Search for charginos and neutralinos with $B-L$ $R$-parity violating decays in $\sqrt{s}=13$ TeV and $13.6$ TeV $pp$ collisions with the ATLAS detector

The ATLAS experiment analyzed 140 fb$^{-1}$ of 13 TeV and 56 fb$^{-1}$ of 13.6 TeV proton-proton collision data to search for charginos and neutralinos decaying via $B-L$ $R$-parity-violating couplings into Higgs bosons and leptons, finding no evidence of new physics and setting 95% confidence level exclusion limits on chargino and neutralino masses up to 1100 GeV.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful "particle smasher." It smashes tiny protons together at nearly the speed of light, creating a chaotic explosion of energy. Usually, this energy turns into particles we know and understand, like electrons and quarks. But physicists suspect that hidden within this chaos are "superpartners"—ghostly, heavier twins of the particles we know, predicted by a theory called Supersymmetry (SUSY).

This paper is a report from the ATLAS experiment, a giant detector at the LHC that acts like a high-speed, 360-degree camera trying to catch a glimpse of these ghostly twins. Specifically, the team was looking for two types of superpartners: charginos and neutralinos.

The Mystery: The "R-Parity" Rule

In many versions of this theory, there's a rule called R-parity. Think of R-parity like a strict bouncer at a club.

• Normal particles (like electrons) have an "R-value" of +1.
• Superpartners have an "R-value" of -1.
• The Rule: If R-parity is conserved, superpartners must be created in pairs, and they can never decay into just normal particles. The lightest superpartner would be stable and invisible, escaping the detector like a ghost.

However, this paper explores a different scenario: R-parity violation (RPV). Imagine the bouncer gets tired and lets the superpartners slip out and decay directly into normal particles. In this specific model, the charginos and neutralinos are predicted to decay into a Higgs boson (a famous particle that gives others mass) and a lepton (an electron, muon, or tau).

The Hunt: Finding the "Higgs Signature"

The ATLAS team set up a very specific trap to catch these decays. They knew that if a chargino or neutralino decayed into a Higgs boson, that Higgs boson would almost immediately split into two bottom quarks (which manifest as "jets" of particles in the detector).

So, the search strategy was like looking for a specific pattern in a messy room:

1. The Leptons: They looked for events with one or two high-energy electrons or muons (the "leptons" from the decay).
2. The Higgs Twins: They looked for at least three "jets" that were tagged as coming from bottom quarks. Since the signal involves two superpartners decaying, they expected to see two Higgs bosons, which means four bottom-quark jets.
3. The Missing Piece: In some scenarios, a neutrino (an invisible particle) is also produced, carrying away some energy. The detector measures this as "missing transverse momentum."

The Data: A Massive Library of Collisions

The team analyzed a massive library of data:

• Timeframe: Collisions from 2015 to 2023.
• Energy: Two different energy levels (13 TeV and 13.6 TeV).
• Volume: They looked at 196 "inverse femtobarns" of data. To visualize this, imagine taking a snapshot of every single collision that happened during those years. It's a dataset so large it would take a supercomputer years to process without the specialized tools ATLAS built.

The Results: The Ghosts Remain Hidden

After sifting through millions of events, the team found no evidence of these charginos or neutralinos.

• The Comparison: They compared what they saw in the data against what the Standard Model (our current best theory of physics) predicts. The data matched the Standard Model predictions perfectly. It's like looking for a specific type of alien in a forest and finding only deer, trees, and birds—exactly what you'd expect to see.
• The Exclusion: Because they didn't find the particles, they could set a "fence" around where these particles cannot be. They concluded that if these charginos and neutralinos exist and decay this way, they must be heavier than 1,100 GeV (roughly 1,100 times the mass of a proton). If they were lighter than that, the ATLAS detector would have seen them by now.

The Conclusion

The paper concludes that for the specific scenario where these superpartners decay into Higgs bosons and leptons, the "light" versions (between 150 and 1,100 GeV) do not exist.

In simple terms: The ATLAS team looked very hard for a specific type of heavy, ghostly particle that breaks the usual rules of physics. They found nothing but the expected background noise. While this doesn't prove these particles don't exist at all, it tells us they are either much heavier than we thought, or they don't decay in the way this specific theory predicted. The search for the "new physics" continues, but this particular door remains closed for now.

Technical Summary

Technical Summary: Search for Charginos and Neutralinos with $B-L$ $R$-parity Violating Decays in $\sqrt{s}=13$ TeV and 13.6 TeV $pp$ Collisions with the ATLAS Detector

Problem and Motivation
Supersymmetry (SUSY) offers a solution to the hierarchy problem within the Standard Model (SM), but the full parameter space allows for baryon ($B$) and lepton ($L$) number violation. While $R$-parity conservation is often enforced to prevent rapid proton decay, phenomenologically viable $R$-parity violating (RPV) theories exist. Specifically, the $B-L$ Minimal Supersymmetric Standard Model introduces a $U(1)_{B-L}$ gauge symmetry and right-handed sterile neutrinos. In this framework, the spontaneous breaking of $U(1)_{B-L}$ violates lepton number, allowing superpartners to decay directly into SM particles.

