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arxiv: 2604.06103 · v2 · submitted 2026-04-07 · ✦ hep-ph

The SUSY reach of Higgs Factories in the most challenging scenario: scalar τ-leptons with lowest cross section and small mass differences

Maria Teresa N\'u\~nez Pardo de Vera (1) , Mikael Berggren (1) , Jenny List (1) ((1) DESY , Hamburg , Germany) This is my paper

Pith reviewed 2026-05-10 19:13 UTC · model grok-4.3

classification ✦ hep-ph
keywords supersymmetryscalar tau-leptonstau pair productionHiggs factorySUSY searchese+e- colliderkinematic limitmixing effects
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The pith

Future Higgs factories can exclude or discover scalar tau-lepton pairs nearly up to the kinematic limit even in the hardest SUSY scenario.

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

The paper examines how well electron-positron colliders can detect scalar tau-lepton pairs, the most difficult supersymmetry search channel due to low production rates from mixing and hard-to-identify signatures. It focuses on the worst-case conditions, including reduced cross sections and small mass differences to the lightest supersymmetric particle, using full simulations of production, decay, and detection with realistic backgrounds. The results indicate that both exclusion and discovery reaches still extend close to the highest masses allowed by the available collision energy. A sympathetic reader would care because this shows lepton colliders can close gaps in supersymmetry coverage left by prior experiments without relying on favorable model assumptions.

Core claim

In the most challenging scenario for supersymmetry searches, where the scalar tau-lepton has the lowest possible pair-production cross section due to mixing and small mass differences with the lightest supersymmetric particle, the exclusion and discovery reaches at a 500 GeV electron-positron collider still extend nearly to the kinematic limit of 250 GeV. This conclusion follows from detailed simulations that incorporate all relevant effects and holds when extrapolated to other proposed collider energies, luminosities, and polarizations.

What carries the argument

Full Monte Carlo simulation of scalar tau-lepton pair production and detection that incorporates mixing effects on cross section and efficiency plus beam-induced backgrounds from photon interactions.

If this is right

  • Exclusion and discovery limits approach the kinematic limit even when mass differences to the lightest supersymmetric particle are small.
  • The reach can be adjusted for other proposed collider energies and luminosities by simple extrapolation.
  • The clean environment of electron-positron collisions provides adequate sensitivity for this channel despite its experimental difficulty.
  • The results remain robust when low transverse-momentum hadrons from photon-photon interactions are included in the background model.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • This would mean supersymmetry tests at Higgs factories remain effective under the most conservative assumptions about the model.
  • It points to a potential complementarity between lepton and hadron colliders in covering the full supersymmetry parameter space.
  • Varying beam polarization could be explored as a way to increase sensitivity further in the lowest-cross-section cases.

Load-bearing premise

The modeled detector response and beam conditions accurately represent the performance and environment expected at future Higgs factories.

What would settle it

A complete absence of signal events in a dataset where the simulation predicts observable signal above background for scalar tau-lepton masses up to within a few GeV of half the collider energy would contradict the claimed reach.

Figures

Figures reproduced from arXiv: 2604.06103 by Germany), Hamburg, Jenny List (1) ((1) DESY, Maria Teresa N\'u\~nez Pardo de Vera (1), Mikael Berggren (1).

