Friday, 13 November 2015

KIAS-CFHEP Workshop Liveblog: Day Five Session Two

The last day of the conference, and the day after the banquet.  As seems to be traditional, I overslept and missed the first session.  Oh well.  I missed two ILC talks that would probably have been interesting, but so be it.

11:20 am: Physics at a 100-TeV Collider, Tao Han

I'm guessing this talk will be similar to the seminar I saw Tao give recently.

HEP is at an extremely exciting time.  The completion of the SM means that for the first time we have a complete and consistent relativistic quantum theory.  Indeed, our theory seems valid up to the Planck scale (possibly).  But we've thought this kind of thing before (19th century).

Indeed, the central questions today are not about details, but fundamental: origin of spacetime, UV/IR connection, real underlying theory.  In this line, let us consider three questions.

1. The Nature of EWSB?  All we really know is the Higgs potential in a tiny region about the vacuum.  What does the full potential really look like?  The first and most important question here is the nature of the Higgs triple coupling.  This can consistently vary by order-1 factors relative to the SM!  This would have huge effects on the cosmology, by allowing a 1st-order phase transition.

2. Naturalness.  Possible theoretically nice explanations: true naturalness and NP at the TeV scale; a fundamental and deep connection between UV and IR physics; some kind of multiverse/anthropic selection effect.

3. Dark Sector.  In particular, Higgs portal will be probed in the next generation of detectors.  Additionally, these can be probed using measurements of Higgs physics at colliders.

The immediate collider future is, of course, the LHC.  Looking further, and ignoring linear colliders, two rough ideas.  First is possible US/European electron-positron collider of few hundred GeV.  But real interest here is, of course, a next generation hadron collider running at 100 TeV.  The Chinese proposal recently confirmed is the one.

Possible Europe timeline: LHC till 2020, VH-LHC till 2030.  Hope to have maybe TLEP ready then for another decade, followed by ...?

Chinese machine recent announcement a surprise to scientists!

100 TeV leads to qualitatively new physics.  Note that top quark at these energies is as massless as the bottom quark was at the Tevatron!  So all SM particles are partons, and really working in unbroken EW phase. Even pure QCD events will be studied in completely new regimes; e.g. dijet invariant masses of 50 TeV, one event per iab.  Guage boson FSR will be like radiation in QCD jets.  Probe of LHC self-coupling can be resolved to order 10%.  Also tests compositeness of Higgs (as a function of scale).

SUSY mass reach improved typically be a factor of 7.  More generally, fine-tuning can be pushed to 10-4, which more or less answers that question.

Dark matter searches good in regions where DD fails: very light particles, or very pure winos/Higgsinos.

Finally, there's the potential to find something unexpected.  Standard resonance searches give us some idea of reach.  Tens of TeV in some channels.

No interesting question, but Tao got provoked into the most animated speech I've ever seen from him.  The crux is that we don't need to find something to justify this kind of experiment; simply understanding nature at a better scale than anyone else ever before is enough, at least it should be for scientists.

12:00 pm: Higgs Physics and Detector Design at CEPC, Manqi Ruan

Plans to run China first as electron-positron tunnel, then upgrade to hadron collider.

Detector goals: high precision vertex tracker, close to IP, for bottom, charm and tau tagging.  High precision tracking to measure transverse momentum for TeV tracks at 10% level.  Calorimeter suitable for jet substructure etc.  Calorimeter benefit from development of microelectronics allowing ultra high granularity.  Improve by ~ 1000 over LHC, gain 3D granularity.

Goal: identify every detector hit.  Substantial pile up.

Measurement of Higgs width using production cross section (shape as function of energy) to resolve the coupling ambiguity.  Also direct measurement of line width (3%) and tag of recoil to measure invisible width (better than 10%?).  Leads to model independent measurements of Higgs couplings.

Hadron machine run benefits from luminosity, lots of tops, and energy to measure top Yukawa.

And we finish 20 minutes late. 

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