Anatomy of a collider detector

Silicon vertex detectors- small but important

Inquiring minds want to know…

• the energy of the final state particles (calorimeters)

• the momentum of the final state particles (tracking detector surrounded by a magnetic field)

• the trajectory of the final state particles (tracking detector)

Once these quantities are known, one can apply conservation of energy and momentum and reconstruct the collision “event”. This is basically freshman physics, with some relativistic corrections thrown in.

One more thing: you would like to know the identity of the final state particles.

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are needed to see this picture.

Nature tends to be simple…following in the tradition of Mendeleev, assume the large number of observed particles are just different combinations of a few fundamental building blocks.

As with the periodic table, this chart predicted that there should be certain combinations of quark states that had not been discovered yet…and these predictions have turned out to be correct.

Governs how quarks can be combined into composite particles.

In analogy with electric charge, except there are more than two

Gluons are the force carriers for the color force.

A color interaction

Particles are color neutral.

Example: decay of the Z into jets

Like the conservation of mass-energy, momentum and charge, there are other quantum numbers that are conserved in an interaction.Lepton number Baryon number

The tau lepton was discovered in 1977. If there are three generations of leptons, there must be three generations of quarks!

Since then, the “botton” and the “top” have been discovered.

• The top was the last quark to be discovered

• It weighs 175 times more than the proton!

• It has essentially no lifetime.

b

W

t

What is a b quark and why do we care?

• b quarks are very massive- more than 4 times the proton mass

• b quarks are relatively long lived ~1 picosecond

• can travel a measurable distance (about 500 microns before decaying)

b

500 microns

Just as the electric and magnetic force were discovered to be related by electromagnetism (and now we know tat they are both carried by the photon). The weak and the electromagnetic force have been discovered to be different manifestation of the same thing.

The weak force is weak and short ranged because its carriers are heavy. At high energies (where the force carriers are real, and their rest mass energies are negligible) the weak and electromagnetic forces have the same strength. These conditions occurred in the early universe.

…and the history of the universe…

If the weak and electromagnetic forces are different manifestations of the same underlying force, are they all? Was there just one force in the early universe?

In physics, any possible interaction can proceed backwards.The process of neutrino capture is the same as the beta decay of a neutron, but in reverse time. The interaction is rare because the W mass is really heavy…80 times the mass of a proton!

The neutrino is observed by “seeing” theproduct of its interaction with matter.

Electron

Muon

e

e

-neutrino

We don’t see the neutrino coming in, but its products are charged particles.

digital optical modules (essentially a camera)

The “wake” of a muon traveling through the ice

Cable that carries power in and information out

The results of searches for “neutrino-less” double beta decay may give us a clue. In this reaction, two beta decays occur simultaneously (a very rare event!) and if they are one another’s own antiparticle, they can annihilate.

The sun is a fusion reactor Neutrinos are produced in fusion reactions, so for many years people counted the number of neutrinos coming from the sun. They saw too few until they realized that neutrinos are “shape-shifters” and can assume different identities.

e

(subject to change)