Fermilab zooms in on the Higgs boson

August 4, 2008 | 9:01 am

Mass range for Higgs excluded by Fermilab search

Scientists working on the CDF and DZero experiments at the Fermi National Accelerator Laboratory are entering Higgs territory. On Sunday, the two groups reported at a conference in Pennsylvania that for the first time their results directly restrict the allowed mass range for the elusive Higgs boson. The results show that the CDF and DZero experiments are sensitive to Higgs signals that may show up as the two collaborations gather and analyze more data.

“These results mean that the Tevatron experiments are very much in the game for finding the Higgs,” said Fermilab Director Pier Oddone. Fermilab issued a press release with the details of these results.

“We have been working toward this exclusion for many years,” said DZero cospokesperson Darien Wood, of Northeastern University. “Of course, our goal is to find the Higgs boson, not just restrict its mass.”

The Standard Model of particles and forces–the theoretical framework for particle physics–predicts the existence of a particle, the Higgs boson, that interacts with other particles of matter–such as electrons and quarks–to give them mass. The mechanism by which particles acquire different mass values is unknown, and finding evidence for the existence of the Higgs boson would address this fundamental mystery of nature.

Sensitivity of the Fermilab experiments to the Higgs boson at various masses

The CDF and DZero collaborations obtained their result by analyzing hundred trillions of proton-antiproton collisions produced by the Tevatron particle collider. While the current data sample did not reveal the Higgs boson, both groups expect to double their data sets in the next couple of years, improving their chance to observe the particle. According to CDF cospokesperson Rob Roser, Fermilab, the two experiments are close to ruling out a Higgs particle with a mass of 165 GeV/c² and 175 GeV/c². (At present, the two experiments look for the Higgs particle in steps of 5 GeV/c².) This graph (right) shows the current sensitivity of the two experiments to the Higgs boson for various mass values. (Because of the way that the Higgs particle is expected to interact with other subatomic particles, it is easier for the Tevatron experiments to exclude–or to find–a Higgs particle with a mass of 160 GeV/c² than, say, 120 or 155 GeV/c².)

The previous constraints on the Higgs mass stem from direct searches at CERN’s Large Electron-Positron (LEP) collider, which operated from 1989-2000, and indirect constraints produced by the LEP and Tevatron experiments. Quantum effects produce indirect constraints. They yield a mathematical relation among the values of the masses of the W boson, the top quark and the Higgs boson. Accordingly, precise measurements of the masses of the W boson and the top quark constrain the Higgs boson mass. Such measurements indicate that the Higgs boson should be lighter than about 200 GeV/c². Direct searches by the LEP experiments, which analyzed electron-positron collisions, have shown that the Higgs boson is heavier than 114 GeV/c².

In 2009, the Large Hadron Collider at CERN will begin its hunt for the Higgs boson. The LHC will produce particle collisions with seven times the energy of the Tevatron collider.

Kurt Riesselmann

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Coming soon: First coffee-table book by a high-energy physics experiment

August 1, 2008 | 5:30 am

Exploring the Mystery of Matter started as an internal photodocumentary of the ATLAS experiment but is now in the hands of a London publisher. Heavy with illustrations and photographs, it may be a handy tool for high-energy physicists, says coordinator, creative director, and photographer Claudia Marcelloni: “If you’re a particle physicist, and you’re at a party and someone says, ‘So what do you do?’ you show them this book and that explains it.”

ATLAS is an enormous detector experiment at the Large Hadron Collider, scheduled to start operating this fall on the Swiss-French border. Among other things, it will examine dark matter candidates and search for the elusive Higgs particle. The 168-page book does include a lot of facts (a proton in the LHC will make 11,245 circumnavigations of the collider ring per second!) and provides a plethora of illustrations and photographs (one chapter titled “Tunnel Vision” has some extraordinary shots of the tunnels being excavated and pieces of the accelerator being put together).  But what makes it a coffee-table book, rather than an information resource or a textbook, is that “the book is not about teaching the science,” Marcelloni told me. “It’s meant to educate people about the collaborators–these brilliant minds that are doing amazing experiments in a very creative fashion.”

ATLAS hired Marcelloni to document the construction of the experiment and the 2200 physicists from 37 nations collaborating on it. Kerry-Jane Lowery, a writer from Geneva, provided the text. The book was meant for the scientists to take home and perhaps share with friends. But a year into the project, Marcelloni and her team realized it might also appeal to the general public. “We started showing the completed chapters to our friends, and the [ATLAS] collaborators were showing it to their friends, and people were getting interested and asking questions and saying, ‘We want to see it when it’s done.’ I think in general physics is not a very popular topic. People can’t relate to it. But I know people can relate to photography; they can relate to images.”

At first, she says, the collaborators were a little confused.  ”Physicists aren’t used to being part of something like this. Here were two girls, who were not a part of the collaboration, running around saying things like ‘So how does it feel to be a part of this?’ They were surprised,” she recalls, laughing. But soon the scientists were thrilled, she says: “We got so much help from the collaboration; we got at least two or three people to read every piece of text and then I got ten or more people to look at the pictures.”

Offering the book to the general public has led Marcelloni and her team of writers, designers, three physicists, and an engineer to strike a new balance. “We still want the collaborators to enjoy it, but we don’t want it to go over the heads of the public,” she says.

Because ATLAS is an international collaboration the book is being printed in English. It will be available on October 4 at CERN, and via the publisher’s Web site sometime before the end of the year.

Calla Cofield

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