LHCb

Abstract:

Photoproduction events which have two or more jets have been studied in the Wγp range 135 GeV < Wγp < 280 GeV with the ZEUS detector at HERA. A class of events is observed with little hadronic activity between the jets. The jets are separated by pseudorapidity intervals (Δη) of up to four units and have transverse energies greater than 6 GeV. A gap is defined as the absence between the jets of particles with transverse energy greater than 300 MeV. The fraction of events containing a gap is measured as a function of Δη. It decreases exponentially as expected for processes in which colour is exchanged between the jets, up to a value of Δη ∼ 3, then reaches a constant value of about 0.1. The excess above the exponential fall-off can be interpreted as evidence for hard diffractive scattering via a strongly interacting colour singlet object.

Abstract:

The tau lepton lifetime has been measured using three different methods with the DELPHI detector. Two measurements of one-prong decays are combined, accounting for correlations, giving a result ofττ = 291.8 ± 3.3 (stat.) ± 2.0(sys.) fs while the decay length distribution of three-prong decays gives the result ττ = 286.7 ± 4.9 (stat.) ± 3.3 (sys.) fs. Combining the results presented here with previous DELPHI measurements, we get ττ = 291.4 ± 3.0 fs and find that the ratio of the coupling constant for tau decay relative to that for muon decay is 0.990 ± 0.009, compatible with lepton universality.

Abstract:

The average lifetime of b-baryons has been studied using 3 × 106 hadronic Z0 decays collected by the DEL-PHI detector at LEP. Three methods have been used, based on the measurement of different observables: the proper decay time distribution of 206 vertices reconstructed with a Λ, a lepton and an oppositely charged pion; the impact parameter distribution of 441 muons with high transverse momentum accompanied by a Λ in the same jet; and the proper decay time distribution of 125 Λc-lepton decay vertices with the Λc exclusively reconstructed through its pKπ, pK0 and Λ3π decay modes. The combined result is: τ(b-baryon) = (1.254-0.109+0.121(stat) ±0.04(syst)-0.05+0.03(syst)) ps where the first systematic error is due to experimental uncertainties and the second to the uncertainties in the modelling of the b-baryon production and semi-leptonic decay. Including the measurement recently published by DELPHI based on a sample of proton-muon vertices, the average b-baryon lifetime is : τ(b-baryon) = (1.255-0.102+0.115(stat) ±0.05) ps. © Springer-Verlag 1996.

Abstract:

Semileptonic decays B → D*- ℓ+νX were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant and non-resonant D*- π+ states was performed and the ratio of the branching fractions: Br(B → D*- ℓ+νX)/Br(B → D*- ℓ+νX) + Br(B0 → D*- ℓ+ν) = 0.19 ± 0.10(stat) ± 0.06(syst) was determined. Taking into account this contribution, the differential decay width of B0 → D*- ℓ+ν was measured as a function of the momentum transfer from the B to the D*- in two separate analyses, using exclusive and inclusive methods of D*- ℓ+ reconstruction. The distributions were fitted over the full momentum transfer range to extract the product of |V cb| times the normalization of the decay form factor F(q max2): F(qmax2)|Vcb| = (35.4 ± 1.9(stat) ± 2.4(syst)) • 10-3. The value of |Vcb| was computed using theoretical calculations of F(q max2), giving: |Vcb| = (38.9 ± 2.0(stat) ± 2.6(syst) ± 1.7(theory)) • 10-3. The total branching fraction Br(B0 → D*- ℓ+ν) was determined to be: Br(B0 → D*- ℓ+ν) = (5.52 ± 0.17(stat) ± 0.68(syst))%.

Abstract:

Three jet events arising from decays of the Z boson, collected by the DELPHI detector, were used to measure differences in quark and gluon fragmentation. Gluon jets were anti-tagged by identifying b quark jets. Unbiased quark jets came from events with two jets plus one photon. Quark and gluon jet properties in different energy ranges were compared for the first time within the same detector. Quark and gluon jets of nearly the same energy in symmetric three jet event topologies were also compared. Using three independent methods, the average value of the ratio of the mean charged multiplicities of gluon and quark jets is 〈 r 〉= 1.241 ± 0.015 (stat.) ± 0.025 (syst.). Gluon jets are broader and produce fragments with a softer energy spectrum than quark jets of equivalent energy. The string effect has been observed in fully symmetric three jet events. The measured ratio Rγ of the charged particle flow in the qq̄ inter-jet region of the qq̄g and qq̄γ samples agrees with the perturbative QCD expectation. The dependence of the mean charged multiplicity on the hadronic center-of-mass energy was analysed in photon plus n-jet events. The value for αs(MZ) determined from these data using a QCD prediction with corrections at leading and next-to-leading order is αs(MZ) = 0.116 ± 0.003 (stat.) ± 0.009 (syst.). © Springer-Verlag 1996.