Abstract
If the LHC is able to produce dark matter particles, they would appear at the end of cascade decay chains, manifesting themselves as missing transverse energy. However, such ``dark matter candidates'' may themselves decay invisibly. We propose to test for this possibility by studying the effect of particle widths on the observable invariant mass distributions of the visible particles seen in the detector. We consider the simplest nontrivial case of a two-step two-body cascade decay and derive analytically the shapes of the invariant mass distributions, for generic values of the widths of the new particles. We demonstrate that the resulting distortion in the shape of the invariant mass distribution can be significant enough to measure the width of the dark matter ``candidate,'' ruling it out as the source of the cosmological dark matter.
Highlights
Events with missing transverse energy, =ET, at the Large Hadron Collider (LHC) at CERN could be indicative of the production of dark matter particles; the latter are stable and weakly interacting, and, once produced in the collision, will escape without leaving a trace inside the detector
In this paper we address the worst-case scenario, when the dark matter candidate produced at the LHC is unstable and decays invisibly
We derived the effects of nonzero particle widths on the observable invariant mass distribution dN=dm in the case of the decay chain of Fig. 1
Summary
Events with missing transverse energy, =ET, at the Large Hadron Collider (LHC) at CERN could be indicative of the production of dark matter particles; the latter are stable and weakly interacting, and, once produced in the collision, will escape without leaving a trace inside the detector. One possibility is to perform a precise measurement of its mass, and if the mass is consistent with zero, it may just be one of the Standard Model (SM) neutrinos instead of a brand new particle [11] This logic is not ironclad either: there exist examples where the dark matter particles are very light [12,13] and cannot be ruled out just on the basis of their small mass. A more detailed analysis of the exact particle width sensitivity is left for a future study
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