Friday, 30 August 2013

SUSY 2013 Live Blog: Day Five Session Three

We start the last afternoon session of the conference.  I've decided to start out in the Higgs session again.  As I noted last time I was here, this session is heavily subscribed, so we have six fifteen minute talks before the coffee break.  I hope that won't be too much.

2:30pm: Stephane Tourneur, "Searches for Higgs and Higgs-like particles at LHCb"

If LHCb is/was designed for B physics, how can it contribute to the direct search for new physics?  This talk answers by looking at Higgs decaying to taus, to two or four b quarks and for displaced vertices.

LHCb has several times lower luminosity than, for example, ATLAS or CMS.  The corollary is that it has typically only two events per beam crossing, leading to the cleanest LHC events.  It also has a unique acceptance focusing on highly forward events.

Ditau search involves either doubly or singly leptonic events.  Limits placed are above the SM by factor of 10, but do place limits on enhanced decays, e.g. for the MSSM.  But even there the limtis are worse than at ATLAS/CMS.

B-quark events look more promising, since these are very hard at the general purpose detectors.  But I didn't see a limit, even preliminary, presented there.

Long-lived particles is another area where LHCb should do well.  Indeed, the search is focusing on low mass regions below 60 GeV where the other experiments have difficulty triggering.  Some six-jet topologies are only tested at LHCb.  Limits only on the earliest data, not even one inverse femtobarn of luminosity; analysis proceeding on the full data set.

2:45pm: Paratma Bhupal Dev, "H to invisible sneutrinos"

Note that a precise measurement of the total Higgs width is very difficult at the LHC.  We can look at the invisible branching fraction using associated production with a Z, that decays leptonically.  Invisible decays to DM are interesting in light of DAMA/CoGeNT etc.

For the MSSM, light neutralinos require abandoning gaugino mass unification, due to chargino mass limits.  This requires high fine tuning.

We know that neutrinos have masses.  This suggests right-handed neutrinos, so in a SUSY context we get singlet sneutrinos (with small active mixing).  They can be our DM candidate.  Using an inverse seesaw you can have a relatively light (TeV) right-handed neutrino, so light sneutrinos acceptable.

Relic density (in a cMSSM boundary choice for the MSSM parameters) leads to sneutrinos around 50 GeV.  Invisible Higgs decay typically bounded by a few to ten percent branching ratio.

Allowed space is completely within the reach of XENON1T, LUX.  I think there is an upper limit from the relic density which makes this statement not obviously nonsense.

3:00pm: Koji Tsumura, "The Higgs boson mixes with an SU(2) septet"

The T parameter is a tight and non-trivial constraint on the nature of the Higgs sectors.  The septet (with hypercharge 2) is the next smallest multiplet that automatically satisfies the T constraint at tree level, i.e. it predicts the SM relation between the W and Z mass.

Adding a septet adds a lot of new particles; fourteen degrees of freedom, two neutral and six charged scalars.  The maximal charge is 5!

The big problem with this type of model is the presence of an accidental U(1), leading to an exactly massless NG boson.  This can be avoided by adding a dimensions five operator.

Interestingly, this model can have an enhanced coupling to gauge bosons, something impossible in e.g. composite Higgs or 2HDMs.

A smoking gun signal is a coupling of the charged Higgs, W and Z; absent in 2HDMs (or just the MSSM?)

3:15pm: Pradipta Ghosh, "Displaced multileptons at LHC in munuSSM"

μνSSM is like NMSSM, but replaces the singlet with three right-handed neutrinos.  This breaks lepton number and R-parity.  Get Higgs mixing with sneutrinos.  End up with a lot of light sneutrinos.

Possibility of displaced vertices from his sector?

3:30pm: Tania Robens, "The Higgs Singlet extension parameter space in the light of the LHC discovery"

Singlet is simplest extension of the SM gauge sector.  Here the minimal case is taken, with no additional dark sector states nor is the singlet DM (it acquires a VEV).

Second Higgs taken heavy; has SM-like decays from mixing, signal strength goes like the sine of the mixing angle to the fourth power.  We also have to worry about the decay of the heavy Higgs to the SM one.

Impose constraints from perturbative unitarity, ST, vacuum stability.  For heavy Higgs below 700 GeV, strongest constraints come from the SM Higgs couplings; above that from perturbativity of the couplings.

3:45pm: Florian Goertz, "Custodial Leptons and Higgs Decays"

Higgs physics in composite models.  In addition to the modifications of the Higgs properties, we generically have light resonances associated with the right-handed top quark.  We also need to enlarge the fermion sector to protect e.g. Z to b-bbar.

Model as SM plus bidoublets, four new vector-like fermions.  These mix with the SM third generation quarks.  Importantly, this mixing does not affect the Higgs-gluon or Higgs-photon coupling, as discussed this morning.

What if there is lepton compositeness?  This can modify the Higgs-photon coupling.  We might need tau partners if we are trying to explain the lepton Yukawa/mixing structure.  These can have a non-zero effect because the tau is light compared to the Higgs, so the loop functions for the tau and its partners are not equal!

The effect of this compositeness is to reduce the Higgs coupling to photons.  This obviously prefers the CMS measurements.

Also considered non-minimal model with the tau in a 10 of SO(5); this adds new fermions that allows the Higgs-photon coupling to be enhanced in principle, but from a top-down perspective still always lower it.

Indirect bounds (four-fermion operators) are current work in progress.

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