Tuesday, 10 November 2015

KIAS-CFHEP Workshop Liveblog: Day Two Session Four

The final session of the day is, as promised, more SUSY aligned.  We also have a challenging six talks.

4:00 pm: Higgs mass from (super)split supersymmetry, Jae-hyeon Park

Higgs mass in SUSY is well-known problem.  Can obtain measured value for stops at 10 TeV or less, depending on the mixing angle.  In high-scale SUSY, naively expect large loop correction to Higgs mass.  But presence of large log means one should perform a resummation, matching the quartic coupling at the soft scale.  Doing so shows that for a SUSY scale up to 1010 GeV, can obtain correct Higgs mass for tan β = 1.

The bound essentially comes from the instability in the SM Higgs potential: becomes negative at this scale, while leading SUSY contribution is positive definite.  Can either deflect the running, or invoke large negative threshold corrections.  Problems with Landau poles that make this unfeasible for split SUSY.  Requires large tan β > 100 to get up towards Planck scale.

Alternative approach to add large trilinear non-holomorphic coupling.  Easy to find region that gets mass right.  Must worry about CCB minima.  In particular, instabilities do not decouple.

Fine tuning?  Of course.
Unitarity?  Claims that there is no problem.

4:20 pm: LFV Higgs decay and muon g−2 in a general 2HDM, Yuji Omura

Change to the schedule, and this talk is next.  Related to CMS h to τμ excess in a non-SUSY model (due to time constraints).

Model is 2HDM where second doublet does not gain a VEV and couples only to leptons in general way.  Point of work: show that off-diagonal τμ coupling of new Higgs can explain both the LFV decay and the muon g−2 in the same region of parameter space.  Prefer near (but not exact) alignment limit and moderately large off-diagonal Yukawa ~ 0.1.

Examining other observables, but these don't constrain same observables.  Unsurprising constraints that other flavour violating couplings of second Higgs doublet must be small.  Exception is τ to μγ decay.  This requires that flavour-conserving couplings to tau and top must either be small, or tuned to cancel one another.

Two-loop diagrams for muon g−2?  (Relevant in other 2HDMs).  Not relevant here as new scalars are heavy.

4:40 pm: Naturalness-guided gluino mass bound from the minimal mixed mediation of SUSY breaking, Doyoun Kim

Another SUSY study saying the gluino is just above the current limits.

5:00 pm: Closing up a light stop window in natural SUSY at LHC, Mengchao Zhang

Study of indirect and direct stop constraints.  Indirect bounds from Higgs properties (essentially).  Shift to T parameter, b to sγ, and Higgs signal strengths.  Current measurements of Higgs diphoton decays allow stops lighter than tops.  Future HL-LHC measurements (of ratio of diphoton and ZZ decays) will be more stringent.

Direct searches: assuming Higgsino LSP (minimal naturalness requirement).  Some complications compared to simplified topologies used in LHC searches.  Redid analysis using primarily mono-jet and 2b + MET searches.  Find window remains at 8 TeV as shown in usual LHC limits.  However, with 14 TeV data, massless neutralino will be excluded for stop mass below 700 GeV and absolute stop mass bound of 250 GeV.  Looks like extension to 14 TeV just based on simple scaling of signals and backgrounds; too optimistic?

How do you scale? Assumptions that efficiency does not change too much.  State was tested on some points.

5:20 pm: Stoponium on a lattice, Seyoung Kim

Assumption that stop lives long enough to form bound state.  Question is how that modifies production rate.  Can factorise cross section into perturbative stop production times non-perturbative matrix element.  Prior work used NRQCD, assuming dominated by single spin state.  Lattice is obvious improvement.

Use standard tools.  Extract long-time behaviour of correlators.  Uses quenched approximation as only the first study.  Finds a larger enhancement in cross section than previously (by factor ~ 4).

Discrepancy is large; compared to charmonium/bottomonium models to understand? Not clear, possibly due to difference in scales.
Comparison to stop pair production?  Heavily suppressed.

5:40 pm: Equal-velocity scenario for hiding top squark signals at the LHC, Toshifumi Yamada

Working in simplified models with light stops, along the degenerate line mstop = mtop + mLSP.  This region is hard to see as LSP tends to be back-to-back, and hence little MET.

Want to exploit a relation between the missing pT and the top momenta.  For this reason, use the fully hadronic top decay modes; no neutrinos and can resolve the full tt system.  Missing pT parallel to tt total momentum, and further proportional to it with a constant given by a ratio of masses.  Plot events in plane of angle between tt/missing pT, and absolute magnitude.  Signal events concentrated around a single point in that plane, SM signals are not.

Problems arise when either the  mstop = mtop + mLSP relation does not hold exactly, or even when it does the top is off-shell.  This breaks the relation and ruins the signal.  To some extent this can be compensated for using boosted tops with small angular separation.

Ultimately must use Monte Carlo simulation to see how well things stand up to more realistic conditions.  Evidence shows clustering, and suggests simple cut and count methods can double or triple sensitivity.  Main weakness is that study only covers two points in parameter space, so can not estimate reach.

Lower bound?  Yes, SM background focused around 0 missing pt.
What about using ISR for large MET?  Suppresses signal too much.

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