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    New LHC Diboson Excesses Point To Light SUSY !
    By Tommaso Dorigo | July 4th 2014 03:44 PM | 26 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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    Among the many more-or-less boring news from the ICHEP conference (International Conference on High Energy Physics), which is presently going on in Valencia (Spain), one bit today is sending good vibrations through the spine of many of the few phenomenologists who have chosen to remain faithful to the idea of Supersymmetry all the way to the bitter end. It is the excess of diboson events that ATLAS has just reported there.

    W bosons are the massive particles discovered in 1983 by Carlo Rubbia at the CERN SppS collider. They govern weak interactions, the ones responsible for radioactive decays as well as for some of the processes going on in the stars' cores. W bosons can be produced in pairs through quite standard electroweak processes in particle collisions if the available energy is sufficiently high. And indeed, the pair production of W bosons has been studied since the late nineties at the Tevatron, at the LEP II collider, and then of course at the LHC.

    The fact that the Higgs boson may decay into WW pairs is one extra motivation for studying these processes, but in earnest they are quite interesting by themselves. Already the sheer production rate of WW pairs at high energy is a topic on which one could write volumes - the cross section of the process would grow beyond what can be considered logical (the "unitarity bound") if there were no Higgs boson to damp down the rate. But let me stick to today's topic: SUSY!

    It turns out that a paper titled "Stop the ambulance!" was submitted to the Arxiv just one week ago. In it, authors Jong Soo Kim, Krzystof Rolbiecki, Kazuki Sakurai, and Jamie Tattersall argue that the rates of WW production published by ATLAS (in 7-TeV collision data) and CMS (in 7- and 8-TeV collision data) are all above the Standard Model prediction, and go on to explain how a very simple SUSY model including light stop quarks, charginos, and neutralinos could fit the data much better than the Standard Model does.

    The authors of 1406.0858 produce a fit of SM+simplified SUSY which has a maximum likelihood exceeding the SM one by 13.3 units - corresponding to an over 3-sigma preference of the data for the SUSY hypothesis. A first hint of Supersymmetry after all ?

    The matter is serious, as the WW production process occurs in LHC collisions mainly by quark-antiquark annihilations, for which uncertainties are small. It is a very simple electroweak process and SM calculations just cannot be much off. But of course, with dozens of measurements of production rates by the LHC, finding one off cannot be too exciting, can it ?

    Apparently, it can. In fact, the only missing piece to the picture of high-energy WW production cross section measurements, given the LHC Run 1 data, was the ATLAS 8-TeV WW result. And it has just been shown at ICHEP: ATLAS measures the cross section at 71.4+-7.5 pb, considerably higher than the theory prediction (57.3 pb) [For comparison, a year ago CMS published its measurement at 8 TeV as σ(WW)=69.9+-6.9 pb, using only a few inverse fb of 8 TeV data; the result is more precise than the new ATLAS one, but it is systematics-dominated as the competitors', so it is unlikely that CMS will try to improve it soon]. A zoom-in view of the new ATLAS measurement is shown below - I was intrigued by the choice of colours and symbols enough to want to zoom into the new measurement (the third line from top).



    Artistic, isn't it ? The grey bars are SM theory predictions, and the orange one is the new ATLAS measurement. Okay, yes, artistic maybe, but more interesting is the whole graph, which collects many different measurements by ATLAS. See below.



    The graph is busy, but you do not need to go through that whole list. The point is that we have some consistent deviations by the two LHC experiments with respect to the Standard Model; and a rather simple Supersymmetric scenario which appears to fit these deviations effectively. Even more significant is the fact that the model published by the SUSY phenomenologists came before the latest ATLAS result; so it is to some degree a "confirmation" of the picture drawn by the theorists.

    The graph below, taken from the preprint, is rather striking. It is a "temperature plot" showing the difference of the likelihood from the value of the best fit. Values close to zero are in red, and they indicate points of parameter space which give a good fit to the considered ATLAS and CMS measurements. On the horizontal axis you see the stop mass, on the vertical axis the neutralino mass. There is a nice band of red points around M_stop=200, M_neu=150 GeV which is out of the current excluded regions of parameter space (lying below the black, yellow, and purple curves).



    Despite the inspiring graph and the red streak of high-likelihood values lying entirely in non-excluded regions, the significance of the departures from SM rates is so far still too weak to claim anything; and yet, as I said above, some SUSY phenomenologists are not going to bed tonight, preferring to check their models than sleeping over it.

