682 research outputs found
On Epsilon Expansions of Four-loop Non-planar Massless Propagator Diagrams
We evaluate three typical four-loop non-planar massless propagator diagrams
in a Taylor expansion in dimensional regularization parameter
up to transcendentality weight twelve, using a recently
developed method of one of the present coauthors (R.L.). We observe only
multiple zeta values in our results.Comment: 3 pages, 1 figure, results unchanged, discussion improved, to appear
in European Physical Journal
Calculating multiloop integrals using dimensional recurrence relation and D-analyticity
We review the method of the calculation of multiloop integrals recently
suggested in Ref.[Lee2010]. A simple method of derivation of the dimensional
recurrence relation suitable for automatization is given. Some new analytic
results are given.Comment: Talk presented at 10th DESY Workshop on Elementary Particle Theory:
Loops and Legs in Quantum Field Theory, Worlitz, Germany, 25-30 April 201
Neutrino Mass and New Physics
We review the present state of and future outlook for our understanding of
neutrino masses and mixings. We discuss what we think are the most important
perspectives on the plausible and natural scenarios for neutrinos and what may
have the most promise to throw light on the flavor problem of quarks and
leptons. We focus on the seesaw mechanism which fits into the big picture of
particle physics such as supersymmetry and grand unification providing a
unified approach to flavor problem of quarks and leptons. We argue that in
combination with family symmetries, this may be at the heart of a unified
understanding of flavor puzzle. We also discuss other new physics ideas such as
neutrinos in models with extra dimensions and possible theoretical implications
of sterile neutrinos. We outline some tests for the various schemes.Comment: 90 pages and 9 figures; With permission from the Annual Review of
Nuclear and Particle Science. Final version of this material is scheduled to
appear in the Annual Review of Nuclear and Particle Science Vol. 56, to be
published in November 2006 by Annual Reviews (http://www.annualreviews.org);
some references and parts of text update
Master Integrals for Four-Loop Massless Propagators up to Transcendentality Weight Twelve
We evaluate a Laurent expansion in dimensional regularization parameter
of all the master integrals for four-loop massless
propagators up to transcendentality weight twelve, using a recently developed
method of one of the present coauthors (R.L.) and extending thereby results by
Baikov and Chetyrkin obtained at transcendentality weight seven. We observe
only multiple zeta values in our results. Therefore, we conclude that all the
four-loop massless propagator integrals, with any integer powers of numerators
and propagators, have only multiple zeta values in their epsilon expansions up
to transcendentality weight twelve.Comment: 19 pages, 1 figur
Screening of Dirac flavor structure in the seesaw and neutrino mixing
We consider the mechanism of screening of the Dirac flavor structure in the
context of the double seesaw mechanism. As a consequence of screening, the
structure of the light neutrino mass matrix, m_\nu, is determined essentially
by the structure of the (Majorana) mass matrix, M_S, of new super-heavy (Planck
scale) neutral fermions S. We calculate effects of the renormalization group
running in order to investigate the stability of the screening mechanism with
respect to radiative corrections. We find that screening is stable in the
supersymmetric case, whereas in the standard model it is unstable for certain
structures of M_S. The screening mechanism allows us to reconcile the
(approximate) quark-lepton symmetry and the strong difference of the mixing
patterns in the quark and lepton sectors. It opens new possibilities to explain
a quasi-degenerate neutrino mass spectrum, special ``neutrino'' symmetries and
quark-lepton complementarity. Screening can emerge from certain flavor
symmetries or Grand Unification.Comment: 27 pages, 3 figures; references added, discussion of the E6 model
modifie
Theta-13 as a Probe of Mu-Tau symmetry for Leptons
Many experiments are being planned to measure the neutrino mixing parameter
using reactor as well as accelerator neutrino beams. In this
note, the theoretical significance of a high precision measurement of this
parameter is discussed. It is emphasized that it will provide crucial
information about different ways to understand the origin of large atmospheric
neutrino mixing and move us closer towards determining the neutrino mass
matrix. For instance if exact symmetry in the
neutrino mass matrix is assumed to be the reason for maximal
mixing, one gets . Whether or can provide information about the way the
symmetry breaking manifests in the case of normal hierarchy. We also discuss
the same question for inverted hierarchy as well as possible gauge theories
with this symmetry.Comment: 12 pages; no figures; latex; more exact expressions given for some
parameters and minor typos corrected; paper accepted for publication in JHE
Lepton Flavor Violating Z Decays in the Zee Model
We calculate lepton flavor violating (LFV) Z decays Z \to {{e_i^\pm}}e_j^\mp
(i, j = e, \mu, \tau ; i\neq j) in the Zee model keeping in view the radiative
leptonic decays e_i\to e_j\gamma (i = \mu, \tau ; j = e, \mu ; i\neq j), \mu
decay and anomalous muon magnetic moment (\mu AMM). We investigate three
different cases of Zee f_{ij} coupling (A) f_{e\mu}^2 = f_{\mu\tau}^2= f_{\tau
e}^2, (B) f_{e\mu}^2 \gg f_{\tau e}^2 \gg f_{\mu\tau}^2, and (C) f_{\mu\tau}^2
\gg f_{e\mu}^2 \gg f_{\tau e}^2 subject to the neutrino phenomenology.
