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MeV Tau Neutrino Astrophysical and Cosmological Constraints and Mischief a c b c Geza Gyuk and Michael S Turner a Department of Physics University of Chicago Chicago IL b Departments of Physics and of Astronomy Astrophysics Enrico Fermi Institute The University of Chicago Chicago IL c NASAFermilab Astrophysics Center Fermi National Accelerator Lab oratory Batavia IL Terrestrial and Heavenly exp eriments severely constrain the mass and lifetime of an MeV tau neutrino sec the mass of the tau neutrino must b e either approximately MeV Irresp ective of decay mo de for or less than MeV Ma jorana keV Dirac If the dominant decay mo de includes electromagnetic daughter 12 sec keV Dirac pro ducts the mass must b e less than MeV Ma jorana or Dirac provided 6 secm MeV A tau neutrino of mass b etween MeV and MeV can have a host of provided interesting astrophysical and cosmological consequences relaxing the bigbang nucleosynthesis b ound to the baryon density and the number of neutrino sp ecies allowing bigbang nucleosynthesis to accommo date a low 4 4 less than He mass fraction or high greater than deuterium abundance improving signicantly the agreement b etween the cold dark matter theory of structure formation and observations and helping to explain how type I I sup ernovae explo de Exploring the MeV mass range not only prob es fundamental particle physics but also interesting astrophysical and cosmological scenarios astro-ph/9410065 20 Oct 94 INTRODUCTION diative decays to the diuse photon background including the CBR the eects of massive Neutrinos are ubiquitous in the cosmos Their neutrinos and additional neutrino sp ecies up on relic abundance from the big bang is cm primordial nucleosynthesis and the formation of p er sp ecies and type II core collapse sup er structure in the Universe the eects of neu novae which o ccur at a rate of ab out sec in trino emission up on the evolution of red giant and the observable Universe pro duce neutrinos white dwarf stars the eects of radiative de of energy of order MeV p er explosion Even cay mass charge and so on on the neutrino burst ordinary stars like our sun radiate ab out of from SN A detected by the Kamiokande I I K their p ower in neutrinos Neutrinos pro duced in I I and IrvineMichiganBrookhaven IMB water the atmosphere rain down on the earth at a rate Cherenkov detectors A partial summary of the of around one p er cm sec regions of the masslifetime plane that can b e ex Because of all this the heavenly lab can and cluded is shown in Figs has b een used to obtain imp ortant constraints to Neutrinos can have imp ortant astrophysical the prop erties of neutrinos including mass life and cosmological implications For example a time magnetic and transition moments charge neutrino sp ecies of mass eV to eV would velocity of propagation secret ie additional account for the bulk of the mass density of the interactions number of neutrino types and so on Universe and more recently a neutrino sp ecies These limits follow from considering among of mass eV to eV has b een suggested as an other things the cosmic contribution of neu additive to improve the agreement b etween the trinos to the mass density and through their ra Figure Excluded region of the masslifetime Figure Excluded region of the masslifetime plane for a neutrino that decays radiatively plane based up on the contribution to the cosmic from Ref mass density from Ref cold dark matter picture of structure formation just ab ove MeV in the foreseeable future the and observations Sciama has emphasized that tauneutrino mass sensitivity may b e improved a neutrino of mass eV and radiative lifetime to MeV or lower Constraints from primor of around sec would explain how the bulk of dial nucleosynthesis and SN A allow the mass the matter in the present Universe b ecame ion limit for a longlived sec tau neutrino to ized as well as accounting for the dark matter b e lowered to around MeV Ma jorana and Neutrino oscillations provide a very attractive so to around keV Dirac On the other hand lution to the solarneutrino problem and have for masses b etween MeV and MeV there are even b een suggested as a means for explaining lifetimes and decay mo des that led to very in how sup ernovae explo de teresting astrophysical and cosmological conse The topic of neutrinos astrophysics and cos quences relaxing the bigbang nucleosynthesis mology is a very rich one indeed and it is not b ound to the baryon density and to the num our intent to try to summarize it here excellent b er of neutrino sp ecies allowing bigbang nucle reviews exist Rather we will discuss recent osynthesis to accommo date a low less than work concerning the astrophysical and cosmolog He mass fraction