April 18, 2023 M. Fincke-Keeler, Univ. of Victoria

Motivation

A little bit of history

Orders of magnitude

Elementary particles

Tools: Accelerators and Detectors

How to go about

More things to find out….

Exploring the World of Elementary Particles

Motivation:

Some people are justplain curious about

the things around them and like to

understand them at a deeper level.

400-500 BC Democritus - Atoms

~ 1700 Newton: F=ma ~ 1800 Atoms are the smallest building blocks of matter~ 1880 All physics is essentially understood ~ 1900 Big confusion: * What are “rays” ? * Energy comes in “lumps” (Planck). * Atoms are mainly empty (Rutherford). * It is impossible to measure the position and the momentum of a particle simultaneously to any desired precision (Heisenberg).

When we say we “understand something” we usually mean “it makes sense” (within the framework of experiences we

have gathered in our lives so far).

Scientists like to be slightly more objective:

Mathematical frameworks exist, which describe many processes and phenomena very accurately -

i.e. the results of the mathematical formalism agrees with theresults observed in nature or in an experiment.

Such frameworks are called:

“Models”

What do we mean when we say “we understand something” ?

Orders of Magnitude

1m

0.1m (10cm)

10-2 m (1cm)

10-3 m (1mm)

10m

100m

103 m (1km)

10-4 m (0.1mm)

104 m (10km)

Orders of Magnitude

ct’d

10-4 m

10-5 m

10-6 m

10-7 m

10-8 m

10-9 m

10-10 m

10-11 m

10-12 m

10-13m

10-14 m

10-15 m

( )

( ) ( )

( )

10-18 m leptons, quarks (still ?) “pointlike”

10-16 m 10-17 m

( )

( smaller than )

Understanding what matter is made of also involves an understanding of the origins of “mass”. Recent theories for a mechanism that could explain the origin of mass imply the existence of a so farundetected “Higgs particle”

H

Leptons

Quarkst

e

u

c

g

d s b

Z W Electroweak andStrong Exchange Bosons

Today we believe that “matter” as we know it is composed out of Leptons, Quarks and Exchange Bosons of the strong and electroweak forces:

(carry colour, but have to form “colourless” observable particles)

e

The Higgs Mechanism

A room full of physicists chattering quietlyis like

space filled with the Higgs field

A well known scientist walks in, creating a disturbance as he moves

across the room and attracting a clusterof admirers with each step...

...this increases his resistance to movement - he acquires mass, just like a particle moving

through the Higgs field.

© CERN

© CERN

Next, a rumor crossesthe room...

…it creates the same kind of clustering, but this time among the scientists themselves.

In this analogy, these clusters are the Higgs particles.

Some Mesons:

_+ : ud charge: +1 : 2.6 ·10-8 s + +

_ _o : (uu - dd)/2 charge: 0 : 0.8 ·10-16 s o _ _- : du charge: -1 : 2.6 ·10-8 s - -

Some Baryons:

p : uud charge: +1 : > 1031 s _n : udd charge: 0 : 885 s n p e- + : uus charge: +1 : 0.8 ·10-10 s + p o ; n +

What we are able to “see” depends on the wavelength

Visible light: 500 nm (green) = 5 10-7 mThis is good for looking at “macroscopic” things: object

A good optical microscope resolves distances of about 1 m.

Electron microscopes can resolve smaller distances.Make use of the wave attributes of the electron:

= h/pWith: h 4 10-15 eV s and p 20 keV/c

0.06 nm(Resolution of an electron microscope: 0.2nm)

-15 -15 810

3 3

h 4 10 s c 4 10 3 10 mλ= 0.6 10 m 0.06nm

p 20 10 20 10

This experiment has also been done

with electrons!

