⚛️ phenomenology

Lepton flavor violation in Higgs boson decays at the HL-LHC

This paper investigates the discovery potential of lepton flavor-violating Higgs boson decays within a Froggatt-Nielsen framework at the High-Luminosity LHC, projecting that the $h \to e\mu$ and $h \to \tau\mu$ channels could achieve a $5\sigma$ discovery significance with an integrated luminosity of 1000 fb $^{-1}$ , while the $h \to \tau e$ channel remains inaccessible.

Original authors: M. A. Arroyo-Ureña, E. A. Herrera-Chacón, Iran Melendez-Hernández, S. Rosado-Navarro

Published 2026-02-26

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CC BY 4.0

Original authors: M. A. Arroyo-Ureña, E. A. Herrera-Chacón, Iran Melendez-Hernández, S. Rosado-Navarro

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, high-energy particle collider, the Large Hadron Collider (LHC), where scientists smash tiny particles together to see what happens. Inside this machine, there's a famous particle called the Higgs boson. Think of the Higgs as a "celebrity" at a party; it's famous for giving mass to other particles, but it usually behaves very politely, sticking to the rules of the Standard Model (the rulebook of physics).

However, this paper asks a daring question: What if the Higgs breaks the rules?

Specifically, the authors are looking for a phenomenon called Lepton Flavor Violation (LFV). In the particle world, there are three "flavors" of charged particles: the electron (light), the muon (medium), and the tau (heavy). In the current rulebook, a Higgs boson is supposed to decay into a pair of the same flavor (like two electrons or two muons). It's like a strict bouncer who only lets twins into the VIP section.

The Big Idea:
The authors are hunting for a scenario where the Higgs acts like a rebel bouncer, decaying into mismatched pairs, like an electron and a muon ( $h \to e\mu$ ) or a tau and a muon ( $h \to \tau\mu$ ). If they find this, it's a "smoking gun" that proves there is new physics beyond our current understanding.

The Theory: The "Froggatt-Nielsen" Recipe

To explain how the Higgs might break these rules, the authors use a theoretical framework called the Froggatt-Nielsen Singlet Model.

The Analogy: Imagine the universe has a hidden "flavor" ingredient, like a secret spice called a Flavon. In this model, the Higgs mixes with this secret spice.
The Mechanism: The amount of "spice" (called the vacuum expectation value, or VEV) and how much the Higgs "mixes" with it determines how likely it is to break the rules. The theory predicts that the Higgs might occasionally swap flavors, but the probability depends on the specific "recipe" (parameters) of the universe.

The Hunt: The High-Luminosity LHC (HL-LHC)

The authors aren't just guessing; they are planning a massive search at the High-Luminosity LHC, which is the upgraded, super-powerful version of the current collider.

The Goal: They want to see if the Higgs can produce these mismatched pairs often enough to be spotted.
The Challenge: The background noise is huge. It's like trying to hear a whisper in a stadium full of cheering fans. Most particle collisions produce standard pairs (like two muons), and the "mismatched" ones are incredibly rare.

The Results: Who Wins the Race?

The team ran massive computer simulations (using a technique called Boosted Decision Trees, which is like a super-smart AI filter) to separate the "whispers" from the "noise." Here is what they found:

The Star Performer ( $h \to \tau\mu$ ):
- The Story: The Higgs decaying into a tau and a muon.
- The Verdict: This is the most promising candidate. If the "secret spice" amount is just right, the HL-LHC could find this signal with 5-sigma certainty (the gold standard in physics for a discovery) after running for a few years. It's like finding a needle in a haystack, but the needle is glowing.
The Underdog ( $h \to e\mu$ ):
- The Story: The Higgs decaying into an electron and a muon.
- The Verdict: This is also very promising! With enough data, they predict they can find this one too, reaching the 5-sigma discovery threshold. It's a bit harder to spot than the tau-muon pair, but the HL-LHC has the power to do it.
The Loser ( $h \to \tau e$ ):
- The Story: The Higgs decaying into a tau and an electron.
- The Verdict: Unfortunately, this one is a bust. According to their "recipe," this specific mix is so rare that even with the most powerful collider in the world, they probably won't see it. It's like looking for a specific grain of sand on a beach that the ocean has already washed away.

Why Does This Matter?