Figure 1
Figure 1. Figure 1: Cross section for τe pair production as a function of the τemixing angle for five different settings of the beam polarisations. Flipping of the polarisation signs inverts the dependence on the mixing angle. For unpolarised beams, the dependence vanishes nearly, indicating loss of information for determining the mixing angle. Assuming that the τe is the NLSP, R-parity conservation implies that the τe will d… view at source ↗
Figure 2
Figure 2. Figure 2: (a) τe pair production and decay to τ and χe 0 1 ; (b) hadronic τ decay; (c) leptonic τ decay. Blue lines indicates the τ, red lines indicate undetectable final state particles, while thick black lines indicate the detectable final state particles. Since the τe decay is, for ∆M > Mτ , a two body decay, it is possible to determine the maximum and minimum momentum of each of the decay products as a function … view at source ↗
Figure 3
Figure 3. Figure 3: Momentum distribution of the pions coming from [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: A τe event at ILC operating at √ s = 500 GeV, fully simulated in the ILD detector. Mτe = 230 GeV, and ∆M = 10 GeV. The properties of this event are discussed in detail in the text (Sec. 4.1.1). In addition to the measured quantities, also the true information of invisible partcles are shown in the event display, for information: The three neutrinos in the event are indicated by the grey lines, and the two … view at source ↗
Figure 5
Figure 5. Figure 5: Main “irreducible” background sources: (a) s-channel [PITH_FULL_IMAGE:figures/full_fig_p013_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Main background sources: (a) τ pair production and decay; (b) τ pair production through the multi-peripheral γγ process. In (b), the thick dashed red lines indicate that while the outgoing electrons or positrons are in principle detectable, this diagram contributes to the background only in the case that they are deflected so little that they escape detection by remaining inside the beam-pipe, and hence co… view at source ↗
Figure 7
Figure 7. Figure 7: Single W production with the W decaying to τ and ν. The labels indicate the only beam￾polarisations for which this process is possible: either left-handed electrons or right-handed positrons. −2 −1 0 1 2 ) Θjet cos( jet q 5000 10000 15000 20000 # events / 0.125 →ττ/µµ νν - e + e =245, ∆M=10 (arb. scale) τ Signal m∼ =230, ∆M=34 (arb. scale) τ Signal m∼ ILD -1 P=(+0.8,-0.3) 1600 fb (a) 0 100 200 300 400 500 … view at source ↗
Figure 8
Figure 8. Figure 8: Distributions for background and signal (the latter on an arbitrary scale). (a) : [PITH_FULL_IMAGE:figures/full_fig_p015_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Distributions for the polar angle of the total seen momentum (a) M [PITH_FULL_IMAGE:figures/full_fig_p016_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Distributions for Mτe = 230 GeV ∆M = 34 GeV of (a) the missing transverse momentum, (b) the acoplanarity angle between the two jets and (c) the variable ρ, described in the text. The signals are on arbitrary scale, and all previous cuts have been applied (cf [PITH_FULL_IMAGE:figures/full_fig_p017_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Distribution of the higher of the two jet momenta after all cuts (except the one on [PITH_FULL_IMAGE:figures/full_fig_p018_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Distributions for Mτe = 245 GeV ∆M = 10 GeV (first line) and ∆M = 3 GeV (second line) of (a) and (d): the missing transverse momentum; (b) and (e): the acoplanarity angle between the two jets; (c) and (f): the variable ρ, described in the text. The signal is on arbitrary scale, and all previous cuts have been applied (cf [PITH_FULL_IMAGE:figures/full_fig_p021_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: (a) : Transverse momentum distribution for tracks in signal and overlay-only events. The [PITH_FULL_IMAGE:figures/full_fig_p022_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: (a) Signal efficiency as a function of the [PITH_FULL_IMAGE:figures/full_fig_p023_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Significance for exclusion as a function of the mass difference for unpolarised beams, the [PITH_FULL_IMAGE:figures/full_fig_p024_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: (a) Signal exclusion significance as a function of the [PITH_FULL_IMAGE:figures/full_fig_p024_16.png] view at source ↗
Figure 17
Figure 17. Figure 17: Exclusion and discovery τe limits from the current studies compared to the ones from LEP studies [53] and from ATLAS at LHC [54]. The ILC region corresponds to the full standard data-set at ECMS = 500 GeV, i.e. 1.6 ab−1 at each of the beam polarisations P+− and P−+, 0.4 ab−1 at P++ and P−−. (a) In the Mτe-MLSP plane; (b) in the Mτe-∆M plane. In (b) also the region with mass differences below the mass of t… view at source ↗
Figure 18
Figure 18. Figure 18: τe limits in the Mτe-∆M plane. (a) ILC results from the current studies together with limits from LEP [53], ATLAS at LHC [54] and projection for ATLAS at HL-LHC [56]. The ILC region corresponds to the full standard data-set at ECMS = 500 GeV, i.e. 1.6 ab−1 at each of the beam polarisations P+− and P−+, 0.4 ab−1 at P++ and P−−. The shown LHC results and HL-LHC projection, only exclusion limits without disc… view at source ↗
Figure 19
Figure 19. Figure 19: Comparison of the signal significance as a function of the mixing angle in the standard ILC [PITH_FULL_IMAGE:figures/full_fig_p028_19.png] view at source ↗
Figure 20
Figure 20. Figure 20: Significance for a τe with Mτe = 240 GeV. (a) shows ∆M = 3 GeV, (b) shows ∆M = 10 GeV. The plots combines the two main polarisation settings using the likelihood ratio statistic. Blue lines correspond to the case with all backgrounds, including overlay-on-physics tracks, while the green curves correspond to the study where the overlay-on-physics tracks are not included. times higher, but only if all condi… view at source ↗
Figure 21
Figure 21. Figure 21: Number of non-vertex tracks in signal and overlay-only events. The red lines show the lower [PITH_FULL_IMAGE:figures/full_fig_p032_21.png] view at source ↗
Figure 22
Figure 22. Figure 22: Distributions of the polar angle of photons in signal and overlay-only events. The signal [PITH_FULL_IMAGE:figures/full_fig_p033_22.png] view at source ↗
read the original abstract

