No Ambulance Needed: Unexpected Diboson Excess at LHC Keeps Super-Symmetry Alive

The grapevines are buzzing again – and there is again a sliver of hope. Many believe that Super-Symmetry (SUSY), the purported next step in high-energy physics is almost dead, with the LHC, the big boy on the block, finding no signatures of it as yet. Just this week, however, a paper and a presentation at the prestigious global International Conference on High Energy Physics (ICHEP), happening in Valencia, Spain, aims to correct that situation a bit. They shout ‘Stop the Ambulance‘, employing a nice play on words.

An event consistent with the Higgs decay. It can decay into two Z-bosons, which can give the four muons (red lines in the pic).
An event consistent with the Higgs decay. It can decay into two Z-bosons, which can give the four muons (red lines in the pic).

What’s left to know?

Their claim is simple – we have seen an excess number of events, unexpected if the known Standard Model of Particle Physics is all there is. The Standard Model (SM) forms the backbone of known particle physics, and it has been rigorously verified over decades. With the discovery of the Higgs, almost exactly 2 years ago, it is believed to the ‘complete’. Physicists are now looking into the chinks in the armour of the SM, hoping to find a crack here or a broken seam there through which they can glimpse some new physics. So far, the armour has been imposing and flawless, but there are still checks to be done.

One of the most important particles in the SM is the W-boson. It can either be positive or negative. All processes producing these bosons are well-known and calculated. Basically we know how they are produced quite accurately. So far, all measurements confirm our theoretical calculations. However, this group reports on an excess of diboson production seen by the LHC, which basically means that there are more pairs of W-bosons being created than expected from the SM.

There’s more

What is more intriguing is that the single W-boson production rates, the single Z-boson (a companion to the W, but neutral in charge), WZ production and a pair of Z-boson productions rates have matches with the predicted SM rates. The pair of W-bosons just don’t seem to comply – and the discrepancy isn’t too small. What more, the discrepancies are in the same direction for both the CMS and ATLAS collaborations of the LHC.

The authors of the paper have taken up a simplified model, consisting of the SM and a few SUSY particles and have showed that their model fits the data way better than the SM. Their model has a light stop, winos and binos.

That said, the data is still quite inadequate to claim anything solid. Everyone is eyeing the 2015 restart of the LHC runs. Let’s hope that SUSY survives till then.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.