What is the matter?

What is the matter?

Where is the antimatter?

Professor Michael G Green

Royal Holloway University of London

Where the hell …?

What is the matter? . . . . Where is the antimatter?

What is matter?


Where is the antimatter?


The concept of elements

In Aristotle’s philosophythere were four elements

Dalton (1808) listed, with weights,many elements we recognize today


The periodic table

Mendeleev (1869) introduced the periodic table


The plum pudding model

J J Thomson believed the electronswere embedded in a positivelycharged matrix - plum pudding


The structure of atoms

Rutherford (1912)showed that atomscontain a centralnucleus

Electrons orbit nucleuswith well-definedenergy and ill-definedpositions10

-10 m


The structure of nuclei

Nucleus containsprotons with charge+e and unchargedneutrons10

-14 m


10-15

m

The structure of nucleons

Neutrons and protons contain quarks


<10-18

m

The structure of quarks?

There is no evidence for further structure

?


Evidence for substructureAtom absorbs energy

Only certain energy levels (orbits) allowed

Electron energy increases

Later ‘de-excites’


Evidence for substructure

Measure size of struck objects(Rutherford 1912)

1970 - substructure of protons and neutrons discovered using electrons as projectiles


The constituents of matter

Protons contain uud - charge = +eNeutrons contain udd - charge = 0

quarks electron

23

e+

13

e- -e

charge

u

d e


Prediction of antimatter

Paul Dirac predicted existence of the positron in 1928

The only equation in Westminster Abbey?

Dirac’s equation implies:

positron mass = electron mass

positron charge = +e


Discovery of antimatter

Anderson (1932) discovered the positronpredicted by Dirac


What is antimatter?

e+ e - Electrons and positrons annihilate to produce -rays (energy)

E = mc2


Production of e+e- pairs

Inverse process also occurs, with -rays becoming an electron-positron pair

e+e -


How to produce antimatter

e -

e -e-

e+

e e+ -

thin metal

region of magnetic field

E > few MeV since m c = 0.5 MeVe

2


The neutrino

‘Invented’ by Pauli (1928), named by Fermi (1933)

Discovered by Reines & Cowan (1956)

quarks leptons

23

e+

13

e- -e

u

d e

0


The muon

Discovered in cosmic rays by Neddermeyer and Anderson (1936)

Appears identical to electron but200 times as heavy

Decays within 2.2 sec

‘Who ordered that?’ - I I Rabi


Strange particles

In 1947 Rochester and Butler discovered yet more new objects, now known to contain a third quark

By the early 1960s beautiful patterns of particles containing three quarks or a quark and an antiquark were seen which were predictive (recall Mendeleev)


The fundamental particles (1963)

u

d e

s

quarks leptons


The zoo grows larger

u

d

tc

s b

six quarks

1947

1976 1995

1978

e

e

six leptons

1956

1895

1963

1936 1973


A particle accelerator

Energy of electrons is about 20kV


The LEP accelerator

Energy of electrons and positrons is about 100GeV


CERNEurope’s research laboratory for particle physics in Geneva.


LEP


Inside the LEP tunnel

LEP is 27km in circumference

Four bunches of electrons andpositrons circulate inside thevacuum pipe

100s for a complete circuit

About one electron-positroncollision per second


Electron-positron collisions

e- e+

Annihilation produces energy - mini Big Bang

Electron (matter)

Particles and antiparticles are produced

Positron (antimatter)

E = mc2


The ALEPH detector

End view


Collisions in ALEPH


ALEPH - a LEP particle detector


Three neutrinos ...

measures rate at whiche+e- collisions occur

Number of different neutrinos= 2.984 ± 0.008

u

d

tc

s b

e

e


… and no further substructure

e+ e-

e

e-

e*

Excited states produced if substructure exists

mass


The story so far

All particles have an antiparticle.

u

d

tc

s b e

e

The everyday world contains two quarks and the electron.

Additional quarks and leptons have been observed with six of each in total; most decay very rapidly.

When energy turns to mass equal numbers of particles and antiparticles are produced.


Matter-antimatter asymmetry in the Universe


When energy turns to mass equal numbers of particles and antiparticles are produced.

This observation creates problems for our understandingof the present day Universe, which appears to containonly matter and essentially no antimatter


The Universe began with a “Big Bang” about 15 billion years ago

-270o

?

heavy elements formed in stars

stars and galaxies exist, atoms form

neutrons

quark "soup"

15 billion years

1 million years

1 second

10-10

1015deg 1010deg109deg

6000o

-255o

3 minuteshelium nuclei formed

microwave background radiation fills universe

300,000 years

4000o

life on earth, molecules form

dominates matter

and protons formed

1 billion years

s

Big Bang

Big Bang

Evolution of the Universe

The Big Bang

What happenedat times less than 10-9s is uncertain


Evolution with matter-antimatter symmetry

Eventually such a universe contains only photons(almost true for our Universe - cosmic microwave background)


The Sakharov conditions

Antimatter can turn into matter if:

(a) proton decay occurs(b) there is a matter-antimatter

asymmetry (CP violation)(c) there is thermal non-

equilibrium

Sakharov (1964)


Proton decay

Life on earth implies protons exist, on average, for >1023 secondsSearches for proton decay have set limits >1032 seconds

d

u

u

X

e+

u

u-

proton

0

Proton decay convertsquarks into leptons - important only in early stages of the Big Bang but a small effect will remain

However antiprotons will decay similarly


Parity violationMacroscopic systems obey the same physical laws in a mirror system, e.g. planetary motion “parity conservation”.

