Left-handed Doublets Research Articles

Twin Pati-Salam theory of flavour with a TeV scale vector leptoquark

We propose a twin Pati-Salam (PS) theory of flavour broken to the G4321 gauge group at high energies, then to the Standard Model at low energies, yielding a TeV scale vector leptoquark {U}_1^{mu } (3, 1, 2/3) which has been suggested to address the lepton universality anomalies {R}_{K^{left(ast right)}} and {R}_{D^{left(ast right)}} in B decays. Quark and lepton masses are mediated by vector-like fermions, with personal Higgs doublets for the second and third families, which may be replaced by a two Higgs doublet model (2HDM). The twin PS theory of flavour successfully accounts for all quark and lepton (including neutrino) masses and mixings, and predicts a dominant coupling of {U}_1^{mu } (3, 1, 2/3) to the third family left-handed doublets. However the predicted mass matrices, assuming natural values of the parameters, are not consistent with the single vector leptoquark solution to the {R}_{D^{left(ast right)}} anomaly, given its current value.

Revisiting R-parity violating interactions as an explanation of the B-physics anomalies

In the last few years, the ratios R_{D^{(*)}} and of R_{K^{(*)}} have reportedly exhibited significant deviations from the relevant standard model predictions, hinting towards a possible violation of Lepton Flavor Universality and a window to New Physics. We investigate to what extent the inclusion of R-parity violating couplings in the minimal supersymmetric standard model can provide a better fit to the anomalies simultaneously. We perform this analysis employing an approximate, non-abelian mathcal {G}_f=U(2)_q times U(2)_ell flavour symmetry, which features a natural explanation of the appropriate hierarchy of the R-parity violating couplings. We show that, under the requirement of a supersymmetric spectrum with much heavier left-handed doublet superpartners, our assumption favors a considerable enhancement in the tree-level charged-current B rightarrow D^{(*)} tau overline{nu }, while the the anomalies induced by b rightarrow sell ^+ell ^- receive up to an approximate 30 % improvement. The consistency with all relevant low-energy constraints is assessed.

Higgs-like mechanism for spontaneous spacetime symmetry breaking

The study of spontaneous breakdown of spacetime symmetries leads to the discovery of another type of Higgs mechanism operating in a chiral SU(2) model. Some of the Nambu-Goldstone vector mesons emergent from simultaneous violations of gauge and Lorentz symmetries are, in this case, absorbed by a left-handed doublet and endow one of the fermions with a right-handed state, while another part becomes emergent as photons. Accordingly, this mechanism allows a chiral fermion to acquire a mass, and it may enable the emergent theory to reproduce the electromagnetism equivalent to the QED sector in the standard theory. It is also mentioned that the "fermion-boson puzzle" known in the presence of a 't Hooft--Polyakov monopole does not exist in our theory.

Dark matter, LFV and neutrino magnetic moment in the radiative seesaw model with fermion triplet

In this paper we work in the framework of a radiative seesaw model with triplet fermion Σ. Due to the Z2 discrete flavor symmetry, the lightest neutral component of Σ is stable and thus can be a dark matter candidate. Its mass can be solely determined by the dark matter relic abundance, which is bout 2.594 TeV. It can still constitute 30% of the dark matter when considering constraints from dark matter indirect detection experiments. The model also predict a dark matter-nucleus scattering cross-section that would be accessible with future dark matter direct detection searches. We further investigate constraints on the parameter space of the model from the lepton-flavor-violating processes and neutrino transition magnetic moments, induced by the Yukawa interaction of the Σ with the left-handed lepton doublets.

Algebraic description of external and internal attributes of fundamental fermions

To describe external and internal attributes of fundamental fermions, a theory of multi-spinor fields is developed on an algebra, a triplet algebra, which consists of all the triple-direct-products of Dirac γ-matrices. The triplet algebra is decomposed into the product of two subalgebras, an external algebra and an internal algebra, which are exclusively related with external and internal characteristic of the multi-spinor field named triplet fields. All elements of the external algebra which is isomorphic to the original Dirac algebra Aγ are invariant under the action of permutation group S3 which works to exchange the order of the Aγ elements in the triple-direct-product. The internal algebra is decomposed into the product of two 42 dimensional algebras, called the family and color algebras, which describe the family and color degrees of freedom. The family and color algebras have fine substructures with "trio plus solo" (3 + 1) conformations which are irreducible under the action of S3. The triplet field has trio plus solo family modes with ordinary tricolor quark and colorless solo lepton components. To incorporate the Weinberg-Salam mechanism, it is required to introduce two types of triplet fields, a left-handed doublet and right-handed singlets of electroweak iso-spin. It is possible to qualify the Yukawa interaction and to make a new interpretation of its coupling constants naturally in an intrinsic mechanism of the triplet field formalism. The ordinary Higgs mechanism leads to the Dirac mass matrices which can explain all data of quark sector within experimental accuracy.

