arXiv subjects

K. S. Babu

Publications and source records attributed to K. S. Babu.

At least 91 records · Page 5Linked to original sources

arXiv2013

Post-Sphaleron Baryogenesis and an Upper Limit on the Neutron-Antineutron Oscillation Time

A recently proposed scenario for baryogenesis, called post--sphaleron baryogenesis (PSB) is discussed within a class of quark--lepton unified framework based on the gauge symmetry SU(2)_L x SU(2)_R x SU(4)_c realized in the multi--TeV scale. The baryon asymmetry of the universe in this model is produced below the electroweak phase transition temperature after the sphalerons have decoupled from the Hubble expansion. These models embed naturally the seesaw mechanism for neutrino masses, and predict color-sextet scalar particles in the TeV range which may be accessible to the LHC experiments. A necessary consequence of this scenario is the baryon number violating ΔB=2 process of neutron--antineutron (n-\bar{n}) oscillations. In this paper we show that the constraints of PSB, when combined with the neutrino oscillation data and restrictions from flavor changing neutral currents mediated by the colored scalars imply an upper limit on the n-\bar{n} oscillation time of 5 x 10^{10} sec. regardless of the quark--lepton unification scale. If this scale is relatively low, in the (200-250) TeV range, τ_{n-\bar{n}} is predicted to be less than 10^{10} sec., which is accessible to the next generation of proposed experiments.

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arXiv2012

Coupling Unification, GUT-Scale Baryogenesis and Neutron-Antineutron Oscillation in SO(10)

We show that unification of the three gauge couplings can be realized consistently in a class of non-supersymmetric SO(10) models with a one-step breaking to the Standard Model if a color-sextet scalar field survives down to the TeV scale. Such scalars, which should be accessible to the LHC for direct detection, arise naturally in SO(10) as remnants of the seesaw mechanism for neutrino masses. The diquark couplings of these scalars lead to ΔB = 2 baryon number violating processes such as neutron-antineutron oscillation. We estimate the free neutron-antineutron transition time to be τ_{n-\bar{n}} \approx (10^9-10^{12}) sec., which is in the interesting range for next generation n-\bar{n} oscillation experiments. These models also realize naturally the recently proposed (B-L)-violating GUT scale baryogenesis which survives to low temperatures unaffected by the electroweak sphaleron interactions.

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arXiv2012

Realistic Fermion Masses and Nucleon Decay Rates in SUSY SU(5) with Vector-Like Matter

We show that by adding a vector-like 5+5bar pair of matter fields to the spectrum of the minimal renormalizable SUSY SU(5) theory the wrong relations for fermion masses can be corrected, while being predictive and consistent with proton lifetime limits. Threshold correction from the vector-like fields improves unification of gauge couplings compared to the minimal model. It is found that for supersymmetric spectra lighter than 3 TeV, which would be testable at the LHC, at least some of the nucleon decay modes should have partial lifetimes shorter than about 2.10^34 yrs., which is within reach of ongoing and proposed experiments.

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arXiv2012

B-L Violating Proton Decay Modes and New Baryogenesis Scenario in SO(10)

We show that grand unified theories based on SO(10) generate quite naturally baryon number violating dimension seven operators that violate (B-L), and lead to novel nucleon decay modes such as n \to e^-K^+, e^- π^+ and p \to νπ^+. We find that in two-step breaking schemes of non-supersymmetric SO(10), the partial lifetimes for these modes can be within reach of experiments. The interactions responsible for these decay modes also provide a new way to understand the origin of matter in the universe via the decays of GUT scale scalar bosons of SO(10). Their (B-L)-violating nature guarantees that the GUT scale induced baryon asymmetry is not washed out by the electroweak sphaleron interactions. In minimal SO(10) models this asymmetry is closely tied to the masses of quarks, leptons and the neutrinos.

