Posted on by Jason Major

Breaking the Speed of Light

Particle Collider
Today, CERN announced that the LHCb experiment had revealed the existence of two new baryon subatomic particles. Credit: CERN/LHC/GridPP

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It’s been a tenet of the standard model of physics for over a century. The speed of light is a unwavering and unbreakable barrier, at least by any form of matter and energy we know of. Nothing in our Universe can travel faster than 299,792 km/s (186,282 miles per second), not even – as the term implies – light itself. It’s the universal constant, the “c” in Einstein’s E = mc2, a cosmic speed limit that can’t be broken.

That is, until now.

An international team of scientists at the Gran Sasso research facility outside of Rome announced today that they have clocked neutrinos traveling faster than the speed of light. The neutrinos, subatomic particles with very little mass, were contained within beams emitted from CERN 730 km (500 miles) away in Switzerland. Over a period of three years, 15,000 neutrino beams were fired from CERN at special detectors located deep underground at Gran Sasso. Where light would have made the trip in 2.4 thousandths of a second, the neutrinos made it there 60 nanoseconds faster – that’s 60 billionths of a second – a tiny difference to us but a huge difference to particle physicists!

The implications of such a discovery are staggering, as it would effectively undermine Einstein’s theory of relativity and force a rewrite of the Standard Model of physics.

The OPERA Neutrino Detector. Credit: LGNS.

“We are shocked,” said project spokesman and University of Bern physicist Antonio Ereditato.

“We have high confidence in our results. We have checked and rechecked for anything that could have distorted our measurements but we found nothing. We now want colleagues to check them independently.”

Neutrinos are created naturally from the decay of radioactive materials and from reactions that occur inside stars. Neutrinos are constantly zipping through space and can pass through solid material easily with little discernible effect… as you’ve been reading this billions of neutrinos have already passed through you!

The experiment, called OPERA (Oscillation Project with Emulsion-tRacking Apparatus) is located in Italy’s Gran Sasso facility 1,400 meters (4,593 feet) underground and uses a complex array of electronics and photographic plates to detect the particle beams. Its subterranean location helps prevent experiment contamination from other sources of radiation, such as cosmic rays. Over 750 scientists from 22 countries around the world work there.

Ereditato is confident in the results as they have been consistently measured in over 16,000 events over the past two years. Still, other experiments are being planned elsewhere in an attempt to confirm these remarkable findings. If they are confirmed, we may be looking at a literal breakdown of the modern rules of physics as we know them!

“We have high confidence in our results,” said Ereditato. “We have checked and rechecked for anything that could have distorted our measurements but we found nothing. We now want colleagues to check them independently.”

A preprint of the OPERA results will be posted on the physics website ArXiv.org.

Read more on the Nature article here and on Reuters.com.

UPDATE: The OPERA team paper can be found here.

 

 

Posted on by Nancy Atkinson

Cosmologists Provide Closest Measure of Elusive Neutrino

Slices through the SDSS 3-dimensional map of the distribution of galaxies. Earth is at the center, and each point represents a galaxy, typically containing about 100 billion stars. Galaxies are colored according to the ages of their stars, with the redder, more strongly clustered points showing galaxies that are made of older stars. The outer circle is at a distance of two billion light years. The region between the wedges was not mapped by the SDSS because dust in our own Galaxy obscures the view of the distant universe in these directions. Both slices contain all galaxies within -1.25 and 1.25 degrees declination. Credit: M. Blanton and the Sloan Digital Sky Survey.

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Cosmologists – and not particle physicists — could be the ones who finally measure the mass of the elusive neutrino particle. A group of cosmologists have made their most accurate measurement yet of the mass of these mysterious so-called “ghost particles.” They didn’t use a giant particle detector but used data from the largest survey ever of galaxies, the Sloan Digital Sky Survey. While previous experiments had shown that neutrinos have a mass, it is thought to be so small that it was very hard to measure. But looking at the Sloan data on galaxies, PhD student Shawn Thomas and his advisers at University College London put the mass of a neutrino at no greater than 0.28 electron volts, which is 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.

