[Paper Review] Search for supersymmetry in a final state containing two photons and missing transverse momentum in $\sqrt{s}$ = 13 TeV $pp$ collisions at the LHC using the ATLAS detector

ATLAS Collaboration|arXiv (Cornell University)|Jun 29, 2016

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a search for supersymmetry in proton-proton collisions at √s = 13 TeV using the ATLAS detector, focusing on events with two photons and large missing transverse momentum. No signal events were observed; the observed upper limit of 0.93 fb at 95% CL implies a lower limit of 1650 GeV on the gluino mass in a generalized gauge-mediated SUSY model with a bino-like next-to-lightest supersymmetric particle.

ABSTRACT

A search has been made for supersymmetry in a final state containing two photons and missing transverse momentum using the ATLAS detector at the Large Hadron Collider. The search makes use of 3.2 fb$^{-1}$ of proton-proton collision data collected at a centre-of-mass energy of 13 TeV in 2015. Using a combination of data-driven and Monte-Carlo-based approaches, the Standard Model background is estimated to be $0.27^{+0.22}_{-0.10}$ events. No events are observed in the signal region; considering the expected background and its uncertainty, this observation implies a model-independent 95% CL upper limit of 0.93 fb (3.0 events) on the visible cross section due to physics beyond the Standard Model. In the context of a generalized model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest supersymmetric particle, this leads to a lower limit of 1650 GeV on the mass of a degenerate octet of gluino states, independent of the mass of the lighter bino-like neutralino.

Motivation & Objective

• To search for physics beyond the Standard Model in final states with two isolated photons and large missing transverse momentum using 3.2 fb⁻¹ of 13 TeV pp collision data.
• To test a generalized gauge-mediated supersymmetry (GGM) model where the next-to-lightest supersymmetric particle (NLSP) is bino-like and decays predominantly to a photon and gravitino.
• To set constraints on the mass of a degenerate octet of gluinos in the context of this GGM model.
• To improve on prior searches using 8 TeV data by leveraging higher center-of-mass energy and improved detector performance.
• To provide a model-independent upper limit on the visible cross section for new physics in the diphoton + E_T^miss final state.

Proposed method

• Employed data-driven and Monte Carlo-based techniques to estimate the Standard Model background, resulting in an estimate of 0.27⁺⁰.²²₋₀.₁₀ events.
• Defined a signal region enriched in diphoton + missing transverse momentum events, applying kinematic and isolation criteria to photons and requiring high E_T^miss.
• Used Herwig++ 2.7.1 with CTEQ6L1 PDFs to simulate signal events for GGM models with degenerate gluino octets and bino-like NLSPs.
• Calculated signal cross sections at next-to-leading order in QCD with soft-gluon resummation for strong production processes.
• Applied a simplified model framework where only the gluino and bino-like NLSP are accessible at 13 TeV, with all other sparticles set to masses too high for production.
• Assumed the NLSP decays 100% to γ + gravitino at low masses, with branching fraction approaching cos²θ_W at high masses, ensuring dominant diphoton final states.

Experimental results

Research questions

• RQ1 What is the observed rate of events with two isolated photons and large missing transverse momentum in 13 TeV pp collisions?
• RQ2 What is the upper limit on the visible cross section for new physics in the diphoton + E_T^miss final state at 95% confidence level?
• RQ3 What lower bound can be placed on the gluino mass in a generalized gauge-mediated SUSY model with a bino-like NLSP?
• RQ4 How does the diphoton + E_T^miss signature constrain the parameter space of GGM models at 13 TeV?
• RQ5 What is the sensitivity of the ATLAS detector to gluino pair production with subsequent decays to photons and gravitinos?

Key findings

• No signal events were observed in the signal region, consistent with the expected Standard Model background of 0.27⁺⁰.²²₋₀.₁₀ events.
• A model-independent 95% CL upper limit of 0.93 fb was set on the visible cross section for new physics in the diphoton + E_T^miss final state.
• In the context of a generalized GGM model with a bino-like NLSP, a lower limit of 1650 GeV was established on the mass of a degenerate octet of gluino states.
• This mass limit is independent of the mass of the lighter bino-like neutralino, demonstrating robustness to NLSP mass variations.
• The analysis achieves improved sensitivity compared to previous 8 TeV searches due to higher center-of-mass energy and enhanced detector performance.
• The branching fraction of the bino-like NLSP to γ + gravitino is dominant across the mass range studied, validating the diphoton signature as a clean probe.