Quark Sector Research Articles

Adding Flavor to the SMEFT

We study the flavor structure of the lepton and baryon number-conserving dimension-6 operators in the Standard Model effective field theory (SMEFT). Building on the work of [1], we define several well-motivated flavor symmetries and symmetry-breaking patterns that serve as competing hypotheses about the ultraviolet (UV) dynamics beyond the SM, not far above the TeV scale. In particular, we consider four different structures in the quark sector and seven in the charged lepton sector. The set of flavor-breaking spurions is (almost) always taken to be the minimal one needed to reproduce the observed charged fermion masses and mixings. For each case, we explicitly construct and count the operators to the first few orders in the spurion expansion, providing ready-for-use setups for phenomenological studies and global fits. We provide a Mathematica package SMEFTflavor (https://github.com/aethomsen/SMEFTflavor) to facilitate similar analyses for flavor symmetries not covered in this work.

The first string-derived eclectic flavor model with realistic phenomenology

Eclectic flavor groups arising from string compactifications combine the power of modular and traditional flavor symmetries to address the flavor puzzle. This top-down scheme determines the representations and modular weights of all matter fields, imposing strict constraints on the structure of the effective potential, which result in controlled corrections. We study the lepton and quark flavor phenomenology of an explicit, potentially realistic example model based on a \U0001d54b6/ℤ3× ℤ3 orbifold compactification of the heterotic string that gives rise to an Ω(2) eclectic flavor symmetry. We find that the interplay of flavon alignment and the localization of the modulus in the vicinity of a symmetry-enhanced point leads to naturally protected fermion mass hierarchies, favoring normal-ordered neutrino masses arising from a see-saw mechanism. We show that our model can reproduce all observables in the lepton sector with a small number of parameters and deliver predictions for so far undetermined neutrino observables. Furthermore, we extend the fit to quarks and find that Kähler corrections are instrumental in obtaining a successful simultaneous fit to the quark and lepton sectors.

Flavor-alignment in an S 3-symmetric Higgs sector and its RG-behavior* *NC acknowledges support from DST, India, under Grant Number IFA19-PH237 (INSPIRE Faculty Award). IC acknowledges support from DST, India, under grant number IFA18-PH214 (INSPIRE Faculty Award)

A three Higgs-doublet model exhibiting -symmetry can predict the observed pattern of quark masses and their mixings. However, the same symmetry also introduces potential flavor-changing neutral currents (FCNCs) at the tree level. In this study, we assume that the scalar potential contains appropriate soft -breaking terms to maintain flexible choices of scalar masses. We identify the parameters in the Yukawa Lagrangian of the quark sector responsible for such FCNCs and constrain them using data from flavor physics observables, such as meson-decays and meson-mixings. We also validate the corresponding model parameter space via renormalization group evaluation.

Implications of the zero 1-3 flavour mixing hypothesis: predictions for {theta}_{23}^{mathrm{PMNS}} and δPMNS

We revisit mixing sum rule relations in the lepton and quark sectors under the assumption that the 1-3 elements of the flavour mixing matrices ( {V}_L^u , {V}_L^d , {V}_L^e , {V}_L^{nu } ) are zero in the flavour basis. We consider the exact relations resulting from the validity of this “zero 1-3 flavour mixing hypothesis” and analyse their implications based on the current experimental data, including effects from RG running. In particular, we analyse how the existing precise measurement of {theta}_{13}^{mathrm{PMNS}} allows to derive predictions for {theta}_{23}^{mathrm{PMNS}} in models with constrained {theta}_{12}^{mathrm{e}} . As examples, we calculate the predictions for {theta}_{23}^{mathrm{PMNS}} which arise in 12 classes of Pati-Salam models and SU(5) GUTs that relate {theta}_{12}^{mathrm{e}} to {theta}_{12}^{mathrm{d}} . We also derive a novel “lepton phase sum rule”, valid under the additional assumption of small charged lepton mixing contributions. We furthermore point out that, in the context of GUT flavour models, the quark and lepton CP violating phases δCKM and δPMNS can both be predicted from a single imaginary element in the mass matrices.

