14 research outputs found
Low Q^2 Jet Production at HERA and Virtual Photon Structure
The transition between photoproduction and deep-inelastic scattering is
investigated in jet production at the HERA ep collider, using data collected by
the H1 experiment. Measurements of the differential inclusive jet
cross-sections dsigep/dEt* and dsigmep/deta*, where Et* and eta* are the
transverse energy and the pseudorapidity of the jets in the virtual
photon-proton centre of mass frame, are presented for 0 < Q2 < 49 GeV2 and 0.3
< y < 0.6. The interpretation of the results in terms of the structure of the
virtual photon is discussed. The data are best described by QCD calculations
which include a partonic structure of the virtual photon that evolves with Q2.Comment: 20 pages, 5 Figure
Hadron Production in Diffractive Deep-Inelastic Scattering
Characteristics of hadron production in diffractive deep-inelastic
positron-proton scattering are studied using data collected in 1994 by the H1
experiment at HERA. The following distributions are measured in the
centre-of-mass frame of the photon dissociation system: the hadronic energy
flow, the Feynman-x (x_F) variable for charged particles, the squared
transverse momentum of charged particles (p_T^{*2}), and the mean p_T^{*2} as a
function of x_F. These distributions are compared with results in the gamma^* p
centre-of-mass frame from inclusive deep-inelastic scattering in the
fixed-target experiment EMC, and also with the predictions of several Monte
Carlo calculations. The data are consistent with a picture in which the
partonic structure of the diffractive exchange is dominated at low Q^2 by hard
gluons.Comment: 16 pages, 6 figures, submitted to Phys. Lett.
Search for compositeness, leptoquarks and large extra dimensions in e q contact interactions at HERA
The reaction e+p --> e+X is studied with the H1 detector at Hera. The data
cover momentum transfers Q^2 between 200 GeV^2 and 30,000 GeV^2 and correspond
to an integrated luminosity of 35.6 pb^(-1). The differential cross section
dsigma/dQ^2 is compared to the Standard Model expectation for neutral current
scattering and analysed to search for bar(e)e bar(q)q contact interactions. No
evidence for new phenomena is observed. The results are used to set limits on
scales within models of electron--quark compositeness, quark form factors and
the exchange of virtual heavy leptoquarks. A search for gravitational effects
mediated through the exchange of virtual gravitons which propagate into large
extra dimensions is presented.Comment: 21 pages, 6 figures, 2 tables, Update to reference
Measurement of open beauty production at HERA
The first observation of open b production in ep collisions is reported. An
event sample containing muons and jets has been selected which is enriched in
semileptonic b quark decays. The visible cross section \sigma(ep -> b \bar{b}X
-> \mu X') for Q^2 < 1 GeV^2, 0.1 < y < 0.8 is measured to be
0.176+-0.016(stat.)+0.026-0.017(syst.) nb for the muons to be detected in the
range 35 deg 2.0 GeV in the laboratory
frame. The expected visible cross section based on a NLO QCD calculation is
0.104+-0.017 nb. The cross sections for electroproduction with Q^2<1 GeV^2 and
photoproduction are derived from the data and found to be \sigma(ep-> e
b\bar{b}X) = 7.1+-0.6(stat.)+1.5-1.3(syst.) nb and \sigma(\gamma p-> b\bar{b}
X) = 111+-10(stat.)+23-20(syst.) at an average \sim 180 GeV,
respectively.Comment: 11 pages, 4 figures, 0 tables, submitted to Physics Letters
Photoproduction with a leading proton at HERA
The total cross section for the photoproduction process with a leading proton in the final state has been measured at γp centre-of-mass energies W of 91, 181 and 231GeV. The measured cross sections apply to the kinematic range with the transverse momentum of the scattered proton restricted to pT≤0.2GeV and 0.68≤z≤0.88, where z=Ep′/Ep is the scattered proton energy normalised to the beam energy. The cross section dσγp→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)) μb 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 αi(0)=0.33±0.04(stat)±0.04(syst). The total cross section for the interaction of the photon with this mixture (γαi→X) can be described by an effective trajectory of intercept αk(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 γp data. The measured cross sections are compared with deep inelastic scattering leading proton data in the same region of z and pT for photon virtuality Q2 > 2.5GeV2. The ratio of the cross section for leading proton production to the total cross section is found to rise with Q2. © 2001 Elsevier Science B.V.0H1 Collaboration.info:eu-repo/semantics/publishe
Diffractive photoproduction of ψ(2S) mesons at HERA
Results on diffractive photoproduction of ψ(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 ψ(2S) cross section is found to be similar to or possibly somewhat steeper than that for J/ψ mesons. The dependences of the elastic and proton dissociative ψ(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 ebt, yields bψ(2S) el = (4.31 ± 0.57 ± 0.46) GeV-2, compatible with that of the J/φ. For the proton dissociative channel the result bψ(2S) pd (0.59 ± 0.13 ± 0.12) GeV-2 is 2.3 standard deviations smaller than that measured for the J/ψ. With proper account of the individual wavefunctions theoretical predictions based on perturbative QCD are found to describe the measurements well. © 2002 Elsevier Science B.V. All rights reserved.0H1 Collaboration.info:eu-repo/semantics/publishe
Measurement of dijet cross-sections in photoproduction and photon structure
The production of hard di-jet events in photoproduction at HERA is dominated
by resolved photon processes in which a parton in the photon with momentum
fraction x_gamma is scattered from a parton in the proton. These processes are
sensitive to the quark and gluon content of the photon. The differential di-jet
cross-section dsigma/dlog(x_gamma) is presented here, measured in tagged
photoproduction at HERA using data taken with the H1 detector, corresponding to
an integrated luminosity of 7.2 pb^(-1). Using a restricted data sample at high
transverse jet energy, E_(T,jet)>6 GeV, the effective parton density
f_gamma,eff(x_gamma) = [q(x_gamma) + bar(q)(x_gamma) +9/4g(x_gamma)] in the
photon in leading order QCD is measured down to x_gamma=0.05 from which the
gluon density in the photon is derived.Comment: 19 pages, 5 figures and 2 table
Three Jet production in deep inelastic scattering at HERA
Three-jet production is studied for the first time in deep-inelastic
positron-proton scattering. The measurement carried out with the H1 detector at
HERA covers a large range of four-momentum transfer squared 5 < Q^2 < 5000
GeV^2 and invariant three-jet masses 25 < M_(3jet) < 140 GeV. Jets are defined
by the inclusive k_T algorithm in the Breit frame. The size of the three-jet
cross section and the ratio of the three-jet to the dijet cross section R_(3/2)
are described over the whole phase space by the predictions of perturbative QCD
in next-to-leading order. The shapes of angular jet distributions deviate
significantly from a uniform population of the available phase space but are
well described by the QCD calculation.Comment: 17 pages, 5 figures, 1 tabl
Search for excited electrons at HERA
A search for excited electron e* production is described in which the
electroweak decays e*->e gamma, e*->e Z and e*->nu W 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/Lambda. These limits extend the excluded region to
higher masses than has been possible in previous direct searches for excited
electrons.Comment: 13 pages, 4 figures, 1 tabl
Search for new physics in e±q contact interactions at HERA.
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 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