arxiv: 2605.08501 · v1 · submitted 2026-05-08 · ✦ hep-ph · hep-ex

Recognition: no theorem link

Lepton Flavor Violating Higgs decays at the Compact Linear Collider

Francisca Garay , Gabriel Vega , Philipp Roloff

Authors on Pith no claims yet

Pith reviewed 2026-05-12 01:26 UTC · model grok-4.3

classification ✦ hep-ph hep-ex

keywords lepton flavor violationHiggs decaysCLICbranching fractionssensitivity studyfuture collidersbeyond standard model

0 comments

The pith

The Compact Linear Collider could set 95% CL upper limits of 10^{-4} to 10^{-5} on branching fractions for lepton flavor violating Higgs decays.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper examines the reach of the proposed Compact Linear Collider for Higgs boson decays that convert one lepton flavor into another. Such processes do not occur in the Standard Model but can appear in many extensions that introduce new sources of flavor violation. The authors simulate the production of the Higgs boson at CLIC's two main operating energies together with the three decay channels h to e mu, h to tau mu, and h to e tau. Accounting for expected detector performance and background processes, they project that the planned data samples would allow upper limits in the 10^{-4} to 10^{-5} range if no signal appears. These sensitivities would test and potentially exclude regions of parameter space in beyond-Standard-Model scenarios that predict observable lepton flavor violation.

Core claim

In the absence of any observation, 95% CL upper limits in the region of 10^{-4} to 10^{-5} could be placed on the branching fractions of the processes h to e mu, h to tau mu, and h to e tau, assuming integrated luminosities of 4 ab^{-1} at sqrt(s) = 1.4 TeV and 5 ab^{-1} at sqrt(s) = 3 TeV.

What carries the argument

Monte Carlo simulation of signal and background events for Higgs production and the three lepton-flavor-violating decay modes, using the nominal CLIC detector performance at the two design collision energies.

Load-bearing premise

The projections assume that background processes are perfectly modeled and that detector efficiencies and resolutions match the design values used in the simulation.

What would settle it

Direct measurement of background rates in the signal regions that exceed the simulated predictions by more than the assumed uncertainties would prevent the quoted limits from being reached.

read the original abstract

Lepton flavor violating Higgs decays could appear in scenarios beyond the Standard Model of particle physics. In this article, we study the sensitivity of a future Compact Linear Collider (CLIC) to such processes, namely, $h\rightarrow e\mu$, $h\rightarrow\tau\mu$, and $h\rightarrow e\tau$. In the absence of any observation, 95\% CL\ upper limits in the region of $10^{-4}$ to $10^{-5}$ could be placed on the branching fractions of these processes, assuming integrated luminosities of $4$\,ab$^{-1}$ at $\sqrt{s}=1.4$ TeV and $5$\,ab$^{-1}$ at $\sqrt{s}=3$ TeV.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

Standard CLIC sensitivity projections for LFV Higgs decays using conventional Monte Carlo methods and idealized assumptions.

read the letter

The paper projects CLIC's sensitivity to lepton flavor violating Higgs decays h→eμ, h→τμ, and h→eτ. With the assumed luminosities of 4 ab^{-1} at 1.4 TeV and 5 ab^{-1} at 3 TeV, it claims 95% CL upper limits on the branching ratios in the 10^{-4} to 10^{-5} range if nothing is seen. This is not a new theoretical result or measurement. It is a standard fast-simulation study that takes the usual approach of generating signal and background events, applying cuts or efficiencies, and setting limits. Similar work exists for the LHC and ILC, so the novelty is mainly in plugging in CLIC's energies and luminosities. The paper does this part well. The limits are presented clearly for each channel and each energy stage. The assumptions are stated up front, and the statistical method for setting the limits is the conventional one. The main limitation is the idealized detector and background modeling. The projections assume backgrounds are perfectly known and that the detector performs exactly as designed. Any real-world deviations would make the actual limits weaker. This is a common feature of these studies rather than a unique flaw here. Readers who care about the physics reach of proposed linear colliders or who are building models with LFV in the Higgs sector will find the specific numbers useful. It is not essential reading for the broader community. I would take this to a reading group focused on collider phenomenology as a maybe. It is not something I would cite in my own work soon, since it does not introduce new ideas or data. But it is the kind of paper that deserves peer review to check the simulation details and confirm the numbers are correctly derived from the inputs.

