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FCC-FS EPOL group and FCCIS WP2.5 meeting 28
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Europe/Zurich
Description
The FCC technical and financial feasibility study comprises a work package (EPOL) on precision determination of the centre of mass energy at FCCee. using resonant depolarisation of the beams, in conjunction with precise measurement of the energy spread and other parameters using physics events in the detectors, and other beam diagnostics in particular to control the collision parameters. Specific equipment involves polarimeters for both beams, polarisation wigglers, and depolarising RF kickers. The possible mono-chromatization of the beams in view of a measurement of the e+ e- —> H (125) process will also be studied and special requirements investigated.
Short group meetings are foreseen at 16:30 on Thursday typically every three weeks.
FCC-FS EPOL group meeting
Zoom Meeting ID
63437787216
Host
Alain Blondel
Alternative host
Jacqueline Keintzel
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R. Kieffer discusses about the polarimeter location. He reminds the CE deadline for requirements or requests is in September this year. The current baseline is that the laser hutch is installed only in PA for both beams, which requires additional CE. Compared to the CDR the LIR is shorter in the MTR, where discussions with K. Oide are ongoing for the optics design. To access the hutch additional 2 x 830m would be required. Discussions with CE will take place. Alternatively the laser could be on ground level, resulting to additional land requests, 830 m away from the IP, which seems very challenging to find extra space.
Since a klystron gallery is already foreseen in PL, and so no extra tunnel is required. Installing the polarimeter in PL seems only feasible for Z and W energy, since for higher energy collider RF would need to be installed there, leading to too much noise. J. Wenninger comments that vertical crossing could have negative effect on the emittance, where K. Oide replies that this could be reduced with a dedicated chikane.
LIR with a small angle could have laser output and input at the same location, this was based on a very short pulse. With a larger angle the LIR could be made shorter, which could also be beneficial for impedance.
With 1000 Compton electrons per bunch crossing, 100 turns are sufficient to detect the RDP process with 10 % initial polarization level with 100 keV per point. J. Wenninger comments that one scans during a few MeV per one RDP measurement. J. Wenninger suggests to simulate if one can rely with only 2 to 3 % polarization. T. Lefevre suggests to see which parameters we need on the laser to reduce systematic errors below statistical ones. Injecting polarized bunches could also be studied, but since the HEB is on top of the collider the spin could be rotated. The laser energy could be mJ with 530.
J. Keintzel asks if this laser could be combined with intensity control. T. Lefevre and J. Wenninger comment that this might be challenging in terms of hardware and impact on the electron and photon detectors.
Z. Duan presents polarization at the CEPC. Polarization rotation from the HEB to the main ring is not yet studied. Achieving polarization at the Higgs energy is currently being studied. Measuring up-down asymmetry by flipping laser helicity has shown feasible with <2% statistical uncertainty for 5 min data taking. First experimental tests are foreseen in 2025. A polarized electron gun could achieve 85% polarization. A laser plasma accelerator plus damping ring is under design. Polarized beam experiments are expected to start in 2027. A Compton polarimeter will be implemented in BEPCII.
D. Barber suggests concerning polarization at Higgs, to work on theory, perform massive MC simulations with multiple particles or single particles. This will be followed up by collaborators in the US. Updates will be presented at the FCC week.
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