Families Of Leptons Research Articles

LEPTON NUMBERS IN THE FRAMEWORK OF NEUTRINO MIXING

In this short review we discuss the notion of lepton numbers. The strong evidence in favor of neutrino oscillations obtained recently in the super-Kamiokande atmospheric neutrino experiment and in solar neutrino experiments imply that the law of conservation of family lepton numbers Le, Lμ and Lτ is strongly violated. We consider the states of flavor neutrinos νe, νμ and ντ and we discuss the evolution of these states in space and time in the case of nonconservation of family lepton numbers due to the mixing of light neutrinos. We discuss and compare different flavor neutrino discovery experiments. We stress that experiments on the search for νμ→ντ and νe→ντ oscillations demonstrated that the flavor neutrino ντ is a new type of neutrino, different from νe and νμ. In the case of neutrino mixing, the lepton number (only one) is connected with the nature of massive neutrinos. Such a conserved lepton number exists if massive neutrinos are Dirac particles. We review possibilities to check in future experiments whether the conserved lepton number exists.

Muon anomalous magnetic moment and lepton flavor violation

A non-universal interaction, which involves only the third family leptons induces lepton flavor violating couplings and contributes to the anomalous magnetic moment of muon. In this paper, we study the effects of non-universal interaction on muon (g-2) and rare decay $\tau \to \mu \gamma$ by using an effective lagrangian technique, and a phenomenological $Z^\prime$ model where $Z^\prime$ couples only to the third family lepton. We find that the deviation from the theory can be explained and the induced $\tau \to \mu \gamma$ rate could be very close to the current experimental limit. In the $Z^\prime$ model, $M_{Z^\prime}$ has to be lighter than 2.6 TeV.

A NEW SEARCH FOR LEPTON FLAVOR VIOLATION IN HIGH-Q2 POSITRON-PROTON COLLISIONS

A search for Lepton Flavor Violation in Deeply Inelastic positron-proton scattering was conducted at the HERA Colider, using the ZEUS detector. This process permits the probing of the four-fermion interaction, in particular those terms that involve two quarks and two leptons. Strong limits exist from rare decays and from lower-energy experiments, but they involve the lighter quarks. At HERA, limits can be set that also involve interactions with heavier sea quarks, and all three lepton families. these limits define the energy scale probed in the effective Lagrangian, and, in a more specific model, involving leptoquarks. While no events survived the selection criteria, two interesting candidate events are discussed.

Resolving the solar neutrino problem: Evidence for massive neutrinos in the Sudbury Neutrino Observatory

or more than 30 years, experiments have detected neutrinos produced in the thermonuclear fusion reactions which power the Sun. These reactions fuse protons into helium and release neu­ trinos with an energy of up to 15 MeY. Data from these solar neutrino experiments were found to be incompatible with the predictions of solar models. More precisely, the flux of neutrinos detected on Earth was less than expected, and the relative intensi­ ties of the sources of neutrinos in the sun was incompatible with those predicted by solar models. By the mid-1990's the data were beginning to suggest that one could not even in principle adjust solar models sufficiently to account for the effects. Novel proper­ ties of neutrinos seemed to be called for. With the recent measurements of the Sudbury Neutrino Observatory (SNO), it has finally become possible to test the solar model predictions and the particle properties of neutrinos independently. Solar models that simulate the interior of the Sun and explain stellar evolution have been developed using experimental and theoretical inputs from nuclear physics, astrophysics, and particle physics. These models are based on the assumption of light ele­ ment fusion in the Sun. As more and more astrophysical data have become available, solar models were tested through a variety of observables and found to be successful in many respects. A variety of hypotheses that require new particle physics have been postulated to explain the discrepancy between the solar model expectations and the apparent deficit of solar neutrinos detected on Earth. In the Standard Model, neutrinos belong to the family ofleptons. Neutrinos were believed to be massless particles with three distinct flavors (electron, muon, and tau) depending on the weak interaction process that created them. One flavor could not transform into another. All three types of neutrinos SNO

Search for the lepton family number violating process nu(mu)e(-) --> mu(-)nu(e).

