CP violation in the neutrino sector

CP violation in the neutrino sectorLecture 4: New sources of CP violation?

Walter Winter

Nikhef, Amsterdam, 06.03.2014

Walter Winter | CPV Amsterdam | 06.02.2014 | Page 2

Contents (overall)

> Lecture 1:Introduction to neutrino physics, sources of CP violation

> Lecture 2:Neutrino oscillations in vacuum, measurement of dCP

> Lecture 3:Matter effects in neutrino oscillations: “extrinsic CP violation”

> Lecture 4:New sources of CP violation?

References:

> WW: “Lectures on neutrino phenomenology“, Nucl. Phys. Proc. Suppl. 203-204 (2010) 45-81

> Giunti, Kim: “Fundamentals of neutrino physics and astrophysics“, Oxford, 2007


Contents (lecture 4)

> Sterile neutrinos

>Non-standard interactions+ Is it plausible that new physics shows up in neutrino sector only

>Non-unitarity


Sterile neutrinos


Evidence for light sterile neutrinos?(addl. generations, not weakly interacting)

> LSND/MiniBooNE

>Reactor+gallium anomalies

Global fits

(Min

iBoo

NE

@ N

eutr

ino

2012

)

(B. F

lemin

g, TA

UP

2011)

(Kopp, Maltoni, Schwetz, 1103.4570)


Neutrino oscillations in vacuum

>Master formula:

“mass squared difference“ F(L,E)=L/E “spectral dependence“

> For antineutrinos: U U*

>Works for sterile neutrinos as well!

> At short distances: Only large mass squared terms non-vanishing


Example: 3+1 framework, parameterization-independent

Well known tension between appearance and disapp. data (appearance disappearance in both channels)

Need one or more new experiments which can test ne disappearance (Gallium, reactor anomalies) nm disappearance (overconstrains 3+N frameworks) ne-nm 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


Again: Necessary conditions for the observation of CPV

> Since

need spectral info!

> Since for a=b

need to observe flavor transitions

>Need (at least) three flavors(actually conclusion in quark sector by Kobayashi, Maskawa, Nobel Prize 2008) No CP violation in two flavor subspaces! Need to be sensitive to (at least) two mass squared splittings at the same time!

>Require at least two (light) sterile neutrinos; then new sources of CPV (but: do not really release tension in data …)

~ Jarlskog invariant


Non-standard interactions


Non-standard interactions

> Typically described by effective four fermion interactions (here with leptons)

>May lead to effects in oscillations (for g=d=e)

>May also lead to source/detector effects

How plausible is a modelleading to such NSI(and showing up in

neutrino sector only)?

acc: SM matter effect (later)


Lepton flavor violation (d=6)

>Charged leptonflavor violation

> Strongbounds

e e

ne nm

NSI

e e

e mCLFV

ne nm

4n-NSIEx.:

ne ne

>Non-standard neutrino interact.

> Effects in neutrino oscillations in matter

>Non-standard int. with 4n

> 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?


Gauge-inv. 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

H, L: SU(2) doublets


Systematic analysis for d=8

>Decompose all d=8 leptonic operators systematically

The 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


On current NSI bounds

> The bounds for the d=6 (e.g.scalar-mediated) operators are strong (CLFV, Lept. univ., etc.)(Antusch, Baumann, Fernandez-Martinez, arXiv:0807.1003)

> The model-independent bounds are much weaker(Biggio, Blennow, Fernandez-Martinez, arXiv:0907.0097)

>However: note that here the NSI have to come from d=8 (or loop d=6?) operators e ~ (v/L)4 ~ 10-4 natural?

> “NSI hierarchy problem“?

t sector least constrained


Source NSI with nt at a NuFact

> Probably most interesting for near detectors: eets, emt

s (no intrinsic beam BG)

>Near detectors measure zero-distance effect ~ |es|2

> Improving current bounds requires substantial equipment

(Tang, Winter, arXiv:0903.3039)

ND5: OPERA-like ND at d=1 km, 90% CL

This correlation is always present if:- NSI from d=6 operators- No CLFV (Gavela et al,arXiv:0809.3451;see also Schwetz, Ohlsson, Zhang, arXiv:0909.0455 for a particular model)


Other types of source NSI

> In particular models, also other source NSI (without nt detection) are interesting

> Example: (incoh.)eem

s from addl.Higgs triplet asseesaw (II) mediator

1 kt, 90% CL, perfect CID

(Malinsky, Ohlsson, Zhang, arXiv:0811.3346)

Requires CID!

Geometric effects? Effects of std.

oscillations

Systematics(CID) limitation?CID important!


CP violation in non-standard interactions

>Discovery potential for new CP violation

(WW, Phys.Lett. B671 (2009) 77)


For what fraction of the phases can CPV be discovered?

> If additional phases are present, the chances to discover CPV are good!

(WW, Phys.Lett. B671 (2009) 77)


Non-unitarity


Non-unitarity of mixing matrix

> Integrating out heavy fermion fields, 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 e with N=(1+e) U

> Similar effect to NSI, but source, detector, and matter NSI are correlated in a particular, fundamental way (i.e., process-independent)

also: „MUV“


Impact of near detector

> Example: (Antusch, Blennow, Fernandez-Martinez, Lopez-Pavon, arXiv:0903.3986)

> nt near detector important to detect zero-distance effect

>Magnetization not mandatory, size matters

Curves: 10kt, 1 kt, 100 t, no ND


NSI versus NU

> For a neutrino factory, leptonic NSI and NU may have very similar correlations between source and matter effects, e.g.

NU (generic, any exp.)NSI (d=6, no CLFV, NF)

>Difficult to disentangle with NuFact alone SB?

(Meloni, Ohlsson, Winter, Zhang, JHEP 1004 (2010) 041)

NU NSI


Is it possible/plausible that “new physics” shows up in the neutrino sector only?

>Possible? Yes, but at least non-standard four-fermion interactions require quite some

fine-tuning

It is difficult to find models which would not produce effects elsewhere (LHC, EWPT, …)

>Plausible?

Additional sterile generations are perhaps the most plausible new physics effect:

Short-baseline anomalies (eV steriles)Caveat: would show up in cosmology … (problem or feature?)

Warm dark matter (keV steriles)

Non-unitarity (>> GeV steriles)

Leptogenesis (GUT-scale steriles)


Summary

> If no new physics will be found at the LHC, the lepton sector may still provide enough CP violation for successful leptogenesis

>Most plausible candidates: see-saw mechanisms in different versions; no necessary implications at the TeV scale; basically “neutrino Standard Model(s)“

> Physics beyond the neutrino Standard Model may also provide new sources of CP violation; however: this kind of physics may have implications elsewhere

>Most interesting candidate, perhaps: sterile neutrinos


Discussion topics

>Do neutrinos really oscillate? After all, the mass eigenstates travel with different velocities …

>What happens over extremely long distances? Can one measure CP violation then?

>Why are neutrino masses “physics beyond the Standard Model“? Are they? Really?

>Why is there only one CP phase in the lepton mixing matrix, and three if neutrinos are Majorana particles? [after all, a general unitary matrix is parameterized including six phases]

>What do we actually learn from neutrinoless double beta decay?

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CP violation in the neutrino sector

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