding to all three generations of leptons over the full HERA running period are now available. The combined H1+ZEUS results for searches for electrons and muons are compatible with the Standard Model.
Isolated hard photons have been measured with jets in Deep Inelastic Scattering using the ZEUS detector at HERA. Preliminary results for the cross sections are presented.
A search for events with one or more isolated leptons in the final state is performed on a data sample collected in e+-p collisions with the H1 and ZEUS detectors at the HERA collider. The data sample corresponds to an integrated luminosity of ~1 fb-1, representing the full HERA high-energy data set. The yields of single, di- and tri-lepton events are measured and compared to the Standard Model predictions, looking for possible deviations. No significant discrepancy with respect to the Standard Model expectations is observed.
Events with isolated leptons and missing transverse momentum are searched for in electron- or positron-proton collisions at HERA by the H1 and ZEUS experiments. Such events may be explained within the Standard Model by W boson production, followed by leptonic decay. The analysis performed including HERA I and the recently collected data at HERA II results in clear evidence of W production. However, H1 collaboration observes an excess of events in e^+p collisions, not supported so far by ZEUS. If the difference between the H1 and ZEUS observations is attributed to a statistical fluctuation, the…
A search for events with an isolated high-energy lepton and large missing transverse momentum has been performed with the ZEUS detector at HERA using a total integrated luminosity of 504 $\mathrm{pb}^{-1}$. The results agree well with Standard Model predictions. The cross section for production of single $W$ bosons in electron-proton collisions with unpolarised electrons is measured to be $0.89^{+0.25}_{-0.22}$ (stat.) $\pm{0.10}$ (syst.) pb.
The photoproduction of isolated photons, both inclusive and together with a jet, has been measured with the ZEUS detector at HERA using an integrated luminosity of $374\, \mathrm{pb}^{-1}$. Differential cross sections are presented in the isolated-photon transverse-energy and pseudorapidity ranges $6 < E_T^γ< 15$ GeV and $-0.7 < η^γ< 0.9,$ and for jet transverse-energy and pseudorapidity ranges $4 < E_T^{jet} < 35$ GeV and $-1.5 < η^{jet} < 1.8,$ for exchanged-photon virtualities $Q^2 < 1$ GeV$^2$. Differential cross sections are also presented for inclusive isolated-photon production as funct…
The ZEUS detector at HERA has been used to measure the photoproduction of isolated photons in diffractive events. Cross sections are evaluated in the photon transverse-energy and pseudorapidity ranges 5 < E_T^gamma < 15 Gev and -0.7 < eta^gamma < 0.9, inclusively and with a jet with transverse-energy an pseudorapidity in the ranges 4 < ETjet < 35 GeV and -1.5 < etajet < 1.8, for an integrated luminosity of 374 pb^{-1}. Further kinematic variables studied include the fractions of the incoming photon energy and of the colourless exchange ("Pomeron") energy that are imparted to a photon-jet final…
The analysis of events with isolated leptons and missing transverse momentum in the H1 experiment is discussed for the electron, muon and tau channels. In the Standard Model (SM) framework, production of real W-bosons gives rise to such topologies. Contributions to the background are dominated by QCD processes. An excess of observed signal over background presents a chance of the discovery of new physics. The results using the HERA 1994-2006 data set corresponding to 341 pb^-1 are presented. While the e- p sample shows good agreement between data and SM expectation, in e+ p collisions an exces…
A search for events with an imbalance in transverse momentum and with isolated high energy leptons has been carried out at the positron-proton collider HERA. One event with an e^- and five events with a mu^+- are found together with evidence for undetected particles carrying transverse momentum. Within the Standard Model the dominant origin of events with this kind of topology is the production of W bosons with subsequent leptonic decay. Three of the six events are within measurement errors found in a region of phase space likely to be populated by this process, while the remaining events show…
