Novel probes for electron-muon flavor violation from exotic Higgs decays

This paper proposes two novel multilepton signatures arising from exotic Higgs decays into a light pseudoscalar that subsequently decays into an electron-muon pair, demonstrating that collider-based searches in the Type-III Two-Higgs-doublet model can provide stronger constraints on electron-muon flavor violation than current low-energy precision measurements.

The Gist

Imagine the Higgs boson as a massive, shy celebrity who usually only interacts with a very specific, predictable group of fans (the Standard Model particles). For a decade, scientists have been watching this celebrity to see if they ever break character. The big question is: Does the Higgs strictly follow the rules, or does it have a secret life involving "forbidden" interactions?

This paper is about hunting for one specific type of forbidden interaction: Lepton Flavor Violation (LFV). In the normal world of particle physics, an electron is an electron and a muon is a muon; they never swap identities. But if the Higgs boson is part of a "secret society" (a theoretical model called the Type-III Two-Higgs-Doublet Model), it might be able to decay into a pair of particles that shouldn't be together, like an electron and a muon.

Here is the story of their hunt, explained through simple analogies:

1. The Setup: A Secret Door and a Light Ghost

The authors propose a scenario where the Higgs boson (the 125-GeV particle we know) has a secret exit door. Instead of decaying into normal particles, it sneaks out a "light ghost" called a pseudoscalar (let's call it A).

• The Analogy: Imagine the Higgs is a heavy bouncer at a club. Usually, he only lets out standard guests. But in this theory, he has a secret tunnel that lets out a lightweight, invisible ghost (Particle A).
• The Twist: This ghost isn't stable. It immediately pops into two particles: an electron and a muon. Since electrons and muons are supposed to be different "flavors" (like different flavors of ice cream that never mix), seeing them born together from a single Higgs is a smoking gun for new physics.

2. The Two New Clues (Signatures)

The paper suggests looking for this "ghost" in two specific ways, which create very clean, easy-to-spot patterns in the data from the Large Hadron Collider (LHC).

Clue A: The "Higgs + Z" Dance ($h \to AZ$)

Sometimes, the Higgs doesn't just release the ghost; it releases the ghost and a Z boson (another heavy particle) at the same time.

• The Scene: The Higgs splits into a Z boson and the ghost (A). The Z boson decays into a normal pair of electrons or muons. The ghost (A) decays into the forbidden electron-muon pair.
• The Result: You see four particles flying out: two normal ones and the forbidden electron-muon pair.
• The Detective Work: The authors realized that if you look at the "weight" (invariant mass) of these four particles, it will perfectly match the Higgs boson's weight. Furthermore, the forbidden electron-muon pair will have a specific weight matching the ghost (A). By filtering for these exact weights, they can ignore almost all the background noise (the "crowd" of normal particles).

Clue B: The "Double Ghost" Party ($h \to AA$)

Sometimes, the Higgs is so generous it releases two ghosts at once.

• The Scene: The Higgs splits into two ghosts (A and A). Each ghost immediately turns into an electron-muon pair.
• The Result: You see four particles: two electrons and two muons.
• The Detective Work: In normal physics, if you see two pairs of particles, they usually look identical. But here, because they come from the forbidden decay, the pairs might look slightly different. The authors proposed a new way to search for this: looking for a very specific pattern where the background noise is almost non-existent. It's like finding a needle in a haystack where the haystack has been magically removed.

3. The Competition: Low-Energy vs. High-Energy

Scientists have been looking for these forbidden electron-muon swaps in two ways:

1. Low-Energy: Watching muons decay in atoms over a long time (like watching a slow leak in a pipe). This is very sensitive but assumes no new heavy particles exist.
2. High-Energy (The LHC): Smashing particles together to create the Higgs directly.

The Paper's Finding:
The authors ran the numbers and found that their new "High-Energy" search methods are actually stronger than the current "Low-Energy" limits in certain scenarios.

• The Metaphor: Imagine trying to find a thief. The "Low-Energy" method is like checking the security logs of a quiet neighborhood. It's good, but it assumes the thief doesn't have a getaway car. The "High-Energy" method is like setting up a roadblock at the highway exit. The authors found that by looking for the specific "ghost" patterns (the four-lepton signatures), they can catch the thief even if they have a getaway car (new heavy particles).

4. The Conclusion: A Better Net

The paper concludes that by using these specific "optimized" searches—focusing on the exact weights of the particles and the specific combinations of electrons and muons—the Large Hadron Collider can set much tighter rules on how often the Higgs can break the rules.

• The Impact: Their new method improves the sensitivity by a factor of two for the "Higgs + Z" search and by a factor of ten for the "Double Ghost" search.
• The Takeaway: Even though we haven't found this new physics yet, the authors have handed the experimentalists a much sharper net. If the Higgs is hiding a secret life involving electron-muon swaps, these new searches are the best way to catch it.

In short, this paper is a blueprint for how to spot a very specific, rare, and forbidden party crash at the world's biggest particle collider, using the unique "footprints" left behind by a light, invisible ghost.

Technical Summary

Technical Summary: Novel Probes for Electron-Muon Flavor Violation from Exotic Higgs Decays

Problem Statement
The discovery of the Higgs boson has ushered in an era of precision measurements aimed at identifying deviations from the Standard Model (SM), particularly in the Higgs sector. While the SM strictly forbids Lepton Flavor Violation (LFV), many Beyond the Standard Model (BSM) theories predict such processes. Current experimental efforts at the LHC (ATLAS and CMS) have searched for direct two-body LFV Higgs decays (e.g., $h \to \mu^\pm e^\mp$), yielding null results and setting upper limits on branching ratios at the $O(10^{-5})$ level. However, these collider bounds are often interpreted within Effective Field Theory (EFT) frameworks that assume no new degrees of freedom exist near the electroweak scale. This assumption is not generally valid; the presence of new particles at the electroweak scale can significantly alter both low-energy and high-energy observables. Furthermore, low-energy precision measurements (such as $\mu \to e\gamma$ and $\mu \to e$ conversion) currently provide constraints orders of magnitude stronger than direct collider searches for the $e\mu$ channel. The challenge is to identify a consistent UV-complete framework where collider-based searches for exotic Higgs decays can provide competitive or complementary constraints to low-energy precision experiments.

