Particle Detectives Announce First Results

by Kurt Riesselmann

Finding a particular person in a sold-out stadium can be a daunting task. You scan thousands and thousands of faces,many looking similar from far away.

You might be looking for someone with dark hair – but are you sure your friend isn’t wearing a hat today? If you remember the color of your friend’s favorite jacket, your task may be easier. But what if he wears a different coat for a change? Or even worse, have you considered the possibility that he hasn’t yet arrived at the stadium?

Particle physicists face pretty much the same dilemma as they look for their favorite particle friends, such as the top quark and the Higgs boson.They’ve studied their characteristic appearances and know what these friends “look ” like, but don’t know when a particular particle will appear and which jacket — decay mode, in expert’s terminology — it will favor at that moment.

The only chance to spot a rare particle friend is to frequently create large crowds of particles and take pictures of them. Scientists can analyze such pictures and identify unique signatures — the particle faces, in the crowd.

That’s exactly what scientists at Fermilab have been doing since the beginning of Collider Run II in March 2001. Having tuned up their two “cameras", CDF and DZero, systems of particle-detecting devices that are as big as three-story houses,they have begun to take more than one million pictures of particle crowds per second. Although scientists have stored only the most promising pictures, about 100 per second, they are filling up their electronic storage space at a rate of one CD-Rom every few minutes.

“To me, it’s like the work of a detective,” said Italian INFN physicist Patrizia Azzi, who leads the top quark physics group of the CDF collaboration at Fermilab, together with Pierre Savard from University of Toronto.“It ’s never the same.”

TOP IN AMSTERDAM
In the last six months, CDF and DZero scientists have achieved the first findings of their detective work. At the end of July, they announced the results of their analyses of the first Run II data. And the results are promising.

“At the ICHEP physics conference in Amsterdam, our collaboration showed the first top quark candidates of Run II,”said Azzi, who worked as a graduate student on Run I.“Run I saw the discovery of the top quark in 1995. For Run II,we are now in the position of doing precision top quark physics. Our goal is to measure the top quark mass with a precision of about two percent."

More than 800 particle physicists from across the world attended the International Conference on High Energy Physics. About 20 CDF and DZero scientists had the opportunity to give talks on a variety of physics topics, highlighting the progress that Run II presents compared to Run I.

“About eight years ago,we embarked upon a very aggressive program for the detector upgrades,” said Kevin Pitts, assistant professor at the University of Illinois at Urbana-Champaign. “It seems it will pay off in a big way. We will measure physics processes that we didn’t have access to before.”

In Run I,which took place from 1992 to 1996,the CDF and DZero collaborations each sifted through more than 100 inverse picobarns (pb of data,a measure for the total number of collisions.Since the beginning of Run II, each collaboration has analyzed 20 pb^-1 of data. Scientists expect to exceed the Run I mark in spring of next year,with much more data to come over the next five years.

To create top quarks and other particles, Fermilab scientists create powerful collisions of protons and antiprotons at the center of the CDF and DZero detectors. Each collision produces hundreds of particles in new combinations and geometries, which the detectors record.

Looking for a specific particle created in these collisions is challenging. Scientists expect to spot a particle like the top quark, which is 175 times heavier than a proton, only in a small number of “pictures.” During Run I, scientists observed a few hundred collisions that produced top quarks. For Run II, however, they expect to find thousands.

ZOOMING IN ON THE HIGGS
At the Amsterdam conference, both CDF and DZero collaborations also announced their first Run II results on processes involving W and Z particles, the carriers of the weak force. “We presented our first measurements of the W and Z cross sections at the new collision energy of 2 TeV,” said Fermilab physicist John Womersley, cospokesperson of the DZero collaboration.“Run II produces collisions with ten percent more energy than Run I, the highest collision energy in the world. The W and Z results are important as they are the first step in understanding Higgs and top quark events.”

Although the current number of Run II “pictures ”is still smaller than the sample accumulated during Run I, the quality of the data is much better.

