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Open research questions in Particle physics theoretical and experimental studies

50 unresolved questions extracted from the limitations and future-work sections of 513 Particle physics theoretical and experimental studies papers in our library. Each links back to the study that raised it.

What the literature leaves open

  • We find, empirically, that significantly-enhanced triple Higgs boson production is limited to the range of masses m3 À 650 GeV or m2 À 450 GeV, and that it can be excluded for a subset of these points, while single heavy scalar production will exclude the majority of the investigated points.

    Triple Higgs boson production with two heavy scalars at the LHC via a simplified approach · 2026 · DOI

  • Our results establish softly broken GCP-symmetric three-Higgs-doublet models as phenomenologically viable extensions of the Standard Model and provide a framework for further studies of their flavor phenomenology.

    Scrutinizing the Mass Matrices in Three-Higgs-Doublet Models with Generalized CP Symmetries · 2026

  • While this shift is non-negligible and exceeds the originally quoted uncertainty from this source, it remains insufficient to explain the full discrepancy observed in the CDF measurement.

    Modeling uncertainties on the Z boson background in the context of high precision W boson mass measurements · 2026 · DOI

  • Peskin, “ILC: Open Questions and New Ideas”, talk at the Snowmass Energy Frontier Workshop on Open Questions and New Ideas, Fermilab, USA, July 20-22, 2020; [34] CLICdp Collaboration and ILD Concept Group, A.

    Probing neutral triple gauge couplings via <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>Z</mml:mi> <mml:mi>Z</mml:mi> </mml:math> production at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:msup> <mml:mi>e</mml:mi> <mml:mo>−</mml:mo> </mml:msup> </mml:math> colliders with machine learning · 2026 · DOI

  • Our results indicate that physically structured representations of QCD radiation retain discriminating value in the transformer era, motivating further study into how different aspects of jet dynamics are encoded by deep learning algorithms.

    Particle-Lund Multimodality in Jet Taggers · 2026

  • The unified study of the dead cone across charm, bottom, and top quarks demonstrates the predictive power of perturbative QCD in describing radiation dynamics over a wide range of masses and lifetimes. The momentum-space suppression observed in charm and bottom jets and the newly developed method for top-quark jets together establish the dead cone as a robust and experimentally accessible QCD signature. The clear separation between the charm and bottom curves on the one hand and the strongly suppressed top quark distributions on the other hand highlights the unique role of the top quark as a laboratory for studying QCD radiation in the presence of a very large mass and a finite lifetime. In this respect, top-quark jets provide direct access to the interplay between production radiation, decay radiation, and colour coherence effects, opening new possibilities for precision tests of perturbative QCD. Future measurements at high-energy lepton colliders, such as the FCC, and at the LHC, combined with modern jet substructure techniques, will allow these effects to be probed with increasing precision. In particular, differential studies of ξ distributions, transverse-momentum spectra, and angular correlations in heavy-quark jets will make it possible to test the dead-cone dynamics beyond leading-logarithmic accuracy and constrain nonperturbative effects in the ultrasoft region. From a broader perspective, the dead cone provides a unifying link between heavy-quark fragmentation, jet structure, and fundamental QCD radiation patterns. In this way, the dead cone not only deepens our understanding of QCD dynamics but also contributes directly to the precision program of current and future collider experiments. Funding Open Access funding enabled and organized by Projekt DEAL. Data Availability The manuscript has associated data in a data repository. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

    The dead cone effect in heavy-quark jets: a unified study from charm and bottom to top · 2026 · DOI

  • Tables II and VIII make this explicit by tying each open question to (i) an observable (or set of observables), (ii) an approximate sensitivity scale that would qualitatively change the inference, and (iii) the cross-checks required for robustness.

