[Paper Review] Search for dark matter at the LHC using missing transverse energy

Sarah Malik|arXiv (Cornell University)|Jun 4, 2012

Particle physics theoretical and experimental studies2 references3 citations

TL;DR

This paper presents a search for dark matter at the LHC using monojet and monophoton final states with missing transverse energy, analyzing 5 fb⁻¹ of 7 TeV proton-proton collisions with the CMS detector. In the absence of observed excess events, it sets the most stringent 90% CL limits to date on the dark matter-nucleon scattering cross section, extending coverage below 3.5 GeV for spin-independent interactions and surpassing prior constraints for 1–200 GeV in spin-dependent interactions.

ABSTRACT

Results are presented for a search for dark matter at the LHC using the signatures of a monojet plus missing transverse energy and a monophoton plus missing transverse energy. The data were collected by the CMS detector at the LHC with pp collisions at a centre-of-mass energy of 7 TeV and an integrated luminosity of 5 inverse femtobarns. In the absence of an excess of events in the data compared to the Standard Model prediction, limits are set on the dark matter-nucleon scattering cross section which can be directly compared with bounds from the direct detection experiments.

Motivation & Objective

• To search for dark matter production in proton-proton collisions at the LHC using monojet and monophoton signatures with missing transverse energy.
• To set model-independent limits on dark matter-nucleon scattering cross sections using an effective field theory framework.
• To improve constraints on weakly interacting massive particles (WIMPs) in the low-mass regime, particularly below 3.5 GeV for spin-independent interactions.
• To compare results directly with direct detection experiments and extend the excluded parameter space for dark matter.
• To validate background estimation techniques using control samples from Z→μμ+jets and W→μν+jets events.

Proposed method

• Utilizes the CMS detector to collect 5 fb⁻¹ of pp collisions at √s = 7 TeV.
• Applies event selection requiring high missing transverse energy (E_T^miss > 350 GeV for monojet, >130 GeV for monophoton) and a high-p_T jet or photon.
• Implements stringent jet and photon identification, isolation, and timing requirements to suppress QCD and instrumental backgrounds.
• Estimates dominant SM backgrounds (Z→νν+jets and W→ℓν+jets) using control samples from Z→μμ+jets and W→μν+jets events.
• Applies corrections for detector efficiency, acceptance, and branching ratios to extrapolate control sample yields to the signal region.
• Converts observed cross-section limits into bounds on the effective contact interaction scale Λ, then translates to dark matter-nucleon scattering cross sections.

Experimental results

Research questions

• RQ1 What are the limits on the dark matter-nucleon scattering cross section for spin-independent and spin-dependent interactions using monojet and monophoton signatures?
• RQ2 How do the CMS results compare with previous direct detection experiments and collider searches in the low-mass dark matter regime?
• RQ3 To what extent do the observed event yields exceed SM background expectations in the monojet and monophoton final states?
• RQ4 What is the sensitivity of the monojet and monophoton searches to dark matter masses below 3.5 GeV?
• RQ5 How effective are the background suppression techniques, particularly the use of control samples and isolation criteria?

Key findings

• No significant excess of events is observed in either the monojet or monophoton final states above the Standard Model background predictions.
• The 90% CL upper limit on the spin-independent dark matter-nucleon scattering cross section is extended into the previously inaccessible region below 3.5 GeV.
• For spin-dependent interactions, the CMS limits surpass all prior constraints in the 1–200 GeV dark matter mass range.
• The observed limits on the effective contact interaction scale Λ are translated into conservative bounds on the dark matter-nucleon scattering cross section.
• Background estimates are validated using control samples, with the Z→νν+jets background estimated at 900 ± 94 events and W+jets at 312 ± 35 events.
• The monophoton analysis observes 73 events, consistent with the predicted 75.1 ± 9.5 background yield.