Charged Particle Optics in Circular Higgs Factory∗

Similar to a super B-factory, a circular Higgs factory will require strong focusing systems near the interaction points and a low-emittance lattice in arcs to achieve a factory luminosity. At electron beam energy of 120 GeV, beamstrahlung effects during the collision post an additional challenge to the collider design. In particular, a large momentum acceptance at 2 percent level is necessary to retain an adequate beam lifetime. This turns out to be the most challenging aspect in the design of circular Higgs factory. In this paper, an example will be provided to illustrate the beam dynamics in circular Higgs factory, emphasizing on the chromatic optics. Basic optical modules and advanced analysis will be presented. Most important, we will show that 2% momentum aperture is achievable. INTRODUCTION Since the discovery of the Higgs particle at LHC, the recent results for ATLAS and CMS have shown that the discovered particle resembles the Higgs boson in the standard model of elementary particles. Because of this remarkable discovery, it becomes increasingly important to precisely measure the property of the particle that gives themass to all and to study the nature of the spontaneous symmetry breaking in the standard model. The relatively low mass of the Higgs boson provides an opportunity to build an e+ and e− collider to efficiently and precisely measure its properties. In the production channel of e+e− → H Z , the beam energy required for such a collider is about 120 GeV, which is only 15% higher than the energy reached about two decades ago at LEP2. Can we design and build a circular Higgs factory (CHF) within a decade? What are the major challenges in the design? In this paper, we will address these questions. LUMINOSITY In a collider, aside from its energy, its luminosity is the most important design parameter. For Gaussian beams, we can write the bunch luminosity as Lb = f0 N2 b 4πσxσy Rh , (1) where f0 is the revolution frequency, Nb the bunch population, σx ,y transverse beam sizes, and Rh is a factor of geometrical reduction due to a finite bunch length σz and is given by