### 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, butthe 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.

Closing in on singlet scalar dark matter: LUX, invisible Higgs decays and gamma-ray linesLei 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.

* the wimp-nucleon cross section reported on second figure scales as (a2/ms2)Aaron Manalaysay For the LUX Collaboration

*.*

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