Referee Report

1 major / 1 minor

Summary. The manuscript investigates the potential of the Compact Linear Collider (CLIC) to probe lepton flavor violating Higgs decays h→eμ, h→τμ, and h→eτ through a fast simulation study. It concludes that, in the absence of observation, 95% CL upper limits on the branching fractions could reach the 10^{-4} to 10^{-5} level with the assumed luminosities at 1.4 TeV and 3 TeV.

Significance. These projections are significant for the physics program of future linear colliders, as they demonstrate the potential to set stringent limits on LFV processes that are highly suppressed in the Standard Model. The work uses standard Monte Carlo techniques for sensitivity estimates, which is a strength for reproducibility in principle, but the lack of detailed documentation reduces the immediate impact.

major comments (1)

[Simulation and Analysis] The description of the Monte Carlo simulation, background modeling, detector efficiencies, and the statistical procedure for setting the 95% CL limits is insufficient. This prevents assessment of whether the projected limits in the abstract are supported by the setup.

minor comments (1)

The abstract could specify the detector model or simulation software used to allow better context for the assumed performance.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and for the constructive feedback. We address the major comment below and will revise the paper to improve clarity and completeness.

read point-by-point responses

Referee: [Simulation and Analysis] The description of the Monte Carlo simulation, background modeling, detector efficiencies, and the statistical procedure for setting the 95% CL limits is insufficient. This prevents assessment of whether the projected limits in the abstract are supported by the setup.

Authors: We agree that the current level of detail on the simulation chain is insufficient for full reproducibility and assessment. The study employs a fast simulation framework standard for CLIC projections, but the manuscript does not adequately document the specific generators, background samples, efficiency parametrizations, and limit-setting procedure. In the revised version we will expand the relevant sections to include: (i) the event generators and parton-level processes used for signal and all relevant backgrounds; (ii) the detector simulation and reconstruction assumptions, including any efficiency and resolution parametrizations; (iii) the selection cuts and background modeling approach; and (iv) the statistical method (including the treatment of systematic uncertainties) used to derive the 95% CL limits. These additions will directly support the quoted sensitivity numbers. revision: yes

Circularity Check

0 steps flagged

No circularity: standard Monte Carlo sensitivity projections

full rationale

The paper derives 95% CL upper limits on LFV Higgs branching fractions from forward simulation of signal and background events at specified CLIC energies and luminosities, followed by standard statistical limit-setting. No step reduces a claimed prediction to a fitted input by construction, no self-citation chain carries the central result, and no ansatz or uniqueness theorem is imported to force the outcome. The derivation remains self-contained against external benchmarks of detector performance and background modeling.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard Monte Carlo simulation of Standard Model backgrounds and assumed CLIC detector performance; no new free parameters, axioms beyond domain conventions, or invented entities are introduced.

axioms (2)

domain assumption Standard Model processes provide the dominant irreducible backgrounds for the rare LFV signals.
The upper-limit calculation assumes that any excess would be attributed to signal only after subtracting well-known SM backgrounds.
domain assumption CLIC detector performance matches the design values for efficiency and resolution used in the simulation.
Projected limits depend on the assumed reconstruction efficiencies and background rejection power of the future detector.

pith-pipeline@v0.9.0 · 5415 in / 1400 out tokens · 54970 ms · 2026-05-12T01:26:13.773869+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

60 extracted references · 60 canonical work pages · 2 internal anchors

[1]

Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC

Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC. Phys.Lett. B716 (2012) 1. doi:https://doi.org/10.1016/j.physletb.2012.08.020

work page internal anchor Pith review doi:10.1016/j.physletb.2012.08.020 2012
[2]

Phys.Lett

Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC. Phys.Lett. B716 (2012) 30. doi:https://doi.org/10.1016/j.physletb.2012.08.021

work page doi:10.1016/j.physletb.2012.08.021 2012
[3]

Broken symmetry and the mass of gauge vector mesons

Broken Symmetry and the Mass of Gauge Vector Mesons. Phys.Rev.Lett.13 (1964) 321. doi:https://doi.org/10.1103/PhysRevLett.13.321

work page doi:10.1103/physrevlett.13.321 1964
[4]

Broken symmetries, massless particles and gauge fields. Phys. Lett. 12 (1964) 132. doi:https://doi.org/10.1016/0031-9163(64)91136-9

work page doi:10.1016/0031-9163(64)91136-9 1964
[5]

Broken Symmetries and the Masses of Gauge Bosons. Phys. Rev. Lett. 13 (1964) 508. doi:https://doi.org/10.1103/PhysRevLett.13.508

work page doi:10.1103/physrevlett.13.508 1964
[6]

Spontaneous Symmetry Breakdown without Massless Bosons

Spontaneous Symmetry Breakdown without Massless Bosons. Phys.Rev.145 (1966) 1156. doi:https://doi.org/10.1103/PhysRev.145.1156

work page doi:10.1103/physrev.145.1156 1966
[7]

Global conservation laws and massless particles

Global Conservation Laws and Massless Particles. Phys.Rev.Lett.13 (1964) 585. doi:https://doi.org/10.1103/PhysRevLett.13.585

work page doi:10.1103/physrevlett.13.585 1964
[8]

Symmetry Breaking in Non-Abelian Gauge Theories. Phys. Rev.155 (1967) 1554. doi:https://doi.org/10.1103/PhysRev.155.1554

work page doi:10.1103/physrev.155.1554 1967
[9]

Phys.Rev.Lett.38 (1977) 622

Mechanism for Non conservation of Muon Number. Phys.Rev.Lett.38 (1977) 622. doi:https://doi.org/10.1103/PhysRevLett.38.622

work page doi:10.1103/physrevlett.38.622 1977
[10]

Phys.Rev.D 62 (2000) 116005

Lepton flavor violating decays of Higgs bosons beyond the standard model. Phys.Rev.D 62 (2000) 116005. doi:https://doi.org/10.1103/PhysRevD.62.116005

work page doi:10.1103/physrevd.62.116005 2000
[11]

JHEP09 (2013) 160, (Erratum:JHEP 10 (2015) 192)

Non-decoupling SUSY in LFV Higgs decays: a window to new physics at the LHC. JHEP09 (2013) 160, (Erratum:JHEP 10 (2015) 192). doi:https://doi.org/10.1007/JHEP09(2013)160

work page doi:10.1007/jhep09(2013)160 2013
[12]

Comprehensive analysis on lepton flavor violating Higgs boson to decay insupersymmetry without R parity. JPhys. Rev.D 87 (2013) 015025. doi:https://doi.org/10.1103/PhysRevD.87.015025

work page doi:10.1103/physrevd.87.015025 2013
[13]

JHEP04 (2016) 162

Large BR(h ) in the MSSM?. JHEP04 (2016) 162. doi:https://doi.org/10.1007/JHEP04(2016)162

work page doi:10.1007/jhep04(2016)162 2016
[14]

Phys.Rev.D 80 (2009) 075016

Composite Higgs-mediated flavor-changing neutral current. Phys.Rev.D 80 (2009) 075016. doi:https://doi.org/10.1103/PhysRevD.80.075016

work page doi:10.1103/physrevd.80.075016 2009
[15]

