intro particle physics

8/13/2019 Intro Particle Physics

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Introduction to ParticlePhysics


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Particle Physics

This is an introduction to the

Phenomena (particles & forces) Theoretical Background (symmetry)

Experimental Methods (accelerators &

detectors)

of modern particle physics

That is, it is not a real introduction to

particle theory (there are other modules!)

Rather, it will attempt to give you theinformation and tools needed to understand

and appreciate the history and new results

in the field


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Particle Physics

Elementary particle physics is

concerned with the basic forces ofnature

Combines the insights of our deepest

physical theories Special Relativity

Quantum Mechanics

Matter, at its deepest level, interactsby the exchange of particles


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Hierarchies of Nature

Animal Life

Biology Chemistry

Atomic Physics

Nuclear Physics Subatomic physics

Particle physics does not and will notexplain everything in nature.

It does provide strong constraints onwhat nature can do


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Probing structure

We see with our eyes by

Light scattered from objects

Light emitted from objects

The size of the objects we can see are

limited by the wavelength of visiblelight

How do we see smaller structure?


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Accelerators and Detectors

Accelerators provide a consistent source of

charged particles traveling at speeds nearthat of light

The energy of the accelerated particlesdictates the kind of physics you are probing

Atomic scale10s of eV (Hydrogen) Nuclear physics10s of MeV (Binding energy)

Particle physics100s of MeV (exciting protonstructure) 100s of GeV (Electroweakunification)

At the lower scales, particles are reallyparticles since you do not perceive theirsubstructure or excited states


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Conserved Quantities: Mechanics

Noethers theorem

For every continuous symmetry of the lawsof physics, there must exist a conservationlaw.For every conservation law, there mustexist a continuous symmetry.

Invariance under Time translationEnergy

Space TranslationMomentum

RotationAngular momentum These quantities are obeyed in any systemon any level

Easiest assumption is that they are obeyedlocally!


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Fundamental Matter Particles

QUARKSEPTONS


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What is a Force?

Every law of physics you have learned

boils down to involving two classes ofphenomena:

Conserved quantities:

Mechanical Energy, momentum, angular momentum

Related to time, translation, and rotation

invariance

Number

Charge conservation, law mass action in

chemistry


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Forces of Nature

Now we know what there is

How do they talk to each other?

We have managed to find four forces:


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How did we get here?

This picture of the world didnt just

emerge naturally It is the synthesis of a wide variety of

experimental data

It is worthwhile to consider howcertain things were discovered


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Radioactivity

End of the 19thcentury

Discovery of three particles emittedby nuclei

Alpha Turned out to be 4He

Beta Turned out to be an electron

Gamma Turned out to be a photon

Amazingalready the strong, weak,and electromagnetic interactions

were visible But they were not distinguishable at this

point


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Proton & Neutron

Rutherford identified the proton as the

nucleus of the hydrogen atom Neutron was discovered by James

Chadwick by bombarding beryllium

with alpha particles

2

4He 4

9Be6

12C 0

1n


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Nucleus

Before Rutherford, people thought the

atom was a diffuse cloud of protonsand neutrons

Rutherford found that there wasscattering off of a point source in theatom

Short distances allowed large momentumtransferseven back-scattering

Like firing a cannonball at tissue paper,and having it bounce back!


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The Electron

Thomson identifiedthe cathode rays as a

new type of matter

Same charge as a

proton

Much lighter!


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Mesons & The Strong Force

But what held the nucleus together

Coulomb forces should repel theprotons

Something stronger must be present

Yukawa postulated a force similar tothe photon, but massive

Strong, but limited in range

Nuclear size suggested / ~ 100R m MeV

f S


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Particles from the Sky! Up in the mountains of

Europe, scientists

detected high-energyparticles in emulsionand cloud chambers

Discovered newparticles which werelighter than nucleonsbut much heavier thanelectrons

New particles

Pion Muon

Similar in mass, butinteracted verydifferently

Th M


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The Muon

Did not suffer nuclear interactions

Rather, was quite penetrating Like an electron, but slower (more

massive) at the same momentum

105.7m MeV

dE

dx 42

NA

Z

A

z2( c)2

mev

2 ln

2mev

2

2

I

v2

c2

Ionization energy lossof charged particles

Th Pi


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The Pion

Other meson events appeared to show a negative

particle which stopped in the emulsion, was

absorbed by a nucleus, and then exploded into

stars (D.H. Perkins was one who observed these!)

The positive particles seemed to stop and then

decay into the previously-seen muons

These had a similar mass to the mesons, but

clearly had different interactions

Recognized as strongly-interacting particles, more

like Yukawas predictions!

