Ultra-rare decay confirmed in LHC

LHCb LHCb observed particles which decay about three times in every billion collisions

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Scientists have confirmed one of the rarest phenomena of decay in particle physics, found about three times in every billion collisions at the LHCb.

They are now certain of the rarity of a transformation of subatomic particles hinted at previously.

The way this unfolds casts doubt on versions of the theory of physics known as Supersymmetry (Susy).

It was hoped Susy could explain gaps in the most established theory of how the Universe works.

The vast LHC machine, housed in a circular tunnel that runs for 27km beneath the French-Swiss border, smashes beams of protons together at close to light speeds.

Detectors positioned at key points around the underground "ring" are then used to scour the wreckage of these collisions for signs of new particles and physical phenomena.

Supersymmetry Supersymmetry predicts heavy versions of all the particles we know about - "super particles"

The theory Susy proposes that each particle has a heavier version of itself which could explain the ever mysterious dark matter, believed to make up a quarter of our Universe.

Needle in a haystack

However, the rate of decay found was predicted by the Standard Model - the framework for particle physics devised in the 1960s and 1970s - even though it's now seen as an incomplete description of nature.

It is not yet able to explain gravity, or indeed the dark matter and dark energy which together make up 95% of the Universe.

Some particles naturally decay into others and the types of decay can help physicists refine key theories. Here scientists found a particle called a Bs meson decaying into two muons for the first time.

The findings were announced at the EPS conference in Stockholm and had the 5-sigma level of significance required to reach the level of a formal discovery.

Statistics of a 'discovery'

Five Swiss francs
  • Particle physics has an accepted definition for a "discovery": a five-sigma level of certainty
  • The number of standard deviations, or sigmas, is a measure of how unlikely it is that an experimental result is simply down to chance, in the absence of a real effect
  • Similarly, tossing a coin and getting a number of heads in a row may just be chance, rather than a sign of a "loaded" coin
  • The "three sigma" level represents about the same likelihood of tossing nine heads in a row
  • Five sigma, on the other hand, would correspond to tossing more than 21 in a row
  • Unlikely results are more probable when several experiments are carried out at once - equivalent to several people flipping coins at the same time
  • With independent confirmation by other experiments, five-sigma findings become accepted discoveries

This builds on a previous announcement of the findings which had lesser statistical significance as the team had not yet analysed all the data.

The observations at LHCb and CMS were so rare that Bs mesons only decayed into two muons about three times in every billion collisions.

The LHCb team announced: "Finding particle decays this rare makes hunting for a needle in a haystack seem easy."

This is due to the hundreds of millions of collisions the LHC produced every second, with each one producing hundreds of new particles that leave electrical signals in the giant detectors.

Quantum loop

Val Gibson, leader of the Cambridge particle physics group and member of the LHCb experiment, told BBC News that it was the rarest decay they have observed so far.

"The reason it's so rare is the fact that it doesn't decay easily into the final quark particles we know about. It has to go through a loop process, like a quantum loop. It's not a straight road but it has to go round a roundabout before it can get to the final state particles.

"Because it's got this roundabout in it, it means that other heavy supersymmetric particles [could potentially] enter the roundabout and make a big difference to the decay rate," Prof Gibson added.

But the quarks did not have heavy particles blocking the decay.

Shy physics

"There was no observation of Supersymmetry, you would have to fine-tune the theory to explain the measurements found," Prof Gibson explained.

"The Supersymmetry theorists have not given up, however it is becoming harder and harder for them to explain these findings.

"Measurements of this very rare decay significantly squeeze the places new physics can hide. The UK LHCb team are now looking forward to the LHC returning at even higher energy and to an upgrade to the experiment so that we can investigate why new physics is so shy."

Tara Shears from the University of Liverpool also works with the LHCb, but was not involved with this particular discovery. She said: "Supersymmetry is starting to look less likely to be a good description of the universe."

"The catch is that Supersymmetry is quite a loosely defined theoretical model which means it has many uncertainties in it. It's impossible to rule it out altogether.

"This result has has really put the squeeze on the possibilities of the different ways Supersymmetry could be possible," she told BBC News.

But John Ellis, professor of theoretical physics at King's College London, told BBC News that the results were not evidence against Supersymmetry.

"It is as if the experiments had looked through a powerful telescope and not seen a new star they had been looking for.

"If the telescope is pointed in the wrong direction, it will not find it. Supersymmetry is a complicated theory with many parameters, and there are directions in parameter space where the Bs meson into two muons 'telescope' sees nothing."

The Standard Model

Diagram of the Standard Model

The Standard Model is the simplest set of ingredients - elementary particles - needed to make up the world we see in the heavens and in the laboratory

Quarks combine together to make, for example, the proton and neutron - which make up the nuclei of atoms today - though more exotic combinations were around in the Universe's early days

Leptons come in charged and uncharged versions; electrons - the most familiar charged lepton - together with quarks make up all the matter we can see; the uncharged leptons are neutrinos, which rarely interact with matter

The "force carriers" are particles whose movements are observed as familiar forces such as those behind electricity and light (electromagnetism) and radioactive decay (the weak nuclear force)

The Higgs boson came about because although the Standard Model holds together neatly, nothing requires the particles to have mass; for a fuller theory, the Higgs - or something else - must fill in that gap

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