An introduction to Flavour Physics

!

Part 4 !

Thomas Blake Warwick Week 2015

1

An introduction to Flavour Physics

• What’s covered in these lectures:

1. An introduction to flavour in the SM.!

2.Neutral meson mixing and sources of CP violation.

3.CP violation in the SM.

4.Flavour changing neutral current processes.!

➡Neutral meson mixing, rare decays, lepton flavour violation and constraints on new particles.

2

The flavour problem• If there is TeV-scale new physics (and we hope there is),

why doesn’t it manifest itself in flavour changing neutral currents (FCNCs)?

3

No FCNC at tree level in SM

• Charged current interaction is the only flavour changing process in the SM.

• Flavour changing neutral current processes are therefore forbidden at tree level (require a loop process involving a virtual W exchange).

�

qi qjqi qiqi qi qiqi

WZ0 gVij

4

Effective theories• In mesons/baryons there is a clear separation of scale.

!

• We want to study the physics of the mixing/decay at or below a scale Λ, in a theory in which contributions from particles at a scale below and above Λ are present. Replace the full theory with an effective theory valid at Λ,

5

⇤

E

�L

�HmW � mb > ⇤QCD

L(�L,�H) ! L(�L) + Le↵ = L(�L) +X

i

CiOi(�L)

operator product expansion

Fermi’s theory• Example:

➡ In the Fermi model of the weak interaction, the full electroweak lagrangian (which was unknown at the time) is replaced by the low-energy theory (QED) plus a single operator with an effective coupling constant.

6

d u d u

⌫ e ⌫ e

W LEW ! LQED +GFp2(ud)(e⌫̄)

FCNC processes• Two types of FCNC process:

➡ ∆F = 2, meson anti-meson mixing.

➡ ∆F = 1, e.g. Bs → 𝝁+𝝁- . Commonly described as rare decays.

!

• In the SM these processes are suppressed: ➡ Loop processes which are CKM

suppressed and can be highly GIM suppressed.

B0s B0

s

B0

K⇤0

µ+

µ�

µ+

µ�

B0s

7

FCNC processes• Two types of FCNC process:

➡ ∆F = 2, meson anti-meson mixing.

➡ ∆F = 1, e.g. Bs → 𝝁+𝝁- . Commonly described as rare decays.

!

• In the SM these processes are suppressed: ➡ Loop processes which are CKM

suppressed and can be highly GIM suppressed.

B0s B0

s

B0

K⇤0

µ+

µ�

µ+

µ�

B0s

8

Mixing

9

Reminder: GIM mechanism• Take mixing diagram as an

example, have an amplitude

10

Bq Bq

Vtb

V ⇤tq

t

W Wt̄

q̄

q = s, db

b̄V ⇤tq

Vtb

(c, u)

A(B0 ! B0) =X

q,q0

(V ⇤qbVqd)(V

⇤q0bVqd)Aqq0

A(B0 ! B0) =X

q

(V ⇤qbVqd)[V

⇤tbVtd(Atq �Auq) + V ⇤

cbVcd(Acq �Auq)]

summing over the internal up-

type quarks Can then plug-in V ⇤ubVud + V ⇤

cbVcd + Vtb ⇤ Vtd = 0

for the B system the top dominates / mqmq0/m2W

New physics in B mixing?• Introduce a multiplicative factor

11

M12 = M12,SM ·�s,d

consistent with SM (Re ∆ =1, Im ∆ = 0)

New physics in B mixing?• Introducing new physics with at some higher energy scale ΛNP

with coupling 𝜅NP

12

SM contribution from box diagram

NP contribution (at dimension 6), can include tree level FCNC

Le↵ = LSM +X

i

NP

⇤d�4NP

O(d)i

(V ⇤tbVtd)

2 g4m2t

16⇡2m4W

c.f.NP

⇤2NP

Mixing constraints• Everything is consistent with the SM, so instead can set

constraints on NP scale from mixing.

