Search for contact interactions in deep inelastic e^+p\to e^+X scattering at HERA

In a search for signatures of physics processes beyond the Standard Model, various eeqq vector contact–interaction hypotheses have been tested using the high– deep inelastic neutral–current scattering data collected with the ZEUS detector at HERA. The data correspond to an integrated luminosity of of interactions at 300 GeV center–of–mass energy. No significant evidence of a contact–interaction signal has been found. Limits at the 95% confidence level are set on the contact–interaction amplitudes. The effective mass scales corresponding to these limits range from 1.7 TeV to 5 TeV for the contact–interaction scenarios considered. Received: 18 May 1999 / Revised version: 14 January 2000 / Published online: 25 February 2000     

Search for baryonic resonances decaying to Ξπ in deep-inelastic scattering at HERA

A search for narrow baryonic resonances decaying into Ξ^-π^- or Ξ^-π⁺ and their antiparticles is carried out with the H1 detector using deep inelastic scattering events at HERA in the range of negative photon four-momentum transfer squared 2 < Q² < 100 GeV². No signal is observed for a new baryonic state in the mass range 1600–2300 MeV in either the doubly charged or the neutral decay channels. The known Ξ(1530)⁰ is observed through its decay mode into Ξ^-π⁺. Upper limits are given on the ratio of the production rates of new baryonic states, such as the hypothetical pentaquark states Ξ^– _5q or Ξ⁰ _5q, relative to the Ξ(1530)⁰ baryon state.     

Diffractive jet production in deep-inelastic e^+p collisions at HERA

A measurement is presented of dijet and 3-jet cross sections in low- diffractive deep-inelastic scattering interactions of the type , where the system X is separated by a large rapidity gap from a low-mass baryonic system Y. Data taken with the H1 detector at HERA, corresponding to an integrated luminosity of 18.0 pb, are used to measure hadron level single and double differential cross sections for and . The energy flow not attributed to jets is also investigated. The measurements are consistent with a factorising diffractive exchange with trajectory intercept close to 1.2 and tightly constrain the dominating diffractive gluon distribution. Viewed in terms of the diffractive scattering of partonic fluctuations of the photon, the data require the dominance of over states. Soft colour neutralisation models in their present form cannot simultaneously reproduce the shapes and the normalisations of the differential cross sections. Models based on 2-gluon exchange are able to reproduce the shapes of the cross sections at low values. Received: 21 December 2000 / Published online: 6 April 2001     

A search for leptoquark bosons and lepton flavor violation in e^+ p collisions at HERA

A search for new bosons possessing couplings to lepton-quark pairs is performed in the H1 experiment at HERA using 1994 to 1997 data corresponding to an integrated luminosity of pb. First generation leptoquarks (LQs) are searched in very high neutral (NC) and charged (CC) current data samples. The measurements are compared to Standard Model (SM) expectations from deep-inelastic scattering (DIS). A deviation in the spectrum previously observed in the 1994 to 1996 dataset at GeV remains, though with less significance. This deviation corresponded to a clustering in the invariant mass spectrum at GeV which is not observed with the 1997 dataset alone. The NC DIS data is used to constrain the Yukawa couplings of first generation scalar and vector LQs in the Buchmüller–Rückl–Wyler effective model. Scalar LQs are excluded for masses up to 275 GeV for a coupling of electromagnetic strength, . A sensitivity to coupling values is established for masses up to 400 GeV for any LQ type. The NC and CC DIS data are combined to constrain for arbitrary branching ratios of the LQ into eq in a generic model. For a decay branching ratio into pairs as small as 10%, LQ masses up to 260 GeV are ruled out for . LQs possessing couplings to mixed fermion generations, which could lead to signals of lepton flavor violation (LFV), are searched in events with a high transverse momentum or . No or event candidate is found that is compatible with LQ kinematics. Constraints are set on the Yukawa coupling involving the and lepton in a yet unexplored mass range. Received: 2 July 1999 / Published online: 28 September 1999     

Measurement of high-Q^{2} neutral-current e^+p deep inelastic scattering cross-sections at HERA

The neutral-current deep inelastic scattering differential cross-sections , for GeV, and , for , 2500 and 10000 GeV, have been measured with the ZEUS detector at HERA. The data sample of 47.7 pb was collected at a center-of-mass energy of 300 GeV. The cross-section, , falls by six orders of magnitude between and 40000 GeV. The predictions of the Standard Model are in very good agreement with the data. Complementing the observations of time-like contributions to fermion-antifermion annihilation, the data provide direct evidence for the presence of exchange in the space-like region explored by deep inelastic scattering. Received: 18 May 1999 / Published online: 3 August 1999     

Search for high-mass resonances decaying to emu in pp collisions at square root = 1.69 TeV

We describe a general search for resonances decaying to a neutral emu final state in pp collisions at a center-of-mass energy of 1.96 TeV. Using a data sample representing 344 pb(-1) of integrated luminosity recorded by the Collider Detector at Fermilab II experiment, we compare standard model predictions with the number of observed events for invariant masses between 50 and 800 GeV/c2. Finding no significant excess (5 events observed vs 7.7 +/- 0.8 expected for M(emu) > 100 GeV/c2 ), we set limits on sneutrino and Z' masses as functions of lepton family number violating couplings.     

