fcnc processes - a window to new physics

FCNC processes - a window to new physics

Gela Devidze

High Energy Physics Institute Tbilisi State University

Tbilisi, Georgia

First Autum School & Workshop on Particle Phenomenology 23 September 2013, Tbilisi

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Gela Devidze – FCNC Processes . . .

Nanouniverse 10-9 m

Femtouniverse 10-15 m

Attouniverse 10-18 m

10-20 m

Zeptouniverse 10-21 m


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Exciting time is for fundamental physics, after the LHC experiments announced about discovery of neutral Higgs particle on July the 4th. Many in the community expect a new paradigm to emerge around the TeV scale, be it some variant of SUSY or of Technicolour or something even more radical, like extra (space) dimensions. Those novel structures can manifest themselves directly through the production of new quanta or the topology of events or indirectly by inducing forces that modify rare weak decays. Such indirect searches are not a luxury. We consider it likely that to differentiate between different scenarios of New Physics, one needs to analyze their impact on flavor dynamics.

First Autum School & Workshop on Particle Phenomenology

23 September 2013, Tbilisi

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Huge progress in experimentally measuring and theoretically understanding flavor physics has been achieved.

No evidence for new physics has been established!

The flavor structure of new physics at the TeV scale is strongly constrained.



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Within the Standard Model (SM), there are four different types of particles, each coming in three flavors:

• Up-type quarks (u, c, t);

• Down-type quarks (d, s, b);

• Charged leptons (e, μ, τ );

• Neutrinos in the (ν1, ν2, ν3).

The term “Flavor Physics” refers to interactions that distinguish between flavors.

Within the Standard Model, Flavor-Physics refers to the weak and Yukawa interactions.



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The term “Flavor Changing” refers to processes where the initial and final flavor-numbers are

different.

In “flavor changing charged current” processes, both up-type and down-type flavors,and/or

both charged lepton and neutrino flavors are involved.

Examples are

Within the Standard Model, these processes are mediated by the W-bosons and occur at tree

level.



e K

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In “flavor changing neutral current” (FCNC) processes, either up-type or down-type flavors

but not both, and/or either charged lepton or neutrino flavors but not both, are involved.

Example are

Within the Standard Model, these processes do not occur at tree level, and are often highly

suppressed.



e K

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Processes, which are highly suppressed in the Standard Model (SM), such as decays mediated by flavour changing neutral currents (FCNC) allow stringent tests of our current understanding of particle physics. These transitions are forbidden at tree level in the SM, as all electrically neutral particles have only diagonal couplings in the flavor space.

FCNC processes are therefore only allowed through loop contributions and probe the underlying fundamental theory at the quantum level, where they are sensitive to masses much higher than that of the b-quark.

Historically, many observations have first been indicated by FCNC processes, examples include the existence of the charm quark or the high top quark mass.

Enhancements of the decay rates of these FCNC decays are predicted in a variety of different New Physics models.


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In order to identify new physics through flavour physics and generally through high precision experiments we need:

• Many high precision measurements of many observables and precise theory,

• Identification of patterns of flavour violation in various NP models, in particular correlations between many flavour observables that could distinguish between various NP scenarios,

• Identification of correlations between low energy flavour observables and observables measured in high energy collisions.

First Autum School & Workshop on Particle Phenomenology, Tbilisi 2013, September 23

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• Heavy quarks rare decays.• Lepton flavour violating decays like μ →e, t →e , t → , decays with three leptons in the final state

and m −e conversion in nuclei.• Electric dipole moments of the neutron, the electron, atoms and leptons.• Anomalous magnetic moment of the muon Having experimental results on these decays and observables with sufficient precision accompanied by

improved theoretical calculations will exclude several presently studied models reducing thereby our exploration of short distance scales to a few avenues.

The correlations between all these observables and the interplay of flavour physics with direct searches for new physics (NP) and electroweak precision studies will tell us hopefully one day which is the proper extension of the SM



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The fundamental Lagrangian of the SM consists of four pieces LSM = Lgauge+Lfermion+LHiggs+LYukawa,

• What is the dynamical origin of the observed electroweak symmetry breaking, of relatedfermion masses and the reason for their hierarchy and hierarchy of their flavour-changinginteractions?

• Will the dynamics of electroweak symmetry breaking be driven by an elementary Higgs and be calculable within perturbation theory?• Will these dynamics be related to a new strong force with a composite Higgs or without Higgs at all?• Could these dynamics help us to explain the amount of matter-antimatter asymmetry and

the amount of dark matter observed in the universe?• Will these dynamics help us to explain various anomalies observed recently in the flavour data?


