Orthogonal U(1)'s, Proton Stability and Extra Dimensions

In models with a low quantum gravity scale, one might expect that all operators consistent with gauge symmetries are present in the low-energy effective theory. If this is the case, some mechanism must be present to adequately suppress operators that violate baryon number. Here we explore the possibility that the desired suppression is a consequence of an additional, spontaneously-broken, non-anomalous U(1) symmetry that is orthogonal to hypercharge. We show that successful models can be constructed in which the additional particle content necessary to cancel anomalies is minimal, and compatible with the constraints from precision electroweak measurements and gauge unification. If unification is sacrificed, and only the new U(1) and its associated Higgs fields live in the bulk, it is possible that the gauge field zero mode and first few Kaluza-Klein excitations lie within the kinematic reach of the Tevatron. For gauge couplings not much smaller than that of hypercharge, we show that these highly leptophobic states could evade detection at Run I, but be discovered at Run II. Our scenario presents an alternative to the `cartographic' solution to baryon number violation in which leptons and quarks are separated in an extra dimension.Comment: 16 pages LaTeX, 4 figure

Physical Parameters and Renormalization of U(1)_a x U(1)_b Models

We analize the structure of models with unbroken and spontaneously broken U(1)_a x U(1)_b gauge symmetry. We show that the quantum corrections to the 2N gauge charges, with N = #fermions + #scalars, can be absorbed in the redefinition of three independent gauge couplings (g_a,g_b and g_ab). We establish the (one-loop) conditions on the matter cotent for g_ab= = 0 (a value usually assumed in the literature) and we show that in the minimal extensions of the Standard Model with an extra U(1) symmetry the choice g_ab = 0 is not stable under radiative corrections induced by the standard Higgs fields. Moreover, g_ab = 0 to all orders seems to require an exact symmetry. The spontaneous breaking of the gauge symmetry induces further mixing between the two gauge bosons and introduces a fourth independent physical parameter. A consequence of our analysis is that the usual tree-level description with only three physical parameters (i.e., two gauge couplings and one gauge boson mixing angle) is not in general a justified zero order limit of the treatment including radiative corrections.Comment: 24 pages, tex, 1 figur

U(2)-like Flavor Symmetries and Approximate Bimaximal Neutrino Mixing

Models involving a U(2) flavor symmetry, or any of a number of its non-Abelian discrete subgroups, can explain the observed hierarchy of charged fermion masses and CKM angles. It is known that a large neutrino mixing angle connecting second and third generation fields may arise via the seesaw mechanism in these models, without a fine tuning of parameters. Here we show that it is possible to obtain approximate bimaximal mixing in a class of models with U(2)-like Yukawa textures. We find a minimal form for Dirac and Majorana neutrino mass matrices that leads to two large mixing angles, and show that our result can quantitatively explain atmospheric neutrino oscillations while accommodating the favored, large angle MSW solution to the solar neutrino problem. We demonstrate that these textures can arise in models by presenting a number of explicit examples.Comment: 20 pages RevTex4, 2 figure

Diagnostics of the structure of AGN's broad line regions with reverberation mapping data: confirmation of the two-component broad line region model

We re-examine the ten Reverberation Mapping (RM) sources with public data based on the two-component model of the Broad Line Region (BLR). In fitting their broad H-beta lines, six of them only need one Gaussian component, one of them has a double-peak profile, one has an irregular profile, and only two of them need two components, i.e., a Very Broad Gaussian Component (VBGC) and an Inter-Mediate Gaussian Component (IMGC). The Gaussian components are assumed to come from two distinct regions in the two-component model; they are Very Broad Line Region (VBLR) and Inter-Mediate Line region (IMLR). The two sources with a two-component profile are Mrk 509 and NGC 4051. The time lags of the two components of both sources satisfy $t_{IMLR}/t_{VBLR}=V^2_{VBLR}/V^2_{IMLR}$, where $t_{IMLR}$ and $t_{VBLR}$ are the lags of the two components while $V_{IMLR}$ and $V_{VBLR}$ represent the mean gas velocities of the two regions, supporting the two-component model of the BLR of Active Galactic Nuclei (AGN). The fact that most of these ten sources only have the VBGC confirms the assumption that RM mainly measures the radius of the VBLR; consequently, the radius obtained from the R-L relationship mainly represent the radius of VBLR. Moreover, NGC 4051, with a lag of about 5 days in the one component model, is an outlier on the R-L relationship as shown in Kaspi et al. (2005); however this problem disappears in our two-component model with lags of about 2 and 6 days for the VBGC and IMGC, respectively.Comment: 7 pages, 5 figures. Accepted for publication in the Special Issue of Science in China (G) "Astrophysics of Black holes and Related Compact Objects