The search for events containing isolated leptons (electrons or muons) and missing transverse momentum produced in e^{\pm}p 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^{-1}, 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 \pm 11.2. At large hadronic transverse momentum P_{T}^{X} > 25 GeV in the e^{+}p data…
Events containing an isolated prompt photon with high transverse energy, together with a balancing jet, have been observed for the first time in photoproduction at HERA.The data were taken with the ZEUS detector, in a $γp$ centre of mass energy range 120--250 GeV. The fraction of the incoming photon energy participating in the production of the prompt photon and the jet, $x_γ$, shows a strong peak near unity, consistent with LO QCD Monte Carlo predictions. In the transverse energy and pseudorapidity range $5\le \eTg < 10$ GeV, $-0.7 \le η^γ< 0.8$, $\eTj\ge 5$ GeV, and $-1.5\le η^{jet}\le 1.8$,…
The production of isolated photons in deep-inelastic scattering $ep\to e γX$ is measured with the H1 detector at HERA. The measurement is performed in the kinematic range of negative four-momentum transfer squared $4<Q^2<150 $~GeV$^2$ and a mass of the hadronic system $W_X>50$ GeV. The analysis is based on a total integrated luminosity of 227~pb$^{-1}$. The production cross section of isolatedphotons with a transverse energy in the range $3 < E_T^γ< 10$ GeV and pseudorapidity range $-1.2 < η^γ< 1.8$ is measured as a function of $E_T^γ$, $η^γ$ and $Q^2$. Isolated photon cross sections are also …
The photoproduction of isolated photons has been measured in diffractive events recorded by the ZEUS detector at HERA. Cross sections are evaluated in the photon transverse-energy and pseudorapidity ranges $5 < E_T < 15$ GeV and $-0.7 < η< 0.9,$ inclusively and also with a jet with transverse energy and pseudorapidity in the ranges $4 < E_{jet} < 35$ GeV and $-1.5 < η_{jet} < 1.8,$ using a total integrated electron-proton luminosity of 456 pb$^{-1}$. A number of kinematic variables were studied and compared to predictions from the Rapgap Monte Carlo model. An excess of data is observed above t…
A search for events containing isolated tau leptons and large missing transverse momentum, not originating from the tau decay, has been performed with the ZEUS detector at the electron-proton collider HERA, using 130 pb^-1 of integrated luminosity. A search was made for isolated tracks coming from hadronic tau decays. Observables based on the internal jet structure were exploited to discriminate between tau decays and quark- or gluon-induced jets. Three tau candidates were found, while 0.40 +0.12 -0.13 were expected from Standard Model processes, such as charged current deep inelastic scatteri…
A search for events containing an isolated electron or muon and missing transverse momentum produced in ep collisions is performed with the H1 and ZEUS detectors at HERA. The data were taken in the period 1994-2007 and correspond to an integrated luminosity of 0.98 fb^-1. The observed event yields are in good overall agreement with the Standard Model prediction, which is dominated by single W production. In the e+p data, at large hadronic transverse momentum PtX >25 GeV, a total of 23 events are observed compared to a prediction of 14.0 \pm 1.9. The total single W boson production cross sectio…
The search for events containing isolated leptons (electrons or muons) and missing transverse momentum produced in $e^{\pm}p$ collisions is performed with the H1 detector at HERA in the period 1994--2005. The analysed data sample corresponds to an integrated luminosity of 279 pb$^{-1}$, which includes 53 pb$^{-1}$ of $e^{+}p$ data and 107 pb$^{-1}$ of $e^{-}p$ data from the new HERA~II phase. A total of 40 events are observed in the data, compared to a Standard Model (SM) prediction of 34.3 $\pm$ 4.8. At large hadronic transverse momentum $P_{T}^{X} >$ 25 GeV, a total of 17 events are observed…
The photoproduction of isolated photons has been measured in diffractive events recorded by the ZEUS detector at HERA. Cross sections are evaluated in the photon-proton transverse-energy and pseudorapidity ranges 5<ET(gamma)<15 GeV and -0.7<eta(gamma)<0.9 inclusively and also with a jet with transverse energy and pseudorapidity in the range 4<ET(jet)<35 GeV and -1.5<eta(jet)<1.8, using a total integrated electron-proton luminosity of 374 pb-1. A number of kinematic variables were studied and compared to predictions from the RAPGAP Monte Carlo model. An excess of data is observed above the RAPG…
