[Show abstract][Hide abstract] ABSTRACT: The diffractive photoproduction of J/ψ mesons is measured with the H1 detector at the ep collider HERA using an integrated luminosity of 78 pb−1. The differential cross section dσ(γp→J/ψY)/dt is studied in the range 2<|t|<30 GeV2, where t is the square of the four-momentum transferred at the proton vertex. The cross section is also presented as a function of the photon–proton centre-of-mass energy Wγp in three t intervals, spanning the range 50<Wγp<200 GeV. A fast rise of the cross section with Wγp is observed for each t range and the slope for the effective linear Pomeron trajectory is measured to be α′=−0.0135±0.0074(stat.)±0.0051(syst.) GeV−2. The measurements are compared with perturbative QCD models based on BFKL and DGLAP evolution. The data are found to be compatible with s-channel helicity conservation.
Physics Letters B 06/2003; 568:205-218. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Deep-inelastic e±p scattering at high squared momentum transfer Q2 up to 30 000 GeV2 is used to search for eq contact interactions associated to scales far beyond the Hera centre of mass energy. The neutral current cross section measurements dσ/dQ2, corresponding to integrated luminosities of 16.4 pb−1 of e−p data and 100.8 pb−1 of e+p data, are well described by the Standard Model and are analysed to set constraints on new phenomena. For conventional contact interactions lower limits are set on compositeness scales Λ ranging between 1.6–5.5 TeV. Couplings and masses of leptoquarks and squarks in R-parity violating supersymmetry are constrained to M/λ>0.3–1.4 TeV. A search for low scale quantum gravity effects in models with large extra dimensions provides limits on the effective Planck scale of MS>0.8 TeV. A form factor analysis yields a bound on the radius of light quarks of Rq<1.0×10−18 m.
Physics Letters B 05/2003; 568:35-47. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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 channels compared to an expectation of 12.4±1.7 dominated by W production (9.4±1.6). Whilst the overall observed number of events is broadly in agreement with the number predicted by the Standard Model, there is an excess of events with transverse momentum of the hadronic system greater than 25 GeV with 10 events found compared to 2.9±0.5 expected. The results are used to determine the cross-section for events with an isolated electron or muon and missing transverse momentum.
Physics Letters B 02/2003; 561:241-257. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Multi-electron production is studied at high electron transverse momentum in positron- and electron-proton collisions using the H1 detector at HERA. The data correspond to an integrated luminosity of 115 pb-1. Di-electron and tri-electron event yields are measured. Cross sections are derived in a restricted phase space region dominated by photon-photon collisions. In general good agreement is found with the Standard Model predictions. However, for electron pair invariant masses above 100 GeV, three di-electron events and three tri-electron events are observed, compared to Standard Model expectations of 0.30 \pm 0.04 and 0.23 \pm 0.04, respectively.
European Physical Journal C 01/2003; 31:17-29. · 5.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A search for excited electron (e*) production is described in which the electroweak decays e* --> egamma, e* --> eZ and e* -- -> nuW are considered. The data used correspond to an integrated luminosity of 120 pb(-1) taken in e(+/-)p collisions from 1994 to 2000 with the H1 detector at HERA at centre-of- mass energies of 300 and 318 GeV. No evidence for a signal is found. Mass dependent exclusion limits are derived for the ratio of the couplings to the compositeness scale, f/A. These limits extend the excluded region to higher masses than has been possible in previous direct searches for excited electrons. (C) 2002 Published by Elsevier Science B.V.
Physics Letters B 11/2002; 548:35-44. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Results on diffractive photoproduction of psi(2S) mesons are presented using data collected between 1996 and 2000 with the H1 detector at the HERA ep collider. The data correspond to an integrated luminosity of 77 pb(-1). The energy dependence of the diffractive psi(2S) cross section is found to be similar to or possibly somewhat steeper than that for J/psi mesons. The dependences of the elastic and proton dissociative psi(2S) photoproduction cross sections on the squared momentum transfer t at the proton vertex are measured. The t-dependence of the elastic channel, parametrised as e(bt), yields b(el)(psi(2S)) = (4.31 +/- 0.57 +/- 0.46) GeV-2, compatible with that of the J/psi. For the proton dissociative channel the result b(pd)(psi(2S)) = (0.59 +/- 0.13 +/- 0.12) GeV-2 is 2.3 standard deviations smaller than that measured for the J/psi. With proper account of the individual wavefunctions theoretical predictions based on perturbative QCD are found to describe the measurements well. (C) 2002 Elsevier Science B.V. All rights reserved.
