[Show abstract][Hide abstract] ABSTRACT: With the H1 detector at the ep collider HERA, D* meson production cross sections have been measured in deep inelastic scattering with four-momentum transfers Q2 > 2 GeV2 and in photoproduction at energies around Wγp ≈ 88 GeV and 194 GeV. Next-to-Leading Order QCD calculations are found to describe the differential cross sections within theoretical and experimental uncertainties. Using these calculations, the NLO gluon momentum distribution in the proton, xgg(xg), has been extracted in the momentum fraction range 7.5 × 10-4 < xg < 4 × 10-2 at average scales μ2 = 25 to 50 GeV2. The gluon momentum fraction xg has been obtained from the measured kinematics of the scattered electron and the D* meson in the final state. The results compare well with the gluon distribution obtained from the analysis of scaling violations of the proton structure function F2.
[Show abstract][Hide abstract] ABSTRACT: Single particles and jets in deeply inelastic scattering at low x are measured with the H1 detector in the region away from the current jet and towards the proton remnant, known as the forward region. Hadronic final state measurements in this region are expected to be particularly sensitive to QCD evolution effects. Jet cross sections are presented as a function of Bjorken-x for forward jets produced with a polar angle to the proton direction, θjet, in the range 7° < θjet < 20°. Azimuthal correlations are studied between the forward jet and the scattered lepton. Charged and neutral single particle production in the forward region are measured as a function of Bjorken-x, in the range 5° < θ < 25°, for particle transverse momenta larger than 1 GeV. QCD based Monte Carlo predictions and analytical calculations based on BFKL, CCFM and DGLAP evolution are compared to the data. Predictions based on the DGLAP approach fail to describe the data, except for those which allow for a resolved photon contribution.
Full-text · Article · Jan 1999 · Nuclear Physics B
[Show abstract][Hide abstract] ABSTRACT: Photoproduction data collected with the H1 detector at HERA in 1994 are used to study the cross-sections for inclusive charged particle production and the structure of the photon. The differential cross-sections dσ/dp2T, for |η| < 1 in the HERA laboratory frame, and dσ/dη for PT > 2GeV/c and PT > 3 GeV/c have been measured. Model calculations of these cross-sections, based on perturbative QCD, indicate that the results are sensitive to the parton densities of the photon as well as to higher order effects, which are phenomenologically treated by multiple interactions. This sensitivity is exploited to determine the leading order cursive Greek chiγ distribution of partons in the photon using a new method based on high PT charged particles. The gluon content of the photon is extracted and found to rise with decreasing cursive Greek chiγ.
[Show abstract][Hide abstract] ABSTRACT: Deep-inelastic scattering events with a leading baryon have been detected by the I-IL experiment at HERA using a forward proton spectrometer and a forward neutron calorimeter. Semi-inclusive cross sections have been measured in the kinematic region 2 less than or equal to Q(2) less than or equal to 50GeV(2), 6 x 10(-5) less than or equal to x less than or equal to 6 x 10(-3) and baryon p(T) less than or equal to 200 MeV, for events with a final-state proton with energy 580 less than or equal to E' less than or equal to 740 GeV, or a neutron with energy E' greater than or equal to 160 GeV. The measurements are used to test production models and factorization hypotheses. A Regge model of leading baryon production which consists of pion, pomeron and secondary reggeon exchanges gives an acceptable description of both semi-inclusive cross sections in the region 0.7 less than or equal to E'/E-p less than or equal to 0.9, where E-p is the proton beam energy. The leading neutron data are used to estimate for the first time the structure function of the pion at small Bjorken-x.
Full-text · Article · Jan 1999 · European Physical Journal C
[Show abstract][Hide abstract] ABSTRACT: Interactions of the type ep → eXY are studied, where the component X of the hadronic final state contains two jets and is well separated in rapidity from a leading baryonic system Y. Analyses are performed of both resolved and direct photoproduction and of deep-inelastic scattering with photon virtualities in the range 7.5 < Q
2 < 80 GeV2. Cross sections are presented where Y has mass M
< 1.6 GeV, the squared four-momentum transferred at the proton vertex satisfies |t| < 1 GeV2 and the two jets each have transverse momentum p
Tjet > 5 GeV relative to the photon direction in the rest frame of X. Models based on a factorisable diffractive exchange with a gluon dominated structure, evolved to a scale set by the transverse momentum \(\hat p_T \) of the outgoing partons from the hard interaction, give good descriptions of the data. Exclusive \(q\bar q\) production, as calculated in perturbative QCD using the squared proton gluon density, represents at most a small fraction of the measured cross section. The compatibility of the data with a breaking of diffractive factorisation due to spectator interactions in resolved photoproduction is investigated.
Full-text · Article · Jan 1999 · European Physical Journal C
[Show abstract][Hide abstract] ABSTRACT: Jet event rates in deep inelastic ep scattering at HERA are investigated applying the modified JADE jet algorithm. The data are corrected for detector and hadronization effects and then compared with perturbative QCD predictions using next-to-leading order calculations. The strong coupling constant αs(M2Z) is determined evaluating the jet event rates. Values of αs(Q2) are extracted in four different bins of the negative squared momentum transfer Q2 in the range from 40 GeV2 to 4000 GeV2 . A combined fit of the renormalization group equation to these several αs(Q2) values results in αs(M2Z) = 0.117 ± 0.003 (stat)+0.009-0.013 (sys) + 0.006 (jet algorithm).
