Tuesday, 6 May 2014

Pheno 2014 Liveblog: Day Two, Session 1

The conference moves to its second day.  Last night was the banquet, so I missed the first talk this morning as a result.  But it was an LHCb talk, so I regret nothing.  The morning session is mostly about flavour physics, with experiment, theory and lattice.

9:20 am: Flavour Physics in the LHC Era, Alexey Petrov

Start with the connection between flavour physics and baryo-/leptogenesis.  Within the SM, the CKM mechanism is well established.  But the whole theoretical basis for this is simply unclear; why are their large hierarchies among the SM flavour couplings?  All of this points to NP in the flavour sector, but then we have the long-standing problem of not seeing it anywhere, even when the SM process is highly suppressed.

This motivates model building, which then motivates placing limits.  Three types of analyses:

  1. Relations among processes allowed in SM, e.g. CKM unitarity triangle.
  2. Processes suppressed in SM, e.g. penguin diagrams.
  3. Processes forbidden in SM, e.g. D0 to p π ν.
Some examples, none of these look unusual.

9:55 am: Modern Lattice QCD: Progress and Prospects, Ruth Van de Water

Many SM precision measurements (especially flavour) are still limited by theory uncertainties.  This is because QCD is an opaque mess.  Hence, precision lattice calculations are still of high priority in probing new physics in precision experiments.

Example: lattice calculations needed to measure all CKM measurements except Vtb.  Some recent progress:
  • Vcb from B to D decays, theory and experimental error commensurate
  • Vus from K to π decays, theory ~ 1.5 times experimental error; error on Vus now smaller then error on Vud!
These processes are good as they involve one hadron to one hadron, with hadrons that are stable under QCD.  Other processes still less good.

Looking to the future, many applications but focus on two: muon g-2 and Higgs decay modes.  The  muon g-2 is a long standing discrepancy between theory and experiment.  The theory calculation is a colossal effort, but is now at the point where hadronic effects in loops are limiting the accuracy.  In particular, light-by-light scattering is a definite problem.  The hadronic contribution to the vacuum polarisation is also a problem.

Some discussion of the plans and progress in this direction, but cut short for time reasons.  This is promising, though.  Cleaning up this theory error will help clarify if this discrepancy is a true one, or just yet another fake teasing and taunting us.

For the Higgs side of things, we are concerned with the uncertainties in quark masses, which of course amount to uncertainties in the Yukawas and hence theory uncertainties in the Higgs partial widths.  This is only an issue at future Higgs factories, of course; the accuracy available at the LHC, even the HL-LHC, probably isn't good enough.

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