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Physics Today - Search and Discovery September 2003
http://www.physicstoday.org/pt/vol-56/iss-9/p19.html
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Four Experiments Give Evidence of an Exotic
Baryon With Five Quarks
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It’s been a long-standing puzzle that the quantum numbers of all the
known mesons and baryons could be attributed to bound states of two
or three quarks. But now the first exception has apparently been
found.
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Convincing evidence has been accumulating in recent
months for the existence of a quite new kind of elementary
particle--a so-called exotic hadron. Four groups of
experimenters have now reported the observation of a
baryon with strangeness S = +1 produced at accelerators of
modest energy.1-4 What’s so exotic about a baryon with
positive strangeness, and why is it exciting great interest
among particle physicists?
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Before this year, every one of the hundreds of
well-established baryons and mesons (collectively called
hadrons) could be described either as a bound triplet of
quarks (the baryons) or a bound quark-antiquark pair (the
mesons). Any exception would be labeled an exotic.
The new baryon, which has been named +, is being
hailed as the first manifestly exotic hadron. Its charge is +1,
and its mass, about 1540 MeV, roughly 60% more than the
proton’s, is quite ordinary for a baryon. But its observed
+
strong decay to a neutron plus a K meson marks it as
something radically new. A three-quark baryon couldn’t
possibly decay that way.
Also this month
Four Experiments Give
Evidence of an Exotic
Baryon with Five
Quarks
Composite Molecules
Store Rewritable Digital
Data
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Uncover Solar Flare
Surprises
Quarks come in six flavors. In increasing mass, the quarks
are called: up (u), down (d), strange (s), charmed (c),
bottom (b), and top (t). Except for the ultramassive t quark,
which doesn’t live long enough to form hadrons, each one
resides in dozens of well-attested mesons and baryons.
Because three quarks make a baryon, we say that each
quark has baryon number B = + 1/3. For antiquarks, B = 1/3. The strong interactions never change a quark’s flavor,
and net B is strictly conserved in all known physical
processes.
What makes the + exotic is the fact that the s quark has
strangeness S = -1, that is, the same strangeness as the Kmeson. There are many baryons with negative strangeness
that couple to the K . But to get S = +1, the strangeness of
+
the K , one needs the antistrange quark s-, and the s- has
negative baryon number. So the fewest quarks that could
constitute an S = +1 baryon is five. For charge +1, this
"pentaquark" configuration would be uudds-. (The electric
charge of the u is +2/3; the d and s quarks have charge -1/3,
and antiquarks have the opposite charges.)
The absence of exotic hadrons has been something of a
conundrum ever since Murray Gell-Mann and George
Zweig introduced the idea of quarks as the building blocks
of the hadrons in 1964. Quantum chromodynamics (QCD),
the standard theory of the strong interactions, does not
obviously forbid qqqqq- baryons or qqq-q- mesons. Indeed,
scattering experiments that probe nucleon structure have
long given indirect evidence of the role of extra qq- pairs in
hadron dynamics. But it was not clear why extra quarks
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never seemed to show up as manifestly exotic combinations
of quantum numbers.
Extracting detailed predictions from QCD is notoriously
difficult. That’s because, unlike the standard theory of the
electromagnetic and weak interactions, QCD has, in
general, no small dimensionless parameter that permits
convergent perturbation expansions. The strong interaction
is simply too strong. So theorists trying to predict the
spectrum of baryon and meson states have to make do with
a patchwork of approximation schemes whose ranges of
validity are not always clear. The discovery of a pattern of
exotic hadrons would do much to elucidate the strong
interactions.
A seminal prediction
One particular prediction, published in 1997 by theorists
Dmitri Diakonov, Victor Petrov, and Maxim Polyakov at
the Petersburg Nuclear Physics Institute in Russia,
instigated the current excitement.5 Their work was based on
an effective field theory first proposed in 1962 by Tony
Skyrme at Birmingham University in England. Skyrme
pointed out that a semiclassical theory of the pion field,
without explicit nucleons, surprisingly yielded a
particle-like soliton wave that emulated the nucleon and
other low-mass baryons with impressive fidelity. With the
advent of quarks two years later, the "skyrmion" model lay
largely dormant until 1983, when Edward Witten showed
that its efficacy for low-energy strong interactions could be
understood in terms of QCD.
