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Search prospect for electrophilic bosons at the $\mu$TRISTAN

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Amit Adhikary, Anindya Datta, Dilip Kumar Ghosh

High Energy Physics - Phenomenology · Submitted 7 Oct 2026 · 94 references

Pith's verdict: The quoted 10^-5 coupling reaches do not follow from the paper's own Tables 2 and 3; the rest of the analysis is coherent but this inconsistency is load-bearing.

T0 · AI review

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arXiv ID2610.10185 (version 1)
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CitationAmit Adhikary, Anindya Datta, Dilip Kumar Ghosh. "Search prospect for electrophilic bosons at the $\mu$TRISTAN." arXiv:2610.10185 (2026). https://pith.science/paper/GN3UTUHN
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The paper

The paper claims

A proposed 346 GeV muon-electron collider mode could probe electron-coupled bosons far below current limits.

The reading

The paper aims to close the least-explored window for new bosons that couple only to electrons: mediator masses from roughly ten to a few hundred GeV, a region above the reach of the B factories that hadron colliders struggle with and that existing $e^+e^-$ data cover only partially. Its proposal is the asymmetric $\mu^+e^-$ mode of the future $\mu$TRISTAN collider, where a 1 TeV muon beam meets a 30 GeV electron beam at $\sqrt{s} = 346$ GeV, putting an electron in the initial state from which the new boson can be radiated directly. The paper shows that both minimal realizations — a pseudoscalar axion-like particle and a $Z'$ vector boson — give the same clean four-lepton signature, $e^-\mu^+ \to e^-e^+e^-\mu^+$, and that a resonance search in the $e^+e^-$ invariant mass for masses between 5.5 and 300 GeV reaches 95% CL couplings of order $10^{-5}$ at the light end. If these projections hold, the region between the kinematic limits of the B factories and the LEP contact-interaction bounds, currently the least constrained part of the electrophilic-mediator parameter space, would become testable at a lepton collider with a hadron-free initial state.

What carries the argument

The load-bearing object is the asymmetric beam configuration itself: the $\mu^+e^-$ mode of $\mu$TRISTAN with $E_e = 30$ GeV against $E_\mu = 1$ TeV gives $\sqrt{s} = 346$ GeV with an electron in the initial state, so an electrophilic mediator can be produced at tree level in a hadron-free environment. The common signature is the four-lepton final state $e^-\mu^+ \to e^-e^+e^-\mu^+$; because both the ALP and the $Z'$ decay promptly to $e^+e^-$, one analysis chain serves both spins. That chain rests on three pieces: a boosted decision tree (the XGBoost implementation) that tags the spectator electron using nineteen production-level kinematic variables while excluding $m_{ee}$ itself, so the tagging cannot sculpt the discriminant; a double-sided Crystal Ball fit to the $m_{ee}$ peak that defines the signal window $[\mu-\sigma, \mu+\sigma]$ for each benchmark; and the Asimov significance formula that converts signal and background yields into 95% CL limits, with detector response emulated by Gaussian momentum smearing tuned to the ILD concept.

Extended reading notes

Core claim

Both mediators analyzed here are produced by radiation off the incoming electron and decay promptly back into an electron-positron pair, so the two scenarios share one narrow-resonance signature: a peak in the reconstructed $e^+e^-$ invariant mass on top of a smooth photon/$Z$-mediated continuum. The paper's central technical ingredient is identifying which of the two final-state electrons is the decay partner of the positron; a boosted-decision-tree tagger, trained on production kinematics alone with $m_{ee}$ deliberately excluded, selects the correct pair in 83% of events on average for the ALP and 75% for the $Z'$ across the full mass range, where no single kinematic rule works. Fitting the resulting peak with a double-sided Crystal Ball function and applying an Asimov significance formula, the paper derives 95% CL exclusion limits for fifteen benchmark masses at four luminosities and two systematic assumptions. With 1000 fb$^{-1}$, the coupling reach is $g_{aee} \lesssim 6.5\times10^{-5}$ and $g_{Z'} \lesssim 4.2\times10^{-5}$ at the light end, degrading to the $10^{-2}$ range at 300 GeV; the degradation is set by the phase-space fall-off of the production cross-section, not by analysis performance. Against existing bounds, the projections are comparable to the BaBar search below about 10 GeV, surpass all current constraints above it — by more than four orders of magnitude for the ALP relative to perturbative-unitarity limits — and provide the only constraint on the heaviest $Z'$ masses above roughly 220 GeV.

Load-bearing premise

The projections assume that a detector for $\mu$TRISTAN, which does not exist yet, will measure lepton momenta with the resolution of the ILC's ILD concept and that backgrounds from the stored muon beam will not add events to the $e^-e^+e^-\mu^+$ final state; the authors acknowledge that only a dedicated detector design and a beam-background study can settle this.

