Search for non-pointing and delayed photons in the diphoton and missing transverse momentum final state in 8 TeV $pp$ collisions at the LHC using the ATLAS detector

TL;DR

This paper searches for long-lived particles decaying into photons and invisible particles in 8 TeV proton-proton collisions, using ATLAS detector data, and sets exclusion limits within supersymmetry models.

Contribution

It introduces a novel analysis technique exploiting calorimeter measurements to detect displaced photon vertices and sets new limits on long-lived neutralinos.

Findings

No excess over Standard Model background observed.

Exclusion limits set for neutralino lifetimes from 250 ps to 100 ns.

Constraints placed on gauge-mediated supersymmetry breaking models.

Abstract

A search has been performed, using the full 20.3 fb$^{-1}$ data sample of 8 TeV proton--proton collisions collected in 2012 with the ATLAS detector at the LHC, for photons originating from a displaced vertex due to the decay of a neutral long-lived particle into a photon and an invisible particle. The analysis investigates the diphoton plus missing transverse momentum final state, and is therefore most sensitive to pair-production of long-lived particles. The analysis technique exploits the capabilities of the ATLAS electromagnetic calorimeter to make precise measurements of the flight direction, as well as the time of flight, of photons. No excess is observed over the Standard Model predictions for background. Exclusion limits are set within the context of Gauge Mediated Supersymmetry Breaking models, with the lightest neutralino being the next-to-lightest supersymmetric particle and decaying into a photon and gravitino with a lifetime in the range from 250 ps to about 100 ns.