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W+charm production nailed December 7, 2007

Posted by dorigo in news, physics, science.
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W production in association with heavy flavor jets is a very important process at hadron colliders. The presence of heavy quarks in the final state may mimic the signature of important physics processes such as top production, Higgs bremsstrahlung off W bosons, and other more exotic mechanisms. Until recently, the various components of W+heavy flavor were only estimated with Monte Carlo simulations using theoretical predictions which had little experimental verification. Then, W+b \bar b production has been measured with good precision at the Tevatron. Even more recently, the elusive W + c production process has been accurately measured by CDF.

W+charm production occurs via interactions whereby a strange quark from the proton sea is converted into a charm by charged weak interaction, with a direct manifestation of its carrier, the W. The two leading order diagrams are shown on the right here and below. One clearly sees that the production only involves positive-charged W bosons and anticharm quarks, or negative-charged W bosons and charm quarks: there is therefore a very striking sign correlation, which can be readily exploited if the charge of the charm quark is measured.


Indeed, with the use of a soft lepton tagging algorithm – which finds the lepton originated from the semileptonic decay of the charmed hadron in the jet – it is possible to verify the charge correlation with the primary lepton from W decay, thus measuring the cross section and determining some interesting kinematical properties of the process.

The analysis is straightforward: from a W sample collected by requiring missing Et and a high-Pt electron or muon, events with one or two additional hadronic jets of E_T>10 GeV are selected. In 1.8/fb of Run II data CDF finds 1822 events where one of the two jets is tagged by the SLT algorithm, which searches for a soft electron or muon inside the jets. Of these, 1059 jets have the soft lepton charge opposite to the W lepton charge: that alone indicates a large asymmetry.

To measure W+charm cross section, the difference of opposite sign lepton pairs minus same charge lepton pairs in the data is subtracted by the same difference estimated from background processes. Backgrounds are mostly charge-symmetric, with the exception of non-W QCD events where the two leptons have a charge correlation due to their common origin (for instance, in the decay of a b \bar b pair), and Drell-Yan production, where the leptons also have opposite charge. Accounting for backgrounds and systematic effects, CDF measures \sigma(Wc) B(W \to e \nu) = 28.5 \pm 8.2^{+4.1}_{-4.4} \pm 1.7 pb,  where the first uncertainty is statistical, the second systematical, and the third is due to luminosity uncertainty of the dataset. This favourably compares with theoretical predictions obtained at leading order with the ALPGEN generator, equal to 22.2 \pm 1.2(PDF) ^{+3.8}_{-3.0}pb. Below you can see some kinematical distributions which show the good understanding of the data. The Wc process is in good agreement with its expected properties.

Above, the transverse momentum of the SLT muon contained in the charm jet has the expected characteristics from charm quark decay: it peaks at the low Pt threshold, as it should, while other backgrounds have harder spectra.

The azimuthal angle between missing Et and soft muon is another variable capable of distinguishing Wc production from the main backgrounds. In fact, the angle is large for the former on average, while it is very small for non-W backgrounds. Notice how there are negative entries in the plot, due to the OS-SS subtraction procedure by which the data is obtained bin by bin in the histogram.

Finally, the missing transverse energy, signalling the escape of a energetic neutrino from W decay, is another good indicator of the nature of the selected processes. Hats off to another nice analysis by CDF. 

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