| 31. |
- Aaron, F. D., et al.
(författare)
-
Three- and four-jet production at low x at HERA
- 2008
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 54:3, s. 389-409
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Tidskriftsartikel (refereegranskat)abstract
- Three- and four-jet production is measured in deep-inelastic ep scattering at low x and Q(2) with the H1 detector using an integrated luminosity of 44.2 pb(-1). Several phase space regions are selected for the three-jet analysis in order to study the underlying parton dynamics from global topologies to the more restrictive regions of forward jets close to the proton direction. The measurements of cross sections for events with at least three jets are compared to fixed order QCD predictions of O(alpha(2)(s)) and O(alpha(3)(s)) and with Monte Carlo simulation programs where higher order effects are approximated by parton showers. A good overall description is provided by the O(alpha(3)(s)) calculation. Too few events are predicted at the lowest x similar to 10(-4), especially for topologies with two forward jets. This hints to large contributions at low x from initial state radiation of gluons close to the proton direction and unordered in transverse momentum. The Monte Carlo program in which gluon radiation is generated by the colour dipole model gives a good description of both the three- and the four-jet data in absolute normalisation and shape.
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| 32. |
- Aktas, A., et al.
(författare)
-
Measurement of inclusive jet production in deep-inelastic scattering at high Q(2) and determination of the strong coupling
- 2007
-
Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 653:2-4, s. 134-144
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Tidskriftsartikel (refereegranskat)abstract
- Inclusive jet production is studied in neutral current deep-inelastic positron-proton scattering at large four momentum transfer squared Q(2) > 150 GeV2 with the H1 detector at HERA. Single and double differential inclusive jet cross sections are measured as a function of Q(2) and of the transverse energy E-T of the jets in the Breit frame. The measurements are found to be well described by calculations at next-to-leading order in perturbative QCD. The running of the strong coupling is demonstrated and the value of alpha(s)(M-Z) is determined. The ratio of the inclusive jet cross section to the inclusive neutral current cross section is also measured and used to extract a precise value for ces (M-Z) = 0.1193 +/- 0.0014(exp.)(-0.0030)(+0.0047) (th.) +/- 0.0016(pdf). (c) 2007 Elsevier BX All rights reserved.
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| 33. |
- Aktas, A., et al.
(författare)
-
Tau lepton production in ep collisions at HERA
- 2006
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 48:3, s. 699-714
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 34. |
- Aktas, A, et al.
(författare)
-
A general search for new phenomena in ep scattering at HERA
- 2004
-
Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 602:1-2, s. 14-30
-
Tidskriftsartikel (refereegranskat)abstract
- A model-independent search for deviations from the Standard Model prediction is performed in e(+) p and e(-) p collisions at HERA using H1 data corresponding to an integrated luminosity of 117 pb(-1). For the first time all event topologies involving isolated electrons, photons, muons, neutrinos and jets with high transverse momenta are investigated in a single analysis. Events are assigned to exclusive classes according to their final state. A statistical algorithm is developed to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta or invariant mass of final state particles and to quantify their significance. A good agreement with the Standard Model prediction is observed in most of the event classes. The most significant deviation is found for a topology containing an isolated muon, missing transverse momentum and a jet, consistent with a previously reported observation.
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| 35. |
- Aktas, A, et al.
(författare)
-
Diffractive photoproduction of J/psi mesons with large momentum transfer at HERA
- 2003
-
Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 568:3-4, s. 205-218
-
Tidskriftsartikel (refereegranskat)abstract
- The diffractive photoproduction of J/psi mesons is measured with the HI detector at the ep collider HERA using an integrated luminosity of 78 pb(-1). The differential cross section dsigma(gammap --> J/psiY)/dt is studied in the range 2 < < 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 W-gammap in three t intervals, spanning the range 50 < W-gammap < 200 GeV. A fast rise of the cross section with W-gammap is observed for each t range and the slope for the effective linear Pomeron trajectory is measured to be alpha' = -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. (C) 200 Published by Elsevier B.V.
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| 36. |
- Aktas, A, et al.
(författare)
-
Evidence for a narrow anti-charmed baryon state
- 2004
-
Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 588:1-2, s. 17-28
-
Tidskriftsartikel (refereegranskat)abstract
- A narrow resonance in D*(-)p and D*(+)(p) over bar invariant mass combinations is observed in inelastic electron-proton collisions at centre-of-mass energies of 300 GeV and 320 GeV at HERA. The resonance has a mass of 3099 +/- 3(stat.) +/- 5(syst.) MeV and a measured Gaussian width of 12 +/- 3(stat.) MeV, compatible with the experimental resolution. The resonance is interpreted as an anti-charmed baryon with a minimal constituent quark composition of uudd (c) over bar, together with the charge conjugate. (C) 2004 Elsevier B.V. All rights reserved.