Enhanced tau lepton signatures at LHC in the constrained supersymmetric seesaw model

We discuss the possible enhancement of the tau lepton events, with the emphasis on the signals with three or more hadronic taus, at LHC when the left-handed stau doublet becomes light or even lighter than the right-handed one. This is the natural outcome of the supersymmetric seesaw models where the slepton doublet mass is suppressed by the effects of a large neutrino Yukawa coupling. We study a few representative parameter sets in the sneutrino coannihilation regions where the tau sneutrino is next-to-lightest supersymmetric particle and the stau coannihilation regions where the stau is next-to-lightest supersymmetric particle both of which yield the thermal neutralinolightest supersymmetric particle abundance determined by Wilkinson Microwave Anisotropy Probe.

The precision electroweak data in warped extra-dimension models

The Randall–Sundrum scenario with Standard Model fields in the bulk and a custodial symmetry is considered. We determine the several minimal quark representations allowing to address the anomalies in the forward–backward b-quark asymmetry A FB b , while reproducing the bottom and top masses via wave function overlaps. The calculated corrections of the Z b ¯ b coupling include the combined effects of mixings with both Kaluza–Klein excitations of gauge bosons and new b ′ -like states. It is shown that the mechanism, in which the left-handed doublet of third generation quarks results from a mixing on the UV boundary of introduced fields Q 1 L and Q 2 L , is necessary for phenomenological reasons. Within the obtained models, both the global fit of R b with A FB b [at the various center of mass energies] and the fit of last precision electroweak data in the light fermion sector can simultaneously be improved significantly with respect to the pure Standard Model case, for M KK = 3 , 4, 5 TeV (first KK gauge boson) and a best-fit Higgs mass m h ⩾ 115 GeV i.e. compatible with the LEP2 direct limit. The quantitative analysis of the oblique parameters S , T , U even shows that heavy Higgs mass values up to ∼500 GeV may still give rise to an acceptable quality of the electroweak data fit, in contrast with the Standard Model. The set of obtained constraints on the parameter space, derived partly from precision electroweak data, is complementary of a future direct exploration of this parameter space at the LHC. In particular, we find that custodians, like b ′ modes, can be as light as ∼ 1200 GeV i.e. a mass lying possibly in the potential reach of LHC.

CKM pattern from localized generations in extra dimension

We revisit the issue of the quark masses and mixing angles in the framework of large extra dimension. We consider three identical standard model families resulting from higher-dimensional fields localized on different branes embedded in a large extra dimension. Furthermore we use a decaying profile in the bulk different form previous works. With the Higgs field also localized on a different brane, the hierarchy of masses between the families results from their different positions in the extra space. When the left-handed doublet and the right-handed singlets are localized with different couplings on the branes, we found a set of brane locations in one extra dimension which leads to the correct quark masses and mixing angles with the sufficient strength of CP-violation. We see that the decaying profile of the Higgs field plays a crucial role for producing the hierarchies in a rather natural way.

A model realizing the Harrison-Perkins-Scott lepton mixing matrix

We present a supersymmetric model in which the lepton mixing matrix $U$ obeys, at the seesaw scale, the Harrison--Perkins--Scott \textit{Ansatz}--vanishing $U_{e3}$, maximal atmospheric neutrino mixing, and $\sin^2{\theta_\odot} = 1/3$ ($\theta_\odot$ is the solar mixing angle). The model features a permutation symmetry $S_3$ among the three lepton multiplets of each type--left-handed doublets, right-handed charged leptons, and right-handed neutrinos--and among three Higgs doublets and three zero-hypercharge scalar singlets; a fourth right-handed neutrino, a fourth Higgs doublet, and a fourth scalar singlet are invariant under $S_3$. In addition, the model has seven $\mathbbm{Z}_2$ symmetries, out of which six do not commute with $S_3$. Supersymmetry is needed in order to eliminate some quartic terms from the scalar potential, quartic terms which would make impossible to obtain the required vacuum expectation values of the three Higgs doublets and three scalar singlets. The Yukawa couplings to the charged leptons are flavour diagonal, so that flavour-changing neutral Yukawa interactions only arise at loop level.

