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Plot of the Day – Large Extra Dimensions

March 12, 2011

The CMS experiment has been producing a host of new results in time for the last of the winter 2001 conferences. One of these results is a search for large extra dimensions.

Large extra dimensions (LED) are en vogue since the end of the nineties, when a few papers proposed them as an intriguing extension of the subnuclear physical world as we currently understand it. They have a number of interesting properties, among them the desirable explanation of the hierarchy problem.

The theory that CMS tested is the ADD model (from the initials of Arkani-Hamed, Dimopoulos, and Dvali, who proposed it in 1998). In the theory, gravity propagates through compactified extra dimensions, ones of which we are totally unaware, since we live on a 3-D “brane” of the multi-dimensional space. I will omit discussing the details of the theory (for evident lack of the necessary insight), but will just mention that the model implies virtual graviton exchange at leading order, producing an enhancement of non-resonant production of lepton pairs at high energy.

Lepton pairs of high energy is something which CMS is very well equipped to discover. And so muon pairs were sought in 40 inverse picobarns of proton-proton collision data produced by the Large Hadron Collider in 2010.

The relevant figure that constitutes today’s plot is shown below. It is a dimuon mass spectrum, with data shown as black points with error bars, and different processes contributing to the dataset are shown stacked one atop the other. A signal of Large Extra Dimensions is also shown, to evidence its utter disagreement with the observed dimuon pairs.

As you can see, after the selection of events with two energetic muons, and a cut to remove cosmic-ray candidates, only Drell-Yan production (in purple) and top pair production (in cyan) are expected to survive. A possible LED signal is shown in blue. The analysis required the invariant mass of signal-like events to be above 600 GeV, where no event is observed and 0.48 are expected from SM backgrounds . From the agreement of the data with the background-only hypothesis, limits can be set on the effective Planck scale governing the size of the extra dimensions. The limits are tighter than existing Tevatron ones, and extend up to 2.1 TeV, depending on the number of extra dimensions.

The CMS paper is available here.

(Posted by T.Dorigo)


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