Testing lepton-flavor-violating decay of doubly charged Higgs bosons in type-II seesaw via photon fusion at the high-energy LHC

This paper investigates the potential of the 100 TeV high-energy LHC to probe lepton-flavor-violating decays of doubly charged Higgs bosons produced via photon fusion in the type-II seesaw model, demonstrating that an integrated luminosity of 3 ab$^{-1}$ could exclude triplet scalar masses up to approximately 1150 GeV under the inverted neutrino mass hierarchy.

The Gist

The Big Picture: Hunting for a "Ghostly" Particle

Imagine the Standard Model of physics as a completed puzzle, but with one tiny, missing piece: neutrinos. We know these tiny particles exist and have mass, but the original puzzle didn't explain how they got it.

This paper proposes a solution called the Type-II Seesaw. Think of this mechanism like a seesaw in a playground. On one side, you have the heavy, new particles (the "triplet scalars"). On the other side, you have the tiny neutrinos. The heavier the new particle is, the lighter the neutrino becomes. This paper focuses on hunting for the heavy side of that seesaw: a specific, exotic particle called the doubly charged Higgs boson (let's call it the "Double-Plus" particle).

The Strategy: A High-Speed "Photon Fusion" Game

Usually, scientists try to find new particles by smashing protons together like two cars crashing in a demolition derby (this is called Drell-Yan production). It's messy, loud, and full of debris (background noise).

This paper suggests a different, cleaner approach: Photon Fusion.

• The Analogy: Imagine two high-speed trains (protons) passing each other on parallel tracks without touching. As they zoom by, their magnetic fields (which act like invisible flashlights) flash at each other. These flashes are actually beams of light (photons).
• The Collision: If the flashes are bright enough, they can crash into each other and create new particles (the "Double-Plus" particles) right in the middle of the tracks, while the trains themselves keep rolling forward, completely intact.
• The Advantage: Because the trains don't crash, the "debris" is much cleaner. It's like finding a rare coin in a quiet library rather than a noisy construction site.

The Clue: The "Wrong" Flavor

Once these "Double-Plus" particles are created, they immediately decay (break apart). The scientists are looking for a very specific, rare breakup pattern:

• The Decay: The particle splits into two pairs of electrons and muons (types of electrons).
• The Twist: The paper focuses on Lepton-Flavor-Violating (LFV) decay. In the normal world, an electron stays an electron, and a muon stays a muon. But this exotic particle might mix them up, creating an electron and a muon together.
• The Rarity: This mixing is like finding a chameleon that suddenly changes color to match a background it shouldn't be able to see. It's extremely rare (happening less than 1% of the time in some scenarios), which makes it a perfect "smoking gun" to prove this new physics exists.

The Hunt: Looking in the Future (100 TeV)

The authors are planning this search for the High-Energy LHC, a future version of the Large Hadron Collider that will be much more powerful (100 TeV) than the current one.

1. The Setup: They simulate billions of these "train passing" events using powerful computers.
2. The Filters: They use a series of "sieves" to filter out the noise:
   • The Proton Check: Did the two trains (protons) survive and get caught by special detectors at the front of the line? If yes, it's a good candidate.
   • The Energy Check: Did the particles have the right amount of energy?
   • The Mass Check: When they put the pieces (the electron and muon) back together, do they add up to the weight of the "Double-Plus" particle?
3. The Result: By applying these filters, they found that even though the "mixing" decay is rare, a powerful machine with enough data could spot it.

The Findings: How Heavy Can It Be?

The paper calculates how heavy this new particle could be before we lose the ability to see it.

• Current Limits: Previous experiments have ruled out particles lighter than about 1,080 GeV (a unit of mass).
• New Limits: With this new "photon fusion" method at the 100 TeV collider, they could potentially rule out (or find) particles up to 1,150 GeV.
• The Catch: This works best if the universe follows a specific pattern of neutrino masses (called the "Inverted Hierarchy"). If the pattern is different, the signal is weaker, but the method still improves our search capabilities significantly.

Summary

In short, this paper says: "If we build a super-powerful collider and look for these new particles using a clean, 'photon-flash' method instead of a messy crash, we might be able to find a heavy, exotic particle that explains why neutrinos have mass. Even though the particle breaks apart in a very rare and weird way (mixing electron flavors), our new strategy gives us a better chance of spotting it than ever before."

Technical Summary

Technical Summary: Testing Lepton-Flavor-Violating Decay of Doubly Charged Higgs Bosons in Type-II Seesaw via Photon Fusion at the High-Energy LHC

Problem and Motivation
The observation of neutrino oscillations necessitates physics beyond the Standard Model (SM), specifically mechanisms to explain the origin and extreme smallness of neutrino masses. The type-II seesaw mechanism offers a theoretically elegant solution by introducing an $SU(2)_L$ triplet scalar field ($\Delta$), which predicts the existence of doubly charged scalar components ($\Delta^{\pm\pm}$). While traditional searches at the Large Hadron Collider (LHC) rely on Drell-Yan production, these are often limited by overwhelming QCD-dominated backgrounds. Furthermore, experimental constraints on the triplet vacuum expectation value ($v_\Delta$) and the mass hierarchy of neutrinos (Normal vs. Inverted) dictate specific decay patterns. In scenarios with a small $v_\Delta$ ($< 10^{-4}$ GeV), the dileptonic decay channels dominate. However, the lepton-flavor-violating (LFV) decay $\Delta^{\pm\pm} \to e^\pm \mu^\pm$ is predicted to have a very small branching ratio (approx. 0.5% for Normal Hierarchy and 2.6% for Inverted Hierarchy), making it difficult to probe with standard search strategies. This paper investigates the potential to probe these rare LFV decays using a complementary production mechanism: elastic photon fusion in ultraperipheral collisions (UPCs) at a future high-energy LHC (100 TeV).

