Neutrino mass: 'Ghost particle' sized up by cosmologists

Jun 22, 2010

(PhysOrg.com) -- Cosmologists at UCL (University College London) are a step closer to determining the mass of the elusive neutrino particle, not by using a giant particle detector, but by gazing up into space.

Although it has been shown that a neutrino has a mass, it is vanishingly small and extremely hard to measure - a neutrino is capable of passing through a light year (about six trillion miles) of lead without hitting a single atom.

New results using the largest ever survey of in the universe puts total neutrino mass at no larger than 0.28 electron volts - less than a billionth of the mass of a single hydrogen atom. This is one of the most accurate measurements of the mass of a neutrino to date.

The research is due to be published in an upcoming issue of the journal Physical Review Letters, and will be presented at the Weizmann:UK conference at UCL on 22-23 June 2010. It resulted from the PhD thesis of Shaun Thomas, supervised by Prof. Ofer Lahav and Dr. Filipe Abdalla.

Professor Ofer Lahav, Head of UCL’s Astrophysics Group, said: “Of all the hypothetical candidates for the mysterious , so far provide the only example of dark matter that actually exists in nature. It is remarkable that the distribution of galaxies on huge scales can tell us about the mass of the tiny neutrinos”.

The work is based on the principle that the huge abundance of neutrinos (there are trillions passing through you right now) has a large cumulative effect on the matter of the cosmos, which naturally forms into “clumps” of groups and clusters of galaxies. As neutrinos are extremely light they move across the universe at great speeds which has the effect of smoothing this natural “clumpiness” of matter. By analysing the distribution of galaxies across the universe (i.e. the extent of this “smoothing-out” of galaxies) scientists are able to work out the upper limits of neutrino mass.

Central to this new calculation is the existence of the largest ever 3D map of galaxies, called Mega Z, which covers over 700,000 galaxies recorded by the Sloan Digital Sky Survey and allows measurements over vast stretches of the known universe.

The Cosmologists at UCL were able to estimate distances to galaxies using a new method that measures the colour of each of the galaxies. By combining this enormous galaxy map with information from the temperature fluctuations in the after-glow of the Big Bang, called the Cosmic Microwave Background radiation, they were able to put one of the smallest upper limits on the size of the neutrino particle to date.

Dr. Shaun Thomas commented: “Although neutrinos make up less than 1% of all matter they form an important part of the cosmological model. It's fascinating that the most elusive and tiny particles can have such an effect on the Universe.”

Dr. Filipe Abadlla added: "This is one of the most effective techniques available for measuring the neutrino masses. This puts great hopes to finally obtain a measurement of the mass of the neutrino in years to come."

The authors are confident that a larger survey of the Universe, such as the one they are working on called the international Dark Energy Survey, in which UCL is heavily involved, will yield an even more accurate weight for the neutrino, potentially at an upper limit of just 0.1 electron volts.

Explore further: Ultrafast imaging of complex systems in 3D at near atomic resolution nears

More information: Physical Review Letters - prl.aps.org/

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frajo
5 / 5 (1) Jun 22, 2010
The Cosmologists at UCL were able to estimate distances to galaxies using a new method that measures the colour of each of the galaxies.
That's the most interesting statement in the article. IMHO.
axemaster
5 / 5 (1) Jun 22, 2010
I'm confused. Why don't they know the mass with great precision already? I thought you could simply measure the amount of missing energy in fusion reactions. Or are they trying to find out how much energy is in the rest mass, and how much is in kinetic energy? I'm confused.

Oh, here's an idea for how to measure it. Find a pulsar and measure the pulse rate of the emitted light. Then build a neutrino detector and measure the pulse rate of the neutrinos. By looking at the offsets on several pulsars, you should be able to find the mass of the neutrinos (offset because the neutrinos travel slightly slower than the light).
yyz
5 / 5 (1) Jun 22, 2010
frajo,

AFAIU, they're talking about photometric redshifts here, which actually have been around for a while ( http://en.wikiped...redshift ).

Far less accurate than spectroscopically derived redshifts, I think their use here indicates accuracy sufficient for the purposes of the study.
frajo
not rated yet Jun 22, 2010
they're talking about photometric redshifts here, which actually have been around for a while
Thanks for the hint. That "using a new method" was misleading me.
eachus
5 / 5 (3) Jun 22, 2010
I'm confused. Why don't they know the mass with great precision already? I thought you could simply measure the amount of missing energy in fusion reactions.

The problem is that the rest mass of the neutrino is so much less than that of the other particles in any reactions--and it is impossible to detect the neutrino associated with any particular reaction. It is possible to set an upper limit on the mass of the neutrino by looking at beta decays, but this limit is much smaller.