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• What is the fundamental dynamics behind the electroweak symmetry breaking that very likely plays also an important role in flavour physics? • Are there any new flavour symmetries that could help us to understand the existing hierarchies of fermion masses and the hierarchies in the quark and lepton flavour violating interactions? Are they local or global? Are they continuous or discrete? Are there any flavour violating interactions that are not governed by the SM Yukawa couplings? In other words, is the Minimal Flavour Violation (MFV) the whole story? • Are there any additional flavour violating and CP-violating (CPV) phases that could explain certain anomalies present in the flavour data and simultaneously play a role in the explanation of the observed baryon-antibaryon asymmetry in the universe (BAU)? • Are there any flavour conserving CPV phases that could also help in explaining the flavour anomalies in question and would be signalled in this decade through enhanced electric dipole moments (EDMs) of the neutron, the electron and of other particles?


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• Are there any new sequential heavy quarks and leptons of the 4th generation and/or new fermions with exotic quantum numbers like vector-like fermions? • Are there any elementary neutral and charged scalar particles with masses below 1 TeV and having a significant impact on flavour physics? • Are there any new heavy gauge bosons representing an enlarged gauge symmetry group?

• Are there any relevant right-handed (RH) weak currents that would help us to make our fundamental theory parity conserving at short distance scales well below those explored by the LHC?


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• Heavy quarks rare decays.Br(t →cg)=5 10∙ -11, Br(t →c)=5 10∙ -13, Br(t →cZ)=1.3 10∙ -11 , Br(t →ch)= 10-13 , Even single experimental observation for any top rare processes could be

breakthrough beyond the SM physics.

• Lepton flavour violating decays like μ →e, t →e , t → , decays with three leptons in the final state and −e conversion in nuclei.

Br(μ →e)SM < 10-54, Br(μ →e)exp < 10-13



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e

eee

e e

Charged Lepton Flavour Violation

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Lepton flavour violation (LFV) processes first arise in the Standard Model (SM) with neutrino

mixing at the one loop level with the exchange by W-bosons and leptons .

In the charged lepton sector the branching ratios for LFV processes are suppressed by factor

and bounded by neutrino oscillation experiments. 2 2/ Wm M

22*

2

( )3( )32

iei i

W

mBr e V VM

54( ) 10Br e



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2013 < 5.7 ∙10-13 MEG collaboration

2011 < 2.4 ∙10-12 J. Adam et al. (MEG collaboration) arXiv:1107.55547[hep-px]

1999 < 1.2 ∙10-11 Brooks et al. Phys.Rev.Lett.83:1521-1524,1999.

1988 < 4.9 ∙10-11 Bolton R.D. et al., Phys. Rev. Lett. Vol.56. P.2461-2464

1982 < 1.7∙10-10 W.W.Kinnison et al., Phys. Rev. D25 (1982) p.2846

1980 < 1.0∙10-9 S.Lokonathan, J. Steinberger, Phys.Rev. 98 (1955) p.240

1977 < 3.6∙10-9 S.Lokonathan, J. Steinberger, Phys.Rev. 98 (1955) p.240

1955 < 10-5 S.Lokonathan, J. Steinberger, Phys.Rev. 98 (1955) p.240

1947 < 10% Hincks E.P., B.Pontecorvo, Phys. Rev. 73 (1948) p.257-258.

( )Br e


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COMET (COherent Muon to Electron Transition), to search for coherent neutrino-less conversion of muons to electrons (μ−−e− conversion), in the presence of a nucleus, μ− + N(A,Z) → e− + N(A,Z), with a single event sensitivity B(μ−N → e−N) < 10−16.

Currently, the search for possible flavor violation of the muon is considered to be a powerful and the most sensitive tool to reveal or restrict a new physics beyond the SM.

μ− + N(A,Z) → e − + N(A,Z),The muon system is considered to be the best system in which to study cLFV experiments because of following

reasons:• Intense muon beam can be obtained at meson factories• Muon life time is rather long .• Final states are very simple and can be precisely measured.• Observation of cLFV would indicate a clear signal of physics beyond the SM.• Search cLFV is especially promising to probe the Tev-scale physics.

-e conversion at level 10-16-18

The big challenge and great discovery petential!First Autum School & Workshop on Particle Phenomenology


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From common theoretical sense it is expected that LFV processes would possible enhance in case when particles running in the appropriate loops have close masses . Loop amplitudes with comparable masses of intermediate particles running in the loop seem to be quite large because the generic quadratic suppression factor is changed to a linear one.

Such a situation with comparable masses in principle is realizable in the models with extra dimensions.

On the other hand it is not obvious without specific calculations how would be changed the SM estimate of the above processes in the models with extra dimensions. Some details of the models can enhance suitable amplitudes and others can cause suppression.


