Little is known on radiative decays of numu and nutau. Lifetimes can be
constrained, or the process discovered, by a search for flavour conversion in a
superconducting RF cavity installed in a neutrino beam. The lifetimes tested
with such a method can be of interest for astrophysics and cosmology. In
particular the conjecture which explains both the LSND and the SuperKamiokande
signals by radiative decays of numu can be straightforwardly put to the test.Comment: 4 pages, 1 figure, contributed paper to the "Rencontres de Blois",
  Blois, France, June 28 - July 3 199

Vannucci, F.

English

arXiv

Little is known on radiative decays of numu and nutau. Lifetimes can be constrained, or the process discovered, by a search for flavour conversion in a superconducting RF cavity installed in a neutrino beam. The lifetimes tested with such a method can be of interest for astrophysics and cosmology. In particular the conjecture which explains both the LSND and the SuperKamiokande signals by radiative decays of numu can be straightforwardly put to the test

Vannucci, François

CERN Document Server

SEARCH FOR STIMULATED NEUTRINO CONVERSION WITH AN RFCAVITY aF. VannucciLPNHE Universite´ Paris 7,Paris, FranceLittle is known on radiative decays of νµ and ντ . Lifetimes can be constrained, or the pro-cess discovered, by a search for flavour conversion in a superconducting RF cavity installedin a neutrino beam. The lifetimes tested with such a method can be of interest for astro-physics and cosmology. In particular the conjecture which explains both the LSND and theSuperKamiokande signals by radiative decays of νµ can be straightforwardly put to the test.1 INTRODUCTIONNeutrino decays require flavour violation and the existence of massive neutrino species. Afterthe recent announcement of an evidence for oscillations presented by the SuperKamiokandecollaboration 1, these two points seem fullled.Radiative decays with the emission of one photon represent the most plausible mode betweenneutrinos having masses in the range of a few eV or less. Radiative decays in vacuum have beencalculated long ago 2. The process is GIM suppressed and has an immeasurably long lifetimefor masses of present interest.Few experimental limits exist. One comes from the SN1987A burst and applies to e sinceonly e have been detected in that circumstance. The limit is 3:0=m > 6 1015 s/eVA limit on µ decay comes from a Los Alamos measurement 4:0=m > 15:4 s/eVBoth limits apply to decays between neutrinos having a strong mass hierarchy. For mass-degenerate neutrinos, two results have been published. The rst one obtained at a nuclearreactor applies to e 5. The second one has been set with solar neutrinos during a total eclipse6. All these limits are very far from the theoretical expectations.It has been recently recognised that the radiative decay probability is very much amplied indense media. For example matter eects can hugely stimulate the process 7. The amplicationfactor relates the lifetime in vacuum 0 to the one in matter m by the formula:0=m = 8:6 1023F (v)(Ne=1024cm−3)2(1eV=m)4(m2=m2)2where Ne is the electron density of the medium, m is the mass of the decaying neutrino andm2 the dierence between the squared masses of the participating particles (the same m2 asthe one appearing in oscillatiom searches). For relativistic neutrinos F(v) takes the value 4m/E,E being the neutrino energy. The formula applies to the case of mass-degenerate neutrinos. Forthe case of mass hierarchy, one should take m2 = m2.aContributed paper to the \Rencontres de Blois", Blois, France, June 28-July 3 1999Enhanced decays in media could have important consequences for astrophysical consider-ations. In particular, it has been conjectured that the LSND appearance of e, and the Su-perKamiokande disappearance of µ could have the same origin in the radiative decay of µamplied in matter 8. The process in question is:µ → γ + e within a scheme of mass-degenerate neutrinos.The corresponding lifetime in matter is found to be :m = 4 10−13 s/eV.This seems extremely small, but with the amplication factor due to the presence of matter,it corresponds to the equivalent lifetime in vacuum:0 = 9:5 104m2=(m2)2Thus 0 can be very large if m2 is small. For example with the value 10−5eV 2 favouredby the MSW solution of the solar decit, and with m of the order of a few eV as required bycosmology, the lifetime reaches a value close to the age of the universe.2 RF STIMULATED CONVERSIONAn ecient method to stimulate conversions between neutrino flavours consists in sending ahigh energy neutrino beam through a strong electromagnetic eld. The case of a high qualityRF cavity has been considered and the probability of conversion calculated in Ref 9. We showhow this idea can set interesting limits.In a high energy µ beam, a high quality RF cavity gives a target of very low energy photonswhich can induce the conversion:µ → e or µ → τExperimentally, if neutrinos are Majorana particles, the rst process shows as an excess ofinteractions producing a positron. If neutrinos are Dirac particles, or if a τ is produced, theprocess results in a disappearance of CC µ interactions. The transition rate depends linearlyon the quality factor Q of the cavity and on the power P stored into it. To optimize the search,the cavity must have the highest possible Q*P and a large surface area intersecting the beam inorder to maximize the number of crossing neutrinos. A superconducting cavity is required.In the fundamental mode TM010, the diameter of a cavity is inversely proportional to itsfrequency. This favours a frequency in the range 300-500 Mhz for which Q is limited by theresidual resistance