Monday, 4 May 2015

Phenomenology 2015 Liveblog: Day 1 Session 1

If there's one thing that gets me to blog every year, it's attending a conference.  I'm again at Phenomenology, the sixth year I've been here.  As usual, we start with some experimental talks; these are pretty much standard reviews.  No new results are going to be announced here, and as everyone well understands we've only found SM stuff so far.

8:45 am: Standard Model Physics at the LHC, Ian Hinchcliffe

We start with some total cross section plots for all SM processes, all in the correct parametric region (except maybe for the total pp cross section, which we can't reliably predict).  Then it's on to the most interesting results, the Higgs measurements.  The fact that we are already talking about Higgs measurements rather than discovery shows how far we have come in three years.

Some tension in mass measurements in different Higgs channels, but given the large errors on sub-processes it's not significant.  Agreement at the 10% level.

Spin measurements (from decay kinematics) currently able to test specific models (e.g. for different spin-2 states) but a completely general search not yet possible.  All tested models except pure scalar, including pure pseudoscalar, ruled out now.

Higgs couplings non-universal, unlike all other SM states.  Looks like SM.  Now have evidence 4.5-σ Higgs couples to leptons (taus) but only limits for electrons and muons.  Also, experimentalist refers to tau as lepton!

Experimentalist limits on invisible width at 30% level (some theorists have claimed stronger limits based on theoretical assumptions).  Also, measurements of total width based on rates on- and off-resonance; bounds already at 22 MeV (compare ~ 3 MeV in SM).

Preliminary studies of ZH and WH production, still statistics limited.  ttH also runs into problem from complexity of final states.  A good reason why the anomaly there is probably nothing, not that that will stop me writing a paper on it.  Cross section for ttH at run-2 will relatively increase much more compared to single Higgs, so if genuine will quickly probe it.

Gauge Bosons
Precision tests of SM physics.  Not a whole lot to say.  Some issues remain with the Monte Carlo predictions of event shapes (jets), but with the large systematic uncertainties, it's not inconsistent.

Top Mass
Important variable, one of the big areas for improvement.  Main experimental limit in the b-jet energy scale.  Measurements now exist in pretty much all channels.

Looks like the Monte Carlos work well here, given the large dynamic range (rates vary by 12 orders of magnitude).  This gives confidence in background estimates for NP searches, though of course most data-driven methods.

Improvements on Higgs width: take a very long time to get down to SM.

Uncertainty on b-jet energy scale noted in top mass measurements is dominantly theoretical.  Calorimeter response to jets depends on hadronisation; light jets can be calibrated with W decay but no analogous process occurs for b jets.

9:20 am: New Physics Searches at CMS and ATLAS, Greg Landsberg

Summary: we found nothing.  200 searches at ATLAS and CMS have now been done and returned null results.

Blah blah naturalness blah.

Natural SUSY
Gluino limits at least 600 GeV for any decay mode.  Away from the highly degenerate limit, bounds closer to 1.4 TeV.  Stop limits much weaker; a lot more gaps in the excluded regions near the degenerate limits.  Without that you get 600-700 GeV limits.  Electroweakinos similar, with fewer degenerate gaps than for stops but more than for gluinos.

Some highlights:
  • Searching for degenerate stops: ATLAS looked at tt spin correlations.
  • New searches using Higgses in final state; now starting to make progress.  Useful for electroweakino searches.
  • Limits on RPV/Stealth models put squarks at TeV.
  • Searches for scharm using charm-tagging.  That has a surprising 20% efficiency!
  • Searches for VBF electroweakinos; might be only probe for degenerate Higgsinos, mostly preparation for run-2.
Long-Lived Particles
Searches for various states.  New searches based on jet pair with displaced vertices.  Very, very hard to trigger so an impressive acheivement.  Also searches for neutral particles hadronizing late in the detector; this is very important for something I'm working on.  Finally, lepton-jet searches making new limits in e.g. hidden photon parameter space.

Mono-X Signals
Dark matter searches.  Nothing looks particularly new here.

Can also be interpreted as searches for large extra-dimensions, with graviton production.  However, limits weaker than KK graviton with leptonic decays, as well as dijet distributions.

Here's a single slide on a search!  Here's another slide on a different search (or two)!  There's nothing anywhere!  This 35-minute talk (including questions) has over 60 slides.  I'm reading a news article instead.

9:55 am: Higgs Phenomenology, Christoph Englert

The final talk of this session is theory.  Crux of the talk: given the Higgs, we ask: is there space for BSM Higgs? Can we find it? What will things look like after run 2?

The traditional line of thought is, of course, based on concrete models.  But perhaps more powerful (and non-overlapping with other plenary talks to come) to consider the effective field theory language.  Well-defined approach provided no new light states, and easy to constrain Wilson coefficients (at ~ 10%) using Higgs signal strengths.  Bounds comparable to Λ ~ 800 GeV.  This can be improved at linear colliders by a factor of up to 5 in best channels.

This isn't really good enough, so how can we do better?  Differential distributions offer more clues, look at events with high momentum transfer.  Not possible at run-1 due to statistics, possible at run-2.  Off-shell Higgs is newest tool here, see Higgs width mentioned above.

Biggest problem with this language: evolution of scales.  Known from flavour physics that running not only changes coefficients, but mixes operators.  Problems are: what is the high energy scale Λ?  (Lagrangian term is c/Λ, so suppression of operator not the same).  What is measurement scale?  The LHC marginalizes over a range of energy scales; compare flavour physics where high (mW) and low (mb) energy scales are scales of measurement.  Effects can be at 10% range.

Higgs off-shell measurement probes non-decoupling properties of the amplitude.  In particular, in SM terms with and without Higgs have interesting destructive interference at high energies.

The future will include probing multi-Higgs production.  DiHiggs is possible at the VLHC.  This is all to constrain the Higgs trilinear coupling.  However, even with total lifetime data, the bounds will be well above the SM.  The 100 TeV experiment can do much better in this regard.

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