Poster excerpt 8: Search for Sterile Neutrino Mixing in Tau Neutrino Appearance with the Opera Detector
(The following text has been produced by M.Tenti for the Opera collaboration)
Motivation: Experimental anomalies exist in neutrino oscillations data that cannot be accommodated in the standard three-neutrino oscillations framework (SAGE, GALLEX, nuclear reactor anomaly, MiniBooNE, LSND). They may hint to the existence of sterile neutrino(s) with a mass of the order of 1 eV. The OPERA experiment can test the sterile neutrino hypothesis looking for deviations from the predictions of the three-neutrino oscillations model.
The OPERA experiment: OPERA is designed to observe the nm → nt oscillations identifying the t lepton decays in nt CC interactions . From 2008 to 2012 the OPERA detector, hosted in the Gran Sasso Underground Laboratory of INFN, was exposed, at a distance of 730 km from neutrino source, to the almost pure nm CNGS beam.
Analysis in the 3+1 neutrino model: OPERA collected data corresponding to 1.8 ´ 1020 protons on target. The analysed data sample amounts to ~75% of the total statistics.
Four nt CC interaction candidates have been observed with a total expected background of 0.23 ± 0.05 events . The number, μ, of expected nt events is evaluated using GLoBES .
The parameter Dm221 is set to 7.54 ´ 10-5 eV2 and Dm231 is assumed Gaussian distributed with mean and sigma from global best fit , i.e. (2.47 ± 0.06) ´ 10-3 eV2 for Normal (N.H.) and
(-2.34 ± 0.06) ´ 10-3 eV2 for Inverted (I.H.) hierarchy.
The likelihood L is defined as:
where P is a Poisson distribution depending on the number, μ, of expected events and the number, n, of observed events while p is the Gaussian prior on the nuisance parameter Dm231. The results are given in terms of the new squared mass difference (Dm241) and the effective mixing (sin 2qmt = 2|Um4||Ut4|).
The 90% Confidence Level (CL) excluded region on [Dm241, sin22qmt] plane is lowered down to values of 10-2 eV2 for sin22qmt > 0.5 (Fig. 1). The excluded regions are very similar but with hierarchies exchanged if Dm241 is assumed negative or positive .
Figure 1: The 90% CL excluded region in [fmt, sin22qmt] plane for Normal (red) and Inverted (blue) hierarchy of the three standard neutrinos. The grey and black lines are the excluded region of the NOMAD and CHORUS experiments, respectively
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