and does not necessitate the lowest temperature of superfluid helium. Notethat the cavity is used in a continuous mode. This is very dierent from the pulsed modecompulsory to accelerate beams. The power supplied to the cavity is very limited and onlynecessary to compensate for the losses in the superconductor. The stored energy is proportionalto V 2c , where Vc is the accelerating tension.A superconducting cavity developped for the French synchrotron radiation project Soleilcould be available for the present search. A good working point is obtained for Vc = 6 MV=mfor which Q = 3 109 and the energy is calculated to be 33 J. For the neutrino beam, we willconsider the case of the K2K beam built at KEK 10. It aims at the SuperKamiokande detectorand is devoted to conrming the signal of oscillations found with atmospheric neutrinos. Onthe KEK site the beam is essentially composed of µ with a peak energy around 1.4 GeV. Theintensity reaches 1:5105 neutrinos /cm2 /spill at a distance of 300 m from the production targetwhere two front detectors have been built.The cavity covers a surface area of 0:45 m2. The neutrino flux crossing this area results inmore than 100 CC µ events per day or 3000 during a one month exposure time in the existingfront detectors. This also produces 30 CC e events from normal beam contamination.3 ACHIEVABLE LIMITSThe proposed test consists in comparing the flux of neutrinos of various flavours with and withoutthe cavity switched on. The experiment measures the number of CC µ interactions and theratio electron/muon in the detectors with the cavity on and o, and looks for a dierence. A 2%conversion seems accesible for the e appearance mode. This would result in 60 excess electron(positron) events above the expected 30. For the disappearance channel a 5% limit is assumed.This level corresponds to a statistical eect of 3. With these limits it is possible to extract alower limit on the radiative lifetimes as shown in Figure 1.The curves labelled 10−5 and 10−10 assume the m2 favoured by the decit of solar neutrinos(MSW and just-so solutions respectively). They relate to the mode:µ → γ + eVery interesting limits can be reached, for example:0 > 5 1018 s for m2 = 10−5eV 2 and m = 1 eVThe curve corresponding to m2 = 310−3 assumes the value of m2 given by the atmosphericneutrino decit and applies to the mode:τ → γ + µThese curves are obtained for mass-degenerate neutrinos in the range of masses interestingfor cosmology (1-10 eV). One can also extract a limit for the case of a strong mass hierarchy.From the signal of atmospheric neutrinos, one infers a τ mass of 50 meV. For such a mass thelifetime limit is:0 > 8 107 sThe limit becomes:0 > 5 103 sfor a mass of 1 eV. These limits apply to both µ and τ . For µ, this is more than twoorders of magnitude better than the present experimental limit. For τ , this would constitutethe rst laboratory limit.The measured conversion rate in the cavity R is related to the lifetime in vacuum by theformula:0 = 100=R (m=m2)3With the lifetime found in interpreting the LSND and SuperKamiokande results 8, the prob-ability of conversion becomes:R = 10−3 (m=m2)For m2 = 10−5eV 2 and m = 1 eV, the conversion is maximal and the signal easy to nd. Ifsuch a very favourable situation arises, it becomes possible to measure the absolute value of mby varying the power of the cavity (and consequently R) provided that m2 is known otherwise.4 CONCLUSIONThe idea of installing a high quality RF cavity in a high energy neutrino beam is discussed. Thecase is applied to the K2K beam recently built at KEK. With an existing cavity, it is possibleto test radiative decays of neutrinos and to put stringent limits on the processes (or discover!):µ → γ + e and τ → γ + µThe limits reach a level which is of interest for astrophysics and cosmology.Oscillations only give information on the dierence of squared masses. Decays may be theonly way to estimate the absolute scale of neutrino masses. In particular, if the radiative decayof µ is responsible for the LSND and SuperKamiokande signals, and assuming that m2 ismeasured with solar neutrinos, then the stimulated conversion could x with good accuracy theabsolute masses of the participating neutrinos.References1. Y. Fukuda et al, Phys. Rev. Lett. 433, 9 (1998).2. P.B.Pal and L. Wolfenstein, Phys. Rev. D 25, 768 (1982).3. E.L. Chupp et al, Phys. Rev. Lett. 62, 505 (1989).4. D.A. Krakauer et al, Phys. Lett. B 252, 177 (1990).5. J. Bouchez et al, Phys. Lett. B 207, 217 (1983).6. C. Birnbaum et al, Phys. Lett. B 397, 143 (1997).7. J.C. D’Olivo, J.F. Nieves and P.B. Pal, Phys. Rev. Lett. 64, 1088 (1990).G. Giunti, C.W. Kim and W.P. Lam, Phys. Rev. D 43, 164 (1991).8. F. Vannucci, Preprint hep-ph/9903487.9. S. Matsuki and K. Yamamoto, Phys. Lett. B 289, 194 (1992).M.C. Gonzales-Garcia, F. Vannucci and J. Castromonte, Phys. Lett. B 373, 153 (1996)10. Y. Oyama, Preprint hep-ex/9803014.

Search for Stimulated Neutrino Conversion with an RF Cavity

4 pages, 1 figure, contributed paper to the "Rencontres de Blois", Blois, France, June 28 - July 3 1999International audienceLittle is known on radiative decays of numu and nutau. Lifetimes can be constrained, or the process discovered, by a search for flavour conversion in a superconducting RF cavity installed in a neutrino beam. The lifetimes tested with such a method can be of interest for astrophysics and cosmology. In particular the conjecture which explains both the LSND and the SuperKamiokande signals by radiative decays of numu can be straightforwardly put to the test

Search for Stimulated Neutrino Conversion with an RF Cavity

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