neutrino physics lecture 2: theory of neutrino mass, and physics bsm? herbstschule für...
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Neutrino physicsLecture 2: Theory of neutrino mass, and physics BSM?
Herbstschule für Hochenergiephysik
Maria Laach04-14.09.2012
Walter Winter
Universität Würzburg
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Contents
Measuring neutrino mass
The seesaw paradigm
Neutrino mass models at the TeV scale:What ingredients do “natural” models need?
Is it possible that new physics shows up in the neutrino sector only?
How does the large 13 affect our understanding of (lepton) flavor?
Summary and conclusions
Measuring neutrino mass
4
Tritium end point experiments
(Guido Drexlin, NOW 2008)
Direct test of neutrino mass by decay kinematics Current bound:
1/250.000 x me (2 eV) TINY! Future experiment:
KATRIN (Karlsruhe Tritium Neutrino Experiment) 1/2.500.000 x me (0.2 eV)
~8800 km
5
n
n
Two times simple beta decay:
Neutrinoless double beta decay:
Neutrinoless double beta decay… if the neutrino is ist own antiparticle, a Majorana neutrino!
p
e-
W-
p
n
e-
W-
p
e-
W-
2 x 2 x e
0 x 2 x e
n
p
e-
W-
=Caveat: discovering 0does not mean that one has actually
seen Majorana neutrinos!
6
Cosmological tests
Example:Relativistic neutrinos damp the formation of structure
Essentially sensitive to sum of neutrino masses
Information from different cosmological datasets used in literature
Limit ~ eV(S. Hannestad)
… might finally be used rule to out that neutrino physics in charge of 0!
The seesaw paradigm
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Why is the neutrino mass so small?
Why are the neutrinos morethan 250.000 times lighter than the electron?
Cannot be described in simple extensions of the Standard Model Is the neutrino mass the lowest order perturbation of physics BSM?
Seesaw mechanism: Neutrino mass suppressed by heavy partner, which only exists in the early universe (GUT seesaw)?
Decay of MR origin of matter-antimatter-asymmetry?CP violation? Test in neutrino oscillations!Requires Majorana nature of neutrino!
Test in neutrinoless double beta decay (0)
Other SM particles
Heavy partner
9
Leptonflavor
violation (LFV)
BSM physics described by effective operators in the low-E limit (gauge invariant):
Effective field theories
: Scaleof new physics
Neutrinomass(LNV)
But these are no fundamental theories (non-renormalizable operators). Idea: Investigate fundamental theories (TeV completions) systematically!
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Neutrino mass from d=5 (Weinberg) - Operator Fundamental theories at tree level:
Neutrino mass ~ Y2 v2/ (type I, III see-saw) For Y = O(1), v ~ 100 GeV: ~ GUT scale For ~ TeV scale: Y << 10-5
Interactions difficult to observe at LHCCouplings “unnaturally“ small?
Seesaw mechanism
Type I Type II
Type III Seesaw
LL
?
Neutrino masses at the TeV scale?
… and physics at the LHC …
12
Neutrino masses at the TeV scale
Goals: New physics scale “naturally“ at TeV scale
(i.e., TeV scale not put in by hand) Testable at the LHC?!
Yukawa couplings of order one
Requires additional suppression mechanisms. The typical ones:
1) Radiative generation of neutrino mass (n loops)2) Neutrino mass from higher than d=5 effective operator3) Small lepton number violating contribution (e.g.
inverse see-saw, RPV SUSY models, …)
13
Example (suppression 3):
Type-II, inverse seesaw
(Florian Bonnet@GGI Florence 2012)
14
Additional suppression (mechanisms 1+2): Loops versus dimension
Tree 1-loop 2-loop
d=5
d=7
d=8
d=11
Loop suppression, controlled by 1/(16 2)
Suppression by d, controlled by 1/
2
Type I, II, IIseesaw
Depends on scale: > 4v ~ 3 TeV?
Discrete symmetryto forbid d=5?
How can I make sure that no lower order operators are generated?
Depends onmediators/int.
Zee, 1980;Ma, 1998; …
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Example: Neutrino mass from higher dimensional operators
Approach: Use higher dimensional operators, e.g.
Leads to
Estimate: for ~ 1 – 10 TeV and m linear in Yukawas (worst case): d = 9 sufficient if no other suppression mechanism d = 7 sufficient if Yukawas ~ me/v ~ 10-6 allowed
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The loop issue
Loop d=5 contribution dominates for or > 3 TeV
Conclusion: If assumed that d=7 leading, one effectively has to put << 3 TeV by hand(see e.g. Babu, Nandi, Tavartkiladze, 2009)
Can one avoid this?
