HC 209H: Discovery of Fundamental Particles and Interactions Final Lecture: Open Questions in

Chris Potter

University of Oregon

HC 209H: Final Lecture – p.1/12 A Spectacularly Successful Theory...

Credit: Wikimedia HC 209H: Final Lecture – p.2/12 ...But the Theory Blows Up!

Credit: Wikipedia

 The Feynman diagram above, showing a with a top quark loop, is called the Higgs boson self energy diagram.  When this diagram is calculated, it is found to diverge - the answer is infinity! To paraphrase David Kaplan (Particle Fever), there is a sickness deep in our theory.  This is not just a mathematical problem unrelated to physics. It suggests our understanding of physics is incomplete.

HC 209H: Final Lecture – p.3/12 A Supersymmetric Solution

 One solution to the divergence of the Higgs self energy is to postulate a new symmetry, called (or SUSY).  SUSY predicts that for each fermion, there is a bosonic partner, while for each boson, there is a fermionic partner.  Formally, this looks very much like antimatter, except that the masses of particle and partner are not identical (broken symmetry).  Then there is an additional Feynman diagram for the Higgs self energy with a stop squark loop (right) which almost exactly cancels the diagram with a top quark loop.  There will also be the other squarks, sleptons and gauginos, and a higgsino.

HC 209H: Final Lecture – p.4/12 Stop Squark Exclusion at the LHC

Credit: CERN/ATLAS HC 209H: Final Lecture – p.5/12 Matter/Antimatter Asymmetry: Why?

Credit: Symmetry Magazine

 Matter and antimatter should be created in equal amounts at the start of the universe, then annihilated in equal amounts over time.  But we observe (luckily!) that after annihilation of all antimatter some matter remains. Why?

HC 209H: Final Lecture – p.6/12 Dark Matter: 85% of the Matter in the Universe!

Credit: Phys.org

 The Standard Model particles comprise only 15% of the matter in the universe.  We know dark matter has mass and zero charge, but that’s about all. Is it a SUSY particle?  Royal Institution Lecture with Frank Close: Looking For Dark Matter

HC 209H: Final Lecture – p.7/12 Fermion Generations: Why 3? Why Any?

 The fermions repeat a pattern in three generations. Why?  The situation is very much like with the Periodic Table of the elements: order and pattern, but no underlying explanation.  Is there some fundamental structure we do not yet understand that explains why? Presumably yes.  Are there more generations? Must both quarks and leptons have another generation?  The Large Electron Positron Collider (LEP) measured the Z cross section (right).

 Assuming 2mν < mZ , more than three generations is ruled out. Credit: CERN/ALEPH HC 209H: Final Lecture – p.8/12 Muon Magnetic Moment: QED Fail or NP?

 The magnetic moment ~m of an object is a measure of how strongly it reacts to an aligning magnetic field B.  The torque ~τ the object feels is ~τ = ~m × B~ .  The muon magnetic moment has been calculated very precisely with many Feynman diagrams (some at right), to very high precision in QED, with small uncertainty.  The measurement of the muon magnetic moment has also been measured with very high precision, and the result is discrepant with the QED calculation at a significant level.  Is QED broken (unlikely) or is there new physics (NP) which give extra Feynman diagrams? Like SUSY? Credit: Science 2.0

HC 209H: Final Lecture – p.9/12 Grand Unification: Doesn’t Happen Without NP

Credit: Stephen Martin, SUSY Primer

 U(1) is electromagnetic coupling, SU(2) is weak coupling, SU(3) is strong coupling.  As you run to higher energies (or, back to the early universe), the couplings evolve.  No unification with only SM particles (dashed), but maybe include NP. SUSY (red/blue)?

HC 209H: Final Lecture – p.10/12 Concluding Remarks

 Open Questions: We’re Not Done Here  What new physics remedies the Higgs self energy sickness deep in the Standard Model?  If it’s SUSY, the non-excluded masses are getting out of reach of the LHC.  Why is there an imbalance in the matter/antimatter ratio? Why are we even here?  What is dark matter? It has nonzero mass but zero charge. Can we see it in colliders?  Why are there 3 fermion generations, each the same but with different masses? What is the underlying order?  Future Colliders: We’re Not Done Here  International Linear Collider (ILC): based in Japan with international participation  (CLIC): based in Europe? with international participation  (FCC) or Circular Electron Positron Collider (CEPC): Where?  You: We’re Not Done Here  UO punches above its weight in particle physics. Bug Jim Brau (ILC) or Laura Jeanty (ATLAS SUSY) if you want to get involved. Or me.  It has been a pleasure teaching a group of motivated, enthusiastic and conscientious students during this very strange global crisis.  To the extent I know you, you underestimate yourselves. Don’t be afraid to fail. Be bold.

HC 209H: Final Lecture – p.11/12 Democritus: The Laughing Philosopher

Credit: Wikipedia Paintings of Democritus by ter Brugghen (1628) and Rembrandt (1629). Democritus was known as the laughing philosopher for his capacity to see humor in human folly. When will we find his atom?

HC 209H: Final Lecture – p.12/12