Friday, 8 May 2015

Friday wrap-up: Collisions, displaced Higgs decays...

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 LHC has seen collisions at injection energy, 450 GeV per beam. There's a little more you can read at symmetry magazine; still plenty of calibration to be done. Here's one of the events in the ATLAS detector, a far cry from the messy environment we'll see at 13 TeV...

As well, CMS has a visualisation of one of their events on YouTube.
• Pheno 2015 happened this week in Pittsburgh. Definitely worth perusing the interesting talks on the indico page.
• I uploaded an arXiv preprint on Monday, "Constraining portals with displaced Higgs decay searches at the LHC." One of the primary purposes of the LHC is to study the properties of the newly discovered Higgs boson in great detail. Even though we have now measured its mass to within ~0.2%, it is still possible that it is decaying to exotic particles 20% of the time! So it is clearly sensible to search for exotic Higgs decays. I concentrated on one possibility: the decay to a pair of long-lived particles which each subsequently decay around 1 metre from the beam pipe...

Such long-lived particles are well-motivated; typically all you need is an approximate symmetry (which is by definition technically natural à la 't Hooft) in your model which if exact would result in a stable particle. This appears to be coming into vogue at the moment as we see natural SUSY being pushed into compressed and long-lived areas of parameter space; for example, if you violate R-parity just slightly then the would-be neutralino dark matter candidate can become long-lived. I became interested in this sort of phenomenology from a much simpler standard model (SM) extension: by a real singlet scalar field $S$. In that case you can write down a potential term $\zeta \phi^\dagger\phi S^2$ which mixes the Higgs boson and a new mass eigenstate $s$ after symmetry breaking. As $\zeta\to 0$ the $S$ field decouples from the SM and becomes stable, so for small $\zeta$ it is long-lived. It is possible that the $S$ also directly couples to some dark sector uncharged under the SM forces, so-called Higgs portal models. Anyway, the Higgs we know and love can decay to two $s$ particles which, in the simplest case, decay to SM particles somewhere in the middle of the detector. Else the $s$ could have some complex cascade decay into hidden sector states which subsequently decay in the middle of the detector (often called hidden valley models).

You can do a similar thing with a massive dark photon, the so-called vector portal. In fact, the possibilities are many and varied, which presents two complementary challenges: how do collaborations present their results in the most model-independent way possible? and how do phenomenologists reinterpret the results in the context of their own models? The point of my paper was to explore these questions...

So I had a go at reinterpreting two searches already performed by ATLAS, in a very simple way: by running Monte Carlo simulations, calculating decay probabilities, and folding in the provided particle reconstruction efficiencies as a function of the decay distance of the long-lived particles. As phenomenologists we are reliant upon these provided efficiencies, as there is no tool available to reliably calculate them ourselves. As we move on the journey which constitutes the bulk of the paper, we learn some valuable life lessons about what efficiency table information the collaborations could provide to make the life of phenomenologists wanting to reinterpret their searches (and there are a few of us out there!) much easier. Those life lessons are summarised in the conclusion.

As well, at the end of the day I was able to make my own contribution to the portal model exclusion space, with the pretty pictures below...

• There's an article at Nautilus on "The Admiral of the String Theory Wars" AKA Peter Woit. As might be intuited, the article describes the string theory wars around the time when Woit released his book, "Not Even Wrong." It touches on his arXiv trackback controversy and feud with Polchinski. (Also I learned that Motl once compared Woit to bin Laden...). Woit said a few words about the article on his blog. There is also some discussion there on the following...
• You should be able to find Amanda Peet's hour long Perimeter Institute public lecture on string theory on YouTube within a day or so.