Luminosity scans for beam diagnostics

Abstract

A new type of fast luminosity separation scans ("Emittance Scans") was introduced at the CERN Large Hadron Collider (LHC) in 2015. The scans were performed systematically in every fill with full-intensity beams in physics production conditions at the Interaction Point (IP) of the Compact Muon Solenoid (CMS) experiment. They provide both transverse emittance and closed orbit measurements at a bunch-by-bunch level. The precise measurement of beam-beam closed orbit differences allowed a direct, quantitative observation of long-range beam-beam PACMAN effects, which agrees well with numerical simulations from an improved version of the TRAIN code. II. BUNCH-BY-BUNCH DIAGNOSTICS AT THE LHC A. Luminosity MeasurementsIn the following, the main observable used for the analysis of Emittance Scans is the bunch-by-bunch luminosity as

Probing LHC halo dynamics using collimator loss rates at 6.5 TeV

“…This may be attributed to the different loss mechanisms, i.e., is related to the position in the batch and, therefore, to the different collision patterns and different number of long-range beam-beam (LRBB) encounters in the highluminosity insertions (IR1 and IR5). For the same case, no clear correlation was found for the computed diffusion coefficient with the LRBB orbits shifts [31,32] nor with the different bunch collision classes (PACMAN [33]). This is not completely unexpected, as recent analyses of the typical lifetimes of the various bunches did not reveal any hint of a LRBB pattern during long periods without changes of the crossing angle or β Ã [34].…”

Emittance Scans for CMS Luminosity Calibration in run 2

“…As shown in Fig.5, the evolution of the first 10-12 bunches in the train is different from the other bunches, especially noticeable in X. Understanding the measured emittance evolution is also of high importance for benchmarking the simulation code (TRAIN) for the beam evolution prediction [7] and improving the luminosity model [8].…”

Performance of the BRIL luminometers at CMS in Run 2

“…It is important to mention that CMS online luminometers are equipped with 40 MHz readout systems, without which it would not be possible to derive the per bunch crossing corrections listed above. From 2017 onwards CMS performed emittance scans in almost every physics fill, these are similar to vdM scans but of shorter duration and extent [5,6]. Emittance scans are not giving a precise enough absolute luminosity calibration, but are powerful for studying bunch effects, to monitor the stability and to measure the nonlinearity of the luminometers.…”