The NuTeV experiment at Fermilab has used a sign-selected neutrino beam to perform a search for the lepton number violating process nu(mu)e(-)-->mu(-)nu(e), and to measure the cross section of the standard model inverse muon decay process nu(mu)e(-)-->mu(-)nu(e). NuTeV measures the inverse muon decay asymptotic cross-section slope sigma/E to be (13.8 +/- 1.2 +/- 1.4) x 10(-42) cm(2)/GeV. The experiment also observes no evidence for lepton number violation and places one of the most restrictive limits on the cross-section ratio sigma(nu(mu)e(-)-->mu(-)nu(e))/sigma(nu(mu)e(-)-->mu(-)nu(e)) < or = 1.7% at 90% C.L. for V-A couplings and < or = 0.6% for scalar couplings.

Extra families, Higgs spectrum, and oblique corrections

The standard model accommodates, but does not explain, three families of leptons and quarks, while various extensions suggest extra matter families. The oblique corrections from extra chiral families with relatively light (weak-scale) masses, $M_{f} \sim <H> $, are analyzed and used to constrain the number of extra families and their spectrum. The analysis is motivated, in part, by recent N = 2 supersymmetry constructions, but is performed in a model-independent way. It is shown that the correlations among the contributions to the three oblique parameters, rather than the contribution to a particular one, provide the most significant bound. Nevertheless, a single extra chiral family with a constrained spectrum is found to be consistent with precision data without requiring any other new physics source. Models with three additional families may also be accommodated but only by invoking additional new physics, most notably, a two-Higgs-doublet extension. The interplay between the spectra of the extra fermions and the Higgs boson(s) is analyzed in the case of either one or two Higgs doublets, and its implications are explored. In particular, the precision bound on the SM-like Higgs boson mass is shown to be significantly relaxed in the presence of an extra relatively light chiral family.

Neutrino mass matrix in Anti-GUT with see-saw mechanism

We have investigated the predictions of neutrino oscillations within extended Anti-GUT, based on a large gauge group - the Standard Model and an B-L abelian group assumed with separate gauge fields coupling to each family of quarks and leptons. We take into account corrections concerning a crude way of accounting for the number of ways the vacuum expectation values of the Higgs fields of the model can be ordered in the Feynman diagrams yielding the mass matrix elements. Performing these corrections in a balanced way between the charged fermion sector and the neutrino sector (case I) leads to the previous version of our model that was a fit to the MSW small angle solution region. The fit considered here is marginally worse that the previous fit inasmuch as it predicts a somewhat lower solar neutrino mass square difference (which might though be the only way of avoiding the day-night exclusion region). The other two versions (case II and case III) are characterised by the heaviest of the see-saw neutrino matrix elements that are composed either of \nu_{R\mu} and \nu_{R\tau} or two \nu_{R\tau}. These cases II and III fit the small mixing angle MSW scenario very well.

The solar LMA neutrino oscillation solution in the Zee model

We examine the neutrino mass matrix in the version of Zee model where both Higgs doublets couple to the leptons. We show that in this case one can accommodate the large mixing angle (LMA) MSW solution of the solar neutrino problem, while avoiding maximal solar mixing and conflicts with constraints on lepton family number-violating interactions. In the simplified scenario we consider, we have the neutrino mass spectrum characterized by m 1≃m 2≃ Δm 2 atm / sin2θ and m 3/ m 1≃cos2 θ, where θ is the solar mixing angle.

Search for spontaneous R-parity violation at [formula omitted] GeV and 189 GeV

Searches for spontaneous R-parity-violating signals at s =183 GeV and s =189 GeV have been performed using the 1997 and 1998 DELPHI data, under the assumption of R-parity breaking in the third lepton family. The expected topology for the decay of a pair of charginos into two acoplanar taus plus missing energy was investigated and no evidence for a signal was found. The results were used to derive a limit on the chargino mass and to constrain the allowed domains of the MSSM parameter space.