13

Operator Re(⇤) Im(⇤) Re(c) Im(c) Constraint(sL�µdL)(sL�µdL) 9.8⇥ 102 1.6⇥ 104 9.0⇥ 10�7 3.4⇥ 10�9 �mK , "K

(sRdL)(sLdR) 1.8⇥ 104 3.2⇥ 105 6.9⇥ 10�9 2.6⇥ 10�11 �mK , "K(cL�µuL)(cL�µuL) 1.2⇥ 103 2.9⇥ 103 5.6⇥ 10�7 1.0⇥ 10�7 �mD, |q/p|, �D

(cRuL)(cLuR) 6.2⇥ 103 1.5⇥ 104 5.7⇥ 10�8 1.1⇥ 10�8 �mD, |q/p|, �D

(bL�µdL)(bL�µdL) 5.1⇥ 102 9.3⇥ 102 3.3⇥ 10�6 1.0⇥ 10�6 �md, SJ/ K0S

(bRdL)(bLdR) 1.9⇥ 103 3.6⇥ 103 5.6⇥ 10�7 1.7⇥ 10�7 �md, SJ/ K0S

(bL�µsL)(bL�µsL) 1.1⇥ 102 1.3⇥ 102 7.6⇥ 10�6 7.6⇥ 10�6 �ms

(bRsL)(bLsR) 3.7⇥ 102 3.7⇥ 102 1.3⇥ 10�5 1.3⇥ 10�5 �ms

coupling (c = 𝜅) when ΛNP = 1TeV ΛNP in TeV when coupling =1

Small couplings?• New flavour violating sources (if there are any) are highly tuned,

i.e. must come with a small coupling constant or must have a very large mass.

14

NP ⇠ 1 ⇤NP >⇠ 2⇥ 104 TeV

⇠ 1

(4⇡)2⇤NP >⇠ 2⇥ 103 TeV

⇠ (ytV⇤tiVtj)

2 ⇤NP >⇠ 5TeV

⇠(ytV ⇤

tiV⇤tj)

2

(4⇡)2⇤NP >⇠ 0.5TeV

generic tree-level

generic loop-order

tree-level with “alignment”

loop-order with “alignment”

Minimal Flavour Violation• One good way to achieve small couplings is to build models that

have a flavour structure that is aligned to the CKM. ➡ Require that the Yukawa couplings are also the unique source

of flavour breaking beyond the SM.

• This is referred to as minimal flavour violation.

• The couplings to new particles are naturally suppressed by the Hierarchy of the CKM elements.

15

Rare decays

16

Rare b → s decays• Can write a Hamiltonian for the effective theory as

17

He↵ = �4GFp2

VtbV⇤ts

↵e

4⇡

X

i

Ci(µ)Oi(µ) ,

Local operator with different Lorentz structure (vector, axial vector current etc)

Wilson coefficient (integrating out scales above μ)

Rare b → s decays• Can write a Hamiltonian for the effective theory as

!

!

!

!

• Conventional to pull SM loop contributions out the front as constants.

18

He↵ = �4GFp2

VtbV⇤ts

↵e

4⇡

X

i

Ci(µ)Oi(µ) ,

CKM suppressionLoop suppression (1/4π)2

Weak decay (1/mW)2

Beyond the SM• In the same way can introduce new particles that give rise to

corrections

!

!

!

• Once again, the constant, 𝜅, can share some, all or none of the suppression of the SM process.

19

�He↵ =NP

⇤2NP

ONP

NP scale local operator

Operators

20

O7 =mb

es̄�µ⌫PRbFµ⌫ , O0

7 =mb

es̄�µ⌫PLbFµ⌫ ,

O8 = gsmb

e2s̄�µ⌫PRT

abGaµ⌫ , O0

8 = gsmb

e2s̄�µ⌫PLT

abGaµ⌫ ,

O9 = s̄�µPLb ¯̀�µ` , O0

9 = s̄�µPRb ¯̀�µ` ,

O10 = s̄�µPLb ¯̀�µ�5` , O0

10 = s̄�µPRb ¯̀�µ�5` .

right handed currents (suppressed in SM)photon penguin

vector and axial-vector current

Operators (beyond SM)• Scalar and pseudo-scalar operators (e.g. from Higgs penguins)

!