135m MeV

A ti tt


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Antimatter As soon as Dirac combined

Special Relativity

Quantum Mechanics

in a way that was symmetric in

space & time, he found that his

equation described spin-1/2

particles

It also predicted negative energysolutions for fermions

Predicted anti-particles in

nature, with opposite charge but

same mass

Anti-electron positron wasdiscovered in cosmic rays

Andersons cloud chamber

Curvature gives momentum

Length gives rate of energy loss

Only consistent withlight positive particle


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A l t


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Accelerators

Cyclotron Linear Accelerator Synchrotron

D t t


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Detectors

Making subatomic particles visible to human

senses

Most commonly-used principles

Scintillationcharged particle produces light

Ionizationcharged particle produces charged ions

Magnetic spectrometerstracking a particle through a

magnetic field: p (MeV) = .3 qB(kG)R(cm)

B bbl Ch b


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Bubble Chamber The bubble chamber was the

most instructive detector of

the early years Liquid kept under overpressure,

but below the boiling point

When particles passed through,

stopper pulled out, reducing

boiling point and bubbles

formed around tracks

Photograph of tank created a

full image of the event

However, slow and difficult to

extract only the events you

wanted (e.g. for rare particles)

These days, the granularity andcomplexity of the collisions

have made the bubble chamber

obsolete

But excellent for pedagogy!

St P ti l


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Strange Particles

In cloud chamber, bubble

chamber and emulsion

experiments new particles

were being discovered at a

fast rate in the 40s and 50s

Some particles appeared to be

Produced immediately (strong

interactions)

Decaying only after a

considerable time (weak

interaction)

Produced in pairslooks like a

quantum number

Given name strangeness

C d titi


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Conserved quantities

Without detailed understanding of theinteractions, particles were classified bytheir quantum numbers, in the hope thatsome scheme would emerge

Multiplicative

Paritybehavior of wave function under spatialinversion

Charge conjugationsymmetry if charges wereflipped

Additive Isospinused to group particles into doublets

and triplets, like an internal spin

Strangenesscharacteristic of long livedparticles


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Th P ti l Z


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The Particle Zoo

Pre-standard model particle physics was

characterized by an increasing particle zoo

Quark Model


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Quark Model Gell-Mann and Neeman

explained the spectrum ofhadronic states with similar

quantum number by means ofquarks

Baryons (p, n, L) have 3 quarks

Mesons have one quark, and oneanti-quark

Transform states into each

other using rotations UpDown

DownStrange

StrangeUp

Particles with similar spin andparity fell into multiplets

SU(3) symmetry increasinglybroken with increasingstrangeness

Predicted unobserved states,like W

D

S

I3

DDD

S S S

Wq qq

~ 1230m MeV D

~ 1385m MeV S

~ 1530m MeV

~ 1672m MeV W


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The Later Years


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The Later Years After the quark model, the zoo reduced to six microbes. Then

it became chase after heavier and heavier particles

t

Weak and Strong Interactions


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Weak and Strong Interactions

While weak and strong interactions

were now extensively studied, andtheoretical concepts existed for their

deeper structure, experiments were

still limited in energy Thus, difficult to probe

Force carriers of weak interactions

Substructure of hadrons

Partons


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Partons For a long time, quarks were seen as simply a convenient

mathematical tool to account for quantum numbers

No evidence for free quarks in nature Scattering experiments at SLAC did the same thing as Rutherford

Found that large momentum transfers were possibleas if the proton has

pointlike consituents

Measured structure functions that characterize the momentum

distributions of the pieces of the proton

Electroweak Unification


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Electroweak Unification

Many features of the weak interactions

Long lifetimes

Parity violation

Isotropic decays

Explained by

Heavy intermediate bosons (like the Yukawa force, but

much shorter range) Coupled to left-handed fermions

The features were then unified with the

electromagnetic force by Glashow, Salam and

Weinbergwho received the Nobel in 1979 The weak force is carried by W and Z bosons of M~90 GeV

The massless photon is induced by the presence of a

condensate of Higgs bosons, that spontaneously breaks

the symmetry of the interaction

Charmed Particles


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Charmed Particles A case where theory led

experiment

Weak interactions seemed torequire a change of strangeness

Neutral currents not seen in

decays of kaons to pions Always

a change in charge

This was explained naturally bythe existence of a fourth quark

The J/Yparticle (M~3.1 GeV!) was

found near-simultaneously at BNL

and SLAC in 1974!

Not just a new quark: Completed the second family of

quarks and leptons

Nobel prize awarded in 1976 (just

two years later)

llK

llK 0

p p

Tau & Bottom


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Tau & Bottom

As energies increased in both e+e- colliders and

fixed target proton beams, new particles started

appearing in the mid-70s

Mark II observed strange events with one electron

and one muon

Suggested new lepton that decayed into e or m

Leon Lederman et al observed new peaks around

10 GeV.

Suggestive of yet another quark m~5 GeV

A new family was found

Required another neutrino and another quark

Took around 20 years to find both!

ee t tt t


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W&Z


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W&Z Electroweak unification

required W and Z

Found by Carlo Rubbia andcollaborators at the CERN

SppS exactly whereexpected!

MW~ 80 GeV

MZ ~ 90 GeV

Another case of theoryleading experiment.

But experimentalists got theNobel in 1984 (3 years later!)

The collider era had reallybegun!

eW e

0Z e e

Colliders in Use


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Colliders in Use

Tevatron, p p 2 TeV

HERA e p 30 900 GeV LEP, e e- 91-209 GeV

RHIC, Au Au 200 GeV/N


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Neutrino Oscillations


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Neutrino Oscillations Super-Kamiokande is

originally designed to

search for proton decay 50k tonnes of water

11k phototubes to detectlight

98 Detected a

significant deficit ofmuon neutrinos,especially when comingthrough the earth

Fit hypothesis ofneutrinos oscillatingchanging flavor Not part of the standard

modelyet!


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