HL-LHC: Parameter Space, Constraints, and Possible Options

This paper reviews the most promising ingredients to boost the LHC integrated luminosity, including smaller beta*, higher beam intensity, crab crossing, long-range beam-beam compensation, large Piwinski angle, flat longitudinal profile, and variations of bunch length, transverse emittance, crossing angle, and bunch spacing. It discusses how various ingredients conspire or compete, and how they pose different requirements on new LHC hardware and on the beams from the injectors, as well as their relative importance. Special emphasis is given to luminosity-levelling schemes. Finally a proposed roadmap towards HL-LHC and branching points in the research for a solution are sketched. In particular, this paper points out that raising the beam current is important for reaching a high integrated luminosity, and that long-range beam-beam compensation should be pushed as a simple tool to boost HL-LHC luminosity performance. Presented at Chamonix 2011 LHC Performance Workshop, 24-28 January 2011