PH 103

Dr. Cecilia VogelLecture 24 From the particle adventure

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Review

Outline

Particles Antimatter conservation laws Mesons and baryons

forces: 4 fundamental interactions particles affected force carriers

The Four Fundamental ForcesGravity

Electromagnetic StrongWeak

forces are interactionsWhat about friction? VanderWaals? etc

These are not fundamental,just consequences of electromagnetic interaction of electrons in atoms.

What about strong nuclear force?Just a consequence of the strong force between the quarks in p’s and n’s.

GravityWhat particles feel gravity?

Anything that has mass. Ex: electron, galaxyAnything that has energy. Ex: photon, .Pretty much everything!

Have we seen its effects?Earth goes around SunApples fall on people’s headsNot a sizeable force for microscopic objects

Often ignored in atomic, nuclear, and particle physics

ElectromagneticWhat particles feel EM force?

Anything that has electric charge (or is made up of charged particles)

Have we seen its effects?Electrons are held in atoms.Electrons flow in circuits.Static-y clothes stick together.Magnets stick to fridges.Compasses point North

StrongWhat particles feel strong force?

Anything that has color chargei.e. All quarks

and anything made up of quarksAnd gluon… to be introduced later

Have we seen its effects?Nuclei are held together by itQuarks are held together by it

held so strongly, they cannot be isolated

Decays: alpha decays occur by strong interaction

WeakWhat particles feel weak force?

Anything that has flavorFlavor = what distinguishes one quark or lepton from another.i.e. all matter and antimatter

Have we seen its effects?Does not hold anything together!

too weak!Decays: beta-plus and beta-minus decays occur by weak interaction

Recall Quarks and Leptonsupcharmtop

downstrangebottom

electron (e-)muon (-)tau ( -)

(electron) neutrino (e)mu neutrino ()tau neutrino ()

What forces does each particles feel?

How do Forces Work? One particle is here, another

over there, how do they interact, how are

they aware of each other? Spooky action-at-a-distance? No — they “communicate” by

exchanging particles.

Virtual Particles Exchange particles come into

existence, even if particles exchanging

them don’t lose energy If exchange particle has mass,

the mass energy created from nowhere

What??? Energy from nowhere? Isn’t energy conserved??

Virtual Particles If exchange particle has mass,

the mass energy created from nowhere, but…

HUP allows non-conservation of energy for a very short period of time

E

ht

2

E

Virtual Particles If exchange particle has mass, HUP allows it to exist

for a very short period of time

Where E = (mass of virtual particle)c2

Virtual particle can’t go far, so forces with massive exchange

particles are very short range.

E

ht

2

Exchange Particles AKA force-carriers, gauge

bosons, field quanta. Particles exert forces on each

other by exchanging these particles.

Force-carriers are not “matter” particles

even though some have mass Force carriers come and go;

their number is not conserved. You can create or destroy as many as you want. (unlike quarks and leptons)

Carriers for Each Force Electromagnetic

photon No surprise: Photon is quantum of

electromagnetic energy. Strong

gluon Weak

W-particles and Z-particles Gravity

graviton? are there gravity waves like EM wave? if there are, are they quantized?

Gauge BosonsThere are six different gauge

bosonsphoton gluon ggraviton

Z0

W+

W-

What forces does each particle feel?NOTE: By “feel” we are excluding the fact that these particles are created and destroyed in exchange.

massless massive

Fundamental Particles We now have all the fundamental

particles that have been found (plus one that hasn’t).

Matter 6 quarks

+ and - 6 leptons

charged and neutral Antimatter

ditto 6 Gauge Bosons

3 massless 3 massive

Search for the Higgs Recall that the mass of a

nucleus is less than the mass of its constituent protons and neutrons.

Also, the mass of a proton is only partly due to constituent quarks.

Interactions contribute to mass. What if all mass comes from

interactions? interactions with what?

hypothetical Higgs boson!