    Fundamental physics in 2025: status, decisive targets, and path forward · 2026 · DOI

  • In this Letter, we have introduced a novel modelbuilding framework in which the observed smallness of light neutrino masses arises “gravity-assisted”. While neutrino masses generated by a Planck-suppressed Weinberg operator are expected to be too small, and the standard type-I seesaw mechanism with order one couplings points to high-scale neutrino mass generation far above observational capabilities, the “gravity-assisted” scheme opens up new possibilities for constructing “low-scale” neutrino mass generation models, where light neutrino masses are “symmetry protected” and therefore technically natural. No small model parameters or cancellations are required in order to arrive at models that explain the small light neutrino masses while being within reach of the (HL-)LHC and future colliders and testable by precision experiments (e.g. on cLFV). Gravity-assisted neutrino masses arise in the presence of a global symmetry that is spontaneously broken at an intermediate scale between the electroweak and Planck leaving a remnant discrete symmetry that forscales, bids light neutrino masses. After spontaneous symmetry breaking, masses and mixing with active neutrinos are generated for the HNLs, potentially suppressed by powers of ⟨ϕ⟩ /Λ. Nevertheless, light neutrinos remain massless due to the residual discrete symmetry, and pairs of right-chiral neutrinos form (at this level exact) heavy Dirac states. Light neutrino masses finally arise through gravity, when the residual symmetry is broken by Plancksuppressed effective operators. The result is small light neutrino masses and pseudo-Dirac pairs of heavy neutrinos with masses that can lie within the reach of future collider experiments, far below the usually expected high seesaw scale. We described the proposed new scheme explicitly for the case of 2 HNLs and a global U(1) symmetry broken spontaneously to a Z3 subgroup. It leads to a plethora of new models with expected rich phenomenology that depends on the symmetry assignments. We illustrated “gravity-assisted” neutrino mass generation and its phenomenological implications for collider searches in a simple benchmark realisation, leaving a more comprehensive exploration of the model class and its extensions for future work. In addition to predictions for future collider 5 searches and precision tests such as cLFV, which may allow to test and distinguish specific model realisations, various cosmological consequences also offer interesting discovery potential. For example, the breaking of the U(1) symmetry, spontaneously as well as explicitly by gravity, leads to a Pseudo-Nambu Goldstone boson with mass linked to MR. This breaking may additionally induce networks of hybrid topological defects composed of cosmic strings and domain walls, which may generate a testable contribution to the stochastic gravitational wave background.

    Gravity-assisted neutrino masses · 2026 · DOI

  • Future work will focus on incorporating an advanced approach to systematic uncertainty estimation, exploring probabilistic models to quantify the uncertainty in ML-based fake factor predictions beyond simple statistical variations available through data resampling techniques (bootstrapping, Jackknife, etc.

    Neural fake factor estimation using data-based inference · 2026 · DOI

  • The paper validates the SCETLIB + DYTURBO correction against standalone MINNLOPS predictions in Z → μμ events (Extended Data Fig. 5) but does not report comparison of the corrected pT predictions with alternative resummed calculations (MATRIX + RADISH, CuTe-MCFM) propagated through the full W boson mass extraction analysis with identical experimental systematics treatment, limiting quantification of theoretical prediction dependence on the specific resummation implementation.

    High-precision measurement of the W boson mass with the CMS experiment · 2026 · DOI

  • The perturbative uncertainty in the fixed-order matching is evaluated using 7-point scale variations in DYTURBO, but the paper states this uncertainty is correlated between W and Z boson decay channels yet uncorrelated between W and Z production. The physical justification for this asymmetric correlation structure and whether alternative approaches (e.g., simultaneous DYTURBO + SCETLIB scale variation or higher-point scale variation schemes) would change the extracted mW value has not been explored.

    High-precision measurement of the W boson mass with the CMS experiment · 2026 · DOI

  • The beam function TNPs in the SCETLIB calculation are parameterized separately for five partonic splitting channels (qqV, qg, qqV, qqS, qqΔS), but the paper does not quantify the correlation structure between these beam function components or validate whether treating their normalization as independent scalar variations correctly captures correlations from the underlying renormalization group equations in the W boson mass fit.

