Chris Parkes 1

Part III

CP Violation and K Physics

Chris Parkes

Chris Parkes 2

Outline

PHENOMENOLOGY AND EXPERIMENTS

III. CP violation and Kaon physicsT-violation: CPLEAR

An angle (phase difference) measurement: CPLEAR

ε’ : NA48

Rare kaon decays

IV. CP violation and B physics

V. CP Violation and D physics

VI. Concluding remarksPresent status and future ahead

Mixing in the

neutral kaon system

Chris Parkes 4

Neutral kaon system K0 – K0 in nature (1/2)

If CP was conserved:

In nature, CP violated:

oo

oo

KKK

KKK

2

12

1

2

1

with 22

11

1

1

KKCP

KKCP

e quantifies

degree of CP violation

flavour eigenstates, CP conjugated

1,2

1

q

pqp

1q

p

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Neutral kaons “identity card”:

Branching fractions:

KS : p+p- 69% , p0p0 31% KL : p0p0p0 21% , p+p-p0 13% , pn 66%

Mixing parameters:

GeV10)009.0483.3(

s100014.05301.0=

s1004.017.51

s100008.08934.01

15

110

SL

8

LL

10

SS

mmm

Neutral kaon system K0 – K0 in nature (2/2)

~600 times larger!

21 95.0

M

x

KSoo

KS+-

KL+-o

KLooo

CP=+1

CP=-1

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d

su, c, t W-

W+_

s

d_u, c, t

d

s u, c, tW- W+

_

s

d_

u, c, t___

K0K0 -

Neutral kaon mixing - oscillations

K0 K0-

Feynman (box) diagrams for neutral kaon mixing:

t=0:initial K0 beam

KS decay early Pure KL beamafter a while!

At t>0:superposition of states

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CPLEAR T-invariance(can also think of it in terms of CP violation)

Rate difference Ko Ko Ko Ko is T violation

8

Experiment at LEAR ring at CERN 1990-1996

Pions from kaon decay

9

1) Identify Ko / Ko at production:

produced in association with K+/K-

2) Identify Ko / Ko at decay from charge of lepton:

CPLEAR

KK

KKpp

0

0

)su(K

)su(K

measure

Get positron:

s

d

Ko

u

d

e+ν

W+

π -

d d

Or electron:

sKo

u

e-ν

W-

π+

Final states f, f are

particle /anti-particles

Oscillation

10

t=0 t

K0

K0

K0

K0

K0

K0

Oscillation & Decay

11

t=0 t

K0

K0

K0

K0

K0

K0

s

d

Ko

u

d

e+ν

W+

π -

d d

sKo

u

e-ν

W-

π+

CPLEAR T-Invariance

12

T or equivalently CP

is violated by this tiny amount

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CPLEAR : decay to CP eigenstate

- Illustrate 3 types of CP violation

- Measuring an angle (phase difference)

14

1) Identify Ko / Ko at production:

produced in association with K+/K-

2) Decay to π+π- final state reachable from Ko and Ko

CPLEAR – decay to π+π-

KK

KKpp

0

0

)su(K

)su(K

measure

Interference of route with/without oscillations

Same final state f in both cases

Oscillation

15

t=0 t

K0

K0

K0

K0

K0

K0

Oscillation & Decay

16

t=0 t

K0

K0

K0

K0

K0

K0

Amplitude

Amplitude

Rate

Rate

Three methods of CP Violation Revisited

Rates

17

1. CP violation in decay

2. CP violation in mixing

3. CP violation in interference mixing/decay

18

(since ) Hence we are measuring an angle ϕ+-

c.f. B system

discussion later

19

NA48: Direct CP Violation

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CP violation first seen though existence of decay modes

CP conservation would imply

…. but experimentally these parameters are found ~ 10-3

CP violation due solely to mixing would be independent of the decay to the final state

SW

LW

SW

LW

KH

KH

KH

KH00

00

00,

00

The e and e’ parameters (1/2)

000

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To separate CP violation in decay from CP violation in mixing, the following is introduced:

… to separate the channel-independent term e

from the channel-dependent term e’

Measurement of rates

yields

The e and e’ parameters (2/2)

S

L2

K

K

N

N 00

S

00L2

00 K

K

N

N

'2,' 00

Direct CP violation(i.e. in decay)

CP violationin mixing

KL/Ks rates:

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The NA48 experiment at CERN (1/2)

Study of direct CP violation in the neutral kaon system

Study of direct CP violation in the neutral kaon system

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Start with a pure Ko beam After a few meters (short time) all Ks component decayed

Introduce slab of material in beam Several reactions can occur with the material:

1) elastic scattering

2) charge exchange

3) Hyperon production

Þ K0 absorbed more strongly!

f amount of K0 absorbed,

f amount of K0 absorbed

pKnK

nKpK

0

0

00 pK

sL

emerge

KffKff

KfKfK

2

1

2

12

1 00

Neutral kaon regeneration

Ks regenerated!

A consequence of particle-antiparticle

mixing

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Measurements of e’/e : Discovery of Direct CP violation

Average: 1.68 ± 0.14 10-3

Not easy to compare with SM theory

- CP violation frontier now in B and D

4

2

00

103.27.16

16

1Re

Direct

CP Violation

Rare kaon decays

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Rare Kaon Decays

Look for extremely suppressed decays in SM –

where new physics could contribute

Precision currently at parts per trillion !

Some interesting modes

Standard Model New Physics ExampleSame experimental signature

Very light/massless neutralino

– neutral SUSY particle

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Mainly (and very) sensitive to Vtd Predicted B. R. ~ 8x10-11

K

One such event

Enhancement over SM

of central value, but not significant

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Rare kaon decays – NA62 (successor to NA48) NA62 at CERN’s SPS currently commissioning, to measure

Start after CERN (LHC) accelerator shutdown K

K+ p+

n

n

~11 MHz of K+ decays

Expect

Also Koto expt, Japan

Chris Parkes 30

Key Points – Kaon section

• Ks (short lived) , KL (long lived) are mixture of K1 (CP +ve), K2 (CP –ve)

• ε quantifies degree of CP violation

• CPLEAR measurement of T (or equiv. CP) Violation

• Ko Ko Ko Ko

• CPLEAR CP violation in a CP eigenstate π+π-

• same final state f, defined Parameter η+-

• How to measure an angle

• NA48 Direct CP Violation

• Difference in CP violation in π+π-, π0π0

• Quantified by e’/e

• Rare Decays, extremely suppressed in SM, new physics can contribute

• NA62 K

'2,' 00