Flavour-changing neutral currents in models with extraZ′ boson

New neutral gauge bosonsZ′ are the features of many models addressing the physics beyond the standard model. Together with the existence of new neutral gauge bosons, models based on extended gauge groups (rank > 4) often predict new charged fermions also. A mixing of the known fermions with new states, with exotic weak-isospin assignments (left-handed singlets and right-handed doublets) will induce tree-level flavour-changing neutral interactions mediated byZ exchange, while if the mixing is only with new states with ordinary weak-isospin assignments, the flavour-changing neutral currents are mainly due to the exchange of the new neutral gauge bosonZ′. We review flavour-changing neutral currents in models with extraZ′ boson. Then we discuss some flavour-changing processes forbidden in the standard model and new contributions to standard model processes.

Parametrizing and rephasing neutrino mixing

Neutrino mixing in the standard model extensions, both renormalizable and effective, with arbitrary numbers of the singlet and left-handed doublet neutrinos is investigated in a systematic fashion. The charged and neutral (the Z and Higgs mediated)lepton currents are written under general Majorana condition, and the odservable independence of the choice of the condition, the rephasing invariance, is studied. A parametrization of the neutrino mixing matrices in the doublet-singlet factorized form is developed. Its relationship with the see-saw mechanism is shown in the limit of a small doublet-singlet mixing. The structure of the mixing matrices relevant to the neutrino oscillation experiments is explicated.

First direct observation of the tau neutrino

History I n 1930 W. Pauli, in a famous letter to his colleagues, postulated the existence of a new particle the neutrino. Pauli presented the neutrino as an explanation ofthe observation that the decay of the neutron produced a continuous B-spectrum. Since then, great progress has been made in particle physics to describe the most fundamental constituents and interaction of matter. Nevertheless, many fundamental questions about neutrinos need to be answered. For example: Are neutrinos massive? Do they have a magnetic moment? Do they oscillate? Does a sterile neutrino exist? Is there direct evidence for the existence ofthe tau neutrino? The last question was finally answered in July 2000 by a group of 52 physicists from the DONUT Collaboration (Ferrnilab experiment E872) at Ferrnilab near Chicago. The tau neutrino was postulated in 1975 after the discovery of the tau lepton by M. 1. Perl et al. as the third generation neutrino in the Standard Model ofelectroweak interactions [1]. Glashow, Salam and Weinberg proposed the Standard Model in the late 1960s as a unified theory of the electromagnetic and weak interactions based on the gauge group SU(2) xU(l). This model has been successful in describing all recent experimental observations. Neutrinos enter this model as massless, neutral fermions. From measurements ofthe ZO decay width, it was determined that there are three light neutrinos in nature. The charged and neutral leptons in the Standard Model are represented by three left-handed doublets ofweak isospin.

Parameter counting for neutrino mixing

The content of physical massess, mixing angles and CP-violating phases in the lepton sector of extended standard model, both renormalizable and non-renormalizable, with arbitrary numbers of the singlet and left-handed doublet neutrinos is systematically analyzed in the weak basis.

SU(2)L×U(1)Y×S3×Dmodel for atmospheric and solar neutrino deficits

Motivated by the recent Super-Kamiokande experiment on atmospheric and solar neutrino oscillation we propose a see-saw model of three generations of neutrinos based on the gauge group $SU(2)_{L}\times U(1)_{Y}$ with discrete symmetries $(S_{3} \times D)$ and three right handed singlet neutrinos so that this model can accommodate the recent Super-Kamiokande data on atmospheric and solar neutrino oscillations. The model predicts maximal mixing between $\nu_{\mu}$ and $\nu_{\tau}$ with $sin^{2}2{\theta_{\mu\tau}}$ = 1 as required by the atmospheric neutrino data and small mixing between $\nu_{e}$ and $\nu_{\mu}$ with $sin^{2}2{\theta_{e\mu}} \sim (10^{-2}-10^{-3})$ as a possible explanation of the solar neutrino deficit through the MSW mechanism. The model admits two mass scales of which one breaks the electroweak symmetry and the other is responsible for the breaking of the lepton number symmetry at GUT scale leading to small Majorana mass of the left handed doublet neutrinos.

