Linear Standard Model (SM) extensions, defined as new particles that can couple linearly to SM fields, form a motivated and finite set of simplified models for exploring phenomenology Beyond the SM (BSM). Heavy BSM particles may be integrated out to obtain their low-energy effects in the SM Effective Field Theory (SMEFT) parametrised by the Wilson coefficients of higher-dimensional operators. We compute and map the dimension-6 SMEFT operator structure of all scalar and fermion linear SM extensions up to one-loop order, thus extending the existing tree-level dictionary of results. Explicit analytic matching expressions for the Wilson coefficients are provided as both Python and Mathematica code in a GitHub repository accessible through links embedded in our main table for each coefficient and within a Python package. We apply our map to highlight the sensitivity to heavy new physics of a $\textit{Z}$-pole run at a future Tera-$\textit{Z}$ factory; at one loop, with unit couplings, all linear SM extensions can be indirectly probed by electroweak precision measurements up to $$ \mathcal{O}(10) $$ TeV.

Spain

Australia

John Gargalionis

France

Jérémie Quevillon

Germany

Pham Vuong

United Kingdom

Tevong You

Linear Standard Model extensions in the SMEFT at one loop and Tera-$\textit{Z}$

We discuss the phenomenology at hadron colliders of a pseudoscalar Higgs boson <italic>A</italic>, as predicted in many extensions of the Standard Model. In particular, we highlight the aspects that makes it different from the case of a quasi-bound toponium state. We apply the discussion to the excess of <math altimg='si1.svg'><mi>t</mi><mover accent='true'><mrow><mi>t</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover></math> threshold events recently observed at the LHC by the CMS collaboration <cross-ref refid='br0010' id='crf0090'>[1]</cross-ref>, which could in principle be due to either or both possibilities, although Occam's razor favours the toponium scenario. Working in an effective theory in which only an additional pseudoscalar <italic>A</italic> boson is present in the spectrum, with a mass above the <math altimg='si2.svg'><mn>2</mn><msub><mrow><mi>m</mi></mrow><mrow><mi>t</mi></mrow></msub></math> threshold and a significant coupling to top quarks, we discuss the interference between <italic>A</italic> production in the dominant gluon-fusion process <math altimg='si3.svg'><mi>g</mi><mi>g</mi><mo stretchy='false'>&#8594;</mo><mi>A</mi></math> with subsequent <math altimg='si4.svg'><mi>A</mi><mo stretchy='false'>&#8594;</mo><mi>t</mi><mover accent='true'><mrow><mi>t</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover></math> decays, and the QCD continuum background, <math altimg='si5.svg'><mi>g</mi><mi>g</mi><mo stretchy='false'>&#8594;</mo><mi>t</mi><mover accent='true'><mrow><mi>t</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover></math>. While this interference is absent in the case of toponium, it is essential for evaluating <italic>A</italic> production. It is difficult to resolve the peak/dip structure that it generates because of the experimental smearing of the <math altimg='si1.svg'><mi>t</mi><mover accent='true'><mrow><mi>t</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover></math> invariant mass spectrum. However, by comparing the total <italic>A</italic> production rates for different integration domains of the <math altimg='si1.svg'><mi>t</mi><mover accent='true'><mrow><mi>t</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover></math> invariant mass or, eventually, at different center-of-mass energies, one may be able to observe its effects. We then discuss additional mechanisms for <italic>A</italic> production in <italic>pp</italic> collisions, including loop-induced production in association with the lighter <italic>h</italic> boson, <math altimg='si6.svg'><mi>g</mi><mi>g</mi><mo stretchy='false'>&#8594;</mo><mi>h</mi><mi>A</mi></math>, and production in association with top-quark pairs, <math altimg='si7.svg'><mi>g</mi><mi>g</mi><mo stretchy='false'>/</mo><mi>q</mi><mover accent='true'><mrow><mi>q</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover><mo stretchy='false'>&#8594;</mo><mi>t</mi><mover accent='true'><mrow><mi>t</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover><mi>A</mi></math>. These mechanisms have small cross sections at the LHC, and their observation will necessitate higher luminosities or collider energies.

Abdelhak Djouadi

Switzerland

John Ellis

Contrasting pseudoscalar Higgs and toponium states at the LHC and beyond

We show that the QCD axion couples to the electromagnetic kinetic term at one loop. The result is that if axions make up dark matter, they induce temporal variation of the fine structure constant <math><mi>α</mi></math>, which is severely constrained. We recast these constraints on the QCD axion parameter space. We also discuss how to generalize our finding to axionlike particles, and the resulting constraints.

Carl Beadle

Sebastian A. R. Ellis

CERN

Pham Ngoc Hoa Vuong

Quadratic coupling of the axion to photons

We derive constraints on the four dimensional energy-momentum tensor from gravitational and gauge anomalies. Our work can be considered an extension of Duff’s analysis [1] to include parity-odd terms and explicit symmetry breaking. The constraints imply the absence of the parity-odd   <math> <mi>R</mi> <mover> <mi>R</mi> <mo>~</mo> </mover> </math>  $$ R\overset{\sim }{R} $$  and   <math> <mi>F</mi> <mover> <mi>F</mi> <mo>~</mo> </mover> </math>  $$ F\overset{\sim }{F} $$  terms, for theories whose symmetries are compatible with dimensional regularisation, in a model-independent way. Remarkably, even in the case of explicit symmetry breaking the □R-anomaly is found to be finite and unambiguous after applying the symmetry constraints. We compute mixed gravity-gauge anomalies at leading order and deduce phenomenological consequences for vector bosons associated with global chiral symmetries.

