Families Of Leptons Research Articles

Stability of the minimal heterotic standard model bundle

The observable sector of the "minimal heterotic standard model" has precisely the matter spectrum of the MSSM: three families of quarks and leptons, each with a right-handed neutrino, and one Higgs-Higgs conjugate pair. In this paper, it is explicitly proven that the SU(4) holomorphic vector bundle leading to the MSSM spectrum in the observable sector is slope-stable.

Lepton family symmetries for neutrino masses and mixing

I review some of the recent progress (up to December 2005) in applying non-Abelian discrete symmetries to the family structure of leptons, with particular emphasis on the tribimaximal mixing ansatz of Harrison, Perkins and Scott.

Search for High-Mass Resonances Decaying toeμinpp¯Collisions ats=1.96 TeV

We describe a general search for resonances decaying to a neutral e-mu final state in ppbar collisions at a center-of-mass energy of 1.96 TeV. Using a data sample representing 344 pb^-1 of integrated luminosity recorded by the CDF-II experiment, we compare Standard Model predictions with the number of observed events for invariant masses between 50 and 800 GeV/c^2. Finding no significant excess (5 events observed vs. 7.7 +/- 0.8 expected for e-mu invariant masses > 100 GeV/c^2), we set limits on sneutrino and Z' masses as functions of lepton family number violating couplings.

The exact MSSM spectrum from string theory

We show the existence of realistic vacua in string theory whose observable sector has exactly the matter content of the MSSM. This is achieved by compactifying the E_8 x E_8 heterotic superstring on a smooth Calabi-Yau threefold with an SU(4) gauge instanton and a Z_3 x Z_3 Wilson line. Specifically, the observable sector is N=1 supersymmetric with gauge group SU(3)_C x SU(2)_L x U(1)_Y x U(1)_{B-L}, three families of quarks and leptons, each family with a right-handed neutrino, and one Higgs-Higgs conjugate pair. Importantly, there are no extra vector-like pairs and no exotic matter in the zero mode spectrum. There are, in addition, 6 geometric moduli and 13 gauge instanton moduli in the observable sector. The holomorphic SU(4) vector bundle of the observable sector is slope-stable.

A heterotic standard model

AbstractWithin the context of the E8 × E8 heterotic superstring compactified on a smooth Calabi‐Yau threefold with an SU(4) gauge instanton, we show the existence of simple, realistic N = 1 supersymmetric vacua that are compatible with low energy particle physics. The observable sector of these vacua has gauge group SU(3)C × SU(2)L × U(1)Y × U(1)B‐L, three families of quarks and leptons, each with an additional right‐handed neutrino, two Higgs‐Higgs conjugate pairs, a small number of uncharged moduli and no exotic matter.

A maximal atmospheric mixing from a maximal CP violating phase

We point out an elegant mechanism to predict a maximal atmospheric angle, which is based on a maximal CP violating phase difference between second and third lepton families in the flavour symmetry basis. In this framework, a discussion of the general formulas for θ12, |Ue3|, δ and their possible correlations in some limiting cases is provided. We also present an explicit realisation in terms of an SO(3) flavour symmetry model.

Standard model compactifications of IIA [formula omitted] orientifolds from intersecting D6-branes

We discuss the construction of chiral four-dimensional T 6 / ( Z 3 × Z 3 ) orientifold compactifications of IIA theory, using D6-branes intersecting at angles and not aligned with the orientifold O6 planes. Cancellation of mixed U ( 1 ) anomalies requires the presence of a generalized Green–Schwarz mechanism mediated by RR partners of closed string untwisted moduli. In this respect we describe the appearance of three quark and lepton family SU ( 3 ) C × SU ( 2 ) L × U ( 1 ) Y non-supersymmetric orientifold models with only the massless spectrum of the SM at low energy that can have either no exotics present and three families of ν R 's ( A ′ -model class) or the massless fermion spectrum of the N = 1 SM with a small number of massive non-chiral colour exotics and in one case with extra families of ν R 's ( B ′ -model class). Moreover we discuss the construction of SU ( 5 ) , flipped SU ( 5 ) and Pati–Salam SU ( 4 ) c × SU ( 2 ) L × SU ( 2 ) R GUTS—the latter also derived from adjoint breaking—with only the SM at low energy. Some phenomenological features of these models are also briefly discussed. All models are constructed with the Weinberg angle to be 3 / 8 at the string scale.

A KNOT MODEL SUGGESTED BY THE STANDARD ELECTROWEAK THEORY

We attempt to go beyond the standard electroweak theory by replacing SU (2) with its q-deformation: SU q(2). This step introduces new degrees of freedom that we interpret as indicative of nonlocality and as a possible basis for a solitonic model of the elementary particles. The solitons are conjectured to be knotted flux tubes labeled by the irreducible representations of SU q(2), an algebra which is not only closely related to the standard theory but also plays an underlying role in the description of knots. Each of the four families of elementary fermions is conjectured to be represented by one of the four possible trefoils. The three individual fermions belonging to any family are then assumed to occupy the three lowest states in the excitation spectrum of the trefoil for that family. One finds a not unreasonable variation of q among the lepton and quark families. The model in its present form predicts a fourth generation of fermions as well as a neutrino mass spectrum. The model may be refined depending on whether or not the fourth generation is found.

