## Friday, 3 May 2013

### Brookhaven Forum 2013 Day 2 Blog II

This session is more on strongly interacting stuff, so I'm less optimistic, this being somewhat away from my primary interests.  We'll see.

Talk 3: 10:50am: Tim Gershon, "Flavour Physics Circa 2013"

Tim's part of the LHCb collaboration.

The basic flavour problem is that the CKM matrix has obvious structure but lacks explanation.  It must become more relevant as naturalness recedes.

Two relevant flavour approaches to new physics:
• CP violation: must exist (baryon asymmetry), but not necessarily in the quark sector.
• Rare decays; expected for generic new physics, but lack of observation is problematic.  Still, often sensitive to higher scales than the standard energy frontier.
Interesting point: a lot of work still to be done with the datasets of the B factories, Tevatron, even CLEO.  Should know this, I've had models constrained by the first of these.

But the LHC was built to be the best, and the only real limit right now is the time it takes to get through the data.

Hah, I was right; LHCb was the first LHC experiment to see a new state (new hadronic states).  Looking at this slide, it's still up three-to-one versus the general-purpose detectors.  Or wait, maybe I should actually read the slides: two discoveries at LHCb versus 1 (plus the Higgs) at ATLAS and CMS each.

Unconventional states:
• X(3872): above the open charm threshold, but narrower than expected.  Obvious question, can it be addressed outside of the lattice?
• JPC = 1++, supports "molecular" interpretation but that raises more questions.
• Charged bottomonium states: show up in decays of Upsilon(5S); also possibly molecular states.
Unitarity triangle; emphasising success.  I remember a few years ago there were suggestions it was starting to fail, did those results go away?
• Direct observation of CP and T violation, and thus a test of the CPT theorem.
• Results all seem very good for the SM.  Why does that surprise me?  I must be going daft.
• Something I didn't quite realise.  In fifty years since CP violation was first observed, CP violating behaviour has still been seen in only four particles.
• Ah, some NP: the semileptonic asymmetry in B mixing.  We saw this already at this conference in the Tevatron talks yesterday.
• However, LHCb has reduced the significance suggesting it was just a statistical effect.  Sometimes it feels like we're cursed.
Still some questions:

• CPV in Charm sector
• B to tau nu and B to D tau nu (3.4 sigma in latter case)
Rare decays
• Bs → μ μ apparently gold standard
• 3 sigma significance observation at LHCb
• Still consistent with SM, but large error bars.
• Oh look, yet another thing killing off large regions of the MSSM parameter space.
• Interesting hint?  B to K mu mu, isospin asymmetry which seems, when integrated, to be a 4 sigma effect.  Didn't give a good value for the deviation from SM...
Future prospects:
• Tau-charm factories?  I need to read up on that.
• LHCb detector, esp. trigger being upgraded.  Needed to properly gain the benefits of higher instantaneous luminosity.
In summary, this was an interesting and well-given talk despite being mostly review.  Part of that might just be that the material reviewed is not so familiar to me.  I should probably look at flavour physics in more detail, since there are several hints there.

Q & A: A good point raised: the LHCb can set interesting results in lepton physics.  This is especially true for taus.

Talk 4: 11:30am: Aude Gehrmann-De Ridder, "Perturbative QCD for the LHC"

I'm half-tempted to skip this talk, to be honest.

Perturbative QCD is important at the multi-purpose experiments, because it sets backgrounds.  Which we can't ignore.  Grumble, grumble.

Looking through the slides, it looks like this talk will hit the recent NLO calculations involving high numbers of final state jets.  This work is very impressive, and I should probably find it more interesting than I do.  I'm stealing a figure to illustrate the rate of recent progress:
It took years to go from one to two final states at NLO, or from two to three.  Now?  An extra state nearly every month.

These calculations have the nice factor that they are guaranteed to be useful.  Cutting the theoretical uncertainties down to a few percent, or order one percent at NNLO, is useful to any experimental measurement.

I'm sorry I'm not writing much here.  As I said, I can recognise the importance of this work but it just isn't clicking for me.

There's a good coverage of how this is relevant for Higgs studies.  We're in or approaching the percent-level domain already, and full studies will require understanding e.g. Higgs plus one jet.  Diphoton production also relevant here; a theoretical prediction for the Higgs background would be nice.  But it seems that there are non-trivial issues with photon isolation (photons must be isolated from jets at hadronic colliders, to avoid fakes from neutral pion decays).

I've just noticed: it seems like every paper she's citing is authored by Lance Dixon (with collaborators, of course, but still).