Coherent elastic scattering between neutrinos and nuclei

Transcription

1 NuFact - Rio de Janeiro, Brazil - August 0 Coherent elastic scattering between neutrinos and nuclei Bruno Miguez Instituto de Fisica Gleb Wataghin Universidade Estadual de Campinas, Unicamp Campinas, SP, Brazil Orlando L. G. Peres Instituto de Fisica Gleb Wataghin Universidade Estadual de Campinas, Unicamp Campinas, SP, Brazil and The Abdus Salam International Centre for Theoretical Physics, Strada Costiera, 0 Trieste, Italy. (Dated: April 6, 06) 7

2 NuFact - Rio de Janeiro, Brazil - August 0 Abstract In e limit of low momentum transfer for neutrino nuclei elastic scattering is expected to observe a coherent superposition at increases e nucleus cross section. This effect was already observed for scattering for electrons, but due to experimental difficulties it have never been verified in neutrino scattering. The next generation of dark matter detectors probably will be sensitive to is interactions. A study of e expected signal produced by coherent elastic scattering for neutrinos from different sources (solar, atmosphere, diffuse flux from supernovae, reactors and accelerators) was made. Considering a conservative reshold kev, as a detectable nuclear recoil, e most promising source to be observed is e 8 B neutrinos produced in e Sun, wi an expected rate of 0 events/(ton year). Anoer promising source is e reactor, but e use of is source is quite dependent of e possible distance between reactor and detector. For a detector far km from e reactor it is expected events/(ton year), but if be possible decrease is distance for 0 m e rate would be increased to 00 events/(ton year). INTRODUCTION The coherent elastic scattering is e consequence of constructive interference on e interaction of neutrinos and nuclei. It is expected at when e transfered momentum (Q) is small when compared wi e nucleus radius (R 0 ), QR 0 <, e neutrino loses e capacity to distinguish among individual nucleons and interact wi e atom nucleus as a whole, increasing his cross-section [, ]. The equation at describes e neutrino-nucleus coherent elastic scattering as a function of nuclear recoil (T ) and incident neutrino energy (E ν ) is: where G F ( dσ dt = G F π M MT ) Q w Eν F (Q ) () is e Fermi Constant and M is e nuclear mass. This equation includes a term, Q w = N Z ( sin θ W ), at describes e coherent superposition of nucleons cross-section (N is number of neutrons and Z is e number of protons and sin θ W 0. is e weak mixing angle) and a form factor, F (Q ) = QR 0 e (Qs) ( sin(qr0 ) (QR 0 cos(qr ) 0) ) QR 0, to describe e loss of coherence wi increasing momentum transfer (R 0 = (.A / ) s fm 7

3 NuFact - Rio de Janeiro, Brazil - August 0 and s = 0. fm are constants from adopted nuclear model). The form factor can be described as a function of e transfered momentum using e relation Q = MT = E ν( + cosθ). ATMOSPHERIC NEUTRINOS AND DIFFUSE FLUX FROM SUPERNOVAE NEUTRINOS Atmospheric neutrinos are produced by cosmic ray interactions in e atmosphere []. Neutrinos from diffuse flux from supernovae are a stationary flux originated on e superposition of bursts emitted by all supernovae []. As can be seem in Fig., bo categories present relatively low fluxes, making difficult eir detection. Recently, ey attracted more interest once ey can imitate e WIMP expected signal in direct search for dark matter []. DFSN T= MeV DFSN T= MeV Flux (cm*s*mev) DFSN T=6 MeV DFSN T=8 MeV Atm ν e Atm ν µ Atm Anti ν e Atm Anti ν µ DFSN T=6 MeV DFSN T=8 MeV Atm ν e Atm ν µ Atm Anti ν e Atm Anti ν µ (kev) E FIG. : Fluxes and expected number of events over reshold for atmospheric neutrinos and diffuse flux from supernovae neutrinos. SOLAR NEUTRINOS Solar neutrinos are produced in e nuclear reaction in e solar core and quickly reach e Ear. Their fluxes are relativily large, but eir energies go just up to 0 MeV, what limitates eirs detection once e maximum nuclear recoil (T max ) for a nucleus wi mass (M) produced by a neutrino wi energy E ν is given by T max = Eν/M, as showed in Fig.. 7

4 NuFact - Rio de Janeiro, Brazil - August 0 In is work was used e fluxes from Bahcall Website ( jnb/) and e monoenergetic fluxes are not considered. PP PP Flux (cm*s) B8 N F7 B8 N F7 9 7 O hep O hep E (kev) FIG. : Fluxes and expected number of events over reshold for solar neutrinos. REACTOR NEUTRINOS An preliminary extension of is study was made considering reactor neutrinos. These sources have eir fluxes quite dependent of e used setup (distance, power, etc). In is work, to estimate a rate of coherent elastic scattering on xenon due to reactor neutrinos was used e neutrino spectrum presented in [6], considering also 0% U composition, GW ermal power and km of distance between e reactor and e detector. The results are showed in Fig.. CONCLUSIONS It is expected at neutrino-nuclei coherent elastic scattering be observed soon. The improvement in detection techniques allows to observe nuclear recoils wi energies in order of kev in ton scale detectors. The neutrino background represents a limit on e sensitivity at direct dark matter detectors can reach, once ey can interact rough coherent elastic scattering wi nuclei. In is work we reproduced e results in [] wi good agreement and extended e study to neutrinos from reactors and accelerators. 7

5 NuFact - Rio de Janeiro, Brazil - August 0 9 Flux (cm*s*mev) E (kev) FIG. : Fluxes and expected number of events over reshold for reactor neutrinos. The best source to detect neutrino-nuclei coherent elastic scattering considering a reshold of kev should be e solar neutrinos from 8 B wi 0 events/(ton year). Anoer good source should be e reactor, but in is case e viability is too much dependent of setup, mainly e distance. The case showed in Fig. presents a rate of events/(ton year), but is number can be increased up to 00 events/(ton year) if e detector could be placed 0 m far from reactor. We would also like to ank FAPESP, CAPES, CNPq and Unicamp for several financial supports. O.L.G.P. anks e support of FAPESP funding grant 0/689. The auors anks e support of FAPESP funding grant 0/0-.. Presented at NuFact, Aug 0, Rio de Janeiro, Brazil [C-08-.] bmiguez@ifi.unicamp.br; Speaker [] D. Z. Freedman, Physical Review D 9, 89 (97). [] A. J. Anderson et al., Phys. Rev. D8, 008 (0). [] T. K. Gaisser and M. Honda, Ann. Rev. Nucl. Part. Sci., (00). [] S. Ando and K. Sato, New J. Phys. 6, 70 (00). [] J. Billard, L. Strigari, and E. Figueroa-Feliciano, Phys. Rev. D89, 0 (0). [6] H. Murayama and A. Pierce, Phys.Rev. D6, 00 (00). 7