crossingangle scan possible staging scenarios Michael Benedikt Anton

crossing-angle scan & possible staging scenarios Michael Benedikt, Anton Bogomyagkov, Erk Jensen, Eugene Levichev, Rogelio Tomas, Uli Wienands, Frank Zimmermann 8 April 2015 FCC-ee Vidyo meeting

crossing angle issues Large crossing angle Small crossing angle + allows a large Piwinski angle & + minimizes luminosity loss due crab waist scheme with small to crossing angle beta*, high bunch charge, high tune shift, and high luminosity + facilitates beam separation and design of final quadrupole + about the same emittance at all energies - very short bunches without collisions (excessive HOM power) - strong dipoles and high synchrotron radiation in the final focus at high energies - very small emittances at the Z - ey due to synchrotron radiation in solenoid/antisolenoid body& fringe + bunch length about the same with and without collision + weaker bending magnets and less synchrotron radiation in final focus - lower luminosity

parameters for FCC-ee running at 91 (Z) and 240 Ge. V centre-ofmass energy (ZH), considering two IPs, and an rf frequency of 400 MHz

solid: xy, dashed: xx

question: can we find a crossing angle which would support both modes of operation?

staging possible purposes: • stretch the construction cost (RF, magnets) • optimize operation at the various energies • allow for gradual improvements in machine tuning and optics control (e. g. vertical emittance, beta*, beeam-beam)

possible staging approaches • installing RF system gradually and reconfigure it later • reducing beta* in steps • reducing arc emittance in steps • using different crossing angle at low energy and at high energy? • stage = one c. m. energy or several (overlapping) energies on each stage ?

possible stages at 45. 5 Ge. V with a constant crossing angle of 11 mrad and varying emittance, b y* and rf voltage

possible 4 stages:

possible operational stages of FCC-ee. The total crossing angle is held constant, equal to θc =11 mrad.

comments: • we could repeat these calculations for a different crossing angle • different energies per stage perhaps do not make sense (? )

Directly measuring the Higgs width The possibility of a special run with monochromatic collisions on the e+e- → H resonance is also indicated in the previous table. The luminosity value quoted is obtained in presence of large horizontal IP dispersion (Dx*≈ 2 m). The resulting relative c. m. energy spread is ∼ 10 -5, i. e. 3 times smaller than the standard-model width of the Higgs boson.