Tevatron top mass
March 21, 2006
Tommaso Dorigo points us at a new Tevatron top mass combination (hep-ex/0603039), using Run I and II data from CDF and DØ — get your copy while it’s hot! The headliner is a combined 2.3 GeV uncertainty — or, more exciting, a 1.3% relative uncertainty on the top mass. Sure to slightly-more-tightly constrain the mtop-mW plane for a while to come.
Some features of note:
- It uses Run II preliminary results for mtop. Since preliminary propagates, the combination is preliminary also.
- The breakthrough in reducing the systematic uncertainties has been the new technique of calibrating the absolute jet energy scale using the dijet W decays from the top sample (Cornellians may remember the talk given by Pekka Sinervo a while back). This cleans up the QCD background enough so the no-go theorem prohibiting W→jj reconstruction at a hadron collider is violated. This procedure in a sense makes the jet energy scale a measurement, and so the energy scale is actually uncorrelated between statistically independent samples. Quite neat. It’s not absolutely perfect; they still need relative calibration from dijet and gamma-jet balancing, and b-jets have a somewhat different response from light quark jets, but it’s still pretty amazing, and the key to pushing the uncertainties down so far.
- The combination is dominated by the Run II lepton+jets values which have had this magic in situ calibration, with some slight help from the Run I DØ lepton+jets and Run II CDF dilepton results. Through a miracle of statistics, the CDF Run I lepton+jets result contributes with a negative weight, which we are assured is good for us. Unfortunately Cornell doesn’t have NIM electronically before 1995, and it’s a bit late for a trudge to the library to find out what negative weights might mean.
- The note number TEVEWWG/top 2006/01 shows that the Tevatron slash-and-abbrev brigade is as powerful as always.
In summary, they threaten us with better than 1% uncertainties on the top mass from the full Run II dataset. Here’s hoping.