LSU researchers see an indication of a new type of neutrino oscillation at the T2K experiment

T2K experiment has first results

BATON ROUGE – LSU Department of Physics Professors Thomas Kutter and Martin Tzanov, and Professor Emeritus William Metcalf, along with graduate and undergraduate students, have been working for several years on an experiment in Japan called T2K, or Tokai to Kamioka Long Baseline Neutrino Oscillation Experiment, which studies the most elusive of fundamental subatomic particles – the neutrino. The team announced they have an indication of a new type of neutrino transformation or oscillation from a muon neutrino to an electron neutrino.

In the T2K experiment in Japan, a beam of muon neutrinos – one of the three types of neutrinos, which also include the electron and tau – was produced in the Japan Proton
Accelerator Research Complex, or J-PARC, located in Tokai village, Ibaraki prefecture, on the east coast of Japan. The beam was aimed at the gigantic Super-Kamiokande underground detector in Kamioka, near the west coast of Japan, 295 km, or 185 miles away from Tokai. An analysis of the detected neutrino-induced events in the Super-Kamiokande detector indicated that a small number of muon neutrinos traveling from Tokai to Kamioka transformed themselves into electron neutrinos.

As part of the experiment, high energy protons were directed onto a carbon target, where their collisions produced charged particles called pions, which travelled through a helium-filled volume where they decayed to produce a beam of the elusive neutrinos. These neutrinos then flew about 200 meters through the earth to a sophisticated detector system capable of making detailed measurements of their energy, direction and type.
"It took the international collaboration about ten years to realize the project and bring it from first idea to first results," said Kutter, leader of the T2K project at LSU. "The entire LSU team is honored to be part of the collaboration and proud to contribute to the experiment. We expect many more results in the near future and look forward to the new research opportunities which are likely to arise from the tantalizing indication of this new neutrino oscillation."

LSU physicists have been part of a number of measurements over the last decade, which include Super Kamiokande, SNO, KamLAND that have shown that neutrinos possess the strange property of neutrino oscillations – one flavor of neutrino can transform into another as it travels through space. This is significant because neutrinos were first predicted theoretically in 1930, first actually detected in 1956 and for 50 years were assumed to have zero mass. But neutrino oscillations require mass.
With mysterious linkage between the three types, neutrinos challenge the understanding of the fundamental forces and basic constituents of matter. They may be related to the mystery of why there is more matter than anti-matter in the universe, and are the focus of intense study worldwide.

Precision measurements of neutrino oscillations can be made using artificial neutrino beams. This was pioneered in Japan by the K2K neutrino experiment in which neutrinos were produced at the KEK accelerator laboratory near Tokyo and were detected using the Super-Kamiokande neutrino detector, a 50,000 ton tank of ultra-pure water located more than half a mile underground in a laboratory 183 miles away near Toyama.
T2K is a more powerful and sophisticated version of the K2K experiment, with a more intense neutrino beam derived from the newly-built main ring synchrotron at the J-PARC accelerator laboratory. The beam was built by physicists from KEK in cooperation with other Japanese institutions and with assistance from American, Canadian, UK and French T2K institutes. The beam is aimed once again at Super-Kamiokande, which has been upgraded for this experiment with new electronics and software.

Before the neutrinos leave the J-PARC facility, their properties are determined by a sophisticated "near" detector, partly based on a huge magnet donated from the CERN accelerator laboratory in Geneva. The CERN magnet was earlier used for the UA1 experiment, which won the Nobel Prize for the discovery of the W and Z bosons which are the basis of neutrino interactions. The LSU team was responsible for building major components of the "near" detector, which provided an important ingredient to the oscillation analysis.
During the next several years, the search will be improved, with the hope that the three-mode oscillation will allow a comparison of the oscillations of neutrinos and anti-neutrinos, probing the asymmetry between matter and anti-matter in the universe.

Source  EurekaAlert!

Second team does not see Tevatron's mystery signal

Is particle physics, like beauty, in the eye of the beholder? You would be forgiven for thinking that now that two teams have analysed data from Fermilab's Tevatron collider and come to the exact opposite conclusion about whether that data hints at a new particle.

A task force is being formed to figure out the discrepancy, but the final arbiter may be the Large Hadron Collider in Switzerland, which will ultimately collect more data than the Tevatron.

In April, members of the Tevatron's CDF experiment reported finding a curious signal in the debris from eight years' worth of collisions between protons and antiprotons. The signal hinted at the existence of a particle that was not predicted by the standard model, the leading theory of particle physics. Theorists scrambled to come up with possible explanations, writing dozens of papers on the topic in the following weeks.

