ethan@utastro.UUCP (Ethan Vishniac) (07/24/85)
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I was thumbing through the New Scientist yesterday and came across a
brief article about a talk by Rubbia. It claimed that the decay widths
of Z and W particles determined at CERN could now be used to limit the
number of neutrino species to a number "consistent with cosmological
predictions" (based on primordial nucleosynthesis). If this is true
it's exciting since it would be the first cosmological prediction concerning
particle physics that has ever been confirmed. The particle physicists
here seem to all be on vacation. Has anyone else heard this rumor?
--
"Don't argue with a fool. Ethan Vishniac
Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan
Department of Astronomy
University of Texascpf@lasspvax.UUCP (Courtenay Footman) (07/31/85)
In article <411@utastro.UUCP> ethan@utastro.UUCP (Ethan Vishniac) writes: >... claimed that the decay widths >of Z and W particles determined at CERN could now be used to limit the >number of neutrino species to a number "consistent with cosmological >predictions" (based on primordial nucleosynthesis). If this is true >it's exciting since it would be the first cosmological prediction concerning >particle physics that has ever been confirmed. The particle physicists >here seem to all be on vacation. Has anyone else heard this rumor? > Here is the status of the number of neutrino species, N(v), as of a few months ago. Most of this is based on data from the 1983 run; the 1984 run is still being analyzed. There are two methods of determining N(v) the CERN SPS data. One is simply to look at the width of the Z. The more (light) neutrino species there are, the wider the width. (That is, the more possible decay channels, the shorter the lifetime, which is equivalent to a larger width.) Unfortunately, the statistics are not very good. The number that I remember is an upper limit of seventeen neutrino species with a 90% confidence level. That number might be 15 or 19, and the C.L. might be 95%, but that is the right ballpark. This method is reasonably theory independent. The other method is to look at the relative leptonic branching ratios of the Z and W: BR(Z->e+e)/BR(W->e+v) (I use v for neutrino, lacking a nu key. Insert charges and anti-particle signs as needed.) The more nuetrinos there are, the smaller the Z BR will be, since there are more competing channels, while additional neutrinos do not affect the W BR because the leptons with these neutrinos would be too massive to be produced. However, we cannot measure that ratio directly; all that can be measured is the product of ratios BR(pp->Z)BR(Z->e+e)/BR(pp->W)BR(W->e+v), and use theory to calculate the additional terms. There is something unappealing about this. (Note: I am a particle theorist.) When this is done, the result claimed is that N(v) is 2+-1. Since we know that N(v) >= 3, this result, if believed, pins down N(v) rather well. -------------------------------------------------------------------------- Note on my address: Cornell's internal structure has recently changed. Only new Internet host tables will get mail to me. Would you believe lnsvax.tn.cornell.edu? Neither would I. -- Courtenay Footman arpa: cpf@lnsvax Newman Lab. of Nuclear Studies usenet: cornell!lnsvax!cpf Cornell University