Their 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.

A neutrino is capable of passing through a light year –about six trillion miles — of lead without hitting a single atom.

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.

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

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.

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

“This is one of the most effective techniques available for measuring the neutrino masses,” said Dr. Filipe Abadlla. “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, will yield an even more accurate weight for the neutrino, potentially at an upper limit of just 0.1 electron volts.
The results are published in the journal Physical Review Letters.

Source: University College London

Posted on by Jean Tate

Antineutrino

IceCube neutrino detector.

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The antineutrino (or anti-neutrino) is a lepton, an antimatter particle, the counterpart to the neutrino.

Actually, there are three distinct antineutrinos, called types, or flavors: electron antineutrino (symbol ̅νe), muon antineutrino (symbol ̅νμ), and tau antineutrino (symbol ̅ντ).

Beta Decay which produces electrons also produces (electron) antineutrinos. Wolfgang Pauli proposed the existence of these particles, in 1930, to ensure that beta decay conserved energy (the electrons in beta decay have a continuum of energies) and momentum (the momentum of the electron and recoil nucleus – in beta decay – do not add up to zero); Enrico Fermi – who developed the first theory of beta decay – coined the word ‘neutrino’, in 1934 (it’s actually a pun, in Italian!). It would be a quarter of a century before the (electron) antineutrino was confirmed, via direct detection (Cowan and Reines did the experiment, in 1956, and later got a Nobel Prize for it).

Another Nobel Prize – for Leon Lederman, Melvin Schwartz, and Jack Steinberger, in 1988 – came from experimental work in the 1960s which showed that muon antineutrinos are not the same as electron antineutrinos.

And in 2002, Davis and Koshiba shared the Nobel Prize (with Giacconi, for work in x-ray astronomy) for their detection of cosmic antineutrinos (a 40-year task!), which lead to the discovery of flavor oscillations (in which an antineutrino of one kind changes into another – electron antineutrino to muon antineutrino, for example).

Are neutrinos their own antiparticles? No … but perhaps there is an as yet undiscovered kind of neutrino that is (called a Majorana neutrino)? So β– (electron) decay produces antineutrinos (lepton number is conserved: 1 + (-1) = 0), and β+ (positron) decay produces neutrinos.

No Guide to Space article would be complete without some ‘Further Reading’, would it? KamLAND (the Kamioka Liquid-scintillator Anti-Neutrino Detector) is a wonderful place to start! For one of the greatest physics detective stories of the 20th century, check out my idol John Bahcall’s webpage. Applied Antineutrino Physics (Lawrence Livermore National Laboratory) – great stuff there too.

You won’t find ‘antineutrino’ in many Universe Today articles … but you’ll find plenty on neutrinos! That’s OK … remember that it’s very common to use the word ‘neutrino’ in a generic sense, one that includes the meaning ‘antineutrino’. Some examples: Neutrino Evidence Confirms Big Bang Predictions , Seeing Inside the Earth with Neutrinos, and Do Advanced Civilizations Communicate with Neutrinos?

Two Astronomy Cast episodes give you more insight into the antineutrino, Antimatter, and The Search for Neutrinos.

Sources:
Stanford University KamLAND
Wikipedia

Posted on by Jean Tate

What are WIMPs?

The Hubble Space Telescope distribution of dark matter - indirect observations (HST)

WIMPs are Weakly Interacting Massive Particles, hypothetical particles which may be the main (or only) component of Dark Matter, a form of matter which emits and absorbs no light and which comprises approx 75% of all mass in the observable universe.

The ‘weakly’ is a bit of a pun; WIMPs would interact with themselves and with other forms of mass only through the weak force (and gravity); get it? More plays on words: WIMP, the word, was created after the term MACHO (Massive Astrophysical Compact Halo Object) entered the scientific literature.

