New CP Violation in Neutrino Oscillations

Measurements of CP–violating observables in neutrino oscillation experiments have been studied in the literature as a way to determine the CP– violating phase in the mixing matrix for leptons. Here we show that such observables also probe new neutrino interactions in the production or detection processes. Genuine CP violation and fake CP violation due to matter effects are sensitive to the imaginary and real parts of new couplings. The dependence of the CP asymmetry on source–detector distance is different from the standard one and, in particular, enhanced at short distances. We estimate that future neutrino factories will be able to probe in this way new interactions that are up to four orders of magnitude weaker than the weak interactions. We discuss the possible implications for models of new physics. [email protected] [email protected] [email protected] [email protected] 0 I. NEW CP VIOLATION IN NEUTRINO INTERACTIONS In the future, neutrino oscillation experiments will search for CP–violating effects [1–22]. The Standard Model, extended to include masses for the light, active neutrinos, predicts that CP is violated in neutrino oscillations through a single phase in the mixing matrix for leptons. This effect is suppressed by small mixing angles and small mass differences. It is not unlikely, however, that the high-energy physics that is responsible for neutrino masses and mixing involves also new neutrino interactions. Such interactions provide new sources of CP violation. In this work we study CP–violating effects due to contributions from new neutrino interactions to the production and/or detection processes in neutrino oscillation experiments. We investigate the following questions: (i) How would new, CP–violating neutrino interactions manifest themselves in neutrino oscillations? (ii) Are the effects qualitatively different from the Standard Models ones? In particular, can we use the time (or, equivalently, distance) dependence of the transition probability to distinguish between Standard Model and new CP violation? (iii) How large can the effects be? In particular, do the new interactions suffer from suppression factors related to mixing angles and mass differences? (iv) Can the new CP violation be observed in proposed experiments? What would be the optimal setting for these observations? (v) Which models of New Physics can be probed in this way? The plan of this paper goes as follows. In section II we present a parameterization of the New Physics effects that are of interest to us and explain the counting of independent CP–violating phases in our framework. In section III we evaluate the New Physics effects on the transition probability in neutrino vacuum oscillation experiments. (A full expression for the transition probability, without any approximations concerning mixing angles and mass differences, is given in Appendix A.) In section IV we investigate the resulting CP asymmetry and compare the New Physics contribution to the standard one (that is, the contribution to the asymmetry from lepton mixing). In sections V and VI we evaluate the New Physics effects on, respectively, the transition probability and CP asymmetry, in neutrino matter oscillations. In section VII we study how these effects can be observed in future neutrino factory experiments. In particular, we estimate a lower bound on the strength of the new interactions that can be observed in these experiments. This lower bound is compared to existing model–independent upper bounds in section VIII. We summarize our results and discuss some of the implications that would arise if a signal is experimentally observed in section IX. II. NOTATIONS AND FORMALISM In this section we give a model–independent parameterization of New Physics effects on production and detection processes in neutrino oscillation experiments. We put special emphasis on CP–violating phases. We denote by |νi〉, i = 1, 2, 3, the three neutrino mass eigenstates. We denote by |να〉 the weak interaction partners of the charged lepton mass eigenstates α (α = e, μ, τ):