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 level, corresponding to an upper bound on the anomalous magnetic coupling $κ_{tuγ} < 0.14$.
In this talk a search for the single production of top quarks performed at the H1 experiment is presented. No evidence for anomalous top production via FCNC processes is found and limits on the anomalous magnetic coupling constant κ_{tuγ} are given.
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…
A search for single top quark production is performed in e^\pm p collisions at HERA. The search exploits data corresponding to an integrated luminosity of 118.3 pb^-1. A model for the anomalous production of top quarks in a flavour changing neutral current process involving a t-u-gamma coupling is investigated. Decays of top quarks into a b quark and a W boson are considered in the leptonic and the hadronic decay channels of the W. Both a cut-based analysis and a multivariate likelihood analysis are performed to discriminate anomalous top quark production from Standard Model background process…
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…
A search for anomalous single-top production, ep -> etX, t -> Wb, has been made with the ZEUS detector at HERA using an integrated luminosity of 130 pb^-1. No evidence for top-quark production was found in either the leptonic or the hadronic decay channel of the W boson. The resulting constraints on single-top production via quark-flavour changing neutral current (FCNC) transitions exclude a substantial region in the FCNC t-u-gamma coupling not ruled out by other experiments.
To resolve the long-standing discrepancy between the precision measurement of bottom quark forward-backward asymmetry at LEP/SLC and the Standard Model prediction, we propose a novel method to probe the $Zb\bar{b}$ coupling by measuring the single-spin asymmetry $A_e^b$ of the polarized lepton cross section in neutral current DIS processes with a $b$-tagged jet at HERA and EIC. Depending on the tagging efficiency of the final state $b$-jet, the measurement of $A_e^b$ at HERA can already partially break the degeneracy found in the anomalous $Zb\bar{b}$ coupling, as implied by the LEP and SLC pr…
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.
A search for single top quark production is performed in the full ep data sample collected by the H1 experiment at HERA, corresponding to an integrated luminosity of 474 pb^-1. Decays of top quarks into a b quark and a W boson with subsequent leptonic or hadronic decay of the W are investigated. A multivariate analysis is performed to discriminate top quark production from Standard Model background processes. An upper limit on the top quark production cross section via flavour changing neutral current processes sigma (ep -> etX) < 0.25 pb is established at 95% CL. Limits on the anomalous coupl…
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 HERA collider experiments H1 and ZEUS have established extensive measurement programs for diffractive vector-meson production processes during the first six years of their operation. The results provide stringent phenomenological tests of quantum chromodynamical descriptions of hard diffraction. We discuss recent topical results on Upsilon photoproduction, on decay-angle analyses of rho^0, phi and J/psi electroproduction and on rho^0 and phi photoproduction at high momentum transfer.
A search for single-top production, ep --> etX, has been made with the ZEUS detector at HERA using an integrated luminosity of 130.1 pb-1. Events from both the leptonic and hadronic decay channels of the W boson resulting from the decay of the top quark were sought. For the leptonic mode, the search was made for events with isolated high-energy leptons and significant missing transverse momentum. For the hadronic decay mode, three-jet events in which two of the jets had an invariant mass consistent with that of the W were selected. No evidence for top production was found. The results are used…
A summary of the recent searches for anomalous single top-quark production and lepton flavour violation in high energy ep collisions is presented. Single top-quark production via an anomalous flavour changing neutral current gamma-u-t coupling would lead to the reaction ep->etX, t->Wb. H1 observed an excess of events above the Standard Model prediction in the leptonic decay channel of the W boson, leading to an isolated lepton, large missing transverse momentum and a jet with large transverse momentum. Five of these events are compatible with single top-quark production, compared to 1.8 events…
Recent results from HERA are presented on a range of topics: charged multiplicities, production of non-strange mesons and strange particles, charm fragmentation, baryons decaying to strange particles, antideuteron production, Bose-Einstein correlations, and new interpretations of results on prompt photon production in DIS.
