Z. Phys. C 64, 499-507 (i994) ZEITSCHRIFT FOR PHYSIK C 9 Springer-Verlag 1994 Squark and gluino decays for large tan/3 A. Bartll, W. Majerotto 2, W. Porod 1 1 Institut fiir Theoretische Physik, Universit~it Wien, A-1090 Vienna, Austria 2 Institut fiir Hochenergiephysik, 13sterreiLchische Akademie der Wissenschaften, A-1050 Vienna, Austria Received: 24 June 1994 Abstract. We have studied stop, sbottom and gluino decays in a scenario with large tan t, where both {L-{R and bL-b:~ mixing as well as the Yukawa couplings have to be taken into account. In particular we have worked out the case that D1 is the lightest squark. The decay patterns of sbottom and stop are very different from those of the squarks of the first and second generation. 1 Introduction At the LHC the production cross sections for gluinos and squarks are expected to be the highest ones of all supersym- metric particles. Gluino and squaxk production and their de- cays have therefore been studied quite extensively in the last years [1, 2, 3, 4]. Monte Carlo studies have been performed to find the most promising signatures. In most studies either degenerate masses for all squark flavours were assumed, or only five flavours were taken (leaving out the top squark). Due to the large top Yukawa coupling {L-{R mixing can- not be neglected and one of the stop mass eigenstates can be much lighter than all other squarks. The production of a light stop in p/3 (pp) collisions was considered in Ref.[5]. Pair production of squarks is also possible in e+e - annihila- tion if the energy is high enough 116].Here the case of a light stop was studied in Ref.[7]. The present experimental limit is m~ > 45 GeV from LEP [8] if the mixing angle 0t is not in the range 0.85 < 0t < 1.15. The VENUS collaboration at KEK obtained the bound m~, > 28 GeV [9]. In most analyses usually moderate values for tan fl (< 10) were taken. However, it is known that the bottom Yukawa coupling can be quite big for tanfl > 10 [10]. In this case also bL-bR mixing can become quite large. It is even possible that the lighter sbottom mass eigenstate is the lightest of the Squark states, in particular also lighter than the stop. This interesting case has not been treated so far, and we shall study in the following some important phenomeno- logical aspects of such a scenario. Due to the influence of the Yukawa couplings and the squark mixing the decay pattern will be changed and will be different from other down-type squarks. Our framework is the Minimal Supersymmetric Stan- dard Model (MSSM)[11, 12]. In addition to gluinos, squarks and sleptons this model contains two pairs of charginos X{, i = 1,2, four neutralinos X ~ i = 1, .., 4 and five Higgs particles (h ~ H ~ A ~ H+). The main parameters determin- ing the phenomenology of the sbottom and stop decays are M O, M O, Mr), tan/3, At, Ab, #, M', M, mA. M O, M O, MI3, At and AD are parameters of the soft-breaking poten- tial and enter the squark mass matrices (see Eq.(1)-(7) ). tan/3 = v2/vl where vi are the vacuum expectation val- ues of the two Higgs doublets. # is the usual Higgs mass parameter in the superpotential. M t and M are the U(1) and SU(2) gaugino masses. We shall use the GUT relation M t = 5/3 tan20w M, M = a2/Olsm~ ~ 0.3m~, m~ being the gluino mass. mA is the mass of the pseudoscalar Higgs boson A ~ In the next section we present the formulae for the squark mixing and for the various decays of the sbottoms and the stops into charginos, neutralinos, W + , Z ~ H +, h ~ H ~ A ~ taking into account the Yukawa coupling of the top as well as of the bottom, and the mixing of { and b. In section 3 we give numerical results for the decay branching ratios of bl, b2, t'l and {2 in the light sbottom scenario and discuss the important signatures. In section 4 we study the influence of stop and sbottom mixing on the decays of the gluino, which has not been considered so far in the literature. 2 Squark mixing and decays The mass matrices for the stop and sbottom system in the (EL, {R) or (bL, DR) basis, respectively, read [13, 14]: O,t/77,tl~t21,) with b, m -2 (1) ' ab mb bR m2 2 28( 89 ~ sin 2 ew ) +mta ,L = v4 cos m-2tR =Mu+2 2 rn2 cos 2fl sin2 0w +mt2 m2 2 m~cos28( 89 - 1 bL = M~ - g sin 2 0w ) + m 2 (2) (3) (4)