Tuesday, 21 May 2013

Planck 2013 Day Two Live Blog 3

After lunch, for the first parallel session of the conference.  I've chosen to go to the LHC session before the coffee break.

2:00pm: Riccardo Torre, "Light RPV Stops Hiding in the LHC Data"

Working within a natural SUSY perspective, from an effective point of view.  In particular, not wedding to a particular model of natural SUSY.

His arguments about RPV are unclear to me.  He doesn't seem to properly address the usual problems with RPV, at least I didn't see that.  Okay, proton decay will be forbidden by assuming lepton number conservation.  Anomalies?

Focus on pair production of stops that decay to two jets.  Four jet background too large, so as yet unobservable.

Even when decay is preferred to light jets, the fraction of bottom decays means the b-tagging can improve the limits in a useful way. Also, ATLAS/CMS searches use a quite high jet pT cut, about 100 GeV, which intrinsically prevents looking at stops below about 200 GeV.

The details of the analysis all looks fine.  No obvious flaws beyond those intrinsic to being a theorist.  The interpretation looks a bit suspect; S/B of ten percent and no features against a QCD background does not sound particularly discoverable to me.  There's also the question of whether the relevant events were recorded.

Oh good, he's addressing that point.  But comparing to a resonance is still questionable.

In all, though, this talk encourages me to read the relevant paper.  It might have some applicability to something I've been thinking about.

2:30pm: Krzysztof Rolbiecki, "Light stops emerging in WW cross section measurements?"

At Brookhaven and Pheno, the WW cross section excess was explained with sleptons.  I'm curious to see how stops can do anything here, though.

Mentions slepton explanation; problem relating to the LEP limit on chargino mass.

Stops require nearly-degenerate charginos, so that when stop decays to bottom-chargino the b-jet is soft enough to evade the tags.  Chargino decays to lepton plus neutralino ... ?  Main advantage is the larger stop cross section, so things can be heavier.

Some LHC limits, but room (just) to avoid them.  Considered both stop and wino searches.

Fit, typically 200 GeV stop and 100 GeV LSP.

Most interesting part comes at the end; how to disprove this possibility.  Key point comes from spins (scalar versus vector), so W are more forward than stops.  Of course, have to track this through decays and cuts.

Related to old problem of distinguishing SUSY and UED; use rapidity tool from that old analysis.  Smart!  Previous analysis based on quarks, here we can use leptons so should be even better.  Or perhaps not; plots are very similar.  Can increase sensitivity by using events produced at high center of mass energy.  Need a cut of about 150 to 200 GeV on (effective) root s.  Can define a reasonable asymmetry that has different signs for stops versus Ws.

Discovery or exclusion not likely at 8 TeV, should be done quickly at 14 TeV.

3:00 pm: Jose Santiago, "Top and bottom partner production in composite Higgs models at the LHC"

Last talk, a slightly different take.

Main message: small modifications to SM and SUSY-motivated searches are easily converted into powerful probes of "exotic" models.

By top and bottom partners, we mean the objects in the composite sector that mix with the elementary top and bottom, allowing us to get a large top Yukawa.  These fields are naturally vector-like, as they gain mass without EWSB.  They should be light if naturalness is a useful point of view.

Colour must be an unbroken symmetry of the composite sector, for the top/bottom partners to exist.  So the gluon will be partially composite as well.  This is not clear to me.

Signals (finally).  Production of top-top partner through gluon partner, followed by top partner decay to top-Higgs.  Signal: Higgs-top-antitop, an important SM search.  Now unrelated to top Yukawa.  Coupling can be large (related to composite sector), so we get strong constraints on gluon partner masses; already up to almost 3 TeV, compare to traditional direct searches.  Nearly double the reach at 14 TeV.

Similar channel: Higgs-b-bbar.  Four b jets, looks nasty from point of view of background; but jets are so hard (hardest at 1 TeV) that it is possible.  Higgs boosted, so fat jet.  Analysis does not use substructure techniques, to simplify understanding of background.  Exclusions up to about 2 TeV.

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