71 research outputs found
Gravitational field equations near an arbitrary null surface expressed as a thermodynamic identity
Previous work has demonstrated that the gravitational field equations in all
Lanczos-Lovelock models imply a thermodynamic identity TdS=dE+PdV (where the
variations are interpreted as changes due to virtual displacement along the
affine parameter) in the near-horizon limit in static spacetimes. Here we
generalize this result to any arbitrary null surface in an arbitrary spacetime
and show that certain components of the Einstein's equations can be expressed
in the form of the above thermodynamic identity. We also obtain an explicit
expression for the thermodynamic energy associated with the null surface. Under
appropriate limits, our expressions reduce to those previously derived in the
literature. The components of the field equations used in obtaining the current
result are orthogonal to the components used previously to obtain another
related result, viz. that some components of the field equations reduce to a
Navier-Stokes equation on any null surface, in any spacetime. We also describe
the structure of Einstein's equations near a null surface in terms of three
well-defined projections and show how the different results complement each
other.Comment: v2, 25 pages, no figures, to appear in JHE
Tribimaximal Mixing From Small Groups
Current experimental data on the neutrino parameters is in good agreement
with tribimaximal mixing and may indicate the presence of an underlying family
symmetry. For 76 flavor groups, we perform a systematic scan for models: The
particle content is that of the Standard Model plus up to three flavon fields,
and the effective Lagrangian contains all terms of mass dimension <=6. We find
that 44 groups can accommodate models that are consistent with experiment at 3
sigma, and 38 groups can have models that are tribimaximal. For one particular
group, we look at correlations between the mixing angles and make a prediction
for theta13 that will be testable in the near future. We present the details of
a model with theta12=33.9, theta23=40.9, theta13=5.1 to show that the recent
tentative hints of a non-zero theta13 can easily be accommodated. The smallest
group for which we find tribimaximal mixing is T7. We argue that T7 and T13 are
as suited to produce tribimaximal mixing as A4 and should therefore be
considered on equal footing. In the appendices, we present some new
mathematical methods and results that may prove useful for future model
building efforts.Comment: 44 pages, 7 figures. Typos corrected, references added, figures
update
A Scan for Models of Neutrino Mixing from Non-Abelian Discrete Symmetries
The structure of the neutrino mixing matrix is indicative of an underlying
family symmetry that interrelates the three generations of fermions in the
Standard Model. We systematically scan the parameter space of 76 discrete
non-Abelian family symmetries and construct all models with the Standard Model
particle content and up to three flavon fields where we include
non-renormalizable interactions of mass dimension five and six. We find that of
the 76 groups that we considered, 44 groups can accommodate models that are
consistent with experiment at 3sigma, and 38 groups can have models that are
tribimaximal. One immediate consequence is that A4 is not "special", but should
be considered on equal footing with other groups such as T7 that is the
smallest group for which we find tribimaximal mixing, and T13 that has the
largest fraction of TBM models. We present the details of a model with
theta12=33.9, theta23=49.1, theta13=5.1 to show that a non-zero theta13 can
easily be accommodated.Comment: Contribution to the proceedings of "The 2011 Europhysics Conference
on High Energy Physics-HEP 2011", July 21-27, 2011, Grenoble, Franc
The Structure of the Gravitational Action and its relation with Horizon Thermodynamics and Emergent Gravity Paradigm
If gravity is an emergent phenomenon, as suggested by several recent results,
then the structure of the action principle for gravity should encode this fact.
With this motivation we study several features of the Einstein-Hilbert action
and establish direct connections with horizon thermodynamics. We begin by
introducing the concept of holographically conjugate variables (HCVs) in terms
of which the surface term in the action has a specific relationship with the
bulk term. In addition to g_{ab} and its conjugate momentum \sqrt{-g} M^{cab},
this procedure allows us to (re)discover and motivate strongly the use of
f^{ab}=\sqrt{-g}g^{ab} and its conjugate momentum N^c_{ab}. The gravitational
action can then be interpreted as a momentum space action for these variables.
We also show that many expressions in classical gravity simplify considerably
in this approach. For example, the field equations can be written in a form
analogous to Hamilton's equations for a suitable Hamiltonian if we use these
variables. More importantly, the variation of the surface term, evaluated on
any null surface which acts a local Rindler horizon can be given a direct
thermodynamic interpretation. The term involving the variation of the dynamical
variable leads to T\delta S while the term involving the variation of the
conjugate momentum leads to S\delta T. We have found this correspondence only
for the choice of variables (g_{ab}, \sqrt{-g} M^{cab}) or (f^{ab}, N^c_{ab}).
We use this result to provide a direct thermodynamical interpretation of the
boundary condition in the action principle, when it is formulated in a
spacetime region bounded by the null surfaces. We analyse these features from
several different perspectives and provide a detailed description, which offers
insights about the nature of classical gravity and emergent paradigm.Comment: 31 pages, published version with typos fixe
A Minimal Model of Neutrino Flavor
Models of neutrino mass which attempt to describe the observed lepton mixing
pattern are typically based on discrete family symmetries with a non-Abelian
and one or more Abelian factors. The latter so-called shaping symmetries are
imposed in order to yield a realistic phenomenology by forbidding unwanted
operators. Here we propose a supersymmetric model of neutrino flavor which is
based on the group T7 and does not require extra Z_N or U(1) factors, which
makes it the smallest realistic family symmetry that has been considered so
far. At leading order, the model predicts tribimaximal mixing which arises
completely accidentally from a combination of the T7 Clebsch-Gordan
coefficients and suitable flavon alignments. Next-to-leading order (NLO)
operators break the simple tribimaximal structure and render the model
compatible with the recent results of the Daya Bay and Reno collaborations
which have measured a reactor angle of around 9 degrees. Problematic NLO
deviations of the other two mixing angles can be controlled in an ultraviolet
completion of the model
Comments on the classification of the finite subgroups of SU(3)
Many finite subgroups of SU(3) are commonly used in particle physics. The
classification of the finite subgroups of SU(3) began with the work of H.F.
Blichfeldt at the beginning of the 20th century. In Blichfeldt's work the two
series (C) and (D) of finite subgroups of SU(3) are defined. While the group
series Delta(3n^2) and Delta(6n^2) (which are subseries of (C) and (D),
respectively) have been intensively studied, there is not much knowledge about
the group series (C) and (D). In this work we will show that (C) and (D) have
the structures (C) \cong (Z_m x Z_m') \rtimes Z_3 and (D) \cong (Z_n x Z_n')
\rtimes S_3, respectively. Furthermore we will show that, while the (C)-groups
can be interpreted as irreducible representations of Delta(3n^2), the
(D)-groups can in general not be interpreted as irreducible representations of
Delta(6n^2).Comment: 15 pages, no figures, typos corrected, clarifications and references
added, proofs revise
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