The Z lineshape challenge: ppm and keV measurements

Abstract

The FCC-ee offers powerful opportunities for direct or indirect evidence for physics beyond the standard model, via a combination of high-precision measurements and searches for forbidden and rare processes and feebly coupled particles. A key element of FCC-ee physics program is the measurement of the Z lineshape from a total of $$5\times 10^{12}$$ 5 × 10 12 Z bosons and a beam-energy calibration with relative uncertainty of $$10^{-6}$$ 10 - 6 . With this exceptionally large event sample, five orders of magnitude larger than that accumulated during the whole LEP1 operation at the Z pole, the defining parameters—$$m_\mathrm{Z}$$ m Z , $$\Gamma _\mathrm{Z}$$ Γ Z , $$N_\nu $$ N ν , $$\sin ^2\theta _\mathrm{W}^\mathrm{eff}$$ sin 2 θ W eff , $$\alpha _\mathrm{S}(m_\mathrm{Z}^2)$$ α S ( m Z 2 ) , and $$\alpha _\mathrm{QED}(m^2_\mathrm{Z})$$ α QED ( m Z 2 ) —can be extracted with a leap in accuracy of up to two orders of magnitude with respect to the current state of the art. The ultimate goal that experimental and theory systematic errors match the statistical accuracy (4 keV on the Z mass and width, $$3\times 10^{-6}$$ 3 × 10 - 6 on $$\sin ^2\theta _\mathrm{W}^\mathrm{eff}$$ sin 2 θ W eff , a relative $$3\times 10^{-5}$$ 3 × 10 - 5 on $$\alpha _\mathrm{QED}$$ α QED , and less than 0.0001 on $$\alpha _\mathrm{S}$$ α S ) leads to highly demanding requirements on collider operation, beam instrumentation, detector design, computing facilities, theoretical calculations, and Monte Carlo event generators. Such precise measurements also call for innovative analysis methods, which require a joint effort and understanding between theorists, experimenters, and accelerator teams.

FCC-ee overview: new opportunities create new challenges

“…The point-to-point centre-of-mass-energy uncertainties of the resonance scan, which can be verified in particular by means of the muon pair invariant mass reconstruction, are most relevant for the Z width and the forward-backward asymmetries. This sets stringent constraints on the stability of the momentum reconstruction over time and scan points [62]. -W mass determination The W mass can be obtained from a scan of the WW pair threshold.…”

Challenges for FCC-ee luminosity monitor design

“…The FCC-ee programme [9] includes four major phases with precision measurements of the four heaviest particles of the Standard Model: (i) the Z boson, with 5 × 10 12 Z decays collected around the Z pole, (ii) the W boson, with 10 8 WW pairs collected close to threshold, (iii) the Higgs boson, with 1.2 × 10 6 e + e − → HZ events produced at the cross section maximum, and (iv) the top quark, with 10 6 t t pairs produced at and slightly above threshold. In particular, the first two phases, with their superior statistics, call for the highest achievable systematic accuracy [10,11]. Ambitious precision goals have been set to 10 −4 for the absolute luminosity measurement and one order of magnitude better for the relative measurement between energy scan points.…”