Panel probes new particle results

16 June 2011 Last updated at 23:49 GMT By Paul Rincon Science Editor, BBC News Website The Tevatron was, until the advent of the LHC, the highest-energy accelerator in the world The head of the US' biggest particle physics lab has appointed an expert committee to establish whether or not a new, unanticipated sub-atomic particle has been detected by scientists.

Such a discovery, hinted at by experts in April, would mark one of the most radical changes to physics in years.

But separate science teams at the Tevatron accelerator are at loggerheads over the matter.

The Tevatron is the US rival to Europe's Large Hadron Collider (LHC).

The American machine is operated by the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois.

Two separate multi-purpose detectors - or experiments - analyse data from particle collisions at the Tevatron: DZero and CDF.

Each can cross-check the other team's discoveries.

The committee, set up by Fermilab director Pier Oddone, will aim to resolve the differences between scientists working on the CDF and DZero experiments.

The panel members will "compare notes", aiming to determine the cause of the signal seen by CDF team members, but not by DZero physicists.

Professor Giovanni Punzi, co-spokesperson for the CDF team, told BBC News: "DZero did not confirm our result, but it is not clear they reject it either. So we need to do more work and look at more data."

He said the estimated size of the "excess" - the possible signal of a new particle - seen by CDF was affected by a large statistical uncertainty. Because of this, he said, "it is quite likely that the disagreement [between the CDF and Dzero analyses] is not nearly as large as it appeared at first glance".

Comparing notes

The CDF team was analysing data from collisions between protons and their anti-matter counterparts antiprotons. In these collisions, particles known as W bosons are produced, along with a pair of "jets" of other particles.

It was in these jets that the unexpected "bump" in the team's data came to light, potentially representing a particle that the widely accepted theory of particle physics - known as the Standard Model - does not anticipate.

Confirmation of the CDF results would have signalled a radical change in physics. But last week, an independent analysis of the data carried out by the DZero team failed to find support for the observation.

Professor Stefan Soldner-Rembold, spokesperson for the DZero collaboration, commented: "The probability that the CDF effect is really new physics is very low."

He told BBC News that the data bump might have been caused by the procedure used to to remove the "background" from the soup of interactions produced by particle collisions in the Tevatron.

However, he added: "We would also be open to other possible explanations."

As spokespeople for the CDF and DZero collaborations respectively, Professor Punzi and Professor Soldner-Rembold will sit on the committee.

Fermilab theoreticians Estia Eichten and Keith Ellis have also been appointed to the panel.

There is currently no timeline for the committee to report back, but one source said they hoped the matter would be resolved soon.

Paul.Rincon-INTERNET@bbc.co.uk

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Particle 'flips to all flavours'

15 June 2011 Last updated at 09:04 GMT By Jonathan Amos Science correspondent, BBC News The detector works by looking for flashes of light from particles that exceed the speed of light in the water An important breakthrough may be imminent in the study of neutrinos.

The multinational T2K project in Japan says it has seen indications in its data that these elementary particles can flip to any of their three types.

The results are provisional because experiments had to be suspended in the wake of the Tohoku earthquake in March.

But if confirmed, they would open the door to further research on where the matter in the Universe came from.

Specifically, such studies would ask why the cosmos is composed of normal matter rather than its opposite - antimatter - which theorists say must have been created in equal amounts at the Big Bang.

"It's a step on the road," explained Professor Dave Wark, of Imperial College London and the STFC's Rutherford Appleton Laboratory, which leads the UK involvement in T2K.

"We want to address this asymmetry, but first we have to show that the different 'flavours' of neutrinos can spontaneously change into each other - something we call 'neutrino oscillation'. So far, our experiments have been very positive," he told BBC News.

Detecting 'ghosts'

Neutrinos are among the fundamental building blocks of matter. They swarm all about us.

The Sun, for example, releases them in huge quantities when it fuses hydrogen to make helium - the raw nuclear process at its core.

They are, however, very difficult to study because they interact so weakly with normal matter. Hence, their nickname - "ghost particles".

Nonetheless, scientists have been able to discern three flavours - electron neutrinos, muon neutrinos, and tau neutrinos.

Previous research has characterised two forms of oscillations.

The T2K experiment has now seen hints for a third transformation - that of a muon neutrino turning into an electron neutrino.

Side to side

T2K is an extraordinary set-up built in two parts.

At one end is the Japan Proton Accelerator Research Centre (J-Parc) on the country's east coast. It generates a beam of muon neutrinos that it fires under the ground for 295km to the mammoth Super-Kamiokande facility on the west coast.

The Super-K, as it is sometimes called, is a tank of 50,000 tonnes of ultra-pure water surrounded by sensitive optical detectors.

These photomultiplier tubes pick up the very rare, very faint flashes of light emitted when passing neutrinos interact with the water.

In experiments this year that ran on T2K until the quake damaged equipment, scientists saw an excess of electron neutrinos turning up at Super-K.

In other words, it appears the muon neutrinos sent from J-Parc had changed flavour in flight.

The statistics are not big enough to claim a discovery, but it has the T2K collaboration excited and keen to get up an running again.

Symmetry violation

"The oscillations of neutrinos are governed by three angles which you can think of like pitch, yaw and roll for an aeroplane," explained Professor Wark.

"In the past, we've measured pitch and yaw, but we had no sign that roll was different from zero. If these turn out to be real events, then we'll have shown that roll is not equal to zero."

The missing angle is actually referred to as "theta-one-three". If it really does have a non-zero quantity, it opens the possibility that the oscillations of neutrinos and their antiparticles (antineutrinos) could be different.

