Determining the spin of supersymmetric particles at the LHC using lepton charge asymmetry

If signals suggesting supersymmetry (SUSY) are discovered at the LHC then it will be vital to measure the spins of the new particles to demonstrate that they are indeed the predicted super-partners. A method is discussed by which the spins of some of the SUSY particles can be determined. Angular distributions in sparticle decays lead to charge asymmetry in lepton-jet invariant mass distributions. The size of the asymmetry is proportional to the primary production asymmetry between squarks and anti-squarks. Monte Carlo simulations are performed for a particular mSUGRA model point at the LHC. The resultant asymmetry distributions are consistent with a spin-0 slepton and a spin-1 2 ˜ χ 0 2 , but are not consistent with both particles being scalars. 1 Spin correlations and charge asymmetry A recent publication [1] describes the method by which spin correlations were added to the HERWIG [2,3] Monte Carlo event generator. It includes an example of part of a supersymmetric decay chain, ˜ q L → ˜ χ 0 2 q L → ˜ l ± R l ∓ q L (1) in which spin correlations can play a significant role in the kinematics of the emitted particles. When the decay of the slepton is also considered, (fig. 1), the final state consists of two opposite-signed leptons of the same family, a quark jet, and missing energy from the undetected˜χ 0 1. In this Letter I examine the observability of spin effects in this decay chain at the LHC. I first consider the distribution of invariant mass of the quark or anti-quark jet from the squark decay, and the lepton or anti-lepton from the˜χ 0 2 decay. Using the terminology of [4], this lepton is referred to as the 'near' lepton, as opposed to the lepton from the slepton decay, which is called the 'far' lepton. Initially I investigate the parton-level distributions which one would obtain if the sign of the squark and the identity of the lepton were known. In section 4 the experimentally observable distributions are introduced. These have added complications due to the difficulty in (a) distinguishing which lepton came from which decay, and (b) measuring the charge of the (s)quark. In the approximation in which the quark and lepton are massless, and the SUSY particles are on-mass-shell, then the l near q invariant mass has a simple and direct interpretation in terms of the angle, θ * , between …