W-spin measured in a low-background environment

The signal templates for left-handed, longitudinal and right-handed W bosons together with the background template for the dilepton selection.

One of the phenomena CDF studies is the production of top and anti-top quark pairs. The top quark decays almost immediately into a W boson and a bottom quark. Thus in a top-anti-top event, two Ws are produced – one from each quark. The W is identified by its subsequent decay into an electron or a muon and a neutrino. The neutrino is not directly identified because it does not interact in the detector. However, the event’s missing energy is a sign that a neutrino was produced.

This relatively rare decay of top and anti-top into muons or electrons, two neutrinos and two bottom quarks, known as a top dilepton event, can be reconstructed.

The top quark, a relatively massive particle, has a particular spin state. When it first decays into a W and a bottom, it transfers its spin properties to the W since, between it and the bottom quark, the W is far heavier.

From the reconstruction we determine the rotational states of the W, of which there are three possibilities. They are the right-handed state, where the motion of the W particle and its spin are in the same direction; left-handed, where the motion of the W particle and its spin are in the opposite direction; and longitudinal, where the motion of the particle and its spin are perpendicular. The fractional probabilties of these three spin states must add up to one. Any given W can exist in only one state. What is measured is essentially the angle between the direction of the momentum of the lepton  – the decay electron or muon – and that of the top quark in the W boson’s rest frame (see figures).

The comparison of the cos ϑ* distribution between data and the expected Standard Model top + anti-top signal and background.

Using 5.1 inverse femtobarns of data we find the probability that a W is observed in a longitudinal rotational state is 71 +18/-17%. The right-handed fraction is -7 ± 9%. We further find that at the 95% confidence level that the right-handed fraction is less than 7%.

This is in very good agreement with the Standard Model prediction of a longitudinal fraction of 69.8% and a right-handed prediction of essentially zero (3.7 x 10-4).

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—edited by Andy Beretvas

These physicists were responsible for this analysis.

First row from left: Jaroslav Antos (Institute of Experimental Physics, Kosice, Slovakia), Andy Beretvas (Fermilab), Yen-Chu Chen (Academia Sinica, Taipei, Taiwan)

Second row: Soo-Bong Kim (Seoul National University, Korea), Roman Lysak (Institute of Experimental Physics, Kosice, Slovakia), Chang-Seong Moon (Seoul National University, Korea)