Physics Letters B 09/2002; 541:251-264. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dijet events in photon-proton collisions in which there is a large pseudorapidity separation, Deltaeta > 2.5 between the two highest E-T jets are studied with the H1 detector at HERA. The inclusive dijet cross sections are measured as functions of the longitudinal momentum fractions of the proton and photon which participate in the production of the jets, x(p)(jets) and x(gamma)(jets) respectively, Deltaeta, the pseudorapidity P separation between the two highest E-T jets, and E-T(gap), the total summed transverse energy between the jets. Rapidity gap events are defined as events in which E-T(gap) is less than E- T(cut), for E-T(cut) varied between jets 0.5 and 2.0 GeV. The fraction of dijet events with a rapidity gap is measured differentially in Deltaeta, x(p)(jets) and x(gamma)(jets). An excess of events with rapidity gaps at low values of E-T(cut) is observed above the expectation from standard photoproduction processes. This excess can be explained by the exchange of a strongly interacting colour singlet object between the jets.
European Physical Journal C 09/2002; 24:517-527. · 5.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A search for contributions from odderon - photon fusion to exclusive single pi0 production via ep --> epi0N* in the photoproduction regime at HERA, at an average photon - proton centre-of-mass energy = 210 GeV, and for photon virtualities of Q2 < 0.01 GeV2 is reported. The measurement proceeded via detection of the pi0 decay photons, the leading neutron from the N* decay, and the scattered electron. No indication for an odderon contribution was found. An upper limit is presented for the cross section integrated in the "visible" range 0.02 GeV2 < -t < 0.3 GeV2. It was derived as sigma(gammap --> pi0N*) < 39 nb at 95 % CL. This excludes an odderon - photon fusion cross section of 400 nb as predicted by theory.
Physics Letters B 09/2002; 544:35-43. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A search for contributions to the reaction ep→eπ0N∗ from photon–odderon fusion in the photoproduction regime at HERA is reported, at an average photon–proton centre-of-mass energy 〈W〉=215 GeV. The measurement proceeds via detection of the π0 decay photons, a leading neutron from the N∗ decay, and the scattered electron. No π0 signal is observed and an upper limit on the cross section for the photon–odderon fusion process of σ(γp→π0N∗)<49 nb at the 95% confidence level is derived, integrated over the experimentally accessible range of the squared four-momentum transfer at the nucleon vertex 0.02<|t|<0.3 GeV2. This excludes a recent prediction from a calculation based on a non-perturbative QCD model of a photon–odderon fusion cross section above 200 nb.
Physics Letters B 06/2002; 544:35-43. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dijet cross sections as functions of several jet observables are measured in photoproduction using the HI detector at HERA. The data sample comprises e(+)p data with an integrated luminosity of 319 pb(-1). Jets are selected using the inclusive k(perpendicular to) algorithm with a minimum transverse energy of 25 GeV for the leading jet. The phase space covers longitudinal proton momentum fraction x(p) and photon longitudinal momentum fraction x(gamma) in the ranges 0.05 < x(p) < 0.6 and 0.1 < x(gamma) < 1. The predict ions of next-to- leading order perturbative QCD, including recent photon and proton parton densities, are found to be compatible with the data in a wide kinematical range.
European Physical Journal C 01/2002; 25:13-23. · 5.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The helicity structure of the diffractive electroproduction of rho mesons, e + p --> e + rho + Y, is studied in a previously unexplored region of large four-momentum transfer squared at the proton vertex, t: 0 < t' < 3 GeV2, where t' = \t\ - \t\ min. The data used are collected with the HI detector at HERA in the kinematic domain 2.5 < Q(2) < 60 GeV2, 40 < W < 120 GeV No t dependence of the r(00)(04) spin density matrix element is found. A significant t dependent helicity non-conservation from the virtual photon to the rho meson is observed for the spin density matrix element combinations r(00)(5) + 2r(11)(5) and r(00)(1) + 2r(11)(1). These t dependences are consistently described by a perturbative QCD model based on the exchange of two gluons. (C) 2002 Elsevier Science B.V. All rights reserved.