Seesaw mechanism in the R-parity-violating supersymmetric standard model with the gauged flavor U(1)X symmetry

Abstract We study the seesaw mechanism in the supersymmetric standard model (SSM) with the ${\mathbb {Z}}_3$ symmetry called matter triality (M3). The Abelian discrete symmetry prohibits the baryon number violation operators at the (non-)renormalizable level, which ensures proton longevity. The cubic coupling by right-handed neutrinos generates the Majorana mass term after the right-handed sneutrinos develop into the vacuum expectation values. Due to the R-parity violation, the active neutrino masses can be generated not only from the mixings with the right-handed neutrinos but also from those with the minimal SSM neutralinos. We propose a model where M3 is embedded in the gauged flavor symmetry. The flavor charges are constrained by the anomaly cancellation conditions and the requirements to realize the fermion masses and mixing angles in the quark and lepton sectors. We analyze the assignments of the flavor charge, and show that the sterile neutrino mass is allowed to be below the soft supersymmetry-breaking scale, which is constrained by the half-lifetime of the neutrinoless double beta decay.

Longitudinal dynamics for mesons on the light cone

We survey a set of proposed light-front models for confinement in the valence quark sector of the mesons and portray similarities as well as differences. We present the spectroscopies for the light mesons that result from a selection of longitudinal confinement forms. We note that the Sturm-Liouville theory provides a unifying framework for many elements of this comparison.

Searching for light new physics at the LHC via lepton-number violation

We study the collider phenomenology of a simplified model containing a right-handed W in which the WR couples predominantly to the third generation in the quark sector. The model also includes a light Majorana neutrino, with M1 ∼ mathcal{O}(100) GeV, giving rise to lepton-number-violating signatures that are visible at the LHC. Taking into account all the searches from the LHC and Tevatron, we find that this WR can still be as light as MR ∼ 300 GeV. We show that this type of new physics, and others like it, can be detected at the LHC using final states with three same-sign same-flavour leptons.

A Bottom-up Approach: The Data Driven Flavour Model$${}^{\mathbf{\#}}$$

A bottom-up approach has been adopted to identify a flavour model that agrees with present experimental measurements. The charged fermion mass hierarchies suggest that only the top Yukawa term should be present at the renormalisable level. The flavour symmetry of the Lagrangian including the fermionic kinetic terms and only the top Yukawa is then a combination of $$U(2)$$ and $$U(3)$$ factors. Lighter charged fermion and active neutrino masses and quark and lepton mixings arise considering specific spurion fields. The associated phenomenology is investigated and the model turns out to have almost the same flavour protection of the minimal flavour violation, in both quark and lepton sectors. Promoting the spurions to be dynamical fields, the associated scalar potential is also studied and a minimum is identified such that fermion masses and mixings are correctly reproduced.

Parametrization of quark and gluon generalized parton distributions in a dynamical framework

We present a parametrization of the chiral even generalized parton distributions, $H$, $E$, $\stackrel{\texttildelow{}}{H}$, $\stackrel{\texttildelow{}}{E}$, for the quark, antiquark, and gluon, in the perturbative QCD (pQCD)-parton framework. Parametric analytic forms are given as a function of two equivalent sets of variables $x$, $\ensuremath{\xi}$, $t$ (symmetric frame) and $X$, $\ensuremath{\zeta}$, $t$ (asymmetric frame), at an initial scale, ${Q}_{o}^{2}$. In the $X>\ensuremath{\zeta}$ region, a convenient and flexible form is obtained as the product of a Regge term $\ensuremath{\propto}{X}^{\ensuremath{-}\ensuremath{\alpha}+{\ensuremath{\alpha}}^{\ensuremath{'}}t}$, describing the low $X$ behavior, times a spectator model-based functional form depending on various mass parameters; the behavior at $X<\ensuremath{\zeta}$ is determined using the generalized parton distributions symmetry and polynomiality properties. The parameters are constrained using data on the flavor separated nucleon electromagnetic elastic form factors, the axial and pseudoscalar nucleon form factors, and the parton distribution functions from both the deep inelastic unpolarized and polarized nucleon structure functions. For the gluon distributions we use, in particular, constraints provided by recent lattice QCD moments calculations. The parametrization's kinematical range of validity is $0.0001\ensuremath{\le}X\ensuremath{\le}\phantom{\rule{0ex}{0ex}}0.85$, $0.01\ensuremath{\le}\ensuremath{\zeta}\ensuremath{\le}0.85$, $0\ensuremath{\le}\ensuremath{-}t\ensuremath{\le}1\text{ }\text{ }{\mathrm{GeV}}^{2}$, $2\ensuremath{\le}{Q}^{2}\ensuremath{\le}100\text{ }\text{ }{\mathrm{GeV}}^{2}$. With the simultaneous description of the quark, antiquark, and gluon sectors, this parametrization represents a first tool enabling a global QCD analysis of deeply virtual exclusive experiments.

