THE NEUTRINO FACTORY F. CERN, Geneva,DYDAK Swi tzerland The discovery of neutrino oscillations marks a major milestone in the history of neutrino physics, and opens a window to what lies beyond the Standard Model. Many current and forthcoming experiments will answer open questions however a major step forward, up to and possibly including CP violation in the neutrino-mixing matrix, will be offered by the intense and well-understood neutrino beams from a neutrino factory. 1 History and introduction The neutrino factory is, in essence, a muon storage ring with long straight sections along which decayir1g muons produce well-defined neutrino beams. The idea is a natural offspring of the concept of a muon collider, first proposed by Budker 1 at the 1969 Yerevan Conference, and developed further by Skrinsky. 2 The firstproposal of a genuine neutrino factory was put forward in quite some detail as early as by Koshkarev. 1974 3 It was not until the nineties that the idea of the muon col!ider was again taken up, this time going well beyond the level of paper studies. The US-led Muon Collider Collaboration 4 developed a conceptual layout of a multi-Te V muon collider, including all stages from muon production to the storage of high-energy muons in a circular collider, and launched an ambitious R & D programme. Figure 1 gives an impression of the complexity of their proposed accelerator network to achieve multi-TeV muon collisions. In the context of this work, the idea of a neutrino factory was developed further and high­ lighted in a comprehensive paper by Geer. 5 The coincidence between the evidence of oscillations of solar 6 and atmospheric 7•8 Ve V and the possible oscillation of accelerator-produced as claimed by the LSND experimentµ. vµ. 9 on the one hand, and the intriguing potential of the neutrino factory for the study of neutrino 95 16 GeV/c 1.5 x 1022 (\ protons Proton Accelerator I year � ) ...__.... Pion Production Target � and Capture Solenoid Pion Decay Channel D D D D Muon Ionization D D D Cooling Channel D D D D D c D D D Stopped Muon 1021 MeV/c 1.5x "--� 100 Physics muons / year muons � Muon Accelerators 100 MeV 2 TeV -> TeV/c 2 � muons Muon Collider Higgs, t t WW, 2 2 TeV x I , ... j Figure Schematic layout of the US design of a 2 TeV muon collider. 1: 2 x 96 oscillations on the other hand, initiated a European study group on the neutrino factory within the context of an ECFA-sponsored 'Prospective Study of Muon Storage Rings in Europe' 10 led by Autin, Blonde! and Ellis. Much of the material presented below arose from the presentations and discussions in this study group. The concept of a muon collider lends itself naturally to a three-step scenario, with increasing technical (and financial) complexity: the neutrino factory at the low-energy end; the Higgs factory at intermediate energy; and the genuine muon collider at the high-energy end (the last two options are discussed by Janot 11 at this conference). While the concepts appear feasible in principle, it is recognized that formidable technical difficulties have yet to be overcome and the practical feasibility has not yet been demonstrated. The concept, if ever realized, lies far in the future. Specifically for the neutrino factory, which is considered the easiest part, it is estimated that from the technical point of view, a functioning machine is ten years away (let alone political and financial considerations). 2 The physics case The physics case for the neutrino factory is largely - but not solely - driven by neutrino oscillations. What do we know and what are the open questions? 2. 1 It is largely accepted that we observe neutrino oscillations with the following characteristics: Ve 2 1 2 Solar disappear with Cl.m 2 10-4 to 10-5 eV , or with Cl.m 2 � 10- 0 eV , with either • 2 � small- or large-mixing sin 2812. i i 2 2 Atmospheric vµ. disappear with Cl.m 3 10-3 eV and large-mixing sin 2823, but do • � not oscillate into Ve . � Therefore, at least all three known neutrino families participate actively in the oscillations. • This suggests, as a minimum, a neutrino-mixing matrix analogous to the CKM matrix in the quark sector. There would be a minimum of six independent variables: three angles 11 2, 913, 823, a CF-violating phase 5 as well as two mass-squared differences·Cl.m and 1 . 