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In the wake
of Witten’s
benediction,
many
theorists,
Diakonov and
Petrov among
them,
revisited the
skyrmion
model and
soon
extended it to
Figure 1
incorporate
the
approximate three-flavor symmetry of the three lightest
quarks: u, d, and s. It was soon recognized that the
three-flavor skyrmion model predicted the existence of an
"antidecuplet" of presumably undiscovered baryon species
that all have the same spin (1/2) and intrinsic parity (+) as
the nucleon (see figure 1). This predicted multiplet of 10
different charge and strangeness states is called an
antidecuplet because, in contrast to the decuplet whose
prediction by Gell-Mann in 1962 led to the historic
discovery of the
baryon (with S = - 3) two years later,
the antidecuplet has its lightest, rather than its heaviest,
member at the apex.
Not only the + at the apex of the predicted antidecuplet,
but also the heavier baryon states at the two bottom
vertices, would have to be manifestly exotic: Their
eccentric combinations of charge and strangeness could not
be attributed to three-quark configurations. Why didn’t
experimenters rush off to look for these three exotic
baryons? In the absence of detailed calculations of mass and
width, it was generally assumed that they were probably too
massive and certainly too wide to be easily detected.
What’s a wide hadron? Unlike the most familiar baryons
and mesons, which cannot decay via strong interactions, the
great majority of hadrons live for only about 10 -23 s.
Therefore their masses have intrinsic widths of tens, or even
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hundreds of MeV. Those widths manifest themselves in the
resonant peaks in invariant-mass distributions of the
hadron’s decay products or of the colliding particles that
form it. (The invariant mass of a system is its energy in its
own center-of-mass frame.) Very wide hadronic states can
be hard to distinguish from nonresonant backgrounds.
Diakonov and company predicted a mass of about 1530
MeV for the + , based largely on the symmetry properties
of the three-flavor skyrmion model and on the bold
conjecture that the nonexotic charge-doublet member of the
antidecuplet in figure 1 was the already known N(1710), a
spin-1/2 baryon. (The number in parentheses is the mass in
+
MeV.) The predicted
mass was not a great surprise.
Earlier predictions had ranged from 1500 to 1700 MeV.
What was quite new, however, was the prediction that the
+ would have a width of less than 15 MeV, which
promised experimenters an attractively clear signal. To
calculate that width, the St. Petersburg group needed much
more than just the symmetry arguments that sufficed for the
mass estimate. To that end, they invoked a detailed
dynamical realization of the skyrmion idea which they call
the chiral quark soliton model.
"Finding a baryon of such modest mass and width," recalls
Diakonov, now at the Nordita institute in Copenhagen,
"seemed so easy that we didn’t understand how it could
have been overlooked--if it really exists. So we were afraid
to publish until we’d thoroughly searched the old
experimental literature."
It turns out, however, that the low mass and narrow width
were actually problematic in the 1960s and 70s, when
experimenters were most actively searching for
positive-strangeness baryons. A + of mass 1530 MeV
would show up as a resonant peak in the energy dependence
+
of the cross section for K mesons scattering off neutrons,
+
at a K beam momentum of about 440 MeV. But that’s an
inconveniently low momentum. It’s not relativistic enough
for Lorentz time dilation to significantly lengthen the
kaon’s 10-nanosecond lifetime, and kaon beams of
well-defined low momentum were hard to extract from
proton accelerators. And the available beams had, by
today’s standards, painfully meager fluxes.
"By the summer of 1962 it seemed clear, to our chagrin,
that our K + interactions, quite unlike Luis Alvarez’s K data, were producing no strange-baryon resonances," recalls
Gerson Goldhaber (University of California, Berkeley).
