Thursday, 7 May 2015

Pheno 2015 Liveblog: Day 3 Session 2

Last night was the conference banquet, and unfortunately as a consequence I missed the first session this morning.  So we come to the final session of the conference, which is both reflective of the current status and also looking to the future.

11:00 pm: Where is SUSY? Howard Baer

LHC7/8 were a great success, but there remain problems on the theory side.

Claim SUSY could have been ruled out by gauge unification measurements (LEP), top mass (EWSB), and the Higgs mass.

Discussion of naturalness, and claim that SUSY can still be natural.  This would be much more believable if we'd heard these ideas even five years ago.  Also, it's really unfair to state that ruling out SUSY would be a "Sensational claim".  Discovering SUSY would be sensational.

Now let's move on to less annoying claims.  In particular, a discussion of different measures of fine tuning.
  • ΔEW: different tree and loop-level contributions to Higgs mass, evaluated at weak scale, should not be much larger than Higgs mass.  Wants large At term.
  •  ΔHS: neglect certain pieces, allowing to integrate running of mHu up to high scale.  Large-log approximation.  Wants light squarks and small At term.  But neglects input value of mHu, which affects its own running.  Combining dependent terms this reduces to ΔEW.
  • ΔBG: take logarithmic derivative of Z-mass with respect to input parameters.  For low-scale models, reduces to ΔEW.  For high-scale models, suggests high dependence on high-energy soft parameters, but this can be reduced if those parameters are dependent, i.e. there is some high-scale symmetry.  Indeed, in a supergravity theory all soft terms are functions of the gravitino mass; taking different soft parameters simply parameterizes our ignorance of the details of the hidden sector that mediates SUSY breaking.
This leads to talk's conclusion: ΔEW is effectively only measure of SUSY fine tuning, and only requires light Higgsinos.

Next claim: only non-fine-tuned model is NUHM and generalisations.  This has soft mass inputs at TeV-scale to reproduce observations.  Oh look, we get a spectrum with coloured sparticles just above the current bounds.  Fancy that! Only things that are sub-TeV are the electroweakinos.  Little hierarchy is consequence of Higgsino mass being small compared to gravitino mass.  The question is then where μ comes from; advocating a PQ symmetry and relation to axion mass.  Axions (or saxions/axinos) allow DM given the tendency for Higgsinos to underproduce the thermal relic density.

True exclusion of this model requires ILC1000, to rule out 350 GeV Higgsinos.  Also some reach for expected axion scale at ADMX.

Preferred axion mass? Doesn't directly answer, just refers to ADMX search range.

11:35 am: Physics and Perspectives of the ILC, Shigeki Matsumoto

Year ago, after some preliminary discussion, committee (ACE) established in Japan to explore ILC potential and to plan for it.  This has had several meetings, but will likely make its final report by March of next year.

Physics working group subset of ACE devoted to studying physics case.  Several possible directions, but can be divided into three main areas: precision Higgs, precision top and DM searches.

As a linear lepton collider, ILC can measure the Higgs couplings in a model independent way.  The total cross section comes from measureing HZ production with the Z decaying to muons.  The total with can be measured using VBF with the Higgs decaying to WW.  In most channels, percent-level accuracy is achievable.  If a 1 TeV run is also done, this improves the two worst couplings, to tops and photons, to the same level.  Even the self-coupling can be found to ~ 10% accuracy.

The goal for the top mass measurement is at least 100 MeV.  Further, threshold scans make the interpretation of the top mass cleaner than in hadron colliders.  The width can be done to ~ 30 MeV.  These are important for determining the stability of the Higgs potential.  Additionally, precise measurements of the chiral top couplings are very important for many models which modify them.

Dark Matter is easiest when DM is a SU(2) multiplet, e.g. Higgsino, MDM.  In this case, two channels for detection.  Direct production of the charged multiplet partners leads to a signal of a hard ISR photon, MET and possibly soft tracks from the charged state decay products.  This has reach pretty much to the beam energy.  An indirect measurement has higher reach, based on loop corrections to ee to ff production.

Singlet DM is, obviously, much harder.  Must look for non-renormalisable operators to produce DM.  This leads to more model dependence.  Study ongoing.

How do you do Higgsino search? Direct production.
Limits look better than before; include luminosity upgrade?  I didn't quite catch the answer, I think it was yes though.

12:10 pm: HEP: Future Perspectives, James Wells

The key question of our time is the electroweak vacuum.  The discovery of the Higgs is an achievement like few, if any, in the history of scientific thought.  And yet, there is a Higgs mode in condensed matter systems associated with the superconductivity transition.  In those theories, there is no explicit Higgs in the Lagrangian; yet there is a boson that is seen!

Superconductivity was originally (1950) described by Ginzburg-Landau theory: a φ4 theory that was just a simple placeholder to explain phenomenology.  It wasn't until the BCS theory (1957) that a true description of microscopic dynamics was obtained.

If SM can be consistent to Planck theory, can SM be answer? No,  due to e.g. baryogenesis.  But philosophically, if we have one scalar than we have others and that is unstable.
Strong CP probelm was not on slides?  Axions are a pretty good solution.

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