New analysis of SUSY dark matter scenarios at ILC

Applying realistic veto eciencies for the low angle electromagnetic calorimeter located in the very forward direction of the future international linear collider, we revisited the Standard Model background contributions studied previously in stau analyses with supersymmetrical dark matter scenarios. In supersymmetry (SUSY) models with R-parity conservation, the lightest SUSY particle neutralino, ~ 0 , is often considered as the best candidate to satisfy the cosmological constraints on cold Dark Matter (DM) of the universe. In two previous studies [1, 2], one of the most challenging scenarios analyzed concerns the benchmark point D 0 [3] in the so-called co-annihilation region. In the mSUGRA model, the mass spectrum depends on two parameters m0 and M1=2, the common masses of scalars and gauginos superpartners at the unication scale. The parameter , dening the higgsino mass, is derived, in absolute value, by imposing the electroweak symmetry breaking condition in terms of these two parameters and of tan , the ratio of the vacuum expectations which appear in the two Higgs doublets of SUSY. In scenario D 0 , these parameters take the value m0 = 101 GeV, M1=2 = 525 GeV, tan = 10 and sign( ) < 0. The resulting ~ 0 has a mass value of 212 GeV and the next lightest SUSY particle stau, ~ , has a mass value of 217 GeV. The mass dierence is only 5 GeV. When the mass dierence is small, the co-annihilation process ~ 0~ ! becomes the dominant process for regulating the relic DM density of the universe. It is therefore crucial to measure precisely the mass values of ~ 0 and ~ . The ~ 0 mass can be measured [2] using the end-point method with a precision down to 170 MeV (80 MeV) relying on e + e ! ~ + ~ ! + ~ 0 ~ 0 (~ + ~ ! e + ~ 0e ~ 0) for the modied SPS 1a scenario with a mass value of ~ or ~ e of 143 GeV and ~ 0 of 135 GeV under the following experimental conditions: a center-of-mass energy (Ecm) of 400 GeV, an integrated luminosity (L) of 200 fb 1 and a polarized electron (positron) beam at 0:8 (0:6). The stau analyses are more challenging not only because the nal state particle of the tau decay is very soft with missing energy due to undetected neutrino(s) in addition to ~ 0 but also because the Standard Model (SM) background processes have rates which are many orders of magnitude larger than that of the signal. The cross section values of the signal and the dominant SM background processes are given in Table 1. The signal row with Ecm= 442 GeV corresponds to the optimal center-of-mass energy method (referred to hereafter as method one using the cross section measurement or event counting near threshold) proposed in [1] whereas the other signal rows correspond to cases studied in another method (method two relying on the measured energy spectra of the tau decay nal