Friday, 8 July 2016

SUSY 2016 Liveblog: Day Five Session Two

It's already the last session of the conference, a mixed selection of talks.

11:00 am: Belle II Physics and Construction Status, Tom Browder

Belle II is the first new collider since the LHC turned on.  Working, of course, on the intensity rather than the energy frontier.

A line of attack of interest to me is dark photons.  It's good to see efforts being made to address invisibly decaying dark photons.  Of course, that's not that new (I used projected limits in a paper several years ago) but that was based on what theorists thought could be done.

B physics is a major planned program.  Leptonic decays B to τν are a good avenue to probe the Higgs sector, in particular the presence of charged Higgs.  Amusing difference between theorists and experimentalists.  Xerxes Tata stated that type-III 2HDM is "FCNC hell"; for an experimentalist, with so many signals it's FCNC heaven!

Search signal is challenging: B decays to charged track plus nothing.  But pair produce Bs, which helps with reconstruction; only really possible at an e+e- collider.  Still, no observation yet from Belle/BaBar, only observation.  But prospective limits are still much, much better than direct searches at the LHC are likely to be.

B to D decays important as we've seen some anomalies there.  Indeed, BaBar paper from 2012 saw clear discrepancy with SM while inconsistent with any type-II 2HDM (tau fraction of decays for D and D* final states).  Current status of combined results is at 3.9σ from the SM.  Belle II can shrink the uncertainties by a factor of 4 to 5, which should resolve things one way or the other.

Asymmetries in B to Kll decays are another current interesting area.  As discussed yesterday, there is a discrepancy in the kinematic distributions.  Worth emphasising that the discrepancy lies in one of two parity-violating observables.  (There is a very mild tension in the other PV observable, the forward-backward asymmetry.)

Why should we see a discrepancy here?  Well, one think is that the loops generating this process in the SM involve the heavy particles (W, Z, t).  Also, interference could be important: linear effect.

Severable observables are seeing 4+ sigma discrepancies from the SM.  So what's the problem?  There are some theoretical issues with calculating the SM predictions.  There's some dependence on hadronic form factors that should be checked on the lattice.  Checks using HQET also continue to show need for NP.  But still need more data and theory to trust everything.

Some details on Belle II experiment/detector.  Most important point: first collisions anticipated in December 2017, with the physics run to begin late 2018.

Complementarity with direct searches interesting.  Note also probing SUSY parameter space, worth presenting results to show that.

11:30 am: The 750 GeV Anomaly, Alessandro Strumia

This looks like the talk he gave at Moriond; the slides are more optimistic than I expected.  Though the spoken part is not so much ...

Diphoton has got us excited because we don't have anything else to play with.  It cannot be systematics; experimentally and theoretically very clean.  So either a statistical fluctuation or a new particle.

Discussions of the expected stuff.  Limits include invisible channels.  The simple model with new fermions/scalars in the loops runs into problems with perturbativity.  Including DM might make things easier because you can hide stuff in invisible decays of the putative resonance.

Composite models based on quark-like bound states have the problem that you'd expect a colour octet near 750 GeV.  Other models which include some more general composite dynamics; best case is to make the resonance composite but have an elementary Higgs.  (I don't know that I agree with that.)

Ultimately the theory statement is that we need some extra charged states.  Why are they light?  There are various possibilities; SUSY, unification, scale invariance or extended gauge groups.  Ultimately it might (if we are lucky) tell us where to go.

Looking ahead, we have to find this thing in other final states; measure its spin; and identify the production mechanism.  Measuring the couplings is important; interference effects could be important.  Double production could also be very important should the state be strongly coupled.  Any extra fermions or scalars should also be looked for.

Final slide: told not to talk about rumours, but essentially tells us what we know anyway. Including plots hinting at the deficit that has been reported in new data.

12:00 pm: Outlook, Hitoshi Muryama 

I can forgive the absence of slides for the summary talk.

We haven't found what we'd hoped.  So why should we still consider SUSY?  Usual list of motivations.  But why not?  Flavour, CP problems; gravitino overproduction; proton decay; Higgs mass.

The Higgs boson is the only spin 0 particle in the SM.  It lacks context but does the most important job in the theory.  SUSY offers the "explanation" of many scalar bosons, of which only one happens to gain a tachyonic mass.  But other explanations (composites or gauge-Higgs unification) do exist.

Higgs mass is peculiar: bad for theorists but good for experimentalists.  Theorists suffer from the fact that no clear evidence; SM potential metastable, not quite consistent with SUSY, etc.  But lots of experimental channels to search.  This at least gives us a potentially powerful probe.

Divergence of masses has been seen before.  In classical electrodynamics, the quantum corrections to the mass from its own field is of the order of GeV.  There would need to be a tuning of at least 0.01% to get the correct mass.  However, quantum mechanics and the associated doubling of states (antiparticles) reduces the divergence from linear to logarithmic.  In this sense, the SUSY explanation of the Higgs mass is the same.

Another successful use of naturalness comes from inflation; why is the Universe so big, flat and entropic?  These theoretical problems were explained by inflation.

Cosmology also gives a time when things where problematic; before Cobe, people were worried about the non-observation of the CMB anisotropy.  Part of the problem is that the CMB quadropole happens to be about 1% tuned smaller than the best-fit curve, which delayed the discovery.  Perhaps we are in the same position right now.

Upper bounds on sparticles?  Mini-split arguments: DM and gauge unification.  unification puts sparticles below 100 TeV, DM requires some states at TeV.  Similarly the Higgs mass points to moderately heavy stops.  Even this sort of situation would reduce the tuning from that in the SM to about 10-4.

No sign of NP means natural and simple models are excluded.  Choice between theory complexity or fine tuning model.  Can perhaps get better SUSY models if we abandon (say) WIMP DM or unification.  Discussion of SIMPs, and their detection prospects using hidden photons.  General problem is that we have almost no idea what the DM mass should be.

Effective operator analysis of BSM physics.  Notable that while first looked at in 1980 by Weinberg, complete set of d = 6 operators only constructed three years ago.  But already we can automate this.  Possible direction for flavour physics?

Susy in the future... We shouldn't panic just yet.  We can still find things and still talk about it for some time!

SUSY 2017 Announcement

Will be held at TIFR in Mumbai in December 2017.

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