2 Cl.m 3· y In the likely limit Cl.m 3 Cl.m 2, the solar and the atmospheric neutrino oscillations can • � » effectively be described by the two-family oscillation formalism. Taking into account all � y three families, however, the solar-neutrino oscillation is governed by 1112, Cl.m 2 and 813, while the atmospheric-neutrino oscillation is governed by 823, Cl.m 3 and 813 . The two oscillation phenomena are linked through the angle 813. y � If the LSND claim of a transition Ve Vµ. with Cl.m2 1 eV2 is correct, a fourth neutrino • ---> � is needed which must be sterile since it is not seen in Z decay. The open questions are, apart from the question of Dirac- or Majorana-type neutrinos, and accepting that the phenomenon of neutrino oscillation is real, is there a sterile neutrino? • what are the values of the neutrino masses? • what are the values of the mixing angles? • is there CP violation in the neutrino-mixing matrix? • 97 2.2 What will we learn within the next fiveyears? From ongoing solar-neutrino experiments and from SuperKamiokande, but even more so from an impressive number of new experiments coming into operation (BOREXINO, ICARUS-600, I216(?), KamLAND, K2K, MiniBooNe, MINOS and SNO), we can expect the following infor­ mation within the next five years: Neutrino oscillations will be ultimately confirmed, andsterile neutrinos will be ruled out • (MINOS, SNO, and SuperKamiokande together with K2K: from the ratio of the flavour­ blind neutral-current rate to the charged-current rate; direct check from MiniBooNE and 1216(?) of the LSND claim). A decision will be made as to which which of the various sets of possible solar-neutrino • oscillation parameters is correct (all solar-neutrino experiments, including BOREXINO and KamLAND in their capacity as long-baseline reactor neutrino experiments). Of course, the above is the conservative view. The LSND claim may actually be confirmed, with important consequences forcosmology. Or the Homestake solar-neutrino experiment, which claims a larger deficitof solar neutrinos than all other solar-neutrino experiments and hasa large weight when the energy dependence of the deficitis exploited, may revise its results. In any such case, things would become even more interesting and would underline rather than undermine the physics casefor the neutrino factory. 2.3 What will be the questions in ten years frnm now? Adopting for the sake of the argument the conservative view, we may expect the main open questions in ten years from now to be: what is the value of B1a ? • is there CP violation in the neutrino-mixing matrix? • As a consequence, the operating parameters of the neutrino factory should not be driven by today's questions but rather by the likely questions which will be asked in ten years from now. As will be argued later, the neutrino factory is the best machine known today to address both questions. 3 Neutrinos from muon decay The decay of the muon is a well-known and precisely calculable process, there are only leptons as involved: The neutrino energy spectrum in µ+ decay is shown in Fig. 2 for unpolarized muons, together with the monoenergetic line of vµ in 71'+ decay. While the fiµ peaks at the kinematic limit of 53 MeV, the spectrum peaks at 2/3 of it. Ve The neutrinos from muons decaying in the straight sections are Lorentz-boosted in the fonrnrd direction towards the detector which is located at some distance L. The neutrino flux in the forward direction is proportional to E�/ L2, and half of the neutrinos are contained within a cone angle of l/1 mµ/ Eµ . As the detector will be located at a distance of several 100 km and = even more, it will be much smaller than the transverse size of the beam. Taking into account the neutrino cross-section which rises linearly with energy, the event rate in the detector scales with M x E�/ where Mis the useful target mass of the detector. L2, 98 µ++ Dµ , , le++ue+Uµ , I , ' , ' , , ' <1l , I \ , \ , Ql \ I 0.. \ t5!/) D ' \ 0 e I \ c I , \ I . \ I • E I ::i . I Ql • I . , z • , I I I I I I I , , , , 0 10 20 30 40 50 Neutrino energy (MeV) Figure 2: Neutrino energy spectra in "+ and µ+ decay. The energy spectrum of the neutrino flux in the distant detector is the same as shown in Fig. 2, but with the energy scale multiplied by the boost factor Eµ/(53 MeV). The momentum of the circulating muons is in principle a free parameter. Thanks to the dependence of the neutrino rates in the far detector, one always wins with higher momentumE! of the circulating muons, at any neutrino energy. Therefore, there is a strong incentive to run the neutrino factory at the highest possible muon momentum. In Table 1 a comparison is made between a conventional beam from and K decay, and v rr a beam from the neutrino factory. The comparison is made specificallyµ for the proposed NGS beam 12 from CERN to the Laboratorio Nazionale del Gran Sasso (hereafter called Gran Sasso) 732 km away, with a neutrino factory sending a beam over the same distance.