0000000000201412
AUTHOR
A Tilquin
showing 21 related works from this author
B* production in Z decays
1995
The decay B*→Bγ has been observed with the DELPHI detector at LEP, where the B* meson is produced in Z boson decays. The combination of inclusively reconstructed B mesons with well-measured converted photons yields a measurement of the flavour-averaged B*-B mass difference of 45.5±0.3 (stat.) ±0.8 (syst.) MeV/c2. 95% confidence level upper limits at 6 MeV/c2 are placed on both the isospin (i.e. B+-B0) and the Bs-Bud splitting of the mass difference. The production ratio of B* to B mesons in Z decays is measured to be 0.72±0.03 (stat.) ±0.06 (syst.). Limits on the production cross-section of other hypothetical excited B hadron states decaying radiatively are established. The differential B* …
Search for pair-production of long-lived heavy charged particles in e+e− annihilation
1997
A search for pair-production of long-lived, heavy, singly-charged particles has been performed with data collected by the ALEPH detector at a centre-of-mass energy of 172 GeV. Data at \sqrt{s} = 161, 136, and 130 GeV are also included to improve the sensitivity to lower masses. No candidate is found in the data. A model-independent 95% confidence level upper limit on the production cross section at 172 GeV of 0.2-0.4pb is derived for masses between 45 and 86 GeV/c^2. This cross section limit implies, assuming the MSSM, a lower limit of 67 (69) GeV/c^2 on the mass of right- (left-) handed long-lived scalar taus or scalar muons and of 86 GeV/c^2 on the mass of long-lived charginos.
Investigation of the splitting of quark and gluon jets
1998
The splitting processes in identified quark and gluon jets are investigated using longitudinal and transverse observables. The jets are selected from symmetric three-jet events measured in Z decays L with the {\sc Delphi} detector in 1991-1994. Gluon jets are identified using heavy quark anti-tagging. Scaling violations in identified gluon jets are observed for the first time. The scale energy dependence of the gluon fragmentation function is found to be about two times larger than for the corresponding quark jets, consistent with the QCD expectation $C_A/C_F$. The primary splitting of gluons and quarks into subjets agrees with fragmentation models and, for specific regions of the jet resol…
Search for charged Higgs bosons in $e^+ e^-$ collisions at centre-of-mass energies from 130 to 172 GeV
1998
Abstract The data collected at centre-of-mass energies ranging from 130 to 172 GeV by ALEPH at LEP, corresponding to an integrated luminosity of 27.5 pb−1, are analysed in a search for pair-produced charged Higgs bosons H±. Three analyses are employed to select the τ + ν τ τ − ν τ , c s τ − ν τ and c s s c final states. No evidence for a signal is found. Mass limits are set as a function of the branching fraction B (τν) for H±→τν. Charged Higgs bosons with masses below 52 GeV/c2 are excluded at 95% C.L. independently of B (τν) , thus significantly improving on existing limits.
Measurements of the Lineshape of the $Z^{0}$ and Determination of Electroweak Parameters from its Hadronic and Leptonic Decays
1994
Abstract: During the LEP running periods in 1990 and 1991 DELPHI has accumulated approximately 450000 Z0 decays into hadrons and charged leptons. The increased event statistics coupled with improved analysis techniques and improved knowledge of the LEP beam energies permit significantly better measurements of the mass and width of the Z0 resonance. Model independent fits to the cross sections and leptonic forward-backward asymmetries yield the following Z0 parameters: the mass and total width M(Z) = 91.187 +/- 0.009 GeV, GAMMA(Z) = 2.486 +/- 0.012 GeV, the hadronic and leptonic partial widths GAMMA(had) = 1.725 +/- 0.012GeV, GAMMA(l) = 83.01 +/- 0.52 MeV, the invisible width GAMMA(inv) = 51…
DETERMINATION OF ALPHA(S) FOR B-QUARKS AT THE Z(0) RESONANCE
1993
The strong coupling constant for b quarks has been determined, and its flavour independence, as predicted by QCD, investigated. The analysis involved events with lepton candidates selected from approximately 356 000 hadronic decays of the Z0, collected by the DELPHI detector at LEP in 1990 and 199 1. A method based on a direct comparison of the three-jet fraction in a b enriched sample, selected by requiring leptons with large momenta and transverse momenta, to that of the entire hadronic sample, illustrated the significant effect of the b quark mass on the multi-jet cross section, and verified the flavour independence of the strong coupling constant to an accuracy of +/- 6%. A second proce…
Measurement of inclusive pi(0) production in hadronic Z(0) decays
1996
An analysis is presented of inclusive \pi^0 production in Z^0 decays measured with the DELPHI detector. At low energies, \pi^0 decays are reconstructed by \linebreak using pairs of converted photons and combinations of converted photons and photons reconstructed in the barrel electromagnetic calorimeter (HPC). At high energies (up to x_p = 2 \cdot p_{\pi}/\sqrt{s} = 0.75) the excellent granularity of the HPC is exploited to search for two-photon substructures in single showers. The inclusive differential cross section is measured as a function of energy for {q\overline q} and {b \bar b} events. The number of \pi^0's per hadronic Z^0 event is N(\pi^0)/ Z_{had}^0 = 9.2 \pm 0.2 \mbox{(stat)} \…
Search for scalar leptons in e+e- collisions at centre-of-mass energies up to 209 GeV
2002
A search for selectron, smuon and stau pair production is performed with the data collected by the ALEPH detector at LEP at centre-of-mass energies up to 209 GeV. The numbers of candidate events are consistent with the background predicted by the Standard Model. Final mass limits from ALEPH are reported.
