Contact: Qizhong Li
The measurement of the W boson production cross section times branching ratio to tau lepton and neutrino can be used with the corresponding result from the electron channel to test one of the fundamental concepts in the standard model: the universality of the leptonic couplings to the weak charged current.
We select taus which decay hadronically resulting in narrow jets in the DØ calorimeter. The signature for W -> tau nu events is a narrow tau jet plus missing ET. We use the jet width variable W and the jet profile variable P to select tau jets. W is the width of the jet in eta-phi space, while P is defined as P = (ET1 + ET2 )/ET, where ET, and ET1, ET2 are the transverse energies of the jet and the two towers within the jet with the largest ET. After subtracting the backgrounds due to QCD events in which one of the jets mimics the tau jet, calorimeter noise, Z -> tau tau and W -> e nu, we are able to measure the W -> tau the cross section times brancing ratio:
sigma(p pbar -> W + X) * B(W -> tau nu) = 2.22 +- 0.09(stat) +- 0.10(syst) +- 0.10 (lum) nb
We find the ratio of tau and electron electroweak charged current couplings to the W boson to be
g_tau / g_e = 0.980 +- 0.032
in agreement with lepton universality, which predicts a ratio of 1.
Our results are published in:
A measurement of the W->tau nu Production Cross
Section in pbarp Collisions at Sqrt(s)=1.8 TeV
Phys. Rev. Lett. {84}, 5710 (2000). PDF
(Copyright by the APS)
FERMILAB-Pub-99/373-E, hep-ex/9912065. PS
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The profile (P) distributions of (a) the $\tau$ sample from data, and (b) the QCD background sample. |
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a) The tau jet $E_T$ distribution for data passing all the selection cuts, and (b) the distribution of the transverse mass of the tau jet and the missing-ET for DBMC events (histogram) and for data (points) passing all the selection cuts and with QCD background subtraction. |
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The distribution of jet width (W) for DBMC tau jets (solid histogram) and QCD jets (dashed histogram) (a) before the profile cut (with arbitrary scale), and (b) after the profile cut. The jet width distribution for data (points) is also shown in (b). The QCD distribution in (b) has been scaled up by a factor of 10. |
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Summary of measurements of g_tau / g_e from UA1~[1], UA2~[2], CDF~[3], and this measurement (D0). |
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