Nothing new from the dielectron spectrum February 20, 2007Posted by dorigo in news, physics, science.
CDF released a few days ago a new result in the search of high-mass resonances decaying to electron-positron pairs, based on 1.3/fb of Run II data. There are quite a few models of new physics that may yield the searched signature: lots of Z’ models, and Randall-Sundrum gravitons.
The search is quite straightforward from an experimental standpoint, and indeed you can see in the plot below that the mass spectrum is simply understood as the sum of the dominant Drell-Yan production – with the outstanding Z boson at 91 GeV- and fake electron processes, which contaminate the sample at a minimum level.
The agreement of data with Standard Model processes is quite nice, or disturbing, depending on what side you are rooting for. Indeed, no fluctuation in the data appears anywhere in the spectrum, as quantitatively described by the following plot:
In it, you can see that the minimum agreement of the data with expectation occurs at 370 GeV, where the probability of observing as many events as the data is of the order of 1%. Strange ? Absolutely not: quite in line, in fact, with the expected range of minimum observed probability (the region boxed in blue), given that we are checking a wide spectrum.
So from the agreement Z’ models – which allow the computation of an expected cross section for these resonances, as a function of their unknown mass – can be ruled out: you can see it in the following plot.
You can see that the black curve, which is the limit on the cross section of a Z’ decay to a electron-positron pair (the y axis) obtained from the data, is lower than the expected cross section for all the Z’ models up to 730 GeV (for the model which predicts the smallest cross section).
Also interesting is to compare the prediction for a Randall-Sundrum graviton with the cross section limit. It is done in the following plot:
Here, to plot the data several choices were made on the ratio k/Mp, a parameter of the model. K describes the “scale of warping of the extra dimension”, and Mp is the effective Planck scale. You can see that more warping means a less observable graviton – so we can exclude a RS graviton up to 820 GeV if k/Mp=0.1, but much less so if k is smaller.
All in all, just another nice agreement with SM and Tevatron data. The 1000 dollars I bet on the lack of new Physics at the LHC feel comfortable in my safe.