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arXiv2012

Higgs boson of mass 125 GeV in GMSB models with messenger-matter mixing

We investigate the effects of messenger-matter mixing on the lightest CP-even Higgs boson mass m_h in gauge-mediated supersymmetry breaking models. It is shown that with such mixings m_h can be raised to about 125 GeV, even when the superparticles have sub-TeV masses, and when the gravitino has a cosmologically preferred sub-keV mass. In minimal gauge mediation without messenger-matter mixing, realizing m_h = 125 GeV would require multi-TeV SUSY spectrum. The increase in $m_h$ due to messenger-matter mixing is maximal in the case of messengers belonging to 10+\bar{10} of SU(5) unification, while it is still significant when they belong to $5+\bar{5}$ of SU(5). Our results are compatible with gauge coupling unification, perturbativity, and the unification of messenger Yukawa couplings. We embed these models into a grand unification framework with a U(1) flavor symmetry that addresses the fermion mass hierarchy and generates naturally large neutrino mixing angles. While SUSY mediated flavor changing processes are sufficiently suppressed in such an embedding, small new contributions to K^0-\bar{K^0} mixing can resolve the apparent discrepancy in the CP asymmetry parameters \sin2βand ε_K.

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arXiv2012

Fundamental Physics at the Intensity Frontier

The Proceedings of the 2011 workshop on Fundamental Physics at the Intensity Frontier. Science opportunities at the intensity frontier are identified and described in the areas of heavy quarks, charged leptons, neutrinos, proton decay, new light weakly-coupled particles, and nucleons, nuclei, and atoms.

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arXiv2012

B-L Violating Nucleon Decay and GUT Scale Baryogenesis in SO(10)

We show that grand unified theories based on SO(10) generate naturally the next-to-leading baryon number violating operators of dimension seven. These operators, which violate B-L, lead to unconventional decays of the nucleon such as n -> e^-K^+, e^- π^+ and p -> νπ^+. In two-step breaking schemes of non-supersymmetric SO(10), nucleon lifetime for decays into these modes is found to be within reach of experiments. We also identify supersymmetric scenarios where these decays may be accessible, consistent with gauge coupling unification. Further, we show that the (B-L)-asymmetry generated in the decays of GUT scale scalar bosons and/or gauge bosons can explain consistently the observed baryon asymmetry of the universe. The induced (B-L)-asymmetry is sphaleron-proof, and survives down to the weak scale without being erased by the electroweak interactions. This mechanism works efficiently in a large class of non-SUSY and SUSY SO(10) models, with either a 126 or a 16 Higgs field employed for rank reduction. In minimal models the induced baryon asymmetry is tightly connected to the masses of quarks, leptons and neutrinos and is found to be compatible with observations.

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arXiv2011

Radiative Neutrino Mass Generation through Vector-like Quarks

A new model of radiative neutrino masses generated via two-loop diagrams is proposed involving a charge 2/3 vector-like quark and a doublet of leptoquark scalars. This model predicts one of the neutrinos to be massless and admits both the normal and inverted neutrino mass hierarchies with correlated predictions for l_i \rightarrow l_j + γbranching ratios. New contributions to CP violation in B_s-\bar{B}_s mixing arise in the model through leptoquark box diagrams, which can explain the anomalous dimuon events reported by the DO collaboration. These leptoquarks, with masses below 500 GeV, also provide a natural resolution to the apparent discrepancy in the measured values of the CP violation parameters \sin2βand ε_K.

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arXiv2011

Variety of SO(10) GUTs with Natural Doublet-Triplet Splitting via the Missing Partner Mechanism

We present a new class of unified SO(10) models where the GUT symmetry breaking down to the standard model gauge group involves just one scale, in contrast to the conventional SO(10) models which require two scales. Further, the models we discuss possess a natural doublet-triplet splitting via the missing partner mechanism without fine tuning. Such models involve $560+\ov{560}$ pair of heavy Higgs fields along with a set of light fields. The $560+\ov{560}$ are the simplest representations of SO(10) besides the $126+\ov{126}$ which contain an excess of color triplets over $SU(2)_L$ doublets. We discuss several possibilities for realizing the missing partner mechanism within these schemes. With the $126+\ov{126}$ multiplets, three viable models are found with additional fields belonging to ${210 + 2 \times 10 + 120}$, ${45 + 10 + 120}$, or ${210 + 16 + \ov{16} + 10 + 120}$. With the $560+\ov{560}$, a unique possibility arises for the missing partner mechanism, with additional ${2\times 10+ 320}$ fields. These models are developed in some detail. It is shown that fully realistic fermion masses can arise in some cases, while others can be made realistic by addition of vector--like representations. Naturally large neutrino mixing angles, including sizable $θ_{13}$, can emerge in these models. The couplings of the $H_u(H_d)$ Higgs doublets of the MSSM which give masses to the up quarks (down quarks and leptons) are not necessarily equal at the grand unification scale and would lead to a new phenomenology at the low energy scales.

