Wilson Coefficients Research Articles

Impact of new physics on momentum-dependent particle widths and propagators

We investigate the impact of momentum-dependent particle widths and propagators on gauge and Higgs bosons and the top quark within the Standard Model (SM) and its effective field theory (SMEFT) extensions near thresholds. By incorporating self-energy corrections via Dyson resummation, we quantify deviations from the fixed-width approximation and assess their implications for collider observables. While effects on the Higgs boson are negligible and the W boson shows percent-level deviations in reconstructed transverse mass distributions, the top quark exhibits significant sensitivity near its mass threshold. Future lepton colliders, e.g., electron-positron machines or muon colliders, can offer sensitivity to these effects, enabling constraints on SMEFT Wilson coefficients. We perform a representative case study for the precision frontier available with a staged future muon collider. Our results highlight that momentum dependencies can provide additional sensitivity at precision-era experiments, enhancing the potential for discovering new physics there. Published by the American Physical Society 2025

Bootstrapping string models with entanglement minimization and machine learning

We present a new approach to bootstrapping stringlike theories by exploiting a local crossing symmetric dispersion relation and field redefinition ambiguities. This approach enables us to use mass-level truncation and to go beyond the dual resonance hypothesis. We consider both open and closed strings, focusing mainly on open tree-level amplitudes with an integer-spaced spectrum, and two leading Wilson coefficients as inputs. Using entanglement minimization in the form of the minimum of the first finite moment of linear entropy or entangling power, we get an excellent approximation to the superstring amplitudes, including the leading and subleading Regge trajectories. We find other interesting S matrices that do not obey the duality hypothesis, but exhibit a transition from Regge behavior to power law behavior in the high energy limit. Finally, we also examine machine-learning techniques to do bootstrap and discuss potential advantages over the present approach. Published by the American Physical Society 2025

Constraints on standard model effective field theory for a Higgs boson produced in association with W or Z bosons in the H →bb¯ decay channel in proton-proton collisions at s = 13 TeV

A standard model effective field theory (SMEFT) analysis with dimension-six operators probing nonresonant new physics effects is performed in the Higgs-strahlung process, where the Higgs boson is produced in association with a W or Z boson, in proton-proton collisions at a center-of-mass energy of 13 TeV. The final states in which the W or Z boson decays leptonically and the Higgs boson decays to a pair of bottom quarks are considered. The analyzed data were collected by the CMS experiment between 2016 and 2018 and correspond to an integrated luminosity of 138 fb−1. An approach designed to simultaneously optimize the sensitivity to Wilson coefficients of multiple SMEFT operators is employed. Likelihood scans as functions of the Wilson coefficients that carry SMEFT sensitivity in this final state are performed for different expansions in SMEFT. The results are consistent with the predictions of the standard model.

SMEFT predictions for semileptonic processes

The SU(2)L×U(1)Y invariance of the Standard Model effective field theory (SMEFT) predicts multiple restrictions in the space of Wilson coefficients of U(1)em invariant effective Lagrangians such as the low-energy effective field theory (LEFT), used for low-energy flavor physics observables, or the Higgs effective field theory (HEFT) in unitary gauge, appropriate for weak-scale observables. In this work, we derive and list all such predictions for semileptonic operators up to dimension 6. These predictions can be expressed as linear relations among the HEFT/LEFT Wilson coefficients (WCs), that are completely independent of any assumptions about the alignment of the mass and flavor bases. We find seven sets of relations among the WCs of vector operators, nine sets with scalar and tensor operators, and two sets with the Z,W± couplings. These correspond to 2223 linear relations among the complex WCs when the quark and lepton generation indices are included. They connect diverse experimental searches such as rare meson decays, high-pT dilepton searches, top decays, Z-pole observables, charged lepton flavor violating observables, and nonstandard neutrino interaction searches. We demonstrate how these relations can be used to derive strong indirect constraints on multiple WCs that are currently either weakly constrained from direct experiments or have no direct bound at all. They also imply, in general, that evidence for new physics in a particular search channel must be accompanied by correlated anomalies in other channels. For example, the observed excess in B→Kνν¯ would imply possible anomalies in B→D(*)ℓνℓ, B→K(*)ℓ+ℓ−, Bq→τντ, Bs→τ+τ−, Ds→τ+ντ, t→cℓ+ℓ−, t→uℓ+ℓ−, etc., correlated according to the relations obtained in this paper. Published by the American Physical Society 2025