    What can I say... If they're roses, they will flourish. I very much hope they will, but I remain sceptical. It ill be very interesting to see if ATLAS and CMS will make an effort to investigate more in detail the SUSY scenarios which would give exactly the observed excesses. Of course, the LHC SUSY searches are already extremely wide-range, but some corner of phase space might still be looked at in more detail... And needless to say, the 13-TeV LHC run of 2015 is getting closer. So stay tuned for what could fizzle out or become the discovery of a young but already eventful century!

    Comments

    To my surprise, I find in myself a fervent wish that this thing will fizzle. A discovery of super symmetry at this point would be unbelievably boring. Imagine a whole new wave of pop sci books explaining it for the billionth time. The horror!

    The discovery of SUSY, here or in any other channel, would be one of the most profound discoveries of humankind. Only ignorants could consider this boring.

    Sven

    Oh yeah,really exciting. Now all we need is another few decades of experiments to nail down the correct version of super symmetry out of all the possible versions. Exciting. YAWN!!!

    dorigo
    Dear Elbi,

    if this does not excite you, I think you're reading the wrong blog ;-)

    Cheers,
    T.
    Not just the wrong blog but the wrong topic/career. You can't do science if you find boring to figure out the way nature is.

    elbi

    Let's step back for a few seconds and perform an experiment. Hold your laptop (phone or tablet) four feet off the ground and drop it. Make note of the results. There's your experimental verification for physics beyond the standard model: the SM does not explain gravity.

    The discovery of supersymmetry would be a very profound and important step in theoretical physics. It would imply that there was still some physics left within our experimental grasp who's resolution could potentially shed light on the hard problems that experiment can't help us with.

    This was why the Bicep signal was so exciting for so many physicists. It was a window into areas that we couldn't access. Similarly the structure and pattern of a supersymmetric spectrum would be incredibly important to elucidating a host of questions.

    Hfarmer
    On one level I hope that SUSY can be confirmed.  Not for any logical reason.  It would be too depressing to think of so many years of so many very smart people wasted on research almost exclusively using SUSY.  All that work on M theory being a total waste without SUSy
    On the other hand, SUSY has felt like the applied Phelbotinum of the real world.  I can be invoked to explain so many things that it seems too good to be true, and with so many versions it can't be disproven. 

    What are the odds that SUSY exist but does not play the huge role it was originally cast for?
    Science advances as much by mistakes as by plans.
    dorigo
    Hello Hontas,

    if Nature is supersymmetric and it does not solve any of the problems that called for its
    discovery by theorists, it would be 1) incomprehensible 2) chaotic 3) it would destroy the
    paradigm of logical inferences that has so far helped us develop our science. But I see it
    as a really remote possibility...
    Cheers,
    T.
    Interestingly, the SUSY particle masses favored by this analysis/these measurements (light stos and liight electroweak particles) are in good agreement with both, naturalness and (g-2)_mu. Everything looks consistent. :-)

    Sven

    Sven,

    Intriguing points indeed. But what hampers my hope for a breakthrough is that we've seen excess rates before that seemed to have evaporated with more data added in. Recall the di-photon excess in the initial Higgs decays? How about the CDF muon anomaly, the PAMELA positron anomaly, the excess of soft pions in Z-decays?...

    Cheers,

    Ervin

    Ervin,

    every excess can "evaporate", of course also this one. But also (nearly) every discovery starts with a few sigma excess. I would not bet my life on this one, but it looks nicer and more consistent than others. :-)

    Sven

    If di-boson and lepton-flavor violating anomalies are real, along with the measured deviation of the muon magnetic moment, we should definitely see first signs of BSM physics in 2015. The next LHC run is going to be truly exciting!

    Ervin

    dorigo
    You're asking for too much Ervin - all real ? Usually 999 out of 1000 2-3 sigma results turn out to be flukes or bad systematics. We're going to be deliriously happy if one of them is real, that'd be enough!

    Cheers,
    T.
    You are right, Tommaso. I meant to refer to "either one" of these anomalies, not to all of them at once.