That would be an example of what physicists call CP violation and a potential explanation for why the normal matter and antimatter created at the Big Bang did not simply annihilate each other, but instead left an excess of normal matter.

"There have to be some laws of physics we don't know about because all the laws we do know about cannot produce any significant excess of matter over antimatter. There are a number of places these laws could be hiding, and one of the most promising places to look is neutrinos," said Professor Wark.

"But the first step has to be to prove that theta-one-three is not equal to zero. If it were then neutrinos and antineutrinos would have to oscillate the same way - there just aren't enough degrees of freedom to allow them to be different."

Repairs at J-Parc should have the experiments gathering data again by the end of the year.

Jonathan.Amos-INTERNET@bbc.co.uk

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Tests 'reject new particle claim'

10 June 2011 Last updated at 14:04 GMT By Paul Rincon Science reporter, BBC News The Tevatron was, until the advent of the LHC, the highest-energy accelerator in the world Cross-checks on data that hinted at the discovery of a new sub-atomic particle have failed to find support for the observation.

In May, researchers working on the CDF experiment at the US Tevatron "atom smasher" announced they had detected tantalising hints of an unanticipated particle.

But independent checks using a separate experiment called DZero have not been able to corroborate the findings - dealing a blow to the idea.

A confirmation would have heralded one of the most radical changes to physics in years.

The DZero result comes several days after a CDF team member presented updated results showing the signal had strengthened - not disappeared - after analysing about double the amount of data.

Scientists from both Tevatron experiments will now have to "compare notes" with the aim of reaching a consensus.

The Tevatron is the only major competition to Europe's Large Hadron Collider machine. It is operated by the Fermi National Accelerator Laboratory (Fermilab), based in Batavia, Illinois.

Independent working

The independent check by DZero is revealed in a seminar at Fermilab on Friday. A paper describing the findings is set to be published on the Arxiv repository, and has been submitted to the Physical Review Letters journal for review.

Continue reading the main story Particle physics has an accepted definition for a "discovery": a five-sigma level of certaintyThe number of sigmas is a measure of how unlikely it is that an experimental result is simply down to chance rather than a real effectSimilarly, tossing a coin and getting a number of heads in a row may just be chance, rather than a sign of a "loaded" coinThe "three sigma" level represents about the same likelihood of tossing more than eight heads in a rowFive sigma, on the other hand, would correspond to tossing more than 20 in a rowA five-sigma result is highly unlikely to happen by chance, and thus an experimental result becomes an accepted discoveryProfessor Stefan Soldner-Rembold, spokesperson for the DZero collaboration, told BBC News: "We looked at the data-set that CDF originally published... We inject a signal in our simulation which looks like what we would have observed if CDF had seen the real thing.

"We analysed the data accordingly and, as observed, there is no enhancement. We can exclude something like what CDF observed to a relatively high probability."

Dr Giovanni Punzi, co-spokesperson for the CDF collaboration at Fermilab, told BBC News: "It has taken a step forward and a step back. But [DZero] has used only half of the data we currently have. So they are now showing a result with our old sample size."

When BBC News spoke to Dr Punzi, who is from the University of Pisa, Italy, he was responding to rumours that DZero had seen a "smaller excess" and had not yet seen DZero's results.

The CDF team was analysing data from collisions between protons and their anti-matter counterparts antiprotons. In these collisions, particles known as W bosons are produced, along with a pair of "jets" of other particles.

It was in these jets that the unexpected "bump" in the team's data came to light, potentially representing a particle that the widely accepted theory of particle physics - known as the Standard Model - does not anticipate.

As such, confirmation of the CDF results would have signalled a radical change in physics. But researchers stress the finding is definitely not the elusive Higgs boson - which explains why other particles have mass. The Higgs is the last missing "jigsaw piece" needed to complete the Standard Model.

DZero is a multi-purpose detector based at the Tevatron facility

When CDF's result was first announced, it was said to be at the "three sigma" level of certainty. This means there is roughly a 1 in 1,000 chance that the result is attributable to some statistical fluctuation in the data.

At a conference at Blois, France, on 30 May, Dr Punzi, announced that after analysing much more data, the "excess" was just below a five sigma level of certainty.

Five sigma means there is about a one-in-one-million chance that the "bump" is just a fluke and is the level generally required for a formal discovery.

Theorists have already weighed in on the CDF data peak. According to one school of thought, it could have provided support for a fifth fundamental force of nature known as "technicolour".

Technicolour is similar to the "strong force", which binds particles known as quarks together inside the nuclei of atoms. It could also give particles their mass - making the Higgs boson unnecessary.

Top stuff?

Other researchers suggested an effect called "top background" could explain away the "bump" seen by CDF.

They suggested that researchers might have underestimated the number of top quarks - a fundamental heavy particle - being produced at the Tevatron and that this could have yielded the data peak. But Dr Punzi said this idea had now been tested and ruled out.

Professor Soldner-Rembold, from the University of Manchester, UK, explained: "This is why it is good to have two experiments [at the Tevatron].

"What we see here is the scientific process at work. If one experiment sees something, another one has to verify it, and currently, we cannot verify it."

US Department of Energy budget cuts are forcing the Tevatron to shut in September this year; by the time the machine accelerates its last particles, it will have been operating for some 28 years.

The decision to close the facility was taken despite the recommendations of a scientific panel that the Tevatron's lifetime be extended by three years in order to continue hunting for the Higgs boson.

Paul.Rincon-INTERNET@bbc.co.uk

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