Precision luminosity measurement at LHCb with beam-gas imaging

The luminosity is the physical quantity which relates the cross-section to the production rate in collider experiments. The cross-section being the particle physics observable of interest, a precise determination of the luminosity is required. This work presents the absolute luminosity calibration results performed at the Large Hadron Collider beauty (LHCb) experiment at CERN using a novel method based on beam-gas interactions with data acquired at a center of mass energy $\sqrt{s}=8$ TeV and $\sqrt{s}=2.76$ TeV. Reconstructed beam-gas interaction vertices in LHCb are used to measure the beam profiles, thus making it possible to determine the beams overlap integral. An important element of this work was to install and use a neon gas injection system to increase the beam-gas interaction rate. The precision reached with the beam-gas imaging method relies on the two-dimensional beam shape determination developed in this work. For such precision, the interaction vertex resolution is an important ingredient. Therefore, a new method has been developed using all reconstructed vertices in order to improve the understanding of the vertex resolution. In addition to the overlap integral, the knowledge of the colliding bunch populations is required to measure the luminosity. The determination of the bunch populations relies on LHC instruments to measure the bunch population fractions and the total beam intensity. Studies performed as part of this work resulted in a reduction of the bunch current normalization uncertainty from $\pm2.7$ % to $\pm0.2$ % and making it possible to achieve precision luminosity measurements at all LHC experiments. Furthermore, information on beam-gas interactions not originating from nominally filled bunches was analyzed to determine the charge fraction not participating in bunch collisions. The knowledge of this fraction is required to correct the total beam intensity. The reference cross-section of \emph{pp} interactions with at least two tracks in the vertex detector was measured with the beam-gas imaging method. The result is $\sigma_{\text{Track}} = 60.6\pm0.9\,\text{mb}$ at a center-of-mass energy of $\sqrt{s}=8$ TeV. The same measurement performed at $\sqrt{s}=2.76$ TeV results in a cross-section of $\sigma_{\text{Track}} = 52.7\pm1.2\,\text{mb}$. The luminosity measurement at $\sqrt{s}=8$ TeV presented here, with an uncertainty of 1.4 %, is to date the most precise luminosity calibration performed at the LHC and at any other bunched-beam proton collider.