What’s the matter with anti-matter? A short history of CP violation Gabriella Sciolla M.I.T....

What’s the matter with anti-matter?

A short history of CP violation

Gabriella Sciolla M.I.T.

Outline: The physics of CP violation

What is CP and why is it interesting? CP in particle physics

The “Standard Model”: a crush course Measurements of CP violation

Tests of Standard Model and probe to New Physics

Conclusion Summary & Prospects

November 13, 2003 What can we learn from CPV? G. Sciolla – M.I.T.

Symmetries and Physics Symmetries are a constant theme in Physics

“For every continuous symmetry of the laws of physics, there must exist a conservation law” Noether’s theorem

Continuous Symmetries Translational symmetry:

All positions in space are physically indistinguishable Consequence: momentum p is conserved

Rotational symmetry: All directions in space are physically indistinguishable Consequence: Angular Momentum L is conserved


Discrete Symmetries Parity (P)

Flips the space coordinates: Example:

Time Reversal (T) Physics invariant when

Charge Conjugation (C) Transforms matter into corresponding anti-

matter

x xy yz z

( ) ( )P e p e p

t t

( ) ( )C e p e p


Are C, P and T good symmetries of Nature?

Strong and Electromagnetic Interactions Conserve separately P and C

Weak Interaction Known to maximally violate both C and P

Wu et al. 1957 Only left-handed and right-handed anti- exist

C,P,T: Symmetries of Nature?


CPT Theorem and CP conservation

The CPT Theorem “In any relativistic quantum field theory the

product CPT is always conserved”

Consequence Since time reversal T was universally expected

to be conserved CP was expected to be conserved too

What’s the big deal with CP? It has to do with our own existence…


The CP symmetry What is CP?

CP = C × PC: Charge ConjugationParticle Anti-particle

P: ParityInverts space coordinates

( ) ( )CP e p e p


The importance of CP violation The Big Bang model predicts:

matter and anti-matter produced in equal amounts matter and anti-matter annihilated into energy

This obviously goes against experimental evidence…

e-

e+


Looking Out, Looking BackThe Universe exists…

Where did all the anti-matter go?

…and it’s made of matter!

8


The last echo of the Big Bang

The Cosmic Microwave Background, a result of matter/anti-matter annihilation near the beginning

of time

Image taken by the Wilkinson Microwave Anisotropy Probe (WMAP)


The matter dominated Universe

0.3 -10-0.2

Baryonic Asymmetry of the Universe: / ( 6.1 ) 10 Bn n

Only 1/109 particles survived!

What caused this tiny asymmetry?

November 13, 2003 What can we learn from CPV? G. Sciolla – M.I.T.11

Courtesy of Alan Chou (SLAC)

CP violation

Remember CPT theorem: CP expected to be conserved!


CP violation in K decaysIn 1964 Fitch and Cronin discovered CP violation in

the decays of KL mesons: KL+-

KL CP=-1} CP=+1}


The importance of CP violation discovery Revolutionary result:

Fitch and Cronin awarded the Nobel Prize in 1980

1980 NOBEL PRIZEJ.CroninV. Fitch

symmetry between matter and anti-matter is broken

How could this phenomenon be explained?


Standard Model of Particle Physics 12 elementary particles

6 leptons 6 quarks

4 forces mediated by various bosons e

e

bt

sc

du

Force Carrier(s) Mass (GeV/c2) Electric Charge

Electromagnetic photon 0 0

Weak W+, Z0 80/90 +1/0Strong gluon 0 0

Gravitational Graviton? 0? 0?


Standard Model’s minor addition: minor addition: anti-matter

Antiparticle vs. particle Same mass, but opposite quantum numbers

Example: electrical charge flips

e e

Matter Anti matter

e e

Charge=-1 Charge=+1

Charge=+2/3 Charge=-2/3

Charge=+1/3Charge=-1/3

Matter Anti matter

u c t u c td s b d s b


Photo courtesy of SLAC

SLAC 40-inch bubble chamber, 1971


SLAC 40-inch bubble chamber, 1971

Photo courtesy of SLAC


Standard Model’s recipes: how to make particles

The 6 quarks are the building blocks of matter

Baryons: 3 quarks Proton: (uud) Neutron: (udd) Lambda: (sud)

Mesons: 1 quark (q) + 1 anti-quark (q) Charged pions: + - Neutral Pion 0

u c td s b

uu d

Proton

ud

Charge=+2/3

Charge=-1/3

uu d

Anti-Proton

ud

uu


Standard Model’s recipes: how particles decay

Most of the particles produced in the lab decay Usually in a tiny fraction of a second!

