The Plot Of The Week - Higgs Windows Of Opportunity
    By Tommaso Dorigo | March 4th 2013 09:29 AM | 5 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    Last week a new important paper appeared in the Arxiv: "MSSM Higgs Boson Searches at the LHC:Benchmark Scenarios after the Discovery of a Higgs-like Particle", by M.Carena, S.Heinemeyer, O. Stal, C.Wagner, and G.Weiglein. The paper fills a void that was created after the discovery of the Higgs particle last July by the ATLAS and CMS experiments: a thorough assessment of what constraints on the allowed chunks of SUSY parameter space in the light of the existence of a neutral scalar at 125 GeV.

    The issue is complicated, because Supersymmetry is not just a theory, but a framework within which one can build quite different phenomenologies, depending on the exact value of a multitude of free parameters. So hard is the problem of characterizing this wide 100+-dimensional space that before one ventures to study the model predictions one must decide on a set of "test points" which can be used as benchmarks. These are called "benchmark scenarios", and agreeing on  the particular values of the defining parameters is important for a number of reasons: understanding how strong are the experimental constraints of subsequent experiments, comparing the sensitivity of different searches, and figuring out what may be the most distinctive signatures of the considered models.

    Hence the study, which is quite extensive and covers several possibilities for some of the experimental signatures of the Higgs sector of the MSSM (the minimal Supersymmetric extension of the standard model, already a restrictive choice in the mare magnum of possibilities) which can be nailed early on by LHC measurements: for example, rates of specific decays of the Higgs-like particle discovered last July. The study also considers the possibility that the particle is not the lightest neutral scalar, but actually the heavy one; such hypothesis creates of course wholly different scenarios.

    One bit of the discussion on which I am personally interested (although, as you might recall, I remain a strong SUSY sceptic) is the scenario where the discovered light Higgs boson can be produced in pairs by the decay of a heavier counterpart: H->hh. Such a circumstance is possible in a wide chunk of parameter space and would lead to quite interesting experimental signatures. Given that I was involved in the search for the MSSM bbH->bbbb production process, which resulted in a recent publication (cited by the article discussed here), it is clear that the option of studying the H->hh->bbbb process in the same final state (one involving four b-flavoured jets) is quite attractive. We'll see what we end up doing there...

    Anyway, this article is titled "the plot of the week", so let me pick a representative graph from the paper to stimulate you to give a deeper look. The graph is a representation of the plane of two of the parameters defining the MSSM: tan(β) and μ in the so-called "low-mH scenario". Different coloured swaths of the plane correspond to areas that experimental searches have already excluded (such as the purple one at the bottom, which is excluded by direct LHC searches for MSSM particles, or the blue one excluded by LEP experiments, or the red one excluded by limits on the H->ττ decay by ATLAS and CMS; the most interesting regions are those in green, which correspond to values of the Higgs boson measured by the experiments. The black area instead correspond to parameter space points which would predict rates of Higgs production too high with respect to what has been observed.

    All in all one gets the impression that the "window of opportunity" for the MSSM is closing down. But if you read the paper (written by MSSM enthusiasts) you might get a different idea !


    But what measurements actually affect the odds (this particular version of) the MSSM is correct? An analogy to clarify:

    You've dropped your car keys somewhere and it seems most likely you did it while walking across a football field. Direct searches obviously affect the odds you actually did drop them there, but increasingly accurate measurements of which path you walked across the field don't. They obviously limit the part of the field where the keys could be, but knowing the exact path you took would "only" limit your search to a line rather than an area, but not affect the odds of the keys being in the field at all.

    Do the measurements of Higgs mass and production rate really affect the odds of Susy being right?

    Correction: it's not the Higgs particle, it's the Higgs-like particle
    Correction: it's not the MSSM, it's the MSSM-like failed-like theory.

    Those purple, red, blue and black exclusion regions look impressively ominous to me. If I were a SUSY practitioner I would be feeling definitely claustrophobic... But then, I guess one might also think that the fact that the window is closing down only makes discovery more imminent. After all, that's what happened with the Higgs. Pity we'll have to wait until the end of 2015, at least, to find out.

    The "closing window" only refers to the case, where the *heavy* CP-even Higgs is interpreted as the newly discovered state at 125.5 GeV. That possibility might indeed be ruled out by non-SM Higgs searches. The more "conventional" interpretation in which the light CP-even Higgs has a mass of 125.5 GeV remains a perfect possibility (as also mentioned in the article arXiv:1302.7033.

    Skeptics believe that this story of the cure child is not really conclusive. They say that the girl - although at high risk for contracting the virus from her mother - was not actually infected. This story shows no proof that the child was indeed born with HIV. On the other hand it mentions that the girl, being at high risk of infection, was placed on treatment even before laboratory investigations had been done. That being said, doctors agree that the child was most likely infected, so it's still a story of hope :)