Editorial extensions

If this is right

  • At 1000 fb$^{-1}$ and null systematics, an electrophilic ALP with $5.5 \le m_a \le 100$ GeV and $g_{aee}$ between $6.5\times10^{-5}$ and $2.7\times10^{-3}$ would be excluded at 95% CL, a region that no current experiment constrains directly.
  • The $Z'$ reach, $g_{Z'} \lesssim 4.2\times10^{-5}$ at low mass, would improve on the LEP mono-photon, LEP-2 contact-interaction, and IceCube bounds by up to two orders of magnitude, and above $m_{Z'} \simeq 220$ GeV the $\mu^+e^-$ search would be the only constraint on the parameter space.
  • Since both scenarios share the same final state, reconstruction chain, and statistical treatment, a single run simultaneously probes pseudoscalar and vector electron couplings across 5.5–300 GeV.
  • A 5% systematic uncertainty on the background degrades the coupling limits by at most about 25%, so the headline reach does not hinge on precise background control as long as the 22.3 fb continuum estimate is right.
  • The reach is dominated by the production cross-section rather than the selection, so the same pipeline extends to other beam energies and mediator masses without redesign.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The $\mu^+\mu^+$ mode at $\sqrt{s} = 2$ TeV, which the paper explicitly leaves to future work, could carry the same four-lepton search above 300 GeV once photon-initiated electron pairs supply the radiating electron, at the price of extra powers of the electromagnetic coupling.
  • The honest reading of the quoted reach inherits the paper's own caveats: the detector response is an ILC-like stand-in and muon-beam-induced backgrounds are not simulated, so a dedicated detector study could move the limits in either direction, and how far they move decides whether the four-order-of-magnitude improvement over current bounds is realised in practice.
  • The recipe here — an energetic electron beam in an asymmetric lepton collider, a resonance-blind tagger, and a narrow invariant-mass window — generalizes to any future electron-beam machine, meaning the search strategy, not just this machine, is the transferable asset.
Original abstract

Mediators coupled predominantly to the electron remain weakly constrained above the kinematic reach of the $B$~factories, a region that is difficult to access at hadron colliders and only partially covered by existing $e^+e^-$ data. We point out that the asymmetric $\mu^+e^-$ mode of the proposed $\mu$TRISTAN collider, operating at $\sqrt{s}=346$~GeV, is well suited to this window, and quantify its reach for two minimal realisations: an axion-like particle and a $Z'$ gauge boson, treated as effective interactions with the electron current. Both are radiated off the electron line and decay to an electron-positron pair, giving a common four-lepton signature whose Standard Model counterpart is under good control. A resonance search in the electron-positron invariant mass, performed for mediator masses between $5.5$ and $300$~GeV, yields projected $95\%$~CL sensitivities to couplings of order $10^{-5}$ at the light end of this range, improving on existing bounds by more than four orders of magnitude for the pseudoscalar and by up to two orders of magnitude for the vector, and providing the only constraint on the heaviest masses considered.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-08: 1 request, 3,996 input tokens.

What the abstract itself states, as probabilities: claims first 0.15 · states limitation 0.24 · human participants 0.02 · controlled comparison 0.14 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.96 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Mediators coupled predominantly to the electron remain weakly constrained above the kinematic reach of the $B$~factories, a region that is difficult to access at hadron colliders and only partially covered by existing $e^+e^-$ data. We point out that the asymmetric $\mu^+e^-$ mode of the proposed $\mu$TRISTAN collider, operating at $\sqrt{s}=346$~GeV, is well suited to this window, and quantify its reach for two minimal realisations: an axion-like particle and a $Z'$ gauge boson, treated as effective interactions with the electron current. Both are radiated off the electron line and decay to an electron-positron pair, giving a common four-lepton signature whose Standard Model counterpart is under good control. A resonance search in the electron-positron invariant mass, performed for mediator masses between $5.5$ and $300$~GeV, yields projected $95\%$~CL sensitivities to couplings of order $10^{-5}$ at the light end of this range, improving on existing bounds by more than four orders of magnitude for the pseudoscalar and by up to two orders of magnitude for the vector, and providing the only constraint on the heaviest masses considered.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Mediators coupled predominantly to the electron remain weakly constrained above the kinematic reach of the $B$~factories, a region that is difficult to access at hadron colliders and only partially covered by existing $e^+e^-$ data. gap 0.62 0.05 0.04 asserted 1.00
s1 We point out that the asymmetric $\mu^+e^-$ mode of the proposed $\mu$TRISTAN collider, operating at $\sqrt{s}=346$~GeV, is well suited to this window, and quantify its reach for two minimal realisations: an axion-like particle and a $Z'$ gauge boson, treated as effective interactions with the electron current. contribution 0.97 0.84 0.36 asserted 0.98
s2 Both are radiated off the electron line and decay to an electron-positron pair, giving a common four-lepton signature whose Standard Model counterpart is under good control. method 0.93 0.31 0.04 asserted 1.00
s3 A resonance search in the electron-positron invariant mass, performed for mediator masses between $5.5$ and $300$~GeV, yields projected $95\%$~CL sensitivities to couplings of order $10^{-5}$ at the light end of this range, improving on existing bounds by more than four orders of magnitude for the pseudoscalar and by up to two orders of magnitude for the vector, and providing the only constraint on the heaviest masses considered. result 1.00 0.87 0.78 asserted 0.98