Last week, evidence for the signal, or "bump" in the data, seemed to get even stronger. The CDF team reported that it had analysed twice as much data as in April and had still found the bump.

But now, a rival team performing an independent analysis of Tevatron data has turned up no sign of the bump. It is using the same amount of data as CDF reported in April, but this data was collected at a different detector at the collider called DZero.

"Nope, nothing here – sorry," says Dmitri Denisov, a spokesman for DZero.

Different detectors

When the CDF collaboration came out with its result in April, DZero researchers spent a couple of days doing a quick check of their data and saw no bump. But to make sure they were comparing like with like, they spent the next two months painstakingly checking that their analysis resembled that of CDF's as closely as possible.

Today, they are reporting that their analysis shows no bump. "We're basically excluding a signal at well over the 95 per cent confidence level," Denisov told New Scientist. The result shows "good agreement with the standard model".

"Now of course the most interesting question is where are these differences coming from?" he says.

The fact that the detector is different should not come into play, he argues. "It would be really puzzling if it's a physical process that's supposed to exist in nature, and one experiment sees it and another doesn't," he says. "Protons and antiprotons don't know what detector they're colliding in."

Modelling issue?

He does not know what is causing the discrepancy but suspects it may be due to differences in the models that each team uses to describe the data. The studies look at how often collisions between protons and antiprotons produce a W boson, which transmits the weak nuclear force, and a pair of jets of subatomic particles called quarks.

The CDF team found an unexpected abundance of these events – a W boson and a pair of jets – where the mass of the jet pair was about 145 gigaelectronvolts, suggesting that a new particle of that same mass was created (see graph). But other events involving different combinations of particles can mimic the signal of a W boson and a pair of jets. If the CDF team incorrectly modelled the number of those "background" signals, it might make it look as if there were a bump in the data, says Denisov.

Rob Roser, a CDF spokesman, acknowledges that "it could be a modelling issue", but says it is too soon to discount CDF's result. "It's disappointing that the peak didn't just jump out at them too," he told New Scientist. "But just because it didn't doesn't mean there's not something there."

Task force

Roser has not yet had time to look carefully at DZero's paper (pdf), which has been submitted to Physical Review Letters. But he says a cursory look suggests that the discrepancies between the two results may not be as large as it seems. The CDF team estimated that the potential new particle is produced at a certain rate in proton-antiproton collisions, but there is some uncertainty in that rate. If the real rate is at the lower end of CDF's range of possibilities, for example, the discrepancy between the two teams' results is smaller.

"I think we have more work ahead of us before we understand what is going on," says Roser.

A task force, made up of members of both experiments, along with Fermilab theorists Estia Eichten and Keith Ellis, will now try to get to the bottom of the discrepancy, a process that could take months, Roser says.

Roser says the CDF team has only analysed 70 per cent of the data it expects to collect by September and will continue crunching the numbers to see if the bump appears in the full data set. But Denisov says DZero is satisfied with the analysis it has already done. "We do not plan to continue the analysis with the same rigour as we did over the last two months," he says. "We are concentrating on many other things."

Even if the task force's investigation proves inconclusive, both Denisov and Roser say the Large Hadron Collider at CERN will eventually collect enough data to settle the question of whether there is a bump at 145 GeV.

They disagree over when this will happen, though. The LHC collides particles together at higher energies than the Tevatron, which produces more debris in which a new particle could be lurking. This leads Denisov to estimate that the LHC will collect enough data to look for the first evidence of the bump in the next few months.

Roser points out that the LHC collides protons together rather than protons and antiprotons, as happens at the Tevatron. That means that even though it produces some collisions between quarks and antiquarks – which could be important in reproducing the bump – it generates them in smaller numbers than the Tevatron. So Roser estimates it could take the LHC more than a year to make its ruling.

So are the two teams going to be arguing bitterly until then? "It's been a little bit tense over the last few days," laughs Denisov. "But I think it's friendly. I would say it's like two sports teams competing."

"I think it's going to be fun," says Roser. "This is science in progress."

Source New Scientist

Stick Up: Antimatter Atoms Trapped for More Than 15 Minutes

CERN physicists have forced flighty atoms of antihydrogen to stick around, potentially affording a better look at how antimatter behaves.

This artist's conception depicts the confined path of an antimatter atom inside the ALPHA trap in light blue. The white tracks depict outflying particles that originate from a matter-antimatter annihilation when the antiatom is released from its trap.

