|
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. |