CYCLIC FAMILY SYMMETRY AND LEPTON HIERARCHY IN SUPERSYMMETRY

A cyclic symmetry among the left-handed doublets of the three families is proposed. This symmetry can naturally result in a realistic hierarchical pattern of the fermion masses within the framework of supersymmetry with nonvanishing sneutrino vacuum expectation values.

Expansion of the weak mixing matrix in powers of 1/mband 1/mt

We perform a 1/${\mathit{m}}_{\mathit{b}}$ and 1/${\mathit{m}}_{\mathit{t}}$ expansion of the Cabibbo-Kobayashi-Maskawa mixing matrix. Data suggest that the dominant parts of the Yukawa couplings are factorizable into sets of numbers \ensuremath{\Vert}r〉, \ensuremath{\Vert}s〉, and \ensuremath{\Vert}s'〉, associated, respectively, with the left-handed doublets, the right-handed up singlets, and the right-handed down singlets. The first order expansion is consistent with Wolfenstein parametrization, which is an expansion in sin${\mathrm{\ensuremath{\theta}}}_{\mathit{c}}$ to third order. The mixing matrix elements in the present approach are partitioned into factors determined by the relative orientations of \ensuremath{\Vert}r〉, \ensuremath{\Vert}s〉, and \ensuremath{\Vert}s'〉 and the dynamics provided by the subdominant mass matrices. A short discussion is given of some experimental support and a generalized Fritzsch model is used to contrast our approach.

AN SU(2)L×U(1)L×U(1)R MODEL WITH FINITE DIRAC NEUTRINO MASSES

An SU(2)L×U(1)L×U(1)R model of electroweak interactions is presented in which the conventional fermions of the standard model are left-handed doublets under SU(2)L× U(1)L and are right-handed singlets under U(1) R . The triangle anomalies are canceled by adding vector-like singlet fermions. Neutrinos are massless at the tree level due to a discrete symmetry and acquire tiny finite masses at the one-loop level due to the exchange of two charged scalar singlets. The singlet scalars carry two units of lepton number.

ELECTROWEAK INTERACTIONS WITH CHIRAL HYPERCHARGES

We consider an extension of the electroweak gauge interactions from SU (2) L × U (1)Y to SU (2) L × U (1) L × U (1) R . This extension is motivated by the observed structure of the quarks and leptons, i.e., they form left-handed doublets and right-handed singlets eigenstates of electroweak interactions. In this new extension, the U (1) R serves only the right-handed singlet fermions. Consistent with the measurements of the mass of the standard massive neutral boson Z0 at the SLAC and CERN colliders and the neutral current couplings involving neutrinos and electrons, the additional massive neutral gauge boson can be as light as a few hundred GeV. The model employs the generalized see-saw mechanism to give masses to all the fermions of the theory.

Detection of the tau-neutrino

Indirect evidence frome + e −→τ+τ− annihilations and from neutrino reactions is discussed which predicts that a tau-neutrino if it exists must transform as the weak isospin +1/2 member of a left-handed doublet under SU(2) rotation and that it has tau-lepton flavour. At present there is no experimental evidence for its existence. A conceptual design is presented aiming at detecting the tau-neutrino in the reactionv τ N→τX by identifying the τ lepton by its characteristic mean life.

Extensions ofSU(2)L×U(1) models and the form factors of the massive Dirac neutrinos

After the discovery of violation of parity andCP-conserving in weak interactions, many authors studied the interaction of a charged spin-1/2 particle with a weak electromagnetic field which has four static couplings described by its charge, magnetic moment, charge radius and the anapole moment. In the standardSU(2)×U(1) model, the neutrinos are predicted to be massless. However, there are many possible extensions of the standard model to give massive neutrinos (Dirac and Majorana neutrinos). Assuming the standardSU(2)×U(1) model with the leptons in left-handed doublets and right-handed singlets plus a single Higgs doublet and using 't Hooft-Feynman gauge we calculate the magnetic moment and the anapole moment of massive (Dirac) neutrinos.