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How We Study Particles. Laura Gilbert. Fundamental particles: FORCE (“gauge bosons”). The basics of particle physics!. Matter is all made up of particles…. Fundamental particle: LEPTON. Fundamental particles: QUARKS. The basics of particle physics!. Matter is all made up of particles….

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How we study particles

How We Study Particles

Laura Gilbert


The basics of particle physics

Fundamental particles: FORCE

(“gauge bosons”)

The basics of particle physics!

Matter is all made up of particles…

Fundamental particle: LEPTON


The basics of particle physics1

Fundamental particles: QUARKS

The basics of particle physics!

Matter is all made up of particles…


Fundamental particles: FORCE

Take a closer look at a proton:

  • Three Quarks (two “up”, one “down”)

  • Held together by “gluons”: Strong force


Why do we want to study particles

Generation:

I

II

III

Forces:

u

c

t

γ

QUARKS

g

d

s

b

e

μ

τ

Z

LEPTONS

υe

υμ

υτ

W

Why do we want to study particles?

The “Standard Model”: particles we have detected

charge

+2/3

EM

-1/3

Strong

-1

Weak

0

Weak

We think we know how these interact with each other.


Why do we want to study particles1
Why do we want to study particles?

We are looking for a “Theory of everything”. So

what’s missing?

  • Are these particles “fundamental”?

  • Are there more?

  • What is gravity? (force particle? “superstring”?)

  • How do we get mass?

  • Why is there more matter than antimatter in the universe?

  • How did the universe begin?



Detecting small things
Detecting Small Things

  • To see small things, we need short wavelengths (<~size of object):

target


Detecting small things1

Detecting Small Things

  • To see small things, we need short wavelengths (<~size of object):

target


How do we study the world around us1
How do we study the world around us?

“fixed target”

detectors

target

Particles

Source of high energy particles


How do we study the world around us2
How do we study the world around us?

detectors

“colliding beam”

accelerator


How do we get high energies
How do we get high energies?

Acceleration!

We give particles kinetic energy (and mass) by accelerating them.

It is simple to accelerate charged particles using electric fields – electrons gain 1eV of energy per volt (=1.6x10-19J)

Charged plates

Acceleration

Constant velocity

Constant velocity

Potential difference

-V

+V


Particle accelerators

-

-

-

+

+

-

-

+

+

+

Particle Accelerators

  • Linear array of plates with holes: alternating high energy field applied.

  • As particles approach a plate they are accelerated towards it by an opposite charge on the plate.

  • As they pass through the plate, polarity is switched: plate now repels them. They are accelerated towards the next plate.

“Bunch” of +ve protons


Particle accelerators1

-

-

+

+

+

Magnetic fields curve particle paths

Electric fields accelerate

Particle Accelerators

  • To allow greater acceleration the accelerator is circular.

  • The path of a charged particle is curved in the presence of a magnetic field. The tracks of the particles are curved to fit using dipole magnets:

  • Linear array of plates with holes: alternating high energy field applied.

  • As particles approach a plate they are accelerated towards it by an opposite charge on the plate.

  • As they pass through the plate, polarity is switched: plate now repels them. They are accelerated towards the next plate.


Cern birthplace of the world wide web
CERN (birthplace of the World Wide Web!)

Super Proton Synchrotron (SPS) accelerates protons. Large Hadron Collider (LHC) accelerates further and collides them.

A television is an accelerator in which electrons gain around 10 keV

(10 000 eV).

The SPS will accelerate protons to around 7 TeV (7 000 000 000 000 eV).

The SPS

8.5km

ATLAS – proton beams collide here

The path of the LHC… 100m below ground


The atlas experiment at cern
The ATLAS experiment at CERN

Two protons collide at very high energy, producing

new particles for us to trap and study.

?


Detecting particles
Detecting Particles

We can see particles when they interact:


Electromagnetic
Electromagnetic

Ionisation:

We need to make ionisation "visible“.


Electromagnetic1
Electromagnetic

Ionisation:

We need to make ionisation "visible“.


Electromagnetic2
Electromagnetic

Ionisation:

The addition of highly charged wires turns it into a “Drift Chamber”. The electrons form a detectable current.


Strong and weak
Strong and Weak

Uncharged (neutral) particles are unaffected by electromagnetic force. They only interact via strong and weak interactions.

We can tell where neutral particles are indirectly as missing tracks:

“Kink” in track

Charged particle – detect ionisation

Particles appear from nowhere!

Charged particle decays into charged + neutral


Identifying particles
Identifying particles

Particles can be identified (almost) UNIQUELY by their mass and charge.

These are what we need to measure.


How do we measure

v

-ve charged particle

F

B into picture

F

+ve charged particle

v

How do we measure…

Charge? Use a magnetic field.

For a charged particle in a magnetic field, the force is perpendicular to velocity → particle moves in circular path. The direction of curvature tells us the sign of the charge (“Flemming’s left hand rule”).

Mass? Indirectly…


How do we measure1
How do we measure…

Momentum? Magnetic Field again.

From the radius of curvature of the tracks.

Energy? Calorimeters.

Dense transparent materials. Energetic particles interact,

producing “showers” of thousands of secondary particles

– particles are stopped dead and energy is absorbed.

“Scintillator” material puts out light that can be measured.

shower

Particles slow down gradually


Example bubble chamber

-ve particles: anticlockwise

+ve particles: clockwise

Example – Bubble chamber

B

Charged particles

B field causes paths of charged particles to curve!


Let s identify some particles

Neutral particle decay

-ve particles

π

+ve particles

Let’s identify some particles…

B

electron

electrons from ionisation


The particle zoo

Name

Symbol

Charge (1.6x10-19C)

Mass (9.1x10-31kg)

Electron

e-

-1

1

Atomic matter

Proton

p

+1

1836

Neutron

n

0

1839

Photon

γ

0

0

Pions

π0 / π±

0 / ±1

264/273

Kaons

K0 / K±

0 / ±1

974/966

Muon

μ-

-1

207

The “Particle Zoo”

Identify particles by their charge and mass!

etc…


Over to you
Over to you…

You will be asked to identify some particles

from an e+e- annihilation.

Which turns into a particle/antiparticle pair:

e+

γ

μ+π+ K+ or p

μ-π- K- or p

e-

From their radius of curvature in a B field, find momentum (p =rQB) and then mass (E2=p2+m2)

Electron + positron of known energy

annihilate producing a photon


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