The Radiative Flavor Template at the LHC g-2 and W-mass Giacomo Cacciapaglia1 2Antimo Cagnotta3 4yRoberta Calabrese3 4z

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The Radiative Flavor Template at the LHC:

g-2 and W-mass

Giacomo Cacciapaglia,1, 2, ∗Antimo Cagnotta,3, 4, †Roberta Calabrese,3, 4, ‡

Francesco Carnevali,3, 4, §Agostino De Iorio,3, 4, ¶Alberto Orso Maria

Iorio,3, 4, ∗∗ Stefano Morisi,3, 4, †† and Francesco Sannino3, 4, 5, 6, ‡‡

1Institut de Physique des 2 Inﬁnis de Lyon (IP2I), UMR5822,

CNRS/IN2P3, F-69622 Villeurbanne Cedex, France

2University of Lyon, Universit´e Claude Bernard Lyon 1, F-69001 Lyon, France

3Dipartimento di Fisica “Ettore Pancini”, Universit`a degli studi di Napoli

“Federico II”, Complesso Univ. Monte S. Angelo, I-80126 Napoli, Italy

4INFN - Sezione di Napoli, Complesso Univ. Monte S. Angelo, I-80126 Napoli, Italy

5Scuola Superiore Meridionale, Largo S. Marcellino, 10, 80138 Napoli NA, Italy

6CP3-Origins and Danish-IAS, Univ. of Southern Denmark,

Campusvej 55, 5230 Odense M, Denmark

arXiv:2210.07131v2 [hep-ph] 19 Apr 2023

Abstract

The Standard Model of particle physics and its description of nature have been recently chal-

lenged by a series of precision measurements performed via diﬀerent accelerator machines. Sta-

tistically signiﬁcant anomalies emerged when measuring the muon magnetic momentum, and very

recently when deducing the mass of the Wboson. Here we consider a radiative extension of the

Standard Model devised to be suﬃciently versatile to reconcile the various experimental results

while further predicting the existence of new bosons and fermions with a mass spectrum in the TeV

energy scale. The resulting spectrum is, therefore, within the energy reach of the proton-proton

collisions at the LHC experiments at CERN.

The model investigated here allows us to interpolate between composite and elementary exten-

sions of the Standard Model with an emphasis on a new modiﬁed Yukawa sector that is needed to

accommodate the anomalies. Focusing on the radiative regime of the model, we introduce inter-

esting search channels of immediate impact for the ATLAS and CMS experimental programs such

as the associate production of Standard Model particles with either invisible or long-lived parti-

cles. We further show how to adapt earlier supersymmetry-motivated searchers of new physics to

constrain the spectrum and couplings of the new scalars and fermions. Overall, the new physics

template simultaneously accounts for the bulk of the observed experimental anomalies while sug-

gesting a wide spectrum of experimental signatures relevant for the current LHC experiments.

∗g.cacciapaglia@ip2i.in2p3.fr

†antimo.cagnotta@unina.it

‡rcalabrese@na.infn.it

§francesco.carnevali@unina.it

¶agostino.deiorio@unina.it

∗∗ albertoorsomaria.iorio@unina.it

†† smorisi@na.infn.it

‡‡ sannino@cp3.sdu.dk

I. INTRODUCTION

In the ﬁrst two decades of this millennium, increasing experimental evidence of the exis-

tence of new physics (NP) beyond the Standard Model (SM) has been accumulated. Several

standard deviations from the SM predictions have been observed when determining the

anomalous magnetic moment (g−2) of the muon by the E821 experiment [1] at Brookhaven

National Laboratory, and in the semileptonic decays B →D(∗)lν [2–4] and B →K(∗)ll [5] by

the BaBar, Belle and LHCb Collaborations. These anomalies have been conﬁrmed by the

most recent measurement performed by the Muon g−2 Collaboration at Fermilab [6] and at

ﬂavor experiments [7,8]. Notably, the anomalies in the semileptonic Bmeson decays emerge

due to the observed diﬀerences between the decay rates in diﬀerent lepton families, encoded

in the ratios RK(∗)and RD(∗). In the third decade of this century the CDF II Collaboration at

Fermilab oﬀered an unexpected sign of new physics by unveiling the high-precision measure-

ment of the Wboson mass [9] from decade-old Tevatron data. The result is seven standard

deviations away from the SM prediction. Even after taking into account higher-order cor-

rections [10] and the proper average with previous measurements [11,12], which reduce the

tension to a little less than 4 standard deviations [12,13], this result clearly points toward

a tension with the SM [14–16]. The most recent LHCb results [17,18] do, however, show a

compatibility of RK(∗)with the SM within two standard deviations. All this evidence [12]

adds to long-standing indirect hints of NP, most notably the dark matter problem, the origin

of neutrino masses that emerge from the observation of neutrino oscillations, and the origin

of the baryon asymmetry in the Universe.

Models with additional fermions and scalars, which couple to a single SM quark or lepton

via new Yukawa-like couplings not involving the SM Brout-Englert-Higgs ﬁeld, are prime

candidates to explain the g−2 anomalies while taking into account the B meson data [19–25].

The NP contributions arise at loop level, while additional bounds come, most notably, from

B0–B0mixing and eﬀects on the Zand Higgs bosons couplings to muons. Such models,

depending on the quantum numbers of the new states, can also contain a dark matter

candidate [23]. A model of this kind has been used in Ref. [26] as a template interpolating

perturbative models and strongly interacting models, like technicolor like ones and models

of fundamental partial compositeness [27–29]. In the latter limit, the multiplicity of the new

fermions and scalars is due to a new conﬁning gauge interaction. The inclusion of the RD(∗)

anomalies in this class of models requires further investigation, as loop eﬀects can hardly

compete with the tree-level contribution to the B →D(∗)l ν decay processes in the SM.

