Studying Very Light Gravitino at ILC Ryo Katayama (Tokyo) Collaborators: Shigeki Matsumoto (IPMU), T. Moroi (Tokyo), K. Fujii (KEK), T. Suehara (ICEPP),T. Tanabe (ICEPP), S. Yamashita (ICEPP)

Motivation The very light gravitino (~O(10eV)) is quite attractive from the view of point cosmology. The next lightest supersymmetry particle(NLSP) life time directly gives the gravitino mass. We focus on the case that the NLSP is stau from now. However, it is difficult to measure the NLSP lifetime at LHC. It is possible to determine the NLSP lifetime at ILC. [arXiv: ]

NLSP : Stau From the stau mass and lifetime, we can get the gravitino mass. To estimate the gravitino mass precision is goal. ~ e+e+ τ−τ− τ+τ+ τ−τ− ~ Z ＊, γ* e-e- g NLSP lifetime example.   ～ 100 x M pl 2 x m 3/2 2 /m NLSP 5 ～ O(10 –13 ) sec.  c  ～ 100  m Tau lepton lifetime.  c  ～ 80  m g ~ ~ τ+τ+

Impact Parameter 1st layer Gravitino Stau  Hadronic decay (π ±, K ±, etc.) Impact parameter  16mm IP Long lifetime means large impact parameter. Thus, by observing the impact parameter distribution of the tau decay products, we can measure the stau lifetime. Leptonic decay (e ±, μ ±, ν.)

Background Signal & Background processes [Signal] e + e –  +  – [  -BG] e + e – e + e –  e + e –  +  – (+ ISR) ~ ~ First, evaluate the tau pair background. Tau is reconstructed in the following 1-prong modes: [PDG]

D0 distribution Display the impact parameter distribution with the mode of e + e - ->τ + τ - (black) and e + e - ->μ + μ - (red). The difference come from the existence of decay products.

D0 distribution2 Cut Conditions 1. (θ = Scattering angle of the τ-jet.) Entries of 1-Prong mode Selection (Φ=Azimuthal angle of the τ-jet.) Before Cut After Cut

Conclusion(Schedule) 1.Genarate the events of stau pair creation as signal. 2.Include total background into the impact parameter distribution. 3.Investigate the suitable cuts and its efficiency. 4.Measure the gravitino mass precision.