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| 37. |
- Aktas, A., et al.
(författare)
-
Inclusive D*(+/-) meson and associated dijet production in deep-inelastic scattering at HERA
- 2007
-
Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 51:2, s. 271-287
-
Tidskriftsartikel (refereegranskat)abstract
- Inclusive D*(+/-) production is measured in deep-inelastic ep scattering at HERA with the H1 detector. In addition, the production of dijets in events with a D*(+/-) meson is investigated. The analysis covers values of photon virtuality 2 <= Q(2) <= 100 GeV2 and of inelasticity 0.05 <= y <= 0.7. Differential cross sections are measured as a function of Q(2) and x and of various D*(+/-) meson and jet observables. Within the experimental and theoretical uncertainties all measured cross sections are found to be adequately described by next-to-leading order (NLO) QCD calculations, based on the photon - gluon fusion process and DGLAP evolution, without the need for an additional resolved component of the photon beyond what is included at NLO. A reasonable description of the data is also achieved by a prediction based on the CCFM evolution of partons involving the kT- unintegrated gluon distribution of the proton.
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| 38. |
- Aktas, A, et al.
(författare)
-
Inclusive production of D+, D-0, D-s(+) and D*(+) mesons in deep inelastic scattering at HERA
- 2005
-
Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 38:4, s. 447-459
-
Tidskriftsartikel (refereegranskat)abstract
- Inclusive production cross sections are measured in deep inelastic scattering at HERA for meson states composed of a charm quark and a light antiquark or the charge conjugate. The measurements cover the kinematic region of photon virtuality 2 < Q(2) < 100 GeV2, inelasticity 0.05 < y < 0.7, D meson transverse momenta p(t)( D) greater than or equal to 2.5 GeV and pseudorapidity |eta( D)| less than or equal to 1.5. The identification of the D-meson decays and the reduction of the combinatorial background profit from the reconstruction of displaced secondary vertices by means of the H1 silicon vertex detector. The production of charmed mesons containing the light quarks u, d and s is found to be compatible with a description in which the hard scattering is followed by a factorisable and universal hadronisation process.
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| 39. |
- Aktas, A, et al.
(författare)
-
Measurement of anti-deuteron photoproduction and a search for heavy stable charged particles at HERA
- 2004
-
Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 36:4, s. 413-423
-
Tidskriftsartikel (refereegranskat)abstract
- The cross section for anti-deuteron photoproduction is measured at HERA at a mean centre-of-mass energy of W-gammap=200 GeV in the range 0.2 < p(T)/M < 0.7 and y < 0.4, where M, p(T) and y are the mass, transverse momentum and rapidity of the anti-deuteron in the HERA laboratory frame, respectively. The numbers of anti-deuterons per event are found to be similar in photoproduction to those in central proton-proton collisions at the CERN ISR but much lower than those in central Au-Au collisions at RHIC. The coalescence parameter B-2, which characterizes the likelihood of anti-deuteron production, is measured in photoproduction to be 0.010+/-0.002+/-0.001, which is much higher than in Au-Au collisions at a similar nucleon-nucleon centre-of-mass energy. No significant production of particles heavier than deuterons is observed and upper limits are set on the photoproduction cross sections for such particles.
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| 40. |
- Aktas, A, et al.
(författare)
-
Measurement of dijet production at low Q(2) at HERA
- 2004
-
Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 37:2, s. 141-159
-
Tidskriftsartikel (refereegranskat)abstract
- Triple differential dijet cross sections in e(+/-)p interactions are presented in the region of photon virtualities 2 < Q(2) < 80 GeV2, inelasticities 0.1 < y < 0.85, jet transverse energies E*(T1) > 7GeV, E*(T2) > 5 GeV, and pseudorapidities - 2.5 < η(1)*, η(2)* < 0. The measurements are made in the gamma*p centre-of-mass frame, using an integrated luminosity of 57 pb(-1). The data are compared with NLO QCD calculations and LO Monte Carlo programs with and without a resolved virtual photon contribution. NLO QCD calculations fail to describe the region of low Q(2) and low jet transverse energies, in contrast to a LO Monte Carlo generator which includes direct and resolved photon interactions with both transversely and longitudinally polarised photons. Initial and final state parton showers are tested as a mechanism for including higher order QCD effects in low E-T jet production.
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