Search for exotic leptons in final states with two or three leptons and fat-jets at 13 TeV LHC

This paper proposes and evaluates a search strategy for heavy exotic leptons in large gauge multiplets at the 13 TeV LHC using final states with two or three leptons and fat-jets, demonstrating that such an approach can achieve $5\sigma$ discovery reach for masses up to approximately 2 TeV with the full High-Luminosity LHC dataset.

The Gist

Imagine the universe is a giant, bustling construction site. For decades, we've had a very successful blueprint called the Standard Model that explains how the basic building blocks (like electrons and quarks) and the forces holding them together work. It's like a perfect instruction manual for a house.

But, there are some cracks in the foundation. We know things like "dark matter" exist, and we don't know why neutrinos have mass. The manual is missing a few pages. Physicists suspect that hidden behind the walls of our known universe are Exotic Leptons—giant, heavy, strange cousins of the electrons we know.

This paper is a detective story about how to find these giants at the Large Hadron Collider (LHC), the world's biggest particle accelerator.

The Problem: The "Invisible" Giants

These exotic leptons are incredibly heavy (heavier than a thousand protons). If they exist, they are created in collisions at the LHC, but they don't stick around. They immediately decay (fall apart) into lighter, familiar particles: regular leptons (like electrons) and bosons (force carriers like W and Z bosons).

The Challenge:
Usually, when a heavy particle decays into two things, those two things fly apart in different directions, like two skaters pushing off each other. We can see both of them easily.

But because these exotic leptons are so heavy, the debris they leave behind is moving at nearly the speed of light. It's like a supersonic jet breaking the sound barrier. The particles from the decay are so tightly packed together that they don't look like two separate skaters anymore; they look like a single, blurry blob.

In the language of particle physics, instead of seeing two distinct "jets" of particles, we see one giant, fat "Fat-Jet."

The Solution: The "Fat-Jet" Search Strategy

The authors of this paper realized that previous searches were looking for the "two separate skaters" pattern. They were missing the "blurry blob" pattern.

They proposed a new search strategy: Look for events with 2 or 3 regular leptons (the skaters) and 1 or 2 Fat-Jets (the blurry blobs).

Think of it like this:

• Old Search: Looking for a crime scene with two distinct footprints.
• New Search: Looking for a crime scene with two footprints and a giant, smeared tire track.

This new signature is much "cleaner." The background noise (random accidents that happen in the LHC) rarely produces these specific combinations of leptons and fat-jets. It's like finding a needle in a haystack, but the needle is glowing neon blue, while the hay is all gray.

The Nine Suspects (The Models)

The researchers didn't just look for one type of exotic lepton. They considered nine different "suspects" based on different theoretical models.

• Some are Triplets (groups of 3).
• Some are Quadruplets (groups of 4).
• Some are Quintuplets (groups of 5).

Each group has different members with different electric charges (some are neutral, some are single-charged, some are double-charged). The paper simulates what would happen if any of these nine groups were the ones hiding in the universe.

The Detective Work (The Simulation)

Since they can't actually go to the LHC and wait for the particles to appear (yet), they used powerful computers to simulate millions of collisions.

1. The Setup: They programmed the computer to act like the LHC, smashing protons together at 13 TeV (a huge amount of energy).
2. The Filter: They applied a set of rules (cuts) to filter out the boring background noise. For example: "Only keep events where the particles have very high energy" or "Only keep events where the fat-jet looks like it came from a W or Z boson."
3. The Reconstruction: They tried to piece the puzzle back together. If they found the right combination of leptons and fat-jets, they could calculate the mass of the original exotic lepton, just like a detective reconstructing the speed of a car from skid marks.

The Results: How Heavy Can They Be?

The paper answers the big question: "If these particles exist, how heavy can they be before we would have already seen them?"

• For the current amount of data (300 "units" of data): They could find these particles if they weigh up to about 1,000 to 1,650 GeV (roughly 1,000 to 1,650 times the mass of a proton).
• For the future amount of data (3,000 "units"): With more data, they could find them if they weigh up to 1,345 to 2,020 GeV.

The Bottom Line

This paper is a roadmap. It tells the experimental teams at the LHC (like ATLAS and CMS): "Stop looking for the two separate skaters. Start looking for the blurry tire tracks (Fat-Jets) alongside the skaters. If you do this, you might just catch a glimpse of these giant, exotic leptons that could solve the biggest mysteries of our universe."