!

!

• Tensor operators

!

!

• All of these are vanishingly small in SM.

• In principle could also introduce LFV versions of every operator.

21

OS = s̄PRb ¯̀̀ , O0S = s̄PLb ¯̀̀ ,

OP = s̄PRb ¯̀�5` , O0P = s̄PLb ¯̀�5`

OT = s̄�µ⌫b ¯̀�µ⌫` , OT5 = s̄�µ⌫b ¯̀�

µ⌫�5` ,

Generic ∆F = 1 process• In the effective theory, we then have

!

!

!

!

• For inclusive processes can relate sum over exclusive states to calculable quark level decays,

!

• For exclusive processes, need to compute form-factors / decay constants etc.

22

A(B ! f) = V ⇤tbVtq

X

i

Ci(MW )U(µ,MW )hf |Oi(µ)|Bi

Hadronic matrix element

B(B ! Xs�) = B(b ! s�) +O(⇤2QCD/m2

B)

Generic ∆F = 1 process• Inclusive processes are theoretically clean, but

experimentally challenging.

• Exclusive processes are theoretically challenging. ➡ Involve non-perturbative quantities.

• Estimate form-factors using, light-cone-sum-rules and Lattice QCD.

23

Form factors• Unfortunately, we don’t just have free quarks and we need to

compute hadronic matrix elements (form-factors and decay constants). ➡ Non-pertubative QCD, i.e. difficult to estimate.

24

fBFortunately we have tools to help us in different kinematic regimes.

e.g how to deal with this:

Theoretical tools (crib sheet)• Lattice QCD

➡ Non-perturbative approach to QCD using discretised system of points in space and time. As the lattice becomes infinitely large and the points infinitely close together the continuum of QCD is reached.

• Light-Cone-Sum-Rules ➡ Exploit parton-hadron duality to compute form-factors and

decay constants.

• Operator product expansions. ➡ Used to match physics to relevant scales.

25

Theoretical tools (crib sheet)• Heavy quark expansion.

➡ Exploit the heaviness of the b-quark,

• QCD factorisation. ➡ Light quark has large energy in the meson decay frame, e.g.

quarks in π have large energy in B → π decays in the B rest frame.

• Soft Collinear Effective Theory. ➡ Model system as highly energetic quarks interacting with soft

and collinear gluons.

• Chiral perturbation theory.

26

mb � ⇤QCD

Which processes?• Will mainly focus on recent measurements of B decay processes,

b → s transitions are some of the least well tested.

• You can also study FCNC decays of charm and strange mesons.

• The GIM mechanism is more effective in both charm and strange meson decays: ➡ For charm mesons the masses and mass differences, e.g. (mb -

ms), are small. ➡ For strange mesons top contribution is suppressed relative to B

meson decays because Vts ≪ Vtb.

27

Bs → 𝝁+𝝁-

• Golden channel to study FCNC decays.

• Highly suppressed in SM.

1.Loop suppressed.

2.CKM suppressed (at least one off diagonal element)

3.Helicity suppressed (pseudo-scalar B to two spin-1/2 muons)

B0s

µ+

µ�

Z0

t

W

b

s̄

28

neutral current (axial-vector)

also receives contributions from W box diagrams

Bs → 𝝁+𝝁-

• Golden channel to study FCNC decays.

• Highly suppressed in SM.

1.Loop suppressed.

2.CKM suppressed (at least one off diagonal element)

3.Helicity suppressed (pseudo-scalar B to two spin-½ muons)

B0s

µ+

µ�b

s̄

29

Interesting probe of models with new or enhanced scalar operators (no helicity suppression), e.g. SUSY at high tan β.

b

h0

Bs → 𝝁+𝝁- in the SM• Only one operator contributes in SM:

!