Improving the global SMEFT picture with bounds on neutrino NSI

We analyze how neutrino oscillation and coherent elastic neutrino-nucleus scattering data impact the global SMEFT fit. We first review the mapping between the SMEFT parameters and the so-called NSI framework, commonly considered in the neutrino literature. We also present a detailed discussion of how the measurements for the normalization of neutrino fluxes and cross sections, that will also be affected by the new physics, indirectly impact the measured oscillation probabilities. We then analyze two well-motivated simplified scenarios. Firstly, we study a lepton flavour conserving case, usually assumed in global SMEFT analyses, showing the complementarity of neutrino oscillation and CEνNS experiments with other low-energy observables. We find that the inclusion of neutrino data allows to constrain previously unbounded SMEFT operators involving the tau flavour and confirm the improvement of the constraint on a combination of Wilson coefficients previously identified. Moreover, we find that neutrino oscillation constraints on NSI are improved when embedded in the global SMEFT framework. Secondly, we study a lepton flavour violating scenario and find that neutrino data also improves over previously derived global constraints thanks to its sensitivity to new combinations of Wilson coefficients.

Search for same-charge top-quark pair production in pp collisions at s = 13 TeV with the ATLAS detector

A search for the production of top-quark pairs with the same electric charge (tt or tt¯) is presented. The analysis uses proton-proton collision data at s = 13 TeV, recorded by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb−1. Events with two same-charge leptons and at least two b-tagged jets are selected. Neural networks are employed to define two selections sensitive to additional couplings beyond the Standard Model that would enhance the production rate of same-sign top-quark pairs. No significant signal is observed, leading to an upper limit on the total production cross-section of same-sign top-quark pairs of 1.6 fb at 95% confidence level. Corresponding limits on the three Wilson coefficients associated with the Otu1, OQu1, and OQu8 operators in the Standard Model Effective Field Theory framework are derived.

Matching and positivity beyond minimal coupling in effective theories of photons and gravitons

We study the low-energy effective theories of photons and gravitons, matching them at one loop to a UV completion with a massive spinning matter particle. The matter is allowed to have non-minimal electromagnetic and gravitational interactions. We construct the one-loop 4-photon and 4-graviton helicity amplitudes with matter in the loop carrying anomalous multipole moments, and we read off the Wilson coefficients in the effective theory below the matter particle mass. Much as in the case of minimal coupling, the Wilson coefficients beyond the leading order turn out to be confined to small islands in the much larger theory space allowed by existing positivity constraints.

QCD running in lepton number violating meson and tau decays

Below the electroweak scale, new physics that violates the lepton number in two units (ΔL=2) and is mediated by heavy particle exchange can be parametrized by a dimension-nine low-energy effective Lagrangian. Operators in this Lagrangian involving first-generation quarks and leptons contribute to the short-range mechanism of neutrinoless double beta decay (0νββ) and therefore they are strongly constrained. On the other hand, operators with other quark and lepton families are bounded by the nonobservation of different lepton number violating meson and tau decays, such as M1−→M2+ℓ1−ℓ2− and τ−→ℓ+M1−M2−. In this work, we calculate renormalization-group-evolution (RGE)-improved bounds on the Wilson coefficients involved in these decays. We calculate QCD corrections to the dimension-nine operator basis and find RGE matrices that describe the evolution of the Wilson coefficients across different energy scales. Unlike the running of operators involved in 0νββ decay, the general flavor structure leads to the mixing of not only different Lorentz structures but also of different quark-flavor configurations. Additionally, operators that vanish for the identical lepton case need to be added to the operator basis. We find new constraints on previously unbounded operators and the enhancement of bounds for specific Wilson coefficients. We also find new bounds coming from the mixing between operators with different quark-flavor configurations. Published by the American Physical Society 2025

Causality in the presence of stacked shockwaves

Relativistic causality constrains the S-matrix both through its analyticity, and by imposing lower bounds on the scattering time delay. These bounds are easiest to determine for spacetimes which admit either a timelike or null Killing vector. We revisit a class of pp-wave spacetimes and carefully determine the scattering time delay for arbitrary incoming states in the eikonal, semiclassical, and Born approximations. We apply this to the effective field theory of gravity in arbitrary dimensions. It is well-known that higher-dimension operators such as the Gauss-Bonnet term, when treated perturbatively at low energies, can appear to make both positive and negative contributions to the time delays of the background geometry. We show that even when multiple shockwaves are stacked, the corrections to the scattering time delay relative to the background are generically unresolvable within the regime of validity of the effective field theory so long as the Wilson coefficients are of order unity. Phrased the other way, this can be read as a bound on the Wilson coefficient from infrared causality which are consistent with positivity/bootstrap bounds. Published by the American Physical Society 2025

Search for charged-lepton flavor violation in the production and decay of top quarks using trilepton final states in proton-proton collisions at s=13 TeV

