| 61. |
- Aktas, A, et al.
(författare)
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Search for leptoquark bosons in ep collisions at HERA
- 2005
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 629:1, s. 9-19
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Tidskriftsartikel (refereegranskat)abstract
- A search for scalar and vector leptoquarks coupling to first generation fermions is performed using the e(+) p and e(-) p scattering data collected by the H1 experiment between 1994 and 2000. The data correspond to a total integrated luminosity of 117 pb(-1). No evidence for the direct or indirect production of such particles is found in data samples with a large transverse momentum final state electron or with large missing transverse momentum. Constraints on leptoquark models are established. For leptoquark couplings of electromagnetic strength, leptoquarks with masses up to 275-325 GeV are ruled out at 95% confidence level. These limits improve and supercede earlier H1 limits based on subsamples of the data used here.
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| 62. |
- Aktas, A., et al.
(författare)
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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
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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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| 63. |
- Aktas, A., et al.
(författare)
-
Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA
- 2007
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 51:3, s. 549-568
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Tidskriftsartikel (refereegranskat)abstract
- Measurements are presented of differential dijet cross sections in diffractive photoproduction (Q(2) < 0.01 GeV2) and deep-inelastic scattering processes (DIS, 4 < Q2 < 80 GeV2). The event topology is given by ep -> eXY, in which the system X, containing at least two jets, is separated from a leading low-mass baryonic system Y by a large rapidity gap. The dijet cross sections are compared with NLO QCD predictions based on diffractive parton densities previously obtained from a QCD analysis of inclusive diffractive DIS cross sections by H1. In DIS, the dijet data are well described, supporting the validity of QCD factorisation. The diffractive DIS dijet data are more sensitive to the diffractive gluon density at high fractional parton momentum than the measurements of inclusive diffractive DIS. In photoproduction, the predicted dijet cross section has to be multiplied by a factor of approximately 0.5 for both direct and resolved photon interactions to describe the measurements. The ratio of measured dijet cross section to NLO prediction in photoproduction is a factor 0.5 +/- 0.1 smaller than the same ratio in DIS. This suppression is the first clear observation of QCD hard scattering factorisation breaking at HERA. The measurements are also compared to the two soft colour neutralisation models SCI and GAL. The SCI model describes diffractive dijet production in DIS but not in photoproduction. The GAL model fails in both kinematic regions.
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| 64. |
- Aaron, F. D., et al.
(författare)
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A search for excited neutrinos in e(-) p collisions at HERA
- 2008
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 663:5, s. 382-389
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Tidskriftsartikel (refereegranskat)abstract
- A search for first generation excited neutrinos is performed using the full e(-) p data sample collected by the HI experiment at HERA at a centre-of-mass energy of 319 GeV, corresponding to a total luminosity of 184 pb(-1). The electroweak decays of excited neutrinos nu* -> nu gamma, v* -> nu Z and nu* -> eW with subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for excited neutrino production is found. Mass dependent exclusion limits on nu* production cross sections and on the ratio of the coupling to the compositeness scale f/boolean AND are derived within gauge mediated models. A limit on f/boolean AND A, independent of the relative couplings to the SU(2) and U(1) gauge bosons, is also determined. These limits extend the excluded region to higher masses than has been possible in previous excited neutrino searches. (c) 2008 Elsevier B.V. All rights reserved.
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| 65. |
- Aaron, F. D., et al.
(författare)
-
Study of charm fragmentation into D (* +/-) mesons in deep-inelastic scattering at HERA
- 2009
-
Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 59:3, s. 589-606
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Tidskriftsartikel (refereegranskat)abstract
- The process of charm quark fragmentation is studied using D (* +/-) meson production in deep-inelastic scattering as measured by the H1 detector at HERA. The parameters of fragmentation functions are extracted for QCD models based on leading order matrix elements and DGLAP or CCFM evolution of partons together with string fragmentation and particle decays. Additionally, they are determined for a next-to-leading order QCD calculation in the fixed flavour number scheme using the independent fragmentation of charm quarks to D (* +/-) mesons. Two different regions of phase space are investigated defined by the presence or absence of a jet containing the D (* +/-) meson in the event. The fragmentation parameters extracted for the two phase space regions are found to be different.
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| 66. |
- Abdallah, J, et al.
(författare)
-
Charged particle multiplicity in three-jet events and two-gluon systems
- 2005
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; C:44, s. 311-331
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Tidskriftsartikel (refereegranskat)abstract
- The charged particle multiplicity in hadronic three-jet events from Z decays is investigated. The topology dependence of the event multiplicity is found to be well described by a modified leading logarithmic prediction. A parameter fit of the prediction to the data yields a measurement of the colour factor ratio C-A/C-F with the result C-A/C-F = 2.261 +/- 0.014(stat). +/- 0.036(exp). +/- 0-066(theo). in agreement with the SU(3) expectation of QCD. The quark-related contribution to the event multiplicity is subtracted from the three-jet event multiplicity resulting in a measurement of the multiplicity of two-gluon colour-singlet states over a wide energy range. The ratios r = N-gg(s)/Ng (g) over bar (s) of the gluon and quark multiplicities and r((1)) = N'(gg)(s)/N'g (g) over bar (s) of their derivatives are compared with perturbative calculations. While a good agreement between calculations and data is observed for r((1)), larger deviations are found for r indicating that non-perturbative effects are more important for r than for r((1)).
