Generations Of Fermions Research Articles

If the weak were strong and the strong were weak

We explore the phase structure of the Standard Model as the relative strengths of the SU(2)SU(2) weak force and SU(3)SU(3) strong force are varied. With a single generation of fermions, the structure of chiral symmetry breaking suggests that there is no phase transition as we interpolate between the SU(3)SU(3)-confining phase and the SU(2)SU(2)-confining phase. Remarkably, the massless left-handed neutrino, familiar in our world, morphs smoothly into a massless right-handed down-quark. With multiple generations, a similar metamorphosis occurs, but now proceeding via a phase transition. In the second half of the paper we introduce a two-parameter extension of the Standard Model, a chiral gauge theory with gauge group U(1)\times Sp(r)\times SU(N)U(1)×Sp(r)×SU(N). We again explore the phase structure of the theory as the relative strengths of the Sp(r)Sp(r) and SU(N)SU(N) gauge couplings vary.

Bounding the charm Yukawa coupling

The study of the properties of the observed Higgs boson is one of the main research activities in High Energy Physics. Although the couplings of the Higgs to the weak gauge bosons and third generation quark and leptons have been studied in detail, little is known about the Higgs couplings to first and second generation fermions. In this article, we study the charm quark Higgs coupling in the so-called $\kappa$ framework. We emphasize the existence of specific correlations between the Higgs couplings that can render the measured LHC Higgs production rates close to the SM values in the presence of large deviations of the charm coupling from its SM value, $\kappa_c = 1$. Based on this knowledge, we update the indirect bounds on $\kappa_c$ through a fit to the precision Higgs measurements at the LHC. We also examine the limits on $\kappa_c$ arising from the radiative decay $H \to J/\psi + \gamma$, the charm quark-associated Higgs production, charm quark decays of the Higgs field and charge asymmetry in $W^{\pm} + H$ production. Estimates for the future LHC sensitivity on $\kappa_c$ at the high luminosity run are provided.

Sub-GeV dark matter model

We propose an extension of the Standard Model gauge symmetry by the gauge group $U(1)_{T3R}$ in order to address the Yukawa coupling hierarchy between the third generation fermions and the first two generation fermions of the SM. We assume that only the right-handed fermions of the first two generations are charged under the $U(1)_{T3R}$. In addition to the new dark gauge boson, we have a dark scalar particle whose vacuum expectation value (vev) breaks the $U(1)_{T3R}$ symmetry down to $Z_2$ symmetry and also explains the hierarchy problem. A vev of $\cal O$(GeV) is required to explain the mass parameters of the light flavor sector naturally. The dark matter (DM) particle arising from the model naturally has mass in the $\mathcal {O}$(1-100) MeV range. The model satisfies all the current constraints. We discuss the various prospects of the direct detection of the dark matter. We get both elastic and inelastic spin independent DM-nucleon scattering. The dark matter obtains the correct thermal relic density by annihilation.

5d Dirac fermion on quantum graph

In this paper, we investigate a five-dimensional Dirac fermion on a quantum graph that consists of a single vertex and N loops. We find that the model possesses a rich structure of boundary conditions for wavefunctions on the quantum graph and they can be classified into distinct categories. It is then shown that there appear degenerate four-dimensional chiral massless fermions in the four-dimensional mass spectrum. We briefly discuss how our model could naturally solve the problems of the fermion generation, the fermion mass hierarchy and the origin of the CP-violating phase.

Kinetic mixing effect in noncommutative B \u2212 L gauge theory

It is well established that the SU(P)L gauge symmetry for P ≥ 3 can address the question of fermion generation number due to the anomaly cancellation, but it neither commutes nor closes algebraically with electric and baryon-minus-lepton charges. Hence, two U(1) factors that determine such charges are required, yielding a complete gauge symmetry, SU(P)L ⊗ U(1)X ⊗ U(1)N, apart from the color group. The resulting theory manifestly provides neutrino mass, dark matter, inflation, and baryon asymmetry of the universe. Furthermore, this gauge structure may present kinetic mixing effects associated to the U(1) gauge fields, which affect the electroweak precision test such as the ρ parameter and Z couplings as well as the new physics processes. We will construct the model, examine the interplay between the kinetic mixing and those due to the symmetry breaking, and obtain the physical results in detail.

