Search for new physics using single-lepton events with high multiplicities of jets and b jets in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Using 138 fb$^{-1}$ of 13 TeV proton-proton collision data from the CMS detector, this study searches for $R$-parity violating supersymmetry in single-lepton events with high jet and b-jet multiplicities by analyzing large-radius jet masses, finding no significant excess and excluding gluino masses below 1890 GeV at 95% confidence level.

The Gist

The Big Picture: Hunting for "Ghost" Particles in a Cosmic Pinball Machine

Imagine the Large Hadron Collider (LHC) at CERN as the world's most powerful cosmic pinball machine. Scientists smash two streams of protons together at nearly the speed of light. Usually, these collisions create a predictable shower of particles that follow the rules of the "Standard Model" (the rulebook of physics we already know).

But sometimes, the rulebook might be incomplete. This paper describes a search for new physics—specifically a theory called Supersymmetry (SUSY)—that might explain things the current rulebook can't.

The Mystery: The "Missing Money" Problem

In many versions of Supersymmetry, when new heavy particles are created, they decay into a stable, invisible particle (like a dark matter candidate). Because this invisible particle flies away without hitting any detectors, it looks like missing money in a bank account. Scientists usually look for this "missing money" (called missing transverse momentum) to find new physics.

However, this paper investigates a different version of the theory called R-parity Violating (RPV) SUSY.

• The Analogy: Imagine a bank robber who doesn't just steal money and vanish. Instead, they steal the money and immediately spend it all on visible items (like gold bars and jewels) before running away.
• The Result: There is no "missing money" left behind. The thief is gone, but the pile of gold and jewels (the particles) is huge and very obvious.

Because there is no "missing money" to look for, the scientists had to change their strategy. They stopped looking for empty space and started looking for massive piles of debris.

The Strategy: Counting the Debris

The scientists focused on a specific scenario where a heavy particle called a gluino (think of it as a super-heavy "glue" particle) is created and then explodes.

• The Explosion: When the gluino explodes, it doesn't just make a few crumbs; it creates a chaotic storm of jets (sprays of particles).
• The Specifics: The theory predicts that each explosion creates a top quark, a bottom quark, and a strange quark. The bottom quarks are like "heavy gold bars" in this storm.
• The Signal: The scientists looked for events with:
  1. One Lepton: A single electron or muon (like a single, distinct spark in the storm).
  2. High Jet Multiplicity: A huge number of particle sprays (the storm itself).
  3. Many "b-jets": Many of those sprays containing heavy bottom quarks (the gold bars).
  4. No Missing Energy: The "thief" didn't leave with any invisible loot.

To measure the size of this storm, they used a special tool called $M_J$ (the sum of the masses of large particle clusters). If new physics exists, this number should be very high, creating a "mountain" of data that doesn't fit the normal background hills.

The Method: The "Data-Driven" Detective Work

The hardest part of this experiment is knowing what "normal" looks like. The background noise comes from standard particle collisions (like top quark pairs) that can accidentally look like the signal.

Instead of relying entirely on computer simulations (which can sometimes be wrong about the "tails" of the distribution), the team used a data-driven approach:

1. Control Regions: They looked at areas of the data where they knew only background noise existed (like looking at a quiet street to understand the sound of traffic).
2. Calibration: They measured how the background behaved in these quiet areas and used that to predict what the background should look like in the "Signal Regions" (the busy streets where they hoped to find the new physics).
3. The Fit: They compared the actual data in the Signal Regions against their predictions.

The Results: The Silence of the Gluinos

After analyzing 138 units of data (an enormous amount of collision history collected between 2016 and 2018), the scientists found:

• No Surprise: The data matched the background predictions perfectly. There was no "mountain" of new physics.
• The Exclusion: Because they didn't see the signal, they could rule out certain possibilities. They concluded that if these specific gluinos exist, they must be heavier than 1,890 GeV (about 2,000 times heavier than a proton).
• The Takeaway: Any gluinos lighter than that have been "excluded" (ruled out) by this search.

Summary

This paper is a high-stakes game of "Where's Waldo?" in a massive crowd of particles. The team looked for a specific type of "thief" (a gluino) that leaves behind a massive pile of visible evidence (jets and bottom quarks) but no invisible loot. They checked every corner of the data, calibrated their search using real-world examples, and found nothing. Consequently, they declared that if these particles exist, they are too heavy to have been caught in this specific net. The search for lighter versions of these particles has come up empty.

