Standard Model Effective Theory Research Articles

Faking ZZZ vertices at the LHC

Searches for anomalous neutral triple gauge boson couplings (NTGCs) provide important tests for the gauge structure of the standard model. At the LHC, NTGCs are searched for via the process pp → ZZ → 4l, where the two Z-bosons are on-shell. In this paper, we discuss how the same process can occur through tree-level diagrams just adding a vector-like quark (VLQ) to the standard model. Since NTGCs are generated in standard model effective theory (SMEFT) only at 1-loop order, vector like quarks could be an important alternative interpretation to, and background for, NTGC searches. Here, we construct a simple example model, discuss low-energy constraints and estimate current and future sensitivities on the model parameters from pp → ZZ → 4l searches.

Dimension-8 SMEFT contact-terms for vector-pair production via on-shell Higgsing

We derive the dimension-8 standard-model effective theory (SMEFT) contact terms relevant for vector-pair production at the LHC and lepton colliders. We first list the relevant dimension-8 massless SMEFT amplitudes, and then obtain the low-energy amplitudes using on-shell Higgsing. In all cases, the contributions we calculate are the leading-order contributions to 4-point contact-terms; the dimension-6 SMEFT merely corrects the three-point couplings entering the amplitudes. Since they are given in terms of physical quantities, namely momenta and polarizations, the results allow for a direct mapping of EFT effects to low-energy observables. The vector amplitudes are sensitive to both anomalous vector couplings and Higgs self-couplings. The left-handed fermion amplitudes feature SU(2) violating effects first generated at dimension-8. We also compare our results to HEFT predictions. Interestingly, the dimension-8 SMEFT populates almost all the novel structures generated by the dimension-8 HEFT.

Constraining νSMEFT coefficients: The case of the extra U(1)′

We study the constraints on low-energy coefficients of the νSMEFT generalization of the Standard Model effective theory in the simple case of a U(1)′ enlargement of the Standard Model gauge group. In particular, we analyze the constraints imposed by the requirement that the extended theory remains free of gauge anomalies. We present the cases of explicit realizations, showing the obtained correlations among the coefficients of d=6 operators. Published by the American Physical Society 2024

More synergies from beauty, top, Z and Drell-Yan measurements in SMEFT

We perform a global analysis of Beauty, Top, Z and Drell-Yan measurements in the framework of the Standard Model effective theory (SMEFT). We work within the minimal flavor violation (MFV) hypothesis, which relates different sectors and generations beyond the SU(2)L-link between left-handed top and beauty quarks. We find that the constraints on the SMEFT Wilson coefficients from the combined analysis are stronger than the constraints from a fit to the individual sectors, highlighting synergies in the global approach. We also show that constraints within MFV are strengthened compared to single-generation fits. The strongest bounds are obtained for the semileptonic four-fermion triplet operator Clq3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {C}_{lq}^{(3)} $$\\end{document}, probing scales as high as 18 TeV, followed by the gluon dipole operator CuG with 7 TeV, and other four-fermion and penguin operators in the multi-TeV range. Operators with left-handed quark bilinears receive order one contributions from higher orders in the MFV expansion induced by the top Yukawa coupling as a result of the FCNC b → sμμ anomalies combined with the other sectors. We predict the 68% credible intervals of the dineutrino branching ratios within MFV as 4.25⋅10−6≤BB0→K∗0νν¯≤11.13⋅10−6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ 4.25\\cdot {10}^{-6}\\le \\mathcal{B}\\left({B}^0\ o {K}^{\\ast 0}\ u \\overline{\ u}\\right)\\le 11.13\\cdot {10}^{-6} $$\\end{document} and 2.26⋅10−6≤BB+→K+νν¯≤5.78⋅10−6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ 2.26\\cdot {10}^{-6}\\le \\mathcal{B}\\left({B}^{+}\ o {K}^{+}\ u \\overline{\ u}\\right)\\le 5.78\\cdot {10}^{-6} $$\\end{document}, which include the respective Standard Model predictions, and are in reach of the Belle II experiment. We show how future measurements of the dineutrino branching ratios can provide insights into the structure of new physics in the global fit.