Example of a weak decay: -

Feynman Diagram Solid lines: quarks or leptons Dashed lines: carriers of forces Each vertex carries a coupling constant gW

Its strength determines the probability of the decay

d

u

W

time

gW gW


Weak Interactions: Quark-Lepton Symmetry? Experimental observation in weak interactions:

Leptons always and only couple inside the same family Quarks instead violate this rule in -

How to preserve lepton-quark symmetry? Introducing “Quark Mixing” mechanism…

s

u

W

time u c

d s e

e

d

u

W

time u c

d s e

e


Standard Model: Quark Mixing Mechanism “Weak eigenstates” are linear combinations of

“Flavor eigenstates” Flavor eigenstates: quarks as we defined them so far Weak eigenstates: linear combinations of ordinary quarks

The matrix that transforms flavor eigenstates (d,s) into weak eigenstates (d’, s’) is called mixing matrix Just a 2x2 rotation 1 parameter: Cabibbo angle C

C C

C C

d'= d cos + s sin with s'= -d sin + s cos' 'u cd s

C C

C C

cos sin' -sin cos'd ds s


CP violation in the Standard Model What if extend this mechanism to 3 families of quarks?

Kobayashi and Maskawa, 1973 Parameterization of the 3x3 rotation in complex space:

3 angles (Euler angles) and a complex phase

This complex phase elegantly introduces CP violation in the Model explaining Fitch and Cronin’s results

It also predicted the existence of third quark family

)(

1)1(211

)(211

6

23

22

32

O

AiA

A

iA

VVVVVVVVV

V

tbtstd

cbcscd

ubusud


Success of the CKM predictions The third families of quark and leptons were soon

discovered

L. LedermanNobel Prize 88

Upsilon (bb) – Fermilab ‘77 Tau lepton – SLAC ‘75

M. PerlNobel Prize 95

Top quark – Fermilab ‘95


CKM and CP violation

Why we love CKM Elegant and simple mechanism Good success record The CKM is predictive

CKMMeasure CP violation in channels theoretically very well understood and look for deviations

w.r.t. Standard Model prediction


How to test CKM?

Kaons known for 40 years but… Experimental results very hard to interpret

theoretically: Loose constraints from K measurement No constraints from ’ yet…

…or clear theory but very hard to reach experimentally: BF(KL)~10-11!

Is there a way out? Yes, using B mesons…


Unitarity of CKM implies: V†V = 1 6 unitarity conditions Of particular interest:

All sides are ~ O(1) possible to measure both sides and angles! Angles from CP asymmetries in B meson decays Sides from measurement of decay rates and mixing of B mesons

The Unitarity Triangle

0VVVVVV *tbtd

*cbcd

*ubud

*

*| |ub ud

cd cb

V VV V || *

*

cbcd

tdtb

VVVV


Testing the Standard Model Measuring directly and independently the

angles tests the Standard Model in the CP violation

sector

B oscillationsB decays

CP violation in B decays


Testing the Standard ModelIn practice:

Measure sides first (easier) and identify allowed area for apex of the triangle

Measure one angle and check for consistency

Confirmation of Standard Model


Testing the Standard ModelIn practice:

Measure sides first (easier) and identify allowed area for apex of the triangle

Measure one angle and check for consistency

Discovery of New Physics!!!


The Bs

What are neutral B mesons? Bound states of b and d quarks:

B characteristics: B0 and B0 look identical from outside

Flavor can only be inferred by their decay products B0 and B0 decay after 1.6 ps in other lighter particles

Some of these decays are especially interesting…

Particle Quarks Mass Charg

e Spin

B0 bd 5.28 GeV/c2 0 0B0 bd 5.28 GeV/c2 0 0

bt

sc

du


CP violation in B0 decays If CP is conserved, B0 and B0 will behave exactly the same

Different types of CP violation: “Direct CP violation”:

Decay rates for B0 and B0 to certain final states are not the same

“Time-dependent CP violation”: The decay rate of B0 and B0 to some final states is not the

same over time This kind of CP violation is especially interesting because

the theory can make specific predictions…

0 0( ) ( )Rate B K Rate B K


Time dependent CP asymmetry Consider the B0 decays B0J/KS

Eigenstate of CP Clean experimental signature

Time Dependent CP asymmetry

It can be calculated that for this decay, the ACP(t) has a sinusoidal shape and its amplitude is related to angle

0 0

0 0( ( ) ) ( ( ) )( ) ( ( ) ) ( ( ) )

CP CPCP

CP CP

N B t f N B t fA tN B t f N B t f

0Kb

c

sc

d0B

/J

d

ACP(t) = sin2 sinmt


How to measure CPV at B factories

.

