Scenarios for an entry-level Neutrino Factory Mario Campanelli Geneva University

LSND and its results The LSND experiment took data from 1993 to 1997 at LAMPF, searching for electron appearance in a  beam. L=30 m, 36 MeV<E ν <52.8 MeV   e –using reaction e p->e + n followed by n p-> d γ –Excess of ± 8.0 events (first run) –Total oscillation probability: (0.31 ± 0.12 ± 0.05)%   e –Electron from e C->e - X in range 60<E e <200 MeV –Excess of 27.7 ± 6.4 events –Osc. Probability: (0.26 ± 0.10 ± 0.05)%

Karmen results Karmen (I and II), covering similar parameter space using a pulsed beam did not observe any oscillation. A corner of the parameter space is still permitted by the combination of the two experiments due to the slightly different L/E of the two experiments

MiniBOONE Currently running at Fermilab, using neutrios from booster = 8 GeV, ~1 GeV Main differences wrt LSND: 30 times more energy 20 times more distance Cerenkov light 4 times larger than scintillation Neutron capture not used Neutrinos instead of (mainly) antineutrinos

The MiniBOONE philosophy The experiment aims at large electron signals if LSND is correct Background is also large: 600 events from muons 600 events from pions 1800 events from ν e in beam (controlled changing decay tunnel length) CC sample (1 nominal year): events

After POT (2 nominal years) Build another detector (BOONE) for precision measurements of oscillation parameters Run with anti-neutrinos to exclude non-oscillation effects So far, 10% taken, running at half the planned rate, but improving. Problems due to radiation issues in the booster tunnel.

Interpretation of a positive result Great excitement! Need to add new pieces to the puzzle If signal only in antineutrino mode, lepton flavour and/or CPT violated If seen in both modes, we need sterile neutrino(s). Normally people tend to: (from NuFact’99 A.deRujula)

Scenarios with 4 neutrinos Sterile neutrinos are like cherries, if you have one you probably have more than one. Depending on mass hierarchies, even the simplest case contains several scenarios: 3+1 (disfavored by short-baseline) 2+2 (disfavored by atmos+ solar) 3+1 scheme collapses into 3-family in the weak mixing limit, it can never be really ruled out, while 2+2 not justified if LSND disproved. 3+2 (hep-ph/ ) not studied yet.

Counting parameters Difference anyway irrelevant in oscillation experiments At least 2 masses for CPv Rotations for neutrinos 123 irrelevant Ordering rotation matrices according to mass difference is convenient in case degeneracy is assumed: U MSN (2+2)= U 14 (θ 14 )U 13 (θ 13 )U 24 (θ 24 )U 23 (θ 23,δ 3 )U 34 (θ 34,δ 2 )U 12 (θ 12,δ 1 ) U MSN (3+1)= U 14 (θ 14 )U 24 (θ 24 )U 34 (θ 34 )U 23 (θ 23,δ 3 )U 13 (θ 13, δ 2 )U 12 (θ 12,δ 1 ) One phase present, but only CP-even possible

Bounds from data Atmospherics and solars strongly disfavor pure oscillation into sterile Limits to ν e   transitions (CHOOZ, CDHS etc.) set maximal size of gap crossing angles –In 2+2, c 2 23 sin 2 (2θ 24 )+c 4 24 sin 2 (2θ 23 )<0.2 The LSND signal connects actives and steriles: –In 2+2, <c 2 13 c 2 24 sin 2 (2θ 23 )<10 -2 In 3+1 schemes, only few zones of parameter space marginally allowed, with very small active-sterile mixing –(e.g. c 4 34 s 2 14 sin 2 (2θ 24 )  2  ) ν s, ν μ, ν e, ν τ

Neutrino Factory and 4-ν scenarios Many simultaneous channels of the NF are a clear benefit to study these complicated scenarios. Interest is reciprocal: if LSND is true, larger probabilities mean an entry level NF with reduced fluxes, smaller detectors and shorter baselines Several works done: Donini, Gavela, Hernandez, Rigolin hep-ph/ Barger, Geer, Raja, Whisnant hep-ph/ Donini, Gavela, Hernandez, Rigolin hep-ph/ Donini, Meloni hep-ph/ Donini, Meloni hep-ph/ Donini, Lusognoli, Meloni hep-ph/

One mass-dominance approximation CP is conserved, and only the four angles θ 13 θ 14 θ 23 (2+2 only)θ 24 are relevant. We can assume, from solar and atmospherics: θ 12 = 45°; θ 34 = 45º; Δm 2 12 =10 -4 Δm 2 34 =3.5  (2+2) θ 12 = 22.5°; θ 13 = 13°; θ 34 = 45º (1+3) (Fogli Lisi Marrone Scioscia) Donini-Meloni studied sensitivity to gap-crossing angles in 2+2 and 3+1 schemes, for a NF of E μ =20 GeV, useful muons, 1 ton (1 m 3 water!) detector with ε μ =0.5, ε τ =0.35 placed at 1 km distance. For simplicity, only negative muon decays considered. Wrong-sign background assumed O(10 -5 ).

