
Physics motivation
The mass of the neutrino is one of the most
intriguing questions of particle physics. On the
one hand, there is no fundamental principle for
the neutrino to be massless; on the other hand,
a neutrino mass implies an extension of the Standard
Model (Fig.1) of elementary physics. "An answer" would
represent a fundamental milestone in particle
physics, astrophysics and cosmology. Most likely,
the neutrino mass is too small and therefore out
of reach of precision studies of the kinematics
of decays involving neutrinos. The only possibility
to get information is the observation of a process
which can only occur if the neutrino has non vanishing
mass. Neutrino oscillations are such a process.
For a massive neutrino, a given weak interaction
eigenstate (nu_e, nu_mu, nu_tau) may be seen as
a different eigenstate at some distance from the
source. The corresponding probability has an "oscillatory"
behavior (hence oscillation) with parameters which
can be determined by experiments. In the simplified
scheme in which oscillation "occurs" dominantly
between a pair of neutrino flavours, they are
described by two quantities: the mixing parameter
sin²2(Theta) (related to the oscillation amplitude)
and the mass squared difference Delta m² between
the two mass eigenstates (related to the "frequency
oscillation"). The sensitivity of the experimental
searches to the above parameters depends on the
neutrino energy E and on the distance L of the
detector from the neutrino source. The oscillation
probability is given by the following equation:
The strongest evidence for the existence of neutrino oscillations comes from recent results on the socalled atmospheric neutrino anomaly. The Kamiokande collaboration suggested neutrino oscillations as an explanation of the anomalous ratio of atmospheric muon neutrinos to electron neutrinos and of its zenith angle dependence. The best fit to their data, in terms of nu_mu <> nu_tau oscillations, gives Delta m² = 1.6x102 eV² and sin²2(Theta)=1. The SuperKamiokande experiment has confirmed the Kamiokande results with a statistics several times larger, making a strong claim for the observation of neutrino oscillations (Fig. 2). A global fit to the SuperKamiokande data yields m² = 3.2x103 eV² and sin²2(Theta)=1 (90% CL). The best fit of SuperKamiokande and K2K gives the following result: m² = 2.6x103. Furthermore the experiments Soudan2, MACRO and SNO yield supporting evidence for the neutrino oscillation.
So far only the disappearance of neutrinos has been experimentaly observed. The conclusive test of the nu_mu <> nu_tau oscillation hypothesis will be the direct observation of nu_tau appearance in an initially pure nu_mu beam, as proposed in OPERA. The sensitivity of OPERA covers the Delta m² region allowed by the present atmospheric data (Fig. 1). In the case of a positive signal, the observation of even a few nu_tau events will be significant, because of the very low expected background. The number of observed nu_tau events will provide a measurement of the product of the two oscillation parameters sin²2(Theta) and Delta m² in the two flavour mixing scheme. Given the already known constraint of nearly maximum mixing (sin²2(Theta) ~1), OPERA can improve the determination of Delta m².
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Fig. 1: Standard Modell
(© DESY)





Fig. 2: OPERA sensitivity, 90% CL



