Re: [Phys-l] The speed of neutrinos

[Jack Uretsky <jlu@hep.anl.gov>]

A post-doc and I have done that calculation for relic anti-nus + a 
suitable nucleus (I forget which) going to nucleus* + e+.  Assume the 
nu-bar mass is big enoungh that the nu-bars are non-relativistic or 
stationary in the lab frame.  Their density, as I recall now, is of order 
10^2/cm^3.  The rate for kilograms or nuclei is unobservable with any kind 
of background we could estimate.  I'd welcome another calculation.
				Regards,
					Jack

"Trust me.  I have a lot of experience at this."
		General Custer's unremembered message to his men,
		just before leading them into the Little Big Horn Valley




On Sat, 2 Oct 2010, John Denker wrote:

> On 10/01/2010 11:45 PM, Derek McKenzie wrote:
> > sed, it is usually asserted that they travel very close to the speed
> > of light, which was responsible for people thinking they were
> > probably massless. But the statement 'close to the speed of light' is
> > meaningless. A particle is either traveling at the speed of light or
> > it is not, and there are no degrees of closeness to it. So this
> > choice of phrase obviously means that physicists always observe
> > neutrinos to be traveling close to the speed of light in their
> > lab-frame. But how can *all* neutrinos, no matter how and where they
> > were produced, appear in the lab-frame to be traveling close to the
> > speed of light? Surely there should be plenty of neutrinos floating
> > around at nice slow speeds relative to the E
>
> That depends on what you mean by "slow".
>
> Try this:
>  http://www.google.com/search?q="cold+neutrinos";
>
> which leads to:
>  http://www.cap-lore.com/MathPhys/ColdNeutrino.html
>
> ... which I can summarize:  In analogy to the cosmic background
> microwave radiation, there must be cosmic background neutrinos,
> with an energy on the order of a few kelvin.  Given plausible
> estimates of neutrino mass, they will have speeds on the order
> of 2e7 m/s.  That is non-relativistic, but not exactly slow on
> the human scale, and not even slow compared to planetary,
> stellar, or even galactic escape velocities.
>
> It hardly matters how many of them there are, because you will
> never see them.  Neutrino cross sections scale like energy.
> It takes heroic measures to see the high-energy ones, and the
> low-energy ones are out of the question for now, and for a
> long time to come.
>  http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neutrino3.html
>
> Any neutrinos produced recently (on earth, or in any known
> star, or in any known galaxy) will be moving at very nearly
> the speed of light relative to the lab frame.
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