Four Experiments Give Evidence of an Exotic Baryon With Five Quarks

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.

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?

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 Θ+, 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.

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 Θ+ exotic is the fact that the s quark has strangeness S = -1, that is, the same strangeness as the K- meson. 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 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.

Figure 1

In the wake of Witten's benediction, many theorists, Diakonov and Petrov among them, revisited the skyrmion model and soon extended it to 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 Ω- baryon (with S = - 3) two years later, the antidecuplet has its lightest, rather than its heaviest, member at the apex.

Not only the Θ+ 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 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 Θ+, 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 Θ+ 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 Θ+ 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 Θ+ 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 Θ+ 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 Θ+ at SPring-8. So Nakano's group fired a tagged photon beam at a hydrocarbon target and looked for the reaction.

γn → K+K-n

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 (σ) 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 Θ+, 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 Θ+, 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 Θ+ prediction, Dolgolenko and a handful of intrepid colleagues undertook to rescan the old film for events in which the K+ produced a K0 (that decayed to π+π-) 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 K0p mass peak, centered at 1539 ± 2 MeV, rose 4.4 σ 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 Θ+ 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 Θ+, they conclude that its intrinsic width cannot be larger than about 1 MeV!

The most statistically significant Θ+ 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 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

γd → 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.

Figure 2

The 5.3-σ 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, 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 Θ+ 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 4.8-σ peak in the K+n mass distribution from the reaction

γp → nK+K-0.

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 Θ+(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 negative-parity exotic baryons lighter than the Θ+(1540).

A recent paper by MIT theorists Robert Jaffe and Frank Wilczek, for example, addresses these issues by proposing that the Θ+(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 Ξ 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."