It's a fresh pair of glasses for looking at the universe, potentially revealing the missing pieces of the cosmic puzzle.

Technical Summary

1. Problem Statement

The Standard Model (SM) fails to explain several theoretical and experimental anomalies, such as neutrino masses, the strong CP problem, and dark matter. Many Beyond the Standard Model (BSM) scenarios, particularly those addressing neutrino mass generation (e.g., see-saw mechanisms), predict the existence of exotic leptons residing in large $SU(2)_L$ gauge multiplets (triplets, quadruplets, or quintuplets).

Challenges in Current Searches:

• Mass Scale: These exotic leptons are expected to be heavy (TeV scale).
• Backgrounds: Traditional searches relying on multileptons, missing transverse momentum ($p_T^{miss}$), and resolved jets suffer from large SM backgrounds.
• Kinematic Reconstruction: In events with multiple leptons, jets, and $p_T^{miss}$, reconstructing the mass of the parent exotic particle is often impossible due to unidentifiable origins of particles and kinematic ambiguities.
• Boosted Regime: For TeV-scale particles, decay products (SM bosons like $W, Z, h$) are highly boosted. Consequently, hadronically decaying bosons produce collimated jets that merge into a single "fat-jet" rather than two resolved jets. Standard searches often miss these signatures or misclassify them.

2. Methodology

A. Simplified Models

The authors adopt a "bottom-up" approach, considering nine simplified models where the SM is extended by a single type of new leptonic multiplet at a time. They analyze:

• Triplets ($3_F$): With hypercharges $Y=1, 2$.
• Quadruplets ($4_F$): With hypercharges $Y=1/2, 3/2, 5/2$.
• Quintuplets ($5_F$): With hypercharges $Y=0, 1, 2, 3$.

These models include components ranging from singly-charged ($\chi^\pm$) to quintuply-charged ($\chi^{5\pm}$) leptons. The mass splittings between charged components are generated radiatively (via electroweak gauge bosons) and are small ($\sim$ MeV to GeV), leading to "heavy state transitions" where highly charged leptons decay into lighter ones plus soft pions/leptons.

B. Production and Decay

• Production: Dominated by Drell-Yan-like pair production ($q\bar{q} \to \gamma^*/Z^* \to \chi\chi$) and associated production. Photon-photon fusion becomes significant for multi-charged states.
• Decay Chain:
  • Multi-charged leptons ($|Q| \ge 3$) decay via Category-II (radiative mass splitting) into the next lighter charged state plus soft particles ($\pi, K, \ell\nu$). These soft particles are often undetectable.
  • The cascade effectively enhances the production cross-section of the doubly-charged ($\chi^{\pm\pm}$) and singly-charged ($\chi^\pm$) states.
  • Dominant Decays: $\chi^{\pm\pm} \to \ell^\pm W^\pm$ and $\chi^\pm \to \ell^\pm Z, \ell^\pm h, \nu W^\pm$.
• Final State Signatures: The decay of boosted $W/Z/h$ bosons results in fat-jets. The paper focuses on final states with 2 or 3 leptons and 1 or 2 fat-jets.

C. Simulation and Analysis Strategy

• Tools: Events generated using MadGraph5_aMC@NLO with LUXqed17 PDFs. Detector simulation performed with Delphes.
• Object Reconstruction:
  • Fat-jets: Reconstructed with radius $R=0.8$ using the anti-$k_T$ algorithm, pruned to remove soft QCD radiation, and tagged using N-subjettiness ($\tau_{21}$) to distinguish $W/Z/h$ decays from QCD jets.
  • Leptons: Electrons and muons with $p_T > 10$ GeV, isolated.
• Signal Regions (SRs): The analysis defines six mutually exclusive signal regions to maximize sensitivity:
  1. 3L-1J: 3 leptons + 1 fat-jet.
  2. SSD-1J: Same-sign dileptons + 1 fat-jet.
  3. OSD-1J: Opposite-sign dileptons + 1 fat-jet + high $p_T^{miss}$.
  4. OSD-2J: Opposite-sign dileptons + 2 fat-jets.
  5. 3L-2j & SSD-2j: Secondary regions using ordinary jets (for cases where jets are not collimated enough to form fat-jets).