Recent results from the experiments ZEUS and H1 on charm production in $ep$ collisions are reviewed. The topics are elastic and inelastic $J/ψ$ photoproduction, $D^*$ photoproduction differential cross sections and a first look at the proton structure function F_2^{c\bar{c}}.
The study of heavy flavor production is a central topic of research at HERA and is an important testing ground for perturbative QCD. A selection of results for charm and beauty production in gamma p, using different experimental techniques and compared to different theoretical predictions, obtained by the H1 and ZEUS collaborations will be presented.
Results on open charm and beauty production and on the search for top production in high-energy electron-proton collisions at HERA are reviewed. This includes a discussion of relevant theoretical aspects, a summary of the available measurements and measurement techniques, and their impact on improved understanding of QCD and its parameters, such as parton density functions and charm- and beauty-quark masses. The impact of these results on measurements at the LHC and elsewhere is also addressed.
The H1 Collaboration at the DESY electron-proton collider HERA has observed, in photoproduction and neutral-current deep-inelastic scattering, an unexpected excess of events with isolated leptons and missing transverse momentum, especially at large values of hadronic transverse momentum - a signature typical for single top-quark production. This observation is being substantiated in the HERA II run. Motivated by this, we evaluate the cross section of single top-quark photo- and electroproduction both in the standard model and its minimal supersymmetric extension, considering both minimal and n…
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.
Recently, the PVLAS collaboration has reported evidence for an anomalous rotation of the polarization of light in vacuum in the presence of a transverse magnetic field. This may be explained through the production of a new light spin-zero (axion-like) neutral particle coupled to two photons. In this letter-of-intent, we propose to test this hypothesis by setting up a photon regeneration experiment which exploits the photon beam of a high-power infrared laser, sent along the transverse magnetic field of a superconducting HERA dipole magnet. The proposed ALPS (Axion-Like Particle Search) experim…
A selection of topics on open heavy quark production at HERA are reviewed here. Measurements of charm fragmentation parameters will be presented together with developments in the study of D* di-jet angular distributions. Charm production in deep inelastic scattering (DIS) is also discussed. Finally we deal with recent measurements of b cross sections using impact parameters in both DIS and photoproduction regimes.
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 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 model-independent search for deviations from the Standard Model prediction is performed in e-p collisions at HERA II using H1 data recorded during the years 2005 and 2006, corresponding to an integrated luminosity of 159 inv.pb. 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 used to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta …
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…
Inclusive jet and multijet cross sections measured in ep collisions in different kinematic regions by the H1 and ZEUS experiments are shown. The measurements are used to extract the strong coupling alpha_s as a function of the scale and at the mass of the Z-boson. Results from prompt photon production in comparison with pQCD calculations are shown in photoproduction and deep-inelastic scattering. Other topics address scaled momentum distributions of charged particles, a measurement of the transverse momenta of charged particles and the first measurement of the charge asymmetry at HERA.
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…
A search for scalar top quarks in R-parity violating supersymmetry is performed in e^+ p collisions at HERA using the H1 detector. The data, taken at sqrt{s}=319 GeV and 301 GeV, correspond to an integrated luminosity of 106 pb^-1. The resonant production of scalar top quarks \tilde{t} in positron quark fusion via an R-parity violating Yukawa coupling lambda' is considered with the subsequent bosonic stop decay \tilde{t} -> \tilde{b} W. The R-parity violating decay of the sbottom quark tilde{b} -> d \bar{nu_e} and leptonic and hadronic W decays are considered. No evidence for stop production i…
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 …
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 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…
Anomalous Wtb couplings modify the angular correlations of the top quark decay products and change the single top quark production cross section. We present limits on anomalous top quark couplings by combining information from W boson helicity measurements in top quark decays and anomalous coupling searches in the single top quark final state. We set limits on right-handed vector couplings as well as left-handed and right-handed tensor couplings based on about 1fb^-1 of data collected by the D0 experiment.
