Scalar Stoponium

We study the decays of a scalar (t̃1t̃ ∗ 1) bound state σt̃1 , where t̃1 is the lighter stop eigenstate. If t̃1 has no tree–level 2–body decays, the dominant decay modes of σt̃1 are gg or, if mh < mt̃1 ≪ mt̃2 , a pair of light scalar Higgs bosons h. The best signal for σt̃1 production at hadron colliders is probably its decay into two photons. Contribution to the Proceedings of the Madison–Argonne Hadron Collider Workshop, March to June 1993 Heisenberg Fellow E–mail: NOJIRIN@JPNKEKVX It is by now quite well known that the lighter scalar top (stop) eigenstate t̃1 is expected to be lighter than the superpartners of the first two generations of quarks, and might even be the lightest colored sparticle [1, 2]. There are two reasons for this: Since the top quark is heavy, mt ≥ 110 GeV, mixing between the superpartners t̃L, t̃R of left– and right–handed top quarks cannot be neglected, in contrast to the superpartners of light quarks. Furthermore, if we assume all squarks to have the same mass at some very high (GUT, string or Planck) scale, radiative corrections [3] will reduce the masses of t̃L and t̃R relative to those of the other squarks. Open t̃1 pair production at e e [4] and p̄p [5] colliders has been discussed previously. Here we study possible signals for the production of a scalar (t̃1t̃ ∗ 1) bound state σt̃1 within the Minimal Supersymmetric Standard Model (MSSM) [6]. In ref.[7] it has been pointed out that the branching ratio for σt̃1 → hh might be large, where h is the lighter scalar Higgs boson. Very recently it has been claimed [8] that σt̃1 → W W can have a very large branching ratio, which might give rise to interesting signals at hadron supercolliders. This motivated us to compute all potentially large branching ratios of σt̃1 . We basically agree with the results of ref.[7], but were unable to reproduce those of ref.[8]. The cleanest signal for σt̃1 production at hadron colliders arises from its 2–photon decay, giving rise to a peak in the γγ invariant mass spectrum. The starting point of our discussion is the stop mass matrix. Following the convention of ref.[9] we write it as (in the basis t̃L, t̃R):