Maybe antimatter is finally ready for its close-up. A team of physicists has succeeded in producing rudimentary atoms of antimatter and holding on to them for several minutes, an advance that holds hope for detailed comparisons of how ordinary atoms of matter compare with their exotic antimatter counterparts.

The researchers, from the ALPHA antimatter experiment at CERN, the European laboratory for particle physics, reported last year the first trapping of antihydrogen, the simplest antimatter atom. But the antihydrogen had at that time been confined for less than two tenths of a second. That interval has now been extended by a factor of more than 5,000. In a study published online June 5 in Nature Physics, the ALPHA group reports having confined antihydrogen for 16 minutes and 40 seconds. The more relevant number for physicists, who often deal in powers of 10, is 1,000 seconds. (Scientific American is part of Nature Publishing Group.)

The subatomic particles of everyday matter—protons, neutrons and electrons—have antimatter cousins; when matter meets antimatter the two annihilate in a burst of energy. And just as the neutral hydrogen atom is made of a single proton bound to an electron, an atom of antihydrogen comprises an antiproton and a positron, the antimatter counterparts, respectively.

But the mutual annihilation between those particles and their ubiquitous matter counterparts makes it challenging to hang on to antimatter for very long, and even more challenging to produce and confine bound atomic arrangements of multiple antiparticles. Neutral anti-atoms such as antihydrogen are especially tricky to confine because they are impervious to electric fields, which can be used to steer charged antiparticles such as antiprotons. Experiments such as ALPHA instead use superconducting magnets to trap their quarry.

Cagey as anti-atoms are, physicists would like to pin them down and compare the properties of antihydrogen with hydrogen, the most abundant element in the universe. Those comparisons might involve laser spectroscopy of the anti-atoms or physical tests of how antihydrogen "feels" the influence of gravity. Any discrepancies between hydrogen and antihydrogen might help explain why matter won out over antimatter in our observable universe. "If one could do that, that would be a huge advance in terms of understanding why we live in a world of matter," says Clifford Surko, a physicist at the University of California, San Diego, who wrote a commentary for Nature Physics accompanying the new study. "There's got to be an asymmetry somewhere, so that's a long-term goal."

The lifetime of antihydrogen in the ALPHA trap is probably sufficient to begin those studies. "We think we're in a position to start measuring something," says ALPHA spokesperson Jeffrey Hangst of Aarhus University in Denmark. Initial studies will involve irradiating the anti-atoms with microwaves to try to engage them in a resonant interaction, flipping their spin like a compass needle swinging from north to south.

Critically, the confinement times achieved by ALPHA imply that the antihydrogen atoms have had time to decay into their lowest-energy, or ground, state. "This method of antihydrogen formation creates them in highly excited states," Surko says. "They're fragile, and for really high-precision measurements of antihydrogen you need them in the ground state."

Hangst says that the jump from trapping times measured in milliseconds to those measured in hundreds of seconds did not stem from any one major advance. But getting antihydrogen atoms to stick in the trap much more often was a big help in improving on last year's confinement experiment. It now takes fewer experimental runs to demonstrate that an antihydrogen atom has been trapped. "What was tricky here was not keeping them but trapping enough of them to do the experiment," he says. "The big technological step here is we're much better now at trapping them at all."

Still the efficiency of trapping is somewhat low—for each antiatom confined by the trap, thousands more from the same batch escape. And in 16 trapping experiments of 1,000 seconds each, only seven antihydrogen atoms were detected in total. (The researchers demonstrate the confinement of antihydrogen by quickly shutting down the superconducting magnets, turning the anti-atoms loose, and watching for matter–antimatter annihilations on the walls of the trap.)

A competing antimatter experiment at CERN, known as ATRAP, has been working toward producing larger numbers of antihydrogen atoms with lower kinetic energies, which would facilitate their trapping. But so far that effort has yet to bear fruit. "We think that it would be good to have more atoms than [the ALPHA rate of] fewer than one atom per trial," says Harvard University physicist Gerald Gabrielse, spokesperson for the ATRAP collaboration. "We would hope not to be publishing a paper that says we see 0.6 atom per trial, but 100 atoms per trial."

The ATRAP group, Gabrielse says, made the choice to increase the number of atoms in the trap rather than increasing the sensitivity of the instruments to detect small numbers of anti-atoms, as ALPHA has done. "Maybe we made the wrong one," he says of that decision. "Certainly we made the one that got less publicity." Nevertheless, Gabrielse says he is encouraged by his competitors' success. "I'm glad that they have demonstrated that you can trap antihydrogen atoms," he says. "I think it shows that if we have more atoms, we'll have time to do some things with them."

Source Scientific American