A Study of Final-State Radiation in Hadronic Z Decays

Citation

Kirkby, David P. (1996) A Study of Final-State Radiation in Hadronic Z Decays. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/n7fb-yw42. https://resolver.caltech.edu/CaltechTHESIS:05272021-214010673

Abstract

I have studied hadronic decays of the Z boson that are accompanied by isolated and energetic photon radiation from one of the primary quarks, Z → qqγ . This study enables me to measure the electroweak couplings and charges separately for up-type and down-type quarks.

I have measured the fraction of hadronic Z decays that contain a photon radiated by a primary quark to be

BR(Z → qqγ)/BR(Z → qq) = (2.85 ± 0.14) x 10^-3,

where I required that the photon have an energy between 8 GeV and 44 GeV and be accompanied by less than 100 MeV of hadronic energy within a 20° cone about its direction. Both the statistical and systematic errors in this result are smaller than those of comparable previous results.

I have calculated the energy distribution of isolated final-state radiation at next-to-lowest order, O(α_sα), and performed a fit of this prediction to my measured energy distribution to obtain a constraint on the quark couplings. I have combined this constraint with a second constraint that I have derived from the 13 measurements of cross sections and charge asymmetries at the Z peak. By assuming the Standard Model quark charges, I have measured the couplings of up- and down-type quarks to the Z boson, c ≡ 4 (g_v² + g_A²), to be

C_u = 1.11 ± 0.17 and C_d = 1.52 ± 0.11.

The experimental errors in this result are smaller than those of comparable previous measurements. By parameterizing C_u and C_d in terms of the quark charges, I have measured these charges to be

3|Q_u| = 1.90 ^+0.25_-0.46 and 3|Q_d| = 1.06 ^+0.26_-0.11.

This is the first such measurement of the quark charges at LEP. The quark electroweak couplings and charges that I have measured are consistent with the Standard Model.

I have selected a sample of events containing isolated and energetic photons from 2.76 million hadronic Z decays recorded by the L3 detector between 1991 and 1994. I have analyzed this sample to determine the energy distribution of isolated photons radiated by a primary quark by subtracting initial-state bremsstrahlung and hadronic background, and by applying acceptance and detector corrections. The hadronic background, which consists mostly of decays of isolated neutral pions into photons, is underestimated by existing Monte Carlo models. I have used a new technique of analyzing shower shapes in the L3 electromagnetic calorimeter to study this background directly using data. The discrepancies between data and Monte Carlo predictions that I have observed have implications for detecting the Higgs decay, H → γγ, at the next generation of high-energy experiments at the LHC.

Thesis Files