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FCC-FS EPOL group and FCCIS WP2.5 meeting 22
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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 two weeks.
FCC-FS EPOL group meeting
Zoom Meeting ID
63437787216
Host
Alain Blondel
Alternative host
Jacqueline Keintzel
Useful links
Join via phone
Zoom URL
List of actions:
Experiments:
- Start preparing proposal for tests at KARA with present set-up.
- Could a Compton polarimeter be installed at KARA? At which cost and which design is needed?
- Does the KARA lattice allow generating bumps and localized depolarizing sources?
- What is the available space for polarimeter at KARA and could a beam line be used?
Polarization:
- Vertical dispersion matching to be followed up.
- What is the impact of orbit bumps on vertical dispersion and emittance?
- How does it look for W?
- What is the impact of BPM errors?
Effects on ECM:
- A broad study is required to understand the cross-talk between IPs with different collision offsets and OSVDs.
- What is an acceptable value for a RF-frequency shift (dp/p ~ 1e-4?) for OSVD for colliding bunches?
- Define parameters (strength, rise time, etc.) and suitable locations for kickers measuring OSVD for pilot bunches.
Polarimeter:
- Can we benefit from more than one polarimeter, and if yes, at which cost?
- What are the systematics for RDP and FSP measurements?
- What is the best location for one polarimeter?
- How stable is the laser wavelength's central value, laser polarization and spectral width and what is required for RDP and FSP measurements?
- Which laser systems are applicable for pilot bunches and which for colliding bunches?
- How many scattering events do we need for which photon detection efficiency and polarimeter detector requirements?
Depolarizer:
- What are the systematics for RDP and FSP measurements?
- Which minimum beam polarization level is needed (is less than 10% ok) ?
- What is the presently foreseen operation mode (collision on and off or continuously on)?
- Where is the best location for a depolarizer (would next to the final focus work)?
- If the kicks are not closed, what would be the impact on the optics?
Monochromatization:
- What is the impact of beamstrahlung on monochromatization?
A. Martens addresses the question on how QED corrections affect the parameter extraction from the polarimeter. The typical precision is 0.1% in a few 10s of seconds. In past studies the change of cross-section due to polarization over various energies has been studied, but neither a detailed study of systematics on transverse polarization, nor the feasibility of flipping spin distribution has been studied, which is investigated here. The bias from QED crosses 0 error for 0 % polarization and is in the order of 10-4 for Z, and W-energy for 10% transversely polarized beams, which seems save to perform RDP. The bias for the photon detector is larger than for the electron one. Following a question by G. Wilkinson, A. Martens explains that only the Born cross-section is being considered. A. Blondel asks how big the bias is for measuring longitudinal polarization if beams are not polarized. A. Martens replies that longitudinally, the error is in the order of 10-3. Using asymmetry measurements could help reducing this bias, but demands high experimental precision. This special distribution is obtained via 1 helicity on the beam, and A. Blondel asks how changing the helicity will impact the result. A. Martens comments that this is meant by asymmetry. A. Blondel and D. Gaskell comment one could possibly reduce this by using 2 different techniques to change the helicity. Following a questions by G. Wilkinson, A. Martens comments that controlling laser polarization below 10-3 seems challenging. Following a question by D. Gaskell, A. Martens comments that every change in background, intensity, etc. could introduce some systematic errors. D. Gaskell comments since this stems mainly from the laser, these systematic could be investigated in advance.
Action: How can QED corrections be implemented in the most efficient way in the fit? Do we need to consider higher order QED corrections?
Y. Wu shows updates on polarization studies. With vertical closed orbits below 100 µm the polarization increases linearly with smaller orbits, where no corrections are applied. A. Blondel comments that after the orbit, the vertical dispersion must be corrected before performing HSM. Furthermore, he comments to see the energy shifts with respect to nu0, not the absolute value.
D. Shatilov presents updates on energy shifts from beam-beam interactions. Beam-beam changes the beam energy and also the crossing angle, but not the center-of-mass energy. Energy shift comes from the electric field in combination with the transverse momentum. The generated longitudinal electric field from the bunches also leads to an energy shift. The main contribution from the longitudinal field is proportional to the beam-beam tune shift, the azimut and the betay*. Longitudinally its contribution is about 10-9, and thus, negligible. Vertically however, it could significantly larger. Particles crossing the IP with off-axis and/or off-momentum experience an energy change. The largest energy change is found for off-transverse particles. Long term simulations yield a longitudinal average change of collision energy for 3x10-7 for head-on collisions and 0.02 beam-beam parameter, resulting from the correlation between the energy offset and the vertical beam size. It is estimated that the order of magnitude for the FCC is 10-6. Following a question by A. Blondel, D. Shatilov comments that it changes with the energy spread, bunch population, orbit, tunes, etc, which could lead to a different machine for every measurement. G. Wilkinson comments that one could possibly also try to see that in the experiments.
Action: What is the transverse error on the change of energy and what magnitude has the fluctuation around it?
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