LHC constraints on a (B−L)3 gauge boson

In this paper, we explore the constraints that the LHC can place on a massive gauge boson $X$ that predominantly couples to the third generation of fermions. Such a gauge boson arises in scenarios where the $B-L$ of the third generation is gauged. We focus on the mass range $10 \leq m_X \lesssim 2\,m_W$, where current constraints are lacking, and develop a dedicated search strategy. For this mass range, we show that $b \bar b \tau^+ \tau^-$, where at least one of the $\tau$s decay leptonically is the optimal channel to look for the $X$ at the LHC. The QCD production of $b$ quarks, combined with the cleanliness of the leptons coming from the decay of the $\tau$ allow us to detect $X$ gauge boson with couplings of $g_X \sim(0.005-0.01)$, for $m_X < 50 \ \text{GeV}$, and a coupling of $O(0.1)$ for heavier $X$ gauge boson with $100 \ \text{fb}^{-1}$ of integrated luminosity. This is about a factor of 2-10 improvement over previous constraints coming from the decay of $\Upsilon \to \tau^+ \tau^-$. Extrapolating to the full HL-LHC luminosity of $3000 \ \text{fb}^{-1}$, the bounds on $g_X$ can be enhanced by another factor of $\sqrt{2}$ for $m_X < 50 \ \text{GeV}$.

Constructing the Standard Model fermions

The Standard Model has three generations of fermions and antifermions, each with two states of isospin, and each of these has both a lepton and a quark in three possible colour states. In total there are 48 states. No known system exists for constructing these from first principles. Here, it is suggested that the number of degrees of freedom required is a consequence of the nilpotent complexified vector-quaternion Dirac algebra, which emerges from the representation of the fundamental parameters mass, time, charge and space as a Klein-4 group, and that these degrees of freedom lead to unique structural representations of each of the individual fermions.

Generic loop effects of new scalars and fermions in b \u2192 s\u2113+\u2113\u2212 and a vector-like 4th generation

In this article we investigate in detail the possibility of accounting for the b → sℓ+ℓ− anomalies via box contributions involving with new scalars and fermions. For this purpose, we first write down the most general Lagrangian which can generate the desired effects and then calculate the generic expressions for all relevant b → s Wilson coefficients. Here we extend previous analysis by allowing that the new particles can also couple to right-handed Standard Model (SM) fermions as preferred by recent b → sℓ+ℓ− data and the anomalous magnetic moment of the muon.In the second part of this article we illustrate this generic approach for a UV complete model in which we supplement the Standard Model by a 4th generation of vector-like fermions and a real scalar field. This model allows one to coherently address the observed anomalies in b → sℓ+ℓ− transitions and in aμ without violating the bounds from other observables (in particular Bs − {overline{B}}_s mixing) or LHC searches. In fact, we find that our global fit to this model, after the recent experimental updates, is very good and prefers couplings to right-handed SM fermions, showing the importance of our generic setup and calculation performed in the first part of the article.

A variant two-Higgs doublet model with a new Abelian gauge symmetry

We consider a two-Higgs doublet model extended with a broken Abelian gauge symmetry under which all Standard Model (SM) quarks, fourth generation fermions and a new SM-singlet scalar boson are charged. Such a setup is shown to be able to accommodate the muon anomalous magnetic dipole moment while being consistent with existing constraints of flavor-violating decays of charged leptons and Z boson. The new scalar boson offers a suitable dark matter candidate that interacts with the SM particles via the Higgs portal and the Z′ boson associated with the new gauge symmetry. The dark matter direct detection bound is found to impose a strong constraint on the new gauge coupling.

Three fermion generations with two unbroken gauge symmetries from the complex sedenions

We show that three generations of leptons and quarks with unbroken Standard Model gauge symmetry SU(3)_ctimes U(1)_{em} can be described using the algebra of complexified sedenions {mathbb {C}}otimes {mathbb {S}}. A primitive idempotent is constructed by selecting a special direction, and the action of this projector on the basis of {mathbb {C}}otimes {mathbb {S}} can be used to uniquely split the algebra into three complex octonion subalgebras {mathbb {C}}otimes {mathbb {O}}. These subalgebras all share a common quaternionic subalgebra. The left adjoint actions of the 8 {mathbb {C}}-dimensional {mathbb {C}}otimes {mathbb {O}} subalgebras on themselves generates three copies of the Clifford algebra {mathbb {C}}ell (6). It was previously shown that the minimal left ideals of {mathbb {C}}ell (6) describe a single generation of fermions with unbroken SU(3)_ctimes U(1)_{em} gauge symmetry. Extending this construction from {mathbb {C}}otimes {mathbb {O}} to {mathbb {C}}otimes {mathbb {S}} naturally leads to a description of exactly three generations.

Braided fermions from Hurwitz algebras

Some curious structural similarities between a recent braid - and Hurwitz algebraic description of the unbroken internal symmetries for a single generations of Standard Model fermions were recently identified. The non-trivial braid groups that can be represented using the four normed division algebras are B2 and , exactly those required to represent a single generation of fermions in terms of simple three strand ribbon braids. These braided fermion states can be identified with the basis states of the minimal left ideals of the Clifford algebra Cℓ(6), generated from the nested left actions of the complex octonions on itself. That is, the ribbon spectrum can be related to octonion algebras. Some speculative ideas relating to ongoing research that attempts to construct a unified theory based on braid groups and Hurwitz algebras are discussed.