A model-independent search for deviations from the Standard Model prediction is performed in $e^+ p$ and $e^- p$ collisions at HERA II using all high energy data recorded by the H1 experiment. This corresponds to a total integrated luminosity of 337 pb$^{-1}$. 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 used to search for deviations from the Standard Model in distributions of the scalar …
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…
Neutral and charged current deep inelastic ep scattering with longitudinally polarised lepton beams has been studied with the H1 and ZEUS detectors at HERA. The differential cross sections were measured in the range of four-momentum transfer squared, Q^2, up to 50'000 GeV^2, where electroweak effects become clearly visible. The measurements were used to determine the structure function xF_3 and to constrain vector and the axial-vector couplings of the light quarks to the Z^0 boson. The polarisation dependence of the charged current total cross section was also measured. Limits on flavour chang…
Three- and four-jet production is measured in deep-inelastic $ep$ scattering at low $x$ and $Q^2$ with the H1 detector using an integrated luminosity of $44{.}2 {\rm pb}^{-1}$. Several phase space regions are selected for the three-jet analysis in order to study the underlying parton dynamics from global topologies to the more restrictive regions of forward jets close to the proton direction. The measurements of cross sections for events with at least three jets are compared to fixed order QCD predictions of ${\mathcal{O}}(α_{\rm s}^2)$ and ${\mathcal{O}}(α_{\rm s}^3) $ and with Monte Carlo si…
Measurements are presented of diffractive open charm production at HERA. The event topology is given by ep -> eX Y where the system X contains at least one charmed hadron and is well separated by a large rapidity gap from a leading low-mass proton remnant system Y. Two analysis techniques are used for the cross section measurements. In the first, the charm quark is tagged by the reconstruction of a D*(2010) meson. This technique is used in deep-inelastic scattering (DIS) and photoproduction. In the second, a method based on the displacement of tracks from the primary vertex is used to measure …
We present measurements of the tWb coupling form factors using information from electroweak single top quark production and from the helicity of W bosons from top quark decays in ttbar events. We set upper limits on anomalous tWb coupling form factors using data collected with the D0 detector at the Tevatron ppbar collider corresponding to an integrated luminosity of 5.4 fb^-1.
A search for squarks in R-parity violating supersymmetry is performed in e+/- p collisions at HERA using the H1 detector. 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. The resonant production of squarks via a Yukawa coupling lambda' is considered, taking into account direct and indirect R-parity violating decay modes. No evidence for squark production is found in the multi-lepton and multi-jet final state topologies investigated. Mass dependent limits on lambda' are obtained…
A review of searches for physics beyond the Standard Model carried out at high energy lepton-hadron and hadron-hadron facilities is presented, with emphasis on topics of interest for future data taking at the upgraded Tevatron $p{\bar p}$ and HERA $ep$ colliders. The status and discovery prospects are discussed for leptoquarks, Technicolour and supersymmetry, forbidden lepton and quark flavour-changing processes, extra gauge bosons, excited states of composite fermions, generic contact interactions and extra compactified dimensions.
We give a simple physical argument to understand the observation that the angular distribution of the top decay lepton depends only on the polarisation of the top and is independent of any anomalous $tbW$ coupling to linear order.
Angular distribution of a secondary particle from top-quark decays is studied in a simple and general manner, paying careful attention to how relevant the top-quark production mechanism is. The conditions that lead to the distribution free from any possible anomalous top-quark decay interactions are specified. It is discussed how important the approximations adopted in earlier papers are.
The amount of data collected by the CMS experiment at the CERN LHC in the second data taking period provides the possibility to study absolute and differential cross sections of top quark interaction processes with high precision. Utilizing these results, fundamental standard model parameters such as the top quark mass are extracted. Moreover, given standard model measurements are interpreted in the context of Beyond the standard model theories. Recent results on top quark properties and rare or anomalous top quark interactions by CMS are presented in these proceedings. Furthermore, the implic…