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arXiv2011

Perturbative unitarity constraints on general W' models and collider implications

We study perturbative unitarity constraints on general W' models by considering the high energy behavior of fermion scattering into gauge bosons. In most cases we survey, a Z' boson with a comparable mass must be present for the theory to be consistent, with fixed couplings to the standard model gauge bosons and fermions. Applying these results to a class of W' models which explains the top quark forward-backward asymmetry observed at the Tevatron, we find that a Z' must exist with a mass below 7-8 TeV and sizable coupling to the light quarks. While such a Z' is strongly constrained by existing experiments, we show that the LHC can explore the entire mass range up to the unitarity limit. We also show how it is possible, by raising the Z' mass consistent with unitarity, to explain the CDF Wjj excess in terms of a light W', without generating an excess in Zjj events.

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arXiv2011

Top quark asymmetry and Wjj excess at CDF from gauged flavor symmetry

We show that the scalar sector needed for fermion mass generation when the flavor symmetry of the standard model is maximally gauged can consistently explain two anomalies reported recently by the CDF collaboration - the forward-backward asymmetry in t-tbar pair production, and the dijet invariant mass in the Wjj channel. A pair of nearly degenerate scalar doublets with masses in the range 150-200 GeV explain these anomalies, with additional scalars predicted in the mass range 100-400 GeV. Consistency of such low scale flavor physics with flavor changing processes is shown, and expectations for the LHC are outlined.

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arXiv2011

Recent Progress in SUSY GUTs

After a brief review of the motivations for grand unification, I discuss the main challenges facing realistic SUSY GUT model building. Achieving doublet-triplet splitting without fine--tuning is chief among them. Symmetry breaking should occur consistently without unwanted Goldston bosons, μ-term of order TeV for the MSSM Higgs fileds should emerge naturally, and realistic fermion masses with small quark mixing angles and large lepton mixing angles should be generated with some predictivity. Significant progress has been made over the years towards achieving these goals in the context of supersymmetric SO(10) GUT. A complete SO(10) model is presented along this line wherein, somewhat surprisingly, the GUT scale threshold corrections to the gauge couplings are found to be small. This results in a predictive scenario for proton lifetime. An interesting correlation between the d=6 (p\to e^+π^0) and d=5 (p\to ν-bar K^+) decay amplitudes is observed. This class of models predicts that both proton decay modes should be observable with an improvement in the current sensitivity by about a factor of five to ten.

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arXiv2011

Variations on the Supersymmetric Q6 Model of Flavor

We observe that a recently proposed supersymmetric model with Q6 flavor symmetry admits a new CP violating ground state. A new sum rule for the quark mixing parameters emerges, which is found to be consistent with data. Simple extensions of the model to the neutrino sector suggest an inverted hierarchical mass spectrum with nearly maximal CP violation (|delta_{MNS}| simeq pi/2). Besides reducing the number of parameters in the fermion sector, these models also provide solutions to the SUSY flavor problem and the SUSY CP problem. We construct a renormalizable scalar potential that leads to the spontaneous breaking of CP symmetry and the family symmetry.

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arXiv2011

Radiative Generation of Neutrino Masses and its Experimental Signals

Tiny neutrino masses can arise naturally via loop diagrams. After a brief review of the radiative mass generation mechanism, I present a new model wherein TeV scale leptoquark scalars induce neutrino masses via two--loop diagrams. This model predicts the neutrino oscillation parameter \sin^2θ_{13} to be close to the current experimental limit. The leptoquarks are accessible to experiments at the LHC since their masses must lie below 1.5 TeV, and their decay branching ratios probe neutrino oscillation parameters. Rare lepton flavor violating processes mediated by leptoquarks have an interesting pattern: mu --> e gamma may be suppressed, while mu --> 3 e and mu-e conversion in nuclei are within reach of the next generation experiments. Muon g-2 receives new positive contributions, which can resolve the discrepancy between theory and experiment. New CP violating contributions to B_s- \B_s-bar mixing via leptoquark box diagrams are in a range that can explain the recently reported dimuon anomaly by the D0 collaboration.

