Spin before mass at the LHC

Abstract

What can be said about the spin of new particles without knowing their mass during an initial discovery phase at CERN? We consider this question in a topology where mass measurement is particularly difficult, $pp\to Y\bar{Y}\to lX\bar{l}\bar{X}$, and introduce two new variables $\text{ctv}$ and $\text{Av}$ which we prove are both independent of the mass of $X$. The variable $\text{ctv}$ approximates the polar production angle of $Y$, and we find that it possesses greater statistical power in determining the spin of this particle than a previous, related variable, $\text{ctll}$ \cite{Barr:2005dz}. $\text{Av}$ is an asymmetry which can provide information about the couplings of a spin half $Y$. Because these variables can be used from the outset, without any knowledge of the masses of the new particles, we find here that it is possible to reverse the usual `mass before spin' determination timeline.

Comments: v2 JHEP version - minor changes to typesetting and some phrasing