LHCb

Abstract:

Knowledge of the centre-of-mass energy at LEP2 is of primary importance to set the absolute energy scale for the measurement of the W-boson mass. The beam energy above 80 GeV is derived from continuous measurements of the magnetic bending field by 16 NMR probes situated in a number of the LEP dipoles. The relationship between the fields measured by the probes and the beam energy is calibrated against precise measurements of the average beam energy between 41 and 55GeV made using the resonant depolarisation technique. The linearity of the relationship is tested by comparing the fields measured by the probes with the total bending field measured by a flux loop. This test results in the largest contribution to the systematic uncertainty. Several further corrections are applied to derive the centre-of-mass energies at each interaction point. In addition, the centre-of-mass energy spread is evaluated. The beam energy has been determined with a precision of 25 MeV for the data taken in 1997, corresponding to a relative precision of 2.7 × 10-4. This is small in comparison to the present uncertainty on the W mass measurement at LEP. However, the ultimate statistical precision on the W mass with the full LEP2 data sample should be around 25 MeV, and a smaller uncertainty on the beam energy is desirable. Prospects for improvements are outlined.

Abstract:

Exclusive production of ρ0 and J/ψ mesons in e+p collisions has been studied with the ZEUS detector in the kinematic range 0.25 < Q2 < 50 GeV2, 20 < W < 167 GeV for the ρ0 data and 2 < Q2 < 40 GeV2, 50 < W < 150 GeV for the J/ψ data. Cross sections for exclusive ρ0 and J/ψ production have been measured as a function of Q2, W and t. The spin-density matrix elements r0400, r11-1i and Re r510 have been determined for exclusive ρ0 production as well as r0400 and r041-1 for exclusive J/ψ production. The results are discussed in the context of theoretical models invoking soft and hard phenomena.

Abstract:

The inclusive forward jet cross section in deep inelastic e+p scattering has been measured in the region of cursive Greek chi-Bjorken, 4.5 · 10-4 to 4.5 · 10-2. This measurement is motivated by the search for effects of BFKL-like parton shower evolution. The cross section at hadron level as a function of cursive Greek chi is compared to cross sections predicted by various Monte Carlo models. An excess of forward jet production at small cursive Greek chi is observed, which is not reproduced by models based on DGLAP parton shower evolution. The Colour Dipole model describes the data reasonably well. Predictions of perturbative QCD calculations at the parton level based on BFKL and DGLAP parton evolution are discussed in the context of this measurement.

Abstract:

The data collected with the DELPHI detector at centre-of-mass energies between 130 and 172 GeV, during LEP operation in 1995 and 1996, have been used to determine the hadronic and leptonic cross-sections and leptonic forward-backward asymmetries. In addition, the cross-section ratios and forward-backward asymmetries for flavour-tagged samples of light (uds), c and b quarks have been measured. No significant deviations from the Standard Model expectations are found. The results are interpreted by performing S-matrix fits to these data and to the data collected previously at the energies near the Z° resonance peak (88-93 GeV). The results are also interpreted in terms of physics beyond the Standard Model: contact interactions, R-parity violating SUSY particle exchange and of possible Z′ bosons.

Abstract:

The bb̄ forward-backward asymmetry has been determined from the average charge flow measured in a sample of 3,500,000 hadronic Z decays collected with the DELPHI detector in 1992 - 1995. The measurement is performed in an enriched bb̄ sample selected using an impact parameter tag and results in the following values for the bb̄ forward-backward asymmetry: Abb̄FB (89.55 GeV) = 0.068 ± 0.018 (stat.) ± 0.0013(syst.) Abb̄FB (91.26 GeV) = 0.0982 ± 0.0047(stat.) ± 0.0016(syst.) Abb̄FB (92.94 GeV) = 0.123 ± 0.016 (stat.) ± 0.0027(syst.) The bb̄ charge separation required for this analysis is directly measured in the b tagged sample, while the other charge separations are obtained from a fragmentation model precisely calibrated to data. The effective weak mixing angle is deduced from the measurement to be: sin2θ1eff = 0.23186 ± 0.00083.