• Branching fraction in SM:

30

B(B0d,s ! µ+µ�) ⇡

��V ⇤tbVtq

��2 G2F ↵2

e MBM2µf

2B

16⇡3�H

s

1�4M2

µ

M2B

|C10(mb)|2

CKM factors

Single hadronic matrix element (decay constant)

O10 = (sL�µbL)(µ̄�µ�5µ)

h0|s�µ�5b|Bi = ifBpµ

helicity suppression⇥ M2

µ

M2B

!

Bs → 𝝁+𝝁-• Very rare decay with branching fraction of 10-9:

31

Bs → 𝝁+𝝁- combination• Combining results from CMS

and LHCb experiments from LHC run 1. ➡ Observe Bs signal and

evidence for B0.

32

]2c [MeV/−µ+µm5000 5200 5400 5600 5800

)2 cS/

(S+B

) wei

ghte

d ca

nd. /

(40

MeV

/

0

10

20

30

40

50

60 DataSignal and background

−µ+µ →s0B

−µ+µ →0BCombinatorial bkg.Semileptonic bkg.Peaking bkg.

CMS and LHCb (LHC run I)

]9�[10)�µ+µ⇥0BB(0 0.2 0.4 0.6 0.8

Lln⇤2�

0

2

4

6

8

10SM

]9�[10)�µ+µ⇥s0BB(

0 2 4 6 8

Lln⇤2�

0

10

20

30

40SM

]9�[10)�µ+µ⇥s0BB(

0 1 2 3 4 5 6 7 8 9

]9�[1

0)� µ

+ µ⇥

0B

B(

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

68.27%

95.45%

99.73% 5�10

◊6.3

�1

7�10

◊5.7

�1

9�10

◊2�1

SM

CMS and LHCb (LHC run I)

a

b

c

LHC

b +

CM

S, [a

rXiv

:141

1.44

13]

the end of a long road …

33Year1985 1990 1995 2000 2005 2010 2015

Lim

it (9

0% C

L) o

r BF

mea

sure

men

t

10−10

9−10

8−10

7−10

6−10

5−10

4−10

−µ+µ → 0sSM: B

−µ+µ → 0SM: BD0L3CDFUA1ARGUSCLEO

CMS+LHCbATLASCMSLHCbBaBarBelle

2012 2013 201410−10

9−10

8−10

Testing MFV• Ratio of the rates of the

two decays is a test of MFV which predicts:

34

R0 0.1 0.2 0.3 0.4 0.5

Lln∆2−

0

2

4

6

8

10

SM and MFV

CMS and LHCb (LHC run I)

R(B0/B0s ) /

|Vtd|2

|Vts|2f2B0

f2B0

s

from Lattice

~1/25

Data are consistent with SM/MFV

LHC

b +

CM

S, [a

rXiv

:141

1.44

13]

Photon polarisation• In radiative B decays, angular momentum

conservation allows

!

!

• However, the charged current interaction only couples to left handed quarks. Need to helicity flip the b- or s-quark.

• The right-handed contribution is therefore suppressed by

35

bR(L) sL(R)W−

γL(R)

t

Vtb VtsbL ! sR�R

bR ! sL�L

A(bL ! sR�R)

A(bR ! sL�L)⇠ ms

mb

Inclusive b → s 𝛾• Can perform either a semi-inclusive measurement, summing a

large number of Xs final states (e.g. K+, K*, …) or a fully inclusive measurement where Xs is not reconstructed.

➡ Cuts on M(Xs) or E𝛾 complicate theory calculations.