LIGHT COMPOSITE HIGGS: LCH @ LHC

Here I summarize some of the salient features of technicolor theories with technifermions in higher dimensional representations of the technicolor gauge group. The expected phase diagram as function of number of flavors and colors for the two index (anti)symmetric representation of the gauge group is reviewed. After having constructed the simplest walking technicolor theory one can show that it is not at odds with the precision measurements. The simplest theory also requires, for consistency, a fourth family of heavy leptons. The latter may result in an interesting signature at LHC. In the case of a fourth family of leptons with ordinary lepton hypercharge the new heavy neutrino can be a natural candidate of cold dark matter. New theories will also be proposed in which the critical number of flavors needed to enter the conformal window is higher than in the one with fermions in the two-index symmetric representation, but lower than in the walking technicolor theories with fermions only in the fundamental representation of the gauge group. Due to the near conformal/chiral phase transition the composite Higgs is very light compared to the intrinsic scale of the technicolor theory. For the two technicolor theory the composite Higgs mass is predicted not to exceed 150 GeV.

Light composite Higgs boson from higher representations versus electroweak precision measurements: Predictions for CERN LHC

We investigate theories in which the technifermions in higher dimensional representations of the technicolor gauge group dynamically break the electroweak symmetry of the standard model. For the two-index symmetric representation of the gauge group the lowest number of techniflavors needed to render the underlying gauge theory quasi conformal is two. We confront the models with the recent electroweak precision measurements and demonstrate that the two technicolor theory is a valid candidate for a dynamical breaking of the electroweak symmetry. The electroweak precision measurements provide useful constraints on the relative mass splitting of the new leptons needed to cure the Witten anomaly. In the case of a fourth family of leptons with ordinary lepton hypercharge the new heavy neutrino can be a natural candidate of cold dark matter. We also propose theories in which the critical number of flavors needed to enter the conformal window is higher than the one with fermions in the two-index symmetric representation, but lower than in the walking technicolor theories with fermions only in the fundamental representation of the gauge group. Due to the near conformal/chiral phase transition, we show that the composite Higgs is very light compared to the intrinsic scale of the technicolor theory. For the two technicolor theory we predict the composite Higgs mass not to exceed 150 GeV.

TRIPLICITY OF QUARKS AND LEPTONS

Quarks come in three colors and have electric charges in multiples of one-third. There are also three families of quarks and leptons. Though the first two properties can be understood in terms of unification symmetries such as SU(5), SO(10), or E6, why should there only be three families remains a mystery. We propose how all three properties involving the number three are connected in a fivefold application of the gauge symmetry SU(3).

Cosmological family asymmetry andCPviolation

We discuss how the cosmological baryon asymmetry can be achieved by the lepton family asymmetries of heavy Majorana neutrino decays and how the lepton family asymmetries are related to $CP$ violation in neutrino oscillation, in the minimal seesaw model with two heavy Majorana neutrinos. We derive the most general formula for $CP$ violation in neutrino oscillation in terms of the heavy Majorana masses and Yukawa mass term. It is shown that the formula is very useful to classify several models in which $e$, $\ensuremath{\mu}$, and $\ensuremath{\tau}$ leptogenesis can be separately realized and to see how they are connected with low energy $CP$ violation. To make the models predictive, we take texture with two zeros in the Dirac matrix. In particular, we find some interesting cases in which $CP$ violation in neutrino oscillation can happen while lepton family asymmetries do not exist at all. On the contrary, we can find $e$-, $\ensuremath{\mu}$-, and $\ensuremath{\tau}$-leptogenesis scenarios in which the cosmological $CP$ violation and low energy $CP$ violation measurable via neutrino oscillations are very closely related to each other. By determining the allowed ranges of the parameters in the models, we predict the sizes of $CP$ violation in neutrino oscillation and $|{V}_{e3}^{\mathrm{MNS}}|$. Finally, the leptonic unitarity triangles are reconstructed.

The spectra of heterotic standard model vacua

A formalism for determining the massless spectrum of a class of realistic heterotic string vacua is presented. These vacua, which consist of SU(5) holomorphic bundles on torus-fibered Calabi-Yau threefolds with fundamental group 2, lead to low energy theories with standard model gauge group (SU(3)C × SU(2)L × U(1)Y)/6 and three families of quarks and leptons. A methodology for determining the sheaf cohomology of these bundles and the representation of 2 on each cohomology group is given. Combining these results with the action of a 2 Wilson line, we compute, tabulate and discuss the massless spectrum.

