Abstract
The forward-backward asymmetry of b quarks at the Z pole, A^{0,b}_FB , is the electroweak observable that currently presents the largest deviation with respect to standard model expectations [3] (2.8σ pull). An order of magnitude improved measurement at the FCC-ee could thus become a clean signal of new physics if the deviation in the central value stays. The world average measurement is still dominated by statistical uncertainties (∆A0,b_FB = 0.0016), but is also affected by non-negligible systematic uncertainties (∆A0,b_FB (syst.) = 0.0007). A fraction of it can be reduced at FCC-ee through dedicated studies on high-statistics control samples selected with well understood charge/flavor tagging techniques. A detailed analysis of the detector requirements to maximize heavy-flavor identification capabilities is also mandatory. Exclusive B decays should also be exploited. For instance, about 1e8 B+ decays, not affected by charm contamination or B-mixing effects - will be available at the FCC-ee. Similar approaches, using both inclusive and exclusive tagging capabilities should be followed for the measurement of the forward-backward asymmetry of charm quarks, A0,c_FB .
An irreducible source of uncertainty in the current estimate of A0,b_FB , fully correlated among experiments, is the use of a theoretical QCD factor, of order 1 − αS/π, that corrects the experimentally observed asymmetry. This correction takes into account the unavoidable angular distortions due to QCD radiation in the Z → bb(g) decay. Recent re-evaluations of that uncertainty using modern parton shower tunes seem to be consistent with the initial estimates. New strategies to reduce or constrain experimentally the size of these uncertainties should be developed in order to keep am overall uncertainty target of ∆A0,b_FB ≈ 0.0001.
Contact
Juan Alcaraz Juan.Alcaraz@cern.ch