Beam-Gas Imaging at the LHCb Experiment

This thesis presents a new absolute luminosity calibration measurement at the LHCb experiment for $pp$ collsions at $\sqrt{s} =$ 13 TeV measured via the beam-gas imaging (BGI) technique with data acquired in the 2016 luminosity calibration session. The measured reference cross-sections under these conditions for the $Track$ and $Vertex$ counters are $\sigma_{\textit{Track}} =$65.82$\pm$1.14mb and $\sigma_{\textit{Vertex}} =$60.00$\pm$0.98mb, respectively. The principle of the BGI technique is set out and the BGI measurements from 2016 are presented alongside beam imaging results from a broader set of run 2 calibration data. The procedure for determining the vertex position resolution from collision data via the split vertex method, a crucial element of a precise BGI measurement, is described and measurements of this resolution are reported. With a precise knowledge of the detector resolution it is possible to accurately measure the LHC beam profiles and parameters on a bunch-by-bunch level from their interactions with gas molecules in the LHCb beam pipe and with each other. This allows for a precise evaluation of the overlap integral, a quantity that is directly proportional to the luminosity produced at a collider. The circulating bunch populations are measured by LHC instrumentation and the results relevant for the calibration measurement are presented. Ghost charge results measured via the BGI technique are reported, including the first such measurements made with the VELO subdetector in its open position and with the first xenon beams circulating in the LHC. The utility of the LHCb measurement as a cross-check on measurements from LHC instrumentation is demonstrated and a novel timing technique making use of the LHCb Outer Tracker is set out. The additional timing information from this subdetector permits the first LHCb measurement of the longitudinal LHC beam profile with a time granularity less than 25 ns. The luminosity calibration cross-section results are then reported and the full range of systematic uncertainties affecting this measurement are evaluated. The reference cross-section for the $Track$ counter, used for the luminosity determination in LHCb physics data, is determined to a precision of 1.73%. Two additional studies that make use of beam-gas interactions at LHCb are also included. The first is a collaborative effort in which a cross-calibration of all existing bunch profile measurement techniques at the LHC was performed. The second looks at possible dynamic vacuum effects due to the movement of the VELO subdetector and relies on a novel vertexing approach making use of the LHCb experiment's downstream tracking stations. This approach demonstrates a statistically significant pressure increase within the VELO volume due to its movement, at 2.6 standard deviations.