A heterotic standard model

Within the context of the E8×E8 heterotic superstring compactified on a smooth Calabi–Yau threefold with an SU(4) gauge instanton, we show the existence of simple, realistic N=1 supersymmetric vacua that are compatible with low-energy particle physics. The observable sector of these vacua has gauge group SU(3)C×SU(2)L×U(1)Y×U(1)B–L, three families of quarks and leptons, each with an additional right-handed neutrino, two Higgs–Higgs conjugate pairs, a small number of uncharged moduli and no exotic matter. The hidden sector contains non-Abelian gauge fields and moduli. In the strong coupling case there is no exotic matter, whereas for weak coupling there are a small number of additional matter multiplets in the hidden sector. The construction exploits a mechanism for “splitting” multiplets. The minimal nature and rarity of these vacua suggest the possible theoretical and experimental relevance of spontaneously broken U(1)B–L gauge symmetry and two Higgs–Higgs conjugate pairs. The U(1)B–L symmetry helps to naturally suppress the rate of nucleon decay.

Heterotic standard model vacua

AbstractIn this talk I will present a class of heterotic string theory compactifications which lead to four dimensional N = 1 supersymmetric vacua with standard model gauge group SU(3)C × SU(2)L × U(1)Y. I will compute their massless particle spectrum and show that it contains precisely three families of quarks and leptons and a certain number of SU(2) Higgs doublets.

Dihedral families of quarks, leptons, and Higgs bosons

We consider finite groups of small order for family symmetry. It is found that the binary dihedral group Q_6, along with the assumption that the Higgs sector is of type II, predicts mass matrix of a nearest neighbor interaction type for quarks and leptons. We present a supersymmetric model based on Q_6 with spontaneously induced CP phases. The quark sector contains 8 real parameters with one independent phase to describe the quark masses and their mixing. Predictions in the |V_{ub}|-bar{eta}, |V_{ub}|-sin 2 beta(phi_1) and |V_{ub}|-|V_{td}/V_{ts}| planes are given. The lepton sector contains also 9 parameters. A normal as well as an inverted spectrum of neutrino masses is possible, and we compute V_{e3}. We find that |V_{e3}|^2 > 10^{-4} in the case of a normal spectrum, and |V_{e3}|^2 >8 10^{-4} in the case of an inverted spectrum. It is also found that Q_6 symmetry forbids all Baryon number violating terms of d=4, and the contributions to EDMs from the A terms vanish in this model.

Systematic approach to gauge-invariant relations between lepton flavor violating processes

We analyze four-lepton contact interactions that lead to lepton flavor violating processes, with violation of individual family lepton number but total lepton number conserved. In an effective Lagrangian framework, the assumption of gauge invariance leads to relations among branching ratios and cross sections of lepton flavor violating processes. In this paper, we work out how to use these relations systematically. We also study the consequences of loop-induced processes.

ON EXTENDED ELECTROWEAK SYMMETRIES

We discuss extensions of the Standard Model through extending the electroweak gauge symmetry. An extended electroweak symmetry requires a list of extra fermionic and scalar states. The former is necessary to maintain cancellation of gauge anomalies, and largely fixed by the symmetry embedding itself. The latter is usually considered quite arbitrary, so long as a vacuum structure admitting the symmetry breaking is allowed. Anomaly cancellation may be used to link the three families of quarks and leptons together, given a perspective on flavor physics. It is illustrated lately that the kind of models may also have the so-called little Higgs mechanism incorporated. This more or less fixes the scalar sector and take care of the hierarchy problem, making such models of extended electroweak symmetries quite appealing candidates as TeV scale effective field theories.

DØ top quark results and their dependence on successful Grid computing

The heaviest known Fermion particle—the top quark—was discovered at Fermilab in the first run of the Tevatron in 1995. However, besides its mere existence one needs to study its properties precisely in order to verify or falsify the predictions of the Standard Model. With the top quark's extremely high mass and short lifetime such measurements probe yet unexplored regions of the theory and bring us closer to solving the open fundamental questions of our universe of elementary particles such as why three families of quarks and leptons exist and why their masses differ so dramatically.To perform these measurements hundreds of millions of recorded proton-antiproton collisions must be reconstructed and filtered to extract the few top quarks produced. Simulated background and signal events with full detector response need to be generated and reconstructed to validate and understand the results. Since the start of the second run of the Tevatron the DØ collaboration has brought Grid computing to its aid for the production of simulated events. Data processing on the Grid has recently been added and thereby enabled us to effectively triple the amount of data available with the highest quality reconstruction methods.We will present recent top quark results DØ obtained from these improved data and explain how they benefited from the availability of computing resources on the Grid.

Phenomenology of a leptonic goldstino and invisible Higgs boson decays

Non-linearly realized supersymmetry, combined with the Standard Model field content and SU ( 3 ) × SU ( 2 ) × U ( 1 ) gauge invariance, permits local dimension-six operators involving a goldstino, a lepton doublet and a Higgs doublet. These interactions preserve total lepton number if the left-handed goldstino transforms as an antilepton. We discuss the resulting phenomenology, in the simple limit where the new couplings involve only one lepton family, thus conserving also lepton flavour. Both the Z boson and the Higgs boson can decay into a neutrino and a goldstino: the present limits from the invisible Z width and from other observables leave room for the striking possibility of a Higgs boson decaying dominantly, or at least with a sizable branching ratio, via such an invisible mode. We finally comment on the perspectives at hadron and lepton colliders, and on possible extensions of our analysis.