Discoveries of New Charm Particles at B Factory
 
Researchers of the Belle international collaboration, working at KEK's B factory, have been experiencing pleasant surprises as new charm particles show their appearances one after another for the past few years. Charm particle is a collective name for mesons (made of a quark and an antiquark) and baryons (made of three quarks) with at least one of the constituent quarks being the charm quark. Since hundreds of mesons and baryons have been known to exist, just another addition itself is not so exciting. What stirs curiosity of the Belle researchers, however, is an exciting possibility that two of their findings might be examples of new types of particle that do not quite fit into the standard classification of mesons and baryons in the context of the quark model that has been enjoying an overwhelming success in classifying all known particles and fitting them into predicted slots in its chart of mesons and baryons up to now.
 
The Belle group reported discoveries of (2365) in 2002 (number in parenthesis indicates the particle mass in unit of MeV/ ), (3872) in 2003, (2308) and (2427) in 2004, (2800), (3940), and (3940) in 2005, and (3930) in 2006.
 
The (2308) and (2427) are charm mesons made of a charm quark and a light antiquark (up quark and down quark are called light quark because their masses are the lightest among the quarks).
 
The (2365) and (3930) are mesons consisting of a charm quark and a charm antiquark. They are also called "charmonium"and have zero electric charge. The (2800) is a baryon made of a charm quark, a strange quark, and a light quark. All these particles fit nicely into predicted but vacant slots in the chart of mesons and baryons. The (2800)s were also found to come with three different electric charges (++, +, 0) and decay into and meson of (+, 0, -) charge as the quark model had predicted.
 
The (3872), on the other hand, did not fit into any of the slot in the chart and became known in the world-wide particle physics community as the "mystery meson". After exhaustive and detailed investigations of its properties, it is now thought that the (3872) is probably not a quark-antiquark state but, rather, a new type of meson made of two quarks and two antiquarks. Existence of such exotic states has been postulated based on the quantum chromodynamics (QCD), fundamental theory of quarks and gluons. However, exact nature has been mostly unknown.
 
Last summer the Belle group reported discoveries of two particles with very similar masses. The (3940) was discovered as a peak in the mass spectrum of particles recoiling against the / in electronpositron collisions (see Fig.1). The / is the first-observed charmonium, and the 1976 Nobel Prise in Physics were awarded to Samuel Ting and Burton Richter for its discovery. The recoil mass system must have properties of charmonium since it is produced with another charmonium / from the electronpositron system which behaves like vacuum but with just energy. Examination about what particles are in the recoil mass system revealed that the (3940) mainly decays into a pair of charm meson () and charm antimeson () as expected for charmonium states with this mass, according to the quark model.
 
The (3940) was found in the process of meson decaying into (3940) and meson. The (3940) then decays into a lighter meson and a / (see Fig.2). A sample of 274 million B meson decays produced only 58 (3940) particles. Although the (3940) must have properties as charmonium, it does not seem to follow the pattern of the (3940), and its preference to decay into an and a / is difficult to understand in the context of the quark model. This is why the Belle researchers think they are different particles inspite of their very similar masses.

In fact, what has been exciting the researchers is that the observed decay property of the (3940) resembles that expected for a "hybrid meson": a conjectured particle to be made of a quark, an antiquark, and a gluon. The existence of such states was predicted in 1978 based on the QCD, but none has been seen so far. Predicted mass for a hybrid meson ranges between 4000 and 4500 MeV/ , a little too large for the observed mass of the (3940). Presently available sample of 58 (3940) is too few for detailed investigations. They are eager to accumulate much more data and settle if this particle is a hybrid meson or something else.
 
The B factory, whose main mission has been to produce a large sample of B meson decays for CP violation study, is providing very exciting results in the charm meson sector. Two of the newly discovered particles do not seem to be compatible with the quark model. They may be the first examples of new types of mesons that are not in the quark model but conjectured in the QCD. Detailed studies of their properties, and yet more new findings, may help to significantly advance our understangding of the QCD.
 

Professor Kazuo Abe has been a KEK staff since 1984. For the past ten years, his main research activity has been experimental exploration of the origin of CP violation that is a breakdown of matter-antimatter symmetry and is considered to have played key role for the formation of present universe.