Measurements of the production of events containing isolated high energy leptons (electrons and muons) produced in ep collisions have been performed with the H1 and ZEUS detectors at HERA, using data collected in the period 1994-2007. Topologies with more than one charged lepton or with a charged lepton in coincidence with missing transverse momentum are analysed. The H1 and ZEUS data, corresponding to an integrated luminosity of about 0.5 fb^-1 per experiment, are combined in a common phase space. The observed event yields are compared to the predictions from the Standard Model. In general a …
A search for gravitinos produced in ep collisions is performed using the H1 detector at HERA. The data were taken at a centre-of-mass energy of 319 GeV and correspond to an integrated luminosity of 64.3 pb^{-1} for e^+p collisions and 13.5 pb^{-1} for e^-p collisions. If R-parity is not conserved, the t-channel exchange of a selectron can produce a neutralino, which, in models where the gravitino is the lightest supersymmetric particle, subsequently decays into a photon and a light gravitino. The resulting event signature, which involves an isolated photon, a jet and missing transverse energy,…
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 channel…
A search for the leptonic decays of W bosons produced in the reaction e^+ p\to e^+ W^\pm X at a centre-of-mass energy of 300 GeV has been performed with the ZEUS detector at HERA using an integrated luminosity of 47.7 pb^-1 . Three events consistent with W\to eνdecay are found, giving a cross section of 0.9 +1.0 -0.7 \pm 0.2 pb, in good agreement with the Standard Model prediction. The corresponding 95% C.L. upper limit on the cross section is 3.3 pb. A search for the decay W\to μνhas a smaller selection efficiency and yields no candidate events. Limits on the cross section for W production wi…
Results of a general search for new phenomena at high transverse momentum and of a dedicated search for events with isolated leptons and missing transverse momentum are reported. These searches were performed on a data sample collected at HERA in e+-p collisions with the H1 detector in the period 1994-2004. The results obtained in the isolated lepton search are compared with those obtained by the ZEUS collaboration in the period 1994-2000 and limits on the anomalous FCNC production of single top events are presented.
The search for events containing isolated leptons and missing transverse momentum produced in ep collisions is performed individually and in a common phase space with the H1 and ZEUS detectors at HERA. The combined H1 and ZEUS data sample corresponds to an integrated luminosity of 0.98 fb^-1 and comprises the complete high energy data from the HERA programme. A total of 81 events are observed in the data, compared to a Standard Model prediction of 87.8 \pm 11.0, which is dominated by single W production. At large hadronic transverse momentum P_T^X > 25 GeV in the e+p data, integrated luminosit…
Events with an isolated high $p_T$ lepton, a hadron jet and missing energy as observed in the H1 experiment at HERA, are potentially associated with R-parity violation in supersymmetric theories. However, stringent kinematic constraints must be fulfilled if the production of supersymmetric particles in R-parity violating scenarios were the correct path for explaining these puzzling events. A reference point \RF is specified for which these constraints are illustrated and implications of the supersymmetric interpretation for new classes of multi-lepton events are indicated.
A search for single top production in ep collisions using the complete high energy data from HERA is presented. This search is based on the analysis of events containing isolated leptons (electrons or muons) and missing transverse momentum PTmiss. In the absence of a signal, an upper limit on the top production cross section sigma_ep -> etX < 0.16 pb is established at the 95% confidence level, corresponding to an upper bound on the anomalous magnetic coupling kappa_tug < 0.14. The search is complemented by a search for events containing an isolated tau lepton and PTmiss and the measurement of …
The H1 and ZEUS collaborations have used the HERA I data to search for physics beyond the SM and to test electroweak physics in electron-proton collisions. The new period of data taking (HERA II) has started and first HERA II analyses become available. An overview is given of recent highlights, including isolated lepton and multi-lepton events, tests of models of new physics and the first systematic search for deviations in all final states with high transverse momentum.