Diluting quark flavor hierarchies using dihedral symmetry

We present a D_4 flavored extension of the SM which provides an intuitive reasoning for the masses and mixing patterns in the quark sector. In our model, the Cabibbo mixing angle becomes related to the ratio of two vacuum expectation values. In fact, the orders of magnitude of the CKM matrix elements are readily obtained from the hierarchical nature of the vacuum expectation values. Moreover, we also show that the smallness of the off-Cabibbo elements in the CKM matrix is strongly connected to the heaviness of the third generation of quarks.

Light scalars: Four-quark versus two-quark states in the complex energy plane from Bethe-Salpeter equations

We study the dynamical generation of scalar mesons in the light quark sector ($q\in\{u,d,s\}$) and calculate the masses and widths for the $f_0(500),a_0(980)$ and $f_0(980)$. To this end we study the mixing of conventional $q\bar{q}$ and `exotic' $q\bar{q}q\bar{q}$ states via a coupled set of two-body Bethe-Salpeter equations based on a symmetry-preserving truncation of the underlying Dyson-Schwinger equations. This allows us to determine the dominant components of each state. Furthermore, we extend our previous framework into the complex energy plane such that we can study the analytic structure of the states in question and extract their width. At the physical point of small quark masses, the $\sigma$ meson is predominantly a $\pi\pi$ resonance. Consequently, its mass and width is driven by the effects of chiral symmetry breaking. At larger quark masses, however, the conventional $q\bar{q}$ components take over. Furthermore, we find a strong molecular $K\bar{K}$ component for both, the strange-light $f_0(980)$ and the $a_0(980)$.

A modular A 4 symmetric scotogenic model for neutrino mass and dark matter

Modular symmetries have been impeccable in the neutrino and quark sectors. This motivated us to propose a variant of the scotogenic model based on modular A 4 symmetry and realize the neutrino mass generation at the one-loop level through a radiative mechanism. Alongside, we discuss the lepton flavour violating processes μ → eγ, μ → 3e and μ–e conversion in the nucleus. The lightest Dirac fermion turns out to be a potential dark matter candidate, made stable by the suitable assignment of modular weights. The relic density of this has been computed with annihilations mediated by inert scalars and the new U(1) gauge boson. The LEP-II and ATLAS dilepton constraints on the new gauge parameters are suitably considered to show the consistent parameter region.

Enhancing the Large Hadron Collider sensitivity to charged and neutral broad resonances of new gauge sectors

In scenarios beyond the Standard Model (BSM) characterised by charged (W′) or neutral (Z′) massive gauge bosons with large width, resonant mass searches are not very effective, so that one has to exploit the tails of the mass distributions measured at the Large Hadron Collider (LHC). In this case, the LHC sensitivity to new physics signals is influenced significantly by systematic uncertainties associated with the Parton Distribution Functions (PDF), particularly in the valence quark sector relevant for the multi-TeV mass region. As a BSM framework featuring such conditions, we consider the 4-Dimensional Composite Higgs Model (4DCHM), in which multiple W′ and Z′ broad resonances are present, with strongly correlated properties. By using the QCD tool xFitter, we study the implications on W′ and Z′ searches in Drell-Yan (DY) lepton decay channels that follow from the reduction of PDF uncertainties obtained through combining high-statistics precision measurements of DY lepton-charge and forward-backward asymmetries. We find that the sensitivity to the BSM states is greatly increased with respect to the case of base PDF sets, thereby enabling one to set more stringent limits on (or indeed discover) such new particles, both independently and in correlated searches.

Equation of state of cold and dense QCD matter in resummed perturbation theory

We discuss the hard dense loop resummation at finite quark mass and evaluate the equation of state (EoS) of cold and dense QCD matter in $\ensuremath{\beta}$ equilibrium. The resummation in the quark sector has an effect of lowering the baryon number density and the EoS turns out to have much smaller uncertainty than the perturbative QCD estimate. Our numerical results favor smooth matching between the EoS from the resummed QCD calculation at high density and the extrapolated EoS from the nuclear matter density region. We also point out that the speed of sound in our EoS slightly exceeds the conformal limit.

