Highlights of Short Baseline Physics at a Neutrino Factory

We review the physics potential for experiments with intense neutrino beams at the front–end of a muon storage ring, stressing the way the spin and flavour structure of neutrino interactions with matter can be used to shed light on the structure of the strong and electroweak interactions. Specifically, we discuss precision tests of the standard model, studies of polarized and unpolarized structure functions of the nucleon, and several new and exotic physics items. Invited plenary talk at NuFact 01 Tsukuba, Japan, May 2001 to be published in the proceedings 1 On leave from INFN, Sezione di Torino, Italy Preprint submitted to Elsevier Preprint 1 February 2008 1 Probing Matter with Intense Neutrino Beams Front–end physics at a neutrino factory is based on the realization that, because of the flavour and spin structure of the coupling of neutrinos to weak currents, a neutrino beam is a unique probe of the structure of the standard model and of the structure of the nucleon. A neutrino beam thus has a greater physics potential than conventional electron or muon beams, provided the intensity of the beam is high enough. An accurate assessment of the physics potential of the experiments which could be performed with an intense neutrino beam cannot abstract from the fact that the time scale for the construction of a neutrino factory is of order of ten years: clearly, it is difficult to envisage what the ‘standard model’ will be ten years from now. Here, we will discuss the physics case for these experiments based on present–day knowledge. This is interesting not only because many of the measurements that we will discuss would only be possible at a neutrino factory (for instance, those related to polarized parton distributions), but also because comparison with what we already know will allow us to highlight the peculiar features of physics with neutrino beams. This brief review is largely based on a recent detailed quantitative study performed by a CERN working group [1]; quantitative estimates given here are taken from there unless otherwise stated. Previous studies on the physics potential of neutrino factories have been performed by working groups at Fermilab [2] and Brookhaven [3]. General background on neutrino experiments is in Ref. [4]. Even though we will usually describe the energy and luminosity dependence of our results, we will assume the ‘CERN reference scenario’ [5]: specifically, a 50 GeV μ beam, with 10 muon decays per year along a 100 m straight section. 2 Neutrino interactions with matter The most interesting neutrino–induced reactions in matter are neutrino–electron elastic scattering, and neutrino–nucleon (deep)–inelastic scattering (DIS), i.e. neutrino–quark scattering. The former is a clean purely weak interaction process, while the latter allows one to probe strongly–interacting matter with weak currents. The cross section for elastic neutrino– or antineutrino–electron scattering is