Methodology
The study employs a comprehensive Monte Carlo simulation framework to analyze the signal process $pp \to p(\gamma\gamma \to \Delta^{++}\Delta^{--})p \to p(e^+\mu^+e^-\mu^-)p$.

• Production Mechanism: The signal is generated via elastic photon-photon fusion, where the colliding protons remain intact and are detectable by forward proton detectors (e.g., AFP, CT-PPS). The cross-section is calculated using the Equivalent Photon Approximation (EPA) with photon parton distribution functions ($\gamma$-PDFs).
• Model Parameters: The analysis assumes a degenerate mass spectrum for the triplet scalars and a small $v_\Delta$ to ensure the dominance of leptonic decays. The Yukawa couplings are constrained by best-fit neutrino oscillation parameters from the NuFIT program for both Normal (NH) and Inverted (IH) mass hierarchies.
• Simulation Chain: Events are generated at the parton level using MadGraph5_aMC@NLO (v3.5.6) with Type-II seesaw UFO libraries. Parton showering and hadronization are handled by Pythia-8.2, and detector effects are simulated via Delphes-3.5.0 (configured for ATLAS). The analysis is managed within the CheckMATE2 framework.
• Backgrounds: The study considers four primary SM backgrounds that mimic the four-lepton signature with intact protons: $\gamma\gamma \to \ell^+\ell^- Z/\gamma$, $\gamma\gamma \to W^+W^- Z/\gamma$, $\gamma\gamma \to W^+W^- jj$ (with jet misidentification), and $\gamma\gamma \to t\bar{t}$ (with $b$-jet misidentification).
• Search Strategy: A sequential cut-based analysis is applied:
  1. Pre-selection: Tagging of two intact forward protons.
  2. Lepton Selection: Requirement of two pairs of same-sign, different-flavor leptons ($e^\pm \mu^\pm$).
  3. Kinematic Cuts: Low missing transverse energy ($\not{E}_T < 180$ GeV) to suppress neutrino-rich backgrounds; high transverse momentum for subleading leptons ($p_T(\ell_2) > m_\Delta/8$); and a tight invariant mass window around the triplet mass $m_\Delta$ for the $e^\pm \mu^\pm$ pairs.

Key Contributions

• Novel Production Channel: The paper demonstrates the viability of using elastic photon fusion at a 100 TeV collider to probe doubly charged Higgs bosons, leveraging the clean signature of intact forward protons to suppress QCD backgrounds.
• LFV Sensitivity: It specifically targets the LFV decay $\Delta^{\pm\pm} \to e^\pm \mu^\pm$, a channel with low branching ratios that is often overlooked in favor of same-flavor dilepton searches.
• Mass Hierarchy Dependence: The study quantifies the sensitivity difference between Normal and Inverted neutrino mass hierarchies, showing how the specific branching ratios dictated by neutrino oscillation data impact the exclusion limits.
• Comprehensive Background Analysis: The work provides a detailed cut-flow analysis for four distinct SM background processes in the photon-fusion environment at 100 TeV.

Results

• Signal vs. Background: The kinematic distributions (lepton number, $\not{E}_T$, subleading lepton $p_T$, and invariant mass) show that the signal can be effectively separated from backgrounds, particularly through the invariant mass cut which reduces irreducible backgrounds to negligible levels.
• Exclusion Bounds:
  • At a center-of-mass energy of 100 TeV with an integrated luminosity of 36.1 fb$^{-1}$, the study can probe branching ratios of $\sim 10\%$ for a triplet mass of 1 TeV.
  • With higher luminosities (100 fb$^{-1}$ and 3 ab$^{-1}$), the sensitivity improves to probe branching ratios of $\sim 1\%$ in the TeV mass region.
  • Inverted Hierarchy (IH): Assuming the IH scenario where $Br(\Delta^{\pm\pm} \to e^\pm \mu^\pm) \approx 2.6\%$, the 95% C.L. mass exclusion limit reaches approximately 1150 GeV with 3 ab$^{-1}$ of data. This surpasses the current ATLAS limit of 1080 GeV derived from 13 TeV Run-2 data.
  • Normal Hierarchy (NH): Due to the smaller branching ratio ($\approx 0.5\%$), the mass exclusion limit is significantly lower, reaching only a few hundred GeV.
• Comparison with Drell-Yan: For triplet masses below $\sim 300$ GeV, Drell-Yan production offers better sensitivity. However, for masses in the TeV scale, the photon fusion channel demonstrates superior probing capability, allowing for the exclusion of much smaller branching ratios.

Significance and Claims
The paper claims that the proposed strategy utilizing elastic photon fusion at a 100 TeV LHC offers a powerful and complementary tool to traditional Drell-Yan searches for type-II seesaw scalars. Specifically, it asserts that this method can extend the exclusion bounds on the triplet scalar mass to approximately 1150 GeV under the assumption of an inverted neutrino mass hierarchy and an integrated luminosity of 3 ab$^{-1}$. The authors emphasize that this approach effectively suppresses overwhelming QCD backgrounds, enabling the exploration of rare LFV decay channels that are otherwise difficult to access. The study notes that pile-up effects were not included in the current analysis and are reserved for future work, suggesting that time-of-flight detectors could be employed in subsequent studies to further mitigate combinatorial backgrounds.