Rémy Larue

Roman Zwicky

Gravity-gauge anomaly constraints on the energy-momentum tensor

In this Letter, we propose reactoscope, a novel experimental setup for axionlike particle (ALP) searches. Nuclear reactors produce a copious number of photons, a fraction of which could convert into ALPs via Primakoff process in the reactor core. The generated flux of ALPs leaves the nuclear power plant and its passage through a region with a strong magnetic field results in the efficient conversion to photons that can be detected. Such magnetic field is the key component of axion haloscope experiments. Adjacent nuclear reactor and axion haloscope experiments exist in Grenoble, France. There, the Institut Laue-Langevin research reactor is situated only <math><mrow><mo>∼</mo><mn>700</mn><mtext> </mtext><mtext> </mtext><mi>m</mi></mrow></math> from GrAHal, the axion haloscope platform designed to offer several volume and magnetic field (up to 43 T) configurations. We derive sensitivity projections for photophilic ALP searches with the institute and GrAHal, and also scrutinize analogous realizations, such as the one comprising the Axion Solar Telescope experiment at CERN and the Bugey nuclear power plant. The results that we obtain complement and extend the reach of existing laboratory experiments, e.g., the light-shining-through-walls experiment. While the derived sensitivities are not competitive when compared to the astrophysical limits, our analysis is free from the assumptions associated with those limits.

Italy

Fernando Arias-Aragón

United States

Vedran Brdar

New Directions for Axionlike Particle Searches Combining Nuclear Reactors and Haloscopes

The ATLAS and CMS collaborations at the LHC have recently announced evidence for the rare Higgs boson decay into a Z boson and a photon. We analyze the interference between the process <math><mi>g</mi><mi>g</mi><mo>→</mo><mi>H</mi><mo>→</mo><mi>Z</mi><mi>γ</mi></math> induced by loops of heavy particles, which is by far the dominant contribution to the signal, and the continuum <math><mi>g</mi><mi>g</mi><mo>→</mo><mi>Z</mi><mi>γ</mi></math> QCD background process mediated by light quark loops. This interference modifies the event yield, the resonance line-shape and the apparent mass of the Higgs boson. We calculate the radiative corrections to this interference beyond the leading-order approximation in perturbative QCD and find that, while differing numerically from the corresponding effects on the more studied <math><mi>g</mi><mi>g</mi><mo>→</mo><mi>γ</mi><mi>γ</mi></math> signal, they are generally rather small. As such, they do not impact significantly the interpretation of the present measurements of the <math><mi>H</mi><mo>→</mo><mi>Z</mi><mi>γ</mi></math> decay mode.

Federico Buccioni

Federica Devoto

Lorenzo Tancredi

Interference effects in → → Beyond Leading Order

We compute the trace, diffeomorphism and Lorentz anomalies of a free Weyl fermion in a gravitational background field by path integral methods. This is achieved by regularising the variation of the determinant of the Weyl operator building on earlier work by Leutwyler. The trace anomaly is found to be one half of the one of a Dirac fermion. Most importantly we establish that the potential parity-odd curvature term   <math> <mi>R</mi> <mover> <mi>R</mi> <mo>~</mo> </mover> </math>  $$ R\overset{\sim }{R} $$ , corresponding to the Pontryagin density, vanishes. This is to the contrary of some recent findings in the literature which gave rise to a controversy. We verify, that the regularisation does not lead to (spurious) anomalies in the Lorentz and diffeomorphism symmetries. We argue that in d = 2 (mod 4) P- and CP-odd terms cannot appear and that for d = 4 (mod 4) they are absent at least at leading order.

Trace anomaly of weyl fermions via the path integral

We complete the so-called Universal One-Loop Effective Action (UOLEA) with effects of gravity and provide a systematic approach to incorporate higher dimensional operators in curved spacetime. The functional determinant stemming from the path integral is computed using the Covariant Derivative Expansion (CDE), in a momentum representation that does not rely on a specific choice of coordinate to be defined, as it often is. This efficient approach manifests an interesting novelty as it allows to integrate out chiral fermions in curved spacetime in a direct manner leading to new operators involving the curvature, and provides a new alternative to the use of Feynman diagrams in that regard. The method presented would very well fit in a code that performs CDE, offering the possibility to integrate out at one-loop fields on a curved spacetime background, including spin-2 fields, like the graviton. Eventually these results should provide an interesting way to study low energy effects of UV completions of gravity.

The universal one-loop effective action with gravity

The Effective Field Theory (EFT) Lagrangian for a generic axion-like particle (ALP) has many free parameters, and leaves quite some freedom for the expected phenomenology. In this work, we set up more constrained EFTs by enforcing true axion-like properties for the ALP. Indeed, though the Peccei-Quinn symmetry of the QCD axion is anomalous, it is so in specific ways, and this shows up as consistency conditions between the gauge boson and fermion couplings. We propose to enforce such conditions, as inspired from the DFSZ and KSVZ scenarios, on the generic ALP EFTs. These truly axion-like ALP EFTs are then particularly well-suited as benchmark scenarios, to be used in the search for ALPs both at colliders and at low-energy experiments.

Christopher Smith

Axion-like ALPs

The path-integral measure of a gauge-invariant fermion theory is transformed under the chiral transformation and leads to an elegant derivation of the anomalous chiral Ward-Takahashi identities, as we know from the seminal work of Fujikawa. We present in this work an alternative and illuminating way to calculate the Jacobian in the path-integral measure from the covariant derivative expansion technique used in effective field theory. We present several ways to customize the crucial regularization such that the anomaly is located in the desired current, which is unprecedented within the path integral approach. We are then able to derive, in a transparent and unified way, the covariant, consistent, gravitational, and scale anomalies.