Effective field theory of chirally enhanced muon mass and dipole operators

We study corrections to observables related to the muon in the context of models of new physics which generate mass-enhanced corrections to the muon dipole moments. Working in the Standard Model effective theory, we demonstrate a correlation between the decay of the Higgs boson to muons, and the magnetic and electric dipole moments of the muon generated by the dominant matching corrections. This defines a novel way to classify predictions for a wide variety of models of new physics based on the pattern of deviations of these three observables. In particular, when applied to specific models we find that this correlation has a potential to rule out whole models or set upper bounds on the scale of new physics motivated by the muon anomalous magnetic moment.

Simple rules for evanescent operators in one-loop basis transformations

Basis transformations often involve Fierz and other relations that are only valid in $D=4$ dimensions. In general $D$ spacetime dimensions, however, evanescent operators have to be introduced in order to preserve such identities. Such evanescent operators contribute to one-loop basis transformations as well as to two-loop renormalization group running. We present a simple procedure on how to systematically change basis at the one-loop level by obtaining shifts due to evanescent operators. As an example we apply this method to derive the one-loop basis transformation from the Buras, Misiak and Urban basis useful for next-to-leading order QCD calculations, to the Jenkins, Manohar and Stoffer basis used in the matching to the standard model effective theory.

RGESolver: a C++ library to perform renormalization group evolution in the Standard Model Effective Theory

Renormalization group evolution above the electroweak scale is a crucial ingredient in the phenomenology of the Standard Model Effective Theory. The RGESolver open-source C++ library performs the evolution at leading order for dimension-six operators in the most general flavour scenario (assuming lepton and baryon number conservation). Given its efficiency, RGESolver can be used to include the effects of renormalization group evolution in extensive phenomenological analyses in the framework of the Standard Model Effective Theory.

Global constraints on Yukawa operators in the standard model effective theory

CP-violating contributions to Higgs-fermion couplings are absent in the standard model of particle physics (SM), but are motivated by models of electroweak baryogenesis. Here, we employ the framework of the SM effective theory (SMEFT) to parameterise deviations from SM Yukawa couplings. We present the leading contributions of the relevant operators to the fermionic electric dipole moments (EDMs). We obtain constraints on the SMEFT Wilson coefficients from the combination of LHC data and experimental bounds on the electron, neutron, and mercury EDMs. We perform, for the first time, a combined fit to LHC and EDM data allowing the presence of CP-violating contributions from several fermion species simultaneously. Among other results, we find non-trivial correlations between EDM and LHC constraints even in the multi-parameter scans, for instance, when floating the CP-even and CP-odd couplings to all third-generation fermions.

Uncovering the root of LEFT in SMEFT

At energies below the electroweak scale, baryon number B and lepton number L violating processes are of significant importance in identifying the nature of UV extensions of the Standard Model. The imprint of UV theories on low-energy measurements can be calculated to a high accuracy using the theoretical framework of the Low-Energy Effective Theory (LEFT). Using and the operators' dimensions as classifying characteristics, we construct a network connecting operator classes of the LEFT with the Standard Model Effective Theory (SMEFT). Following the links of this network, the contact interactions described by the effective operators of LEFT can be unambiguously embedded into those of SMEFT, which enables us to constrain the possible realisations of UV theories. In turn, this can help to prioritise low-energy measurements with the aim of comprehensively testing all classes of LEFT and SMEFT operators.

EFT at FASER\u03bd

We investigate the sensitivity of the FASERν detector to new physics in the form of non-standard neutrino interactions. FASERν, which will be installed 480 m downstream of the ATLAS interaction point, will for the first time study interactions of multi-TeV neutrinos from a controlled source. Our formalism — which is applicable to any current and future neutrino experiment — is based on the Standard Model Effective Theory (SMEFT) and its counterpart, Weak Effective Field Theory (WEFT), below the electroweak scale. Starting from the WEFT Lagrangian, we compute the coefficients that modify neutrino production in meson decays and detection via deep-inelastic scattering, and we express the new physics effects in terms of modified flavor transition probabilities. For some coupling structures, we find that FASERν will be able to constrain interactions that are two to three orders of magnitude weaker than Standard Model weak interactions, implying that the experiment will be indirectly probing new physics at the multi-TeV scale. In some cases, FASERν constraints will become comparable to existing limits — some of them derived for the first time in this paper — already with 150 fb−1 of data.

