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L. J. Hall

Publications and source records attributed to L. J. Hall.

15 recordsLinked to original sources
arXiv2013

Discrete Symmetries and Neutrino Mass Perturbations for θ_{13}

The recent measurement of the third lepton mixing angle, θ_{13}, has shown that, although small compared to θ_{12} and θ_{23}, it is much larger than anticipated in schemes that generate Tri-Bi-Maximal (TBM) or Golden Ratio (GR) mixing. We develop a model-independent formalism for perturbations away from exact TBM or GR mixing in the neutrino sector. Each resulting perturbation scheme reflects an underlying symmetry structure and involves a single complex parameter. We show that such perturbations can readily fit the observed value of θ_{13}, which is then correlated with a change in the other mixing angles. We also determine the implication for the lepton CP violating phases. For comparison we determine the predictions for Bi-Maximal mixing corrected by charged lepton mixing and we discuss the accuracy that will be needed to distinguish between the various schemes.

hep-ph↗
arXiv2007

LHC and Dark Matter Signals of Improved Naturalness

The Standard Model Higgs suffers from the hierarchy problem, typically implying new states within the reach of the LHC. If the Higgs is very heavy (~500 GeV) the states that cutoff the quadratic divergence may be beyond the reach of the LHC. However, in this case precision electroweak data require the Standard Model to be augmented with new states at the electroweak scale. We study a very simple model, with no new colored states, that allows a heavy Higgs whilst remaining consistent with experiments, and yielding the correct dark matter abundance. We investigate the possibilities for its discovery at the LHC and future dark matter detection experiments.

hep-ph↗
arXiv1998

Oscillations of solar and atmospheric neutrinos

Motivated by recent results from SuperKamiokande, we study both solar and atmospheric neutrino fluxes in the context of oscillations of the three known neutrinos. We aim at a global view which identifies the various possibilities, rather than attempting the most accurate determination of the parameters of each scenario. For solar neutrinos we emphasise the importance of performing a general analysis, independent of any particular solar model and we consider the possibility that any one of the techniques --- chlorine, gallium or water Cerenkov --- has a large unknown systematic error, so that its results should be discarded. The atmospheric neutrino anomaly is studied by paying special attention to the ratios of upward and downward going nu_e and nu_mu fluxes. Both anomalies can be described in a minimal scheme where the respective oscillation frequencies are widely separated or in non-minimal schemes with two comparable oscillation frequencies. We discuss explicit forms of neutrino mass matrices in which both atmospheric and solar neutrino fluxes are explained. In the minimal scheme we identify only two `zeroth order' textures that can result from unbroken symmetries. Finally we discuss experimental strategies for the determination of the various oscillation parameters.

hep-ph↗
arXiv1996

Unified Theories with U(2) Flavor Symmetry

A general operator expansion is presented for quark and lepton mass matrices in unified theories based on a U(2) flavor symmetry, with breaking parameter of order $V_{cb} \approx m_s/m_b \approx \sqrt{m_c/m_t}$. While solving the supersymmetric flavor-changing problem, a general form for the Yukawa couplings follows, leading to 9 relations among the fermion masses and mixings, 5 of which are precise. The combination of grand unified and U(2) symmetries provides a symmetry understanding for the anomalously small values of $m_u/m_c$ and $m_c/m_t$. A fit to the fermion mass data leads to a prediction for the angles of the CKM unitarity triangle, which will allow a significant test of these unified U(2) theories. A particular SO(10) model provides a simple realization of the general operator expansion. The lighter generation masses and the non-trivial structure of the CKM matrix are generated from the exchange of a single U(2) doublet of heavy vector generations. This model suggests that CP is spontaneously broken at the unification scale --- in which case there is a further reduction in the number of free parameters.

