Its relatively low mass and narrow width has been predicted by Dmitri
Diakonov,
Victor Petrov and Maxim Polyakov in 1997 - see the abstract
of the DPP paper
From the point of view of the chiral
quark soliton model of baryons it was natural to get
a relatively light pentaquark baryon resonance and, what was even more
important,
a very narrow one. This could explain why it was not observed before
despite 30 years
of intensive searches.
In February 2000 during a lunch at a conference in Adelaide, Australia,
Diakonov convinced
Takashi Nakano, the spokesman of the LEPS collaboration at
SPring-8,
Osaka, Japan,
that it is worthwhile to search for this particle in reactions induced
by high-energy
gamma quanta - see the story in New
York Times.
In October 2002 Takashi Nakano reported on the first evidence of the
new baryon
at the PANIC 2002 conference in Osaka. See the abstract
of the Nakano et al. paper .
Independently of the search at SPring-8, Anatoly Dolgolenko et al. at
ITEP, Moscow,
looked into the K+Xe bubble chamber data taken at ITEP back in 1986.
They learned
about the DPP prediction
from a talk given by Polyakov. Rescanning old films took more
than two years, and in December 2002 the group reported on the observation
of a very narrow
Theta-plus. Its mass 1539(2) MeV was compatible with the LEPS result
of 1540(10) MeV,
and the width of the Theta-plus resonance was found to be less than
9 MeV, also compatible
with less than 25 MeV of LEPS -- see the abstract
of the ITEP paper.
That's how the name Theta-plus came into being (April 12, 2003).
Following the PANIC conference, Ken Hicks (Ohio U., USA) organized an
analysis
group at the CLAS collaboration working in Hall B at Jefferson Lab,
Virginia, to search
for the new baryon. In May 2003 Valery Kubarovsky and Stepan Stepanyan
reported
for the CLAS collaboration on the observation of Theta-plus both in
gamma-proton and
in gamma-deutron reactions - see the abstract
of CIPANP 03 talk and also the
abstract of the CLAS
paper.
At this point the world media realized that something dramatic has happened
in
subatomic physics. See the articles that appeared in the beginning
of July 2003 in
USA Today,
Los
Angeles Times (pdf), Le
Figaro, The
Economist and popular science
journals like New
Scientist, Nature,
Physics
World (pdf) , Physics
Today and a few
other which can be found on the homepage of Ken
Hicks, a member of both LEPS
and CLAS collaborations.
In the end of July 2003 a new confirmation came from the SAPHIR collaboration
at ELSA,
Germany -- see the abstract
of SAPHIR paper . All experiments performed that far were
consistent with one another and gave the mass of the pentaquark baryon
with strangeness=+1
at around 1540 MeV and narrow width, in remarkable agreement with the
theoretical prediction.
Here is the list of experiments
where pentaquarks have been observed and also
the list of those where they were not.
Meanwhile, theoretical papers started to appear dealing with various
physical issues related
to Theta-plus, in particular with its quark structure. The rapidly
growing list of references
can be found here.
There is also an ongoing discussion of various aspects of the DPP prediction .
Last updated: 07.05.04
