Probing inside the proton

17 August 2015 | By

W and Z bosons are the massive carriers of the weak force, responsible for radioactive decays. These bosons also couple closely to the Higgs boson. W and Z bosons are produced at a large rate in proton-proton collisions at the LHC, where ATLAS physicists have now measured the rates for W and Z boson production using 13 TeV proton-proton collisions (corresponding to an integrated luminosity of 85 inverse picobarns).

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Figure 1: The measured value of σZ/γ∗×BR (Z/γ∗→ℓ+ℓ−) where the electron and muon channels have been combined, compared to the theoretical predictions. The predictions are shown for both proton-proton and proton-antiproton colliders as a function of the collision energy s√. In addition, previous measurements at proton-antiproton colliders are shown. All data points are displayed with their total uncertainty. The theoretical uncertainties on the cross section predictions are not shown. (Image: CERN)

The decays of W and Z bosons into high-momentum electrons, muons, and neutrinos are easily detected by the ATLAS experiment. Although certain background processes also produce these particles, careful comparison of the data with expectations allows physicists to subtract the contributions from those other events. ATLAS has measured the production cross sections in a sample of 950,000 W candidate events and 80,000 Z candidate events.

Results from the W and Z boson decays into electrons and muons are combined to give the total production cross section measurement. This measurement can be compared to theoretical predictions and to results from collisions at other centre-of-mass energies (see figures below). The W and Z boson production cross sections increase, as expected, by a factor of 1.7 as the LHC moves from a centre-of-mass energy of 8 TeV to 13 TeV.

Furthermore, because the W and Z bosons are produced by interactions of quarks and anti-quarks within colliding protons, these measurements can also be interpreted as a direct probe of the proton’s quark content. Although the current precision of the cross section measurement is limited by our knowledge of the LHC beam luminosity, the ratio of the W+ and W- boson production rates has revealed the relative contributions of the up and down quarks with 1% precision. This deep probe into the proton was made possible due to the LHC’s new high-energy regime.

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Figure 2: Ratio of W+ to W- boson production fiducial cross sections (red line) compared to predictions based on different PDF sets. The inner shaded band corresponds to the statistical uncertainty while the outer band shows statistical and systematic uncertainties added in quadrature. The theory predictions are given with the corresponding PDF uncertainties shown as error bars. Scale uncertainties are not included in the error bars of the predictions. (Image: CERN)