Live and let die (the singlet real scalar...)

...  dark matter model?

One of the arguably most economical extensions to the Standard Model (SM) of particle physics that provide a dark matter (DM) candidate consists of augmenting the SM with a real gauge-singlet scalar S charged under a global Z2 symmetry under which S is charged (S → −S) and all other SM fields are neutral. S is then a prototypical weakly interacting massive particle (WIMP) that interacts with other SM fields via mixing with the SU(2) Higgs. 

Such a minimal extension to the SM has a long history. A first incarnation of a singlet, real scalar extension to the SM was envisioned by Veltman and Yndurain [1] in the context of one-loop radiative corrections to SM processes such as WW scattering. The scalar particle was first considered in a “cosmological” context, to our knowledge, by Silveira and Zee [2], where the relic abundance from thermal freeze-out for a stable real scalar gauge singlet was first calculated; Ref. [2] also computed the “direct detection” cross section, i.e. the scattering cross section for the scalar particle off of baryons, and other quantities relevant for the phenomenology of the mode, such as the impact on the SM Higgs decay and on the flux of Galactic cosmic rays. 

Later incremental work on this setup included Ref. [3], which considered an arbitrary number of complex singlet scalars, and Ref. [4], which focused on collider implications, on possible DM self-interactions effects, and on constraints from the singlet potential of the model. Subsequent studies that focused on the phenomenology of a singlet scalar extension to the SM at colliders and with DM searches include Refs. [5–25]. Not long ago, two of us (SP and LU) have focused [26] on the issue of vacuum stability in this model [27], as well as on a first calculation of the pair-annihilation cross section of the additional singlet into two photons. 

The possibility that this simple extension to the SM might be relevant for models where the baryon asymmetry is produced at the electro-weak phase transition has also been widely explored [28–33]. A strongly first-order electroweak phase transition is in fact a generic possibility that this model entails (see e.g. the recent analysis of Ref. [34] and references therein).  

There are two reasons why it is now timely to scrutinize the singlet real scalar DM model: (i) The discovery of a 125 GeV Higgs by the CMS [35] and ATLAS [36] Collaborations at the Large Hadron Collider, a discovery which effectively removes one degree of freedom from the model parameter space, and (ii) Recent, rapid progress in the area of both direct and indirect dark matter detection, with significant improvements on constraints on the size of the spin-independent scattering cross section of DM particles off of nuclei [37] and on the pair-annihilation cross section of DM into two monochromatic gamma-ray photons [38]. 

As we show in the present study, the singlet real scalar DM model is alive, but the only viable region of parameter space of this model where the DM can be produced as a thermal relic from the early universe is highly constrained and will be thoroughly explored in the very near future.

{Experimental bounds on the two-dimensional parameter space (a2, mS) where a2 is the coupling constant between the singlet and the Higgs boson}. Along the cyan line the real scalar singlet gives the correct dark matter relic abundance. The region below this line corresponds to overabundance and is excluded, while most of the region above is excluded by experimental constraints. The strongest limits are from direct detection (LUX [43]): they exclude the region above the black line. Going to masses below a few GeV the most important constraint comes from invisible Higgs decays searches [40], which exclude the region above the purple line. We show several lines for the constraints from gamma-ray line searches (Fermi [38]): the plain lines correspond to the annihilation SS → γγ, the dashed lines to SS → γZ. The colors correspond to different dark matter density profiles: red is for Einasto, blue for NFW, green for Isothermal. Fermi excludes the area above these lines. The only regions which are not yet excluded are the white areas, one for mS > 110 GeV, the other on the lower left part of the plot, close to the resonance mS = mh/2

Closing in on singlet scalar dark matter: LUX, invisible Higgs decays and gamma-ray lines Lei Feng, Stefano Profumo, Lorenzo Ubaldi(Submitted on 2 Dec 2014 (v1), last revised 7 Mar 2016 (this version, v4))

This article has already been published but the last results from LUX shown below if reported on the figure above would almost* close the white gap between the different excluded regions so that the singlet real scalar dark matter model might be dead right now.

 2016 Limits on Dark Matter from the LUX experiment. The vertical axis expresses the probability of dark matter interacting with normal matter, and the horizontal axis is the mass of the dark matter particle. The region above the bold black line is now excluded. Photograph: LUX

Dark-matter results from 332new live days of LUX data

Aaron Manalaysay For the LUX Collaboration

 * the wimp-nucleon cross section reported on second figure scales as (a2/ms2)².