Wednesday, 6 May 2015

Pheno 2015 Liveblog: Day 2 Session 3

I've decided to spend the first session after lunch today in a single session, Dark Matter III.  It will if nothing else let me charge my laptop.

2:00 am: Baryonic Dark Matter, Michael Duerr

Baryon number violating operators must be suppressed by a huge scale.  Should we take that as evidence that there is no NP all the way up to that scale?  Well, to avoid this idea let us promote baryon (and lepton) numbers to gauge symmetries, ruling out those operators exactly.  This requires new fermions to cancel the various anomalies that this introduces.

The solution presented here, the scalar that gives masses to these new fermions with a VEV has baryon number 3.  Only ΔB = 3 processes are allowed, so proton decay is forbidden.  A residual Z2 stabilizes one of these fermions, DM.  Six parameter DM model with a vector U(1)B mediator.

Vector mediator has ~ 10% branching ratio to tops, rest to either jets or MET.  MET dominates when allowed.

Relic density requires living close to, but somewhat off, the vector resonance. Rough upper bound on the symmetry breaking scale, leading the vector mass less than 35 TeV.  Direct detection sets important bounds, DM above about 500 GeV.

Deuteron decay woud be allowed here?  Limits weaker.
Kinetic mixing? Yes, but likely unimportant.
Can we truly rule out the upper limit?  Maybe.

2:15 pm: Sterile Neutrino Dark Matter at the PeV Scale, Samuel Roland

Traditional neutrino mass explanation involved right-handed neutrinos.  Standard see-saw too heavy to observe.  Lighter NR interesting, MνSM, but why light?  Consider PeV scale; natural scale for sfermions for Higgs mass, (flavour).

Let NR be charged under new U(1) symmetry, to forbid standard see-saw terms.  Add also a scalar that leads to effective terms with scales ~ (v/M)2, with v the new VEV and M a high-energy scale.  PeV-scale VEV leads to keV-scale sterile neutrinos.

Lightest neutrino is Dodelson-Widrow DM.  This is ruled out as composing all DM by X-ray and structure formation constraints.  In typical mass scales, must be less than half DM.  (Assume rest is LSP or axion or similar.)

Can also produce DM through "freeze in" decays of the new heavy scalar.  This leads to GeV-scale cold DM, but can compose all of dark matter.

Model may be motivated by 3.5 keV line and PeV Ice Cube neutrinos.

Lepton Number?  Good question.

2:30 pm: Implications of a Hidden Sector in the Conformal Framework, Juri Smirov

SM has single true scale - Higgs mass.  Cannot remove this and be fully conformal; cannot get correct Higgs VEV by dimensional transmutation.  Need more bosonic degrees of freedom.  These can be gauge bosons of the hidden sector.

Hidden sector will have scalar portal (to transmit scale), vector portal (hidden gauge bosons) and neutrino portal.  Last is because in a conformal theory, neutrino Majorana mass forbidden.

Neutrino-mass sector purely from scalar VEVs.  Heavier fermions at TeV scale; lighter ones at keV.  Usual constraints, but with different set up region of parameter space exists that is allowed.

LNV at LHC is suppressed; heavy neutral fermions are pseudo-Dirac.  Dominant signal is OSDL, not SSDL.  Can explain recent excesses, would also lead to signals in ditop channel.

2:45 pm: The Dark Side of Neutrino Mass, Wei-Chih Huang

A model that predicts DM mass, based on neutrino physics.  Simple dimension-6 operator coupling Majorana DM and neutrinos; this can arise easily from scalar coupling that is integrated out.  Scalar couplings violate lepton number; it is only broken by DM Majorana mass.  Neutrino mass is radiatively generated by the effective coupling, which also determines the DM annihilation rate.  The combination of these two conditions picks out a specific DM mass, slightly below an MeV.

A UV-complete toy extension offered. Problems exist with CMB due to light DM.

Worked out full 3-flavour model yet? No.

3:00 pm: Aspects of Lepton Flavoured Dark Matter, Can Kilic

Instead of connecting DM with the hierarchy problem, can we connect it to the flavour problem?  Can potentially also be extended to baryogenesis.

Basic set up: two dark states, fermion and scalar, with non-trivial flavour structure.  Let flavour number be associated with DM.  For purposes of this work, consider couplings to leptons.  LFV processes like μ to eγ constrain size of couplings.  To make things simple and feasible, work in MFV.  Coupling goes with leading term, universal, but split the DM states with Yukawa squared.  Two states close in mass (e, μ), one somewhat separate (τ).

Direct Detection: for scalar DM, Higgs exchange (gluon loop) and lepton loops can interfere only if DM is asymmetric.  This relates DM to leptogenesis, but there is some additional effort needed (an extra scalar to break an accidental U(1)).  If do not do this, DD places strong constraints.

Indirect Detection absent in asymmetric case.  However, if mass splitting is highly suppressed have multi-component DM with loop- and phase space-suppressed decays.  Leads to prediction of double line in keV region with eV split.  Possible explanation for 3.5 keV line.

Muon anomalous magnetic moment?  Nothing; coupling is to right-handed leptons only.  Two mass insertions suppress signal.
Resolving split lines?  10 ev splitting, astro-H experiment should be able to do it, though same order as thermal broadening.

3:15 pm: Lepton-Flavoured Dark Matter, Jennifer Kile

Two problems: DM scales, relic density prefers 100 GeV to 1 TeV while DD prefers scales over 10 TeV.  Flavour, muon anomalous magnetic moment prefers physics below a TeV while LFV scales are much higher.  Even LF-conserving scales are multi-TeV.  Can we explain them simultaneously?

Gauge lepton flavour symmetry, but try to be model-independent.  Dark sector particles are Dirac fermions.  Assume electrons uncharged under this symmetry.  Most structure of general theory covered by two LFC vectors, one LFV vector.

Relic density points to usual scale.  DD limits loop-level, as only couple to leptons.  This is enough suppression to bypass LUX.  Direct contributions to muon anomalous magnetic moment; in particular, from LFV contributions allows a TeV-scale vector.  This is in some tension with limits from τ decays, so need LFC vector contributions as well.  Cannot use LFC vector alone, due to limits from neutrino trident production.

LEP constraints subleading.  LHC production is loop level; after applying all constraints, would not have expected to see anything yet but possible to see things early in run II.

What of kinetic mixing?  Haven't picked gauge group.  Could be non-abelian.  Or could be allowed, in which case limits conservative.
What about leaving electrons uncharged under the flavour group? Just an assumption.  Toy models suggest it should work.

3:30 pm: Dark Matter in Leptophilic SUSY, David Feld

Really, leptophilic 2HDM.  Scalars can be lighter than in other 2HDM.  Typical constraints (b to and similar) much suppressed on this class of models.

Dark matter comes in SUSY-like set of two EW doublets plus a singlet.  (Higgsino plus Bino.)  Sample point involving a 10 GeV DM (motivated by GCE).  Together with relic density, prefers high tan β of around 50.  This requires a pseudoscalar mass of 25 GeV.

Can explain muon anomalous magnetic moment.

Collider limits from LEP 4τ searches.

Invisible Higgs width? Not shown but OK.

The last talk of this session was cancelled, so that brings us to a conclusion.

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