The results of searches for Supersymmetry in events with two same-sign isolated leptons, hadronic jets, and missing transverse energy in the final state are presented. The searches use pp collisions at 7 TeV collected in 2011 by the CMS experiment.
Results are presented on the production of isolated prompt photons in photoproduction (virtuality Q^2 < 1 GeV^2) and in deep inelastic scattering (Q^2 > 35 GeV^2). The results are reasonably well described by pQCD calculations in next to leading order. Comparisons to the Monte Carlo models PYTHIA and HERWIG are also presented.
Results on multi-lepton events at high transverse momenta, isolated lepton events with large missing transverse energy and single W production reported to this conference are based for the first time on the full data samples taken by two colliding experiments, H1 and ZEUS, at HERA. The data correspond to an integrated luminosity of about 1fb-1 from both experiments.
A model-independent search for deviations from the Standard Model prediction is performed in e^+ p and e^- p collisions at HERA using H1 data corresponding to an integrated luminosity of 117 pb^-1. For the first time all event topologies involving isolated electrons, photons, muons, neutrinos and jets with high transverse momenta are investigated in a single analysis. Events are assigned to exclusive classes according to their final state. A statistical algorithm is developed to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta or invari…
The search for events containing isolated leptons (electrons or muons) and missing transverse momentum produced in e^{\pm}p collisions is performed individually and in a common phase space with the H1 and ZEUS detectors at HERA in the period 1994--2007. The presented H1+ZEUS data sample corresponds to an integrated luminosity of 0.97 fb^{-1}, and comprises the complete high energy data from the HERA programme. A total of 87 events are observed in the data, compared to a Standard Model prediction of 92.7 \pm 11.2. At large hadronic transverse momentum P_{T}^{X} > 25 GeV in the e^{+}p data, lumi…
Recent results of searches for new physics at HERA using the complete data sample corresponding to about 0.5/fb per experiment are reviewed. Searches for leptoquarks, isolated leptons (electrons, muons and taus) and a generic search for new physics in many topologies are reported. No evidence for new physics is found. The ep collisions at HERA are ideally suited to search for resonantly produced squarks in SUSY with R-parity violation. It is highlighted how the present results can be interpreted in this way.
The HERA ep collider and the experiments H1 and ZEUS operated from 1994-2007. A total integrated luminosity of almost 1 fb^-1 was collected at centre-of-mass energies up to 320 GeV. Results from searches for leptoquarks and squarks, final states with an isolated lepton and missing transverse momentum and final states with multi-leptons are presented. The leptoquark limits are interpreted in terms of limits on squark production in SUSY models with R-parity violating couplings.
Results on isolated prompt photon production are presented. The measurements were performed at HERA in deep inelastic ep scattering and photoproduction, as well as at LEP in photon photon collisions. Differential cross sections are shown for inclusive prompt photons and those accompanied by a jet. The results are compared to predictions of perturbative QCD calculations in next to leading order and to predictions of the event generators PYTHIA and HERWIG.
The production of tau leptons in ep collisions is investigated using data recorded by the H1 detector at HERA in the period 1994-2000. Tau leptons are identified by detecting their decay products, using leptonic and hadronic decay modes. The cross section for the production of tau lepton pairs is measured for the first time at HERA. Furthermore, a search for events with an energetic isolated tau lepton and with large missing transverse momentum is performed. The results are found to be in agreement with the Standard Model predictions.