hep-ph↗
arXiv1996

Probing Lepton Flavor Violation at Future Colliders

Supersymmetric theories with significant lepton flavor violation have $\tilde{e}$ and $\tildeμ$ nearly degenerate. In this case, pair production of $\tilde{e}^+ \tilde{e}^-$ and $\tildeμ^+ \tildeμ^-$ at LEPII and at the Next Linear Collider leads to the phenomenon of slepton oscillations, which is analogous to neutrino oscillations. The reach in $Δm^2$ and $\sin^2 2 θ$ gives a probe of lepton flavor violation which is significantly more powerful than the current bounds from rare processes, such as $μ\to eγ$. Polarizable $e^-$ beams and the $e^-e^-$ mode at the NLC are found to be promising options.

hep-ph↗
arXiv1996

A Supersymmetric Theory of Flavor with Radiative Fermion Masses

Supersymmetric theories involving a spontaneously broken flavor symmetry can lead to fermion masses which vanish at tree level but are generated by radiative corrections. In the context of supersymmetric theories with minimal low energy field content we discuss which fermion masses and mixings may be obtained radiatively, and find that constraints from flavor changing phenomenology imply that only the first generation fermion masses and some (but not all) CKM mixings can naturally come from radiative corrections. We also consider general conditions on theories of flavor which guarantee the existence of tree level massless fermions while having non-trivial CKM matrix elements at tree level. Two complete models of flavor are presented. In the first model, all first generation fermion masses are radiatively generated. In the second model, the electron and up quark mass are due to radiative corrections whereas the down mass appears at tree level, as does a successful prediction for the Cabibbo angle $\sin θ_c = \sqrt{m_d/m_s}$.

hep-ph↗
arXiv1995

Flavor Mixing Signals For Realistic Supersymmetric Unification

The gauge interactions of any supersymmetric extension of the standard model involve new flavor mixing matrices. The assumptions involved in the construction of minimal supersymmetric models, both $SU(3) \times SU(2) \times U(1)$ and grand unified theories, force a large degree of triviality on these matrices. However, the requirement of realistic quark and lepton masses in supersymmetric grand unified theories forces these matrices to be non-trivial. This leads to important new dominant contributions to the neutron electric dipole moment and to the decay mode $p \to K^oμ^+$, and suggests that there may be important weak scale radiative corrections to the Yukawa coupling matrix of the up quarks. The lepton flavor violating signal $μ\to eγ$ is studied in these theories when $\tanβ$ is sufficiently large that radiative effects of couplings other than $λ_t$ must be included. The naive expectation that large $\tanβ$ will force sleptons to unacceptably large masses is not borne out: radiative suppressions to the leptonic flavor mixing angles allow regions where the sleptons are as light as 300 GeV, provided the top Yukawa coupling in the unified theory is near the minimal value consistent with $m_t$.

hep-ph↗
arXiv1995

A Complete Supersymmetric SO(10) Model

A complete supersymmetric SO(10) model is constructed, which is the most general consistent with certain $R$, discrete, and $U(1)$ flavor symmetries. The desired vacuum of the theory has vevs which lie in particular directions of group space. This leads to both doublet triplet splitting and to the generation of just four operators for charged fermion masses. The model illustrates how many features of superunification become related in the context of a complete theory. The features discussed here include: the weak mixing angle prediction, the doublet-triplet splitting problem, proton decay, the generation of the $μ$ parameter, neutrino masses and the generation of the operators which lead to charged fermion mass predictions.

hep-ph↗
arXiv1994

Electric dipole moments as a test of supersymmetric unification

In a class of supersymmetric grand unified theories, including those based on the gauge group $SO(10)$, there are new contributions to the electric dipole moments of the neutron and electron, which arise as a heavy top quark effect. These contributions arise from CKM-like phases, not from phases of the supersymmetry breaking operators, and can be reliably computed in terms of the parameters of the weak scale supersymmetric theory. For the expected ranges of these parameters, the electric dipole moments of the neutron and the electron are predicted to be close to present experimental limits.

