Using the (Modified) Matrix Element Method to constrain $L_\mu - L_\tau$ Interactions

Abstract

In this paper, we explore the discriminatory power of the matrix element method (MEM) in constraining the $L_\mu-L_\tau$ model at the LHC. The $Z'$ gauge boson associated with the spontaneously broken $U(1)_{L_\mu-L_\tau}$ symmetry only interacts with the second and third generation of leptons at tree level, and is thus difficult to produce at the LHC. We argue that the best channels for discovering this $Z'$ are in $Z \to 4\mu$ and $2\mu+\displaystyle{\not}E_T$. Both these channels have a large number of kinematic observables, which strongly motivates the usage of a multivariate technique. The MEM is a multivariate analysis that uses the squared matrix element to quantify the likelihood of the testing hypotheses. We find that with $300 \, \text{fb}^{-1}$ of integrated luminosity, we are sensitive to the couplings of $ g_{Z'} \gtrsim 0.002 ~ g_1$ and $M_{Z'} < 20 \text{ GeV}$, and $g_{Z'} \gtrsim 0.005 g_1$ and $20\, \text{GeV} <M_{Z'} < 40 ~ \text{GeV}$, which is about an order of magnitude improvement over the cut-and-count method for the same amount of data.