Interestingly, we find that, although the case (C) satisfies the large excess
value of \mu AMM, however, it is unable to explain the solar neutrino
experimental result, whereas the case (B) satisfies the bi-maximal neutrino
mixing scenario, but confronts with the result of \mu AMM experiment. We also
find that among all the three cases, only the case (C) gives rise to largest
contribution to the ratio B(Z\to e^\pm\tau^\mp)/B(Z\to \mu^\pm \mu^\mp) \simeq
{10}^{-8} which is still two order less than the accessible value to be probed
by the future linear colliders, whereas for the other two cases, this ratio is
too low to be observed even in the near future for all possible LFV Z decay
modes.Comment: 12 pages, RevTex, 2 figures, 3 Tables, typos corrected, reference
added, version to appear in Phys. Rev.
Minimal SUSY SO(10) model and predictions for neutrino mixings and leptonic CP violation
We discuss a minimal Supersymmetric SO(10) model where B-L symmetry is broken
by a {\bf 126} dimensional Higgs multiplet which also contributes to fermion
masses in conjunction with a {\bf 10} dimensional superfield. This minimal
Higgs choice provides a partial unification of neutrino flavor structure with
that of quarks and has been shown to predict all three neutrino mixing angles
and the solar mass splitting in agreement with observations, provided one uses
the type II seesaw formula for neutrino masses. In this paper we generalize
this analysis to include arbitrary CP phases in couplings and vevs. We find
that (i) the predictions for neutrino mixings are similar with as before and other parameters in a somewhat bigger range and (ii) that
to first order in the quark mixing parameter (the Cabibbo angle), the
leptonic mixing matrix is CP conserving. We also find that in the absence of
any higher dimensional contributions to fermion masses, the CKM phase is
different from that of the standard model implying that there must be new
contributions to quark CP violation from the supersymmetry breaking sector.
Inclusion of higher dimensional terms however allows the standard model CKM
phase to be maintained.Comment: 22 pages, 6 figure
Bi-large Neutrino Mixing and Mass of the Lightest Neutrino from Third Generation Dominance in a Democratic Approach
We show that both small mixing in the quark sector and large mixing in the
lepton sector can be obtained from a simple assumption of universality of
Yukawa couplings and the right-handed neutrino Majorana mass matrix in leading
order. We discuss conditions under which bi-large mixing in the lepton sector
is achieved with a minimal amount of fine-tuning requirements for possible
models. From knowledge of the solar and atmospheric mixing angles we determine
the allowed values of sin \theta_{13}. If embedded into grand unified theories,
the third generation Yukawa coupling unification is a generic feature while
masses of the first two generations of charged fermions depend on small
perturbations. In the neutrino sector, the heavier two neutrinos are model
dependent, while the mass of the lightest neutrino in this approach does not
depend on perturbations in the leading order. The right-handed neutrino mass
scale can be identified with the GUT scale in which case the mass of the
lightest neutrino is given as (m_{top}^2/M_{GUT}) sin^2 \theta_{23} sin^2
\theta_{12} in the limit sin \theta_{13} = 0. Discussing symmetries we make a
connection with hierarchical models and show that the basis independent
characteristic of this scenario is a strong dominance of the third generation
right-handed neutrino, M_1, M_2 < 10^{-4} M_3, M_3 = M_{GUT}.Comment: typos correcte
Leptogenesis and low energy observables in left-right symmetric models
In the context of left-right symmetric models we study the connection of
leptogenesis and low energy parameters such as neutrinoless double beta decay
and leptonic CP violation. Upon imposition of a unitarity constraint, the
neutrino parameters are significantly restricted and the Majorana phases are
determined within a narrow range, depending on the kind of solar solution. One
of the Majorana phases gets determined to a good accuracy and thereby the
second phase can be probed from the results of neutrinoless double beta decay
experiments. We examine the contributions of the solar and atmospheric mass
squared differences to the asymmetry and find that in general the solar scale
dominates. In order to let the atmospheric scale dominate, some finetuning
between one of the Majorana phases and the Dirac CP phase is required. In this
case, one of the Majorana phases is determined by the amount of CP violation in
oscillation experiments.Comment: 18 pages, 6 figures. Matches version to appear in PR
- …