A search is performed for charged-lepton flavor violating processes in top quark (t) production and decay. The data were collected by the CMS experiment from proton-proton collisions at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 138 fb−1. The selected events are required to contain one opposite-sign electron-muon pair, a third charged lepton (electron or muon), and at least one jet of which no more than one is associated with a bottom quark. Boosted decision trees are used to distinguish signal from background, exploiting differences in the kinematics of the final states particles. The data are consistent with the standard model expectation. Upper limits at 95% confidence level are placed in the context of effective field theory on the Wilson coefficients, which range between 0.024–0.424 TeV−2 depending on the flavor of the associated light quark and the Lorentz structure of the interaction. These limits are converted to upper limits on branching fractions involving up (charm) quarks, t→eμu (t→eμc), of 0.032(0.498)×10−6, 0.022(0.369)×10−6, and 0.012(0.216)×10−6 for tensorlike, vectorlike, and scalarlike interactions, respectively. © 2025 CERN, for the CMS Collaboration 2025 CERN

CP violation in two-meson Tau decays induced by heavy new physics

We apply the effective field theory formalism that was used to study CP violation induced by heavy new physics in the τ → KSπντ decays to the other two-meson tau decay channels. We focus on the rate and the forward-backward asymmetries, that are predicted using current bounds on the complex Wilson coefficients of the effective Lagrangian. We discuss our outcomes for the modes with (K/π)π0 and KKS, that can be studied at Belle-II and a super-tau-charm facility. Our main finding is that current and forthcoming experiments would be sensitive to the maximum allowed CP rate asymmetry in the K±KS modes if a precision of 5% is reached on this observable, that can check as well the BaBar anomaly in KSπ. For the π±π0 channels, new physics would be difficult to probe at present. Disentangling new sources of CP violation would be most challenging in K±π0 and the other modes.

Flat space gravity at finite cutoff

Abstract We study the thermodynamics of Einstein gravity with vanishing cosmological constant subjected to conformal boundary conditions. Our focus is on comparing the series of subextensive terms to predictions from thermal effective field theory, with which we find agreement for the boundary theory on a spatial sphere, hyperbolic space, and flat space. We calculate the leading Wilson coefficients and observe that the first subextensive correction to the free energy is negative. This violates a conjectured bound on this coefficient in quantum field theory, which we interpret as a signal that gravity does not fully decouple in the putative boundary dual.

Probing right-handed neutrinos via trilepton signals at the HL-LHC

Neutrino oscillation experiments have provided direct evidence for the existence of neutrino masses. The seesaw mechanism explains the smallness of these masses through the introduction of heavy right-handed neutrino (RHN) states. The RHN states can also generate Dirac neutrino masses at tree or loop level. These heavy states can exist at the electroweak scale, approximately in the O(GeV) range, and can be investigated through current and future collider experiments. This scenario, where other new physics interactions occur at scales much higher than the RHN scale, can be described using an effective field theory (EFT) framework known as NR-EFT. This study focuses on constraining the Wilson coefficients of NR-EFT operators, which primarily contribute to trilepton production and missing energy signals at the LHC. We examine both the scenarios where the RHN mass MN is less than and greater than the W boson mass MW, and provide predictions for the High-Luminosity run of the LHC. Published by the American Physical Society 2025

Positivity bounds on parity-violating scalar-tensor EFTs

Using dispersion relations of the scattering amplitudes and semi-definite programming, we calculate causality bounds on the Wilson coefficients in scalar-tensor effective field theories that include parity-violating operators. Particular attention has been paid to the dynamical-Chern-Simons (dCS) and scalar-Gauss-Bonnet (sGB) couplings, along with higher order coefficients, and the interplay between them. For the leading terms, the bounds on the parity-conserving and -violating coefficients are simply projections of the complex coefficients. Some parity-violating coefficients are found to be upper bounded by the parity-conserving counterparts, or the higher order parity-conserving coefficients. While the observational constraints on parity-violating coefficients are weaker than the parity-conserving counterparts, the causality bounds are of comparable strength and thus may play a more prominent role in constraining strong gravity effects in upcoming observations.

