Distinguishing Spins in Supersymmetric and Universal Extra Dimension Models at the Large Hadron Collider

TL;DR

The paper tackles the challenge of distinguishing SUSY and Universal Extra Dimensions (UED) at the LHC by extending Barr's spin-discrimination method to UED. It analyzes spin correlations in a common decay chain, deriving analytic expressions for angular and invariant-mass distributions in both SUSY and UED, and examines how mass spectrum shapes (hierarchical vs quasi-degenerate) affect observability. For SUSY, near-lepton helicity aligns with the quark leading to distinct $dP/d\cos\theta^*$ and $dP/d\widehat{m}$ forms that are mass-independent; for UED, angular distributions depend on mass ratios $x$ and $y$ through $q^*\to qZ^*$ decays with mixed $Z^*$ polarisation, yielding $ql^{\rm near}$ distributions that approach SUSY under certain limits. The study shows that hierarchical mass spectra in SUSY enhance spin discrimination, while quasi-degenerate UED spectra can obscure spin correlations, and it provides concrete analytic and numerical results to guide experimental analyses at the LHC.

Abstract

An interesting alternative to supersymmetry (SUSY) for extending physics beyond the Standard Model is a model with universal extra dimensions (UED), in which the SUSY superpartners are replaced by Kaluza-Klein excitations of the Standard Model particles. If new particles are discovered at the LHC, even if their mass spectrum favours SUSY or UED, it will be vital to distinguish between their spin assignments in the two models as far as possible. We extend the method proposed by Barr [hep-ph/0405052] to the UED case and investigate the angular and charge asymmetries of decay distributions for sample mass spectra of both SUSY and UED types. For hierarchical (`SUSY-type') mass spectra there is a good chance of distinguishing the spin structures of the two models. However, a mass spectrum of the quasi-degenerate type expected in UED would make it difficult to observe spin correlations.