PH 103 Dr. Cecilia Vogel Lecture 24 From the particle adventure webpage

Review Outline  Particles  Antimatter  conservation laws  Mesons and baryons  forces:  4 fundamental interactions  particles affected  force carriers

The Four Fundamental Forces  Gravity  Electromagnetic  Strong  Weak  forces are interactions  What 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.

Gravity  What particles feel gravity?  Anything that has mass. Ex: electron, galaxy  Anything that has energy. Ex: photon,.  Pretty much everything!  Have we seen its effects?  Earth goes around Sun  Apples fall on people’s heads  Not a sizeable force for microscopic objects  Often ignored in atomic, nuclear, and particle physics

Electromagnetic  What 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

Strong  What particles feel strong force?  Anything that has color charge  i.e. All quarks  and anything made up of quarks  And gluon… to be introduced later  Have we seen its effects?  Nuclei are held together by it  Quarks are held together by it  held so strongly, they cannot be isolated  Decays: alpha decays occur by strong interaction

Weak  What particles feel weak force?  Anything that has flavor  Flavor = 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 Leptons up charm top down strange bottom 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

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)c 2  Virtual particle can’t go far, so  forces with massive exchange particles are very short range.

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 Bosons There are six different gauge bosons photon  gluon g graviton Z0W+W-Z0W+W-  What forces does each particle feel?  NOTE: By “feel” we are excludin g the fact that these particles are created and destroyed in exchange. masslessmassive

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!