Exploring observable effects of scalar operators beyond SMEFT in the angular distribution of B→K*0τ+τ−

The SU(2)L×U(1)Y invariance of the Standard Model effective field theory (SMEFT) imposes relations among different low-energy effective field theory Wilson coefficients (WCs), any deviations from which would signal the presence of physics beyond SMEFT. In this work, we investigate two such relations (CS=−CP, CS′=CP′) among the scalar and the pseudoscalar new physics WCs that can contribute to b→sττ processes. We show that, even when new physics violating these relations would not be measurable in the branching ratios of Bs→τ+τ− and B→K(*)τ+τ− at HL-LHC and FCC-ee, it can still manifest itself in the angular distribution of B→K*0τ+τ−. We identify the combinations of angular observables in this decay channel that are sensitive to scenarios beyond SMEFT. We find that the two observables S6c and A7, and their combination, have the potential to identify physics beyond SMEFT. Published by the American Physical Society 2024

Impact of SMEFT renormalisation group running on Higgs production at the LHC

We study the effects of the one-loop renormalisation group running/mixing of the Wilson coefficients in the standard model effective field theory on the predictions for Hj, tt¯H and HH production at the LHC. We focus on a subset of operators closed under QCD corrections and explore the differences between employing a fixed or dynamical scale on the SMEFT predictions for key observables such as the Higgs transverse momentum and the Higgs pair invariant mass. We then study the impact of consistently taking into account renormalisation group effects on the constraints that can be obtained on the Wilson coefficients through current and future measurements at the LHC.

ALP-ine quests at the LHC: hunting axion-like particles via peaks and dips in tt¯ production

We present an analysis of the sensitivity of current and future LHC searches for new spin-0 particles in top–anti-top-quark tt¯ final states, focusing on generic axion-like particles (ALPs) that are coupled to top quarks and gluons. As a first step, we derive new limits on the effective ALP Lagrangian in terms of the Wilson coefficients ct and cG~ based on the results of the CMS search using 35.9 fb−1 of data, collected at s = 13 TeV. We then investigate how the production of an ALP with generic couplings to gluons and top quarks can be distinguished from the production of a pseudoscalar which couples to gluons exclusively via a top-quark loop. To this end, we make use of the invariant tt¯ mass distribution and angular correlations that are sensitive to the tt¯ spin correlation. Using a mass of 400 GeV as an example, we find that already the data collected during Run 2 and Run 3 of the LHC provides an interesting sensitivity to the underlying nature of a possible new particle. We also analyze the prospects for data anticipated to be collected during the high-luminosity phase of the LHC. Finally, we compare the limits obtained from the tt¯ searches to existing experimental bounds from LHC searches for narrow di-photon resonances, from measurements of the production of four top quarks, and from global analyses of ALP–SMEFT interference effects.

Uniting low-energy semileptonic and hadronic anomalies within SMEFT

Two categories of four-fermion SMEFT operators are semileptonic (two quarks and two leptons) and hadronic (four quarks). At tree level, an operator of a given category contributes only to processes of the same category. However, when the SMEFT Hamiltonian is evolved down from the new-physics scale to low energies using the renormalization-group equations (RGEs), due to operator mixing this same SMEFT operator can generate operators of the other category at one loop. Thus, to search for a SMEFT explanation of a low-energy anomaly, or combination of anomalies, one must: (i) identify the candidate semileptonic and hadronic SMEFT operators, (ii) run them down to low energy with the RGEs, (iii) generate the required low-energy operators with the correct Wilson coefficients, and (iv) check that all other constraints are satisfied. In this paper, we illustrate this method by finding all SMEFT operators that, by themselves, provide a combined explanation of the (semileptonic) b¯→s¯ℓ+ℓ− anomalies and the (hadronic) B → πK puzzle.

Neutrino nonstandard interactions and lepton flavor universality violation at SND@LHC via charm production

In this work, we explore the effect of neutrino nonstandard interactions (NSI) involving the charm quark at SND@LHC. Using an effective description of new physics in terms of four-fermion operators involving a charm quark, we constrain the Wilson coefficients of the effective interaction from two and three-body charmed meson decays. In our fit, we include charmed meson decays not only to pseudoscalar final states but also to vector final states and include decays to the η and η′ final states. We also consider constraints from charmed baryon decays. We then study the effect of new physics in neutrino scattering processes, involving charm production at SND@LHC, for various benchmark new physics couplings obtained from the low energy fits. Finally, we also study the effects of lepton universality violation (LUV) assuming that the new physics coupling is not lepton universal.

Accidental suppression of Wilson coefficients in Higgs coupling

Higgs couplings are essential probes for physics beyond the Standard Model (BSM) since they can be modified by new physics, such as through the Higgs portal interaction H2O. These modifications influence Higgs interactions via dimension-6 operators of the form (∂|H|2)2 and |H|6, which are generally expected to be of comparable size. This paper discusses a phenomenon of accidental suppression, where the |H|6 coupling is significantly smaller than (∂|H|2)2. This suppression, arising from the truncation of the tree-level effective potential, lacks a clear symmetry explanation but persists in portal models. This paper aims to inspire further studies on additional instances of accidental suppression without symmetry explanations or a general framework to characterize such suppression. We also discuss constraints, at the HL-LHC and future colliders, on the Wilson coefficients of the two dimension-6 operators for various benchmark scenarios of the concrete model.