Saturday, 31 August 2013

SUSY 2013 Live Blog: Final Session

We end with some review and summary talks.

10:30am: John Ellis, "Outlook for supersymmetry"

A clue about John's opinion: his t-shirt has "MSSM" written on it.

I recognise the opening slide from Cairns.

"Is outlook for SUSY black?  Bullshit!"  Claims Higgs observations provide more evidence for SUSY, not less.

The LHC paradox: a light Higgs plus nothing else?  If there is something else light, why haven't we seen it?  If there is nothing light, is the Higgs unnatural?  There has been a high mortality rate among theories, but not SUSY.  Comment on split-/High scale-/etc. SUSY, "If you can't say something good, say nothing".

SUSY anywhere is better than SUSY nowhere.

Mh-Mt close to stability bound.  Instability sets in at 1010-1013 GeV.  Claim that we do need to fix this.  Natural way to do this is to add a new stop-like scalar, but that only brings more fine-tuning and stability issues.  These are best solved with a Higgsino.

"Anybody sensible would say that the Higgs that has been observed is the lightest supersymmetric Higgs."

Various data that has to go into global fits.  Big question is g-2.  Is there low-scale SUSY, as this suggests?

2012 data does not rule out as much of CMSSM parameter space as you might expect.  Even best-fit point of pre-2012 is still fine.  CMSSM has two preferred regions now, one with gluinos just above the exclusion limits and another at 4-5 TeV; latter is probably too heavy for LHC.  Stops have similar masses.  Stau is NLSP, can be as light as several hundred GeV.  Can be long-lived; small mass splitting with LSP (less than tau mass) is preferred in global fits.

Planck evidence: simple SUSY inflationary models fit the constraints, especially no-scale supergravity.

DM DD: Probe SUSY parameter space by 2020.

DM positrons: probably astrophysics, but can use smooth spectra to place constraints.

What's next?  A Higgs factory; LHC upgrades?  ILC?  CLIC?  TLEP?  TLEP could test SUSY predictions in deviations of Higgs couplings.  Most alternatives could not.  TLEP could, like LEP, be later replaced with hadronic collider (100 TeV).

Note: we had to wait 48 years for the Higgs.  We have only been waiting 40 years for SUSY—keep the faith!

11:10 am: Nima Arkani-Hamed, "Supersymmetry 2033"

The title is based on the fact that this is the 21st SUSY conference, so look 20 years back and 20 years forward; plus, of course, the fact the LHC is giving us results and will change things.

The talk is a motivation for a 100 TeV pp machine.  It is an obvious direction.

Precisely what we need to do depends on what is seen at LHC 13/14.  But "every scenario I can envisage will need a 100 TeV machine."  Not obvious it would have been, e.g. had we found light SUSY.

A lot of technically interesting questions, but focus on the physics question.

1. The ultimate fate of naturalness.  Higgs discovery is crucial; our vacuum is qualitatively different to a random condensed matter system.  In none of our studies of EFTs in the lab do we see a light scalar without deliberate external fine-tuning.

Naturalness has been invoked before the Higgs, and every time it worked.  They are: the classical self-energy of the electron, infinite.  Leads to models based on a cut-off; models where wrong, but there was new physics at that scale (QM); in fact, even earlier than it needed to!  (Due to weak EM coupling.)

Second case is the pion; quadratic QED corrections to the pion mass.  Again, quadratically divergent contributions cut-off by the rho, again came in earlier than it needed to.

Third is Kaon mixing.

It may well have worked before, but where is everybody?  This is not a new problem; there has been a tension from the beginning because of the SM approximate symmetries that cannot be broken at the TeV scale.  Not problems, opportunities!

Naturalness has failed dramatically before.  Aristarchos's model of the solar system was unnatural, since the distant stars had to be ridiculously far away!  More recently, we have nuclear physics; consider the small deuteron binding energy, or the fact that two neutrons are not bound by a 1% factor.  Related to up/down quark masses?  And, of course, the cosmological constant problem.

You can use history to argue either way.  Worth keeping that in mind.

The question of if the Higgs scale is natural is then a deep, structural question about the foundation of fundamenal physics.

a. What if LHC sees nothing else.  Tuning of 1% in Higgs mass.  Is that convincing to kill naturalness?  Well, we have seen this kind of tuning before (neutrons!) and has not forced us to make a radical departure in our philosophy.

We can only properly address this question by going to higher energies.  If we find something, we end the discussion.  If we find nothing, we get a quadratic gain in the tuning with the energy of the machine!  So a 100 TeV collider will push the tuning down to 10-4, hundred times worse than anything seen in particle physics before.  Much much harder to dismiss this.

b. Fine-tuned theories could exist that are not anthropically motivated.  Example: find a TeV-scale Higgs, nothing else up to 100 TeV.  Now have worse tuning, and no anthropic reason!  Force us to really reexamine our entire point of view.

c. If we unlucky and NP is just outside of LHC, obviously 100 TeV machine will find it.  Also, consider split scenarios; minimal picture prefers gluino below about 20 TeV, so should be found.  Also, if found and they decay in the detector at all, that tells us new scale not too far away.

d. What if LHC discovers (relatively) natural spectrum?  Will be too heavy to study at a low-energy lepton collider, while coloured particles will not be produced in large numbers of LHC.  Plus, if we do see a natural spectrum then a number of particles will be too heavy to see at the LHC.

2. Robust probe of few TeV electroweak particles, e.g. WIMPs.  Impossible at LHC.

3. Collider flavour physics.  Can not have theory of flavour at 1 TeV, can at 30-40 TeV.  (e.g. RS).  Not guaranteed, by any means, just possible.  Tops produced in large numbers.  Various low-energy probes of CP-violation, EDM etc; if they find anything, the scale must be in the 10s or 100s of TeV.

Summary: "The scientific questions at stake in our field are the deepest ones we have encountered in 50 years."  Our ambition should match the size of the question.  The time to start thinking about this is now.

And with that, SUSY is effectively over.  We have one final talk telling us about SUSY 2014, which will be in Manchester next July.  However, I don't think I need say much about this.

I will write my own thoughts on the conference as a whole some point in the next few days.  First, I'm indulging in a little sightseeing this weekend, and I also need to write a seminar talk by Thursday!

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