Study of vertex optimization at the CEPC

Abstract

The CEPC is a planned future collider targeting precise Higgs property measurements. Its core physics program includes the determination of Br(H→ bb), Br(H→ cc), and Br(H→ τ+τ−), whose accuracies are sensitive to the vertex (VTX) design and the performance. This study is performed on Higgsstrahlung events e+e−→ ZH at √s = 240 GeV, with subsequent Higgs decay to bb, cc and ττ. Using full simulation, we quantified the dependency between these physics performances and the main VTX geometry parameters: the inner radius, the material budget, and the spatial resolution. We select three benchmark VTX designs, a conservative design according to the upgraded ALICE Inner Tracker System (ITS) geometry, a baseline detector design that is roughly two times better than the upgraded ALICE ITS, and an aggressive design that is roughly four times better. Comparing the physics performance at the aggressive design to the baseline one, we found the Br(H→ cc) measurement is extremely sensitive and the relative accuracy could change up to 40%. The relative accuracy on Br(H→ τ+τ−) changes by 8%. The Br(H→ bb) is not really sensitive to the VTX parameters and the relative accuracy only changes by 1%. Our analyses also show that the reduction of the inner radius is the most efficient way to improve the performance on Br(H→ cc) measurements.