Stopping Gluinos
6/24/2005
129 citations (118 excluding self-citations). Proposed a distinctive experimental signature — stopped particles decaying out of time with collisions — that ATLAS and CMS later searched for directly.
The Problem
Split supersymmetry, proposed by Arkani-Hamed and Dimopoulos in 2004, raised scalar superpartner masses to very high scales while keeping gauginos light. The gluino becomes long-lived because its decay to quarks requires a virtual squark that is now extremely heavy. A gluino with a lifetime longer than nanoseconds hadronizes into R-hadrons — bound states of a gluino with ordinary quarks and gluons — that traverse the detector. Some fraction of these R-hadrons lose enough energy through ionization to stop inside the detector material.
The Key Idea
The paper calculates the stopping rate: for a 300 GeV gluino, roughly 106 R-hadrons per year stop in LHC detectors. When these stopped gluinos eventually decay — potentially hours, days, or weeks after the collision that produced them — they deposit energy in the calorimeter with no activity in the tracker (no incoming track) or the muon chamber. This out-of-time signature is essentially background-free. The gluino lifetime can be measured by recording when the delayed decays occur relative to the original collisions. Even at the Tevatron, several hundred stopped gluinos per year were predicted.
Impact
Both ATLAS and CMS performed dedicated searches for stopped long-lived particles using exactly the out-of-time calorimeter signature described in this paper. The searches examined empty bunch crossings and inter-fill gaps for unexpected energy deposits, setting limits on stopped gluino masses and lifetimes. The paper is part of a pair with “Limits on Split Supersymmetry from Gluino Cosmology”, which used cosmological constraints to bound the gluino lifetime from the other direction.
Recollections
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