A General Method for Model-Independent Measurements of Particle Spins, Couplings and Mixing Angles in Cascade Decays with Missing Energy at Hadron Colliders

TL;DR

This paper presents a comprehensive, model-independent method for measuring particle spins, couplings, and mixing angles in cascade decays with missing energy at hadron colliders, applicable even when traditional charge asymmetry methods fail.

Contribution

It introduces a new approach to determine spins using invariant mass distributions that are independent of couplings and mixing angles, enhancing analysis at colliders like the LHC and Tevatron.

Findings

Derived analytic formulas for invariant mass distributions.

Proposed a basis for spin determination independent of couplings.

Applicable to pp-bar colliders where charge asymmetry is zero.

Abstract

We outline a general strategy for measuring spins, couplings and mixing angles in the case of a heavy partner decay chain terminating in an invisible particle. We consider the common example of a new scalar or fermion D decaying sequentially to other new particles C, B and A by emitting a quark jet j and two leptons ln and lf. We derive analytic formulas for the dilepton {ln,lf} and the two jet-lepton ({j,ln} and {j,lf}) invariant mass distributions for most general couplings and mixing angles of the new partners. We then consider various spin assignments for the particles A, B, C and D, and derive the relevant functional basis for the invariant mass distributions which contains the intrinsic spin information and does not depend on the couplings and mixing angles. We propose a new method for determining the spins of the new partners, using the three experimentally observable distributions {l+,l-}, {j,l+}+{j,l-} and {j,l+}-{j,l-}. We show that the former two only depend on a single model-dependent parameter alpha, while the latter may depend on two other parameters beta and gamma. By fitting these distributions to our set of basis functions, we are able to do a pure measurement of the spins per se. Our method is also applicable at a pp-bar collider such as the Tevatron, for which the previously proposed lepton charge asymmetry is identically zero and does not contain any spin information. In the process of determining the spins, we also obtain an independent measurement of the parameters alpha, beta and gamma, which represent certain combinations of the couplings and the mixing angles of the heavy partners A, B, C and D.