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Sökning: WFRF:(Collins A) > (2005-2009)

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11.
  • Abdallah, J., et al. (författare)
  • Study of multi-muon bundles in cosmic ray showers detected with the DELPHI detector at LEP
  • 2007
  • Ingår i: Astroparticle physics. - 0927-6505 .- 1873-2852. ; 28:3, s. 273-286
  • Tidskriftsartikel (refereegranskat)abstract
    • The DELPHI detector at LEP has been used to measure multi-muon bundles originating from cosmic ray interactions with air. The cosmic events were recorded in "parasitic mode" between individual e(+)e(-) interactions and the total live time of this data taking is equivalent to 1.6 x 10(6) s. The DELPHI apparatus is located about 100 m underground and the 84 metres rock overburden imposes a cutoff of about 52 GeV/c on muon momenta. The data from the large volume Hadron Calorimeter allowed the muon multiplicity of 54,201 events to be reconstructed. The resulting muon multiplicity distribution is compared with the prediction of the Monte Carlo simulation based on CORSIKA/QGSJETOI. The model fails to describe the abundance of high multiplicity events. The impact of QGSJET internal parameters on the results is also studied.
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12.
  • Abdallah, J, et al. (författare)
  • Coherent soft particle production in Z decays into three jets
  • 2005
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - Elsevier. - 0370-2693. ; 605:1-2, s. 37-48
  • Tidskriftsartikel (refereegranskat)abstract
    • Low-energy particle production perpendicular to the event plane in three-jet events produced in Z decays in e(+)e(-) annihilation is measured and compared to that perpendicular to the event axis in two-jet events. The topology dependence of the hadron production ratio is found to agree with a leading-order QCD prediction. This agreement and especially the need for the presence of a destructive interference term gives evidence for the coherent nature of gluon radiation. Hadron production in three-jet events is found to be directly proportional to a single topological scale function of the inter-jet angles. The slope of the dependence of the multiplicity with respect to the topological scale was measured to be 2.211 +/- 0.014(stat.) +/- 0.053(syst.) in good agreement with the expectation given by the colour-factor ratio C-A/C-F = 9/4. This result strongly supports the assumption of local parton-hadron duality, LPHD, at low hadron momentum.
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13.
  • Abdallah, J., et al. (författare)
  • Investigation of colour reconnection in WW events with the DELPHI detector at LEP-2
  • 2007
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 51:2, s. 249-269
  • Tidskriftsartikel (refereegranskat)abstract
    • In the reaction e(+)e(-) -> WW -> (q(1) (q) over bar (2))(q(3)(q) over bar (4)) the usual hadronization models treat the colour singlets q(1)(q) over bar (2) and q(3)(q) over bar (4) coming from two W bosons independently. However, since the. nal state partons may coexist in space and time, cross-talk between the two evolving hadronic systems may be possible during fragmentation through soft gluon exchange. This e. ect is known as colour reconnection. In this article the results of the investigation of colour reconnection e. ects in fully hadronic decays of W pairs in DELPHI at LEP are presented. Two complementary analyses were performed, studying the particle. ow between jets and W mass estimators, with negligible correlation between them, and the results were combined and compared to models. In the framework of the SK-I model, the value for its. parameter most compatible with the data was found to be: (SK)-S-kappa-I = 2.2(-1.3) (+2.5) corresponding to the probability of reconnection P-reco to be in the range 0.31 < P-reco < 0.68 at 68% confidence level with its best value at 0.52.
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14.
  • Abdallah, J., et al. (författare)
  • Masses, lifetimes and production rates of Xi(-) and Xi(+) at LEP 1
  • 2006
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - Elsevier. - 0370-2693. ; 639:3-4, s. 179-191
  • Tidskriftsartikel (refereegranskat)abstract
    • Measurements of the Xi(-) and (Xi) over bar (+) masses, mass differences, lifetimes and lifetime differences are presented. The (Xi) over bar (+) sample used is much larger than those used previously for such measurements. In addition, the S production rates in Z -> b (b) over bar and Z -> q (q) over bar events are compared and the position xi* of the maximum of the distribution in Z -> q (q) over bar events is measured.
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15.
  • Abdallah, J., et al. (författare)
  • Measurement and interpretation of fermion-pair production at LEP energies above the Z resonance
  • 2006
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 45:3, s. 589-632
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents DELPHI measurements and interpretations of cross-sections, forward-backward asymmetries, and angular distributions, for the e(+)e(-) -> f (f) over bar process for centre-of-mass energies above the Z resonance, from root s similar to 130-207 GeV at the LEP collider. The measurements are consistent with the predictions of the Standard Model and are used to study a variety of models including the S-Matrix ansatz for e(+)e(-) -> f (f) over bar scattering and several models which include physics beyond the Standard Model: the exchange of Z' bosons, contact interactions between fermions, the exchange of gravitons in large extra dimensions and the exchange of (v) over tilde in R-parity violating supersymmetry.
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16.
  • Abdallah, J., et al. (författare)
  • Measurement of the mass and width of the W boson in e(+) e(-) collisions at root s=161-209 GeV
  • 2008
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 55:1, s. 1-38
  • Tidskriftsartikel (refereegranskat)abstract
    • A measurement of the W boson mass and width has been performed by the DELPHI collaboration using the data collected during the full LEP2 programme (1996-2000). The data sample has an integrated luminosity of 660 pb(-1) and was collected over a range of centre-of-mass energies from 161 to 209 GeV. Results are obtained by applying the method of direct reconstruction of the mass of the W from its decay products in both the W+W- -> l (nu) over bar (l) and W+W- -> q (q) over bar'(q) over barq'channels. The W mass result for the combined data set is M-W = 80.336 +/- 0.055(Stat.) +/- 0.028(Syst.) +/- 0.025(FSI) +/- 0.009(LEP) GeV/c(2) , where FSI represents the uncertainty due to final state interaction effects in the q (q) over bar'(q) over barq' channel, and LEP represents that arising from the knowledge of the collision energy of the accelerator. The combined value for the W width is Gamma(W) = 2.404 +/- 0.140(Stat.) +/- 0.077(Syst.) +/- 0.065(FSI) GeV/c(2). These results supersede all values previously published by the DELPHI collaboration.
