Quarks, Leptons, Bosons, the LHC and all that.

Tony LissOLLI Lecture

September 23, 2008

Some HE Physicist Principles

• We are reductionists (and proud of it!)– Our worldview is that there are a small

number of fundamental constituents, interacting via a small number of forces, that make up the Universe as we know it.

– This picture has worked extremely well for about 2000 years.

– The modern version has been untangled using particle beams of ever increasing energy.

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The Standard ModelThe matter around us is made up of “quarks” and “leptons”

And held together by four forces, each with a force carrier:

A proton is made of u u d

Add an electron to make a hydrogen atom Electromagnetic

StrongWeak

Gravity

The Standard ModelThe matter around us is made up of “quarks” and “leptons”

particleadventure.org

The marriage of quantum mechanics and special relativity required that antiparticles exist.

Helium Atom

Why High Energy?

• From quantum theory we know ~ 1/p Wavelength is inversely

proportional to momentumIf you want to see small things you need short wavelengths (that’s why electron microscopes were invented) and short wavelengths means high momentum (and energy).

• From relativity we knowE=Mc2

If you want to create a heavy particle (large M) you need a lot of energy.

Unification of the Forces

ElectricMagneticWeakStrong

ElectromagneticElectroweak

“Low Energy”

“High Energy”

“Very (very)High Energy”

Theory works up to ~hereThat’s the region we want to probe with the LHC.

Part way to Einstein’s

dream!

Higgs Bosons born here?

CERN- LHC

Fermilab

Protons & anti-protons collide at 2 TeV (2 x 1012 electron volts)

The worlds highest energy particle accelerator!!

Fermilab Makes Top Quarks

The heaviest known elementary particle.Discovered in 1994!Why is it so heavy??

We don’t know

pp tt

eW b e b W b udb

The Large Hadron Collider

The world’s largest, highest energy, accelerator 300 feet underground outside of Geneva, Switzerland. The LHC collides intense beams of protons 40 million times per second at “14 TeV”

2.7mi

France

Switzerland

Grapes Cows

Inside the Tunnel

ATLAS Detector at CERN

ATLAS is VERY BIG

ATLAS

Who Is ATLAS?

• ATLAS is one of four large experiments at LHC– The ATLAS collaboration consists of

• ~2500 physicists including• ~700 graduate students from• 169 different institutions in• 37 different countries

ATLAS is a United Nations of particle physics.

ATLAS, The Movie

http://atlas.ch

Some of What LHC Can Study• Higgs Boson

– Understanding M• Supersymmetry

– Dark Matter?• Extra Dimensions

– Quantum Gravity/String Theory• Dark Energy

– We don’t even know how to look for this• Heavy gauge bosons

– New forces?• Precision top quark studies

– New physics?• Diboson production

– From the Higgs?• Quark and lepton substructure

– Are fundamental particles fundamental?• etc. etc.

Let’s Pick Two

• Higgs Boson• Supersymmetry

What is “The Higgs”?

• Named after Peter Higgs

• It “gives mass” to the fundamental particles (if, in fact, it exists)

Without the Higgs (or something) the theory requires that all these fundamental particles have M=0. But we know that’s not the case.

F=MaThe idea is that the Higgs field exists throughout all space. As particles try to move through this field they interact with it and are “slowed down”. Heavier particles are those that interact more strongly

M=F/a

In quantum mechanics there is a particle associated with a field (quantum of the field). The photon is the quantum of the electromagnetic field. The Higgs boson is the quantum of the Higgs Field.

Finding the Higgs• The Higgs “couples to mass”

– It decays to the heaviest particles available

Easy, but rareHard, but copious

This is a simulation of the production and decay of a Higgs to two Z bosons.

The Z bosons themselves decay, one to a pair of electrons and the other to a pair of muons.

0 0 0pp H Z Z e+e-

+-

Supersymmetry (SUSY)• Every quark, lepton and force

carrier has a SUSY partner (sparticles).– Sparticles would be made

copiously in the early (HOT) universe.

– They all decay away quickly, except for the lightest one (neutralino), which has nowhere to go.

www.science.doe.gov/hep/EME2004/03-what-is.html

Make SUSY particles at an accelerator:

pp

SUSY & Unified ForcesWhy is this an attractive idea?

• SUSY helps with unifying the forces.• SUSY is a necessary ingredient of quantum

gravity theories.• We know that the universe is filled with dark

matter.– Dark matter is not made of quarks and leptons – the

Standard Model has no dark matter candidates.– Dark matter interacts very weakly with normal matter

(or else we would have found it already).– The lightest SUSY particle is a perfect candidate.

• Einstein’s dream of a “Unified Field Theory”, now needs SUSY:

Energy Stre

ngth

of f

orce No SUSY

Energy Stre

ngth

of f

orce SUSY

EM

weakstrong

Dark Matter

Dark Matter’s Everywhere

Speed of stuff out here

Doesn’t match luminous matter in here!

In Galaxies

Motion of a galaxy out here

Doesn’t agree with luminous matter in here

The “Hydra” Galactic Cluster

And clusters of galaxies

Physics 211

• Momentum is “conserved”

– Before the protons collide they have equal and opposite momentum: The total momentum is zero.

– Therefore: The total momentum of all the stuff created in the collision must also be zero.

A Simulated SUSY Event

Missing momentum carried away by invisible particle

About Those Black Holes…

• Creating microscopic black holes at the LHC would be – A MAJOR BREAKTHROUGH IN SCIENCE!– INCREDIBLY EXCITING– NOBEL PRIZE STUFF– NOT AT ALL DANGEROUS– REALLY

About Black Holes

• The microscopic black holes that might be created at LHC are so small they evaporate instantly according to Steven Hawking.– But what if Hawking’s wrong?

• Cosmic rays reach much higher energies than the LHC and have been having collisions for billions of years – any black holes created have not done much damage.

About Black Holes

• Black holes don’t suck everything in.– Only stuff inside the Schwarzschild radius.– If the sun suddenly became a black hole, the

Earth’s orbit would not change (but we’d get very cold). The Schwarzschild radius of the sun is 3 kilometers.

– The Schwarzschild radius of a 1 TeV black hole is about 10-18 m – that’s about 1/1000th of the size of a proton.

• There’s nothing around such a black hole for it to suck in!

Final Words

• After 25 years of planning and 15 years of design and construction, the LHC is finally about to turn on.

• This is the chance of a lifetime.• Our understanding of the way the

Universe works is about to be revolutionized.– We just don’t know exactly how…