Distinguishing Spins in Decay Chains at the Large Hadron Collider

TL;DR

The paper tackles model-independent determination of spins for new particles produced at the LHC by analyzing a cascade decay chain $D\to C q$, $C\to B l^{\rm near}$, $B\to A l^{\rm far}$ with $A$ invisible. It derives analytic spin-correlated invariant-mass distributions for all spin assignments across 2- and 3-particle observables, in two representative mass spectra, and uses the Kullback-Leibler distance to quantify how many events are needed to discriminate between competing spin hypotheses. One- and three-dimensional analyses show that dilepton observables alone often fail to distinguish certain SUSY-like and UED-like configurations, while jet+lepton and especially the full three-dimensional distributions provide stronger discrimination, depending on the mass spectrum. The methodology yields a detector-agnostic framework, informing experimental prospects and guiding analyses by providing analytic spin-correlation formulas and a principled discrimination metric. The results highlight how spin-determination at the LHC hinges on exploiting multi-dimensional phase-space information and mass spectrum specifics to separate competing new-physics scenarios.

Abstract

If new particles are discovered at the LHC, it will be important to determine their spins in as model-independent a way as possible. We consider the case, commonly encountered in models of physics beyond the Standard Model, of a new scalar or fermion $D$ decaying sequentially into other new particles $C,B,A$ via the decay chain $D\to C q$, $C\to B l^{near}$, $B\to A l^{far}$, $l^{near}$ and $l^{far}$ being opposite-sign same-flavour charged leptons and $A$ being invisible. We compute the observable 2- and 3-particle invariant mass distributions for all possible spin assignments of the new particles, and discuss their distinguishability using a quantitative measure known as the Kullback-Leibler distance.

Figures (1)

• Figure 1: Dilepton mass distributions for (a) mass spectrum I and (b) mass spectrum II.