Recent results on searches for physics beyond the Standard Model obtained by the H1 and ZEUS experiments are reported here. After a brief introduction to the HERA collider, indirect searches for contact interactions and extra-dimensions are presented as well as direct searches for new physics including leptoquarks, lepton-flavour violation, squarks produced by R-parity violation and excited fermions. New results from isolated lepton events and single top searches are also presented. Finally the future prospects of HERA-2 are shown.
A model-independent search for deviations from the Standard Model prediction has been performed in e+p and e-p collisions at HERA using H1 data. All experimentally measurable event topologies involving isolated electrons, photons, muons, neutrinos and jets with high transverse momenta have been investigated. A good agreement with the Standard Model prediction is found in most of the event classes. A new algorithm has been developed to look for regions with large deviations from the Standard Model in the invariant mass and sum of transverse momenta distributions and to quantify the significance…
The results of searches for new physics in events with two same-sign isolated leptons, hadronic jets, and missing transverse energy in the final state are presented. The searches use an integrated luminosity of 35 inverse picobarns of pp collision data at a centre-of-mass energy of 7 TeV collected by the CMS experiment at the LHC. The observed numbers of events agree with the standard model predictions, and no evidence for new physics is found. To facilitate the interpretation of our data in a broader range of new physics scenarios, information on our event selection, detector response, and ef…
The high energy programme of the HERA collider ended in March 2007. During the whole HERA programme, a combined total integrated luminosity of 1 fb$^{-1}$ was collected by the H1 and ZEUS experiments. In this context, an overview of the most recent results of both experiments concerning searches for new physics is presented. The topics covered are searches for contact interactions, leptoquarks and excited leptons, as well as studies of the isolated lepton and multi-lepton topologies, and a general signature based search.
The analysis of $W$ production and the search for anomalous single top production is performed with the H1 detector at HERA with an integrated luminosity of 0.5 fb$^{-1}$, consisting of the complete high energy data from the HERA programme. Production cross section measurements of single $W$ production, as well as $W$ polarisation fractions in events containing isolated leptons and missing transverse momentum are also presented. In the context of a search for single top production an upper limit on the top production cross section $σ_{ep\to etX} < 0.16$ pb is established at the 95% confidence …
A search for first generation scalar and vector leptoquarks produced in ep collisions is performed by the H1 experiment at HERA. The full H1 data sample is used in the analysis, corresponding to an integrated luminosity of 446 pb^-1. No evidence for the production of leptoquarks is observed in final states with a large transverse momentum electron or with large missing transverse momentum, and constraints on leptoquark models are derived. For leptoquark couplings of electromagnetic strength lambda=0.3, first generation leptoquarks with masses up to 800 GeV are excluded at 95% confidence level.
A search for events containing two or more high-transverse-momentum isolated leptons has been performed in ep collisions with the ZEUS detector at HERA using the full collected data sample, corresponding to an integrated luminosity of 480 pb^{-1}. The number of observed events has been compared with the prediction from the Standard Model, searching for possible deviations, especially for multi-lepton events with invariant mass larger than 100 GeV. Good agreement with the Standard Model has been observed. Total and differential cross sections for di-lepton production have been measured in a res…
A search for first-generation leptoquarks was performed in electron-proton and positron-proton collisions recorded with the ZEUS detector at HERA in 2003-2007 using an integrated luminosity of 366 pb^-1. Final states with an electron and jets or with missing transverse momentum and jets were analysed, searching for resonances or other deviations from the Standard Model predictions. No evidence for any leptoquark signal was found. The data were combined with data previously taken at HERA, resulting in a total integrated luminosity of 498 pb^-1. Limits on the Yukawa coupling, lambda, of leptoqua…