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8 2*

8( )

6( ) ( )Wi j ik jk k

W n

MBr l l U U f x

M

601241)(

)( )( kk

W

nk

Wk

xxM

mRM

nxf

2 2( )* * *

2

( ) ( ) ( )12 2

k n k kik jk ik jk ik jk

W W W W W

m Rm mn n nU U U U U URM M RM R n M M

28 2*

8 2( )

( )3( )32

W ki j ik jk

W n W

M mBr l l U UM M

22*

2

( )3( )32

ki j SМ ik jk

W

mBr l l U UM


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On general grounds, one expects an enhancement of the amplitudes, but this expectation is not

fulfilled because of the almost degeneracy of the massive neutrino towers modes from

different generations.

This is not necessarily the last word, though; the black hole can inspire LFV processes and and

enhance them


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Lepton Family Number Violation via Intermediate Black Hole

LFV processes via intermediate black hole (dashed intermediate line ): a) LFV one-photon radiative decay ; b) tree lepton LFV decay ; c),d) muonium muonium oscillation .

e eee ( ) ( )e e


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We accept the conjecture that black holes violate global symmetries including lepton number.

So, black holes could manifest themselves in LFV processes as intermediate states and

enhance them. We assume that black holes with mass lighter than effective Planck mass have

a zero charges (electric, color) and zero angular moment in the classical case and this future is

adopted by quantum gravity too. So, one can write the effective Lagranjian describing

interactions between charged leptons and black hole in the following way . .ij Li Rj bhL g l l h c


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Virtual black hole can induce LFV processes at the one loop level. Through direct calculation we get following expression for branching ratios

e

i jl l

2*2 4

3( ) ( )4 ei i i

F

Br e g g f xG M

2 3

4

2 3 6 6 ln( )24(1 )

x x x x xf xx

2

2

( )ii

m lxM

*5

2

( )6.4 10ei i i

F

g g f xG

M

J. Adam et al. (MEG collaboration) 201313107.5)( eBr


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eee

2 2

2 4( )16

e ee

F

g gBr e e e

G M

12( ) 1.0 10Br e e e

6 24 10e ee Fg g G M


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Muonium gives us perfect opportunity to test fundamental interactions, because it does not contain hadronic constituents. The corresponding effective hamiltonian has the form

( ) ( )e e

5 5(1 ) (1 ) . .2

MMMM

GH e e h c

33 10 FMMG G

2

22 2e

MM

gG

M

23

2 8.5 10eF

gG

M


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Usual hierarchy of LFV processes , seems like It is not excluded vice-versa situation in some BSM approaches, which could be interesting

from the point of view NP. On the other hand, the search decays could be more favourable by some experimental reasons even if is a little less than . In this aspect even more intriguing would be situation with the hierarchy . This case is just the situation which could be predicted by the case of LFV via intermediate black hole processes. It shall be interesting to check this on experiments.

The upcoming experiments aim to test LFV processes at a sensitivity of and they

may reach sensitivity. Our suggestion is to drift from the LFV radiative decays to three lepton LFV decays and/or muonium-antimuonium oscillation.

If extra dimensions scenario realize at O(1 TeV) level, it is possible to discover their traces in LFV processes and three lepton LFV decay and muonium-antimuonium oscillation seem favorable. .

e ( ) ( 3 )Br e Br e

3l l( 3 )Br l l ( )i jBr l l

( 3 ) ( )i jBr l l Br l l

13 1610 10 1810

eee

eee


11105)( cgtBr

13105)( ctBr

13103.1)( cZtBr

1310)( cHtBr


2*44

5

)()4(

)( icitit xfggM

mct 4

32

)1(36ln6632)(

xxxxxxxf

2

2 )(M

qmx upi

2*8 )(104.4)( iciti xfggctBr


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We have studied manifestation of NP in rare processes. Our attention was devoted to lepton flavour violation processes, top quark and neutral B-meson rare decays.

The calculation presented here revealed a moderate to small difference for the branching ratios. Maybe the more relevant statement is that the simplest realization of an extra dimension model cannot affect in a numerically large way. If a large deviation from the SM prediction were observed, it must have a completely different origin and one had to look elsewhere

We have estimated lepton flavour violation processes rates and concluded that three body decays and −e conversion seem more favourable then radiative decays.


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The correlations between flavour physics observables and the interplay of flavour physics with direct searches for new physics and electroweak precision studies will tell us hopefully one day which is the proper extension of the SM.

The interplay of high energy collider results with the flavour precision experiments will allow us to make important steps towards a New Standard Model.


fcnc processes - a window to new physics

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