Saturday, 25 April 2015

Friday wrap-up: SHiP, portals...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

• The SHiP fixed target experiment has uploaded its 200+ page physics case to the arXiv. Working at the intensity frontier, fixed target experiments collide very many (in this case) protons on a heavy target, with a detector placed some distance away after a significant amount of shielding. Because of the sheer number of protons on target, the setup is particularly sensitive to any ≲GeV scale extremely weakly interacting particles (large number × small number = detectable number!). The document is very comprehensive and speaks for itself; evidently the experiment has the capacity to explore some very interesting new physics scenarios, for example...

If one modestly extends the standard model with a vector or a scalar field, it is always possible to write down gauge-invariant operators$$\epsilon F^{\mu\nu}F'^{\mu\nu},\\ \xi\phi^\dagger\phi S^2,$$where $F'^{\mu\nu}$ is a dark field strength operator and $S$ is a real singlet scalar. These are known as portal operators, and in the limit of very small $\epsilon$ or $\xi$ (which restores an enhanced Poincare symmetry and is therefore technically natural) the new states (referred to often as the dark photon and dark Higgs) are very long-lived and very weakly coupled to standard model states, so that they could still have gone undetected even if their masses are sub-GeV. If dark matter couples directly to these new states then they provide a "portal" from the standard model to the dark sector.

So I was very interested to see the reach of the proposed experiment with respect to those portals; that reach is shown below as a function of mass for the case without dark matter, or with $m_{DM}>m_{A},m_S$ (the g* in the singlet case is proportional to the $\xi$ parameter above)...

It is evident that the experiment would explore a significant amount of unexplored (not grey) parameter space (and the results are even stronger for a pseudoscalar). For the dark scalar case, the reach of the experiment comes from the unprecedented (in a fixed target experiment) number of B mesons produced, which can then subsequently decay to the light scalar state at a rate of one in a million or so. The states then live long enough to travel through the (~70m of) shielding before decaying in the detector.

It is of note that unfortunately the widths and branching ratios of the scalar in the region $2m_\pi < m_S \lesssim 4$ GeV have large hadronic uncertainties, and the plot above must assume one theoretical prediction, so the story is not as clear-cut as it seems; luckily the experiment would be sensitive to many final states, and this goes some way to making the reach independent of this uncertainty. (These uncertainties do not exist for the dark photon thanks to measurements of our very own photon!). The most recent theoretical calculation for the dark scalar widths in this region is >20 years old. I wonder if lattice QCD could have something to say if it was applied to the problem?
• Scientific American has an article on self-interacting dark matter on the back of the Abell 3827 cluster "hint" from last week.
• And now that we have warmed up with Hubble here are some photos from the week...