Three new subatomic particles have been found, and all survive for an unusually long time before they decay. Physicists now face the challenge of explaining this within the framework of the existing theory.
“It is as if Cleopatra had fallen from her barge in BC and had not yet hit the water.” Such was the description half a century ago of the discovery of the astonishingly long lifetime (up to about 10−8 seconds) of strange particles. Everyday matter, such as protons and neutrons, is made of two types of quark, known as 'up' and 'down' (Fig. 1). But in 1947, new particles were discovered that contained a third type of quark, called 'strange'1. Today, strange particles are a well-established part of the standard model of particle physics, which now includes six types of quark. We know that their seemingly long lifetimes are a consequence of the 'weak' interaction that they undergo in decaying: if instead they were subject to the powerful 'strong' interaction their lives would be over in around 10−23 seconds. Instead, death by decay is neutered by the presence of strangeness.
In the past two months, three different particles have been discovered, and explaining them has proved a challenge for theorists. Although not as extreme as the above example, each of these new particles has an unusually elongated lifetime. Two of them are 'mesons', each containing a strange anti-quark and a charm quark (the fourth quark type)2,3. The reason for their metastability is understood, but their detailed nature and dynamics remain to be resolved. The third particle4 is a member of the 'baryon' family of particles that also includes the proton and neutron. But, unlike the proton and neutron, this particle has some strange-quark content. In fact, unlike any other baryon known, it has overall one unit of 'positive strangeness'. It is an enigma. The quark model that now underpins the standard model was developed, in part, under the assumption that such things do not exist. And although it may be possible to interpret this particle as a combination of four quarks (two up, two down) and a strange anti-quark (providing that unit of positive strangeness), the challenge is to explain also why this 'pentaquark' does not fall apart more quickly.
Viewed at high resolution (through high-energy particle collisions), mesons and baryons appear to be swarms of quarks, anti-quarks and gluons — the quantum bundles that glue these constituents to one another, according to the theory of quantum chromodynamics (QCD). At lower resolution, the picture is simpler. The mesons and baryons form two distinct classes: mesons consist of a single quark and anti-quark; baryons seem to be formed from just three quarks. In addition, QCD seems to allow more complicated clusters of quarks or anti-quarks — atomic nuclei are familiar examples of quarks bound in multiples of three. An open question is whether there are analogues containing anti-quarks. The simplest would be two quarks balanced by two anti-quarks, in effect a 'molecule' of two conventional mesons, or three quarks accompanied by an additional quark and an anti-quark, making a pentaquark.
Unambiguous evidence for such states in the data is lacking. Their absence is attributed to the ease with which they would fall apart into a pair of conventional mesons, or a meson and a baryon. It is estimated that they would survive for less than 10−24 seconds, which is at the current limit of detection. But the sightings of the three metastable particles, reported by the BaBar2 and CLEO3 experiments in the United States and the SPring-8 experiment4 in Japan, may at last be proof of these states' existence.
The baryon is utterly novel. In 60 years of studying strange particles, no such combination of electrical charge and strangeness (one positive unit of each) with baryon nature has been seen. The original sighting in Japan has been corroborated by two other experiments5,6, but all of the detections are at levels that are currently on the borderline of significance, limited by the amount of data available. The simplest response is to wish it away. But within the next year a high-statistics experiment is planned to establish whether it really exists and, if so, to measure its properties (such as its spin). If it is real, then it may be most naturally explained as a pentaquark, containing a strange anti-quark. Its metastability would require that it is one of a family of particles related through a property called 'isospin'. Because isospin must be conserved (in the same way that, for example, energy and momentum must be conserved when billiard balls collide), the number of ways in which these particles can decay is restricted, and so they cannot decay quickly. If this picture is correct, it implies the existence of more of these baryons with unusual correlations of charge and strangeness, and they could be searched for in moderately high-energy experiments.
By contrast, there is no doubt about the existence of the two meson states, both known as 'DS'. They appear as clear peaks in the data and their spins almost certainly have the values zero and one (thus they are referred to as 'scalar' and 'axial' mesons, respectively). They have all the characteristics of states made from a charm quark and a strange anti-quark, but, for some reason, have masses that are lower than expected — so much lower, in fact, that their natural decay paths (into a charm meson and a strange meson) are energetically closed. This is the cause of their metastability. But the question of why they are so much lighter than their siblings in the charm–strange family is still to be resolved.
One possibility is that they are better described as 'molecules' — bound states of mesons, one containing a charm quark and the other a strange quark, with energies slightly below the fall-apart threshold. This is analogous to a proton and a neutron binding together to form a deuteron, and such behaviour has been seen elsewhere for scalar (spin zero) mesons. The masses of the newly discovered mesons are tantalizingly close to the thresholds for some two-meson states (in the case of the scalar meson, a molecule of a K and a D meson; and in the case of the axial meson, of a K and a D* meson). It seems certain that these meson combinations play some role in lowering the observed masses of these new-found particles.
Further ways of producing these enigmatic states, along with more precise measurement of their properties, are now being pursued to identify the sources of their unexpected longevity.
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Advances in High Energy Physics (2015)
Publications of the Astronomical Society of the Pacific (2004)