HERA is the only high energy electron-proton collider in the world today and hence has unique opportunities both to probe the structure of the proton and to search for physics beyond the Standard Model. Results are presented for measurements of both neutral and charged current cross sections, and for searches for exotic processes involving direct electron-quark interactions (leptoquarks and R-parity violating SUSY), generic coupling models (contact interactions) and exclusive final states (isolated leptons and missing PT, single top production and pentaquarks). Exclusion limits on proposed mod…
A search for scalar and vector leptoquarks coupling to first generation fermions is performed in the H1 experiment at the ep collider HERA. The analysis uses e^- p data collected in 1998 and 1999 at a centre-of-mass energy of 320 GeV, corresponding to an integrated luminosity of 15 pb^-1. No evidence for the direct production of such particles is found in a data sample with a large transverse momentum final state electron or with large missing transverse momentum, and constraints on leptoquark models are established. For a Yukawa coupling of electromagnetic strength leptoquarks are excluded fo…
Recent results from searches for physics beyond the Standard Model (SM) in electron/positron-proton collisions at HERA at center-of-mass energies of 300 and 320 GeV are presented. They were performed on a data sample collected in the period 1994-2004 by the H1 and ZEUS collaborations. The data have been analysed searching for leptoquarks, light gravitinos in R-parity violating supersymmetric models and magnetic monopoles. Results of a general search for new phenomena at high transverse momentum and of a dedicated search for events with isolated leptons and missing transverse momentum are also …
A search for new physics is performed in events with two same-sign isolated leptons, hadronic jets, and missing transverse energy in the final state. The analysis is based on a data sample corresponding to an integrated luminosity of 4.98 inverse femtobarns produced in pp collisions at a center-of-mass energy of 7 TeV collected by the CMS experiment at the LHC. This constitutes a factor of 140 increase in integrated luminosity over previously published results. The observed yields agree with the standard model predictions and thus no evidence for new physics is found. The observations are used…
First inclusive measurements of isolated prompt photons in photoproduction at the HERA ep collider have been made with the ZEUS detector, using an integrated luminosity of 38.4 pb$^{-1}$. Cross sections are given as a function of the pseudorapidity and the transverse energy ($η^γ$, \eTg) of the photon, for $\eTg > $ 5 GeV in the $γp$ centre-of-mass energy range 134-285 GeV. Comparisons are made with predictions from Monte Carlo models having leading-logarithm parton showers, and with next-to-leading-order QCD calculations, using currently available parameterisations of the photon structure. Fo…
Cross sections for the production of two isolated muons up to high di-muon masses are measured in ep collisions at HERA with the H1 detector in a data sample corresponding to an integrated luminosity of 71 pb^-1 at a centre of mass energy of sqrt{s} = 319 GeV. The results are in good agreement with Standard Model predictions, the dominant process being photon-photon interactions. Additional muons or electrons are searched for in events with two high transverse momentum muons using the full data sample corresponding to 114 pb^-1, where data at sqrt{s} = 301 GeV and sqrt{s} = 319 GeV are combine…
Isolated photons with high transverse energy have been studied in deep inelastic $ep$ scattering with the ZEUS detector at HERA, using an integrated luminosity of $326\,$ pb$^{-1}$ in the range of exchanged-photon virtuality $10 - 350$ GeV$^2$. Outgoing isolated photons with transverse energy $4<E_T^γ< 15$ GeV and pseudorapidity $-0.7 <η^γ< 0.9$ were measured with accompanying jets having transverse energy and pseudorapidity $2.5 <E_T^{jet}<35$ GeV and $-1.5<η^{jet}< 1.8$, respectively. Differential cross sections are presented for the following variables: the fraction of the incoming photon e…
The latest results from the H1 and ZEUS collaborations are presented on leptoquark production and rare Standard Model processes. The data were taken in the period 1994-2005, at a centre of mass energy of up to 319 GeV. Intriguing events containing isolated leptons and missing transverse momentum, as well as multi-lepton events, are observed by H1 in regions of phase space where the SM prediction is low. Interpretations of the observed excesses in terms of physics Beyond the Standard Model are also discussed.