If the HL-LHC finds these mismatched decays, it changes everything. It would prove that:

The Higgs boson isn't just a boring mass-giver; it has a complex, hidden personality.
There is a new layer of physics (the "Flavon" or "spice") that explains why particles have the masses they do.
The universe is more interconnected and "flavorful" than we thought.

In Summary:
This paper is a roadmap for the next decade of particle physics. It tells us exactly where to look (the tau-muon and electron-muon channels) and how hard we need to look (the High-Luminosity LHC) to catch the Higgs boson breaking the rules. If they succeed, it's a Nobel Prize-level discovery that opens a new chapter in our understanding of the universe.

1. Problem Statement

The Standard Model (SM) with massless neutrinos strictly conserves lepton flavor numbers. While neutrino oscillations confirm lepton flavor violation (LFV) in the neutral sector, charged LFV (cLFV) processes (e.g., $\mu \to e\gamma$ , $h \to \tau\mu$ ) are highly suppressed in the SM with massive neutrinos, making them pristine probes for Beyond the Standard Model (BSM) physics.
Current experimental limits on cLFV Higgs decays ( $h \to \tau\mu$ , $h \to \tau e$ , $h \to e\mu$ ) are not yet restrictive enough to rule out many BSM scenarios. The High-Luminosity LHC (HL-LHC), expected to deliver an integrated luminosity of $3000 \text{ fb}^{-1}$ , offers a unique opportunity to probe these rare decays. The paper addresses the question: Can the HL-LHC discover lepton-flavor-violating Higgs decays within a specific, theoretically motivated framework?

2. Theoretical Framework: Froggatt-Nielsen Singlet Model (FNSM)

The authors analyze the FNSM, which extends the SM scalar sector with a complex singlet scalar field ( $S_F$ ) and incorporates the Froggatt-Nielsen (FN) mechanism to explain fermion mass hierarchies.

Scalar Sector: The model introduces a CP-conserving scalar potential with a mixing angle $\alpha$ between the SM-like Higgs ( $h$ ) and a heavy CP-even Flavon ( $H_F$ ). A pseudo-scalar Flavon ( $A_F$ ) also exists.
Yukawa Sector: Flavor violation arises naturally through the FN mechanism. The effective Yukawa couplings are non-diagonal, leading to Flavor-Changing Neutral Currents (FCNCs) at tree level.
Key Parameters: The phenomenology is driven by:
- The mixing angle $\alpha$ (constrained to $\cos \alpha \approx 1$ by Higgs signal strength data).
- The singlet vacuum expectation value (VEV), $v_s$ .
- Off-diagonal flavor-violating coupling elements: $\tilde{Z}_{e\mu}$ , $\tilde{Z}_{\tau\mu}$ , and $\tilde{Z}_{\tau e}$ .
Constraints: The parameter space is heavily constrained by low-energy cLFV processes (e.g., $\mu \to e\gamma$ , $\tau \to \mu\gamma$ , $\tau \to 3\mu$ ) and current Higgs measurements.

3. Methodology

The study employs a systematic approach combining theoretical constraints with collider phenomenology:

A. Parameter Space Scanning & Constraints

The authors map the allowed parameter space in the $(\cos \alpha, v_s)$ plane using Higgs signal strength modifiers ( $R_X$ ).
They apply stringent upper limits from experiments (CMS, ATLAS, MEG, etc.) on branching ratios for $\ell_i \to \ell_j \gamma$ and $\ell_i \to 3\ell$ .
Benchmark Points (BP): Specific scenarios are selected where the heavy flavon masses ( $M_{H_F} = M_{A_F}$ ) are set to 0.9, 1.5, 2.0, and 2.5 TeV. The mixing angle is fixed at $\cos \alpha = 0.995$ .

B. Collider Simulation

Signal Processes: Gluon-fusion production of the Higgs followed by cLFV decays:
1. $h \to e^\pm \mu^\mp$
2. $h \to \tau^\pm \mu^\mp$ (with $\tau \to \pi^\pm \nu_\tau$ )
3. $h \to \tau^\pm e^\mp$ (analyzed theoretically but found unobservable).
Backgrounds: Dominant SM backgrounds include Drell-Yan ( $Z/\gamma^* \to \tau\tau$ ), Top-quark pair production ( $t\bar{t}$ ), and Diboson production ($WW, WZ, ZZ$).
Simulation Chain:
- Model implementation: FeynRules.
- Event generation: MadGraph5 (NNPDF2.3LO PDFs).
- Parton showering/Hadronization: Pythia8.
- Detector simulation: Delphes3 (using the HL-LHC configuration).

