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Register a new userRevisiting the LSND anomaly I: impact of new data
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This paper, together with a subsequent paper, questions the so-called LSND anomaly: a 3.8 {sigma} excess of anti-electronneutrino interactions over standard backgrounds, observed by the LSND Collaboration in a beam dump experiment with 800 MeV protons. That excess has been interpreted as evidence for the anti-muonneutrino{to} anti-electronneutrino oscillation in the {Delta}m2 range from 0.2 eV2 to 2 eV2. Such a {Delta}m2 range is incompatible with the widely accepted model of oscillations between three light neutrino species and would require the existence of at least one light sterile neutrino. In this paper, new data on pion production by protons on nuclei are presented, and four decades old data on pion production by neutrons on nuclei are recalled, that together increase significantly the estimates of standard backgrounds in the LSND experiment, and decrease the significance of the LSND anomaly from 3.8 {sigma} to 2.9 {sigma}. In a subsequent paper, in addition the LSND Collaborations data analysis will be questioned, rendering a further reduction of the significance of the LSND anomaly.
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This paper, together with a preceding paper, questions the so-called LSND anomaly: a 3.8 sigma excess of antielectronneutrino interactions over standard backgrounds, observed by the LSND Collaboration in a beam dump experiment with 800 MeV protons. That excess has been interpreted as evidence for the antimuonneutrino to antielectronneutrino oscillation in the Deltam2 range from 0.2 eV2 to 2 eV2. Such a Deltam2 range is incompatible with the widely accepted model of oscillations between three light neutrino species and would require the existence of at least one light sterile neutrino. In a preceding paper, it was concluded that the estimates of standard backgrounds must be significantly increased. In this paper, the LSND Collaborations estimate of the number of antielectronneutrino interactions followed by neutron capture, and of its error, is questioned. The overall conclusion is that the significance of the LSND anomaly is not larger than 2.3 sigma.
The so-called LSND anomaly, a 3.8 sigma excess of anti-nu_e events interpreted as originating from anti-nu_mu -> anti-nu_e oscillation, gave rise to many theoretical speculations. The MiniBooNE Collaboration reported inconsistency of this interpretation with the findings from their search for nu_mu -> nu_e oscillations. Yet the origin of the LSND anomaly was never clarified. A critical issue is the prediction of the background anti-nu_e flux that was used in the analysis of the LSND experiment. For this, decisive input comes from pion spectra measured with the HARP large-angle spectrometer under conditions that closely resemble the LSND situation: a proton beam with 800 MeV kinetic energy hitting a water target.
We report an early result from the ICARUS experiment on the search for nu_mu to nu_e signal due to the LSND anomaly. The search was performed with the ICARUS T600 detector located at the Gran Sasso Laboratory, receiving CNGS neutrinos from CERN at an average energy of about 20 GeV, after a flight path of about 730 km. The LSND anomaly would manifest as an excess of nu_e events, characterized by a fast energy oscillation averaging approximately to sin^2(1.27 Dm^2_new L/ E_nu) = 1/2. The present analysis is based on 1091 neutrino events, which are about 50% of the ICARUS data collected in 2010-2011. Two clear nu_e events have been found, compared with the expectation of 3.7 +/- 0.6 events from conventional sources. Within the range of our observations, this result is compatible with the absence of a LSND anomaly. At 90% and 99% confidence levels the limits of 3.4 and 7.3 events corresponding to oscillation probabilities of 5.4 10^-3 and 1.1 10^-2 are set respectively. The result strongly limits the window of open options for the LSND anomaly to a narrow region around (Dm^2, sin^2(2 theta))_new = (0.5 eV^2, 0.005), where there is an overall agreement (90% CL) between the present ICARUS limit, the published limits of KARMEN and the published positive signals of LSND and MiniBooNE Collaborations.
We develop the consequences of introducing a purely leptonic, lepton number violating non-standard interaction (NSI) and standard model neutrino mixing with a fourth, sterile neutrino in the analysis of short-baseline, neutrino experiments. We focus on the muon decay at rest (DAR) result from the Liquid Scintillation Neutrino Experiment (LSND) and the Karlsruhe and Rutherford Medium Energy Neutrino Experiment (KARMEN). We make a comprehensive analysis of lepton number violating, NSI effective operators and find nine that affect muon decay relevant to LSND results. Two of these preserve the standard model (SM) value 3/4 for the Michel rho and delta parameters and, overall, show favorable agreement with precision data and the electron anti-neutrino signal from LSND data. We display theoretical models that lead to these two effective operators. In the model we choose to apply to DAR data, both electron anti-neutrino appearance from muon anti-neutrino oscillation and electron anti-neutrino survival after production from NSI decay of the positive muon contribute to the expected signal. This is a unique feature of our scheme. We find a range of parameters where both experiments can be accommodated consistently with recent global, sterile neutrino fits to short baseline data. We comment on implications of the models for new physics searches at colliders and comment on further implications of the lepton number violating interactions plus sterile neutrino-standard model neutrino mixing.
We revisit the status of the new-physics interpretations of the anomalies in semileptonic $B$ decays in light of the new data reported by Belle on the lepton-universality ratios $R_{D^{(*)}}$ using the semileptonic tag and on the longitudinal polarization of the $D^*$ in $Bto D^*tau u$, $F_L^{D^*}$. The preferred solutions involve new left-handed currents or tensor contributions. Interpretations with pure right-handed currents are disfavored by the LHC data, while pure scalar models are disfavored by the upper limits derived either from the LHC or from the $B_c$ lifetime. The observable $F_L^{D^*}$ also gives an important constraint leading to the exclusion of large regions of parameter space. Finally, we investigate the sensitivity of different observables to the various scenarios and conclude that a measurement of the tau polarization in the decay mode $Bto Dtau u$ would effectively discriminate among them.
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