Corrections to the Wboson mass can also be explained in the context of this model as

coming from additional fermion loops, providing additional constraints on the new particles

and couplings.

In this paper, we aim to describe the productions and decays of the new families of bosons

and fermions in proton-proton collisions, in the conditions akin to the ones provided by the

Large Hadron Collider (LHC) and future hadron collider projects. We will, in particular, es-

timate the limits on the parameter space from current LHC searches, and identify promising

new channels that deserve further investigation and dedicated searches.

We focus on the radiative case, where the new fermions and scalars can be directly

produced. In the composite case they are conﬁned in new meson and baryon states, that

need to be studied at the LHC via their eﬀective interactions.

This article is structured as follows: Sec. II gives a brief overview of the framework

following the notation adopted in Ref. [26], establishing the terminology and the assumptions

made in the rest of the paper. Section III describes the signatures that could arise in

proton-proton collisions, following diﬀerent hypotheses on the mass hierarchies and on the

parameter range allowed by the precision measurement constraints, while Section IV focuses

on describing in detail the phenomenological implications at the LHC of one speciﬁc decay

channel via the reinterpretation of one existing search by the Compact Muon Solenoid (CMS)

experiment. Finally, Sec. Vreviews the constraints on the anomalies in the context of this

model and draws conclusions on the potential for future studies at the LHC, before the

conclusions in Sec. VI.

II. THEORETICAL FRAMEWORK

Following Ref. [26], the class of models described here naturally interpolates between

dynamical models of electroweak symmetry breaking and perturbative models where radia-

tive corrections contribute to ﬂavor observables. In the latter case, the new strong gauge

symmetry is demoted to a global one, acting on the new fermions and scalars, hence simply

counting their multiplicity. The main contribution to anomalies, therefore, stems from loop

diagrams involving the new fermions and scalars. These loops also contribute to the Yukawa

couplings of the SM fermions; hence, one can design models where light fermion masses and

ﬂavor structures can be radiatively generated [30]. In this work, we will consider a more

general scenario, where direct couplings of the SM fermions to the Higgs boson are also

present, and SM fermion masses emerge from a combination of the tree-level couplings and

the loop contributions. For concreteness, we will consider a model where the SM is extended

by means of a set of Dirac fermions, an SU(2)Ldoublet FLand two SU(2)Lsinglets FN

and FE, and two scalar ﬁelds S`and Sq. Their assigned quantum numbers are shown in

Table I. We can appreciate that the hypercharge of these new fundamental particles is ﬁxed

once the parameter Yis chosen. Integer values of Yfor the radiative case are not allowed,

because they would lead to fractionally charged elementary states, and |Y|>1/2 are not

allowed because they would not admit neutral particles.

In this paper we will focus our attention on the case Y= 1/2 [26].

The NP resides in the Yukawa sector of the theory by means of the most general Yukawa

Lagrangian involving the new fermions that is also, by construction, invariant under the

global GTC symmetry. Besides kinetic terms and masses for the new fermions and scalars,

the SM Lagrangian is complemented by the following set of Yukawa-like interactions:

−LYuk,NP =yij

LLiFLSj

E∗+yij

EEicFc

NSj

E+

yij

QQiFLSj

D∗+yij

DDicFc

NSj

D+yij

UUicFc

ESj

D+

√2k↑FLFc

N+k↓FEFc

LΦH+ h.c.,

(1)

where FL=F↑

L,F↓

LTwith respect to SU(2)Lwhile Q, Uc, Dc, L, Ecare the SM

fermions expressed in terms of chiral left-handed spinors. Note also that the SM ﬂavor

indices i, j are carried by the scalars. The ﬁeld ΦHis the SM Higgs scalar doublet. The

complete Lagrangian also contains a generic scalar potential, to complement the usual SM

Lagrangian. Also, the ﬁelds Rctransform as the representation conjugated to R, for example

Uc= (¯

3,1)−2/3. Further terms that violate GTC can be added to Eq. (1), and they will be

considered in a follow-up work.

For the choice Y= 1/2 we made in this analysis, the electric charges of the new fermions

and scalars are ﬁxed, as reported in Table I. We see that Sqhas the same charge as up-type

quarks, while F↓

Land FEare neutral, and F↑

Land FNcarry positive charge. This implies,

for instance, that Sj

qcan decay into an up-type quark plus a neutral heavy fermion or a

down-type quark and a heavy charged fermion. This will be further explored in Sec. III,

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TheRadiativeFlavorTemplateattheLHC:g-2andW-massGiacomoCacciapaglia,1,2,AntimoCagnotta,3,4,yRobertaCalabrese,3,4,zFrancescoCarnevali,3,4,xAgostinoDeIorio,3,4,{AlbertoOrsoMariaIorio,3,4,StefanoMorisi,3,4,yyandFrancescoSannino3,4,5,6,zz1InstitutdePhysiquedes2InnisdeLyon(IP2I),UMR5822,CNRS/IN2P3,F-6...

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The Radiative Flavor Template at the LHC g-2 and W-mass Giacomo Cacciapaglia1 2Antimo Cagnotta3 4yRoberta Calabrese3 4z

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