Fermilab still in race for Higgs boson
Today is the first day of the Users' meeting at Fermi National Accelerator Laboratory, the largest US national laboratory for particle physics. Several hundred of the 2300 physicists who are using Fermilab's infrastructure and accelerator complex to conduct experiments have gathered here at the laboratory to hear about the latest news in particle physics and to discuss plans for the future of the field. Fermilab provides a live video stream of this meeting.
"We have a packed agenda for the next two days," said Kevin Pitts, chairman of the Fermilab Users Executive Committee, at the beginning of the meeting. The day started out with talks about the Fermilab accelerator complex, which provides the beams for collider experiments as well as neutrino experiments.
Ron Moore, Fermilab Accelerator Division, reported (PDF) that the performance of the Tevatron proton-antiproton collider reached new heights, increasing the opportunity for experimenters to make discoveries. The month of May provided more collisions than ever before, yielding 0.2 inverse femtobarns (the scientific unit used to count the number of collisions). The total number of collisions delivered to the two collider experiments, CDF and DZero, now exceeds 4 inverse femtobarns.
"We are steadily climbing. Things are going very well," said Moore.
Just yesterday, Moore said, the Fermilab antiproton source set a new record for the number of antiprotons produced in one hour, delivering more than 27x1010 in that hour. Last year, the average was around 20x1010 per hr. This year, it is about 25x1010 per hr. This bodes well for the future of the Tevatron program. The more collisions the Tevatron produces, the greater the chance that a significant number of collisions will produce the sought-after Higgs particle.
"We will have a one-week shutdown for maintenance some time this year. Otherwise it is just run, run, run," said Moore.
The 4 inverse femtobarns delivered to each collider experiment, CDF and DZero, so far is about half the projected total by the end of September 2009.
Oscar Gonzalez, of the Spanish research institute CIEMAT, reported (PDF) on some of the most important results obtained by the CDF and DZero experiments in the last year. Of interest for Higgs hunters is the world's most precise, single measurement of the mass of the W boson. Reducing the uncertainty in the size of the W mass reduces the range for the predicted mass of the Higgs boson, the elusive particle that is the key to understanding the origin of the mass of elementary particles. "We [physicists] have a lot of confidence that the Higgs will be there," said Gonzalez. Particle physics measurements made by many experiments indirectly point to the existence of the Higgs boson, confirming the Standard Model of particles and interactions. Measuring the W boson mass with great precision is another test of the Standard Model.
The W boson weighs, in subatomic units, 80,413 MeV/c2, with an uncertainty of 48 MeV/c2, said Gonzalez. To appreciate this achievement, imagine this particle is a person and has a weight of close to 80 kilograms. Then scientists are able to tell you the mass of the particle plus/minus 48 grams. I don't know my weight with that precision!
The CDF and DZero experiments have used only a fraction of the data collected to obtain this result. "Preliminary studies suggest that [a total error of] 25 MeV/c2 is achievable with the current data sample we have," said Gonzalez.
Because physicists know the value of the W boson and the top quark so well, the indirect upper limit for the mass of the Higgs boson is now at 160 GeV/c2 at a 95 percent confidence level. But finding the Higgs is not easy.
Said Gonzalez, "The strategy of the search for the Higgs at the Tevatron has to deal with the small cross section," or probability that a proton-antiproton collision at the Tevatron will produce the Higgs. The search for proton-antiproton collisions that produce a pair of W bosons, with one W boson decaying into a Higgs boson, is currently the best chance to find a Higgs boson. The Tevatron collider experiments are approaching the level of sensitivity needed to exclude a Higgs boson with a mass of 160 GeV/c2. "We expect to have news very soon," said Gonzalez, pointing at the upcoming ICHEP 2008 conference this summer in Philadelphia.
(Because of the way that the Higgs boson interacts with other particles, it is easier for Tevatron experimenters to exclude a Higgs particle with 160 GeV/c2 than, say, a Higgs boson with 120 or 140 GeV/c2.)
In the next couple of years, the CDF and DZero experiments will improve their reach and begin to exclude other mass values as well-or see the first hints of the Higgs particle. Because of searches at the European laboratory CERN (using the LEP collider) scientists know that the Higgs must be heavier than 114 GeV/c2.
According to Gonzalez, the success of the search for the Higgs boson at Fermilab does not depend just on collecting more collision data. He pointed out that the exclusion limit is improving much faster because of improvements in data analyses and new search strategies. "We expect another factor of two in two years," he said.
The numerous physics results churned out by the CDF and DZero collaborations are available on the Web (CDF here, DZero here). Links to many of the talks presented at the Fermilab Users' meeting are online as well.
See all reports from the Fermilab Users' Meeting 2008 here.