May 6, 2005  


Evidence Mounts Against Pentaquarks

By Heather Rock Woods

They’re exotic. So exotic they may not exist.

Pentaquarks are mysterious particles—whose existence is still in question—composed of four quarks and an antiquark. Quarks usually pair up to make mesons (like the B particles PEP-II produces) or flock together in triplets to make baryons like protons. Exotic baryons refer to pentaquarks, whose quantum numbers cannot be reduced down to those of regular baryons.

A meson (shown top) is composed of a quark and an antiquark, a regular baryon (center) consists of three quarks, and a pentaquark (bottom) is proposed to have four quarks and an antiquark.
(Image courtesy of Pat Burchat)

Two years ago, the first claim to see a pentaquark set the high-energy and nuclear physics communities abuzz. Twelve experiments then looked for pentaquarks in various ways and claimed to see them. To date, another 17 experiments have searched but seen nothing, including BABAR, and a Jefferson Lab experiment that announced in April it had found no signs of pentaquarks.

“It’s raising very serious issues of existence for this thing,” said Bill Dunwoodie (EB). “It teaches you again how important it is to do these experiments with very high statistics and very good mass resolution.”

BABAR has both—very large samples (high statistics) of expected mesons and baryons and excellent mass resolution—with the ability to resolve signals from particles with small mass width. BABAR’s negative results, using different search methods, have two orders of magnitude more statistics than experiments using comparable methods that claimed to observe a pentaquark signal.

“We have enough resolution to see a particle with a narrow mass range, the narrow width claimed for pentaquarks. We see nothing, lots of nothing,” said Valerie Halyo (EE), co-convener of BABAR’s Pentaquark Task Force.

As unconfirmed particles go, pentaquarks fall into a nebulous realm, unlike the top quark, for example, whose existence and mass were clearly predicted before discovery.

“There’s no reason for them not to exist. Theories can accommodate them, but theories don’t predict them,” Halyo said.

As the BABARians see it, there are shortcomings to the pro-pentaquark experiments, the biggest being small sample sizes (low statistics) and uncertain understanding of the background (events not of interest but in the same mass region). The smaller the sample, the easier it is to mistake a statistical fluctuation for a signal, or to accidentally include or exclude background events that affect the strength of the signal.

Experiments making positive claims produced only dozens of supposed pentaquarks, while BABAR produced millions of ‘control particles’ (which help calibrate an experiment) with a similar mass but decaying into a regular baryon and a meson.

Another concern is that the experiments were not conducted blind. Long a standard in medical trials, blind experiments in particle physics do not reveal the answers until researchers have finished their analysis. This helps prevent consciously or unconsciously fitting data to the expected or desired solution.

The recent flow of pentaquark papers illustrates that negative search results do not frequently get reported in scientific literature. Only after the first published claims generated interest did some experiments publish old data finding no pentaquark signals.

“Sometimes negative results are the most important; they can refute claims or constrain theories,” said Pat Burchat (BABAR), co-convener of BABAR’s Pentaquark Task Force.

BABAR searched for almost all members of the controversial pentaquark family. In a paper submitted recently to Physical Review Letters, the collaboration showed the lack of any pentaquarks in a sample of 500 million events, and said with a 95 percent confidence level that the production of pentaquarks is suppressed by a factor of four to eight relative to that of regular baryons—if they exist, they would be produced at rates an order of magnitude lower than for regular baryons of similar mass.

BABAR also used their data in an unusual way to take advantage of possible pentaquark production through electro-production. The collaboration selected the events produced when deflected electrons and positrons crash into the walls of the beam pipe (instead of each other). This technique is similar to performing a fixed target experiment, like the HERMES experiment in Germany that claimed to see pentaquarks, except the BABAR experiment yielded more data. At the recent winter conferences, BABAR presented its electro-production results.

“In effect, we did an experiment similar to theirs 200 times and put all the data together. We see there’s no structure (no pentaquark signal),” said Dunwoodie.

The recently announced results from Jefferson Lab’s high-statistics CLAS experiment directly refute SAPHIR, a pentaquark-claiming experiment with two orders of magnitude fewer statistics.

Dunwoodie, whose term on Jefferson Lab’s physics advisory committee just ended, said the CLAS collaboration and lab management have made extensive detector calibration efforts and have emphasized the importance of producing large data samples which are very well understood.

Two earlier low-statistics CLAS experiments, investigating different production mechanisms, claimed to observe pentaquarks. One of those has now gathered 10 times more statistics, and plans to announce the new findings this summer. The other proposes to collect an order of magnitude more statistics next year.

“We’re anxiously awaiting those CLAS results,” said Burchat. “If they still see the pentaquark, and they used techniques to minimize experimenters’ bias, then we have a problem theoretically: why are they produced in some experiments and not in others? If this is confirmed, not only will pentaquarks be exotic in internal quantum numbers, but exotic in dynamics (how they are produced) in ways that will take a lot of explaining.”

In the meantime, Halyo has enjoyed the search at BABAR. “We are a fun detector. We can do B physics and more,” she said.  


The Stanford Linear Accelerator Center is managed by Stanford University for the US Department of Energy

Last update Friday May 06, 2005 by Topher White