Friday, 30 August 2013

SUSY 2013 Live Blog: Day Five

We've reached the last ful day of SUSY 2013.  We start the morning with another pair of LHC talks, this times on the so-called exotic searches.  As this is a SUSY conference, they only get 35 minutes, unlike the 45 minutes the SUSY searches got.  Of course, like the SUSY searches we know going in the final statement; nothing has been found.

9:00am: Sunil Somalway, "Recent results from exotic searches of physics beyond the standard model from the CMS experiment"

Importance of data-driven techniques: backgrounds in many cases are huge, e.g. dileptons have signal to background of 105.  Can't use simulations, systematics aren't that good.

Menu: Resonances, LHC as top factory, heavy quarks, dibosons, displaced jets, monoX.

The problem with these summary talks is that you often end up with "here's a search channel, here's the result, here's the next search channel...".  That's why I often prefer the parallel experimental talks, as they have relatively more time to cover the details.  The CMS talk on SUSY on Wednesday is pretty much the only exception to that pattern I've seen.  Jeff Richman structured his talk around a number of case studies to illustrate larger themes, and was much more accessible as a result.

LHC as top factory: this lets us probe non-standard top decays to an extant never possible before.  The relatively small top sample from the Tevatron means that the top is the least-constrained SM fermion.  There are thus a lot of models exploiting that wiggle room that are ripe for exclusion by new measurements.

New limit of this type: top decaying to charm plus Higgs, zero in SM and typically 0.1 to 1% in BSM.  Observed limit at 0.83%, slightly worse than expectations (0.53%).  Other limits: top to Z plus quark, limit down to 0.05%.  Top goes to "everything but Wb" (looks like actually Wq?); get branching ratio for top to Wb at least 94.5%.  And B-violating decays to muons at less than 1%.

Production of heavy fermions (top/bottom partners) sensitive to decay modes.  I was actually just reading a paper this morning on this point, offering some clever tricks using Higgs production to get sensitivity independent of that systematic.  I might write a post on that later.  The actual searches seem to have limits around 600 GeV, slightly worse than expected.  However, looks like full data set not yet used for bottom partners.

Limits on Q=5/3 top partners, that show up in many Little Higgs models, at around 770 GeV.

Diboson limits, I was at the CMS parallel talk yesterday.

Question: are width effects included for heavy resonances?  Answer: don't know.  10% effect.  Actually, someone else knowns: depends on the search; e.g. not in CMS Z' searches.

9:35am: Cigdem Issever, "Summary of the ATLAS Exotics Searches"

Exotic searches characterised, relative to SUSY by a lack of a unifying model to guide the search.  Models are then important in providing clues where to look; the fact that they may be quite wrong in the details is, to the experimentalist, irrelevant.

Dark Matter Mono-X searches use the usual EFT approach and ISR.  Focus on new result, Mono-W/Z with the vector boson being boosted and decaying hadronically.  Interference effect between radiation of u versus d quarks has an important effect, almost an order of magnitude in the exclusion contours.  We get the usual results, where we beat XENON etc at low mass, assuming the EFT is valid.

Resonance searches; main experimental challenge is pT resolution and efficiency up to the TeV scale.  In particular, no standard candle to serve as a check.  Credibility limits, so Bayesian!  Limits on typical dilepton resonances over 2 TeV now.

Ditau resonances a new result; similar, slightly weaker limits.  Important to use all possible τ decay modes.

Dijets, very much into the high energy regime.  Highest such event has dijet mass of almost 5 TeV!  Excited quark limits almost 4 TeV.

Top searches making extensive use of jet substructure tools.  Worth noting that boosts effects even leptonically decaying tops, where isolation cuts become problematic.  Some tricks using a "variable isolation cone" that gain around 15% in efficiency.  Two-top resonance now excluding KK gluon in RS to 2 TeV.  Relevant as finally starting to directly probe the most popular theoretical choice of KK gluon mass (2.5 TeV).

Inclusive Same-Sign Dilepton searches: paper deliberately does not use model, to emphasise broad applicability and allow theorists to set limits on any model they want.

Summary: worth noting that parton luminosity at 13 TeV will be significantly higher in the TeV regime.  Indeed, just for a TeV particle the pdfs are a factor of 4 larger than in the 2012 run; go up to 2 or 3 TeV and the ratio exceeds ten.  Plus, of course, the plans are for at least ten times integrated luminosity at the higher energies.  This is my view, too; the LHC run is just starting and it's too early to draw the broader negative conclusions that some people have done.

Question: why is there no 2-sigma excess given how many channels have been analysed?  Statistically unlikely.  Didn't quite follow the answer, I think she said that there are even three sigma excesses, but not reported as caused by systematics.

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