The familiar nucleosynthesis constraint on the number of neutrino species, 7V V < 3.4, applies to massless neutrino species. An MeV-mass neutrino can have even greater impact, and we show that primordial nucleosynthesis excludes a r-neutrino mass from 0.3 to 25 MeV (Dirac) and 0.5 to 25 MeV (Majorana) provided that its lifetime r v^ 1 sec, and from 0.3 to 30 MeV (Dirac) and 0.5 to 32 MeV (Majorana) for r v^ 10 3 sec. A modest improvement in the laboratory mass limit-from 35 to 25 MeV-would imply that the r-neutrino mass must be less than 0.5 MeV (provided T V^ 1 sec).PACS numbers: 98.80.Ft, 12.15.Ff, 14.60.Gh, 98.80.Cq The agreement of the predictions of primordial nucleosynthesis for the abundances of the light isotopes D, 3 He, 4 He, with 7 Li with their measured abundances is one of the great triumphs of the hot big-bang cosmology. Because of this success primordial nucleosynthesis has been used as a probe of both cosmology and particle physics [1]. In particular, primordial nucleosynthesis provides a stringent limit to the number of "light" (mass <3CMeV) degrees of freedom, which, expressed in terms of the equivalent number of light neutrino species, is AV:<3.4 [2]. Recent precision measurements of the properties of the Z° have confirmed this in spectacular fashion:In the context of nucleosynthesis, "light" refers to neutrinos of mass much less than 1 MeV, while "heavy" refers to neutrinos of mass of an MeV or greater, a division that traces to the temperature of the Universe when the weak interactions freeze out: 7>~ 1 MeV. The present laboratory limit to the mass of the r neutrino is m v < 35 MeV [4]; if the r-neutrino mass is greater than order 1 MeV, it will not be a light degree of freedom during nucleosynthesis and the above limit does not apply. Since the equilibrium energy density of a massive neutrino species is less than that of a "massless" neutrino species it would seem that the nucleosynthesis limit is irrelevant for the r neutrino.Neutrinos do not stay in thermal equilibrium; they decouple at temperature of order a few MeV [5]. After a massive neutrino species decouples and becomes nonrelativistic, its energy density grows relative to a massless neutrino species:= 7;r 2 r 4 /120, r is the ratio of the number density of massive neutrinos to massless neutrinos after freeze-out, and T v is the neutrino temperature. For an MeV-mass neutrino species r is order unity, and when the neutron-to-proton ratio freezes out (7>~ 1 MeV) its energy density is comparable to or even greater than a massless species. Since the 4 He yield is very sensitive to the value at which the neutron-to-proton ratio freezes out, which depends upon the energy density of the Universe, an MeV-mass neutrino species can have a significant effect on 4 He production.Later, when T<&\ MeV, the energy density of a massive r neutrino is even more significant: When the actual synthesis of the light elements begins in earnest (/ -200 sec, 7 1 -0.1 MeV), it can be 10 times that of a massless species, comparable to the total energy densit...
We report the first evidence for the decay Sigma(+)-->pmu(+)mu(-) from data taken by the HyperCP (E871) experiment at Fermilab. Based on three observed events, the branching ratio is B(Sigma(+)-->pmu(+)mu(-))=[8.6(+6.6)(-5.4)(stat)+/-5.5(syst)]x10(-8). The narrow range of dimuon masses may indicate that the decay proceeds via a neutral intermediate state, Sigma(+)-->pP(0),P0-->mu(+)mu(-) with a P0 mass of 214.3+/-0.5 MeV/c(2) and branching ratio B(Sigma(+)-->pP(0),P0-->mu(+)mu(-))=[3.1(+2.4)(-1.9)(stat)+/-1.5(syst)]x10(-8).
We have compared the p and p angular distributions in 117 x 10(6) Xi- -->Lambdapi- -->ppi-pi- and 41 x 10(6) Xi+ -->Lambda pi+ -->p pi+pi+ decays using a subset of the data from the HyperCP experiment (E871) at Fermilab. We find no evidence of CP violation, with the direct-CP-violating parameter AXiLambda identical with (alphaXialphaLambda-alpha Xialpha Lambda)/(alphaXialphaLambda+alphaXialphaLambda)=[0.0+/-5.1(stat)+/-4.4(syst)] x 10(-4).
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