New Tevatron average of top mass March 16, 2007Posted by dorigo in news, physics, science.
The Tevatron electroweak working group has produced a new average of CDF and D0 results on the top quark mass, which includes the most recent new determinations of that quantity.
So, the new Tevatron-average of the top quark mass is…
M(top) = 170.9 +- 1.8 GeV.
What is left to me is to comment that the apparently small improvement – from 1.2 to 1.1% of relative uncertainty since last year – is the result of painstaking work from the two experiments, and the fruit of 1/fb datasets. With the 5-6 times more data the two experiments expect to collect by the end of 2009, the Tevatron might arrive at a precision in the top mass measurement that LHC would not be able to match for a long time…
On the other hand, think positive: CMS and ATLAS will have a reference point to calibrate their jet energy scale to better than one percent accuracy!
A further comment is about the value itself: last August the top mass was quoted at 171.4+-2.1 GeV . The decrease to 170.9 GeV is small but significant, if you note that the two averages share most of the input. The effect of this small downward shift will be to push further down the fitted value of Higgs boson mass from global electroweak fits, which currently quote it at 76 GeV with +33 and -24 GeV errors . The upper limit, at 95% confidence level, is at 144 GeV – and the inclusion of the new top mass average will bring that number further down.
If the trend should continue, or even if the measured top quark mass stayed the same but the error got to shrink by a factor of two through more data and more great work, it is easy to predict that we would end up in a situation when the “tension” becomes a plain disagreement. What if the Standard Model Higgs gets to be indirectly excluded to be above 114 GeV by global electroweak fits, but it is directly excluded below the same value by LEP II ?
No worry. The limits are just saying there is “only” a 5% chance that the mass is beyond the bounds. We would still be ok for a while…