Technical Summary

Technical Summary: Search for New Physics in Single-Lepton Events with High Jet Multiplicities at $\sqrt{s} = 13$ TeV

Problem and Motivation
The Standard Model (SM) successfully describes fundamental particles but fails to address theoretical challenges such as particle mass hierarchies and gauge coupling unification. Supersymmetry (SUSY) offers a potential solution, yet searches for R-parity conserving SUSY often rely on large missing transverse momentum ($p^{\text{miss}}_T$) signatures, which are absent in R-parity violating (RPV) scenarios. In RPV models, the lightest supersymmetric particle (LSP) can decay into SM particles, eliminating the $p^{\text{miss}}_T$ signature and requiring alternative search strategies. This paper addresses the search for RPV SUSY, specifically focusing on gluino pair production where gluinos decay into top ($t$), bottom ($b$), and strange ($s$) quarks via the $\lambda''_{323}$ coupling in a minimal flavor-violating framework. The analysis targets events with high jet multiplicities, high $b$-jet counts, and a single lepton, without requiring significant missing transverse momentum.

Methodology
The analysis utilizes proton-proton collision data collected by the CMS detector at the CERN LHC during 2016–2018, corresponding to an integrated luminosity of 138 fb$^{-1}$ at a center-of-mass energy of 13 TeV.

• Event Selection: Events are selected requiring exactly one isolated electron or muon ($N_{\text{lep}} = 1$), at least four small-radius jets ($R=0.4$), and a scalar sum of jet transverse momenta ($H_T$) greater than 1200 GeV. A key discriminant is the sum of large-radius jet masses ($M_J$), defined as the sum of masses of jets clustered with $R=1.2$ (including the selected lepton). A baseline requirement of $M_J > 500$ GeV is applied.
• Analysis Regions: The search is performed in bins of jet multiplicity ($N_{\text{jet}}$) and the number of $b$-tagged jets ($N_b$). Signal regions (SRs) are defined in high-$N_{\text{jet}}$ and high-$N_b$ bins (specifically $N_{\text{jet}} \ge 8$ and $N_b \ge 3$), while control regions (CRs) with lower multiplicities are used to constrain background normalizations.
• Background Estimation: The dominant backgrounds are top quark pair production ($t\bar{t}$), W+jets, and QCD multijet events. A data-driven approach is employed where background normalizations are determined via a simultaneous fit to the $M_J$ distribution in all bins.
  • $\kappa$ Factors: To correct for discrepancies between simulation and data in the $M_J$ shape, correction factors ($\kappa_1$ and $\kappa_2$) are derived from independent control samples for QCD multijet, W+jets, and $t\bar{t}$. These factors are constrained in a global fit using the large statistics of low-$N_b$ regions.
  • Normalization: The $t\bar{t}$ and QCD multijet normalizations are treated as free parameters constrained by CRs (including an $N_{\text{lep}}=0$ region with shifted $N_{\text{jet}}$ requirements). W+jets normalization is constrained by a global parameter across all bins.
• Systematic Uncertainties: Uncertainties include jet energy scale/resolution, $b$-tagging efficiency, lepton identification, pileup, and theoretical scale variations. The analysis specifically addresses potential dependencies of the $M_J$ shape on $N_b$ (e.g., from gluon splitting) by assigning separate uncertainties for each $N_b$ bin.

Key Contributions

• Novel Discriminator: The paper demonstrates the efficacy of the sum of large-radius jet masses ($M_J$) as a robust variable for distinguishing high-multiplicity signal events from SM backgrounds in the absence of $p^{\text{miss}}_T$.
• Data-Driven Background Modeling: A refined strategy is presented where backgrounds are normalized separately in each $N_b$ bin. This mitigates systematic uncertainties related to jet flavor composition and gluon splitting, which were limiting factors in previous analyses.
• Comprehensive Fit Framework: The use of a simultaneous binned maximum likelihood fit across $N_{\text{jet}}$, $N_b$, and $M_J$ bins allows for the simultaneous extraction of background normalizations and shape corrections ($\kappa$ factors), maximizing the use of control regions to constrain signal regions.

Results
The observed event yields in the signal regions are consistent with the background predictions. No significant excess of data over the background is observed in the $M_J$ distribution.

• Exclusion Limits: Based on the null result, upper limits on the production cross-section times branching ratio are set at 95% confidence level (CL).
• Mass Limits: For the specific simplified model of gluino pair production ($\tilde{g}\tilde{g} \to tbs \, tbs$), gluino masses below 1890 GeV are excluded at 95% CL. This result improves upon previous limits, including those from the ATLAS experiment (1830 GeV) and earlier CMS results.

Significance
This work provides a stringent test of R-parity violating supersymmetry in a regime characterized by high jet and $b$-jet multiplicities without missing transverse momentum. By excluding gluino masses up to 1890 GeV, the analysis significantly constrains the parameter space of minimal flavor-violating RPV SUSY models. The methodology, particularly the data-driven background estimation using $M_J$ templates and $N_b$-binned normalization, offers a robust framework for future searches for new physics in high-multiplicity final states where traditional missing energy signatures are not applicable. The results are consistent with the Standard Model and represent the most sensitive search to date for this specific gluino decay topology.