Searches for Supersymmetry in the photon(s) plus missing energy channels at $\sqrt{s}$ = 161 GeV and 172 GeV
1997
Searches for supersymmetric particles in channels with one or more photons and missing energy have been performed with data collected by the ALEPH detector at LEP. The data consist of 11.1 pb-1 at $\sqrt{s} = 161 GeV$, 1.1 pb-1 at 170 GeV and 9.5 pb-1 at 172 GeV. The e+e- -> nunu+photon cross section is measured. The data are in good agreement with predictions based on the Standard Model, and are used to set upper limits on the cross sections for anomalous photon production. These limits are compared to two different SUSY models and used to set limits on the neutralino mass. A limit of 71 GeV/c^2 at 95% C.L. is set on the mass of the lightest neutralino ($\tau_{\chi_{1}^{0}} \leq $ 3 ns) fo…
DETERMINATION OF ALPHA-S FROM THE SCALING VIOLATION IN THE FRAGMENTATION FUNCTIONS IN E+E- ANNIHILATION
1993
A determination of the hadronic fragmentation functions of the Z0 boson is presented from a study of the inclusive hadron production with the DELPHI detector at LEP. These fragmentation functions were compared with the ones at lower energies, thus covering data in a large kinematic range: 196 less-than-or-equal-to Q2 less-than-or-equal-to 8312 GeV2 and x (= p(h)/E(beam)) > 0.08. A large scaling violation was observed, which was used to extract the strong coupling constant in second order QCD: alpha(s)(M(Z)) = 0.118 +/- 0.005. The corresponding QCD scale for five quark flavours is: LAMBDA(MS)(5)BAR = 230 +/- 60 MeV.
Test of Colour Reconnection Models using Three-Jet Events in Hadronic Z Decays
2006
Hadronic Z decays into three jets are used to test QCD models of colour reconnection (CR). A sensitive quantity is the rate of gluon jets with a gap in the particle rapidity distribution and zero jet charge. Gluon jets are identified by either energy-ordering or by tagging two b-jets. The rates predicted by two string-based tunable CR models, one implemented in JETSET (the GAL model), the other in ARIADNE, are too high and disfavoured by the data, whereas the rates from the corresponding non-CR standard versions of these generators are too low. The data can be described by the GAL model assuming a small value for the R_0 parameter in the range 0.01-0.02.
Measurement of Event Shape and Inclusive Distributions at $\sqrt{s} =$ 130 and 136 GeV
1997
Inclusive charged particle and event shape distributions are measured using 321 hadronic events collected with the DELPHI experiment at LEP at effective centre of mass energies of 130 to 136 GeV. These distributions are presented and compared to data at lower energies, in particular to the precise Z data. Fragmentation models describe the observed changes of the distributions well. The energy dependence of the means of the event shape variables can also be described using second order QCD plus power terms. A method independent of fragmentation model corrections is used to determine $\alpha_s$ from the energy dependence of the mean thrust and heavy jet mass. It is measured to be: % %\alpha_s…
Search for an invisibly decaying Higgs boson in e+e-collisions at 189 GeV
1999
The data collected in 1998 by ALEPH at LEP at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.2 pb(-1), are analysed to search for invisible decays of a Higgs boson produced in the reaction e(+) e(-) --> hZ. The number of events found in the data and their properties art: in agreement with the Standard Model expectation. This search results in an improved 95% C.L. lower limit on the Higgs boson mass of 95.4 GeV/c(2), assuming it decays totally invisibly and for a production cross section equal to that of the Standard Model. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
Measurement of the W mass in e+e− collisions at 183 GeV
1999
Abstract The mass of the W boson is obtained from reconstructed invariant mass distributions in W-pair events. The sample of W pairs is selected from 57 pb−1 collected with the ALEPH detector in 1997 at a centre-of-mass energy of 183 GeV. The invariant mass distributions of reweighted Monte Carlo events are fitted separately to the experimental distributions in the q q q q and all lν q q channels to give the following W masses: m W hadronic =80.461±0.177 (stat.) ±0.045 (syst.) ±0.056 (theory) GeV /c 2 , m W semileptonic =80.326±0.184 (stat.) ±0.040 (syst.) GeV /c 2 , where the theory error represents the possible effects of final state interactions. The combination of these two measurements…
Measurement of the W boson Mass and Width in $e^{+}e^{-}$ Collisions at LEP
2006
