RWTH Aachen Particle Physics Theory

Supersymmetry or just a bunch of logarithms?

November 20th, 2014 | by

In its first phase from 2010 to 2013, CERN’s Large Hadron Collider has delivered an impressive amount of new results. The LHC experiments ATLAS and CMS have measured a myriad of particle scattering cross sections with unprecedented accuracy. These cross sections reflect the probability of producing certain particles in the collisions of protons, smashed at each other by the LHC. The so-called “Stairway to Heaven” plot below shows the remarkable agreement between experimental cross section measurements (points) and the theoretical predictions (lines) within the Standard Model of particle physics.


Among all those measurements, the cross section for producing a pair of electroweak gauge bosons, pp → WW, is particularly interesting.  Electroweak gauge bosons acquire their mass through the Higgs mechanism, and examining how they are produced and how they decay is an important test of the way the mass of fundamental particles is generated. Interestingly, the experimental WW cross section appears higher than the Standard Model prediction, for both the ATLAS and CMS experiments.

While the discrepancy is not yet significant, and could well be a statistical fluctuation, it is interesting to see if the data may be explained by new exotic physics beyond the Standard Model. One of the most popular new physics models is supersymmetry, which predicts a superpartner for each of the Standard Model particles. No sign for superparticles has been found at the LHC so far, which has prompted some to declare supersymmetry dead. And while certain simple supersymmetric scenarios are indeed strongly disfavoured, my SLAC colleagues JoAnne Hewett, Tom Rizzo and collaborators have shown that a swath of supersymmetric models could have escaped the intensive search at the LHC so far.

Former Bonn/Aachen postdoctoral researcher Jamie Tattersall and collaborators have looked at the WW cross-section excess through their supersymmetric-colored glasses. They have approached the problem in a most pragmatic way by considering so-called simplified models which try to explain a certain experimental observation with the minimal amount of new particles and with as little theoretical bias as possible. As such, simplified models are not real theories, but they rather mediate between theory and data and provide clues as to which kind of new physics could explain an unexpected experimental result. And indeed, Jamie and collaborators could identify a simplified supersymmetric model with few new particles, that could explain the excess in the WW cross section and would not have been discovered otherwise!

In this case, however, I believe the difference between the LHC measurement of the WW cross section and the theoretical prediction points to a shortcoming of the Standard Model calculation, rather than to exotic new physics. To obtain predictions for the very complex reactions at the LHC, we have to rely on approximate calculations of the scattering probabilities. And despite some exciting recent progress, it remains a daunting task to improve upon even the simplest approximations. Notwithstanding the challenges, various teams of researchers have recently presented improved calculations for the WW cross section, including large logarithms of ratios of different energy scales that had previously not been considered. These new calculations show that the approximations used thus far most likely underestimate the exact scattering probability, so one may expect that improved calculations will be able to explain the LHC measurements of the WW cross section without invoking exotic physics.

In any case, there are interesting lessons to be learned: in spite of all rumours, supersymmetry is not dead (OK, maybe it does smell funny…) and could have escaped searches at the LHC; simplified models are a new and powerful way to interpret LHC measurements with less prejudice; and to establish evidence for new, exotic physics we need to continue developing the elaborate techniques to calculate Standard Model scattering reactions accurately.

It will be interesting to see how the discrepancy between the measurement of the WW cross section and the Standard Model prediction evolves with the new data to be expected in run 2 of the LHC starting in 2015, and with improved theoretical calculations. Like many others, I love and miss supersymmetry dearly, but I don’t believe that she will make a grand entry through the WW cross section; most likely, we have just missed a bunch of logarithms in our calculations.

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