Phys.Rev.D 80 (2009) 035016

Higgs mediated flavor changing neutral currents in warped extra dimensions. Phys.Rev.D 80 (2009) 035016. doi:https://doi.org/10.1103/PhysRevD.80.035016

work page doi:10.1103/physrevd.80.035016 2009
[16]

Prog.Theor.Phys

Non-Abelian Discrete Symmetries in Particle Physics. Prog.Theor.Phys. Suppl.183 (2010) 1. doi:https://doi.org/10.1143/PTPS.183.1

work page doi:10.1143/ptps.183.1 2010
[17]

JHEP01 (2009) 077

Natural neutrino masses and mixings from warped geometry. JHEP01 (2009) 077. doi:https://doi.org/10.1088/1126-6708/2009/01/077

work page doi:10.1088/1126-6708/2009/01/077 2009
[18]

JHEP 0909:064 (2009)

Electroweak and Flavour Structure of a Warped Extra Dimension with Custodial Protection. JHEP 0909:064 (2009). doi:https://doi.org/10.1088/1126-6708/2009/09/064

work page doi:10.1088/1126-6708/2009/09/064 2009
[19]

Phys.Lett

Higgs-induced lepton flavor violation. Phys.Lett. B 707(2012)369. doi:https://doi.org/10.1016/j.physletb.2011.12.059

work page doi:10.1016/j.physletb.2011.12.059 2012
[20]

Phys.Rev.D 86 (2012) 113004

Modified Higgs branching ratios versus CP and lepton flavor violation. Phys.Rev.D 86 (2012) 113004. doi:https://doi.org/10.1103/PhysRevD.86.113004

work page doi:10.1103/physrevd.86.113004 2012
[21]

Phys.Rev.Lett.109 (2012) 101802

Evidence for an Excess of B D^ (*) ^- _ Decays. Phys.Rev.Lett.109 (2012) 101802. doi:https://doi.org/10.1103/PhysRevLett.109.101802

work page doi:10.1103/physrevlett.109.101802 2012
[22]

Measurement of the ratio of the B^0 D^ *- ^+ _ and B^0 D^ *- ^+ _ branching fractions using three-prong -lepton decays. Phys. Rev. Lett. 120, 171802 (2018). doi:https://doi.org/10.1103/PhysRevLett.120.171802

work page doi:10.1103/physrevlett.120.171802 2018
[23]

Measurement of the Ratio of Branching Fraction B ( B ^0 D^ *+ ^- _ )/ B ( B ^0 D^ *+ ^- _ ). Phys. Rev. Lett. 115, 111803 (2015). doi:https://doi.org/10.1103/PhysRevLett.131.111802

work page doi:10.1103/physrevlett.131.111802 2015
[24]

JHEP06 (2015)115, (Erratum:JHEP09 (2018)145)

Differential branching fraction and angular analysis of ^0_b ^+ ^- decays. JHEP06 (2015)115, (Erratum:JHEP09 (2018)145). doi:https://doi.org/10.1007/JHEP09

work page doi:10.1007/jhep09 2015
[25]

Phys.Rev.Lett.124 (2020)161803

Measurement of R(D) and R(D^*) with a Semileptonic Tagging Method. Phys.Rev.Lett.124 (2020)161803. doi:https://doi.org/10.1103/PhysRevLett.124.161803

work page doi:10.1103/physrevlett.124.161803 2020
[26]

Tests of lepton universality using B^0 K_s^0 ^+ ^- and B^+ K^. Phys. Rev. Lett. , volume =. 2023 , pages =

work page 2023
[27]

, pages =

Measurement of the branching fraction ratio R_K at large dilepton invariant mass , journal =. , pages =

work page
[28]

Phys.Rev.D 95(2017)115040

LHCb anomaly and B physics in flavored Z' models with flavored Higgs doublets. Phys.Rev.D 95(2017)115040. doi:https://doi.org/10.1103/PhysRevD.95.115040

work page doi:10.1103/physrevd.95.115040 2017
[29]