"We hadn’t expected that discrepancy, but it became an
essential clue leading to the very useful three-quark model
of the baryons."
Found at last?
The first paper reporting evidence for something that
+
looked very much like the predicted
came in January
from Takashi Nakano and coworkers at the SPring-8
synchrotron radiation facility near Osaka, Japan. 1
Ultraviolet laser light backscattered by the synchroton’s
8-GeV electron beam produces intense beams of GeV
photons whose energies can be individually "tagged" by
recording the scattering of the electrons that made them.
Diakonov had urged Nakano to search for photoproduction
of the + at SPring-8. So Nakano’s group fired a tagged
photon beam at a hydrocarbon target and looked for the
reaction.
n
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+ -
K Kn
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from photons hitting neutrons in the carbon nuclei. The
momenta and identities of the kaons were determined by
time-of-flight counters and tracking chambers in a magnetic
field. The scattered neutrons were not seen, but their
momenta could be deduced from the known photon and
kaon momenta. Events with visible recoil protons were
vetoed to eliminate collisions off protons.
A telltale narrow spike in the invariant-mass distribution of
the K+ and the final-state neutron implies that some object
+
of well-defined mass decayed to K n. And that’s what the
SPring-8 group found for the experiment’s final selected
sample of 109 K+K - n events. The peak, centered at 1.54 ±
0.01 GeV, rose 4.6 standard deviations ( ) above the
nonresonant background. The invariant masses were
subjected to a rough correction for the nonzero momenta of
the target neutrons inside the nuclei. Given the experiment’s
resolution, the SPring-8 group could only say that the
peak’s intrinsic width was less than 25 MeV.
If this is indeed the predicted +, it must be an isospin
singlet. That is, it must appear only in the charge state +1.
That seems to be confirmed by the absence of a 1.54 GeV
+
peak in the K p invariant-mass distribution for events
created in an alternative liquid-hydrogen target.
So provocative a result, overturning four decades of
conventional wisdom, cries out for confirmation. The next
reported sighting of the + , appearing on the Web in April,
was almost archaeological.2 On dusty shelves at the
Institute of Theoretical and Experimental Physics (ITEP) in
Moscow lay long-abandoned bubble chamber film from a
1986 experiment in which Anatoli Dolgolenko and
colleagues had exposed a liquid-xenon bubble chamber to
+
an 850-MeV-momentum K beam from ITEP’s small
proton synchrotron.
The bubble chamber is long gone. But in 1999, having
learned of the + prediction, Dolgolenko and a handful of
intrepid colleagues undertook to rescan the old film for
+
0
events in which the K produced a K (that decayed to
+ -) together with a proton and an unseen recoil
nucleus. Protons and pions were identified by the bubble
densities of their tracks, and their momenta were
determined from stopping ranges in the xenon.
All this took three years, there being no money to hire
scanners. In the final sample of 541 events, the K 0p mass
peak, centered at 1539 ± 2 MeV, rose 4.4
above
background. Its measured width yielded an upper limit of
only 9 MeV on the intrinsic width of the resonance, a
tribute to the excellent resolution of old-fashioned bubble
chambers.
+
The unusually narrow width of the
is an issue that
theorists are grappling with. And it’s getting more difficult
every day. In August, Richard Arndt and colleagues at
George Washington University reported a reanalysis6 of the
world’s low-energy K +-nucleon scattering data, mostly
from the 1960s and 70s. From the fact that these old data
+
show no clear evidence of the
, they conclude that its
intrinsic width cannot be larger than about 1 MeV!
The most statistically significant + peak thus far was
reported in July by a group at the Thomas Jefferson
National Accelerator Laboratory’s CLAS large-acceptance
spectrometer facility. 3 Like the SPring-8 group, the CLAS
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collaboration used a high-flux beam of tagged GeV photons
produced at the laboratory’s electron accelerator. The
photon beam hit a deuterium target, and the group looked
for the reaction
d
K +K -n(p).
