Direct Bounds on Electroweak Scale Pseudo-Dirac Neutrinos from $\sqrt s=8$ TeV LHC Data

TL;DR

This paper derives direct collider bounds on electroweak-scale pseudo-Dirac neutrinos by analyzing 8 TeV LHC multilepton data, extending sensitivity to higher masses and providing constraints comparable to indirect methods.

Contribution

It provides the first direct bounds on light-heavy neutrino mixing for pseudo-Dirac neutrinos using LHC multilepton data, covering masses up to 500 GeV.

Findings

Limits extend up to 500 GeV in neutrino mass.

Constraints are comparable to indirect bounds in the lower mass range.

Provides new direct collider bounds on pseudo-Dirac neutrino mixing.

Abstract

Seesaw models with a small lepton number breaking can naturally accommodate electroweak-scale pseudo-Dirac neutrinos with a sizable mixing with the active neutrinos, while satisfying the light neutrino oscillation data. Due to the smallness of the lepton number breaking parameter, the 'smoking gun' collider signature of same-sign dileptons is suppressed, and the heavy neutrinos in these models would manifest at the LHC dominantly through lepton number conserving trilepton final states. Using the recent CMS results for anomalous production of multilepton events at $\sqrt{s}$=8 TeV LHC with an integrated luminosity of $19.5~{\rm fb}^{-1}$, we derive direct upper bounds on the light-heavy neutrino mixing parameter as a function of the heavy Dirac neutrino mass. These limits extend the collider sensitivity to higher heavy neutrino masses up to about 500 GeV. In the lower mass range, our limits are comparable to the existing indirect constraints derived from Higgs and electroweak precision data.