LL
LL
Close loop
d=7 operator d=5 operator
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Forbid lower dim. operators
Define genuine d=D operator as leading contribution to neutrino mass with all operators d<D forbidden
Use new U(1) or discrete symmetry (“matter parity“) Problem: H+H can never be charged under the new
symmetry! Need new fields! The simplest possibilities are probably
(e.g. Chen, de Gouvea, Dobrescu, hep-ph/0612017; Godoladze, Okada, Shafi, arXiv:0809.0703)
(e.g. Babu, Nandi, hep-ph/9907213; Giudice, Lebedec, arXiv:0804.1753)
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Higher dim. operators in THDM
Simplest possibility (d=7): Z5 with e.g.
(SUSY: Z3)
SUSY:only this one
(but: there canbe operators withthe scalar singletin the NMSSM)
Same for d=9
Bonnet, Hernandez, Ota, Winter, JHEP 10 (2009) 076
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Systematic study of d=7
Systematically decompose d=7 operator in all possible ways
Notation for mediators:
SU(2)
Lorentz
Y=Q-I3
Bonnet, Hernandez, Ota, Winter, JHEP 10 (2009) 076
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Generalizations of see-saws
Generalizations of originial see-saws: Duplication of the original see-saws plus scalars
Type I (fermionic singlet)
Type II(scalar triplet)
Type III(fermionic triplet)
Characteristics:Similar phenomenology!
Bonnet, Hernandez, Ota, Winter, JHEP 10 (2009) 076
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A SUSY example
Neutral fermion mass matrix after EWSB in basis
Krauss, Ota, Porod, Winter, Phys. Rev. D84 (2011) 115023
Fermionic doublets#17 from list
Compare to “inverse see-saw“(suppression mechanism 3)
if heavy doublets integrated out
3 2 2 1 1Flavor struct. by Mass states: ni
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Test at the LHC? (example)
Krauss, Ota, Porod, Winter, Phys. Rev. D84 (2011) 115023
Test mediators
Test LFV Test LNV
compare
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Even higher suppression?
Tree 1-loop 2-loop
d=5
d=7
d=8
d=11
Loop suppression, controlled by 1/(16 2)
Suppression by d, controlled by 1/
2
Switched off bydiscrete symmetry
Switched off by discrete symmetry
To beavoided
for
< 3
TeV
Example 1: d=9 at tree levelExample 2: d=7 at two loop Suppression mechanisms 1), 2), and 3)
Bonnet, Hernandez, Ota, Winter, JHEP 10 (2009) 076
Physics at TeV scale with O(1) couplings
Strategies for higher loops:Farzan, Pascoli, Schmidt,
arXiv:1208.2732
New physics in neutrino sector only?Most discussed options in literature: Light sterile neutrinos (aka: light SM singlets) Heavy SM singlets ( non-unitary mixings) Non-standard interactions (aka: flavor changing neutral currents)
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Evidence for sterile neutrinos?
LSND/MiniBooNE Reactor+gallium anomalies
Global fits(M
iniB
ooN
E @
Neu
trin
o 20
12)
(B. F
lemin
g, TA
UP
2011)
(Kopp, Maltoni, Schwetz, 1103.4570)
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Example: 3+1 framework(with addl. m2 ~ 1 eV2)
Well known tension between appearance and disapp. data (appearance disappearance in both channels)
Need one or more new experiments which can test e disappearance (Gallium, reactor anomalies) disappearance (overconstrains 3+N frameworks) e- oscillations (LSND, MiniBooNE) Neutrinos and antineutrinos separately (CP violation? Gallium vs reactor?)
Example: nuSTORM - Neutrinos from STORed Muons (LOI: arXiv:1206.0294) Summary of options: Appendix of white paper arXiv:1204.5379
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Non-unitarity of mixing matrix? Integrating out heavy fermion fields (such as in a type-I TeV
see-saw), one obtains neutrino mass and the d=6 operator (here: fermion singlets)
Re-diagonalizing and re-normalizing the kinetic terms of the neutrinos, one has
This can be described by an effective (non-unitary) mixing matrix with N=(1+) U
Relatively stroung bounds already, perhaps not so good candidate for future measurements(see e. g. Antusch, Baumann, Fernandez-Martinez, arXiv:0807.1003)
also: “MUV“
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Non-standard interactions
Typically described by effective four fermion interactions (here with leptons)
May lead to matter NSI (for ==e)
May also lead to source/detector NSI
How plausible is a modelleading to such NSI(and showing up in
neutrino sector only)?