Constraints on anomalous quartic gauge couplings via Zγjj production at the LHC

In this paper, we investigate the contributions of anomalous quartic gauge couplings (aQGCs) to $Z\gamma jj$ production at the Large Hadron Collider (LHC) in the context of Standard Model effective theory (SMEFT). When energy scale is large, the validity of SMEFT becomes an important issue. To ensure the validity, the unitarity bound is applied in a model independent approach, which is found to have significant suppressive effects on the signals of $O_{M_i}$ operators. The kinematic and polarization features of the aQGC signals are also studied. The polarization effect is useful to highlight the signals of $O_{T_i}$ operators. The sensitivity estimates on dimension-8 operators with unitarity bounds at $\sqrt{s}=14$ TeV are obtained.

Lessons from the B0,+→K*0,+μ+μ− angular analyses

We perform an analysis within the Standard Model of $B^{0,+} \to K^{*0,+} \mu^+ \mu^-$ decays in light of the recent measurements from the LHCb experiment, showing that new data strengthen the need for sizable hadronic contributions and correlations among them. We then extend our analysis to New Physics via the Standard Model Effective Theory, and carry out a state-of-the-art fit of available $b \to s \ell^+ \ell^-$ data, including possible hadronic contributions. We find the case of a fully left-handed operator standing out as the simplest scenario with a significance of almost $6\sigma$.

Probing new physics in dimension-8 neutral gauge couplings at e+e− colliders

Neutral triple gauge couplings (nTGCs) are absent in the standard model effective theory up to dimension-6 operators, but could arise from dimension-8 effective operators. In this work, we study the pure gauge operators of dimension-8 that contribute to nTGCs and are independent of the dimension-8 operator involving the Higgs doublet. We show that the pure gauge operators generate both $Z\gamma Z^*$ and $Z\gamma\gamma^*$ vertices with rapid energy dependence $\propto E^5$, which can be probed sensitively via the reaction $e^+e^- \to Z\gamma$. We demonstrate that measuring the nTGCs via the reaction $e^+e^- \to Z\gamma$ followed by $Z \to q\bar{q}$ decays can probe the new physics scales of dimension-8 pure gauge operators up to the range $(1-5)$TeV at the CEPC, FCC-ee and ILC colliders with $\sqrt{s}=(0.25-1)$TeV, and up to the range $(10-16)$TeV at CLIC with $\sqrt{s}=(3-5)$TeV, assuming in each case an integrated luminosity of 5/ab. We compare these sensitivities with the corresponding probes of the dimension-8 nTGC operators involving Higgs doublets and the dimension-8 fermionic contact operators that contribute to the $e^+e^-Z\gamma$ vertex.

Flavour violating effects of Yukawa running in SMEFT

We study Yukawa Renormalization Group (RG) running effects in the context of the Standard Model Effective Theory (SMEFT). The Yukawa running being flavour dependent leads to RG-induced off-diagonal entries, so that initially diagonal Yukawa matrices at the high scale have to be rediagonalized at the electroweak (EW) scale. Performing such flavour rotations can lead to flavour violating operators which differ from the ones obtained through SMEFT RG evolution. We show, that these flavour rotations can have a large impact on low-energy phenomenology. In order to demonstrate this effect, we com- pare the two sources of flavour violation numerically as well as analytically and study their influence on several examples of down-type flavour transitions. For this purpose we con- sider {B}_s-{overline{B}}_s mixing, b → sγ, b → sℓℓ as well as electroweak precision observables. We show that the rotation effect can be comparable or even larger than the contribution from pure RGE evolution of the Wilson coefficients.

..., 83106786, 114382724, 1509048322, 2343463290, 27410087742, ... efficient Hilbert series for effective theories

We present an efficient algorithm for determining the Hilbert series of an effective theory, which counts the number of operators with a specific field content, and provide a companion code called Eco (Efficient Counting of Operators) in Form. For example, the Hilbert series for the dimension 15 operators in the Standard Model Effective Theory (SMEFT) can be obtained in a minute on a single CPU core. While our implementation focusses on SMEFT, we allow for a flexible user input of the light degrees of freedom. Furthermore, gravity, as well as additional U(1) global or gauge symmetries can be included.