What kind of paper the classifier reports this is: position or commentary 0.50 · evidence from simulation or computation 0.77.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. The leading order (LO) Feynman diagrams for the signal process ((a)&(b)) and the background process ((c)&(d)) in µ +e − → µ +e +e −e − final state. a = 0, b = 2×10−5 GeV−1 and c = 0, which reproduces the momentum resolution expected for the International Linear Collider (ILC) [97] in the ILD detector concept [98]. Since the electron mass is negligible at the energies considered here, the smeared leptons are placed back on their mass shell by setting E = |⃗p |, which guarantees a physical invariant mass for any reconstructed pair. The following Asimov signal significance formula is…
Figure 2. Efficiency of correctly identifying the electron–positron pair originating from the ALP decay, as a function of the ALP mass, for the three analytic prescriptions discussed in the text and for the BDT-based tagger adopted in this analysis. the event itself rather than being supplied externally. The hyperparameters used for the training are collected in the lower part of
Figure 3. Reconstructed mee distributions for the fifteen ALP benchmark masses considered in this work. The black points show the smeared signal events after the BDT-based pairing, and the red curve is the double-sided Crystal Ball fit of Eq. 4.4. The shaded band indicates the range used in the fit, and the fitted values of µ, σ and the resulting χ 2/dof are quoted in each panel. evaluated at the reference coupling gaee = 0.1, with the production cross-section taken from the same calculation used below to convert the cross-section limit into a limit on the coupling. Since σ ∝ g 2 aee, the…
Figure 4. Upper limit at 95% CL on the signal production cross-section, σ(µ +e − → µ +e +e −e −) as a function of ALP mass, ma (a) with null systematics and (b) with 5% systematics, and the ALP-electron coupling, gaee (c) with null systematics and (d) with 5% systematics. The different colors correspond to variation with integrated luminosity. σ ∝ g 2 aee, so that the coupling reach is governed by the rapid fall of the ALP production cross-section rather than by the performance of the analysis: at √ s = 346 GeV, producing a resonance of a few hundred GeV together with the recoiling electron…
Figure 5. The leading-order (LO) Feynman diagrams for the signal process in the case of the Z ′ scenario. The Feynman diagrams for the background process are the same as in
Figure 6. Efficiency of correctly identifying the electron–positron pair originating from the Z ′ decay, as a function of the Z ′ mass, for the three analytic prescriptions discussed in Section 4.1 and for the BDT-based tagger adopted in this analysis. over the entire range, and the relative width of the peak is stable, σ/µ = 1.09 − 1.24%, with only a mild increase above mZ′ = 150 GeV. The signal region is again defined by the interval [µ− σ, µ+σ], and the corresponding signal efficiency, signal yield and background yield are listed in the last three columns of
Figure 7. Reconstructed mee distributions for the fifteen Z ′ benchmark masses considered in this work. The black points show the smeared signal events after the BDT-based pairing, and the red curve is the double-sided Crystal Ball fit of Eq. 4.4. The shaded band indicates the range used in the fit, and the fitted values of µ, σ and the resulting χ 2/dof are quoted in each panel. L = 1000 fb−1 , the excluded cross-section ranges from 0.027 fb at mZ′ = 5.5 GeV to 0.13 fb at mZ′ = 90 GeV without systematics, and from 0.029 fb to 0.18 fb once a 5% uncertainty is included. The shape of the curve c
Figure 8. Upper limit at 95% CL on the signal production cross-section, σ(µ +e − → µ +e +e −e −) as a function of Z ′ mass, mZ′ (a) with null systematics and (b) with 5% systematics, and the Z ′ - electron coupling, gZ′ (c) with null systematics and (d) with 5% systematics. The different colors correspond to variation with integrated luminosity. 5 Interpretation of the results In this section, we compare the projected sensitivities obtained in Sections 4.1 and 4.2 with the constraints already available on electrophilic mediators. The comparison is presented in
Figure 9. The projected 95% CL upper limit on gaee, without any systematics and L = 1000 fb−1 , is shown as a function of the ALP mass, in teal color. Other existing constraints, such as the perturbative limit [112] and reinterpreted BaBar [53] limit for the electrophilic mediator, are also shown. The constraint coming from ATLAS [25] through the loop-induced gaγγ coupling is also shown. state. Above the BaBar kinematic threshold, however, the available constraints are consider￾ably less restrictive. The requirement of perturbative unitarity [112] limits the coupling to gaee ≲ 3, while the…
Figure 10. The projected 95% CL upper limit on gZ′ , without any systematics and L = 1000 fb−1 , is shown as a function of the Z ′ mass, with teal color. Other existing constraints like BaBar [53], LEP [113, 114] are also displayed. Note that the IceCube constraint depends on the νe NSI [115]. acquires an invisible decay mode and contributes to neutrino observables, neither of which is the case for the electrophilic ALP. Both effects are reflected in the figure. The mono￾photon analyses at LEP [113, 114], which search for Z ′ → νν¯ produced in association with a photon, exclude gZ′ ≳ 1.2 ×…

Figures reproduced from arXiv:2610.10185 by the authors.

References

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