The parentheses mean that the deuteron’s proton was
merely a spectator in the photoproduction of the kaon pair
off the neutron.
Typically the spectator’s recoil momentum would be too
low to be detected. The CLAS group used an interesting
trick to eliminate the blurring due to the target neutron’s
momentum inside the deuteron. They selected only that
small fraction of events in which the recoil proton was
kicked up to a detectable momentum, presumably by
final-state collision with the K . For such events, one has
enough information to determine the unseen neutron’s
momentum without having to assume it was initially at rest.
The 5.3- peak
in the resulting
K +n mass
distribution (see
figure 2) is
centered at 1542
± 5 MeV. Its
21-MeV width
is consistent
with the
experiment’s
resolution.
CLAS
spokesman
Kenneth Hicks
(Ohio
University,
Figure 2
Athens) says
that the group finds no evidence for a doubly charged peak
in the K+ p mass distribution from its older hydrogen-target
data. A high-statistics deuterium-target run scheduled for
early next year will continue the search for a possible
isospin partner, and it will address the crucial question of
+
whether the observed
does indeed have the predicted
spin 1/2 and positive parity.
At the end of July, a group at Bonn University’s ELSA
accelerator, an electron-beam facility similar to Jefferson
Lab, piled on with yet another confirmation of the
1540-MeV particle.4 This time the evidence is a 4.8peak in the K+ n mass distribution from the reaction
p
+
0
nK K- .
But they found no corresponding doubly charged peak in
the pK+ mass distribution.
What is it?
Theoretical responses to the experimental reports and
phenomenological reanalyses of ancient data are sprouting
on the preprint server like mushrooms after an autumn rain.
Among other things, theorists are trying to understand the
narrow width of the exotic baryon--now generally called
+(1540)--in alternatives to Diakonov’s Skyrme-inspired
chiral quark soliton model. In any model of five quarks
moving independently in an effective mean potential, the
pentaquark state would fall apart so fast that it would be at
least 100 MeV wide. If its parity were negative, as such
models suggest, we would already have seen lots of other
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negative-parity exotic baryons lighter than the
+ (1540).
A recent paper by MIT theorists Robert Jaffe and Frank
Wilczek, for example, addresses these issues by proposing
+
that the
(1540) is a bound state of two highly correlated
ud diquark pairs plus an s-. 7 That would account for its
positive parity and perhaps its very narrow width.
A specific prediction of the diquark model, which might be
testable in the next round of searches, clearly distinguishes
it from Diakonov’s chiral soliton model. Jaffe and Wilczek
predict that the exotic
baryons at the bottom corners of
the antidecuplet in figure 1 should be 300 MeV lighter, and
therefore more accessible, than what Diakonov and
company predict.
"Whatever the explanation of this first exotic turns out to
be," says CERN theorist John Ellis, "it’s very exciting. It
looks like the beginning of a whole new hadron
spectroscopy. That could become a key testing-ground for
rival quark and skyrmion views of baryon structure."
Bertram Schwarzschild
References
1. T. Nakano et al., Phys. Rev. Lett. 91, 012002
(2003) [SPIN].
2. V. Barmin et al. (DIANA collaboration),
http://arXiv.org/abs/hep-ex/0304040; Yad. Fiz. (in press).
3. S. Stepanyan et al. (CLAS collaboration),
http://arXiv.org/abs/hep-ex/0307018.
4. J. Barth et al. (SAPHIR collaboration),
http://arXiv.org/abs/hep-ex/0307083; Nucl. Phys. B (2003)
(in press).
5. D. Diakonov, V. Petrov, M. Polyakov, Z. Phys. A 359,
305 (1997) [INSPEC].
6. R. Arndt, I. Strakovsky, R. Workman,
http://arXiv.org/abs/nucl-th/0308012. See also S. Nussinov,
http://arXiv.org/abs/hep-ph/0307357.
7. R. Jaffe, F. Wilczek,
http://arXiv.org/abs/hep-ph/0307341.
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