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Lepton flavor violation (d=6)
Charged leptonflavor violation
Strongbounds
e e
e
NSI
e e
e CLFV e
4-NSIEx.:
e e
Non-standard neutrino interact.
Effects in neutrino oscillations in matter
Non-standard int. with 4
Effects in environments with high neutrino densities (supernovae)
BUT: These phenomena are not independent (SU(2) gauge invariance!)Is it possible that new physics is present in the neutrino sector only?
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Idea: d=8 operator?
Decouple CLFV and NSI by SU(2) symmetry breaking with operator
Works at effective operator level, but are there theories allowing that? [at tree level]
Davidson, Pena-Garay, Rius, Santamaria, 2003
Project outneutrino field
Project outneutrino field
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Systematic analysis for d=8
Decompose all d=8 leptonic operators systematicallyThe bounds on individual
operators from non-unitarity, EWPT, … are very strong! (Antusch, Baumann, Fernandez-Martinez, arXiv:0807.1003)
Need at least two mediator fields plus a number of cancellation conditions(Gavela, Hernandez, Ota, Winter, Phys. Rev. D79 (2009) 013007)
Basis (Berezhiani, Rossi, 2001)
Combinedifferent
basis elements
C1LEH, C3
LEH
Canceld=8
CLFV
But these mediators cause d=6 effects Additional cancellation condition
(Buchmüller/Wyler – basis)
Avoid CLFVat d=8:
C1LEH=C3
LEH
Feynman diagrams
Implications of large 13: flavor
33
Short seesaw-I mixing primer
Charged leptonmass terms
Eff. neutrinomass terms
cf., charged current
Rotates left-handed fields
Block diag.
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The TBM “prejudice“
Tri-bimaximal mixings probably most discussed approach for neutrinos (Ul often diagonal)
Can be obtained in flavor symmetry models (e.g., A4, S4)
Consequence: 13=0 Obviously not!
Ways out for large 13?
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Impact on theory of flavor?
Structure:A4, S4, TBM, …
Anarchy:Random draw?
13 very small very large
Different flavor symmetry?
Corrections?CL sector?
RGR running?
Some structure + randomness:
Froggatt-Nielsen?
vs.
Quark-leptoncompementarity:
13 ~ C?
e.g. 12 = 35 + 13 cos (Antusch, King)
36
Anarchy?
Idea: perhaps the mixing parameters are a “random draw“?
Challenge: define parameterization-independent measure
Result: large 13 “natural“, no magic needed
(Hall, Murayama, Weiner, 2000; de Gouvea, Murayama, 2003, 2012)
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“Structure+randomness“:Froggatt Nielsen mechanism?
L/R are SM fermions After integrating out the heavy fermions:
Integer power n is controlled by the (generation/flavor-dependent) quantum numbers of the fermions under the flavor symmetry
K: (complex) generation dependent (random) order one coefficients
Well-suited to describe hierarchies
(F. Plentinger)
Ml ~
Example:
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Hybrid alternatives?
Charged leptons:Strong hierarchy,masses through
SM Yukawas
Quarks:Strong hierarchies
Small mixings
Neutrinos:Mild (no?) hierarchy,
large mixings, Majorana masses?
Origin:physics BSM?LNV operator?
Flavor symmetry,structure?
Tri-bimaximal mixing“paradigm“?
Ansatz suitablefor hierarchies,
such as Froggatt-Nielsen?
Meloni, Plentinger, Winter, PLB 699 (2011) 244
13=0
39
Consequences
Can control the size of 13 by suitable U(1) charges/mass texture for the charged lepton sector:
Challenge:Deviations from TBM 12 typically accompanied by large 13
Re-think zeroth order paradigm (TBM)???
Meloni, Plentinger, Winter, PLB 699 (2011) 244
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Summary and outlook Are neutrinos masses evidence for physics BSM?
Neutrinoless double beta decayTests at the LHC
0 signal + CPV in lepton sector + no evidence for mass at the LHCGUT seesaw, leptogenesis?
Natural TeV neutrino mass model requires additional suppression mechanism; then, however, plausible to discover it at the LHC
Most likely case for new physics in neutrino sector: fourth generation (light sterile neutrino)?
Theory of flavor has to be re-thought after 13 discovery