This paper targets a specific scenario involving a wino-like lightest supersymmetric particle (LSP) and next-to-lightest supersymmetric particle (NLSP). In this model, the chargino ($\tilde{\chi}^\pm_1$) and neutralino ($\tilde{\chi}^0_1$) are nearly mass-degenerate (splitting $< 200$ MeV), which suppresses standard $R$-parity conserving decays. Instead, these particles undergo prompt RPV decays primarily into a Higgs boson ($h$) and a lepton ($\ell$) or neutrino ($\nu$): $\tilde{\chi}^\pm_1 \to h\ell^\pm$ and $\tilde{\chi}^0_1 \to h\nu$. Previous ATLAS searches have targeted RPV decays into $Z$ bosons or top squarks; this analysis provides complementary sensitivity by focusing on the Higgs boson decay channel ($h \to b\bar{b}$), a region of phase space previously less explored for these specific electroweakino production modes.

Methodology
The analysis utilizes proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider (LHC) between 2015 and 2023. The dataset comprises an integrated luminosity of 196 fb$^{-1}$, split between 140 fb$^{-1}$ at $\sqrt{s}=13$ TeV (Run 2) and 56 fb$^{-1}$ at $\sqrt{s}=13.6$ TeV (Run 3).

• Signal Topology: The search targets electroweak pair production ($\tilde{\chi}^\pm_1 \tilde{\chi}^\mp_1$) and associated production ($\tilde{\chi}^\pm_1 \tilde{\chi}^0_1$).
  • The $\tilde{\chi}^\pm_1 \tilde{\chi}^\mp_1$ channel results in a final state with two opposite-sign leptons and two Higgs bosons (four $b$-jets).
  • The $\tilde{\chi}^\pm_1 \tilde{\chi}^0_1$ channel results in one lepton, two Higgs bosons (four $b$-jets), and missing transverse momentum ($E_T^{\text{miss}}$) from the neutrino.
• Event Selection: Events are required to have at least four jets, with at least three identified as $b$-tagged jets using the GN2 algorithm. Lepton selection requires high-$p_T$ electrons or muons ($p_T > 40$ GeV).
  • Two-Lepton Region (SR2$\ell$): Requires two opposite-sign leptons. The analysis reconstructs two Higgs boson candidates and two chargino candidates. The variable $A_C$ (mass asymmetry between the two reconstructed chargino candidates) is used to suppress background.
  • One-Lepton Region (SR1$\ell$): Requires one lepton and significant $E_T^{\text{miss}}$. The neutralino mass is reconstructed via transverse mass ($m_{N,T}$) using a Higgs candidate and $E_T^{\text{miss}}$.
• Background Estimation: The dominant background is top-quark pair production ($t\bar{t}$), particularly $t\bar{t}$ with heavy-flavor jets ($t\bar{t}+b$). Background normalizations are derived from data using control regions (CRs) that invert specific signal selection criteria (e.g., Higgs mass windows or kinematic variables). Validation regions (VRs) confirm the extrapolation of background models.
• Statistical Analysis: A profile-likelihood fit is performed using the pyhf framework. Systematic uncertainties (experimental, theoretical, and MC statistical) are treated as nuisance parameters. Limits are set at 95% confidence level (CL) using the $CL_s$ prescription.

Key Contributions

1. First Constraints on Higgs-Channel RPV: This work presents the first search constraints on chargino and neutralino production where the dominant decay mode is into a Higgs boson and a lepton/neutrino within the $B-L$ MSSM framework.
2. Advanced $b$-tagging: The analysis leverages the GN2 transformer-based $b$-tagging algorithm, which offers improved background rejection compared to previous algorithms, crucial for isolating the $h \to b\bar{b}$ signal.
3. Combined Run 2 and Run 3 Data: The analysis integrates data from both LHC Run 2 (13 TeV) and Run 3 (13.6 TeV), utilizing the increased luminosity and energy to probe higher mass ranges.
4. Flavor-Specific Limits: Beyond the standard assumption of equal branching fractions to all lepton flavors, the analysis derives specific limits for pure-electron, pure-muon, and pure-tau scenarios, demonstrating the impact of lepton reconstruction efficiency on sensitivity.

Results
The observed data are consistent with Standard Model predictions across all signal regions. No significant excess of events was observed.

• Model-Independent Limits: Upper limits were set on the visible cross-section for beyond-the-SM processes in four "discovery regions" (SR2$\ell$, SR2$\ell$700, SR1$\ell$, SR1$\ell$600).
• Model-Dependent Exclusions:
  • For the scenario where charginos and neutralinos decay with 100% branching fraction to $h\ell$ and $h\nu$ respectively, and assuming equal branching fractions to electron, muon, and tau flavors, charginos and neutralinos with masses between 150 GeV and 1100 GeV are excluded at 95% CL.
  • For the pure-electron hypothesis, masses between 150 GeV and 1225 GeV are excluded.
  • For the pure-muon hypothesis, masses between 150 GeV and 1150 GeV are excluded.
  • For the pure-tau hypothesis, masses between 450 GeV and 750 GeV are excluded. The lower sensitivity for the tau channel is attributed to the difficulty in reconstructing hadronically decaying taus and the energy loss in leptonic tau decays, which often results in daughter leptons falling below the $p_T$ threshold.

Significance
The paper claims that these results provide the first constraints on the simplified signal model for charginos and neutralinos decaying via Higgs bosons in the context of $B-L$ $R$-parity violation. The exclusion of masses up to 1100–1225 GeV (depending on the flavor hypothesis) significantly extends the reach of previous searches that targeted $Z$-boson decays (which excluded masses up to ~975 GeV). The analysis demonstrates that the Higgs decay channel, despite the challenging $b$-jet multiplicity and background environment, offers a powerful and complementary probe for electroweakino production in RPV scenarios. The results are interpreted as limits on model-independent cross-sections and specific Minimal Supersymmetric Standard Model parameters, contributing to the broader effort to constrain the SUSY parameter space.