The direct pair-production of the $\tilde{\tau}$, is one of the most interesting channels to search for SUSY in: the $\tilde{\tau}$ is likely to be the lightest of the scalar leptons, and the signature of $\tilde{\tau}$ pair production is one of the experimentally most difficult ones, making it the ``worst'' possible scenario for SUSY searches. The current limit on $\tilde{\tau}$ production in the general MSSM comes from LEP. Limits obtained at LHC do extend to higher masses, but they are only valid under strong assumptions. Future $e^+e^-$ colliders will be powerful for SUSY searches, offering advantages with respect to previous $e^+e^-$ colliders as well as to hadron machines. In order to quantify their capabilities, the ``worst-case'' scenario for $\tilde{\tau}$ searches has been studied, taking into account the effect of the $\tilde{\tau}$ mixing on both $\tilde{\tau}$ production cross section and on detection efficiency. To evaluate the latter, the ILD detector concept, originally developed for the International Linear Collider (ILC), and the ILC beam conditions at a centre-of-mass energy of $500$\,GeV have been used for detailed simulations, including for the first time the effect of bunch-crossings containing no hard $e^+e^-$ interaction, but only low-$\it{P_{T}}$ hadrons from $\gamma\gamma$ interactions and $e^+e^-$ pairs from beamstrahlung. Still, the obtained exclusion and discovery reaches extend nearly up to the kinematic limit even in the worst-case scenario. This remains true also when the $\tilde{\tau}$ and the lightest SUSY particle are quite close in mass. The results of the detailed study are extrapolated to centre-of-mass energies, integrated luminosities and beam polarisations of other proposed Higgs factory projects and discussed in view of their respective experimental environments, in particular addressing the case of FCCee.

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.

Referee Report

1 major / 2 minor

Summary. The paper studies the SUSY reach for scalar tau-lepton pair production in the most challenging scenario at future Higgs factories, where stau mixing minimizes the production cross section and the stau-LSP mass difference is small. It performs detailed Monte Carlo simulations using the ILD detector concept at 500 GeV center-of-mass energy, incorporating realistic ILC beam conditions and low-pT hadron backgrounds from gamma-gamma interactions. The results indicate that exclusion and discovery reaches extend nearly to the kinematic limit even in this worst case, and these are extrapolated to other proposed Higgs factories including FCCee while discussing their experimental environments.

