SELEX finds a new charm-strange meson

A new narrow charm-strange meson – a charm quark bound with a strange antiquark – has been found by the SELEX experiment at Fermilab. The new particle is a heavier relative of similar states found in other experiments last year, and its puzzling behaviour adds another chapter to the continuing story of this intriguing family of mesons.

In spring 2003 the BaBar experiment at SLAC announced the discovery of a new charm-strange meson, the D⁺_sJ(2317), which was swiftly confirmed by CLEO at Cornell and BELLE at KEK. CLEO also found evidence for the existence of a heavier partner, with a mass slightly more than 40 MeV higher. While these mesons had been predicted theoretically, their masses were lower and their lifetimes longer than expected. Following these announcements, the SELEX collaboration began to re-examine its own data from fixed-target collisions at Fermilab’s Tevatron.

SELEX, which had stopped data-taking in 1997, was designed to make high-statistics studies of the production of charm particles in Fermilab’s charged hyperon beam. In this most recent study the collaboration analysed a sample of nearly 10¹⁰ interactions produced by Σ^–. In particular, the team used events containing decays of the charm-strange ground state, D_s^± → K⁺K^–π^±. To search for new excited states of the D_s⁺ they selected events in which it was produced together with an eta meson, identifying the eta through the two photons to which it decays, η →γγ. Then, when the team plotted the mass spectrum of D_s⁺η events, they found a clear peak of some 49 events, with a significance of 7.2 σ, at a mass of 2635.9 ± 2.9 MeV/c² (Evdokimov et al. 2004).

As a particle of this mass could also decay to D⁰K⁺ the team searched for this decay mode in those events in the Σ^– sample that contained the decay D⁰ → K^–π⁺. The events selected in this way clearly showed the state D⁺_sJ(2573), already known, but also revealed a peak with 14 events at the slightly higher mass of 2613.5 ± 1.9 MeV/c². Combining the results of the two decay modes – D_s⁺η and D⁰K⁺ – indicates the existence of a new state, the D⁺_sJ(2632), with a mass of 2362.6 ± 1.6 MeV/c² and a very narrow width. Just why the new state is so narrow remains unclear, as it is massive enough to decay easily to D⁰K⁺. It is also surprising that this decay mode is dominated by the decay to D_s⁺η. However, as the SELEX team points out, if the new state does belong to charm-strange spectroscopy in the usual way, it should have a closely spaced partner. The challenge is now on to investigate this spectroscopy thoroughly.