“Because of the higher energy, we have a thirty-five percent higher production rate for the top,” said physicist Boaz Klima, who coordinates the physics analyses of the DZero collaboration.“In addition, our upgraded detectors are better than during Run I. Hence every recorded picobarn of Run II data is much more valuable than the same amount of Run I data. By spring 2003, we expect to have the first results for top cross section and top mass measurement of Run II.”

Improving the experimental knowledge of the masses of the W particle and the top quark will help scientists zoom in on a particle that nobody has ever seen:the Higgs boson. Named after a theoretical model proposed by Scottish professor Peter Higgs and others, the Higgs particle is the missing ingredient in an otherwise successful model of particles and their interactions.Over several years, the Fermilab mass measurements will slowly narrow the window on the Higgs.If the particle is not too heavy,the Fermilab accelerators will be able to produce it.

“To directly see the Higgs, it will take time and data,” said Klima.“You are not going to jump from nothing directly to discovery. There is still a lot of work ahead of us.”

LOOKING FOR RARE B’S
Another hot research topic is the physics of the bottom quark, a particle that weighs much less than the top quark. Scientists have built B factories in California and Japan, but the heavier types of B mesons, such as those consisting of a bottom quark and a strange quark, will only be produced in large numbers at Fermilab.

“The world expects us to measure B_s oscillations,” said Pitts, who worked on a new detection subsystem for bottom quarks.“Our findings will be complementary to results from the B factories.”

During Run II,the Fermilab accelerators will routinely produce tens of millions of B mesons per day. In addition to identifying B_s particles in this large crowd, scientists expect to catch an occasional glimpse of bottom quarks wearing “jackets” never seen before.

“We know that rare decay modes of the B mesons should exist,” said Pitts.“The question is: how do you isolate them?”

Pitts and Marjorie Shapiro, professor at UC Berkeley, head the B physics group of the CDF collaboration. They coordinate the work of about one hundred scientists that scrutinize the CDF data for information on bottom quarks.

“Our upgraded detector is a brand-new tool,” Pitts said about the CDF experiment.“We now have new ways of finding things. In addition to finding B mesons, we get a lot of charm quark states. In a few weeks of running we’ve got 50,000 decays of a D₀ meson into a kaon and a pion. We’ve also reconstructed D⁺ and D_s⁺ charm states in quantities that have already far surpassed the Run I samples.”

GODPARENTS MAKE THE CALL
Measuring the mass difference between the two D states, particles that contain a charm quark and a lighter partner, might be the subject of the first CDF publication from Run II. Even with a relatively small data sample, the quality of the mass measurement is already comparable to the best results obtained at other labs in the past. The B physics group has written a draft of an article, which is currently being reviewed by an internal board of CDF experts called the godparents. If the godparents approve, the paper will go for review by all collaboration members. The submission of the article to a scientific journal may happen even before the end of the year.

But the new Run II data may hold more treasures. Scientists are searching for phenomena in particle signatures and interactions that cannot be explained by the standard particle theory. From extra dimensions to leptoquarks to super- symmetric partners, physicists are prepared for the unexpected. At Amsterdam, scientists reported on their search strategies.

“Traditionally, scientists begin to look for new particles by identifying collisions that have produced a particle with large momentum perpendicular to the direction of the incoming proton beam,” explained Fermilab physicist Gustaaf Brooijmans, who heads the new phenomena group at DZero together with Greg Landsberg of Brown University. “It’s a rather clean signal.

“To look for leptoquarks, for example, the rule of thumb is: Every time you double the number of collisions in your analysis, you gain 20 GeV in your exclusion limits. Improving our analysis,this winter we could get a limit of 200 GeV — very close to the Run I results.”

Next year, the particle detectives at CDF and DZero will be staking out new territory. Whether they will find any exotic signals remains to be seen.

If they do, they will have pictures to prove it.

On the Web:
CDF Homepage:
www-cdf.fnal.gov
DZero Homepage:
www-d0.fnal.gov
Live collisions and more:
www.fnal.gov/pub/now