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| 67. |
- Abdallah, J, et al.
(författare)
-
Determination of heavy quark non-perturbative parametersfrom spectral moments in semileptonic B decays
- 2006
-
Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 45:1, s. 35-59
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Tidskriftsartikel (refereegranskat)abstract
- Moments of the hadronic invariant mass and of the lepton energy spectra in semileptonic B decays have been determined with the data recorded by the DELPHI detector at LEP. From measurements of the inclusive b-hadron semileptonic decays, and imposing constraints from other measurements on b- and c-quark masses, the first three moments of the lepton energy distribution and of the hadronic mass distribution, have been used to determine parameters which enter into the extraction of vertical bar V-cb vertical bar from the measurement of the inclusive b-hadron semileptonic decay width. The values obtained in the kinetic scheme are: m(b)(1 GeV) = 4.591 +/- 0.062 +/- 0.039 +/- 0.005 GeV/c(2), m(c)(1 GeV) = 1.170 +/- 0.093 +/- 0.055 +/- 0.005 GeV/c(2), m(pi)(2) (1 GeV) = 0.399 +/- 0.048 +/- 0.034 +/- 0.087 GeV2, (p) over tilde (3)(D) = 0.053 +/- 0.017 +/- 0.011 +/- 0.026 GeV3, and include corrections at order 1/m(b)(3). Using these results, and present measurements of the inclusive semileptonic decay partial width of b-hadrons at LEP, an accurate determination of vertical bar V-cb vertical bar is obtained: vertical bar V-cb vertical bar = 0.0421 x (1 +/- 0.014 (meas center dot) +/- 0.014 (fit) +/- 0.015 (th center dot)).
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| 68. |
- Abdallah, J, et al.
(författare)
-
Inclusive single-particle production in two-photon collisions at LEP II with the DELPHI detector
- 2009
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Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 678:5, s. 444-449
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Tidskriftsartikel (refereegranskat)abstract
- A study of the inclusive charged hadron production in two-photon collisions is described. The data were collected with the DELPHI detector at LEP II. Results on the inclusive single-particle p(T) distribution and the differential charged hadrons d sigma/dp(T) cross-section are presented and compared to the predictions of perturbative NLO QCD calculations and to published results. (C) 2009 Elsevier B.V. All rights reserved.
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| 69. |
- Abdallah, J, et al.
(författare)
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Photon events with missing energy in e(+)e(-) collisions at root s=130 to 209 GeV
- 2005
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 38:4, s. 395-411
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Tidskriftsartikel (refereegranskat)abstract
- The production of single- and multi-photon events has been studied in the reaction e(+) e(-) --> gamma(gamma) + invisible particles. The data collected with the DELPHI detector during the years 1999 and 2000 at centre-of-mass energies between 191 GeV and 209 GeV was combined with earlier data to search for phenomena beyond the Standard Model. The measured number of light neutrino families was consistent with three and the absence of an excess of events beyond that predicted by the Standard Model processes was used to set limits on new physics. Both model-independent searches and searches for new processes predicted by supersymmetric and extra-dimensional models have been made. Limits on new non-standard model interactions between neutrinos and electrons were also determined.
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| 70. |
- Aktas, A, et al.
(författare)
-
Search for light gravitinos in events with photons and missing transverse momentum at HERA
- 2005
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Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 616:1-2, s. 31-42
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Tidskriftsartikel (refereegranskat)abstract
- A search for gravitinos produced in e(+/-)p collisions is performed using the H1 detector at HERA. The data were taken at a centre-of-mass energy of 319 GeV and correspond to an integrated luminosity of 64.3 pb(-1) for e(+)p collisions and 13.5 pb(-1) for e(-)p collisions. If R-parity is not conserved, the t-channel exchange of a selectron can produce a neutralino, which, in models where the gravitino is the lightest supersymmetric particle, subsequently decays into a photon and a light gravitino. The resulting event signature, which involves an isolated photon, a jet and missing transverse energy, is analysed for the first time at HERA. No deviation from the Standard Model is found. Exclusion limits on the cross section and on R-parity-violating Yukawa couplings are derived in a Gauge Mediated Supersymmetry Breaking scenario. The results are independent of the squark sector. Neutralinos and supersymmetric partners of the left-handed electron with masses up to 112 GeV and 164 GeV, respectively, can be ruled out at the 95% confidence level for R-parity-violating couplings lambda' equal to 1, in some parts of the parameter space of the considered model.
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