Standard models from heterotic M-theory

We present a class of N = 1 supersymmetric models of particle physics, derived directly from heterotic M-theory, that contain three families of chiral quarks and leptons coupled to the gauge group SU(3)C×SU(2)L×U(1)Y. These models are a fundamental form of “brane-world” theories, with an observable and hidden sector each confined, after compactification on a Calabi–Yau threefold, to a BPS three-brane separated by a five dimensional bulk space with size of the order of the intermediate scale. The requirement of three families, coupled to the fundamental conditions of anomaly freedom and supersymmetry, constrains these models to contain additional five-branes wrapped around holomorphic curves in the Calabi–Yau threefold. These five-branes “live” in the bulk space and represent new, non-perturbative aspects of these particle physics vacua. We discuss, in detail, the relevant mathematical structure of a class of torusfibered Calabi–Yau threefolds with non-trivial first homotopy groups and construct holomorphic vector bundles over such threefolds, which, by including Wilson lines, break the gauge symmetry to the standard model gauge group. Rules for constructing phenomenological particle physics models in this context are presented and we give a number of explicit examples. CERN-TH/99-407, UPR-853T December 1999 ∗current address

New formulation of matter effects on neutrino mixing and CP violation

Within the framework of three lepton families we have derived an exact and compact formula to describe the matter effect on neutrino mixing and CP violation. This model- and parametrization-independent result can be particularly useful for recasting the fundamental lepton flavor mixing matrix from a variety of long-baseline neutrino experiments.

NEUTRINO MASS AND OSCILLATIONS

The neutrino remains as exotic and challenging today as it was seventy years ago when first proposed by Pauli. What is known for certain about neutrinos is minimal indeed. They have spin 12, charge 0, negative helicity, and exist in 3 flavors, electron, mu, and tau. Strictly speaking, only 2 flavors are certain: direct observation of the tau neutrino has not yet been achieved, but an experiment, DONUT, at Fermilab is in progress with this objective [1]. Limits on the masses from direct, kinematic experiments (ones that do not require assumptions about the non-conservation of lepton family numbers) have been steadily lowered by experiments of ever-increasing sophistication over the years, with the results given in Table 1. As will be discussed below, there are stronger limits on the mass of νe from tritium beta decay [2, 3], but the data show curious distortions near the endpoint that are not at present understood.

Update of the search for the neutrinoless decayτ→μγ

We present an update of the search for the lepton family number violating decay τ→μγ using 12.6 million τ+τ− pairs collected with the CLEO detector. No evidence of a signal has been found and the corresponding upper limit is B(τ→μγ)<1.1×10−6 at 90% C.L., significantly smaller than previous experimental limits.Received 25 October 1999DOI:https://doi.org/10.1103/PhysRevD.61.071101©2000 American Physical Society

SO(10) Grand Unified Theory in Generalized Covariant Derivative Formalism

The SO(10) grand unified theory is reformulated in a new field theory with a unified field strength for both the gauge and Higgs fields, and the fine-tuning problem and the condition for the symmetry breakings are investigated. The unified field strength for the gauge and Higgs fields, which takes values in the Dirac algebra, is defined by means of the commutator of the generalized covariant derivative for a multi-spinor field describing all families of quarks and leptons. The bosonic Lagrangian is constructed from a general sum of quadratic invariants of the field strength. Among the Yukawa coupling constants and other parameters related to the Higgs field self-couplings, there exist additional relations that are interpreted as initial conditions for renormalization equations. The grand unified symmetry is broken down to the low energy symmetry by the 210-, 45-, 126- and 10-dimensional Higgs fields. The Higgs potential turns out to have a discrete symmetry among Higgs fields. This symmetry makes it subtle and difficult to solve the fine-tuning problem, which requires accurate adjustments of the parameters included in the generalized covariant derivative. It is shown that the 45-dimensional Higgs field plays an essential role to break the discrete symmetry and to solve the fine-tuning problem.

Non-perturbative vacua in heterotic M-theory

We present a class of N = 1 supersymmetric `standard' models of particle physics, derived directly from heterotic M-theory, that contain three families of chiral quarks and leptons coupled to the gauge group SU(3)C × SU(2)L × U(1)Y. These models are a fundamental form of `brane world' theories, with an observable and hidden sector each confined, after compactification on a Calabi-Yau 3-fold, to a BPS 3-brane separated by a higher-dimensional bulk space with size of the order of the intermediate scale. The requirement of three families, coupled to the fundamental conditions of anomaly freedom and supersymmetry, constrains these models to contain additional 5-branes located in the bulk space and wrapped around holomorphic curves in the Calabi-Yau 3-fold.