Atomki Anomaly in Family-Dependent U(1)′ Extension of the Standard Model

In the context of a gauge invariant, non-anomalous and family-dependent (non-universal) $U(1)'$ extension of the Standard Model, wherein a new high scale mechanism generates masses and couplings for the first two fermion generations and the standard Higgs mechanism does so for the third one, we find solutions to the anomaly observed by the Atomki collaboration in the decay of excited states of Beryllium, in the form of a very light $Z'$ state, stemming from the $U(1)'$ symmetry breaking, with significant axial couplings so as to evade a variety of low scale experimental constraints.

HDECAY: Twenty++ years after

The program HDECAY determines the partial decay widths and branching ratios of the Higgs bosons within the Standard Model with three and four generations of fermions, including the case when the Higgs couplings are rescaled, a general two-Higgs doublet model where the Higgs sector is extended and incorporates five physical states and its most studied incarnation, the minimal supersymmetric Standard Model (MSSM) with real soft SUSY-breaking parameters. The program addresses all decay channels including the dominant higher-order effects such as radiative corrections and multi-body channels. Since the first launch of the program, more than twenty years ago, important aspects and new ingredients have been incorporated. In this update of the program description, some of the developments are summarized while others are discussed in some detail. New version program summaryProgram Title: HDECAYProgram Files doi:http://dx.doi.org/10.17632/3ggyvkjz95.1Licensing provisions: GPLv2Programming language: FORTRAN77Journal reference of previous version: Comp. Phys. Comm. 108 (1998) 56-74.Does the new version supersede the previous version?: YesReasons for the new version: Major updates and extensionsSummary of revisions: Since the first release of the original version of the program some bugs have been fixed, a number of improvements and new theoretical calculations have been implemented. The logbook of all modifications and the most recent version 6.52 of the program can be found on the web page http://tiger.web.psi.ch/hdecay/.Nature of problem: Decay widths and branching ratios for Higgs bosons decays in the Standard Model with three and four generations of fermions and rescaled couplings, a general two-Higgs doublet model and the Minimal Supersymmetric Standard Model are calculated numerically, including loop corrections according to the current theoretical knowledge.Solution method: One- and two-dimensional numerical integration of analytic formulae for Higgs boson decays into off-shell particles. All other decay widths are calculated analytically. The transition between off-shell and on-shell decays is performed by a linear interpolation.

Building models for B-physics anomalies

Several different experiments show evidences of Lepton Flavour Universality violation in semi-leptonic B meson decays, both in charged and neutral currents. These anomalies can be interpreted in terms of new short-distance interactions that involve mainly the third generation of fermions. I will first discuss the present status of the anomalies, and their connection to other low- and high-energy observables, in a model-independent way, adopting an Effective Field Theory approach based on a CKM-like flavour structure. I will then extend the analysis to a few simple UV completions with different types of mediator, discussing the main issues that have to be faced in each case. A few models emerge, which look capable of accommodating both charged- and neutral-current anomalies consistently with all the experimental bounds, with associated flavour and high-pT signatures within the reach of present and future experiments.

Confronting SUSY SO(10) with updated Lattice and Neutrino data

We present an updated fit of supersymmetric SO(10) models to quark and lepton masses and mixing parameters. Including latest results from lattice QCD determinations of quark masses and neutrino oscillation data, we show that fits neglecting supersymmetric threshold corrections are strongly disfavoured in our setup. Only when we include these corrections we find good fit points. We present χ2-profiles for the threshold parameters, which show that in our setup the thresholds related to the third generation of fermions exhibit two rather narrow minima.

Maximally mathcal{N} -extended super-BMS3 algebras and generalized 3D gravity solutions

We consider the maximal mathcal{N} -extended supergravity theory in 3 dimensions with fermionic generators transforming under real but non necessarily irreducible representations of the internal algebra. We obtain the symmetry algebra at null infinity preserving boundary conditions of asymptotically flat solutions, i.e. the maximal mathcal{N} -extended super-BMS3 algebra, which possesses non-linear correction in the anti-commutators of supercharges. We present the supersymmetric energy bound and derive the explicit form of the asymptotic Killing spinors. We also find the most generic circular symmetric ground state of the theory, which corresponds to a non-supersymmetric cosmological solutions and derive their entropy.