Physics Letters B 01/2002; 539:25-39. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Deep-inelastic scattering data in the range 150 < Q(2) < 35000 GeV2 are used to investigate the minimum jet separation necessary to allow accurate description of the rate of dijet production using next-to-leading order perturbative QCD calculations. The required jet separation is found to be small, allowing about 1/3 of DIS data to be classified as dijet, as opposed to approximately 1/10 with more typical jet analyses. A number of precision measurements made using this dijet sample are well described by the calculations. The data are also described by the combination of leading order matrix elements and parton showers, as implemented in the QCD based Monte Carlo model RAPGAP.
European Physical Journal C 01/2002; 24:33-41. · 5.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a search for excited neutrinos using e−p data taken by the H1 experiment at HERA at a center-of-mass energy of 318 GeV with an integrated luminosity of 15 pb−1. No evidence for excited neutrino production is found. Mass dependent exclusion limits are determined for the ratio of the coupling to the compositeness scale, f/Λ, independently of the relative couplings to the SU(2) and U(1) gauge bosons. These limits extend the excluded region to higher masses than has been possible in previous searches at other colliders.
Physics Letters B 01/2002; 525:9-16. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The total cross section for the photoproduction process with a leading proton in the final state has been measured at yp centre-of-mass energies W of 91, 181 and 231 GeV. The measured cross sections apply to the kinematic range with the transverse momentum of the scattered proton restricted to P-T less than or equal to 0.2 GeV and 0.68 less than or equal to z less than or equal to 0.88, where z = E'(p)/E-p is the scattered proton energy normalised to the beam energy. The cross section d sigma (gammap-->Xp')(W, z)/dz is observed to be independent of W and z within the measurement errors and amounts to (8.05 +/- 0.06 (stat) +/- 0.89 (syst)) Pb on average. The data are well described by a Triple Regge model in which the process is mediated by a mixture of exchanges with an effective Regge trajectory of intercept alpha (i)(0) = 0.33 +/- 0.04 (stat) +/- 0.04 (syst). The total cross section for the interaction of the photon with this mixture (gamma alpha (i) --> X) can be described by an effective trajectory of intercept alpha (k) (0) = 0.99 +/- 0.01 (stat) +/- 0.05 (syst). Predictions based on previous triple Regge analyses of pp --> pX data assuming vertex factorisation are broadly consistent with the yp data. The measured cross sections are compared with deep inelastic scattering leading proton data in the same region of z and p(T) for photon virtuality Q(2) > 2.5 GeV2. The ratio of the cross section for leading proton production to the total cross section is found to rise with Q(2). (C) 2001 Elsevier Science B.V. All rights reserved.
Nuclear Physics B 12/2001; 619:3-21. · 3.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A measurement is presented of the cross section for D∗± meson production in diffractive deep-inelastic scattering for the first time at HERA. The cross section is given for the process ep→eXY, where the system X contains at least one D∗± meson and is separated by a large rapidity gap from a low mass proton remnant system Y. The cross section is presented in the diffractive deep-inelastic region defined by 2<Q2<100 GeV2, 0.05<y<0.7, , MY<1.6 GeV and |t|<1 GeV2. The D∗± mesons are restricted to the range GeV and . The cross section is found to be 246±54±56 pb and forms about 6% of the corresponding inclusive D∗± cross section. The cross section is presented as a function of various kinematic variables, including which is an estimate of the fraction of the momentum of the diffractive exchange carried by the parton entering the hard-subprocess. The data show a large component of the cross section at low where the contribution of the boson–gluon-fusion process is expected to dominate. The data are compared with several QCD-based calculations.
Physics Letters B 09/2001; 520:191-203. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A measurement of the derivative (∂ lnF2/∂ lnx)Q2≡−λ(x,Q2) of the proton structure function F2 is presented in the low x domain of deeply inelastic positron–proton scattering. For 5×10−5⩽x⩽0.01 and , λ(x,Q2) is found to be independent of x and to increase linearly with lnQ2.
Physics Letters B 08/2001; 520:183-190. · 6.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A measurement is presented of elastic deeply virtual Compton scattering e++p→e++γ+p at HERA using data taken with the H1 detector. The cross section is measured as a function of the photon virtuality, Q2, and the invariant mass, W, of the γp system, in the kinematic range , and , where t is the squared momentum transfer to the proton. The measurement is compared to QCD based calculations.
Physics Letters B 07/2001; 517:47-58. · 6.02 Impact Factor