Phys.Rev.Lett.116 (2016) 251801

Lepton Flavor and Non-Universality from Minimal Composite Higgs Setups. Phys.Rev.Lett.116 (2016) 251801. doi:https://doi.org/10.1103/PhysRevLett.116.251801

work page doi:10.1103/physrevlett.116.251801 2016
[30]

Eur.Phys

Flavour-changing Higgs couplings in a class of two Higgs doublet models. Eur.Phys. J.C 76(2016)161. doi:https://doi.org/10.1140/epjc/s10052-016-3993-0

work page doi:10.1140/epjc/s10052-016-3993-0 2016
[32]

Search for lepton-flavor violating decays of the Higgs boson in the and e final states in proton-proton collisions at s = 13 TeV. Phys. Rev. D 104, 032013 (2021). doi:https://doi.org/10.1103/PhysRevD.104.032013

work page doi:10.1103/physrevd.104.032013 2021
[33]

Search for the Higgs boson decays h and h e in pp collisions at s =13 TeV with the ATLAS detector. Phys. Lett.B801 (2020) 135148. doi:https://doi.org/10.1016/j.physletb.2019.135148

work page doi:10.1016/j.physletb.2019.135148 2020
[34]

Phys.Rev.Lett.110 (2013) 201801

New Constraint on the Existence of the e Decay. Phys.Rev.Lett.110 (2013) 201801. doi:https://doi.org/10.1103/PhysRevLett.110.201801

work page doi:10.1103/physrevlett.110.201801 2013
[35]

JHEP 03(2013)026

Flavor violating Higgs decays. JHEP 03(2013)026. doi:https://doi.org/10.1007/JHEP03

work page doi:10.1007/jhep03 2013
[36]

Flavour-changing decays of a 125 GeV Higgs-like particle. Phys. Lett.B 712 (2012) 386. doi:https://doi.org/10.1016/j.physletb.2012.05.007

work page doi:10.1016/j.physletb.2012.05.007 2012
[37]

European Physics Journal

Higgs Physics at the CLIC Electron-Positron Linear Collider. European Physics Journal. 2017. doi:https://doi.org/10.1140/epjc/s10052-017-4968-5

work page doi:10.1140/epjc/s10052-017-4968-5 2017
[38]

CERN Yellow Reports: Monographs

The CLIC Potential for New Physics. CERN Yellow Reports: Monographs. doi:https://doi.org/10.23731/CYRM-2018-003

work page doi:10.23731/cyrm-2018-003 2018
[39]

Eur.Phys.J.C71:1742,2011

WHIZARD: simulating multi-particle processes at LHC and ILC. Eur.Phys.J.C71:1742,2011. doi:https://doi.org/10.1140/epjc/s10052-011-1742-y

work page doi:10.1140/epjc/s10052-011-1742-y 2011
[40]

CERN-2016-004

Updated baseline for a staged Compact Linear Collider. CERN-2016-004. 2016. doi:https://doi.org/10.5170/CERN-2016-004

work page doi:10.5170/cern-2016-004 2016
[41]

CERN-2018-005-M

The Compact Linear Collider (CLIC) - 2018 Summary Report. CERN-2018-005-M. 2018. doi:https://doi.org/10.23731/CYRM-2018-002

work page doi:10.23731/cyrm-2018-002 2018
[42]

CLICdp-Note-2018-002

Updated CLIC luminosity staging baseline and Higgs coupling prospects. CLICdp-Note-2018-002. 2018. doi:https://doi.org/10.48550/arXiv.1812.01644

work page doi:10.48550/arxiv.1812.01644 2018
[43]

CERN-2012-007

A Multi-TeV Linear Collider based on CLIC Technology: CLIC Conceptual Design Report. CERN-2012-007. 2012

work page 2012
[44]

DESY 2009-87

The International Large Detector: Letter of Intent. DESY 2009-87. 2010. doi:https://doi.org/10.48550/arXiv.1006.3396

work page doi:10.48550/arxiv.1006.3396 2009
[45]

