Archive for ‘Particle physics’

March 20, 2012

Neutrino oscillation measured

The possibility that neutrinos might travel a smidgen faster than light has been widely publicized. It seems to have already been disproven, but there’s something else that neutrinos do that’s no longer in doubt – and almost as interesting.

Neutrinos come in three “flavors“: electron, muon, and tau (denoted by νe, νμ, ντ.) These three types are parallel with and correspond to the three flavors of better-known leptons: electron, muon, and tau. “Flavor” is a facetious term used by particle physicists to refer the fact that leptons (as well as neutrinos and quarks) come in three distinct types with different masses, but no other distinguishing characteristics. And nobody really knows why different flavors exist, or why there are (or seem to be) exactly three “generations” of them.

But what neutrinos do that is no longer disputable, and which neither other leptons nor quarks do, is to spontaneously change their flavor – undergo “oscillation” – on the fly. More precisely, neutrino flavor is a quantum mechanical property, which has no definite value until it’s measured. And the probability of observing one definite flavor or another varies over time. Not only can neutrinos have flavors different from what they had when created, but even the probability of what will be measured fluctuates.

Neutrino oscillation is a very interesting phenomenon, but rather technical. See this article for a more detailed presentation.

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December 19, 2011

Possible constraints on dark matter particle mass

Although there is very good indirect evidence for the existence of dark matter, it’s an understatement to say that actual detection of dark matter particles has not been easy. Recent research results from two different teams that both studied gamma rays from dwarf galaxy neighbors of the Milky Way provide an illustration of the difficulty.

Both studies are based on the hypothesis that dark matter consists largely of “WIMPs” (Weakly Interacting Massive Particles). If existing indirect evidence for dark matter can be explained by modifications to the theory of gravity (so dark matter doesn’t actually exist), or if it exists in some form other than WIMPs, then the results of these studies are consistent with either possibility, but otherwise provide no additional information.

A further assumption of the WIMP hypothesis is that WIMPs are Majorana fermions. This is true, at least, if WIMPs are supersymmetric neutralinos. What this means is that WIMPs are their own antiparticles, and hence they are annihilated in any collision with another of their kind.

This annihilation can occur in different ways or “channels”, in which different types of known antiparticle pairs are created, such as b and anti-b mesons or τ+ leptons. These particles subsequently decay and produce gamma rays, which the Fermi Gamma-ray Space Telescope is designed to detect. The latest research shows that gamma-ray emissions that can be detected, from 7 to 10 dwarf satellite galaxies of the Milky Way, are not significantly higher than expected background emissions. Consequently, if indeed dark matter consists of WIMPs, then the individual particles must have masses above some lower bounds.

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November 19, 2011

Favored Higgs hiding spot remains after most complete search yet

Are we there yet? Well, no, not quite. Just be patient and try counting the cows, or something.

Despite a number of rumors and preliminary results, the Higgs boson hasn’t yet been found. There’s not even a clear signal yet that could be the real thing, with only a bit more data needed for better statistical confidence.

However, the possible range in energy space where the Higgs could hide keeps shrinking. The search parties are closing in. And it seems meaningful that so far everyone sees the same range where the Higgs might be.

Favored Higgs hiding spot remains after most complete search yet

The CMS and ATLAS experiments at the Large Hadron Collider have backed the Standard Model Higgs boson, if it exists, into a corner with their first combined Higgs search result.

The study, made public today, eliminates several hints the individual experiments saw in previous analyses but leaves in play the favored mass range for the Higgs boson, between 114 and 141 GeV. ATLAS and CMS ruled out at a 95 percent confidence level a Higgs boson with a mass between 141 and 476 GeV. …

“I think it could be an interesting message the data is telling us,” said physicist Eilam Gross of the Weizmann Institute of Science, who shares leadership of the ATLAS experiment’s Higgs group. “Any discovery starts with the inability to exclude.”

Several related measurements indirectly suggest a Standard Model Higgs boson exists at the lower end of the mass range.

It is possible, however, that some answer, based on data already collected, may come out before the end of the year.

Higgs hunt enters endgame

Analysis of the very latest data from this autumn — which Murray isn’t yet ready to share — will scour the range that remains. If it turns out to be empty, physicists may have to accept that the particle simply isn’t there. Working around the clock, the detector teams hope to have this larger data set analysed before the end of December. “We’ll know the outcome within weeks,” says Guido Tonelli, spokesman for the CMS detector.

September 17, 2011

Particle physics: The dark at the end of the tunnel?

Dark matter is certainly nothing if not elusive. If dark matter does not exist in some form or other, there are serious problems with much of what astrophysicists think they know about the evolution of the universe. There are numerous types of astrophysical observations which can be explained by the existence of dark matter. But these explanations are contingent upon both the existence of dark matter in the form of some sort of as yet unknown particles (“WIMPs“) and also the correctness of accepted fundamental laws of physics – such as Newtonian gravity. If dark matter doesn’t exist, then the fundamental laws are also called into question.

Particle physics: The dark at the end of the tunnel? – The Economist

To identify dark matter, experiments like DAMA, CRESST and CoGeNT look for weak-force-mediated collisions between atoms on Earth and WIMPs in the dark-matter halo of the Earth’s home galaxy, the Milky Way. Such collisions should cause individual atomic nuclei to recoil, and with the right apparatus such recoils can be observed. To screen out the confounding effects of cosmic rays, though, such experiments are best located underground.

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September 9, 2011

EXO releases first results on double beta decay

I know this sounds esoteric, but bear with me. Physicists have measured the slowest radioactive decay process yet – (2-neutrino) double-beta decay in xenon-136. The half-life is figured to be 2.11×1021 years. With the age of the universe around 1.37×1010 years, this half-life is more than 100 billion times as long. Don’t wait up for it.

Howwever, as impressive a feat as this is, the experiment is aiming at something even more interesting: a decay process that’s not even predicted by the Standard Model, namely neutrinoless double beta decay. If this actually occurs, it would imply neutrinos are their own antiparticles, which is very non-SM.

EXO releases first results – symmetry breaking

What the EXO 200 team wants to find is another decay process – one that is not only even more fantastically rare than 2nubb, but that no one is certain even exists. It’s called zero-neutrino double-beta decay, or 0nubb, and it is decidedly not a Standard Model process.

In 0nubb, two neutrons once again decay into two protons and two electrons, but the antineutrinos are nowhere to be found. They must have been there; the IRS has nothing on Nature for keeping the books balanced. The two antineutrinos must have annihilated each other, like positrons and electrons can annihilate each other, or protons and anti-protons, or any particle and its antiparticle.

This means in order for 0nubb decay to happen, neutrinos must be their own antiparticles.

Odd as this sounds, the possibility of a particle that could be both itself and its anti-self was hypothesized by an Italian theoretical particle physicist named Ettore Majorana in 1937. Such particles are called Majorana particles, and if they exist physicists would need to get busy revising the Standard Model.

September 1, 2011

A Lighter Higgs, But Chase Continues – Science News

A Lighter Higgs, But Chase Continues – Science News.


“In the hunt for the Higgs boson, the world’s most powerful particle collider has tightened the net. New data collected this year by CERN’s Large Hadron Collider near Geneva narrow the range of allowable masses for the hypothetical particle, whose existence would confirm the mechanism thought to give mass to other particles.”