The Invisible Z' at the LHC

TL;DR

This paper explores the potential to detect invisibly decaying Z' bosons at the LHC through a specific process, enabling model-independent measurements and discovery of hidden sector particles up to 2 TeV mass with feasible data.

Contribution

It introduces a new method to observe invisibly decaying Z' bosons at the LHC, including scenarios where Z' does not couple to leptons, expanding detection possibilities beyond traditional channels.

Findings

Invisibly decaying Z' can be observed with 30 fb^-1 at 5 sigma for 1 TeV mass.

Process can discover Z' up to 2 TeV if it couples to hidden sectors.

Excess hidden decays can be excluded at 95% CL with 100 fb^-1 if they are 20-30% of total invisible decays.

Abstract

We study the feasibility of observing an invisibly decaying Z' at the LHC through the process pp -> ZZ' -> l+l-XX*, where X is any neutral, (quasi-) stable particle, whether a Standard Model (SM) neutrino or a new state. The measurement of the invisible width through this process facilitates both a model independent measurement of Gamma_{Z' -> \bar{nu} nu} and potentially detection of light neutral hidden states. Such particles appear in many models where the Z' is a messenger to a hidden sector, and also if dark matter is charged under the U(1)' of the Z'. We find that with as few as 30 fb^-1 of data the invisibly decaying Z' can be observed at 5 sigma over SM background for a 1 TeV Z' with reasonable couplings. If the Z' does not couple to leptons and therefore cannot be observed in the Drell-Yan channel, this process becomes a discovery mode. For reasonable hidden sector couplings, masses up to 2 TeV can be probed at the LHC. If the Z' does couple to leptons, then the rate for this invisible decay is predicted by on-peak data and the presence of additional hidden states can be searched for. With 100 fb^-1 of data, the presence of excess decays to hidden states can be excluded at 95% C.L. if they comprise 20-30% of the total invisible cross section.