CP violation: The difference between matter and antimatter

Oct 8th, 2003 Gerhard Raven 1

CP violation:CP violation:The difference between matter and The difference between matter and

antimatterantimatter

Gerhard Raven Vrije Universiteit Amsterdam, Subatomic Physics

& NIKHEF


AntimatterAntimatter

Westminster Abbey

1928: Dirac equation unifies Quantum Mechanics and

Special Relativity: 0i m

( )i p xu p e

1) Negative energy solutions can be seen as particles traveling backwards in time, equivalent to anti-particles traveling forward in time (Feynman & Stückelberg)

2) The # of particles is NOT conserved but #particles - #antiparticles is conserved)

Paul A.M. Dirac 2

Dirac Algebra:g 2 2E p m


Discovery of the positronDiscovery of the positronIn 1932, Carl Anderson discovers the positron


E=mcE=mc22: creating Matter and Antimatter: creating Matter and Antimatter

When creating matter from energy, always create equal amount of antimatter

e e e e Z qq


Big Bang CosmologyBig Bang Cosmology

Equal amounts of matter & antimatter

Matter Dominates !


Searches for Antimatter in the UniverseSearches for Antimatter in the Universe• Universe around us is matter dominated:

– Absence of antinuclei amongst cosmic rays– Absence of intense ray emission due to annihilation of

distant galaxies in collision with antimatter

Alpha Magnetic Spectrometer


Searches for Antimatter in the UniverseSearches for Antimatter in the Universe

The visible universe is very much matter dominated


Where did the Antimatter go?Where did the Antimatter go?

0.4 100.3(6.5 ) 10baryons

photons

NN

Almost all matter annihilated with antimatter, producing photons…

, ,lm lmaT Y

WMAP satellite

1 1,T 2 2,T

2.7248K 2.7252K

Cosmic Microwave Background

Angular Power Spectrum

l


Where did the Antimatter go?Where did the Antimatter go?• In 1966, Andrei Sakharov

showed that the generation of a net baryon number requires:1. Baryon number violating

processes (e.g. proton decay)2. Non-equilibrium state during the

expansion of the universe3. Violation of C and CP symmetry

• Standard Model of particle physics does allow for some CP-violation

• However, it is extremely unlikely to be sufficient to explain matter asymmetry in the universe– It means there must be

something beyond the SM in CP violation somewhere, so a good place for further investigation


Three Important Symmetries: C, P and T Three Important Symmetries: C, P and T

• Parity, P– Parity reflects a system through the origin. Converts

right-handed coordinate systems to left-handed ones.– Vectors change sign but axial vectors remain unchanged

• x x , p -p, but L=xp L

• Charge Conjugation, C

– Charge conjugation turns a particle into its anti-particle• e e K K

• Time Reversal, T– Changes, for example, the direction of motion of particles

• t t• CPT Theorem

– One of the most important and generally valid theorems in local quantum field theory.– All interactions are invariant under combined C, P and T– Implies particle and anti-particle have equal masses and lifetimes


““Weak” InteractionsWeak” Interactions

dp uu

du nd

e

e

W


Weak Force breaks C, breaks P, is CP really Weak Force breaks C, breaks P, is CP really OK ?OK ?

• Weak Interaction breaks both C and P symmetry maximally!

• Despite the maximal violation of C and P symmetry, the combined operation, CP, seemed exactly conserved…

• But, in 1964, Christensen, Cronin, Fitch and Turlay observed CP violation in decays of Neutral Kaons!

(1980 Nobel prize)

W+

e+R

L

W+

e+L

R

W

eR

L

W

eL

R

P

C

LR spin

spin


The Standard Model and CP violationThe Standard Model and CP violation• 1973: If there are at least 3

generations of quarks, the Standard Model of particle physics allows for CP asymmetry– All 3 generations have been

observed • c: 1974 (Nobel prize 1976)• : 1975 (Nobel prize 1995)• b: 1977• t: 1994• LEP: 1990 – 1995: there are 3 species of (light, left-handed) neutrinos

– With 3 generations, there is a single parameter in the SM responsible for all CP violating processes

– Very predictive! (in principle)• To explain the observed ratio of

baryons to photons, it falls short by ~8 orders of magnitude

• Ideal place for further research!


Matter-Antimatter OscillationsMatter-Antimatter Oscillations

0B

0B

0B

0B

Oscillation frequency: 0.5/ps,

Average B0 lifetime:1.5 ps

Produce with =0.56, and measure flight distance (1ps ~ 150 m)t(ps

)

mixA

At t=0 produce a B0 and B0 pairFor many such pairs, plot Amix asa function of the decaytime, t

0 0 0 0 0 0

0 0 0 0 0 0mix

N B B N B B N B BA

N B B N B B N B B

But B0 B0 goes as fast as B0B0…


Intermezzo: InterferenceIntermezzo: Interference

Interference allows one to determine phase-differences


Interference due to BInterference due to B00 B B00 oscillations oscillations

0B

0B 0SJ K

0SJ K

0t t

CP

0B2 sin

2i mtie

0B2 sin

2i mtie

0Bcos

2mt

0Bcos

2mt


Coherent Time Evolution at the Coherent Time Evolution at the (4S)(4S)

B-Flavor Tagging

Exclusive B Meson

Reconstruction

PEP-2 (SLAC)

Vertexing &Time DifferenceDetermination


BaBar Silicon Vertex DetectorBaBar Silicon Vertex Detector

Beam pipeLayer 1,2Layer 3Layer 4Layer 5

Beam bending magnets

Readoutchips


BaBar Detector BaBar Detector @ Stanford Linear Accelerator Center (SLAC)@ Stanford Linear Accelerator Center (SLAC)


Example of a fully reconstructed eventExample of a fully reconstructed event

(2S) Ks +- +-

B0 D*+ -fast

D0+

soft

K-+

‘’fish eye’’ view

fast

soft

B0(t)

At t=0 (i.e. when the D* decay happened), the ‘CP’ B was/would have been a B0

EPR!

Kb

c s

In general, use charges of identified•leptons,•kaons,•soft pions

from the “the rest of the event” to tag B flavour


CP violation in the B system is not small!CP violation in the B system is not small!CP violation in B system not small!CP violation in B system not small!

caveat: 100 million (4S) decays needed…

sin2 0 79 0 11β . .

220 events

0 taggedB

0 taggedB

0B

0B

0B

0B

0B

0B

0SJ K

0SJ K


The Result & The Standard ModelThe Result & The Standard Model

Without using sin(2)

One solution for is consistent with the

prediction from the SM

The SM has successfully

survived its first precision test of CP violation!

Standard Model predicts two other distinct phase differences, and

Current research aims to measure using several redundant methods


SummarySummary•CP asymmetry is required to generate a universe with more than just photons…

•CP is included in the Standard Model of particle physics if particles come in (at least) 3 generations

•We have now observed all 3 generations!

•The Standard Model does not allow sufficient CP asymmetry to explain the observed baryon to photon ratio

•The Standard Model prediction for CP violation has survived its first experimental precision test

•Current research aimed at testing the Standard Model predictions in various ways

•Somewhere the Standard Model must be incomplete…


Escher on CP violation…Escher on CP violation…

P

P

C CCP


CollidersColliders

• First collider: 13 cm, 80 KeV, 1931• LHC: 27 km, 14 TeV, 2007

Documents

CP violation: The difference between matter and antimatter