WIMPs are massive particles because they are not light; they would have masses considerably greater than the mass of the proton (for example). Being massive, WIMPs would likely be cold; in astrophysics ‘cold’ doesn’t mean ‘below zero’, it means the average speed of the particles is well below c. Neutrinos are weakly interacting particles, but they are not massive, so they cannot be WIMPs (besides, neutrinos aren’t hypothetical, and they’re hot, very hot … they travel at speeds just a teensy bit below c).

Or maybe they are … if there is a kind of neutrino which is really, really massive (a TeV say) then it would certainly be a WIMP! However, the latest results from WMAP seem to rule out this kind of WIMP-as-neutrino.

There are quite a few experiments – active or planned – looking for WIMPs; some Universe Today stories on them are New Proposal to Search for Dark Matter, Searching for Dark Matter Particles Here on Earth, Digging for Dark Matter, the Large Underground Xenon (LUX) Detector, and A Prototype Detector for Dark Matter in the Milky Way. It may turn out that WIMPs remain entirely hypothetical (the particles which comprise Dark Matter may not be WIMPs, for example) or only a minor component of Dark Matter. In the latter case they’d follow MACHOs not only because they were named at a later time … MACHOs have been detected in the Milky Way halo, but there aren’t anywhere near enough of them to account for the rotation curve of our galaxy.

This section of a review by Princeton University’s Dave Spergel is rather old but still one of the best concise technical descriptions; and this Particle Data Group review (PDF file) lists WIMP searches (though it’s a bit dated). NASA’s Imagine the Universe! has a page on Possibilities for Dark Matter that includes a simple overview of WIMPs.

Want to hear, rather than read, more? Check out the Astronomy Cast episode The Search for Dark Matter!

Source: NASA

Posted on by Jean Tate

Supernova 1987A

SN 1987A Credit: Hubble Space Telescope (2004)

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The first supernova in 1987 (that’s what the “A” means) was the brightest supernova in several centuries (and the first observed since the invention of the telescope), the first (and so far only) one to be detected by its neutrino emissions, and the only one in the LMC (Large Magellanic Cloud) observed directly.

Ian Shelton, then a research assistant with the University of Toronto, working at the university’s Las Campanas station, and Oscar Duhalde, a telescope operator at Las Campanas Observatory, were the first to spot it, on the night of 23/24 February 1987 (around midnight actually); over the next 24 hours several others also independently discovered it.

The IAU’s CBAT went wild! That’s the International Astronomical Union’s Central Bureau for Astronomical Telegrams, the clearing house for astronomers for breaking news. You can read the historic IAUC (C for Circular) 4416 here.

Once the discovery of SN 1987A became known, physicists examined the records from various neutrino detectors … and found three, independent, clear signals of a burst of neutrinos several hours before visual discovery, just as predicted by astrophysical models! Champagne flowed.

Not long afterwards, the star which blew up so spectacularly – the progenitor – was identified as Sanduleak -69° 202a, a blue supergiant. This was not what was expected for a Type II supernova (the models said red supergiants), but an explanation was quickly found (Sanduleak -69° 202a had a lower-than-modelled oxygen abundance, affecting the transparency of its outer envelope).

The iconic Hubble Space Telescope image (above) of SN 1987A shows the inner ring, where the debris from the explosion is colliding with matter expelled from the progenitor about 20,000 years ago; more from the Hubble here.

AAVSO (American Association of Variable Star Observers) has a nice write-up of SN 1987A.

No wonder, then, that SN 1987A features so often in Universe Today stories; for example Supernova Shockwave Slams into Stellar Bubble, XMM-Newton’s View of Supernova 1987A, Supernova Left No Core Behind, and Hubble Sees a Ring of Pearls Around 1987 Supernova.

SN 1987A figures prominently in Astronomy Cast The Search for Neutrinos, and in Nebulae.

References:
AAVSO
University of Oregon