[Paper Review] Search for new physics in dijet final states in ATLAS and CMS

M. Bauce|arXiv (Cornell University)|Sep 14, 2017

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents a comprehensive search for new physics in dijet final states using 13 TeV proton-proton collision data from the ATLAS and CMS experiments during LHC Run 2 (2015–2016). By analyzing dijet invariant mass spectra and angular correlations, the experiments set stringent limits on new resonances, excluding excited quarks up to 6.0 TeV and $W'$ bosons up to 3.6–3.3 TeV, with no significant deviations from the Standard Model observed.

ABSTRACT

Events containing a pair of high energy hadronic jet can provide clear signatures in the search for new physics at high energy hadron colliders. The ATLAS and CMS experiments collected the data from LHC collisions at $\sqrt{s}$= 13 TeV during 2015 and 2016, looking for evidence of new resonances or deviations from the Standard Model predictions. Althoug no hint of new physics was seen, strong limits have been set on the most interesting benchmark models, improving LHC Run1 reach.

Motivation & Objective

• To search for new physics in dijet final states at the LHC using 13 TeV proton-proton collisions.
• To test benchmark models such as $W'$, $Z'$, excited quarks, and quantum black holes via dijet resonance and angular correlation signatures.
• To improve sensitivity in low- and high-mass regions using advanced background modeling and jet substructure techniques.
• To set exclusion limits on new physics models, including simplified Dark Matter models, using dijet signatures.
• To explore the reach of the LHC in detecting new resonances beyond the Standard Model through high-energy dijet final states.

Proposed method

• ATLAS and CMS use high-energy dijet events with $p_T > 30$ GeV and $|\eta| < 2.5$ to search for resonant peaks in the dijet invariant mass spectrum $m_{jj}$.
• CMS applies a $H_T > 900$ GeV trigger and uses a smooth analytical fit $dN/dm_{jj} = p_1(1-z)^{p_2}z^{p_3}z^{p_4\ln z}$ to model the QCD background, with $z = m_{jj}/\sqrt{s}$.
• ATLAS employs a Sliding Window Fit (SWiFt) technique with a simplified analytical function to stabilize background prediction across the $m_{jj}$ spectrum.
• For low-mass resonances, CMS reconstructs boosted dijets as wide jets and applies substructure algorithms (e.g., soft drop) to identify resonance decays.
• ATLAS and CMS analyze events with an ISR jet or photon to access lower $m_{jj}$ regions, using $p_T > 440$ GeV for jets and $p_T > 150$ GeV for photons.
• Limits are set on production cross sections for generic signals and benchmark models using data-driven background estimation and Monte Carlo simulations.

Experimental results

Research questions

• RQ1 What are the exclusion limits on new resonances such as $W'$, $Z'$, excited quarks, and quantum black holes in the dijet final state at $\sqrt{s} = 13$ TeV?
• RQ2 How do dijet angular correlations and jet substructure techniques improve sensitivity to new physics at low and high masses?
• RQ3 Can the presence of anomalous bumps in the $m_{jj}$ spectrum be detected, and what are the implications for new physics models?
• RQ4 How do advanced background modeling techniques like SWiFt and analytical fits enhance the robustness of resonance searches?
• RQ5 To what extent can dijet signatures constrain simplified Dark Matter models?

Key findings

• No significant deviations from the Standard Model background were observed in the dijet invariant mass spectrum across the full range from 50 GeV to 8 TeV.
• Excited quarks were excluded up to a mass of 6.0 TeV by both ATLAS and CMS, representing a significant improvement over Run 1 limits.
• The $W'$ boson was excluded up to 3.6 TeV by ATLAS and 3.3 TeV by CMS, based on dijet resonance searches.
• CMS set limits on $Z'$-like resonances down to 1.7 TeV in the dijet final state, with improved sensitivity in the low-mass region below 100 GeV using wide jet techniques.
• A local 2.9$\sigma$ excess was observed by CMS at 115 GeV in the wide jet mass spectrum, with a global significance of 2.2$\sigma$, but not considered evidence for new physics.
• Generic Gaussian signals were excluded with cross sections greater than 0.1–0.01 pb in the $m_{jj}$ range [303–1493] GeV for ISR jet and photon final states.