What we are able to “see” depends on the wavelength

Good: small compared to object

Not so good: large compared to object

ParticleEnergy

Proton structure as predicted by: de BrogliewavelengthSimple parton model QCD

107 eV

1011 eV

10-14 m

10-16 m

10-17 m

10-18 m

109 eV

2·1010 eV

We can probe different aspects of particles depending on the type of particles we choose to accelerate

Electrons: - no substructure We have a good idea of what actually collides

Protons: - are made up of “partons” It can be difficult to untangle what happened in the collision

We get a chance to probe the partons

Two electrons collide at high energy:

50 GeV 50 GeV

qq pairs

e eZo

“jet”

“jet”

. .

Two Protons collide at high energy:

7 TeV 7 TeV

qq pairs

pp

“spectators”

gluon

“jet”

“jet”

q

qqgq

q qgq

q qqg

g

Some accelerators and experiments:

Accelerator:

SPS

LEP

LHC

Particles:

p p

e+ e-

p p

CM Energy:

5.4·1011eV (540 GeV)

0.9-2·1011eV (90-200 GeV)

1.4·1013eV (14 TeV)

Experiment:

UA1

OPAL

ATLAS

The LEP (e+e-) accelerator at CERN (Geneva)

UA1(1980’s)

OPAL(1990’s)ATLAS

(2007+)LEP/LHC

SPS

Inside theLEP

tunnel

L3 Experiment

OPAL experiment

How do we find things out?

Particle properties that can be measured:

Trajectory - fast moving charged particles leave a trail of ionization in their wake. Mass - particles ionize matter differently, depending on their massCharge - a charged particle has a curved trajectory in a magnetic field.Energy - particles lose some or all of their energy as they travel through matter - this energy can be detected.

These measurements are done simultaneously or consecutively, in order to gather the maximum amount of information.

tracking of charged particles (p,q,vertex)

e.m. calorimeter (e, stop here)

hadronic calorimeter ( stop here)

detectors (with 4T toroidal B field)

coil (2T solenoid)

A typical collider experiment:

Hermeticity <===> missing energy

(jets)

“Signatures” of particles in a

high energy physics detector

OPALcalorimeter

Inside the tracking chamber of the UA1 experiment

Reconstructed particle tracks of the UA1 experiment

e+

e-

Zoq qp p

e+ e-Zo

q

q

e+ e-Zo

+

-

e+ e-Zo

e+

e-

e+

e-Zo

e+, +, q, ...

e-, -, q, ….

~10-25seconds

Many more possibilities:

e+

e-

Zo/

e+

e-

e+

e-

Zo/

e-

e+

and all of them contribute...

Heisenberg’s Uncertainty Relation:

E t ћ p x ћ

If the lifetime t of a particle is very short, it’s rest mass ( in terms of energy/c ) cannot possibly be determined

any more precise than ћ/t .

We can determine the lifetime of the particle by measuring the “width” of the rest mass.

Zo :

E = 2.5 GeV t ћ/E = 6.6·10-25GeV·s/2.5GeV = 2.6 ·10-25s

qe+

e- /Zo

q_

e+

e-

q_q

Hot topics at the moment:

• Looking for a Higgs particle• Neutrinos “oscillate” (and have mass!)• Unification of all forces?• Are there more particles than we know? (e.g. Supersymmetry) - More dimensions?• Gravitational waves ?• How did and does the universe evolve?• Lots more mass in the universe than we had thought … “dark matter” (what actually is that?)

• W H Y …... ?

?

( http://pdg.web.cern.ch/pdg/particleadventure/ )

There are more things in heaven and earth, Horatio,

Than are dreamt of in your philosophy.

Hamlet, Act1, Scene5

A little bit of kinematics:

2 2 2 2 4E p c m c &&&&&&&&&&&&&&For any particle with mass m:

22 2 2 2 4

( , )

o

P pc E

P E p c m c

&&&&&&&&&&&&&&

&&&&&&&&&&&&&&

“Four momentum”:

2 2 2 2 4( ) ( ) oP E pc m c &&&&&&&&&&&&&&

For several particles (eg: decay products):