Generation patterns, modified γ − Z mixing, and hidden sector with dark matter candidates as framed standard model results

A descriptive summary is given of the results to-date from the framed standard model (FSM) which: Assigns geometric meaning to the Higgs field and to fermion generations, hence offering an explanation for the observed mass and mixing patterns of quarks and leptons, reproducing near-quantitatively 17 of SM parameters with only 7. Predicts a new vector boson [Formula: see text] which mixes with [Formula: see text] and [Formula: see text], leading to deviations from the SM mixing scheme. For [Formula: see text] TeV, these deviations are within present experimental errors but should soon be detectable at LHC when experimental accuracy is further improved. Suggests the existence of a hidden sector of particles as yet unknown to experiment which interact but little with the known particles. The lowest members of the hidden sector of mass around 17 MeV, being electrically neutral and stable, may figure as dark matter constituents. The idea is to retrace the steps leading to the above results unencumbered by details already worked out and reported elsewhere. This has helped to clarify the logic, tighten some arguments and dispense with one major assumption previously thought necessary, thus strengthening earlier results in opening up possibly a new and exciting vista for further exploration.

Dark matter and collider signals in an MSSM extension with vector-like multiplets

Motivated by grand unification considerations, we analyse a simple extension of the minimal supersymmetric standard model with additional pairs of vector-like chiral supermultiplets. We focus on the so-called LND setup, which enlarges the particle content of the minimal model by two vector-like pairs of weak doublets (one pair of leptons and one pair of down-type quarks) and one vector-like pair of neutrino singlets. Imposing collider and low-energy constraints, sneutrinos and neutralinos both emerge as possible lightest supersymmetric particles and thus dark matter candidates. We perform a complete analysis of the dark sector and study the viability of these neutralino and sneutrino dark matter options. We show that cosmological considerations (the dark matter relic abundance and its direct and indirect detection signals) restrict neutralino dark matter to exhibit similar properties as in the minimal supersymmetric standard model, and impose the sneutrino dark matter candidate to be singlet-like, rather than doublet-like. Allowing the mixing of the fermionic component of the new supermultiplets with the Standard Model third generation fermions, we moreover demonstrate the existence of collider signals that are distinguishable from other, more minimal, supersymmetric scenarios by virtue of an enhanced production of events enriched in tau leptons. We furthermore show that this signature yields robust LHC signals, that could potentially be differentiated from the background in future data.

A closer study of the framed standard model yielding testable new physics plus a hidden sector with dark matter candidates

This closer study of the FSM (I) retains the earlier results of Ref. 1 in offering explanation for the existence of three fermion generations, as well as the hierarchical mass and mixing patterns of leptons and quarks; (II) predicts a vector boson [Formula: see text] with mass of order TeV which mixes with [Formula: see text] and [Formula: see text] of the standard model. The subsequent deviations from the standard mixing scheme are calculable in terms of the [Formula: see text] mass. While these deviations for (i) [Formula: see text], (ii) [Formula: see text], and (iii) [Formula: see text] are all within present experimental errors so long as [Formula: see text] TeV, they should soon be detectable if the [Formula: see text] mass is not too much bigger; (III) suggests that in parallel to the standard sector familiar to us, there is another where the roles of flavour and colour are interchanged. Though quite as copiously populated and as vibrant in self-interactions as our own, it communicates but little with the standard sector except via mixing through a couple of known portals, one of which is the [Formula: see text] complex noted in (II), and the other is a scalar complex which includes the standard model Higgs. As a result, the new sector appears hidden to us as we appear hidden to them, and so its lowest members with masses of order 10 MeV, being electrically neutral and seemingly stable, but abundant, may make eligible candidates as constituents of dark matter. A more detailed summary of these results together with some remarks on the model’s special theoretical features can be found in the last section of this paper.

Confronting the fourth generation two-Higgs-doublet model with the phenomenology of heavy Higgs bosons

A sequential fourth generation is known to be excluded because the non-decoupling contribution to $\kappa_g$, the Higgs coupling modifier with a gluon pair, is unacceptably large. Recently a new way to save the model was suggested in the Type-II two Higgs doublet model: if the Yukawa couplings of down-type fermions have wrong-sign, the contributions from $t'$ and $b'$ to $\kappa_g$ are cancelled. We study the theoretical and experimental constraints on this model, focusing on the heavy Higgs bosons. Two constraining features are pointed out. First the exact wrong-sign limit does not allow the alignment, which makes the perturbative unitarity for the scalar-scalar scattering put the upper bounds on the heavy Higgs boson masses like $M_H, M_A \lesssim 920$ GeV and $M_{H^\pm} \lesssim 620$ GeV. Secondly, the Yukawa couplings of the fourth generation fermions to the heavy Higgs bosons are generically large as being proportional to the heavy fermion mass and, for the down-type fermions, to $\tan\beta$ as well. The gluon fusion productions of $H$ and $A$ through the fourth generation quark loops become significant. We found that the current LHC data on $pp \to Z Z$ for $H$ along with the theoretical and indirect constraints exclude the model at leading order.