2+2 scheme sin 2 θ1 3 : μ - disappearance sin 2 θ 23 : μ + appearance sin 2 θ 14,sin 2 θ 14 : τ - appearance

3+1 scheme sin 2 θ 34 : τ appearance sin 2 θ 14,sin 2 θ 14 : μ + appearance

CP violation In 3 families, magnitude of effect depends on Δm 2 12, while in 4 families on Δm 2 23, so larger, can go closer and have small matter effects. Here we consider integrated asymmetry, A CP (δ)= (R - -R + ) / (R - +R + ) where R ± =N(l ± α )/N(l ± β ) as a function of δ, considering known the values of the mixing angles

CPV sensitivity in 2+2 schemes e     e   e  

CP violation sensitivity in 3+1 schemes e   e   δ 2 =0,δ 3 =90° e   δ 2 =δ 3 =15°,45°, 90°

Summary on physics Obviously, if LSND is confirmed, neutrino physics will receive a great boost, due to the relative ease to perform measurements.  identification becomes key issue due to complicated parameter space. Simple muon identification gives much worse understanding of oscillation picture. To a first approximation, sensitivity scales as  N Due to the mass governing the effect, the study of CP violation requires detector size and baseline comparable to measuring parameters in a 3-family scenario. “simple” parameter measurement in 4 families requires modest effort- detector 2 order of magnitude closer and 4 lighter.

Entry-level NF? We can consider a 100 ton detector, and lower machine power to 2  μ/y (and 5 years run) Beware: due to beam width of ~1m, detector has to be cigar-shaped! Aiming to such a low intensity, we can assume: No need for a new proton driver (JHF I or even FNAL booster could be sufficient) No special radiation-hard targeting No need for cooling Energy: could be reduced, but not by large amounts (cannot gain like 1/L 2 going closer since already embracing all beam with 1 m detector diameter)

NF scheme Possible savings Detector at km

Pricing Overall, a total saving of about $600M could be envisaged (probably more due to lower cost of other components)

What if MiniBOONE finds nothing? LSND could still be right, and have seen instead of oscillations LFV like  +  e + l  or  -  e - e l. It was shown (Bueno, MC, Laveder, Rico, Rubbia hep- ph/ ) that a low-energy entry-level NF and a 10 ton LAr detector located 100 m from the machine improves limits on these decays, reaching sensitivities relevant to the LSND effect

An even smaller machine A NF with ~10 15 μ /y (muons trapped in the CERN AD or FNAL debuncher) has no interest for neutrino oscillations. However, such a machine could be used to measure neutrino cross section at low-energy with precision O(10%), much better than present data MC Navas-Concha Rubbia, hep-ph/ , μ’s, E μ =2 GeV, 100 ton detector 10 meters from machine

A poor-(wo)man NF? It recently came out (B.Fleming) that neutrinos from muons trapped in FNAL debuncher can be seen in MiniBOONE: 8 GeV negatives captured ps and p s p s decay within 1 st turn m s take ~200 m s to decay to n e s, n m s and es Debuncher captures 1 x antiprotons/hour 1:1 p: m captured 5.2 x n e s produced by debuncher/year (assumes 100 hr/wk) 13% produced off each straight section, the rest in a circle 6.24 x of these hit MiniBooNE Folding in the n e cross section at ~2 GeV 16 n e s/year from Quasi-elastic and single p interactions Only useful to calibrate detector response to muon and electron neutrino and crosscheck flux calculations.

Conclusions After many years of data analysis and checks, LSND effect is still alive and represents arguably the largest “mystery” of experimental neutrino physics In case of positive MiniBOONE result, sterile neutrinos will leave their limbo The neutrino factory coupled with a detector with τ identification is the best machine to explore them Due to larger mass difference scales, physics reach will come with far less effort than in 3 families LFV can be explored if no signal at MiniBOONE Very low-intensity NF in parasitic mode, cross-sections and calibration studies Hope nature prepared a surprise for us, otherwise this talk (and hundreds of papers) would mostly be “sterile”