SU(3)L × U(1)X Model and N-Particle as Dark Matter Particle

We proposed an SU(3)L × U(1)X gauge model for the electroweak interactions. In the model each lepton family contains two neutral particles. We show that the new neutral lepton (N-particle) can play the role of dark matter particle.

SO(3) gauge symmetry and neutrino-lepton flavor physics

Based on the SO(3) gauge symmetry for three family leptons and the general seesaw mechanism, we present a simple scheme that allows three nearly degenerate Majorana neutrino masses needed for hot dark matter. The vacuum structure of the spontaneous SO(3) symmetry breaking can automatically lead to a maximal $\mathrm{CP}$-violating phase. Thus the current neutrino data on both the atmospheric neutrino anomaly and solar neutrino deficit can be accounted for via maximal mixings without conflict with the current data on the neutrinoless double $\ensuremath{\beta}$ decay. The model also allows rich interesting phenomena on lepton flavor violations.

Family unification from universality

A direct consequence of the occurrence of fermion families is the invariance of currents under certain groups of (universality) transformations. We show how these universality groups can themselves be used to find and study grand family unification models. Identifying two independent - weak and strong - universality groups and assuming that the grand unification group is SU(8N), its subgroup respecting either weak or strong universality is shown to be G = SU(2)xU(1)xSU(3). The fundamental representation of SU(8N) decomposes as N families of leptons and quarks. In the G-invariant limit, all fermions are left-handed. A mechanism for generating the correct number of right-handed fermions with the correct couplings so as to give pure vector colour and electromagnetic currents is exhibited. Universality is shown to result most naturally from a preonic structure of fermions. In such a preonic picture there are no ultraheavy gauge bosons and no anomaly or hierarchy problem.

Varying leptonic chemical potentials and spatial variation of primordial deuterium at high z

We try to explain the spatial variation of primordial deuterium suggested by some observations by varying leptonic chemical potentials. The variation of the latter may take place in some scenarios of leptogenesis. The model predicts a large mass fraction of 4He (35–60%) and 7Li (up to 10 −9) in deuterium-rich regions. Because of lepton family symmetry, the angular variations of cosmic microwave background radiation can be sufficiently small although still observable in future measurements.

FERMION-SECTOR FRUSTRATED SU(4) AS A PREONIC PRECURSOR OF THE STANDARD MODEL

We give a model for composite quarks and leptons based on the semisimple gauge group SU(4), with the preons in the 10 representation; this choice of gauge gluon and preon multiplets is motivated by the possibility of embedding them in an N=6 supergravity multiplet, with the preons and antipreons both in the 20 of SU(6). Hypercolor singlets are forbidden in the fermionic sector of this theory; we propose that the SU(4) symmetry spontaneously breaks to SU (3)× U (1), with the binding of triality nonzero preons and gluons into composites, and with the formation of a color singlet condensate that breaks the initial Z12 vacuum symmetry to Z6. The spin ½ fermionic composites have the triality structure of a quark–lepton family, and the initial Z12 symmetry implies that there are six massless families, which mix to give three distinct families below the scale of the condensate. The spin 1 triality zero composites of the color triplet SU(4) gluons, when coupled to the condensate and with the color singlet representation of the 10 acting as a doorway state, lead to weak interactions of the fermionic composites through an SU(2) gauge algebra. The initial Z12 symmetry implies that this SU(2) gauge algebra structure is doubled, which in turn permits the corresponding independent gauge bosons to couple to chiral components of the composite fermions. Since the U(1) couples to the 10 representation as B-L, an effective SU (2)L× SU (2)R × U (1)B-L electroweak theory arises at the condensate scale, with all composites having the correct electric charge structure. Assuming a mechanism for forming composite Higgs bosons, the Z12→ Z6 symmetry breaking chain implies that below the condensate scale there can be two sets of discrete chiral Z6 triplets of Higgs doublets, as required by a phenomenological model for the CKM matrix that we have analyzed in detail elsewhere. A renormalization group analysis of the SU(4) model shows that the conversion by binding of one 10 of SU(4) to 12 triplets of SU(3) can give a very large, calculable hierarchy ratio between the SU(4) and the hadronic mass scales.