or high greater than ical constraints to and interesting consequences deuterium abundance improving signicantly the of an MeV tau neutrino This work is timely for agreement b etween the cold dark matter theory of two reasons The current lab oratory mass limit is structure formation and observations and help cussing on nal states containing ve pions The CLEO data set has such decays and the AR GUS data set has such decays By searching for events close to the kinematic endp oint they are able to set the following CL upp er lim its to the tauneutrino mass MeV ARGUS MeV CLEO Detector and accelerator upgrades at CLEO as well as the study of other decay mo des eg nal states with Kaons should lead to improved mass sensitivity p erhaps as low as MeV or so In addition the LEP collab orations are b e ginning to study tau physics including the tau neutrino mass Finally up coming exp eriments at Bfactories and taucharm factories if built may b e helpful A b eamdump exp eriment at CERN using the BEBC set a very restrictive limit to the decay of tau neutrinos to channels that include electro magnetic daughter pro ducts e and photons The absence of such electromagnetic interactions in the BEBC excludes a radiative decay rate in the interval m m Figure Excluded region of the masslifetime sec sec rad MeV MeV plane for a neutrino that decays radiatively based As we will describ e this limit together with those up on type I I sup ernovae white dwarf co oling and based up on SN A and primordial nucleosyn red giant evolution from Ref thesis all but exclude a tau neutrino that is more massive than ab out MeV and that decays pri marily through radiative mo des ing to explain how type II sup ernovae explo de SN A While the theoretical motivation for an MeV When the core of a massive star exhausts its mass tau neutrino is not strongthere are some nuclear fuel and collapses to form a neutron star mo delswe wish to stress that exploring the MeV most of its binding energy ab out erg is mass range allows tests of intriguing astrophysi released in thermal neutrinos of all three sp ecies calcosmological scenarios A neutron star is so dense that neutrinos b ecome trapp ed and are emitted from a neutrinosphere MASSLIFETIME CONSTRAINTS whose temp erature is ab out MeV In all more than neutrinos p er sp ecies of average en Lab oratory ergy around MeV are emitted during the ini There are two very imp ortant lab oratory con tial sec to sec of co oling The detection of straints that to the mass based up on the kine neutrino events asso ciated with SN A by the matics of taulepton decays and that to the radia IMB and KI I detectors provided dramatic conr tive lifetime based on the Big Europ ean Bubble mation of this picture The enormous ux of neu Chamber BEBC b eamdump exp eriment trinos emitted the b eautiful KI I and IMB data The CLEO and ARGUS collab orations have and our theoretical understanding of type I I core studied the decays of millions of tau leptons fo collapse sup ernovae make SN A a wonderful gammaray detectors which were in op eration at lab oratory for probing neutrino prop erties as has the time Since the neutrino uence on earth was b een summarized elsewhere nearly cm and that of gammarays dur So far as tau neutrinos are concerned there ing the sec interval at the time of the neutrino are three imp ortant SN A constraints The burst was less than ab out cm this leads to a rst involves Dirac neutrinos b ecause of the mis very stringent constraint Additional con match b etween chirality and helicity for a mas straints of this type have b een obtained recently sive neutrino neutrinonucleon scattering deep from GRO Comptel observations of SN A at inside a hot young neutron star can transform late times and of SN J a prop erhelicity neutrino into a wronghelicity Given the tauneutrino mass lifetime and ux neutrino whose interactions are weaker by a fac from a hot neutron star it is a simple matter to tor of m E These wronghelicity neutri derive the constraints that follow from SN A nos are emitted copiously from the core where There is a slight hitch in getting the tauneutrino temp eratures reach MeV or higher and sim ux for masses in the MeV range the neutri ply stream out For a Dirac mass b etween ab out nosphere temp erature is only MeV for a mass keV and MeV and lifetime greater than ab out less neutrino sp ecies so that suppression of the sec m MeV they quickly rob the core of neutrino ux should b ecome imp ortant for masses its thermal reserves leading to a burst of prop er ab ove MeV Recently the neutrinosphere tem helicity neutrinos from the neutrinosphere that p erature and neutrino ux for a massive neutrino is to o short to b e consistent with the KI
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