- The paper presents a new analysis that categorizes events by jet multiplicity and scalar jet momentum to isolate potential signals of new physics.
- It employs data-driven methods to accurately estimate SM backgrounds from Z+jets, W+jets, top pairs, and QCD multijet events.
- The study sets exclusion limits on simplified SUSY models, ruling out gluino masses up to 1.21 TeV and squark masses below 780 GeV in certain scenarios.
Overview of the Search for New Physics in Multijet and Missing Transverse Momentum Events at 8 TeV Proton-Proton Collisions
The paper conducted by the CMS Collaboration focuses on exploring new physics paradigms beyond the Standard Model (SM) by analyzing multijet events associated with significant missing transverse momentum (\MHT) produced in proton-proton collisions at a center-of-mass energy of 8 TeV. The research utilizes a substantial data set corresponding to an integrated luminosity of 19.5 \fbinv collected by the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC).
Analysis Approach
The analysis systematically categorizes events based on jet multiplicity into three distinct categories: 3--5 jets, 6--7 jets, and 8 or more jets. This classification aims to enhance sensitivity across various final-state topologies, which might result from extended squark and gluino decay cascades, leading to multiple jets.
The investigation involves further subdivision of these events into bins of the scalar sum of jet transverse momenta (\HT) and \MHT. These metrics are pivotal in distinguishing potential new physics signals from SM backgrounds.
Background Estimation
The primary background contributions arise from SM processes including \cPZ+jets, \PW+jets, and top quark pairs (\ttbar), where \MHT can be attributed to the presence of neutrinos. Additionally, quantum chromodynamics (QCD) multijet events contribute background when high \MHT results from jet energy mismeasurements or detector imperfections.
Comprehensive data-driven methods are utilized to estimate these backgrounds:
- \cPZ+jets Background: Leveraging the analogy between γ+jets and \cPZ+jets processes for high transverse momenta, with corrections for acceptance and cross-sections.
- \ttbar and \PW+jets Background: Utilizing a muon control sample and appropriately weighing events according to lepton detection efficiencies and acceptances.
- Hadronic τ Lepton Background: Estimating contributions by replacing a muon with a jet sampled from \tauh templates and correcting for efficiencies and kinematics.
- QCD Multijet Background: Employing the "rebalance and smear" method to adjust jet energies and estimate event counts in high-\MHT regions.
Results and Implications
The observed data across 36 search regions was consistent with the predicted SM backgrounds within uncertainties. A statistical excess in a specific region (6--7 jets, $\HT = 500$--800 GeV, $\MHT > 450$ GeV) was not deemed significant when considering the overall analysis.
The paper sets exclusion limits for various simplified supersymmetry (SUSY) models, focusing on gluino and squark pair production. Notably, gluino masses up to 1.16--1.21 TeV and squark masses below 780 GeV were excluded for certain lightest supersymmetric particle (LSP) mass scenarios, demonstrating enhanced sensitivity over previous analyses.
Future Directions
The findings of this analysis contribute to narrowing the parameter space for SUSY and other theories beyond the SM. With the improvement in detector technology and data acquisition, future studies at higher energies and luminosities are expected to further probe the limits of the SM, potentially revealing new physics phenomena. As the field of precision measurements and theoretical models advances, these searches will be critical in addressing unanswered questions in particle physics.