hep-ph↗
arXiv1994

Squark and Slepton Mass Relations in Grand Unified Theories

In the minimal supersymmetric standard model, assuming universal scalar masses at large energies, there are four intragenerational relations between the masses of the squarks and sleptons for each light generation. In this paper we study the scalar mass relations which follow only from the assumption that at large energies there is a grand unified theory which leads to a significant prediction of the weak mixing angle. Two new intragenerational mass relations for each of the light generations are derived. In addition, a third mass relation is found which relates the Higgs masses, the masses of the third generation scalars , and the masses of the scalars of the lighter generations. Verification of a fourth mass relation, involvingonly the charged slepton masses, provides a signal for SO(10) unification.

hep-ph↗
arXiv1994

Signals For Supersymmetric Unification

Using the Yukawa couplings of the minimal supersymmetric SU(5) model, the rates for $μ\to eγ$, $μ\to e$ conversion and $τ\to μγ$ are computed. For a selectron mass of 100 GeV, and without exploring the full parameter space, we find rates which are one order of magnitude beneath present experimental bounds. It is argued that these relatively large rates have a wide applicability, so that lepton flavor violating signals provide a more general test of supersymmetric unification than can be obtained from either proton decay or neutrino masses.

hep-ph↗
arXiv1993

A Systematic SO(10) Operator Analysis for Fermion Masses

A new approach for deducing the theory of fermion masses at the scale of grand unification is proposed. Combining SO(10) grand unification, family symmetries and supersymmetry with a systematic operator analysis, the minimal set of fermion mass operators consistent with low energy data is determined. Exploiting the full power of SO(10) to relate up, down and charged lepton mass matrices, we obtain predictions for 7 of the mass and mixing parameters. The assumptions upon which the operator search and resulting predictions are based are stressed, together with a discussion of how the predictions are affected by a relaxation of some of the assumptions.The masses of the heaviest generation, $m_t,m_b$ and $m_τ$, are generated from a single renormalizable Yukawa interaction, while the lighter masses and the mixing angles are generated by non-renormalizable operators of the grand unified theory. The hierarchy of masses and mixing angles is thereby related to the ratio of grand to Planck scales, $M_G / M_P$. An explicit realization of the origin of such an economical pattern of operators is given in terms of a set of spontaneously broken family symmetries. In the preferred models the top quark is found to be heavy: $M_t = 180 \pm 15$ GeV, and $\tan β$ is predicted to be very large. Predictions are also given for $m_s, m_s/m_d , m_u/m_d, V_{cb}, V_{ub}/V_{cb}$ and the amount of CP violation. Stringent tests of these theories will be achieved by more precise measurements of $M_t, V_{cb}, α_s$ and $ V_{ub}/V_{cb}$ and by measurements of CP violation in neutral B meson decays.

hep-ph↗
arXiv1993

$SO(10)$ Operator Analysis For $ν_μν_τ$ Oscillations

In grand unified theories the flavor mixing angles of leptons are related to those of quarks. In this paper we study SO(10) theories with a precise group theoretic relation at the GUT scale for mixing between the heaviest two generations: $\thetuto = κ\abs{\vcbo}$. A comprehensive operator search yields all possible cases where $κ$ is a group theory Clebsch. The resulting predictions for $ν_μν_τ$ oscillations are scaled from the grand to the weak scales. We find that all but one of the models which have such a relationship between $\thetuto$ and $\vcbo$, and are not already excluded, will be probed by the CHORUS and NOMAD experiments. A more precise mixing measurement, for example by the proposed P803 experiment, could distinguish between the models.

hep-ph↗
arXiv1992

Neutrino masses and mixings in a predictive theory of fermion masses

A framework for predicting charged fermion masses in supersymmetric grand unified theories is extended to make predictions in the neutrino sector. Eight new predictions are made, and their relevance to neutrino oscillation experiments and the solar neutrino problem are discussed.

hep-ph↗