We use the transverse momentum spectrum of leptons in the decay chain t→bW with W→lv to measure the helicity of the W bosons in the top quark rest frame. Our measurement uses a tt̄ sample isolated in 106±4pb-1 of data collected in pp̄ collisions at √s=1.8TeV with the CDF detector at the Fermilab Tevatron. Assuming a standard V-A weak decay, we find that the fraction of W's with zero helicity in the top rest frame is F0=0.91±0.37(stat)±0.13(syst), consistent with the standard model prediction of F0=0.70 for a top mass of 175GeV/c2.
Bibliographical noteFunding Information:
We thank the Fermilab staff and the technical staffs of the participating institutions for their vital contributions. This work is supported by the U.S. Department of Energy and the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, the Istituto Nazionale di Fisica Nucleare of Italy, the Ministry of Education, Science and Culture of Japan, the National Science Council of the Republic of China, and the A. P. Sloan Foundation.