LHCb

Abstract:

Using the ZEUS detector at HERA, we have studied the reaction e+p → e+X for Q2 > 5000 GeV2 with a 20.1 pb-1 data sample collected during the years 1994 to 1996. For Q2 below 15000 GeV2, the data are in good agreement with Standard Model expectations. For Q2 > 35000 GeV2, two events are observed while 0.145 ± 0.013 events are expected. A statistical analysis of a large ensemble of simulated Standard Model experiments indicates that with probability 6.0%, an excess at least as unlikely as that observed would occur above some Q2 cut. For x > 0.55 and y > 0.25, four events are observed where 0.91 ± 0.08 events are expected. A statistical analysis of the two-dimensional distribution of the events in x and y yields a probability of 0.72% for the region x > 0.55 and y > 0.25 and a probability of 7.8% for the entire Q2 > 5000 GeV2 data sample. The observed excess above Standard Model expectations is particularly interesting because it occurs in a previously unexplored kinematic region. © Springer-Verlag 1997.

Abstract:

In 1996 LEP ran at a centre-of-mass energy of 161 GeV, just above the threshold of W-pair production. DELPHI accumulated data corresponding to an integrated luminosity of 9.93 pb-1, and observed 29 events that are considered as candidates for W-pair production. From these, a cross-section for the doubly resonant e+e- → WW process of 3.67+0.97-0.85 ± 0.19 pb has been measured. Within the Standard Model, this cross-section corresponds to a mass of the W-boson of 80.40 ± 0.44(stat.) ±0.09(syst.) ±0.03(LEP) GeV/c2. Alternatively, if mw is held fixed at its current value determined by other experiments, the observed cross-section is used to obtain limits on trilinear WWV(V ≡ γ, Z) couplings. © 1997 Elsevier Science B.V.

Abstract:

A search for stable or long-lived heavy charged particles in e+e- interactions at energies of 130-136, 161 and 172 GeV has been performed using the data taken by the DELPHI experiment at LEP. The search is based on particle identification provided by the Time Projection Chamber and the Ring Imaging Cherenkov detector. Upper limits at 95% confidence level are derived on the cross-section for heavy long-lived pair-produced charge ±e and ±2/3e particles in the range of 0.4-2.3 pb for masses from 45 to 84 GeV/c2. Within supersymmetric extensions of the Standard Model, long-lived charginos with masses from 45 to 84 (80) GeV/c2 for high (low) sneutrino masses can be excluded at 95% confidence level. Left-handed (right-handed) long-lived or stable smuons and staus with masses between 45 and 68 (65) GeV/c2 can be excluded at 95% confidence level. © 1997 Elsevier Science B.V.

Abstract:

Using the ZEUS detector at the HERA electron-proton collider, we have searched for lepton flavor violation in ep collisions at a center-of-mass energy (√s) of 300 GeV. Events of the type e + p → ℓ + X with a final-state lepton of high transverse momentum, ℓ = μ or τ, were sought. No evidence was found for lepton flavor violation in the combined 1993 and 1994 data samples, for which the integrated luminosities were 0.84 pb-1 for e-p collisions and 2.94 pb-1 for e+p collisions. Limits on coupling vs. mass are provided for leptoquarks and R-parity violating squarks. For flavor violating couplings of electromagnetic strength, we set 95% confidence level lower limits on leptoquark masses between 207 GeV and 272 GeV, depending on the leptoquark species and final-state lepton. For leptoquark masses larger than 300 GeV, limits on flavor-changing couplings are determined, many of which supersede prior limits from rare decay processes. © Springer-Verlag 1997.

Abstract:

A search with the DELPHI detector at LEP for excited leptons (e*, μ*, τ* and v*) decaying through γ, W or Z transitions is reported. The data used correspond to an integrated luminosity of 10 pb-1 at an e+e- centre-of-mass energy of 161 GeV. The search for pair produced excited leptons established the limits me* > 79.6 GeV/c2, mμ* > 79.6 GeV/c2, mτ* > 79.4 GeV/c2 and mv* > 56.4 GeV/c2 at 95% confidence level, assuming that the SU(2) and U(1) couplings of excited leptons are in the same ratio as for normal leptons and considering both left and right-handed components. Limits corresponding to other assumptions are also given. The search for single excited lepton production established upper limits on the ratio λ/mℓ* of the coupling of the excited lepton to its mass.