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.     

A search for excited fermions at HERA

A search for excited fermions f of the first generation in scattering at the collider HERA is presented using H1 data with an integrated luminosity of 37 pb. All electroweak decays of excited fermions, f f f W,f Z are considered and all possible final states resulting from the Z orW hadronic decays or decays into leptons of the first two generations are taken into account. No evidence for f production is found. Mass dependent exclusion limits on cross-sections and on the ratio of coupling constants to the compositeness scale are derived. Received: 19 July 2000 / Published online: 23 October 2000     

A direct search for stable magnetic monopoles produced in positron-proton collisions at HERA

A direct search has been made for magnetic monopoles produced in e ⁺ p collisions at a centre of mass energy of 300 GeV at HERA. The beam pipe surrounding the interaction region in 1995-1997 was investigated using a SQUID magnetometer to look for stopped magnetic monopoles. During this time an integrated luminosity of 62 pb^-1 was delivered. No magnetic monopoles were observed and charge and mass dependent upper limits on the e ⁺ p production cross section are set.Received: 25 January 2005, Revised: 13 March 2005, Published online: 12 April 2005     

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     

A search for leptoquarks and squarks at HERA

A search in the H1 experiment at HERA for scalar and vector leptoquarks, leptogluons and squarks coupling to first generation fermions is presented in a data sample corresponding to an integrated luminosity of 425 nb^–1. For masses ranging up to 275 GeV, no significant evidence for the direct production of such particles is found in various possible decay channels. At high masses and beyond the centre of mass energy of 296 GeV a contact interaction analysis is used to further constrain the couplings and masses of new vector leptoquarks and to set lower limits on compositeness scales.Supported by the Bundesministerium für Forschung und Technologie, FRG under contract numbers 6AC17P, 6AC47P, 6DO571, 6HH17P, 6HH271, 6HD171, 6HD271, 6KH7P, 6MP171, and 6WT87P     

Search for lepton flavour violation in ep collisions at HERA

A search for the lepton flavour violating processes ep→μX and ep→τX is performed with the H1 experiment at HERA. Final states with a muon or tau and a hadronic jet are searched for in a data sample corresponding to an integrated luminosity of 66.5 pb^-1 for e⁺p collisions and 13.7 pb^-1 for e^-p collisions at a centre-of-mass energy of 319 GeV. No evidence for lepton flavour violation is found. Limits are derived on the mass and the couplings of leptoquarks inducing lepton flavour violation in an extension of the Buchmüller–Rückl–Wyler effective model. Leptoquarks produced in ep collisions with a coupling strength of λ=0.3 and decaying with the same coupling strength to a muon–quark pair or a tau–quark pair are excluded at 95% confidence level up to masses of 459 GeV and 379 GeV, respectively.     

Tau lepton production in ep collisions at HERA

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.     

Hadron production by ep collisions at HERA

TheP distribution of hadrons from deep-inelastic ep collisions of the H1 experiment determines for the hadrons in the final state, a temperature 277±14 MeV220 MeV of the critical temperature predicted by QCD, suggesting the formation of quark-gluon plasma (QGP). From their forward to backward ratio, it is found an excess of hadrons arising from the deconfinement of quarks inside the proton.     

Some weak processes in electron-proton collisions

Polarization effects are investigated for processes of single production of strange particles in electron-proton collisions e^±+p ⁺+e^±. Special attention is paid to effects associated with possible application for investigation of similar processes of polarized electron beams and polarized nucleon targets. We point out the important role of the study of polarization effects for determination of the matrix element of processes of the type e^±+p ⁺+e^±.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 78–82, September, 1990.     

Search for single top quark production in ep collisions at HERA

The European Physical Journal C - 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...     

Jets and energy flow in photon-proton collisions at HERA

Properties of the hadronic final state in photoproduction events with large transverse energy are studied at the electron-proton collider HERA. Distributions of the transverse energy, jets and underlying event energy are compared to data and QCD calculations. The comparisons show that theγp events can be consistently described by QCD models including — in addition to the primary hard scattering process — interactions between the two beam remnants. The differential jet cross sectionsdσ/dE _T^jet anddσ/dη ^jet are measured. Supported by the Bundesministerium für Forschung und Technologie, FRG under contract numbers 6AC17P, 6AC47P, 6DO571, 6HH17P, 6HH27I, 6HD17I, 6HD27I, 6KI17P, 6MP17I, and 6WT87P     

Diffractive photoproduction of dijets in ep collisions at HERA

Diffractive photoproduction of dijets was measured with the ZEUS detector at the ep collider HERA using an integrated luminosity of 77.2 pb^-1. The measurements were made in the kinematic range Q² < 1 GeV², 0.20<y<0.85 and x_IP<0.025, where Q² is the photon virtuality, y is the inelasticity and x_IP is the fraction of the proton momentum taken by the diffractive exchange. The two jets with the highest transverse energy, E_T ^jet, were required to satisfy E_T ^jet>7.5 and 6.5 GeV, respectively, and to lie in the pseudorapidity range -1.5<η^jet<1.5. Differential cross sections were compared to perturbative QCD calculations using available parameterisations of diffractive parton distributions of the proton.     

