Speaker
Description
Precise luminosity calibration is an essential task for any collider. In hadron colliders, the absolute luminosity is calibrated using the van-der-Meer scan, where the two colliding beams are swept across each other, and the reaction rates and the separations are recorded. As two charged particle bunches approach each other, their electromagnetic beam-beam interaction leads to perturbations in the motion of the colliding bunches. The beam-beam interaction affects the bunch motion as a whole resulting in an orbit shift of the bunch center (coherent effect) as well as the motion of the individual particles inside the bunch resulting in redistribution of the particles inside the bunch which leads to optical distortion such as dynamic beta and tune shift (incoherent effect). This effect influences luminosity in two ways. First, the number of collisions is changed in comparison to expected values; second, the calibration of luminosity performed with van-der-Meer scan is biased. For example, in LHC this bias is at the level more than 1%.
The current estimation of beam-beam effects is based on Gaussian models for particle densities in colliding beams. However, it was found that the actual beam profiles is deviated from Gaussian in LHC. To improve the precision of luminosity calculations, especially for high-luminosity collider projects like the HL-LHC aiming for precision at the level below 1%, it is necessary to consider the non-Gaussian tails of the particle distributions.
The q-Gaussian distributions provides a more realistic description for the bunch profile for LHC and also for the HL-LHC. The Q-Gaussian beams were used to investigate effect of the non-Gaussian tails on the emittance evolution and IBS (S. Papadopoulou, et. al PRAB 2020). In (arXiv:2305.04023 [physics.acc-ph]), we investigated the luminosity calibration of Q-Gaussian beams. In this work, the beam-beam interaction of q-Gaussian beams is considered, models for coherent and incoherent kicks are presented. The impact of beam-beam effect on luminosity is estimated. The progression of the effect during van-der-Meer scan is demonstrated, and the resulting beam-beam bias in the luminosity calibration constant is obtained for CMS and ATLAS van-der-Meer scan beam conditions.
| Young scientist paper | Yes |
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