RevModPhys.84.1477】Tests of the standard electroweak model at the energy frontier

Systematic uncertainties 30 20 10 10 1 0 100 150 200

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Unformatted text preview: κγ 1.5 2 FIG. 13. The (a) photon ET distribution for the D0 W analysis. From Abazov et al., 2008b. The photon ET distribution of events is used to constrain anomalous coupling parameters shown in (b). probability for compatibility between the data and the SM expectation, demonstrating a reasonable level of agreement. To specifically investigate the dip region around Q‘ Â Áy ¼ À1=3 indicative of the RAZ effect, a simple test statistic R is constructed which is the ratio of the number events observed in a bin including the dip region to the number of events observed in a bin with more negative Q‘ Â Áy where a maximum is expected, based on Monte Carlo simulation, from the SM including the total D0 acceptance in this analysis. A value of R ¼ 0:64 is observed in the data and it is determined that 28% of SM pseudoexperiments give a value as large or larger than 0.64. A particular anomalous coupling hypothesis corresponding to  ¼ 0,  ¼ À1 is chosen to test against as a null hypothesis since it is a specific model which leads to a Q‘ Â Áy distribution without a dip as shown in the inset of Fig. 14(b). Only the shape is tested such that the null ‘‘no-dip’’ distribution is normalized to the yield expected from the SM. The R distribution for the SM and no-dip hypotheses are shown in Fig. 14(b) along with the value (0.64) observed in the data. From this analysis, the no-dip hypothesis is excluded at 2:6 Gaussian significance. 3. Z As mentioned in Sec. III.E.1, photons do not directly couple to Z bosons at tree level in the SM. Therefore, observation of such a coupling constitutes evidence for new phenomena. The Z final state at hadron colliders involves a Rev. Mod. Phys., Vol. 84, No. 4, October–December 2012 0 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 R value FIG. 14. (a) The background-subtracted charge-signed rapidity difference of events is shown along with the SM expectation from Abazov et al. (2008b). Consideration of the full covariance matrix gives a 15% 2 probability for compatibility between the data and the SM expectation. (b) Distributions of the R-test statistic for the SM (solid line) and ‘‘no-dip’’ (dashed line) hypothesis pseudoexperiments from Abazov et al. (2008b). The data result is 0.64 shown by the vertical line where only 45 out of 104 no-dip pseudoexperiments had a value smaller as small or smaller than 0.64. The R-test statistic is defined as the ratio of events in the ‘‘Dip Bin’’ to the ‘‘Small Max Bin,’’ as shown in the inset where the solid line is the SM expectation and the dashed line is for the no-dip hypothesis. combination of ZZ and Z couplings. Both CDF and D0 made measurements of the Z cross section in leptonic decay channels of the Z boson. The signature of the Z signal is two isolated high ET charged leptons having the same flavor and opposite charge with invariant mass consistent with decay of a Z boson, and an isolated high ET photon. The dominant background is from Z þ je...
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This document was uploaded on 09/28/2013.

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