Optimization of the beam crossing angle at the ILC for e+e and γγ collisions

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At this time, the design of the International Linear Collider (ILC) is optimized for e+e collisions; the photon collider (γγ and γe) is considered as an option. Unexpected discoveries, such as the diphoton excess (750) seen at the LHC, could strongly motivate the construction of a photon collider. In order to enable the γγ collision option, the ILC design should be compatible with it from the very beginning. In this paper, we discuss the problem of the beam crossing angle. In the ILC technical design [1], this angle is 14 mrad, which is just enough to provide enough space for the final quadrupoles and outgoing beams. For γγ collisions, the crossing angle must be larger because the low-energy electrons that result from multiple Compton scattering get large disruption angles in collisions with the opposing electron beam and some deflection in the solenoidal detector field. For a 2E0 = 500 GeV collider, the required crossing angle is about 25 mrad. In this paper, we consider the factors that determine the crossing angle as well as its minimum permissible value that does not yet cause a considerable reduction of the γγ luminosity. It is shown that the best solution is to increase the laser wavelength from the current 1 µm (which is optimal for 2E0 = 500 GeV) to 2 µm as this makes possible achieving high γγ luminosities at a crossing angle of 20 mrad, which is also quite comfortable for e+e collisions, does not cause any degradation of the e+e luminosity and opens the possibility for a more energetic future collider in the same tunnel (e.g., CLIC). Moreover, the 2 µm wavelength is optimal for a 2E0 = 1 TeV collider, e.g., a possible ILC energy upgrade. Please consider this paper an appeal to increase the ILC crossing angle from 14 to 20 mrad.

Original languageEnglish
Publication statusPublished - 1 Jan 2019
Event2017 International Workshop on Future Linear Collider, LCWS 2017 - Strasbourg, France
Duration: 23 Oct 201727 Oct 2017


Conference2017 International Workshop on Future Linear Collider, LCWS 2017


  • Accelerator modelling and simulations (multi-particle dynamics; single-particle dynamics)
  • Beam dynamics
  • Instrumentation for particle accelerators and storage rings - high energy (linear accelerators)
  • Lasers

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