Selectron and squark production inep collisions at HERA

For the SUSY Standard Model we give a detailed discussion how HERA can be used for disentangling the mixing structure in the neutralino sector. We present 3-dimensional plots for the total cross section and for the polarization asymmetry for selectron-squark production, varying both the sfermion masses and a mixing parameter. The main results are: (i) production rates at HERA are reasonably large for a wide range of mixing parameters; (ii) the polarization asymmetry depends only weakly upon the sfermion masses and thus is a very useful tool in determining mixing parameters or mass splittings between left and right handed selectron masses.     

A study of the fragmentation of quarks in et-p collisions at HERA

Deep inelastic scattering (DIS) events, selected from 1993 data taken by the H1 experiment at HERA, are studied in the Breit frame of reference. The fragmentation function of the quark is compared with those of e⁺e⁻ data. It is shown that certain aspects of the quarks emerging from within the proton in e⁻p interactions are essentially the same as those of quarks pair-created from the vacuum in e⁺e⁻ annihilation. The measured area, peak position and width of the fragmentation function show that the kinematic evolution variable, equivalent to the e⁺e⁻ squared centre of mass energy, is in the Breit frame the invariant square of the four-momentum transfer. We comment on the extent to which we have evidence for coherence effects in pArton showers.     

Search for first generation leptoquarks in ep collisions at HERA

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 λ=0.3, first generation leptoquarks with masses up to 800 GeV are excluded at 95% confidence level.     

Production of excited charm and charm-strange mesons at HERA

The production of excited charm, D ₁(2420)⁰ and D ₂^*(2460)⁰, and charm-strange, D _s1(2536)^±, mesons in ep collisions was measured with the ZEUS detector at HERA using an integrated luminosity of 126 pb⁻¹. Masses, widths and helicity parameters were determined. The measured yields were converted to the rates of c quarks hadronising as a given excited charm meson and to the ratios of the dominant D ₂^*(2460)⁰ and D _s1(2536)^± branching fractions. A search for the radially excited charm meson, D ^*′(2640)^±, was also performed. The results are compared with those measured previously and with theoretical expectations. Dedicated to the memory of our colleague Pavel Ermolov. Deceased. Also affiliated with University College London, UK. Now at BayesForecast, Madrid, Spain. Also at Institut of Theoretical and Experimental Physics, Moscow, Russia. Also at INP, Cracow, Poland. Also at FPACS, AGH-UST, Cracow, Poland. Partly supported by Moscow State University, Russia. Royal Society of Edinburgh, Scottish Executive Support Research Fellow. Also affiliated with DESY, Germany. Also at University of Tokyo, Japan. Now at Kobe University, Japan. Supported by DESY, Germany. Now at University of Salerno, Italy. Partly supported by Russian Foundation for Basic Research grant no. 05-02-39028-NSFC-a. Partially supported by Warsaw University, Poland. This material was based on work supported by the National Science Foundation, while working at the Foundation. Now at University of Kansas, Lawrence, USA. Also at Max Planck Institute, Munich, Germany, Alexander von Humboldt Research Award. Now at KEK, Tsukuba, Japan. Now at Nagoya University, Japan. Member of Department of Radiological Science, Tokyo Metropolitan University, Japan. Now at SunMelx Co. Ltd., Tokyo, Japan. PPARC Advanced fellow. Supported by the research grant no. 1 P03B 04529 (2005–2008). Also at Hamburg University, Inst. of Exp. Physics, Alexander von Humboldt Research Award and partially supported by DESY, Hamburg, Germany. Also at Łódź University, Poland. Member of Łódź University, Poland. Now at Lund Universtiy, Lund, Sweden. This work was supported in part by the Marie Curie Actions Transfer of Knowledge project COCOS (contract MTKD-CT-2004-517186). Now at University of Bonn, Germany. Now at DESY group FEB, Hamburg, Germany. Now at University of Liverpool, UK. Now at CERN, Geneva, Switzerland. Now at Bologna University, Bologna, Italy.     

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     

Selectron and squark production inep collisions at HERA

For the SUSY Standard Model we give a detailed discussion how HERA can be used for disentangling the mixing structure in the neutralino sector. We present 3-dimensional plots for the total cross section and for the polarization asymmetry for selectron-squark production, varying both the sfermion masses and a mixing parameter. The main results are: (i) production rates at HERA are reasonably large for a wide range of mixing parameters; (ii) the polarization asymmetry depends only weakly upon the sfermion masses and thus is a very useful tool in determining mixing parameters or mass splittings between left and right handed selectron masses.