Thursday, March 12, 2015


The ATLAS experiment at the LHC has reported a three sigma excess of events beyond the Standard Model expectation in a particular kind of search for squarks and gluinos, a type of particle predicted by Supersymmetry models.

This is one of the strongest experimental indicators of SUSY phenomena to date amidst an ocean of searches and may simply be an overstated statistical fluke due to look elsewhere effects (i.e. the notion that if you do enough searches, some will come up positive by random chance, undermining the significance of any particular result that is not replicated).  If CMS sees the same thing (and this study failed to reproduce a similar, but slightly weaker excess in the CMS data) then it could very well be real.  If CMS does not see it, it is probably just a fluke.

A five sigma effect is considered necessary to call a finding a "discovery" of a particle.

A summary at the conclusion of the paper states:
This paper presents results of two searches for supersymmetric particles in events with two same-flavour opposite-sign leptons, jets, and E miss T , using 20.3 fb−1 of 8 TeV pp collisions recorded by the ATLAS detector at the LHC. 
The first search targets events with a lepton pair with invariant mass consistent with that of the Z boson and hence probes models in which the lepton pair is produced from the decay Z → ``. In this search 6.4 ± 2.2 (4.2 ± 1.6) events from SM processes are expected in the µµ (ee) SR-Z, as predicted using almost exclusively data-driven methods. The background estimates for the major and most difficult-to-model backgrounds are cross-checked using MC simulation normalised in data control regions, providing further confidence in the SR prediction. Following this assessment of the expected background contribution to the SR the number of events in data is higher than anticipated, with 13 observed in SR-Z µµ and 16 in SR-Z ee. This corresponding significances are 1.7 standard deviations in the muon channel and 3.0 standard deviations in the electron channel. These results are interpreted in a supersymmetric model of general gauge mediation, and probe gluino masses up to 900 GeV. 
The second search targets events with a lepton pair with invariant mass inconsistent with Z boson decay, and probes models with the decay chain χ˜ 0 2 → ` + ` −χ˜ 0 1 . In this case the data are found to be consistent with the expected SM backgrounds. 
No evidence for an excess is observed in the region in which CMS reported a 2.6σ excess [24]. 
The results are interpreted in simplified models with squark- and gluino-pair production, and probe squark (gluino) masses up to about 780 (1170) GeV. 
Lubos Motl offers a cautious but hopeful assessment of the result being really due to SUSY.

UPDATED March 22, 2015: More SUSY exclusions here.


Maju said...

Habemus SUSY? XD

And if we do, will String Theory be again popular and above all plausible? 0.0

andrew said...

If we have SUSY, Sting theory will be plausible and popular. But, I strongly suspect that this is a false alarm.

andrew said...

The narrow window on the kind of SUSY models that this could fit are discussed here. While I haven't exhaustive reviewed the literature, I suspect that some of even this very narrow parameter space has been ruled out by other experimental tests of SUSY, of which there are many, almost all of which with a null result.