New method for fitting coefficients in standard model effective theory

We present an alternative method for carrying out a principal-component analysis of Wilson coefficients in standard model effective field theory (SMEFT). The method is based on singular-value decomposition (SVD). The SVD method provides information about the sensitivity of experimental observables to physics beyond the standard model that is not accessible in the Fisher-information method. In principle, the SVD method can also have computational advantages over diagonalization of the Fisher information matrix. We demonstrate the SVD method by applying it to the dimension-6 coefficients for the process of top-quark decay to a $b$ quark and a $W$ boson and use this example to illustrate some pitfalls in widely used fitting procedures. We also outline an iterative procedure for applying the SVD method to dimension-8 SMEFT coefficients.

B-decay discrepancies after Moriond 2019

Following the updated measurement of the lepton flavour universality (LFU) ratio R_K in Brightarrow Kell ell decays by LHCb, as well as a number of further measurements, e.g. R_{K^*} by Belle and B_s rightarrow mu mu by ATLAS, we analyse the global status of new physics in brightarrow s transitions in the weak effective theory at the b-quark scale, in the Standard Model effective theory above the electroweak scale, and in simplified models of new physics. We find that the data continues to strongly prefer a solution with new physics in semi-leptonic Wilson coefficients. A purely muonic contribution to the combination C_9 = -C_{10}, well suited to UV-complete interpretations, is now favoured with respect to a muonic contribution to C_9 only. An even better fit is obtained by allowing an additional LFU shift in C_9. Such a shift can be renormalization-group induced from four-fermion operators above the electroweak scale, in particular from semi-tauonic operators, able to account for the potential discrepancies in b rightarrow c transitions. This scenario is naturally realized in the simplified U_1 leptoquark model. We also analyse simplified models where a LFU effect in brightarrow sell ell is induced radiatively from four-quark operators and show that such a setup is on the brink of exclusion by LHC di-jet resonance searches.

The rise of SMEFT on-shell amplitudes

We present a map between the tree-level Standard Model Effective Theory (SMEFT) in the Warsaw basis and massive on-shell amplitudes. As a first step, we focus on the electroweak sector without fermions. We describe the Feynman rules for a particular choice of input scheme and compare them with the 3-point massive amplitudes in the broken phase. Thereby we fix an on-shell basis which allows us to study scattering amplitudes with recursion relations. We hope to open up new avenues of exploration to a complete formulation of massive EFTs in the on-shell language.

Model-independent bounds on the standard model effective theory from flavour physics

Meson-antimeson mixing provides the most stringent constraints on baryon- and lepton-number conserving New Physics, probing scales higher than 105 TeV. In the context of the effective theory of weak interactions, these constraints translate into severe bounds on the coefficients of ΔF=2 operators. Generalizing to the effective theory invariant under the Standard Model gauge group, valid above the electroweak scale, the bounds from ΔF=2 processes also affect ΔF=1 and even ΔF=0 operators, due to log-enhanced radiative corrections induced by Yukawa couplings. We systematically analyze the effect of the renormalization group evolution above the electroweak scale and provide for the first time the full set of constraints on all relevant dimension-six operators.

Proton decay at one loop

Proton decay is usually discussed in the context of grand unified theories. However, as is well-known, in the standard model effective theory proton decay appears in the form of higher dimensional non-renormalizable operators. Here, we study systematically the 1-loop decomposition of the $d=6$ $B+L$ violating operators. We exhaustively list the possible 1-loop ultra-violet completions of these operators and discuss that, in general, two distinct classes of models appear. Models in the first class need an additional symmetry in order to avoid tree-level proton decay. These models necessarily contain a neutral particle, which could act as a dark matter candidate. For models in the second class the loop contribution dominates automatically over the tree-level proton decay, without the need for additional symmetries. We also discuss possible phenomenology of two example models, one from each class, and their possible connections to neutrino masses, LHC searches and dark matter.