Z mediated flavor changing neutral currents with a fourth vectorlike family

We discuss $Z$ mediated flavor changing neutral currents within a model where the hierarchical quark and lepton masses are explained via a fourth vectorlike family, together with a scalar sector consisting of two Higgs doublets augmented by a gauge singlet scalar field that spontaneously breaks an extra global $U(1{)}^{\ensuremath{'}}$ symmetry. The $Z$ mediated flavor violating interactions arise from the mixings between the SM fermions and the vectorlike fermions, where the mixing is discussed in an analytic approximation and also exactly numerically. We first discuss charged lepton flavor violating (CLFV) $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$, $\ensuremath{\tau}\ensuremath{\rightarrow}3\ensuremath{\mu}$ and $Z\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ decays and find that they cannot significantly constrain the masses of charged vectorlike leptons. However, the 790 GeV mass bound arising from collider searches on vectorlike lepton doublets can set further constraints on the model parameter space. We also consider rare $t\ensuremath{\rightarrow}cZ$ decays as well as unitarity violation in the CKM mixing in order to constrain the quark sector of the model under consideration.

The BGOOD experiment at ELSA Exotic structures in the strange quark sector?

The BGOOD photoproduction experiment accesses forward meson angles and low momentum exchange kinematics in the uds sector, which may be sensitive to molecular-like hadronic structure. Recent highlights are summarised in these proceedings.

Exploring the Origin of CP Violation in the Standard Model

In this article, we present a very general but not ultimate solution of CPV problem in the standard model. Our study starts from a naturally Hermitian ${\bf M^2}\equiv M^q \cdot M^{q\dagger}$ rather than the previously assumed Hermitian $M^q$. The only assumption employed here is that the real part and imaginary part of $\bf M^2$ can be, respectively, diagonalized by a common $\bf U^q$ matrix. Such an assumption leads to a $\bf M^2$ pattern which depends on only five parameters and can be diagonalized analytically by a $\bf U^q$ matrix which depends on only two of the parameters. Two of the derived mass eigenvalues are predicted degenerate if one of the parameters $\bf C ~(C')$ in up- (down-) quark sector is zero. As the $\bf U^q$ patterns are obtained, thirty-six $V_{CKM}$ candidates are yielded and only eight of them, classified into two groups, fit empirical data within the order of $O(\lambda)$. One of the groups is further excluded in a numerical test, and the surviving group predicts that the degenerate pair in a quark type are the lightest and the heaviest generations rather than the lighter two generations assumed in previous researches. However, there is still one unsatisfactory prediction in this research, a quadruple equality in which four CKM elements of very different values are predicted to be equal. It indicates the $\bf M^2$ pattern studied here is still oversimplified by that employed assumption and the ultimate solution can only be obtained by diagonalizing the unsimplified $\bf M^2$ matrix containing nine parameters directly. The $V_{CKM}$ presented here is already very close to such an ultimate CPV solution.

Flavor changing neutral couplings for leptons

The article reviews the conditions for lepton-flavor conservation in leptonic interactions. The conditions are similar to those developed in the quark sector. For neutrinos the mass eigenstates are unitary superpositions of flavor states which together with small mass differences produce neutrino oscillations. A new property for neutrinos is the fact that their masses are very small suppressing their couplings to neutral scalar fields. The results are demonstrated in several models.

Linking continuum and lattice quark mass functions via an effective charge

The quark mass function is computed both by solving the quark propagator Dyson-Schwinger equation and from lattice simulations implementing overlap and Domain-Wall fermion actions for valence and sea quarks, respectively. The results are confronted and seen to produce a very congruent picture, showing a remarkable agreement for the explored range of current-quark masses. The effective running-interaction is based on a process-independent charge rooted on a particular truncation of the Dyson-Schwinger equations in the gauge sector, establishing thus a link from there to the quark sector and inspiring a correlation between the emergence of gluon and hadron masses.

Quarkyonic stars with isospin-flavor asymmetry

We suggest an extension to isospin asymmetric matter of the quarkyonic model from McLerran and Reddy. This extension allows us to construct the $\beta$-equilibrium between quarks, nucleons and leptons. The concept of the quarkyonic matter originates from the large number of color limit for which nucleons are the correct degrees of freedom near the Fermi surface -- reflecting the confining forces -- while deep inside the Fermi sea quarks naturally appear. In isospin asymmetric matter, we suggest that this new concept can be implemented within a global isoscalar relation between the shell gaps differentiating the nucleon and the quark sectors. In addition, we impose the conservation of the isospin-flavor asymmetry in the nucleon and the quark phases. Within this model, several quarkyonic stars are constructed on top of the SLy4 model for the nucleon sector, producing a bump in the sound speed, which implies that quarkyonic stars are systematically bigger and have a larger maximum mass than the associated neutron stars. They also predict lower proton fraction at $\beta$-equilibrium, which potentially quenches fast cooling in massive compact stars.