C. Multivariate Analysis (MVA)

To separate signal from background, the authors utilize Boosted Decision Trees (BDT) via the XGBoost library.
Input Variables: Kinematic observables including invariant mass, transverse momentum ( $p_T$ ), pseudorapidity ( $\eta$ ), azimuthal angle ( $\phi$ ), missing transverse energy ( $E_T^{miss}$ ), and angular separation ( $\Delta R$ ).
Optimization: The BDT cut is optimized to maximize signal significance ( $Z = S/\sqrt{S+B+(x\cdot B)^2}$ ), accounting for systematic uncertainties (luminosity, lepton ID, PDFs, etc.), totaling $\sim 4.36\%$ .

4. Key Results

A. $h \to e\mu$ Channel

Sensitivity: This channel is highly sensitive to the coupling $\tilde{Z}_{e\mu}$ .
Discovery Potential: For a singlet VEV $v_s \approx 900$ GeV and couplings $|\tilde{Z}_{e\mu}| \sim \mathcal{O}(10^{-2})$ , the HL-LHC can achieve a $5\sigma$ discovery significance with an integrated luminosity of $1000 \text{ fb}^{-1}$ .
Scaling: At $3000 \text{ fb}^{-1}$ , the sensitivity improves, allowing the detection of slightly smaller couplings ( $|\tilde{Z}_{e\mu}| \sim 0.007$ ).
Constraint: The parameter space is limited by the $\mu \to e\gamma$ branching ratio, which imposes $|\tilde{Z}_{e\mu}| \lesssim 0.005$ for lighter flavon masses (0.9 TeV) but allows up to $\sim 0.026$ for heavier masses (2.5 TeV).

B. $h \to \tau\mu$ Channel

Sensitivity: Probed via the final state $\mu^\pm \pi^\mp \nu_\tau$ .
Discovery Potential: Similar to the $e\mu$ channel, a $5\sigma$ significance is projected for $v_s = 500$ GeV and couplings $|\tilde{Z}_{\tau\mu}| \sim \mathcal{O}(10^{-3})$ with $1000 \text{ fb}^{-1}$ .
Constraints: The strongest bounds come from the combination of $\tau \to 3\mu$ , $\tau \to \mu\gamma$ , and direct $h \to \tau\mu$ searches.

C. $h \to \tau e$ Channel

Result: This channel is predicted to be unobservable even at the ultimate HL-LHC luminosity ( $3000 \text{ fb}^{-1}$ ).
Reasoning: Based on the FN hierarchy, the coupling $\tilde{Z}_{\tau e}$ is expected to be significantly smaller than $\tilde{Z}_{e\mu}$ and $\tilde{Z}_{\tau\mu}$ (estimated $< \mathcal{O}(10^{-4})$ ). This results in a production cross section ( $\sim 10^{-10}$ pb) that is orders of magnitude too small to yield detectable events.

5. Significance and Conclusions

Complementarity: The work highlights the complementary nature of low-energy precision experiments (which set tight bounds on couplings) and high-energy colliders (which can directly observe the resulting rare decays).
Discovery Potential: The HL-LHC has the unique capability to discover cLFV Higgs decays, specifically $h \to e\mu$ and $h \to \tau\mu$ , providing a "smoking gun" signature for new physics and the Froggatt-Nielsen mechanism.
Predictive Power: The model makes a specific, testable prediction: while $h \to e\mu$ and $h \to \tau\mu$ are discoverable, $h \to \tau e$ should remain null. Observing this specific pattern would provide strong evidence for the flavor hierarchy structure inherent in the FN mechanism.
Methodological Impact: The study demonstrates the effectiveness of advanced machine learning techniques (BDT) in isolating rare Higgs signals from overwhelming SM backgrounds at the HL-LHC.

In summary, the paper establishes that the HL-LHC is a powerful tool for exploring the flavor structure of the Higgs sector, with the potential to confirm the existence of lepton-flavor-violating Higgs decays within the Froggatt-Nielsen Singlet Model.