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arXiv2010

Two-Loop Neutrino Mass Generation through Leptoquarks

We present a new model of radiative neutrino mass generation wherein TeV scale leptoguark scalars induce tiny neutrino masses as two-loop radiative corrections. The neutrino oscillation parameter \sin^2θ_{13} is predicted to be close to the current experimental limit within the model. Rare lepton flavor violating processes mediated by leptoquarks have an interesting pattern: μ-> e γ$ may be suppressed, while μ-> 3 e and μ-e conversion in nuclei are within reach of the next generation experiments. New CP violating contributions to B_s- \bar{B}_s mixing via leptoquark box diagrams are in a range that can explain the recently reported discrepancy with the standard model. D_s^- -> \ell^- νdecays mediated by leptoquarks brings theory and experiment closer, removing an observed 2 sigma anomaly. Muon g-2 receives new positive contributions, which can resolve the discrepancy between theory and experiment. The leptoquarks of the model are accessible to the LHC, and their decay branching ratios probe neutrino oscillation parameters.

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arXiv2010

Semidirect Product Groups, Vacuum Alignment and Tribimaximal Neutrino Mixing

The neutrino oscillation data are in very good agreement with the tribimaximal mixing pattern: \sin^2θ_{23}=1/2, \sin^2θ_{12}=1/3, and \sin^2θ_{13}=0. Attempts to generate this pattern based on finite family symmetry groups typically assume that the family symmetry is broken to different subgroups in the charged lepton and the neutrino mass matrices. This leads to a technical problem, where the cross-couplings between the Higgs fields responsible for the two symmetry breaking chains force their vacuum expectation values to align, upsetting the desired breaking pattern. Here, we present a class of models based on the semidirect product group (S_3)^4 \rtimes A_4, where the lepton families belong to representations which are not faithful. In effect, the Higgs sector knows about the full symmetry while the lepton sector knows only about the A_4 factor group. This can solve the alignment problem without altering the desired properties of the family symmetry. Inclusion of quarks into the framework is straightforward, and leads to small and arbitrary CKM mixing angles. Supersymmetry is not essential for our proposal, but the model presented is easily supersymmetrized, in which case the same family symmetry solves the SUSY flavor problem.

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arXiv2010

Constraining Proton Lifetime in SO(10) with Stabilized Doublet-Triplet Splitting

We present a class of realistic unified models based on supersymmetric SO(10) wherein issues related to natural doublet-triplet (DT) splitting are fully resolved. Using a minimal set of low dimensional Higgs fields which includes a single adjoint, we show that the Dimopoulos--Wilzcek mechanism for DT splitting can be made stable in the presence of all higher order operators without having pseudo-Goldstone bosons and flat directions. The μterm of order TeV is found to be naturally induced. A Z_2-assisted anomalous U(1)_A gauge symmetry plays a crucial role in achieving these results. The threshold corrections to alpha_3(M_Z), somewhat surprisingly, are found to be controlled by only a few effective parameters. This leads to a very predictive scenario for proton decay. As a novel feature, we find an interesting correlation between the d=6 (p\to e^+π^0) and d=5 (p\to ν-bar K+) decay amplitudes which allows us to derive a constrained upper limit on the inverse rate of the e^+π^0 mode. Our results show that both modes should be observed with an improvement in the current sensitivity by about a factor of five to ten.

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arXiv2009

TASI Lectures on Flavor Physics

This review is based on lectures on flavor physics given at TASI 2008. First I summarize our present knowledge on the fundamental parameters of the flavor sector. Then I discuss various scenarios going beyond the standard model which attempt to explain aspects of the "flavor puzzle". Relating quark masses and mixing angles via flavor symmetry is explored. Explaining the mass hierarchy via the Froggatt-Nielsen mechanism is reviewed and illustrated. Grand unification ideas are pursued to seek a pattern in the observed masses and mixings of quarks and leptons. Generating light fermion masses as radiative corrections is explained and illustrated. The popular solutions to the strong CP problem are summarized. Finally, specific processes in B meson system where significant new flavor contributions can arise are discussed.

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