Tuesday, March 02, 2010

Top quark: 15th birthday

In 1973, Kobayashi and Maskawa pointed out that the observed CP-violation in kaon decays may be explained by a third generation of quarks. For their discovery of the CKM matrix, a complex SU(3) matrix transforming the upper-quark mass eigenstates to the SU(2) partners of the lower-quark mass eigenstates, they were allowed to share the 2008 physics Nobel prize.

In this way, they also postulated the existence of the top quark - that was also called the "truth" for a while.

The Wilson Hall at the Fermilab doesn't really look like a top quark - as bright readers can determine by looking at the reflection in the water - but it could.

The world had to wait for 22 years for the experimental discovery: be sure that it is impossible for the experimenters (and Nature) to guarantee that they will empirically prove every correct theory before a 5-year bureaucratic deadline. Sometimes it can take decades, centuries, or millennia - even in cases when the seemingly modest task is to discover "just another quark". The discovery of the top quark took place exactly 15 years ago, on March 2nd, 1995.
Observation of the top quark (click)
The D0 collaboration has observed 17 di-lepton and single-lepton events while the expected background was around 4 events. This discrepancy meant a 4.6-sigma discovery of the new particle. They could have merged the forces with CDF that previously observed a 2.8-sigma signal.

However, this CDF paper received in February 1995 claimed a 4.8-sigma discovery by the CDF only. So which team was actually faster will remain disputable. The combined body of CDF and D0 surely had a five-sigma evidence for the top quarks well before each individual collaboration separately.

You should notice that the center-of-mass energy used for those discoveries at the Tevatron was 1.8 TeV those 15 years ago: it hasn't increased much. The maximum available energy for particle collisions was just increased from 1.96 TeV (Tevatron) to 2.36 TeV (LHC) a few months ago. In two or three weeks, we should finally see 7 TeV (LHC) collisions - a more appropriate number for the 21st century.

The current mass of the top quark is 173.1 +- 1.3 GeV.

Recall that the United Nations require that fermions must be discovered in America while bosons must be discovered in Europe. That's because fermions respect the individualist "Pauli's principle" while the bosons like to be in a large group: the Bose-Einstein condensate is an extreme example. The rule has worked so far because Barack Obama hasn't yet distorted its validity.

We'll see whether the Higgs bosons or squarks - new bosons - will be discovered at the LHC in Europe before the neutralinos (which may have correctly been discovered in Minnesota) and and/or charginos - fermions. ;-)