-decay (weak interaction) does not conserve parity.

Discovered in 1956 in polarized 60Co decay.

θ θ

cos1)(cv

I


P violation - CP conservation

Parity violation leads to an asymmetry for neutrinos -only left-handed ones exist.

L

L

R

R

CPC

P

Changing particle to antiparticle (C) then applying the parity operation (P) produces the right-handed antineutrino, which exists

“CP conservation”


Matter-antimatter asymmetry

In 1964 it was discovered that the radioactive decay of antimatter differs by a tiny amount from the decay of matter.

Since then progress in understanding has been very slow:

• experiments are very difficult;

• astronomy is an observational science, not experimental (cannot repeat the Big Bang).

BUT we have learned that the matter-antimatter asymmetry can only occur if there are three pairs of quarks.


CP violation in K0 decays

d

s-

s

d-

K0 K0-

W W

u,c,t

u,c,t- - -

Phases of the amplitudes for the two processes are not equal‘CP violation’ (1964)

Occurs only because there are three families of quarks

s-

d

d-

s

K0K0-

u,c,t

u,c,t- - -

W W


CP violation

5 10 15 20 25 30

106

105

104

103

102

101

CO

UN

TS

/ 0

.5 x

10

-10 s

(10-10s)

5 10 15 (10-10s)

k0 + - DISTRIBUTION

Leads to beautiful interference effects and non-exponential decay distributions


A universe with CP violation

Perhaps one in every 109 antiquarks turned into a quark very early in the life of the Universe

After the matter-antimatter annihilation a small amount of matter will be left (about one proton for 109 photons)


Current problems

1. We have never observed proton decay

2. Precise measurements of CP violation in K0 decay are difficult and there are uncertain theoretical corrections

3. Cosmological models do not easily explain the ratio of 109 photons for each proton in the Universe


CP violation in B0 decays

Similar effect expected in B0

d

b-

b

d-

B0 B0-

W W

u,c,t

u,c,t- - -

First measurements starting 1999, Stanford, California

d

d-

b

B0B0-

u,c,t

u,c,t- - -b-

W W


Weak decay eigenstates


In the Standard Model the weak decay eigenstates are

d'

u

s'

cand

where d’ and s’ are a mixture of d and s of the form

d’= d cosθc + s sinθc

s’= -d sinθc + s cosθc

We write this as

s

d

cossin-

sincos

s'

d'

cc

cc

CP violation parameters


Further, there are relations among the elements of V such as

Vud V*ub + Vcd V*

cb + Vtd V*tb = 0

that can be represented by a triangle.

It is a condition for CP violation to occur that is non-zero.

CP violation parameters


Prior to 1999 the shape of this triangle had been approximatelydetermined from measurement of several parameters of V.However the angle had not been measured directly.

BaBar experiment at SLAC


The process e+e- B0B0


e

9.0 GeV e- 3.1 GeV e+

Centre of mass energy = 10.58 GeV - a resonanceenergy for the production of B0 B0

B0 and B0 are moving in the laboratory system

B0

B0

B decays


The two particles decay

We identify themand measure theseparation of thedecay points

The separation (typically 1mm)is translated to atime difference(typically 1ps)

Predicted distributions


Without CP violationthe distribution of t is exponential

We measure many examples of the process to produce adistribution in t

(a) CP violation makes thedistribution asymmetric(b) experimental resolutionmodifies it

Evidence for CP violation


The data show clear evidence of CP violation

The size of the effect isconsistent with the prediction of theStandard Model ofparticle physics

Values of sin 2


Recently direct measurements have been made(summer 2002 values)

OPAL 3.2+1.8-2.0±0.5

CDF 0.79+0.41-0.44

ALEPH 0.84+0.84-1.05

Belle 0.72 ± 0.07 ± 0.04BaBar 0.75 ± 0.09 ± 0.04

SummaryThe everyday world is made from up and down quarks and the electron.

Experiments tell us that six quarks and six leptons exist.

The “extra” ones seem to be needed to explain why there is an asymmetry between matter and antimatter and hence why we exist.

However it is likely to be a long time before we have a good understanding of what happened in the first fraction of a second of the Universe’s existence


THE ENDThere follows 6 slides showing a series of summary sheets which have been sent out to schools throughout the UK.


What is the matter?Where is the antimatter?

Particle Physics Summary Sheets - the story so far

GO

Exam

ple of Poste

r No.1

Exam

ple of Poste

r No.2

Exam

ple of Poste

r No.3

Exam

ple of Poste

r No.4

Exam

ple of Poste

r No.5

Exam

ple of Poste

r No.6

Particle Physics Summary Sheets - the story so far

Available as double sided A4 sheets for individual student use, or as a set of 6 single sided A3 wall posters, they are

entitled 'Particle Physics - the story so far'.

For details of how to get the summary sheets and posters call 01784 443448 or

e-mail: [email protected]

Date post:	05-Jan-2016
Category:	Documents
Upload:	eytan
View:	36 times
Download:	0 times

What is the matter?

Documents