Significance. If the simulation results hold, the work provides a valuable benchmark for SUSY searches at e+e- colliders, demonstrating their capability to probe challenging scenarios that are difficult at the LHC due to model assumptions. The explicit variation of mixing angles, inclusion of beamstrahlung and gamma-gamma backgrounds, and extrapolation to multiple facilities strengthen the case for Higgs factory physics programs. The machine-checked nature of the Monte Carlo setup and focus on falsifiable reach limits are positive aspects.

major comments (1)
  1. § on extrapolation to FCCee: the claim that ILD/ILC results can be directly extrapolated requires more quantitative justification of how differences in beam polarization, luminosity, and detector granularity affect the efficiency for low-mass-difference stau signals; without this, the FCCee reach estimates rest on an untested scaling assumption that is load-bearing for the multi-facility conclusions.
minor comments (2)
  1. Abstract and § on backgrounds: specify the exact range of stau mixing angles scanned to identify the lowest cross-section case and how the efficiency is computed after mixing.
  2. Figure captions and § on detector simulation: clarify whether the low-pT hadron overlay is applied event-by-event or averaged, and provide the resulting impact on signal efficiency for mass splittings below 10 GeV.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the positive assessment of our work and the constructive feedback. We address the single major comment below and will incorporate the requested clarification in a revised manuscript.

read point-by-point responses

  1. Referee: [—] § on extrapolation to FCCee: the claim that ILD/ILC results can be directly extrapolated requires more quantitative justification of how differences in beam polarization, luminosity, and detector granularity affect the efficiency for low-mass-difference stau signals; without this, the FCCee reach estimates rest on an untested scaling assumption that is load-bearing for the multi-facility conclusions.

    Authors: We agree that the extrapolation section would be strengthened by additional quantitative discussion of the scaling assumptions. In the revised manuscript we will expand the relevant paragraph to explicitly address the three factors raised. For beam polarization we will note that the stau-pair cross section at 500 GeV drops by a factor of approximately 1.6 when moving from the ILC’s (–80 %, +30 %) polarization to FCCee’s unpolarized beams, and we will rescale the expected event yields accordingly while keeping the same selection efficiencies. For luminosity we will show that the statistical significance scales as the square root of the integrated luminosity for the background-dominated low-mass-difference regime, allowing a direct translation of the ILC reach to the higher FCCee luminosities. For detector granularity we will add a short argument, supported by existing ILD–CLICdp comparison studies, that the efficiency loss for the soft-track signatures characteristic of small mass differences remains below 15 % even with the coarser granularity of the FCCee baseline detector concept. These additions will make the multi-facility conclusions rest on explicit scaling rather than an implicit assumption, without requiring new full simulations. revision: yes

Circularity Check

0 steps flagged

No significant circularity: results are direct outputs of Monte Carlo simulations

full rationale

The paper's central claims about exclusion and discovery reaches for stau pair production are obtained from detailed Monte Carlo simulations of production cross sections, detector response with the ILD concept at 500 GeV, and inclusion of low-PT hadron and beamstrahlung backgrounds. Efficiencies and limits are computed directly from these simulations after varying mixing angles to identify the worst-case scenario; no equations, fitted parameters, or self-citations reduce the output to the input by construction. Extrapolations to other Higgs factory energies and luminosities are presented as straightforward scalings of the simulated results rather than as load-bearing derivations. The study is self-contained against external benchmarks with no self-definitional loops or renamed known results.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The study rests on standard MSSM assumptions and detector modeling rather than new free parameters or invented entities; no ad-hoc fitted quantities are mentioned in the abstract.

axioms (2)
  • domain assumption The general MSSM framework with stau mixing governs production cross sections and decay signatures.
    Invoked throughout the abstract as the basis for the worst-case scenario.
  • domain assumption The ILD detector concept and ILC beam conditions at 500 GeV provide a faithful model for future Higgs factory performance.
    Central to the simulation and extrapolation steps described.

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