Spectroscopy of D Mesons

The scenario of heavy quark meson spectroscopy underwent recently a major revolution, after the observation of BABAR and CLEO, confirmed by BELLE, of DsJ L=1 excited states, and by further evidences by SELEX. These experimental results have cast doubts on the incarnations of the ideas of Heavy Quark Effective Theory in heavy quark spectroscopy. I shall review the status of experimental data, discuss implications and sketch an outlook. INTRODUCTION This paper reports on recent experimental results on D meson spectroscopy, discussing the recent events that brought to cast doubts to our current understanding of the overall picture. I shall discuss excited non-strange D mesons, namely the observation of jq = 1/2 broad states, the revolutionary observations of excited strange DsJ mesons which are forcing us to switch the paradigm of HQ spectroscopy, discuss the status of debated DsJ(2632) meson observed by SELEX at Fermilab, finally sketch an outlook and draw conclusions. For a detailed review on charm physics including spectroscopy the reader is referred to Ref.[1], for other charm spectroscopy issues such as charmonium states etc. see other up-to-date reviews such as [2, 3, 4]. Let me pay a tribute to cosmic ray physicists and show the — possibly — very first D meson observed by human eye (D+ → K+π0), in nuclear emulsions exposed to cosmic rays in 1971 [5]. After 35 years, here is where we are. HEAVY-LIGHT QUARK SPECTROSCOPY, THE GLOBAL PICTURE A global interpretation scheme for heavy quark meson spectroscopy is provided by the idea of Heavy Quark Symmetry (HQS). In the infinite heavy-quark mass limit, the heavy-light meson can be described as formed by a the still heavy quark, with all the orbital degrees of freedom being due to the light quark. This means that good conserved quantum numbers are the spin of the heavy quark, and the angular momentum jq. Experimentally, for each of the cū, cd̄ and cs̄ systems four P-wave and two n = 2 radial excitations have been studied. There are four L = 1 states, namely two with jq = 1/2 and total spin J = 0,1 and two with jq = 3/2 and J = 1,2. These four states are named respectively D∗0, D1( jq = 1/2), D1( jq = 3/2) and D ∗ 2 (Fig.2). Parity and angular momentum conservation force the ( jq = 1/2) states to decay to the ground states via FIGURE 1. First charm candidate event in nuclear emulsions [5]. Figure from Ref. [6]. 180