The results of a search for squarks and gluinos in final states with an isolated electron or muon, multiple jets and large missing transverse momentum using proton--proton collision data at a center-of-mass energy of $\sqrt{s}$ = 13 TeV are presented. The dataset used was recorded during 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 36.1 $fb^{-1}$. No significant excess beyond the expected background is found. Exclusion limits at 95% confidence level are set in a number of supersymmetric scenarios, reaching masses up to 2.1 Te…
We present an update of a search for supersymmetry in final states containing jets, missing transverse momentum, and one isolated electron or muon, using 1.04 fb^-1 of proton-proton collision data at sqrt{s} = 7 TeV recorded by the ATLAS experiment at the LHC in the first half of 2011. The analysis is carried out in four distinct signal regions with either three or four jets and variations on the (missing) transverse momentum cuts, resulting in optimized limits for various supersymmetry models. No excess above the standard model background expectation is observed. Limits are set on the visible…
Recent results from searches for new physics at HERA are reviewed. Exploiting the uniqueness of lepton-hadron collisions at high energy, searches are performed for electron-quark resonant states (leptoquarks or squarks in R-parity-violating supersymmetry) or excited states of fermions. New phenomena at a very high energy scale, manifested at present energies as effective four-fermion contact interaction, are also investigated, including cases with lepton-flavor violation. Finally, the status of events with a high-energy lepton and missing transverse momentum is presented, resulting in the most…
A search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at sqrt(s) = 7 TeV. The measurement is based on 4.7 fb-1 of data collected with the ATLAS detector at the LHC. Each top squark is assumed to decay to a top quark and the lightest supersymmetric particle (LSP). The data are found to be consistent with Standard Model expectations. Top squark masses between 230 GeV and 440 GeV are excluded with 95% confidence for massless LSPs, and top squark masses around 400 GeV are exc…
The production of final state photons in deep inelastic scattering originates from photon radiation off leptons or quarks involved in the scattering process. Photon radiation off quarks involves a contribution from the quark-to-photon fragmentation function, corresponding to the non-perturbative transition of a hadronic jet into a single, highly energetic photon accompanied by some limited hadronic activity. Up to now, this fragmentation function was measured only in electron-positron annihilation at LEP. We demonstrate by a dedicated parton-level calculation that a competitive measurement of …
A model--independent search for deviations from the Standard Model prediction is performed using the full $e^\pm p$ data sample collected by the H1 experiment at HERA. All event topologies involving isolated electrons, photons, muons, neutrinos and jets with transverse momenta above 20 GeV are investigated in a single analysis. Events are assigned to exclusive classes according to their final state. A dedicated algorithm is used to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta or the invariant mass of final state particles and to qua…
A search for scalar and vector leptoquarks coupling to first generation fermions is performed using the e^+p and e^-p scattering data collected by the H1 experiment between 1994 and 2000. The data correspond to a total integrated luminosity of 117 pb^{-1}. No evidence for the direct or indirect production of such particles is found in data samples with a large transverse momentum final state electron or with large missing transverse momentum. Constraints on leptoquark models are established. For leptoquark couplings of electromagnetic strength, leptoquarks with masses up to 275-325 GeV are rul…
The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider (LHC) are reported. The search is based on proton-proton collision data at a centre-of-mass energy $\sqrt{s} = 8$ TeV collected in 2012, corresponding to an integrated luminosity of 20 fb$^{-1}$. No significant excess above the Standard Model expectation is observed. Limits are set on the parameters of a minimal universal extra dimensions model, excluding a compactification radius of…
This work presents a new inclusive search for supersymmetry (SUSY) by the ATLAS experiment at the LHC in proton-proton collisions at a center-of-mass energy sqrt(s) = 7 TeV in final states with jets, missing transverse momentum and one or more isolated electrons and/or muons. The search is based on data from the full 2011 data-taking period, corresponding to an integrated luminosity of 4.7 inverse fb. Single- and multi-lepton channels are treated together in one analysis. An increase in sensitivity is obtained by simultaneously fitting the number of events in statistically independent signal r…