    High-precision measurement of the W boson mass with the CMS experiment · 2026 · DOI

  • The matching procedure between the resummed SCETLIB + DYTURBO predictions and the fixed-order MINNLOPS calculation uses a transition scale midpoint, and the paper evaluates uncertainty by varying this scale. However, the optimal choice of this transition scale as a function of pT, yV, and mV in the W boson mass extraction has not been systematically explored, and sensitivity to different functional forms of the matching scheme beyond the current implementation needs investigation.

    High-precision measurement of the W boson mass with the CMS experiment · 2026 · DOI

  • The nonperturbative uncertainty contribution to the pT distribution is described as 'most pronounced at low pT' in the W boson mass measurement, but the paper does not specify quantitatively how nonperturbative effects (beyond the parton shower tune) scale as a function of pT in the region below 20 GeV where sensitivity to mW is highest. Precise characterization of the nonperturbative QCD effects in this critical kinematic region using alternative parton shower and hadronization models remains unaddressed.

    High-precision measurement of the W boson mass with the CMS experiment · 2026 · DOI

  • The TNP approach for estimating perturbative uncertainties in W boson mass measurement relies on parameterizing unknown higher-order corrections in the hard, soft, and beam functions. However, the paper only validates the robustness of the N3+0LL, N3+1LL, and N4+0LL schemes against the measured pT distribution in Z → μμ events; direct experimental validation of these TNP variations against independent W boson production datasets at different collision energies or in different kinematic regions (e.g., high rapidity or forward detector acceptance) has not been performed.

    High-precision measurement of the W boson mass with the CMS experiment · 2026 · DOI

  • Multi-parameter SMEFT fits and global combinations across different processes and phase-space selections require further development to ensure robust portability of results and consistent interpretation.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • At the HL-LHC, presenting SMEFT constraints together with kinematic scales that dominate sensitivity and as a function of maximum probed scale will become standard practice but requires further methodological development.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • Sensitivity of reported intervals to modeling assumptions and choice of observables in SMEFT fits requires systematic validation through alternative binnings and reduced observable sets, with further cross-checks needed.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • Translation of confidence intervals on Wilson coefficients into indicative lower bounds on UV scales under coupling assumptions (e.g., ci ∼ g²∗ or ci ∼ 1) is model dependent and requires further development for robust interpretation.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • When likelihood becomes non-parabolic near the SM point in quadratic-dominated fits, asymptotic approximations can be less reliable, and coverage studies or alternative interval constructions may be warranted but are not yet systematically applied.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • Practical aspects of SMEFT simulation and event reweighting, including potential biases when separating squared terms, require further scrutiny and standardization across analyses.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • At the HL-LHC, validity criteria for EFT constraints will become increasingly important as statistical uncertainties are reduced, but systematic approaches for enforcing these criteria across analyses are still being developed.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • Energy-growing operator effects and the use of high-pT tails raise the question of EFT validity; a practical approach comparing typical momentum transfer to inferred Λ and energy-binned fits are increasingly used but remain area of active development.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • For directions in parameter space where interference is suppressed, quadratic terms can dominate, and resulting limits should be interpreted as conditional on the smallness of dimension-eight contributions and on the validity of the EFT in the probed kinematics.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

  • Quadratic terms in the EFT expansion introduce contributions formally of the same order as neglected dimension-eight operators, and bounds derived from purely quadratic fits should be interpreted with the implicit assumption that dimension-eight contributions are subleading in the explored phase space.

    Indirect Probes of Electroweak Interactions in the Top-Quark Sector: Recent SMEFT Results from CMS · 2026 · DOI

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50 open questions have been extracted from the limitations and future-work passages of 513 Particle physics theoretical and experimental studies papers in our library. Each one below links back to the study that raised it, so you can read the original claim in context.

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