D. Background Suppression

• Irreducible Backgrounds: Diboson ($VV$), triboson ($VVV$),$t\bar{t}V$, and Higgsstrahlung.
• Reducible Backgrounds: $Z/\gamma^* + \text{jets}$ and $t\bar{t} + \text{jets}$ (where jets fake leptons).
• Selection Cuts:
  • High $p_T$ cuts on leptons ($>100-300$ GeV) and fat-jets ($>200$ GeV).
  • Invariant mass windows for fat-jets ($70-140$ GeV) to tag $W/Z/h$.
  • $\tau_{21} < 0.5$ to select 2-prong jets.
  • Kinematic variables like $L_T$ (scalar sum of lepton $p_T$) and $M_{eff}$ (effective mass) to separate signal from background.

3. Key Contributions

1. Novel Search Strategy: Proposes a search strategy specifically targeting fat-jets in association with multileptons. This exploits the boosted nature of TeV-scale decays, offering a cleaner signature with lower SM backgrounds compared to traditional resolved-jet searches.
2. Kinematic Reconstruction: Demonstrates that these final states allow for the kinematic reconstruction of the exotic lepton mass. For example, in the OSD-2J channel, the invariant mass of the lepton-fat-jet system peaks at the exotic lepton mass, a feature often lost in $p_T^{miss}$-heavy searches.
3. Comprehensive Coverage: Systematically studies nine distinct simplified models (covering triplets, quadruplets, and quintuplets with various hypercharges), providing a unified framework applicable to a wide range of BSM scenarios.
4. Statistical Rigor: Uses a Profile Likelihood Ratio method with 60 independent search bins (10 bins per SR) to calculate discovery reach, accounting for a 20% systematic uncertainty on backgrounds.

4. Results

Discovery Reach (5$\sigma$):
The study estimates the mass reach for exotic leptons at the 13 TeV LHC for two integrated luminosities: 300 fb$^{-1}$ (Run 3) and 3000 fb$^{-1}$ (High-Luminosity LHC).

• 300 fb$^{-1}$: The 5$\sigma$ discovery reach spans from 985 GeV to 1650 GeV, depending on the specific model and branching ratios.
  • Example: Model-$3_F^1$ reaches ~985–1140 GeV.
  • Example: Model-$5_F^3$ reaches ~1625–1650 GeV.
• 3000 fb$^{-1}$: The reach extends to 1345 GeV to 2020 GeV.
  • Example: Model-$3_F^1$ reaches ~1345–1485 GeV.
  • Example: Model-$5_F^3$ reaches ~1985–2020 GeV.

Key Observations:

• The OSD-2J (Opposite-sign dilepton + 2 fat-jets) channel is the most sensitive, particularly for models dominated by doubly-charged pair production, due to low backgrounds and clean mass reconstruction.
• Models with higher $SU(2)_L$ representations (e.g., Quintuplets) generally offer better discovery reach due to larger effective production cross-sections (enhanced by cascade decays from higher-charged states).
• The sensitivity is largely independent of the branching ratio $\text{BR}(\chi^\pm \to \nu W^\pm)$ for models dominated by doubly-charged pair production (like $5_F^3$), but shows moderate dependence for models where associated production is significant.

5. Significance

• Complementarity: This work provides a crucial complementary search strategy to existing LHC limits. While current searches focus on multileptons + $p_T^{miss}$ + resolved jets, this approach targets the fat-jet regime, which is theoretically motivated for heavy TeV-scale particles but often overlooked.
• Model Independence: By using simplified models, the results are applicable to a broad class of BSM theories (including those addressing neutrino masses and dark matter) without being tied to a specific UV-complete theory.
• Feasibility: The study proves that with the High-Luminosity LHC (3000 fb$^{-1}$), the LHC can probe exotic leptons up to ~2 TeV, significantly extending the current exclusion limits (which are typically a few hundred GeV).
• Theoretical Consistency: The paper also addresses the Renormalization Group (RG) flow of these models, noting that some high-representation models (like $5_F^3$) develop Landau poles at relatively low scales ($\sim 10^6$ GeV), implying these models must be embedded in a larger theory well below the Planck scale.

In conclusion, the paper establishes that searching for multilepton + fat-jet signatures is a powerful and necessary strategy to discover heavy exotic leptons at the LHC, potentially uncovering new physics up to the 2 TeV scale.