The International Linear Collider Technical Design Re- port - Volume 4: Detectors,

The International Linear Collider Technical Design Report - Volume 4: Detectors. ILC-REPORT-2013-040. 2013. doi:https://doi.org/10.48550/arXiv.1306.6329

work page doi:10.48550/arxiv.1306.6329 2013
[46]

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment , volume =

Performance of Particle Flow Calorimetry at CLIC. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment , volume =. 2013 , doi =

work page 2013
[47]

2009 , doi =

Particle flow calorimetry and the PandoraPFA algorithm , journal =. 2009 , doi =

work page 2009
[48]

The Pandora Software Development Kit for Pattern Recognition. Eur. Phys. J. C 75, 439 (2015). doi:https://doi.org/10.1140/epjc/s10052-015-3659-3

work page doi:10.1140/epjc/s10052-015-3659-3 2015
[49]

LCD-Note-2010-009

TauFinder: A Reconstruction Algorithm for Leptons at Linear Colliders. LCD-Note-2010-009. 2010

work page 2010
[50]

Asymptotic formulae for likelihood-based tests of new physics

Asymptotic formulae for likelihood-based tests of new physics. Eur.Phys.J.C71:1554,2011. doi:https://doi.org/10.1140/epjc/s10052-011-1554-0

work page internal anchor Pith review doi:10.1140/epjc/s10052-011-1554-0 2011
[51]

Exact and approximate solutions for the electron nonsinglet structure function in QED. Z. Phys. C. 1991. doi:https://doi.org/10.1007/BF01483573

work page doi:10.1007/bf01483573 1991
[52]

JHEP 05 (2006) 026

PYTHIA 6.4 Physics and Manual. JHEP 05 (2006) 026. doi:https://doi.org/10.1088/1126-6708/2006/05/026

work page doi:10.1088/1126-6708/2006/05/026 2006
[53]

A Comparison of b and uds quark jets to gluon jets. Phys. C - Particles and Fields 69 (1995) 543. doi:https://doi.org/10.1007/BF02907439

work page doi:10.1007/bf02907439 1995
[54]

Physics and Detectors at CLIC: CLIC Conceptual Design Report. 2012. doi:https://doi.org/10.48550/arXiv.1202.5940. arXiv:1202.5940

work page Pith review doi:10.48550/arxiv.1202.5940 2012
[55]

A detector for CLIC: main parameters and performance. 2018. doi:https://doi.org/10.48550/arXiv.1812.07337

work page Pith review doi:10.48550/arxiv.1812.07337 2018
[56]

status report

TAUOLA the library for tau lepton decay, and KKMC / KORALB / KORALZ /... status report. Nucl.Phys.Proc.Suppl. 98 (2001) 96-102. doi:https://doi.org/10.1016/S0920-5632

work page doi:10.1016/s0920-5632 2001
[57]

JHEP 1607 (2016) 059

The Lepton Flavour Violating Higgs Decays at the HL-LHC and the ILC. JHEP 1607 (2016) 059. doi:https://doi.org/10.1007/JHEP07

work page doi:10.1007/jhep07 2016
[58]

Updated CLIC luminosity staging baseline and Higgs coupling prospects. 2018. arXiv:1812.01644

work page arXiv 2018
[59]

CLIC Higgs coupling prospects with 100 Hz operation. 2025. arXiv:arXiv:2503.21857v1

work page arXiv 2025
[60]

Marlin and LCCD: Software tools for the ILC

Gaede, F. Marlin and LCCD: Software tools for the ILC. Nucl. Instrum. Meth. A. 2006. doi:10.1016/j.nima.2005.11.138

work page doi:10.1016/j.nima.2005.11.138 2006
[61]

Hocker, J

Hoecker, Andreas and others. TMVA - Toolkit for Multivariate Data Analysis. 2007. arXiv:physics/0703039

work page arXiv 2007