Oh, here's an idea for how to measure it. Find a pulsar and measure the pulse rate of the emitted light...

Right... The problem is the same as above, trying to tie particular detected neutrinos to specific events. Oh, and who says neutrinos travel slower than light? The only case where detected neutrinos could be tied to a particular astonomical event, supernova 1987A, they arrived 8 seconds before the light did. (Technically, if neutrinos are tachyons, the rest mass would be imaginary.)
thermodynamics
3.7 / 5 (3) Jun 23, 2010
eachus: The reason they can be confident that the particles have a rest mass and travel slower than light is that they can switch between neutrino types. If they actually traveled at the speed of light the particles could not shift because time would stop for them (time dilation). If they were tachyons we would not be able to see them at all because they could never be slowed down below the speed of light to interact (it would take infinite energy to slow a tachyon to the speed of light just like it would take to have a massive particle accelerate to the speed of light). So, if relativity is right they have to have mass and be traveling at less than the speed of light. Of course we have been surprised by other things so there is always the possibility that relativity is wrong. :-)
jsa09
5 / 5 (1) Jun 23, 2010
Once we accept that the neutrino travels slower than the speed of light for any reason we then are open to the suggestion that neutrinos do not travel at a fixed no mass speed. This leads us further down the track that neutrinos can then travel at any speed from zero up to the energetic sub-light speeds. Once we accept that neutrinos have no intrinsic speed, we can see that there may be neutrinos existing at rest mass. Perhaps we will be able to put a bunch of neutrinos into a thimble one day.

Which brings us to slow neutrinos, have we measured neutrinos at different velocities? or do they all seem to be barreling along at high speed?
thermodynamics
3 / 5 (2) Jun 23, 2010
jsa09: As far as I understand it, they are produced in high energy events so have a high initial speed. Since nothing we know of slows them down, they continue at that speed as long as they do not interact. However, we also know they do, slightly, interact (or they would not be observable)so I don't know if that will eventually "cool" them over the life of the universe or not. Can someone answer that question?
Graeme
1 / 5 (1) Jun 25, 2010
A very large number of neutrinos were liberated in the first few minutes of the universe, these could have cooled due to the expansion of the universe. At some early point there should be something like the CMB for neutrinos before which the universe was opaque to them.

Are there fusion reactions that are reversible, sapping heat from a star but emmitting neutrinos? This probably happens in a supernova
eachus
5 / 5 (1) Jul 03, 2010
Are there fusion reactions that are reversible, sapping heat from a star but emmitting neutrinos? This probably happens in a supernova
Yes. Core collapse supernovas are initiated by neutrino emissions cooling the core. Current thinking is that the neutrinos are partially reabsorbed by the shock wave in the outer layers of the star. The inner parts of the star are much, much more dense than lead--or any normal matter. This makes the inner core essentially opaque to neutrinos. If the shock in the outer layers is sufficently dense, it will also absorb energy from neutrinos and the shock will not stall.

It may be that some stars go through several core collapse instances before they succeed in blowing off the outer layers of the star. Eta Carina has had a few significant increases in brightness. In 1820 it started to brighten, reaching peak brightness in April 1843. It has been brightening again recently. Hmmm.
eachus
not rated yet Jul 03, 2010
If they were tachyons we would not be able to see them at all because they could never be slowed down below the speed of light to interact...
A tachyon, if such exist, could interact with normal matter through quantum tunneling. (Or if you prefer, through the uncertainty principle.) In either case a particle with sufficiently low momentum squared could interact with a normal particle. Hmmm. If neutrinos are tachyons, this would be very rare, since neutrinos interact through the electromagnetic force with bosons or leptons. Of course, neutrinos do interact very, very rarely with normal matter.
Of course we have been surprised by other things so there is always the possibility that relativity is wrong. :-)


This is one of those areas where quantum mechanics or relativity is wrong, or perhaps both. Mixing QM and relativity is known not to work. The problem is, we don't have anything better yet.
jsa09
not rated yet Jul 06, 2010
I am thinking that if neutrinos travel slower than the speed of light they would not have intrinsic speed.

This would mean that apart from being slowed down by interactions (as rare as they be) that neutrinos ejected from objects traveling away from us would reach us at much slower speeds again.

Since the universe is supposed to be expanding this would mean that neutrinos from further away would be slower and slower until we had neutrinos from objects retreating at almost light speed would be reaching us at everyday velocities. Like a neutrino traveling at 100 miles per hour of 10 miles per hour etc.
jsa09
not rated yet Jul 06, 2010
This would also mean that neutrinos would only be able to reach us from the closer galaxies. The more distant galaxies would just be retreating too fast for the neutrinos to ever reach us. Something that can be determined through experiment.

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