Models of Neutrino Masses and Mixings:Understanding Lepton Flavor Mixing

Lisa L. EverettU. Wisconsin, Madison

!"#"$#%""& '()*)+,-./0+'1'23"&0+456(5.0+1'7 8

!"#$%& '()*#+*,

Absolute neutrino mass scale?

Neutrino mass squared splittings and angles

Talks by Mohapatra, Valle

The symmetry group of

is D6 , one of the finite groups.

Nakatani, 1936

the first who made snow crystal in a laboratory

Introduction/Motivation

Observation of Neutrino Oscillations:

Neutrinos are massive

Lepton mixing is observable

P!!!!" (L) =!

ij

Ui"U"i#U"j"Uj#e#i!m2

ijL

2E

“ Standard Model ”!

•

•

Standard Model( SM)!

The Flavor Puzzle

• SM flavor puzzle: origin of charged fermion masses, quark mixingsDirac mass terms, parametrized by Yukawas:

• SM flavor puzzle: origin of neutrino masses (Dirac or Majorana), lepton mixings,...

Ultimate Goal: satisfactory, credible, complete flavor theory

YijH · !Li!Rj

very difficult!

!

!

but first, let’s look at the data...

The Data: Neutrino Masses

assume: 3 neutrino mixing!m2ij ! m2

i "m2j

Atmospheric:

Solar:

Normal Hierarchy Inverted Hierarchy

Cosmology (WMAP):!

i

mi < 0.7 eV

Homestake, Kam, SuperK,KamLAND,SNO, SuperK, MINOS,miniBOONE,...

fits: Schwetz, Tortola, Valle

!m2! = |!m2

12| = 7.65+0.23"0.20 ! 10"5 eV2

!m231 = ±2.4+0.12

!0.11 ! 10!3 eV2

12

3

3

12

(best fit ) ±1!

The Data: Lepton Mixing

Maki, Nakagawa, SakataPontecorvoUMNSP = R1(!!)R2(!13, "MNSP)R3(!")P

|UMNSP| !

!

"#cos !! sin !! "

" cos !" sin !! cos !" cos !! sin !"

sin !" sin !! " sin !" cos !! cos !"

$

%&

Solar:

Atmospheric:

Reactor:

(no constraints on CP violation)2 large angles, 1 small angle!

fit: Schwetz, Tortola, Valle ’08

!! = !12 = 33.4" ± 1.4"

!! = !23 = 45.0"+4.0#3.4

! = sin "13, "13 = 5.7!+3.5"5.7

(best fit ) ±1!

Homestake, Kam, SuperK,KamLAND,SNO, SuperK, Palo Verde, CHOOZ, MINOS...

Compare: Quark MixingCabibbo; Kobayashi, Maskawa

UCKM = R1(!CKM23 )R2(!CKM

13 , "CKM)R3(!CKM12 )

!CKM12 = 13.0! ± 0.1! Cabibbo angle !c

!CKM23 = 2.4! ± 0.1!

!CKM13 = 0.2! ± 0.1!

JarlskogDunietz, Greenberg, Wu

J ! Im(U!iU"jU!"iU!

!j)

J (CKM)CP ! sin 2!CKM

12 sin 2!CKM23 sin 2!CKM

13 sin "CKM

!CKM = 60! ± 14!J ! 10!5

CP violation:

Mixing Angles:

3 small angles, 1 large phase!

A paradigm shift

Strikingly different flavor patterns for quarks and leptons!

Mass scales, hierarchies of neutral and charged fermions:•

• Mixing Angles: quarks small, leptons 2 large, 1small

step 1 for theory: suppressing neutrino mass scale

step 2 for theory: understanding lepton mixing pattern

Prototype: Type I neutrino seesaw

M! =!

0 mm M

" m ! O(100GeV)

M ! mm1 !

m2

Mm2 !M " m1

Minkowski;Yanagida; Gell-Mann, Ramond, Slansky

!L! = Y!ijLLiH!Rj +"ij

!(LLiH)(LLjH) +

12(Mij !Ri(!Rj)c + h.c.)

Beyond physics of Yukawa couplings!

but also many other possibilities...

!1,2 ! !L,R +m

M!R,L

Majorana (Type II, III seesaws, double seesaw...), suppressed Dirac masses (most mechanisms exploit SM singlet nature of )!R

This talk: implications of large lepton mixings (step 2)

Flavor Model Building in the SM (I)

Recall for quarks:

••

Standard paradigm: spontaneously broken flavor symmetry

!

UCKM = UuU†d ! 1 +O(!)

Wolfenstein parametrization: ! ! sin "c = 0.22

Froggatt, Nielsen! !

