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Particles and interactions

Particles and interactions

Exotic meson challenges rules

06 Jul 2005

A heavy particle with an unusual decay pattern discovered by the Belle experiment at KEK in Japan is the latest addition to the meson family tree

The Belle detector

If we had to name a modern-day Mendeleev, his name would surely be Murray Gell-Mann. In the 1960s, faced with a bewildering array of particles called hadrons that had been turning up in high-energy experiments around the world, Gell-Mann proposed that the particles were combinations of a few fundamental entities called quarks. This idea brought order into the hadronic chaos, a feat for which Gell-Man was awarded the 1969 Nobel Prize for Physics.

Ever since the quark model was introduced, physicists have wanted to find out how the six different types of quarks – up (u), down (d), strange (s), charm (c), bottom (b) and top (t) – combine to form the hadrons we observe. The simple hadronic structures that we see are easy to define: mesons such as pions and kaons consist of a quark and an antiquark pair, while baryons such as protons and neutrons are made up of three quarks or three antiquarks.

But the theory that describes quarks, quantum chromodynamics (QCD), also permits particles containing four or more quarks. Indeed, a meson can be more generally defined as a hadron that has an integer value of intrinsic angular momentum in quantum units, while baryons have half-integer multiples of spin. Discovering such exotic hadrons, particularly mesons with more than the minimal quark-antiquark structure, would therefore provide crucial information for our understanding of the strong force. In fact, physicists thought they had glimpsed a five-quark state called a “pentaquark” in 2003. Sadly this excitement looks as if it was misplaced, since the latest results from dedicated experimental searches suggest that pentaquarks are a purely statistical phenomenon (see Physics World May p7: print edition only).

Charming discoveries

There have been some hints that nature might be hiding four-quark states under the guise of ordinary mesons, but until recently no compelling evidence for such particles had emerged. That situation changed dramatically two years ago when the BaBar experiment at the Stanford Linear Accelerator Center in the US reported an unusually long-lived excited meson called the DsJ(2317), where the number in brackets is the mass of the particle in MeV (see Physics World June 2003 p3: print edition only). Like atoms, mesons and baryons can exist in ground states and excited states, each of which can have different masses.

The DsJ particle consists of a charm quark – which makes it a type of D-meson – along with a strange antiquark (sbar), and revealed its presence via the way it decayed into lighter particles. These decays are analogous to Mendel’s rules of genetics: a parent carries a variety of attributes that show up differently in its offspring. A particle, on the other hand, can decay into a number of final states, which means that its properties – which are usually described in terms of quantum numbers such as spin and charge – can be inferred from its decay products.

The BaBar team found that the DsJ decayed into a ground state Ds-meson and a neutral π-meson. While this is a common decay mode for D-mesons, in the case of the DsJ the process took an extraordinarily long time because this particular decay does not conserve a quantity known as isotopic spin. What was also surprising about the new meson was that its mass was much lower than theory predicted – so low, in fact, that it could not decay into its preferred daughter products (a D0 and K+ meson) because the combined mass of these two particles is greater than that of the DsJ.

Within weeks, the CLEO experiment at Cornell had confirmed the BaBar discovery, and had also found a heavier partner called the DsJ(2460) that violated the same quantum rules when it decayed. Careful studies showed that both new mesons had the quantum numbers expected for the excited states of a cs-meson, but masses much lower than predicted. One explanation was that these particles might be four-quark states.

Now another unexpected meson called the Y(3940) has turned up, this time at the Belle detector at the KEK laboratory in Japan. The exciting feature of this new particle is that even though its mass is heavier than a D-meson and an excited D-meson (D*), it does not decay into these particles. Instead, in a sample of more than half a billion events the Belle researchers detected 100 examples in which it had decayed into a J / ψ-meson (ccbar) plus an ω-meson (ud).

The decay rate of the Y(3940) was found to be normal for a state of this mass, which tells us that its decay is not inhibited by any quantum selection rules. And the combination of its large mass and the appearance of a J / ψ-meson as a decay product strongly indicates that the Y-meson contains at least a ccw combination. The burning issue now is whether or not any additional quarks are needed to describe it.

The J / ψ and the ω are both spin-1 particles, which suggests that the Y(3940) is spin 0 (which is possible because spins add as vectors). However, if it is, then there must be some angular-momentum barrier that inhibits the decay into a D (spin 0) and a D* (spin 1). In other words, the Y-meson could simply be an excited ccbar state analogous to the excited states of the hydrogen atom.

But this idea flies in the face of three decades of research into the ccbar or “charmonium” system, which has well understood quantum numbers and decay modes. The Y(3940) just does not fit, and appears instead to decay as if it were a four-quark state containing two charm quarks, an idea originally proposed by David Horn of Caltech and Jeff Mandula of MIT back in 1978.

Four quarks or not four quarks?

To test whether the Y(3940) really is an exotic four-quark state, we need to turn to yet another recent meson discovery: the X(3872). This state was first seen at Belle towards the end of 2003 in the decay X → π + π + J / ψ, and was soon confirmed by the CDF collaboration at Fermilab (see Physics World December 2003 p3: print edition only). In contrast to the Y and DsJ mesons, the mass of the X(3872) is right at the threshold to produce a DD* pair.

This led Eric Swanson of the University of Pittsburgh to treat the X(3872) as a weakly bound DD* meson system – i.e. as a loosely bound system of four quarks. Last year, he predicted a series of other features of the decay that have subsequently been confirmed by some lovely experimental work carried out by the Belle group, and it now seems almost certain that the X(3872) is a four-quark system. Might the Y(3940) and the two DsJ systems also be four-quark states of this sort?

The short answer is “no”, because, like the DsJ states, the Y(3940) is tens of MeV lighter than the thresholds for nearby particle decays, compared with the X(3872), which sits right at threshold. Quantum mechanics tells us that related but slightly different forces will therefore act, which could push the DsJ and the Y masses to lower values than the relevant decay threshold. If coupled-channel interactions like these are indeed at work, then the DsJ and Y particles are effectively multiquark mixtures of other meson states.

In a completely different experiment, the SELEX collaboration at Fermilab has recently observed a long-lived cs-meson in interactions between high-mass baryons: the DsJ(2632), which decays into either a Dsη or a D0K. The first of these modes is highly unusual because decays involving h-mesons (which contain u and d quarks) are highly suppressed in conventional η-mesons. Moreover, the DsJ(2632) has not been seen at the Belle or BaBar “B-factories”.

These observations can also be explained by a four-quark model, such as that developed by Luciano Maiani and co-workers at University of Rome. This model makes some interesting predictions for other decay modes that are now being studied by the SELEX group, in the same way that the additional decay modes predicted by Swanson nailed down the character of the X(3872).

Decades ahead

The Y(3940) is an important find because its decay into a J / ψ and a ω is a clear statement that it cannot be a simple ccbar system. Physicists have long wondered why the strong force somehow saturates so that the observed particle spectrum is limited to two-quark mesons and three-quark baryons. This new particle, along with the other unusual D and X mesons that have been discovered, will therefore expand our understanding of meson structure.

The challenge now is to follow up the leads offered by the new mesons to build a consistent theoretical picture of what causes these more complicated states to form, and to find new experimental methods to clarify the systematics of these processes. The next 10 years should be an exciting period of discovery.

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