Accelerator Neutrinos

Abstract:

The KS → γγ decay rate has been measured with the NA48 detector using a high intensity short neutral beam from the CERN SPS. The measured branching ratio BR(KS → γγ) = (2.78 ± 0.06stat ± 0.04syst) x 10^-6, obtained from 7461 ± 172 KS → γγ events, is significantly higher than the O(p⁴) prediction of chiral perturbation theory. Using a KL beam the ratio Γ(KL → γγ)/Γ(KL → π⁰π⁰π⁰) = (2.81 ± 0.02syst) ×10^-3 has been measured. © 2002 Elsevier Science B.V. All rights reserved.

Abstract:

The KL → π⁺π^-e ⁺e^- and KS → π⁺π ^-e⁺e^- decay modes have been studied in detail using the NA48 detector at the CERN SPS. Based on the data collected during the 1998 and 1999 run periods, a sample of 1162 KL → π ⁺π^-e⁺e^- candidates has been observed with an expected background level of 36.9 events, yielding the branching ratio measurement BR(KL → π⁺π ^-e⁺e^-) = (3.08 ± 0.20) × 10 ^-7. The distribution of events in the sinφ cosφ variable, where φ is the angle between the π⁺π^- and the e⁺e^- decay planes in the kaon centre of mass, is found to exhibit a large CP-violating asymmetry with the value Aφ = (14.2 ± 3.6)%. For the KS → π⁺π ^-e⁺e^- decay channel, 621 candidates have been identified in the 1999 data sample with an estimated background contribution of 0.7 event. The corresponding branching ratio has been determined to be BR(K S → π⁺π^-e⁺e^-) = (4.71 ± 0.32) × 10^-5. The combined value of this measurement with the published 1998 result is BR(KS → π ⁺π^-e⁺e^-) = (4.69 ± 0.30) × 10^-5. No asymmetry is observed in this decay mode. Our results are in good agreement with theoretical predictions based on a phenomenological description of radiative kaon decays. The form factor parameters a1/a2 and g̃M1 in the K L → π⁺π^-e⁺e^- direct emission process as well as the value of the K⁰ charge radius have been extracted from the data.

Abstract:

Inclusive momentum distributions of charged particles in restricted cones around jet axes were measured in dijet events with invariant dijet masses in the range 80 to 600 GeV/c2. Events were produced at the Fermilab Tevatron in pp collisions with a center of mass energy of 1.8 TeV and recorded by the Collider Detector at Fermilab. The results were compared to perturbative QCD calculations carried out in the framework of the modified leading log approximation (MLLA) and assuming local parton-hadron duality. It was shown that the data follow theoretical predictions quite well over the whole range of the jet energies included in this analysis. We extracted the MLLA cutoff scale Qe f f and found a value of 230±40 MeV. The theoretical prediction of Ejetsin θc scaling, where u c is the cone opening angle, was experimentally observed for the first time. From the MLLA fits to the data, two more parameters were extracted: the ratio of parton multiplicities in gluon and quark jets, r = Npartonsg-jet /Npartonsq-jet = 1.9±0.5, and the ratio of the number of charged hadrons to the number of predicted partons in a jet, KLPHDcharged=Nhadronscharged/Npartons=0.56±0.10.

Abstract:

We report on a search for the flavor-changing neutral current decay D ⁰→µ⁺µ^- in pp^- collisions at √s = 1.96 TeV using 65 pb^-1 of data collected by the CDF II experiment at the Fermilab Tevatron Collider. A displaced-track trigger selects long-lived D⁰ candidates in the D⁰→µ ⁺µ^- search channel, the kinematically similar D ⁰→П⁺p^- channel used for normalization, the Cabibbo-favored D⁰K^-П^- channel used to optimize the selection criteria in an unbiased manner, and their charge conjugates. Finding no signal events in the D⁰→µ⁺µ^- search window, we set an upper limit on the branching fraction B(D⁰→µ⁺µ^-)≤2.5×10^-6 (3.3×10^-6) at the 90% (95%) confidence level.

Abstract:

With the evidence for massive neutrinos from recent ν-oscillation experiments, one of the most fundamental tasks of particle physics over the next years will be the determination of the absolute mass scale of neutrinos, which has crucial implications for cosmology, astrophysics and particle physics. The KArlsruhe TRItium Neutrino (KATRIN) experiment is the next-generation direct neutrino mass experiment with a sensitivity to sub-eV ν-masses. It combines an ultra-luminous molecular windowless gaseous tritium source with a high resolution electrostatic retarding spectrometer (MAC-E filter) to measure the spectral shape of β-decay electrons close to the endpoint at 18.6 keV with unprecedented precision. If no neutrino mass signal is found, the KATRIN sensitivity after 3 years of measurements is mν < 0.2 eV/c2 (90 % CL.); a ν-mass signal of mν = 0.35 (0.30) eV/c2 can be measured with 5 (3) σ evidence.