Impact of beam-beam effects and linear coupling resonance on the absolute luminosity calibration at the CERN Large Hadron Collider

In colliders such as the CERN Large Hadron Collider (LHC), luminosity represents the ratio between the event rate at a particle detector and the cross section of the particle physics event being measured. Uncertainties in the luminosity directly propagate to uncertainties in the cross section of the event. Luminosity is defined by the accelerator properties and performance. Measurements of the luminosity are performed to define the absolute calibration of the detectors by applying the well known van der Meer (vdM) method. This is performed regularly to improve precision; the bias to such measurements should be corrected for in the detector data. One important bias is the electromagnetic interaction between the two charged beams, the so-called beam-beam effects. This effect causes a measurable deviation from the analytical expression, although recent simulations have proved that the effective bias on luminous region computation was previously overestimated. Moreover, recent studies have shown that the initial assumptions of the vdM method, stating that the particle distributions in the three spacial dimensions are factoriz able, may no longer hold. This is recreated using a new beam simulation framework code called Xsuite. Further deviations from expected results may arise from other beam dynamics effects such as linear and non-linear resonances. One hypothesis is that linear coupling effects introduced by elements in the accelerator lattice create correlated distributions in the transverse plane, therefore producing non-factorizable distributions and distorting the luminosity in transverse scan measurements. This idea originates from the earlier finding that Landau damping is modified by linear coupling resonance. To investigate this effect, firstly numerical tools needed to reproduce the above effects were developed in Xsuite, and previous results obtained with COMBI were reproduced. Secondly, previous studies were extended by exploring the effects of linear coupling and beam-beam effects on luminosity and $\sigma_{vis}$ as a possible source of the non-factorization problem observed in data analysis from the ATLAS and CMS detectors. This effect has been studied for different machine and beam configurations to create a comprehensive picture of the impact to luminosity.