Result

36

10

* (GeV)γE1.6 1.8 2 2.2 2.4 2.6 2.8 3

Even

ts /

100

MeV

310

410

510

610

* (GeV)γE1.6 1.8 2 2.2 2.4 2.6 2.8 3

Even

ts /

100

MeV

310

410

510

610(a)

* (GeV)γE1.6 1.8 2 2.2 2.4 2.6 2.8 3

Even

ts /

100

MeV

0

2

4

6

8

10

12

310×

* (GeV)γE1.6 1.8 2 2.2 2.4 2.6 2.8 3

Even

ts /

100

MeV

0

2

4

6

8

10

12

310×

Continuum MC MCBB

Signal MC

(b)

FIG. 2: Estimated signal and background yields vs. photon energy in the CM frame based on MC simulation, at two stages ofthe event selection: (a) after requiring an unvetoed high-energy photon (logarithmic scale); (b) after all selection requirements(linear scale). The three contributions are shown cumulatively. The signal distribution is for a KN model withmb = 4.65GeV/c2,while the continuum distribution has been scaled as described in Sec. III.

CM Electron Momentum (GeV/c)0 0.5 1 1.5 2 2.5 30

200

400

600

800

1000

1200

Signal MC

MCBBContinuum MC

(a)

)γ* (e,θcos -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

200

400

600

8001000

1200

1400

1600

1800 (b)

CM Muon Momentum (GeV/c)0 0.5 1 1.5 2 2.5 30

100

200

300

400

500

600 (c)

)γ,µ* (θcos -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

200

400

600

800

1000

1200

1400

1600 (d)

FIG. 3: Lepton distributions from MC simulation, after the photon selection requirements but before applying lepton tag andNN criteria. Plots (a) and (b) are for electron tags, plots (c) and (d) for muon tags. Plots (a) and (c) show the CM-framemomentum distributions, with vertical lines indicating the minimum selection requirements. Plots (b) and (d) show the cosineof the CM angle between the lepton and the high-energy photon, after applying the momentum criteria; the vertical lines showthe minimum requirement on this quantity. The signal (black dots) is from a KN model with mb = 4.65GeV/c2. The BBbackground (solid blue histogram) and continuum background (dashed red) are from the MC simulations. Each distribution isseparately normalized to best illustrate its behavior.

shapes in MC

background subtracted data

BaBar, Phys. Rev. D. 86. 112008 (2012)

B(B ! Xs�)E�>1.6 GeV = (343± 21± 7)⇥ 10�6

BF consistent with SM expectation (impressive prediction of the SM)

b → s 𝛾 constraints

37

time dependent CP violation in B ! [K0

S⇡0]�

inclusive branching fraction.

angular distribution of B ! K⇤e+e�

• Constraints on right-handed currents in b → s 𝛾 decays:

Results are consistent with LH polarisation expected in SM

decays

38

!"#$%&$%$"'$(

J/�(1S)

�(2S)C(�)7

C(�)7 C(�)

9C(�)

9 C (�)10

4 [m(µ)]2 q2

d�dq2

)"*(

+,"-(*!.#)"'$(

',"#%!/01,".(&%,2(

)/,3$(,4$"('5)%2(

#5%$.5,6*((

cc̄

b ! cc̄stree levelphoton

penguin

b ! s`+`�

decay rates

39

b ! s`+`�

]4c/2 [GeV2q0 5 10 15 20

]-2

GeV

4 c × -7

[10

2 q/d

Bd

0

0.5

1

1.5

Theory BinnedLHCb CMS

]4c/2 [GeV2q0 5 10 15 20

]2/G

eV4 c ×

-8 [1

02 q

/dBd 0

1

2

3

4

5

LCSR Lattice Data

LHCb−µ+µ+ K→+B B0 ! K⇤0 µ+µ�

• 7 form-factors

• Enhancement at low q2 from virtual photon

• 3 form-factors

• No enhancement at low q2 from virtual photon (angular momentum conservation).

