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PARTICLE PHYSICS: ON THE NON-EXISTENCE OF PENTAQUARKS

The following points are made by Frank Close (Nature 2005 435:287):

1) Correct perceptions differ from mistaken ones in that they become clearer when experimental accuracy is improved -- Irving Langmuir's observation may have gained a new example with the latest report on the question of the "pentaquark" particle. A high-statistics experiment at the Jefferson Laboratory in Virginia finds no evidence to support claims of the existence of this enigmatic object that have been made over the past three years. Final tests of the data remain to be completed, and a second independent experiment is still in progress. But this is a serious setback for the many who had hoped that this novel particle had revealed unexpected phenomena in quantum chromodynamics (QCD), the theory governing the "strong" interactions of quarks -- subatomic particles thought to be elemental and indivisible.

2) The story began in 1997 with the prediction[1] that an analogue of the proton should exist with a mass of "about 1530 MeV" (some 50% more massive than the proton) and with both positive electric charge and a positive value for another fundamental quantum number, strangeness. Such a correlation was not possible within the simplest quark model, where most strongly interacting particles (known as hadrons) are either mesons, which contain a quark and an antiquark, or baryons, which comprise three quarks. To make such a correlation of charge and strangeness requires a particle consisting of four quarks and one antiquark (hence dubbed a "pentaquark"). Such combinations are allowed by QCD but are expected to be highly unstable, with "widths" of many hundreds of MeV. (An inherent property of quantum mechanics is that short lifetimes are correlated with large uncertainties in energy. Thus the mass, or energy at rest, of a short-lived particle is actually a distribution with an intrinsic width --for strongly interacting unstable particles, such widths typically exceed 100 MeV.) No pentaquark had ever been seen with certainty, and their absence had been one of the planks upon which the standard quark model had been developed.

3) A surprising feature of the 1997 prediction was that the particle would have a width of the order of a few MeV and not the hundreds that might have been expected. Initially, the paper[1] received little attention, but the LEPS collaboration at the SPring-8 laboratory in Japan was encouraged to mount an experiment to look for the particle. The first pentaquark sighting was announced by them[2] in early 2003, exactly at the predicted mass and with a narrow width. The experiment involved photon beams interacting with protons or neutrons in a carbon nucleus. There was some surprise that the first sighting of a particle with such a narrow width should have occurred in such a complex environment: the nuclear constituents are bound and have kinetic energy, which tends to smear any signals. However, this stimulated experimentalists elsewhere to look again at their data from earlier experiments to see if these contained evidence for the pentaquark.

4) Within a few months, teams from the Jefferson Lab, from Russia and from the SAPHIR collaboration at the Electron Stretcher Accelerator (ELSA) in Bonn, Germany, all announced that they, too, had spotted tantalizing hints of the particle in data taken in other experiments. For instance, the SAPHIR team's evidence of the pentaquark[3] came from data they had obtained in 1997-98 and confirmed its mass of 1540 MeV. None of these experiments on their own was very significant, but the broad agreement among them created huge excitement.

5) In 2004 a series of theoretical criticisms emerged, centered around some anomalies. On closer inspection there seemed to be small but systematic differences in the mass, and in the width of the signals[4]; also, it was unclear how such a narrow width state was apparently produced so readily. Moreover, reports of negative experimental searches began to appear. The null results tended to come from experiments using nuclei, or hadrons such as mesons or protons, rather than photons, and also included searches involving very-high-energy electron beams. Unlike some of the supposedly positive sightings, the common feature of the null results was that they tended to have rather large statistical samples.

6) Researchers at the Jefferson Lab are currently undertaking dedicated hunts for the pentaquark, including an experiment that repeats their original pentaquark search with much higher statistics. Those data are being analyzed, and the results are expected later this year. If they show a null result, the pentaquark story will probably have come to an end for physicists but will live on as a case-history for historians and philosophers of science.

References:

1. Diakonov, D. et al. preprint at http://www.arxiv.org/hep-ph/9703373 (1997).

2. Nakano, T. et al. (LEPS collaboration) Phys. Rev Lett. 91, 012002 (2003)

3. Barth, J. et al. (SAPHIR collaboration) preprint at http://www.arxiv.org/hep-ex/0307083 (2003)

4. Zhao, Q. & Close, F. E. J. Phys. G 31, L1-L5 (2005)

Nature http://www.nature.com/nature

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PARTICLE PHYSICS: ON THE TOP QUARK

The following points are made by Georg Weiglein (Nature 2004 429:613):

1) The basic building-blocks of matter, as far as we know, are quarks and leptons, together with the force-carrying particles that mediate their interactions. Quarks and leptons (the latter group including the electron) are grouped in three generations; the particles in the second and third generations seem a perfect copy of those of the first generation, except that their masses are much larger. The top quark is the heaviest of all quarks and leptons, and is central to some of the most pressing questions in particle physics. For instance, why is the third-generation top quark more than 300,000 times heavier than the first-generation electron? Why are there two other quarks with precisely the same properties as the top quark but with very different masses? And what is the origin of mass itself?