M

"nij

H · "Li"RjYijH · !Li!Rj

! ! "

M

suggests Cabibbo angle may be a useful flavor expansion parameter

hierarchical masses, small mixings: continuous family symmetries

CKM matrix: small angles and/or alignment

Flavor Model Building in the SM (II)

•

•

!

(Datta,L.E., Ramond,...)

UMNSP = UeU†! !W +O(!!)

flavor expansion parameter

“bare” mixing angles

useful, and motivated in unified/string scenarios, to take

!! = ! ! sin "c

ideas of “Cabibbo haze” and quark-lepton complementarity

(!012, !

013, !

023)

(more shortly...)

Main issue: what is in limit of exact symmetry?UMNSP

for the leptons, large angles suggest

Aside: Lepton Mixing Angles are “non-generic”

Issues: size of , origin of non-maximal

note: lepton mixing angle pattern has the most challenges (w/3 families)

M!

M!large angles

small angles3

3

large,1 small2

1 small2 large,

diagonal!

RankM! < 3!

“anarchical”

fine-tuning, non-Abelian

•

•

•

•

!13 !12

Classify scenarios by the form of in symmetry limitUMNSP

large angles may suggest discrete non-Abelian family symmetries!

Flavor Model Building in the SM (III)

•

•

!

In general:

Classify models by form of :

More variety in choice of bare solar angle :

“bimaximal” mixing

“tri-bimaximal” mixing

“golden ratio” mixing

(quark-lepton complementarity)

UMNSP = UeU†! !W +O(!!)

!023 = 45! !0

13 = 0!

!012

W(!012, !

013, !

023)

(reasonable)

•

•

•

Harrison, Perkins, Scott (HPS)

! = (1 +!

5)/2

Scenario 1. Bimaximal Mixing

U (BM)MNSP =

!

""#

1!2! 1!

20

12

12 ! 1!

212

12

1!2

$

%%&

“bare” solar angle

Requires large perturbations:

!012 = 45! tan !0

12 = 1

!12 = !012 +O(") ! #

4" !c

“quark-lepton complementarity”

Raidal; Minakata, Smirnov; Frampton, Mohapatra; Xing; Ferrandis, Pakvasa; King; L.E., Ramond; Plentinger, Lindner; Dighe, Rodejohann, many others (>100 papers)...

popularity: peaked ~2004-2006

Bimaximal mixing scenarios:

useful framework for exploring Cabibbo effects in quark+lepton sectors

mu

mt! !8

mc

mt! !4

md

mb! !4

ms

mb! !2

me

m!! !5

mµ

m!! !2

mb

mt! !3mb

m!! 1 (GUT scale)

!CKM12 ! " !CKM

23 ! "2 !CKM13 ! "3

!!m2

!!m2

"! !

!13 ! O(")

!!23 < O(")

but implementation in full grand unified theories: very challenging

recent resurgence in context of discrete non-Abelian family symms

Altarelli, Feruglio, and Merlo, ’09,...

Scenario II. Tri-bimaximal (HPS) Mixing

U (HPS)MNSP =

!

""#

$23 ! 1!

30

1!6

1!3

! 1!2

1!6

1!3

1!2

%

&&'

popularity: ~2006-now

“bare” solar angle tan !012 =

1!2

Harrison, Perkins, Scott ’02

!012 = 35.26!

Does not require large perturbations! !12 = !012 +O("2)

amusing note: MNSP looks like Clebsch-Gordan coeffs Meshkov; Zee,...

Naturally obtained from discrete non-Abelian symmetries

Ma and collaborators (earliest in ’01), Altarelli, Feruglio, Babu and He, Valle, Hirsch et al., King et al.,

many, many others...

(subgroups of , ) SO(3) SU(3)

A Few Examples: A4(tetrahedron)

S4

T ! Aranda, Carone, Lebed; Chen, Mahanthappa,...

Luhn, Nasri, Ramond; Ma; King, Ross,...

Ma; Hagedorn, Lindner, Mohapatra; Cai, Yu; Zhang,...

(cube)

!(3n2)A5 L.E., Stuart (in progress)

(icosahedron)

Most popular scenario! many models, elegant results

issue of incorporating quarks: much recent progress

•

Idea: solar angle related to “golden ratio”

Implementation: icosahedral flavor symmetry

•

! = (1 +!

5)/2

I (A5)

Implementation: dihedral flavor symmetry D10

Scenario III. Golden Ratio Mixing

Two proposed scenarios:

!12 = 36!

!12 = 31.72!

cos !12 ="

2

tan !12 =1"

•

L.E., Stuart ’08, in progress

Adulpravitchai, Blum, Rodejohann ’09

popularity: hopefully rising!!