Ingredient #1:Exclusivereconstruction

Ingredient #2:Flavor tagging(coherent state)

Ingredient #3: t determination

e- 4S

B0

B0

e-

+

-

Breco

Btag

e+

z~ c t

+

-


The experimental challenge Rare events:

Experimental accessible BF: ~ 10-6 -10-5

Millions of Bs needed!

Time dependent analysis: Bs produced with a boost

Asymmetric B factory!

Full B reconstruction and flavor tag State of the art detector with excellent

tracking and particle identification: BaBar Detector!


The solution: asymmetric B factories

Asymmetric e+e- B factories First proposed in 1987 by P. Oddone (LBL) Elegant and conceptually simple Challenging for accelerator builders!

2 beam pipes, 2 sets of magnets, difficult interaction region…

Many proposals, only 2 survived: PEP-II at SLAC (California) KEK-B at KEK (Japan)

Similar in design and achievements Will discuss PEP-II and BaBar as an example…


The PEP-II accelerator

Asymmetric B factory 9.0 GeV e- beam 3.1 GeV e+ beam

Very high luminosity 9 BB pairs/second > 500 M B mesons


The SLAC accelerators

BaBar Detector


The BABAR Detector

DIRC (PID))

1.5 T solenoid

CsI(Tl) EMC

Drift Chamber

Instrumented Flux Return

Silicon Vertex Tracker

e+ (3.1GeV)

e- (9GeV)


The BaBar experiment at SLAC

MIT people at work!


Ingredient #1: Reconstruct the B0 decay

Accelerator and detector were completed in 1999

First goal: measure CP violation in the “golden channel” B0J/KS

Experimental challenge: Very rare decays BF (~10-4) It took 2 years to accumulate

enough data to measure CPV

Today’s sample: >4,000 clean B decays


Gold plated event at BaBar

B0 J/ KS B0 K-X

Zoom on interaction region


Distinguish B0 from B0 We are measuring the asymmetry:

B0 and B0 look identical from outside Flavor can only be inferred by their decay products

Strategy: Combine the many sources of flavor tagging information using Artificial

Neural Networks to optimize performance Very tricky business: the measurement is very sensitive to this

ingredient!!!

B0 D*- D0

l +(soft)

K +

B0D*-

(hard)

W+

Ingredient #2:


Ingredient #3:t measurement

Time dependent analysis:

The technique: e- 4S

B0

B0

e+

z~ c t ~ 250 m


The CP fit: sin2 from 88M B0B0

sin(2) = 0.722 ± 0.040stat ± 0.023syst


What did we learn?

Compare measurement of angle with

measurement of sides:

sin2 from BaBar vs. indirect constraints

Excellent agreement with Standard Model expectation


CKM

CKM and CPV: end of the story?

Why we love CKM Elegant and simple mechanism The CMK is predictive

Measurements in B and K sector confirm predictions

Good News: new sources of CP violation must exist besides CKM (New Physics!)

Deviations from CKM expected in some channels

Just one (major) problem:X It fails to explain the matter-antimatter

asymmetry observed in the Universe by several orders of magnitude!


New Physics in penguin decays?

Standard Model predicts ACP(t)

New Physics can modify ACP(t): look for deviations >2.5 effect observed: more data needed…

0

S


sin2charmonium vs penguins

Fluctuation or first signs New

Physics?

…more data will tell…


Other sources of CP violations The explanation of the matter-antimatter

asymmetry doesn’t necessarily reside in the quark sector

Other possible sources of CP violation: CP violation in strong interactions

Search for neutron EDM (electric dipole moment) CP violation in neutrinos

Especially promising: Recent discovery of neutrino mixing: neutrinos have masses! 3x3 mixing matrix as in the quark sector CP violation term as in the CKM mechanism

Interesting experiments planned in the near future Stay tuned!


ConclusionThe mysteries of CP violation are being uncovered

thanks to new studies of B meson decays

Standard Model is still holding well, but…

First unambiguous evidence of CPV in B system First quantitative test of CP side of Standard Model

New Physics must be hiding somewhere: Penguins? Neutrinos? The quest continues…

What’s the matter with anti-matter? A short history of CP violation Gabriella Sciolla M.I.T....

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Transcript of What’s the matter with anti-matter? A short history of CP violation Gabriella Sciolla M.I.T....