2) Precise knowledge of the mass of the top quark and its interactions is a key ingredient in testing theory against experimental data. Recent work(1) reports an improved measurement of the top-quark mass, using data taken at the Tevatron proton antiproton collider at Fermilab, near Chicago. Combining this with previous measurements, the new world average(2) for the mass of the top quark is 178.0 +- 4.3 GeV/c^(2), where c is the speed of light (the mass of the proton expressed in these units is about 1 GeV/c^(2)). Compared with the previous world average(3), the central value of the mass has shifted upwards by about 4 GeV/c^(2). The experimental error has been reduced by about 15%, sharpening our view of the underlying physics.

3) The role of the top quark in disentangling the fundamental principles of nature is twofold. On the one hand, its large mass makes the top quark a prime target in the search for new physics that might so far be unaccounted for. For instance, the long-hypothesized Higgs boson, which is the last missing ingredient of the standard model of particle physics, is predicted to interact with other particles with a strength that is proportional to their masses. So the physics of the heavy top quark would be significantly influenced by its interaction with the Higgs boson. On the other hand, the mass of the top quark is a key parameter in the predictions for many observable quantities. Small deviations between measurement and prediction could be a signal of new physics, so the uncertainty in the predictions that arises from the experimental error on the top-quark mass limits the sensitivity of experiment to new physics.

4) The values of several precisely measured quantities, as predicted by the standard model, depend on the square of the top-quark mass, Mt; their dependence is much weaker on the as yet unknown mass of the Higgs boson (so far, experiment has excluded any mass value below 114.4 GeV/c^(2)) (4). Therefore, in using a so-called global fit of the model predictions to all available data, an improved knowledge of Mt better constrains the likely value of the Higgs-boson mass. In fact, the 4 GeV/c^(2) shift in the central value of Mt has shifted the upper limit on the Higgs-boson mass by more than 30 GeV/c^(2) , to 251 GeV/c^(2) (at 95% confidence level)(1). This upper limit has an important impact on the experimental strategies used to search for the Higgs boson at present and future colliders.(5)

References (abridged):

1. The DOE Collaboration Nature 429, 638 642 (2004)

2. The CDF Collaboration, the DOE Collaboration and the Tevatron Electroweak Working Group. Preprint at http://arxiv.org/abs/hep-ex/0404010 (2004).

3. Hagiwara, K. et al Phys. Rev. D 66, 010001, 271 433 (2002)

4. The ALEPH, DELPHI, L3 and OPAL Collaborations and the LEP Working Group for Higgs Boson Searches Phys. Lett. B 565, 61 75 (2003)

5. Heinemeyer, S., Hollik, W. & Weiglein, G. 124, 76 89 (2000)

Nature http://www.nature.com/nature

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ON HADRONS, LEPTONS, AND QUARKS

The following points are made by James Trefil (citation below):

1) By the 1950s, scientists studying the collisions of protons with nuclei began to realize that when a nucleus is torn apart, all sorts of strange and wonderful things can be found in the debris. There were, of course, the protons and neutrons you would expect to find, but in addition there was a whole collection of new particles. These particles seemed to live inside the nucleus, but when freed from this environment they decayed -- came apart --in very short times.

2) The instability of these particles explains why we were not previously aware of them -- they can be seen only briefly and under very special conditions such as those that exist in the laboratory. It quickly became evident, in fact, that there were two classes of particles in nature. There were particles like the proton, the neutron, and the stuff that was seen in the debris of nuclear collisions. These particles seemed to be at home in the nucleus and to contribute in some way to holding it together. The other class of particles were those like the electron --particles that are normally found not inside the nucleus but outside of it. The former class of particles was christened "hadrons" (after the Greek for "strongly interacting ones"). The latter particles were called "leptons", or weakly interacting ones.

3) It turned out that going into the nucleus after the hadrons had opened a real Pandora's box. Throughout the 1960s and into the 1970s the number of hadrons being discovered skyrocketed. The last time I looked there were over 200, but no one is counting anymore. It's clear that a system of the world with 200 kinds of "elementary" particles just isn't going to work. Some way of ordering the hadrons had to be found. In the late 1960s such a scheme was proposed, and since then this scheme has come to dominate the study of the basic structure of matter. The scheme is actually very similar to the simple proton-neutron-electron picture of the 1930s. The idea is that just as all nuclei are merely different arrangements of the hadrons, all hadrons are themselves simply different arrangements of things more fundamental still, things for which the name "quarks" was invented. The name actually comes from a line in James Joyce's /Finnegan's Wake/ that goes "Three quarks for Muster Mark", and it reflects the fact that in its earliest incarnation the quark model had three different kinds of quarks in it.

Adapted from: James Trefil: "The New Physics and the Universe" in: Byron Preis (ed.): The Universe. Bantam Books, New York 1987.

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