Wednesday, March 13, 2013

Top Quark Physics At Moriond

Today is top quark day at the Moriond conference. 

The latest LHC measurement of the top quark mass is 173.3 +/- 0.5 +/- 1.3 GeV.  The final Tevatron measurement is 173.2 +/- 0.6 +/- 0.8 GeV, which is slightly more accurate.  These are precisiosn of 0.8% and 0.6% respectively.  The fact that the two independent measurements confirm each other to within 0.1 GeV when their stated accuries are much larger is also encouraging.  The top quark has a lifetime of about 0.5 * 10^-24 seconds, which is generally too short for them to hadronize - they almost always decay to bottom quarks.  It has a yukawa coupling to the Higgs boson of approximately one.  (The W boson mass world average is about 80.385 +/- 0.015 GeV).  A Higgs boson mass of 125.7 GeV (within error bars far greater than the experimental values) favors a W mass at the low end of the range of uncertainty, closer to 80.37 GeV and a top mass on the high end of the range of uncertainty, closer to 173.4 GeV. (As an aside, the Z boson mass is about 91.188 GeV with an uncertainty of perhaps +/- 0.033 GeV).

The measured top quark mass less the measured antitop quark mass is -0.272 +/- 0.196 (stat) +/- 0.122 (syst) GeV, which is about 1.2 sigma from zero, which would generally be considered experimentally consistent with zero (a strong theoretical expectation due to CPT symmetry).

The rate of top quark decays to bottom quarks is consistent with the Standard Model theoretical expectation to a high degree of precision, strongly disfavoring 4th generation quarks, charged Higgs bosons, or the other new physics in the mass vicinity of a top quark.  The measured value of bottom quark decays where 1 is the Standard Model expectation is 1.023 +0.036-0.034 with a 95% probability that it is at least 0.945.  If one assumes three generations and CKM unitarity and does a global fit the measurement drop to 1.011 +0.018-0.017 and is greater than 0.972 with a 95% probability. 

The observed CP violation is within about 0.5 sigma of the Standard Model prediction.  All of the top quark results are consist with each other and the Standard Model predictions.

Final analysis of the Tevatron results also found no new physics in its top quark searches.

1 comment:

andrew said...

Update via Lubos:

"In June 2011, Tommaso Dorigo hyped a very bad measurement by CDF at the Tevatron that claimed that the top-antitop mass difference (which must be zero due to the CPT-theorem i.e. essentially due to the Lorentz symmetry, of course) was (3.3±1.4 stat ±1.0 syst)GeV.

Their U.S. colleagues in the D0 collaboration debunked that preposterous result.

But it's interesting to see what CMS may determine with the 2012 data. Their value of the top-antitop mass difference is
(0.27±0.20 stat ±0.12 syst)GeV."

The new result from the CMS experiment at LHC is ten times as precise and consistent with zero.