Tuesday, 5 May 2015

Pheno 2015 Liveblog Day 1 Session 4

For the last session of the day, I'm staying in one place - the Higgs II session.
4:30 pm: The Higgs Cross Section and N3LO, Falko Dulat

In many channels, statistical uncertainties are approaching theoretical ones.  In run II, we will enter a region where errors are theory-dominated.  This reflects badly on us theorists.  This coupling extraction is, after all, a major part of the LHC Run II program.

Focus on inclusive cross section.  (Better understood in QCD.)  Gluon fusion in effective theory; QCD corrections to this process are the dominant uncertainty.  Lots of Feynman diagrams!  Threshold expansion: expand around Higgs mass.

Taylor series computed to 37th term.  While first term shows huge jump, it quickly stabilizes to a trustworthy result.  As a Feynman series, the N3LO term sits in the middle of the N2LO and with smaller error, so it all looks reliable.

This opens up new possible uncertainties.  These now need to be examined.

4:45 pm: The Role of Low-Energy Observables in Precision Higgs Analysis, Zhengkang Zhang

Precise (sub-percent) Higgs measurements are necessary to get any hints of new physics.  But to be sensitive to NP, we need to reduce theory uncertainties to at least the same level.

A big problem is the uncertain in charm and bottom masses.  Induced error in Higgs partial widths is a few percent.

Quark masses come from low energy precision measurements.  Moments of low energy observables (ee to hadrons as function of energy).  Can invert relationships.

Major problem in perturbative uncertainty (finite number of terms in perturbation series) which is itself difficult to estimate.  Need to go to higher orders, try other approaches or possibly look at other low energy observables.

5:00 pm: Search Strategies for Composite Higgs Models at LHC Run II, Jeong Han Kim

Run 1 bounds on top partners at 800 GeV.  Light quark partners bounded from modifications to Higgs signals, from additional production of pairs of quarks.

At run II, everything is boosted so need appropriate tools to overcome the QCD background.  Reach should hit TeV scale in first year of run II.

Interesting to ask best signal channel.  Top Partners decay to tZ, how does Z decay?  Neutrinos actually seems best, due to MET!

5:45 pm: Search for a Non-Standard Model Higgs Boson Decays in Events with Boosted Dimuons, Sven Dildick

A CMS search.  One way to probe the Higgs properties is to look for non-SM decays.  Here look for Higgs decay to two light bosons (plus X) followed by decays to four muons.  This can arise e.g. from Dark SUSY or NMSSM.

Previous analysis based on prompt dimouns.  New analysis looking also for displaced dimuons.  Dark photon lifetime up to 5mm.

Search had downward fluctuation; one event vs 2.2 expected.  Cross section bound at 0.4 fb.  Results will exclude all region for vector masses above muon threshold and kinetic mixing above 0.00001.

6:00 pm: Uncovering Light Scalars with Exotic Higgs Decays to bbμμ, Yiming Zhong

Similar to previous talk, focused on light scalars that Higgs can decay to.  In this case, real singlet mixing with the Higgs.  Couplings inherited from Higgs Yukawas, so most previous focus on bbττ or 4b, 4τ.  Channel with muons offers better triggers, so might offset low BR.  Also, rate can be enhanced in e.g. lepton-specific 2HDM.

Limits can be reasonable, 0.01% Br at 300 ifb for example.  Problems when intermediate scalar is light (<20 GeV), due to lepton collimation.  Use substructure analysis to improve reach, comparable to high mas regime.

These analyses benefit from less reliance on b tagging and no reliance on τ identification.

6:15 pm: Light Charged Higs Bosons in Single-Top Production, Adarsh Pyarelal

Charged Higgs are experimentally challenging.  If sufficiently light, can show up in top decays.  Such light Higgs under serious pressure from various searches; but assume only SM decays (specifically τν).  Decays to light scalars, e.g. to AW, can easily dominate.

Such signals are very difficult.  Key feature is angle between single top and COM lab-frame velocity; this peaks at around 180 degrees, an essential cut for reducing backgrounds.  By using this, with run-II luminosities can cut the branching ratio limits to sub-percent level.  In the context of a 2HDM, this will exclude the low-tan β region of parameter space better than existing searches.

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