BaBar’s new particle shows unexpected traits

31 May 2003

The BaBar experiment at SLAC has revealed a new narrow particle state that has been identified as a charmed meson, that is, a charm quark, c, bound with an antiquark, in this case a strange antiquark, sbar. It joins a select club of four such states, only two members of which are well established. While additional members were expected, the new particle does not seem to fit in quite as predicted.

A member of the BaBar team, Antimo Palano of Bari University and INFN, first found a bump at about 2320 MeV after combining π0 and D+s in an analysis of 91 fb-1 of data collected at the PEP-II asymmetric electron-positron collider at energies around 10.6 GeV. The D+s is the ground state of the csbar system and therefore might be expected to figure in the decays of excited csbar states. However, both the π0 and the D+s decay quickly, and by using the decay vertex information are reconstructed from their decay products. The BaBar analysis team looked for the D+s by combining charged particles corresponding to K+Kπ+, while photons were paired together to yield potential π0s.

After applying various selection criteria, to home in on the desired D+s and π0 particles, the team calculated the mass for the D+sπ0 pairs to obtain a mass distribution. A clear peak appears both for events where D+s → K+Kπ+, and also for events where an additional π0 appears to come from the same decay vertex, corresponding to the decay D+s → K++π0. The peak occurs at a mass close to 2.32 GeV/c2, and its width is narrow, consistent with the resolution of the BaBar detector.

The charm quark is much heavier than the strange quark, so the spectroscopy of the csbar system can be thought of in terms of the total angular momentum (spin plus orbital) of the light quark coupled with the spin of the heavy quark. The ground state, D+s, has zero orbital angular momentum (L = 0), and the spins are antiparallel (1S0 in spectroscopic notation). In the only other well-established state in the spectrum, D*s(2112)+, the spins are parallel and L = 0 again (3S0). Excited states should also occur with L = 1, which combines with spin 1/2 to give a total angular momentum for the light quark of 3/2 or 1/2. When this is in turn combined with the spin 1/2 of the heavy quark, the result is an overall angular momentum J = 2, 1, or 0. With L = 1 all these states will have positive parity.

To conserve parity in its decay to D+sπ0, the new state, which has been designated Ds*J(2317)+ by the BaBar team, must have spin-parity JP in the series (0+,1 2+…). The low mass compared with previously observed related states, which have masses of around 2.5 GeV/c2, suggests that the new state corresponds to JP = 0+. If this is correct, the narrow width and relatively low mass of the state are not in line with theoretical models, which predict masses between 2.4 and 2.6 GeV/c2 with large widths. In this case, the unexpected characteristics of the new particle suggest that the models will need to be changed – or maybe the state is something entirely new, for example consisting of four quarks.

Further reading

B Aubert et al. 2003 .

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