A Hexagonal Theory of Flavor

We construct a supersymmetric theory of flavor based on the discrete gauge group (D_6)^2, where D_6 describes the symmetry of a regular hexagon under proper rotations in three dimensions. The representation structure of the group allows one to distinguish the third from the lighter two generations of matter fields, so that in the symmetry limit only the top quark Yukawa coupling is allowed and scalar superpartners of the first two generations are degenerate. Light fermion Yukawa couplings arise from a sequential breaking of the flavor symmetry, and supersymmetric flavor-changing processes remain adequately suppressed. We contrast our model with others based on non-Abelian discrete gauge symmetries described in the literature, and discuss the challenges in constructing more minimal flavor models based on this approach.Comment: 19 pages, ReVTeX, 1 eps figur

1/Nc Countings in Baryons

The $1/N_c$ power countings for baryon decays and configuration mixings are determined by means of a non-relativistic quark picture. Such countings are expected to be robust under changes in the quark masses, and therefore valid as these become light. It is shown that excited baryons have natural widths of ${\cal{O}}(N_c^0)$. These dominant widths are due to the decays that proceed directly to the ground state baryons, with cascade decays being suppressed to ${\cal{O}}(1/N_c)$. Configuration mixings, defined as mixings between states belonging to different $O(3)\times SU(2 N_f)$ multiplets, are shown to be sub-leading in an expansion in $1/\sqrt{N_c}$ when they involve the ground state baryons, while the mixings between excited states can be ${\cal{O}}(N_c^0)$.Comment: 19 pages, 1 figure An omission that changes the conclusions on configuration mixings has been correcte

Maximal Neutrino Mixing from a Minimal Flavor Symmetry

We study a number of models, based on a non-Abelian discrete group, that successfully reproduce the simple and predictive Yukawa textures usually associated with U(2) theories of flavor. These models allow for solutions to the solar and atmospheric neutrino problems that do not require altering successful predictions for the charged fermions or introducing sterile neutrinos. Although Yukawa matrices are hierarchical in the models we consider, the mixing between second- and third-generation neutrinos is naturally large. We first present a quantitative analysis of a minimal model proposed in earlier work, consisting of a global fit to fermion masses and mixing angles, including the most important renormalization group effects. We then propose two new variant models: The first reproduces all important features of the SU(5)xU(2) unified theory with neither SU(5) nor U(2). The second demonstrates that discrete subgroups of SU(2) can be used in constructing viable supersymmetric theories of flavor without scalar universality even though SU(2) by itself cannot.Comment: 34 pages LaTeX, 1 eps figure, minor revisions and references adde

Weak-scale phenomenology of models with gauge-mediated supersymmetry breaking

We study in some detail the spectral phenomenology of models in which supersymmetry is dynamically broken and transmitted to the supersymmetric partners of the quarks, leptons and gauge bosons, and the Higgs bosons themselves, via the usual gauge interactions. We elucidate the parameter space of what we consider to be the minimal model, and explore the regions which give rise to consistent radiative electroweak symmetry breaking. We include the weak-scale threshold corrections, and show how they considerably reduce the scale dependence of the results. We examine the sensitivity of our results to unknown higher-order messenger-sector corrections. We compute the superpartner spectrum across the entire parameter space, and compare it to that of the minimal supergravity-inspired model. We delineate the regions where the lightest neutralino or tau slepton is the next-to-lightest supersymmetric particle, and compute the lifetime and branching ratios of the NLSP. In contrast to the minimal supergravity-inspired model, we find that the lightest neutralino can have a large Higgsino component, of order 50%. Nevertheless, the neutralino branching fraction to the gravitino and the light Higgs boson remains small, < 10^{-4}, so the observation of such a decay would point to a non-minimal Higgs sector.Comment: 22 pages, 16 figures, published versio

Symmetries of the Standard Model without and with a Right-Handed Neutrino

Given the particle content of the standard model without and with a right-handed neutrino, the requirement that all anomalies cancel singles out a set of possible global symmetries which can be gauged. I review this topic and propose a new gauge symmetry B - 3L_tau in the context of the minimal standard model consisting of the usual three families of quarks and leptons plus just one nu_R. The many interesting phenomenological consequences of this hypothesis are briefly discussed.Comment: 7 pages, no figure, latex, sprocl.sty, talk at the Fifth Workshop on High Energy Physics Phenomenology, Pune, Jan 9

Supersymmetric Flavor Models and the B --> phi K_S Anomaly

We consider the flavor structure of supersymmetric theories that can account for the deviation of the observed time-dependent CP asymmetry in B --> phi K_S from the standard model prediction. Assuming simple flavor symmetries and effective field theory, we investigate possible correlations between sizable supersymmetric contributions to b --> s transitions and to flavor changing processes that are more tightly constrained. With relatively few assumptions, we determine the properties of minimal Yukawa and soft mass textures that are compatible with the desired supersymmetric flavor-changing effect and constraints. We then present explicit models that are designed (at least approximately) to realize these textures. In particular, we present an Abelian model based on a single U(1) factor and a non-trivial extra-dimensional topography that can explain the CP asymmetry in B --> phi K_S, while suppressing other supersymmetric flavor changing effects through a high degree of squark-quark alignment.Comment: 18 pages LaTeX, 3 eps figure