Based on material presented at various meetings (last the neutrino town meeting @CERN) by Alan Bross, Ken Long and others

The main messages the critical elements that are needed for precision measurements of the neutrino oscillation parameters why HyperK and DUNE really need this. how can this be provided by a muon storage ring neutrino beam facility. what concrete steps are being taken other synergies

Alain Blondel T2K latest results and prospects

sin  term is CP violating cos  term is not. sensitivity to  (precision on  measurement) is driven by sin  term if  is close to 0 or   rate asymmetry e vse cos  term if  is close to /2  E shift, common to e ande different systematics! Alain Blondel T2K latest results and prospects

For  measurement : measure e and e in proctice NFAR (e) / NNEAR () which is critically dependent on the energy reconstruction also affects other parameters (cos, m223 , sin2 23 and sin2 213) This will be affected by uncertainties on cross-section and efficiency. In T2K/HyperK as well as NOVA/DUNE we (will) use muon neutrino beams which are ~99% muon neutrino with an (anti) electron neutrino component of (0.3 – 2)% direct measurement of electron neutrino cross-sections and topologies and their differences w.r.t. muon neutrino events are difficult to measure The theoretical difficulty lies in the interplay of the lepton mass effect (106 MeV) with the -- nuclear physics effects: binding energy Eb = 23 MeV, equiv. Fermi Motion PF = 200 MeV/c) -- pion production threshold (140 MeV)  cross-section, selection efficiency and energy recontruction bias are different between  , e ;  ,e and should be measured Parametric model fitting (which is done now) is only as good as the model and errors estimates are problematic.

It has been observed since 1998 that muon storage rings would provide well calibrated neutrino beams -- well known flux and energy scale (from muon storage ring current and magnets) -- rigourously equal number of (anti) [muon and anti-electron] (anti) neutrinos (for - [+] ) m- -> e- ne nm and m+ -> e+ ne nm m polarization controls ne flux: m+ -X> ne in forward direction Furthermore the difference between the fluxes generated by muons of two different energies is narrow spectrum near the falling edge nearly monochromatic beam ideal for energy resolution and energy bias measurements

NUSTORM momentum adjustable between 1-6 GeV -- first flash of muon (anti) neutrinos from the decay of injected pions. -- then muons from pion decay are stored in the ring. -- cross-sections to be measured in near detector similar to HyperK or DUNE ND (water, scintillator and Liq. Argon targets -- also probably iron)  preferably magnetized for energy reconstruction? -- require precise instrumentation for number of stored muons and polarization -- storage time at 6 GeV is 5x144s (fast extraction of short spill required) -- high field magnets would be highly beneficial by reducing the size of the facility.

muons are born longitudinally polarized in pion decay (~18%) Muon Polarization muons are born longitudinally polarized in pion decay (~18%) depolarization is small (Fernow &Gallardo) effects in electric and magnetic fields is (mostly) described by spin tune: which is small: at each kick q of a 200 MeV/c muon the polarization is kicked by n.q = 0.002 q in the high energy storage ring polarization precesses. Interestingly = 0.5 for a beam energy of 45.3112 GeV: at that energy it flips at each turn. Alain Blondel NUFACT03

muon polarization is too small to be very useful for physics (AB, Campanelli) but it must be monitored. In addition it is precious for energy calibration (Raja&Tollestrup, AB) a muon polarimeter would perform the momentum analysis of the decay electrons at the end of a straight section. Because of parity violation in muon decay the ratio of high energy to low energy electrons is a good polarization monitor. Alain Blondel NUFACT03

# positons with E in [0.6-0.8] Em to number of muons in the ring. muon polarization here is the ratio of # positons with E in [0.6-0.8] Em to number of muons in the ring.  There is no RF in the ring. spin precession and depolarization are clearly visible This is the Fourier Transform of the muon energy spectrum (AB) amplitude=> polarization frequency => energy decay => energy spread. DE/E and sE/E to 10-6 polarization to a few percent. Alain Blondel NUFACT03

preliminary study at CERN

possible location at CERN

Final comments: A facility dedicated for direct measurements of the neutrino interaction parameters that are needed for oscillation experiments -- neutrino interaction cross-sections -- neutrino flavour/charge identification -- neutrino selection efficiencies -- neutrino energy reconstructions  clearly spelled out requirement for the future of the long baseline experiments A muon storage ring seems to be able to provide these answers It will also be an important step towards muon colliders first high intensity muon storage ring (development of operation and instrumentation) search for synergies and complementarities neutrino community, accelerator community, technology (High field magnets etc..) make the case that this is worth it for HyperK and DUNE (maybe it is too good?) develop a work program towards the solution  need to produce a cost estimate, reasonable time scale etc….

Neutrino town meeting recommendation https://indico.cern.ch/event/740296/overview

submission to European Strategy for Particle Physics https://arxiv.org/abs/1812.06739

RECOMMANDATION: Extracting the most physics out of DUNE and HyperK will require ancillary experiments: CERN should continue improving NA61/SHINE towards percent level flux determinations; 2) a study should be set-up to evaluate the possible implementation, performance and impact of a percent-level electron and muon neutrino cross-section measurement facility (based one.g. ENUBET or NuSTORM) with conclusion in a few years;