The mass of the W boson is determined from the direct reconstruction of W decays in WW→qq̄qq̄ and WW→ℓνqq̄ events in e+e- collisions at LEP. The data sample corresponds to an integrated luminosity of 683 pb-1 collected with the ALEPH detector at centre-of-mass energies up to 209 GeV. To minimise any effect from colour reconnection a new procedure is adopted in which low energy particles are not considered in the mass determination from the qq̄qq̄ channel. The combined result from all channels is mW=80.440±0.043(stat.)±0.024(syst.)±0.009(FSI)±0.009(LEP)GeV/c2,where FSI represents the possible effects of final state interactions in the qq̄qq̄ channel and LEP indicates the uncertainty in the b…
Measurement of the W-pair cross section in $e^+ e^-$ collisions at 172 GeV
1997
The e(+)e(-) --> W+W- cross section is measured in a data sample collected by ALEPH at a mean centre-of-mass energy of 172.09 GeV, corresponding to an integrated luminosity of 10.65 pb(-1). Cross sections are given for the three topologies, fully leptonic, semi-leptonic and hadronic of a W-pair decay. Under the assumption that no other decay modes are present, the W-pair cross section is measured to be 11.7 +/- 1.2(stat.) +/- 0.3(syst.) pb. The existence of the triple gauge boson vertex of the Standard Model is clearly preferred by the data. The decay branching ratio of the W boson into hadrons is measured to be B(W --> hadrons) = 67.7 +/- 3.1(stat.) +/- 0.7(syst.) %, allowing a determinati…
Single- and multi-photon production in e+e− collisions at a centre-of-mass energy of 183 GeV
1998
The production of final states involving one or more energetic photons from e(+)e(-) collisions is studied in a sample of 58.5 pb(-1) of data recorded at a centre-of-mass energy of 183 GeV by the ALEPH detector at LEP. The e(+)e(-) --> v (v) over bar gamma(gamma) and e(+)e(-) --> gamma gamma(gamma) cross sections are measured. The data are in good agreement with predictions based on the Standard Model and are used to set upper limits on the cross sections for anomalous photon production in the context of two supersymmetric models and for various extensions to QED. In particular, in the context of a super-light gravitino model a cross section upper limit of 0.38 pb is placed on the process e…
Absolute mass lower limit for the lightest neutralino of the MSSM from $e^{+}e^{-}$ data at $\sqrt{s}$ up to 209 GeV
2004
Charginos and neutralinos are searched for in the data collected by the ALEPH experiment at LEP for centre-of-mass energies up to 209 GeV. The negative result of these searches is combined with those from searches for sleptons and Higgs bosons to derive an absolute lower limit of 43.1 GeV/c(2) on the mass of the lightest supersymmetric particle (LSP), assumed to be the,lightest neutralino. This limit is obtained in the framework of the MSSM with R-parity conservation and with gaugino and sfermion mass unification at the GUT scale and assuming no mixing in the stau sector. The LSP limit degrades only slightly to 42.4 GeV/c(2) if stau mixing is considered. Within the more constrained framewor…
Search for invisible Higgs boson decays in e+e- collisions at centre-of-mass energies up to 184 GeV
1999
In a data sample of 78.3 pb(-1) collected in 1996 and 1997 by the ALEPH detector at centre-of-mass energies from 161 to 184 GeV, invisible decays of a Higgs boson have been searched for in the reaction e(+)e(-) --> hZ, where the Z can decay into e(+)e(-), mu(+) mu(-) or . No evidence for a signal is found and limits on the production cross section are derived as a function of the Higgs boson mass. These results are combined with those obtained in an update of the analyses of the ALEPH data taken at LEP 1. For a production cross section equal to that of the minimal standard model Higgs boson, masses below 80 GeV/c(2) are excluded at 95% C.L. (C) 1999 Elsevier Science B.V. All rights reserved.
Measurement of the charged particle multiplicity of weakly decaying B hadrons
1998
From the Z decays recorded in 1994 and 1995 by the DELPHI detector at LEP, the charged particle multiplicity of weakly decaying B hadrons was measured to be: 4.97 +/- 0.03 +/- 0.06 excluding the K-o and Lambda decay products. (C) 1998 Published by Elsevier Science B.V. All rights reserved.
Four-jet final state production in $e^+ e^-$ collisions at centre-of-mass energies ranging from 130 to 184 GeV
1998
The four jet topology is analysed in the ALEPH data taken between November 1995 and November 1997, at centre-of-mass energies ranging from 130 to 184 GeV. While an unexpected accumulation of events with a dijet mas sum around 105 GeV/c**2 had been observed during the first run in 1995 at 130/136 GeV, corresponding to an integrated luminosity of 5.7 pb-1, no significant differences between data and standard model prediction is noticed, either in the high energy runs (81.1 pb-1 taken at centre-of-mass energies from 161 to 184 GeV) or in the 7.1 pb-1 recorded during a new short run at 130/136 GeV in 1997. We have found no other explanation for the earlier reported ``four jet anomaly'' than a s…