    Ervin

    A 3 sigma deviation is a fluke 1 time in 740. So how is it that 999/1000 are flukes?

    dorigo
    Hi Rehbock,

    a 3-sigma deviation (or a n-sigma one, whatever) can be always, or never, a statistical fluctuation, depending on what your null hypothesis is. E.g.: if your null is "the ratio of a circumference divided by a diameter is pi", ALL your 3-sigma-off measurements will be flukes. If your null is "the boiling point of water at standard pressure is 70 degrees Celsius" and your measurement device has a precision of 10 degrees, virtually none of your 3-sigma-off measurements will be flukes.

    When I say that 999 out of 1000 3-sigma deviations of the SM are flukes, I am saying something quite qualitative - just a way to say that the SM is a very good prior.

    Cheers,
    T.
    Sven,

    see my comment further down - unless A_t, M_3, the other sfermion masses etc. are tuned against each other, a stop mass of 212 +/- GeV seems to run away from the chargino mass, with which it needs to be degenerate up to ~10 GeV, very quickly (and possibly in a dangerous negative direction if the gluino is heavy). Is it really natural SUSY to demand such a degeneracy, or don't we introduce a new stop-chargino naturalness problem at the same time?

    That's interesting. If the excess remains it will probably falsify the multiverse -- esplly if SUSY is discovered?

    Hello,

    Where exactly is your plot of diboson cross section measurements from? I can't find it amongst the ICHEP 2014 talks. Could I have a link?

    Thanks,

    Andrew.

    dorigo
    Hi andrew,

    it's Fig.5(right) from https://cds.cern.ch/record/1740976/files/HIG-14-005-pas.pdf which is also linked in the article above.

    Cheers,
    T.
    Hello,

    Thanks for your help, but I think we have a mix-up! Fig. 5 (right) in that link is a plot from your other recent blogpost, a CMS plot of Higgs signal stregnths in http://www.science20.com/a_quantum_diaries_survivor/does_the_higgs_viola...

    I would like to know, if possible, where I can find the talk/paper with the diboson cross section measurements vs data/theory plot in this blog post?

    Thanks,

    Andrew.

    One thing that at first glance makes me a bit pessimistic that it's really such a type of SUSY model that we are observing here, is that the near degeneracy between the stop and the chargino which we require in order to keep the b-jets soft, doesn't seem very stable under renormalization group flows. So the model seems in need of finetuning for small stop mass running and/or a finetuned high scale stop mass, in order to have near-degenerate pole masses. Maybe someone with more sExperience can comment on whether that is a valid concern or easy to circumvent.

    Alex,

    you are asking a different question. The approach here (or in the paper by
    Rolbiecki et al.) was purely phenomenological, i.e. only analyzing the
    low-energy parameters. Once they are pinned down one can ask the next
    question: do what kind of model at a/the GUT scale does this correspond. I
    myself would not worry about specific RGE running at this stage, this can be
    figured out later. However, one thing would be clear already now: the "normal"
    (i.e. simplest) GUT scale relations for M1, M2 and M3 would not hold, as they
    predict (roughly) mcha1 = 2 mneu1. But then again, this relation is not
    sacred, and in principle anything for these parameters is possible. One has to
    see it the other way around: from low-energy parameters we learn about
    unification and the GUT scale.

    Cheers, Sven

    Sven,

    I appreciate their simplified model approach, and agree that once you find a simplified (or say phenomenological) model, it becomes interesting to go one step further - which is exactly what we are doing when we start to speeculate whether we find it likely that this decay chain of three new particles, which we need to explain the phenomenon, comes from a susy model.

    Now that being said, I don't really talk about running to the GUT scale - merely considering the parameters at a TeV or a few TeV which is reasonable if stuff like gluino, heavy higgses and squarks live up there. If you assume a TeV Gluino and for the sake of the naturalness argument assume that the M_3 term in the anomalous dimension is indicative of the mQ3 running, then you barely make it a few dozen GeV in the renorm scale before the degeneracy of the chargino and stop are broken so much that the model is ruled out. Unless I made a mistake :)

    Alex,

    I think you are still making some assumptions here. In their solution, mstop1
    = 212 GeV, you most likely need a relatively heavy stop2, probably at 1-2 TeV
    to get the right light Higgs mass. This points towards very different diagonal
    soft SUSY-breaking parameters in the stop sector. How these are
    connected/unified (or not) at some higher scale we do not know (yet),
    i.e. your "mQ3" might not exist as such.

    Cheers, Sven