Methodology
The authors investigate LFV within the framework of the Type-III Two-Higgs-Doublet Model (2HDM). In this model, the second Higgs doublet ($\Phi_2$) couples to fermions without imposing discrete symmetries for flavor conservation, allowing for tree-level LFV Yukawa couplings ($Y_{e\mu}, Y_{\mu e}$). The study focuses on a specific scenario where a light CP-odd pseudoscalar ($A$) exists with a mass $m_A \lesssim 100$ GeV, while the heavy CP-even Higgs ($H$) and charged Higgs ($H^\pm$) are significantly heavier ($m_H, m_{H^\pm} \gtrsim 300$ GeV).

The analysis proceeds through the following steps:

1. Model Constraints: The authors derive constraints from low-energy LFV observables ($\mu \to e\gamma$ and $\mu \to e$ conversion in nuclei) and electroweak precision tests (specifically the $T$ parameter). They calculate one-loop and two-loop (including Barr-Zee diagrams) contributions to the dipole operators and effective vector/scalar currents.
2. Collider Constraints (Existing): They reinterpret existing LHC searches. This includes CMS searches for resonant $H \to e^\pm \mu^\mp$ production and CMS multilepton analyses (specifically the $4\ell G$ and $4\ell H$ signal regions) targeting pair production of $H$ and $A$ followed by $H \to AZ$ and $A \to e^\pm \mu^\mp$.
3. Novel Signature Proposals: The authors propose two novel decay channels for the SM-like Higgs boson ($h$):
   • $h \to AZ^{(*)} \to e^\pm \mu^\mp \ell^+ \ell^-$ (where $\ell$ is $e$ or $\mu$).
   • $h \to AA \to 2e^\pm 2\mu^\mp$.
4. Simulation and Recasting: Using Feynrules, MadGraph5, Pythia8, and Delphes3, the authors simulate signal events and recast existing analyses (CMS multilepton and ATLAS $h \to AA$ searches). They define "optimized" signal regions that exploit specific kinematic features of the proposed decays, such as the invariant mass of the four-lepton system peaking at $m_h$ and the invariant mass of the opposite-sign different-flavor (OSDF) pair peaking at $m_A$.

Key Contributions

• Identification of Novel Signatures: The paper identifies that in the Type-III 2HDM with a light pseudoscalar, the SM-like Higgs can decay into multilepton final states ($4\ell$) via $h \to AZ$ and $h \to AA$, mediated by LFV couplings. These signatures are distinct from standard flavor-conserving exotic decays.
• Kinematic Optimization: The authors demonstrate that applying specific kinematic cuts—requiring the four-lepton invariant mass ($m_{4\ell}$) to be within $\pm 2.5$ GeV of $m_h$ and the OSDF pair invariant mass ($m_{e^\pm \mu^\mp}$) to be within $\pm 2.5$ GeV of $m_A$—drastically reduces Standard Model backgrounds (such as $t\bar{t}Z$ and multiboson production) to negligible levels.
• Systematic Recasting: The study provides a systematic recasting of current CMS and ATLAS data to constrain these specific LFV channels, rather than relying solely on generic EFT interpretations.

Results

• Low-Energy vs. Collider: The analysis confirms that low-energy constraints ($\mu \to e\gamma$) remain the most stringent limits on the product of the mixing angle squared and the LFV Yukawa couplings ($s_\alpha^2 \sqrt{Y_{e\mu}^2 + Y_{\mu e}^2}$), particularly when the $h \to AA$ channel is closed.
• Impact of Optimized Cuts:
  • For the $h \to AZ^{(*)}$ channel, the optimized signal region improves the bound on the mixing angle $s_\alpha$ by approximately a factor of two compared to the standard CMS multilepton analysis.
  • For the $h \to AA$ channel, the optimized search (focusing on same-sign lepton pairs to suppress backgrounds) improves the upper limit on the branching ratio $\text{BR}(h \to AA)$ by an order of magnitude compared to the recast ATLAS $h \to AA$ analysis.
• Parameter Space: The study identifies regions of parameter space where the light pseudoscalar is viable. For instance, if $A$ has a branching ratio of 50% to $e^\pm \mu^\mp$, the CMS multilepton constraints push the heavy Higgs mass $m_H$ above $\sim 550-850$ GeV, depending on the specific decay topology and assumptions about other decay modes (e.g., $A \to \tau^+\tau^-$).

Significance and Claims
The paper claims that while low-energy precision measurements currently provide the strongest constraints on LFV couplings in the $e\mu$ sector, collider-based probes of exotic Higgs decays offer a powerful and necessary complement. The authors argue that the "novel signatures" ($h \to AZ$ and $h \to AA$) provide access to parameter space regions that might be less constrained by low-energy experiments or where the EFT assumption breaks down. Specifically, the optimized kinematic selections allow the LHC to probe these LFV couplings with significantly enhanced sensitivity, potentially uncovering new physics that precision experiments alone might miss or that requires a UV-complete model interpretation. The work underscores the importance of searching for multilepton final states in exotic Higgs decays as a distinct avenue for discovering lepton flavor violation.