Large theory uncertainties (from QCD)

K+

⇡�K⇤0 ✓K

µ+

µ�

B0

✓`

(a) ✓K and ✓` definitions for the B0decay

µ�

µ+

K+

⇡�B0

K⇤0�

K+ ⇡�

n̂K⇡

�p̂K⇡

µ�

µ+

n̂µ+µ�

(b) � definition for the B0decay

⇡+

K�K⇤0

µ�

µ+

B0

�

K� ⇡+

n̂K⇡

� p̂K⇡

µ�

µ+

n̂µ�µ+

(c) � definition for the B0decay

B →K*0 µ+µ-

• Four-body final state. ➡ Angular distribution provides

many observables that are sensitive to NP.

e.g. at low q2 the angle between the decay planes is sensitive to the photon polarisation.

• System described by three angles and q2.

40

B →K*0 µ+µ-

41

1

d(�+

¯

�)/dq2d

3(�+

¯

�)

d

~⌦

����P

=

9

32⇡

h34 (1� FL) sin

2 ✓K + FL cos2 ✓K +

+

14 (1� FL) sin

2 ✓K cos 2✓l

�FL cos2 ✓K cos 2✓l + S3 sin

2 ✓K sin

2 ✓l cos 2�

+S4 sin 2✓K sin 2✓l cos�+ S5 sin 2✓K sin ✓l cos�

+

43AFB sin

2 ✓K cos ✓l + S7 sin 2✓K sin ✓l sin�

+S8 sin 2✓K sin 2✓l sin�+ S9 sin2 ✓K sin

2 ✓l sin 2�i

Angular distribution:

fraction of longitudinal polarisation of the K*

forward-backward asymmetry of the dilepton system

The observables depend on form-factors for the B → K* transition plus the Wilson coefficients.

Latest LHCb result, shown at Moriond 2015, LHCb-CONF-2015-002

42

Form-factor “free” observables

• Can construct ratios of observables which in QCD factorisation/SCET are independent of form-factors.

43

local tension with SM predictions (vector like NP contribution or unexpected large QCD effect?)

P 05 = S5/

pFL(1� FL)

Late

st L

HC

b re

sult,

sho

wn

at M

orio

nd 2

015,

LH

Cb-

CO

NF-

2015

-002

global fits to b → s data• Global fits to the b → s data favour

a new vector like contribution.

➡ Could be sign of new physics (heavier partner of the Z) or a sign that we don’t understand something about QCD, loop contributions will also look vector-like.

44

cc

branching fractions angular observables

combination

Interpretation of global fits

45

Optimist’s view point Pessimist’s view point

Vector-like contribution could come from new tree level contribution from a Z’ with mass of few TeV (the Z’ will also contribute to mixing, a challenge for model builders)

Vector-like contribution could point to a problem with our understanding of QCD, e.g. are we correctly estimating the contribution for charm loops that produce dimuon pairs via a virtual photon.

More work needed from experiment/theory to disentangle the two

(charged) lepton flavour violation

46

Charged LFV• Essentially forbidden in SM by

smallness of the neutrino mass. Powerful null test of the SM.

• Any visible signal would be an indication of NP.

47

x ⌫µ� e�

�

B(µ ! e�) / m4⌫

m4W

⇠ 10�54

µ LFV• Three different signatures

1. at rest (MEG at PSI).

2. (SINDRUM at PSI).

3. µ conversion in field of nucleus.

48

SINDRUM 1983–1986 @SIN(PSI) B(μ→eee) <1.0×10-12

25 year’s ago

SINDRUM II 1989–1993 @PSI R(μAu→eAu)<7.0×10-13

MEG 2008–2013@PSI B(μ→eγ) <5.7×10-13

µ ! e�

µ ! 3e

𝜏 LFV

• Expect improvement from Belle 2 and for 𝜏 → 3µ from LHCb.

49

large number of experimental signatures

A look into the future

50

New Physics? • As we saw in the last lecture, measurements of the CKM matrix

and the properties (closure) of the Unitarity triangle are consistent with the Standard Model picture of flavour physics. ➡ Nobel prize for Kobayahsi and Maskawa in 2008.

