[Paper Review] Search for resonances in the dijet mass spectrum from 7 TeV pp collisions at CMS

CMS Collaboration, V. Khachatryan+43 more|arXiv (Cornell University)|Oct 1, 2011

Particle physics theoretical and experimental studies26 references162 citations

TL;DR

This paper presents a search for narrow dijet resonances in 1 fb⁻¹ of 7 TeV proton-proton collisions at the LHC using the CMS detector. By analyzing wide jets reconstructed from particle-flow jets to improve mass resolution, the study sets 95% confidence level upper limits on resonance production and excludes several new physics models, including string resonances up to 4.00 TeV and excited quarks up to 2.49 TeV.

ABSTRACT

A search for narrow resonances with a mass of at least 1 TeV in the dijet mass spectrum is performed using pp collisions at √ s = 7 TeV corresponding to an integrated luminosity of 1 fb −1, collected by the CMS experiment at the LHC. No resonances are observed. Upper limits at the 95 % confidence level are presented on the product of the resonance cross section, branching fraction into dijets, and acceptance, separately for decays into quark-quark, quark-gluon, and gluon-gluon pairs. The data exclude new particles predicted in the following models at the 95 % confidence level: string resonances with mass less than 4.00 TeV, E6 diquarks with mass less than 3.52 TeV, excited quarks with mass less than 2.49 TeV, axigluons and colorons with mass less than 2.47 TeV, and W ′ bosons with mass less than 1.51 TeV. Submitted to Physics Letters B ∗ See Appendix A for the list of collaboration members1 The Large Hadron Collider (LHC) has recently delivered an integrated luminosity in excess of 1 fb −1 at a centre-of-mass energy √ s = 7 TeV. This extends considerably the search territory for

Motivation & Objective

• To search for narrow resonances in the dijet mass spectrum from 7 TeV pp collisions at the LHC.
• To set exclusion limits on new physics models predicting resonant dijet states with masses above 1 TeV.
• To improve sensitivity by using wide jets that recover final-state radiation and enhance mass resolution.
• To test specific models such as string resonances, excited quarks, axigluons, and Randall-Sundrum gravitons.
• To provide 95% confidence level upper limits on the product of cross section, branching fraction, and acceptance for quark-quark, quark-gluon, and gluon-gluon decays.

Proposed method

• Reconstructed particle-flow jets using the anti-kT algorithm with R = 0.5 (AK5) and R = 0.7 (AK7) to form wide jets.
• Combined AK5 jets with pT > 10 GeV and ∆R < 1.1 to the two leading AK5 jets to form wide jets, enhancing energy collection.
• Applied selection criteria: |η| < 2.5 and |∆η| < 1.3 for the two leading jets to maximize sensitivity to isotropic decays.
• Used dijet mass defined as m = √[(E₁ + E₂)² − (⃗p₁ + ⃗p₂)²], with m > 838 GeV for signal selection.
• Calibrated jet energies using Monte Carlo simulations, test beam data, and collision data to correct for detector response.
• Validated results with calorimeter jets and confirmed consistency in dijet mass spectra.

Experimental results

Research questions

• RQ1 What are the upper limits on the production cross section times branching fraction times acceptance for narrow dijet resonances at 95% confidence level?
• RQ2 To what mass scale are string resonances excluded by the CMS data from 1 fb⁻¹ of 7 TeV pp collisions?
• RQ3 How does wide jet reconstruction improve the sensitivity to resonant dijet states compared to standard jet reconstruction?
• RQ4 What are the exclusion limits for excited quarks, axigluons, colorons, and W′ bosons in the dijet final state?
• RQ5 How well do the observed dijet mass spectra agree with QCD predictions, and what deviations would indicate new physics?

Key findings

• No significant resonances were observed in the dijet mass spectrum, consistent with the Standard Model prediction.
• The data exclude string resonances with masses below 4.00 TeV at 95% confidence level.
• Excited quarks are excluded up to a mass of 2.49 TeV, assuming compositeness scale equal to the resonance mass.
• Axigluons and colorons are excluded up to 2.47 TeV, while W′ bosons are excluded up to 1.51 TeV.
• E6 diquarks are excluded for masses below 3.52 TeV, and Randall-Sundrum gravitons with κ/MPl = 0.1 are excluded below 3.0 TeV.
• The trigger efficiency exceeds 99.9% for dijet masses above 838 GeV, ensuring high sensitivity to high-mass resonances.