Search for lepton-flavor violation at HERA

A search for lepton-flavor-violating interactions and has been performed with the ZEUS detector using the entire HERA I data sample, corresponding to an integrated luminosity of . The data were taken at center-of-mass energies, , of 300 and . No evidence of lepton-flavor violation was found, and constraints were derived on leptoquarks (LQs) that could mediate such interactions. For LQ masses below , limits were set on , where is the coupling of the LQ to an electron and a first-generation quark q₁, and is the branching ratio of the LQ to the final-state lepton (μ or ) and a quark q. For LQ masses much larger than , limits were set on the four-fermion interaction term for LQs that couple to an electron and a quark and to a lepton and a quark , where and are quark generation indices. Some of the limits are also applicable to lepton-flavor-violating processes mediated by squarks in R-Parity-violating supersymmetric models. In some cases, especially when a higher-generation quark is involved and for the process , the ZEUS limits are the most stringent to date. Received: 1 April 2005, Revised: 13 July 2005, Published online: 18 October 2005     

Search for W' boson resonances decaying to a top quark and a bottom quark

We search for the production of a heavy W' gauge boson that decays to third generation quarks in 0.9 fb-1 of pp collisions at square root(s)=1.96 TeV, collected with the D0 detector at the Fermilab Tevatron collider. We find no significant excess in the final-state invariant mass distribution and set upper limits on the production cross section times branching fraction. For a left-handed W' boson with SM couplings, we set a lower mass limit of 731 GeV. For right-handed W' bosons, we set lower mass limits of 739 GeV if the W' boson decays to both leptons and quarks and 768 GeV if the W' boson decays only to quarks. We also set limits on the coupling of the W' boson to fermions as a function of its mass.     

A search for excited fermions in e + p collisions at HERA

Using the ZEUS detector at HERA, we have searched for heavy excited states of electrons, neutrinos, and quarks in e ⁺ p collisions at a center-of-mass energy of 300 GeV. With an integrated luminosity of 9.4 pb^-1, no evidence was found for electroweak production and decay of such states. Limits on the production cross section times branching ratio and on the characteristic couplings, ?/Λ, are derived for masses up to 250 GeV. For the particular choice ?/Λ = 1/M ?*, we exclude at the 95% confidence level excited electrons with mass between 30 and 200 GeV, excited electron neutrinos with mass between 40 and 96 GeV, and quarks excited electroweakly with mass between 40 and 169 GeV.     

Search for Higgs bosons in {\rm e^+e^-} collisions at 183 GeV

The data collected by the OPAL experiment at GeV were used to search for Higgs bosons which are predicted by the Standard Model and various extensions, such as general models with two Higgs field doublets and the Minimal Supersymmetric Standard Model (MSSM). The data correspond to an integrated luminosity of approximately 54 pb. None of the searches for neutral and charged Higgs bosons have revealed an excess of events beyond the expected background. This negative outcome, in combination with similar results from searches at lower energies, leads to new limits for the Higgs boson masses and other model parameters. In particular, the 95% confidence level lower limit for the mass of the Standard Model Higgs boson is 88.3 GeV. Charged Higgs bosons can be excluded for masses up to 59.5 GeV. In the MSSM, GeV and GeV are obtained for , no and maximal scalar top mixing and soft SUSY-breaking masses of 1 TeV. The range is excluded for minimal scalar top mixing and GeV. More general scans of the MSSM parameter space are also considered. Received: 27 October 1998 / Published online: 19 February 1999     

Search for pentaquarks decaying to Ξπ in deep inelastic scattering at HERA

A search for pentaquarks decaying to Ξ⁻π⁻ (Ξ⁻π⁺) and corresponding antiparticles has been performed with the ZEUS detector at HERA. The data sample consists of deep inelastic ep scattering events at centre-of-mass energies of 300 and 318 GeV, and corresponds to 121 pb⁻¹ of integrated luminosity. A clear signal for Ξ⁰(1530)→Ξ⁻π⁺ was observed. However, no signal for any new baryonic state was observed at higher masses in either the Ξ⁻π⁻ or Ξ⁻π⁺ channels. The searches in the antiparticle channels were also negative. Upper limits on the ratio of a possible () signal to the Ξ⁰(1530) signal were set in the mass range 1650–2350 MeV.     

Search for narrow p¯p resonances in the reaction p¯p → p¯pπ+π−.

The reaction p¯p → p¯pπ⁺π^?. has been studied with high statistics at CERN-LEAR with incident p¯ momenta from 1.65 to 2.0 GeV/c by the JETSET (PS202) experiment. The aim of this paper is to search for narrow resonances decaying to p¯p. No evidence for such structures is found. In particular, an upper limit for the production of a 2.02 GeV state with a width of Γ = 20 MeV, having been seen in other hadroproduction experiments, is established. Our results restrict the cross section for such a peak to be below 200 nb at the 95% confidence level.     

Search for stop production in R-parity-violating supersymmetry at HERA

A search for stop production in R-parity-violating supersymmetry has been performed in e⁺p interactions with the ZEUS detector at HERA, using an integrated luminosity of 65 pb^-1. At HERA, the R-parity-violating coupling λ’ allows resonant squark production, . Since the lowest-mass squark state in most supersymmetry models is the light stop, , this search concentrated on production of , followed either by a direct R-parity-violating decay, or by the gauge decay to . No evidence for stop production was found and limits were set on λ’₁₃₁ as a function of the stop mass in the framework of the minimal supersymmetric standard model. The results have also been interpreted in terms of constraints on the parameters of the minimal supergravity model.