• However, there are some interesting “hints” of new physics: ➡ Tension in Vub (and Vcb).

➡ Enhancement of D(*)𝜏ν.

➡ P5’ anomaly in B →K*0 µ+µ-.

➡ Muon g-2.

• There’s still plenty of room for discoveries …

51

all at ~3𝜎

LHCb running conditions• LHCb is currently running at a luminosity of 4x1032 cm-2s-1 .

• Displacing LHC beams to run at a lower luminosity that ATLAS and CMS.

!

!

• How do we interpret this number?

!

!

• Higher luminosity means more B mesons produced per year and in turn better statistical precision.

52

�(pp ! bb) = (75.3± 5.4± 13.0)µb

⇠ 30 k bb pairs per second

running at a levelled luminosity

in LHCb acceptance

LHCb upgrade• Aim to run at 2x1033.

• Main limitation with the current detector is the 1 MHz readout rate.

• Need to cut much harder maintain the 1MHz rate with higher luminosity (end up removing as much signal and background).

• Ambitious plan to read out the full detector at 40 MHz to a software farm.

• Also plans to replace vertex and tracking detectors to cope with higher luminosity.

• Upgrade planned for LS2 (2018).

53

current trigger scheme

54Peter Križan, Ljubljana

e- 2.6 A

e+ 3.6 A

To obtain x40 higher luminosity

Colliding bunches

Damping ring

Low emittance gun

Positron source

New beam pipe& bellows

Belle II

New IR

TiN-coated beam pipe with antechambers

Redesign the lattices of HER & LER to squeeze the emittance

Add / modify RF systems for higher beam current

New positron target / capture section

New superconducting /permanent final focusing quads near the IP

Low emittance electrons to inject

Low emittance positrons to inject

Replace short dipoles with longer ones (LER)

KEKB to SuperKEKB

Rare kaon decays• Two rare kaon decay experiments are just starting:

➡ KOTO at J-PARC, searching for ➡ NA62 at CERN, searching for

• The main advantage of final states with neutrinos is that there is no contribution from quark loops involving light quarks (which can annihilate to produce charged leptons).

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K+ ! ⇡+⌫⌫̄

K0L ! ⇡0⌫⌫̄

KOTO NA62

also CP

Charged LFV • Upgrade to MEG underway,

aiming for O(10-14). Expected to start data taking soon.

• New µ →3e experiment (Mu3e) at PSI.

• Two new conversion experiments, one at PSI (Mu2e) and one at J-PARC (COMET).

• Expect improvements for LFV 𝜏 decays from Belle 2.

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50 μm thick silicon wafer

Interesting challenges, e.g. very thin detectors

for µ→3e.

Recap• In today’s lecture we discussed:

➡ Flavour changing neutral current processes and constraints on new particles.

➡ Minimal flavour violation. ➡ Charged lepton flavour violation. ➡ Future flavour experiments.

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Further reading• There are a number of good sets of lecture notes on flavour physics

available on arXiv that give a more detailed overview of this field.

➡ A. J. Buras, “Weak Hamiltonian, CP violation and rare decays,” [arXiv:hep-ph/9806471].

➡ A. J. Buras, “Flavor physics and CP violation,” [arXiv:hep-ph/0505175].

➡ G. Isidori, “Flavor physics and CP violation,” [arXiv:1302.0661].

➡ Y. Grossman, “Introduction to flavor physics,” [arXiv:1006.3534].

➡ Y. Nir, “Flavour physics and CP violation,” [arXiv:1010.2666].

➡ M. Neubert, “Effective field theory and heavy quark physics,” [arXiv:hep-ph/0512222].

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Further reading• Most introductory particle physics text books include a basic

introduction to flavour physics.

• There are also more specialist books available, e.g. ➡ I. I. Bigi and A. I. Sanda. “CP violation” ➡ CP violation, G.C.Branco, L.Lavoura & J.P.Silva

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Fin

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