U (GR1)MNSP =

!

""""#

$!!5

!$

1!5!

0

1!2

$1!5!

1!2

$!!5! 1!

2

1!2

$1!5!

1!2

$!!5

1!2

%

&&&&'

Scenario III: GR1

Ramond et al., hep-ph/0306002 (footnote)

Kajiyama, Raidal, Strumia 0705.4559 [hep-ph]

Idea:Z2 ! Z2

L.E. and Stuart, 0812.1057 [hep-ph], and work in progress A5

A5 isomorphic to icosahedral group, A5 ! II

Group elements:

The (Rotational) Icosahedral Group, I ~ A5

Properties of the icosahedron:

20 faces 30 edges

12 vertices

(equilateral triangles)

(3 sides/triangle, 2 triangles/edge)

(3 vertices/triangle, 5 vertices/edge)

Rotations which take vertices to vertices, i.e., by

Rotation by each angle forms a conjugacy class:

e, 12C5, 12C25 , 20C3, 15C2

0,2!

5,

4!

5,

2!

3, !

order=number of elements:

(Schoenflies:

1 + 12 + 12 + 15 + 20 = 60

Ckn =

2!k

nrotation)

I 1 3 3! 4 5e 1 3 3 4 5

12C5 1 ! 1! ! -1 012C2

5 1 1! ! ! -1 020C3 1 0 0 1 -115C2 1 -1 -1 0 1

The (Rotational) Icosahedral Group, I ~ A5

Conjugacy classes: characterized by trace (character)

Character Table

Theorem: group order = sum of squares of irred. reps

1 + 12 + 12 + 15 + 20 = 60 = 12 + 32 + 32 + 42 + 52.(two triplets!)

The (Rotational) Icosahedral Group, I ~ A5

From character table, deduce tensor product decomposition:

3! 3=1" 3" 53! ! 3! =1" 3! " 5

3! 3!=4" 53! 4=3! " 4" 53! ! 4=3" 4" 5

3! 5=3" 3! " 4" 53! ! 5=3" 3! " 4" 5

4! 4=1" 3" 3! " 4" 54! 5=3" 3! " 4" 5" 5

5! 5=1" 3" 3! " 4" 4" 5" 5

Not enough for flavor model building. Need explicit representations!

I not a crystallographic point group, so there was work to be done...

LL : 3! 3 = 1" 3" 5, Le : 3! 3! = 4" 5

Lepton Flavor Model Building with A5

!Lm =aij

MLiHLjH + Y (e)

ij LiejH

With product rules in hand, back to physics:

Charge assignments: natural to have triplets under I

L! 3, e! 3!

(symmetry)

L, e

choose here:

leading order: charged leptons massless, neutrinos degenerate...

Mass terms:

Lepton Flavor Model Building with A5 (II)

!Lmass =!ijk

M2LiHLjH"k +

#ijk

MLiejH$k +

%ijl

MLiejH&l

Fix it at higher order: Froggatt-Nielsen approach

with “minimal” choice of “flavon” fields:

Toy example (bottom-up approach):

! ! 5 ! ! 5, !! 4

LL Le

(!, ", # ! O(1))

Mass terms: flavon vacuum expectation values divided by (high) scale M

Lepton Flavor Model Building with A5 (III)

Explicit example with assumed flavon vevs:

can obtain: realistic neutrino masses (normal hierarchy)

hierarchical charged lepton masses

Rich and virtually unexplored model building territory!

prediction for neutrinoless double beta decay

m!! =m1!!

5+

m2

!!

5.

and “golden prediction” for solar mixing angle

Important questions for this class of models:

dynamics of flavon sector, quark flavor mixing,...L.E. and Stuart, in progress

Scenario III: GR2

Idea: Rodejohann, 0810. 5239 [hep-ph]

complete flavor theory based on dihedral symmetry

Adulpravitchai, Blum, and Rodejohann, 0903.0531 [hep-ph]

U (GR2)MNSP =

!

""""#

!2 ! 1

2

$!5

! 0

12

$52!

!2!

2! 1!

2

12

$52!

!2!

21!2

%

&&&&'

D10

(solar angle prediction based on exterior angle of decagon)

Conclusions

• Lepton data has given us a SM flavor puzzle!

• Theoretically favored mixing angle patterns:

• Bimaximal mixing and Cabibbo-sized effects

• Tri-bimaximal mixing: tetrahedral (+others)

• “golden ratio” mixings: icosahedral/dihedral

• themes: discrete non-Abelian family symmetries, embedding quarks together with leptons...

• lots of interesting work to do! Data will be crucial!

!