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17.
  • Abdallah, J., et al. (författare)
  • Search for a fourth generation b '-quark at LEP-II at root s=196-209GeV
  • 2007
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 50:3, s. 507-518
  • Tidskriftsartikel (refereegranskat)abstract
    • A search for the pair production of fourth generation b'-quarks was performed using data taken by the DELPHI detector at LEP-II. The analysed data were collected at centre-of-mass energies ranging from 196 to 209 GeV, corresponding to an integrated luminosity of 420 pb(-1). No evidence for a signal was found. Upper limits on BR(b'-> bZ) and BR(b'-> bZ) were obtained for b' masses ranging from 96 to 103 GeV/c(2) stop. These limits, together with the theoretical branching ratios predicted by a sequential four generations model, were used to constrain the value of R-CKM=vertical bar V-cb(') /V-tb'V-tb vertical bar there V-cb', V-tb' and V-tb are elements of the extended CKM matrix.
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18.
  • Abdallah, J., et al. (författare)
  • Search for pentaquarks in the hadronic decays of the Z boson with the DELPHI detector at LEP
  • 2007
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - Elsevier. - 0370-2693. ; 653:2-4, s. 151-160
  • Tidskriftsartikel (refereegranskat)abstract
    • The quark model does not exclude states composed of more than three quarks, like pentaquark systems. Controversial evidence for such states has been published in the last years, in particular: for a strange pentaquark Theta(1540)(+); for a double-strange state, the Xi(1862)(--), subsequently called Phi(1860)--; and for a charmed state, the Theta(c)(3100)(0). If confirmed, a full pentaquark family might exist; such pentaquark states could be produced in e(+)e(-) annihilations near the Z energy. In this Letter a search for pentaquarks is described using the DELPHI detector at LEP, characterized by powerful particle identification sub-systems crucial in the separation of the signal from the background for these states. At 95% CL, upper limits are set on the production rates N of such particles and their charge-conjugate state per Z decay: N-Theta+ x Br(Theta(+) -> pK(S)(0)) < 5.1 x 10(-4), N Theta++ < 1.6 x 10(-3), N Phi(1860)-- x Br((P(1860)-- -> Xi(-)pi(-)) < 2.9 x 10(-4), N-Theta c(3100)0 x Br(Theta(c)(3100)(0) -> D*(+)p) < 8.8 x 10(-4). (c) 2007 Elsevier B.V. All rights reserved.
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19.
  • Abdallah, J.,..., et al. (författare)
  • Study of b-quark mass effects in multijet topologies with the DELPHI detector at LEP
  • 2008
  • Ingår i: European Physical Journal C. Particles and Fields. - Springer. - 1434-6044. ; 55:4, s. 525-538
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of the heavy b-quarkmass on the two, three and four-jet rates is studied using LEP data collected by the DELPHI experiment at the Z peak in 1994 and 1995. The rates of b-quark jets and light quark jets (l = uds) in events with n = 2, 3, and 4 jets, together with the ratio of two and four-jet rates of b-quarks with respect to light-quarks, R-n(bl), have been measured with a double-tag technique using the CAMBRIDGE jet-clustering algorithm. A comparison between experimental results and theory (matrix element or Monte Carlo event generators such as PYTHIA, HERWIG andARIADNE) is done after the hadronisation phase. Using the four-jet observabl R-4(bl), a measurement of the b-quark mass using massive leading-order calculations gives: m(b)(M-Z) = 3.76 +/- 0.32 (stat) +/- 0.17 (syst) +/- 0.22 (had) +/- 0.90 (theo) GeV/c(2). This result is compatible with previous three-jet determinations at the M-Z energy scale and with low energy mass measurements evolved to the M-Z scale using QCD renormalisation group equations.
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20.
  • Abdallah, J., et al. (författare)
  • Study of leading hadrons in gluon and quark fragmentation
  • 2006
  • Ingår i: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - Elsevier. - 0370-2693. ; 643:3-4, s. 147-157
  • Tidskriftsartikel (refereegranskat)abstract
    • The study of quark jets in e(+)e(-) reactions at LEP has demonstrated that the hadronisation process is reproduced well by the Lund string model. However. our understanding of gluon fragmentation is less complete. In this study enriched quark and gluon jet samples of different purities are selected in three-jet events from hadronic decays of the Z collected by the DELPHI experiment in the LEP runs during 1994 and 1995. The leading systems of the two kinds of jets are defined by requiring a rapidity gap and their sum of charges is studied. An excess of leading systems with total charge zero is found for gluon jets in all cases, when compared to Monte Carlo simulations with JETSET (with and without Bose-Einstein correlations included) and ARIADNE. The corresponding leading systems of quark jets do not exhibit such an excess. The influence of the gap size and of the gluon purity on the effect is studied and a concentration of the excess of neutral leading systems at low invariant masses (less than or similar to 2 GeV/c(2)) is observed, indicating that gluon jets might have an additional hitherto undetected fragmentation mode via a two-gluon system. This could be an indication of a possible production of gluonic states as predicted by QCD. (c) 2006 Elsevier B.V. All rights reserved.
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