[Show abstract][Hide abstract] ABSTRACT: The electronic system developed for the SpaCal lead/scintillating-fibre calorimeters of the H1 detector in operation at the HERA ep collider is described in detail and the performance achieved during H1 data taking is presented. The 10 MHz bunch crossing rate of HERA puts severe constraints on the requirements of the electronics. The energy and time readout are performed, respectively, with a 14-bit dynamic range and with a resolution of ∼0.4 ns. The trigger branch consists of a nanosecond-resolution calorimetric time of flight for background rejection and an electron trigger based on analog ‘sliding windows’. The on-line background rejection currently achieved is ∼106. The electron trigger allows a low-energy trigger threshold to be set at ∼0.50±0.08 (RMS) GeV with an efficiency ⩾99.9%. The energy and time performance of the readout and trigger electronics is based on a newly developed low noise (σnoise∼0.4 MeV) wideband (f⩽200 mHz) preamplifier located at the output of the photomultipliers which are used for the fibre light readout in the ∼1 T magnetic field of H1.
[Show abstract][Hide abstract] ABSTRACT: 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 (xF) variable for charged particles, the squared transverse momentum of charged particles (), and the mean as a function of xF. These distributions are compared with results in the γ∗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 Q2 by hard gluons.
Full-text · Article · May 1998 · Physics Letters B
[Show abstract][Hide abstract] ABSTRACT: Quasi-elastic (z>0.95) photo-production of ψ(2S) mesons has been observed at HERA for photon-proton centre-of-mass energies in the range 40 to 160 GeV. The ψ(2S) mesons were identified through their decays to ℓ+ℓ−, and to J/ψπ+π−, where the J/ψ subsequently decays to ℓ+ℓ−, the lepton ℓ being either a muon or an electron. The cross-section for quasi-elastic photoproduction was measured to be (18.0±2.8(stat)±3.0(syst)) nb at a photon-proton centre-of-mass energy of 80 GeV. The ratio of the ψ(2S) to J/ψ quasi-elastic cross-sections is (0.150±0.027(stat)±0.022(syst)).
No preview · Article · Mar 1998 · Physics Letters B
[Show abstract][Hide abstract] ABSTRACT: The new lead/scintillating-fibre calorimeter (“SpaCal”) for the backward region of the H1 experiment at HERA (DESY) is equipped with fine mesh phototubes which operate in a magnetic field close to 1 T. A large sample of these tubes of the types Hamamatsu R5505 and R5506, and Hamamatsu R2490-05, have been tested in fields of up to 1.2T. We have investigated the cathode homogeneity with and without magnetic field, the gain loss under the influence of the magnetic field, and stability with time. For a subsample of tubes, we have performed additional studies on stability with respect to temperature changes, variation of gain as a function of the magnetic field, high voltage discharges, single photo-electron response, and linearity. We finally summarize the experience with these tubes after one year of operation in the experiment.
No preview · Article · Feb 1998 · Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment
[Show abstract][Hide abstract] ABSTRACT: Quasi-elastic (z 0.95) photo-production of psi' mesons has been observed at HERA for photon-proton centre-of-mass energies in the range 40 to 160 GeV. The psi' mesons were identified through their decays to l+l- and to J/psi pi+ pi-, where the J/psi subsequently decays to l+l-, the lepton l being either a muon or an electron. The cross-section for quasi-elastic photoproduction was measured to be [18.0 +- 2.8 (stat) +- 3.0(syst)] nb at a photon-proton centre-of-mass energy of 80 GeV. The ratio of the psi' to J/psi quasi-elastic cross-sections is 0.150 +- 0.027 (stat) +- 0.022 (syst).
[Show abstract][Hide abstract] ABSTRACT: 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 μ± 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 kinematic properties which are atypical of all Standard Model processes considered.
[Show abstract][Hide abstract] ABSTRACT: A thrust analysis of Large-Rapidity-Gap events in deep-inelastic ep collisions is presented, using data taken with the H1 detector at HERA in 1994. The average thrust of the final states X, which emerge from the dissociation of virtual photons in the range 10 < Q2 < 100 GeV2, grows with hadronic mass MX and implies a dominant 2-jet topology. Thrust is found to decrease with growing Pt, the thrust jet momentum transverse to the photon-proton collision axis. Distributions of P2t are consistent with being independent of MX. They show a strong alignment of the thrust axis with the photon-proton collision axis, and have a large high-Pt tail. The correlation of thrust with MX is similar to that in e+e- annihilation at √see = MX , but with lower values of thrust in the ep data. The data cannot be described by interpreting the dissociated system X as a qq̄ state but inclusion of a substantial fraction of qq̄g parton configurations leads naturally to the observed properties. The soft colour exchange interaction model does not describe the data.