Collimation down to 2 sigma in special physics runs in the LHC

Abstract

We report on observations with collimation very close to the beam. Primary collimators were moved in small steps down to 2 σ from the beam axis to allow for measurements of very forward proton scattering in special high beta runs in the LHC. We studied the reduction in intensity as a function of collimator position which provides information about the halo shape. After scraping at 2 σ, collimators were retracted to 2:5σ. This allowed for measurements of very forward proton-proton scattering with roman pot detectors at 3 σ from the beam axis at acceptable background levels for about an hour. Good background conditions were restored by another scraping with primary collimators at 2σ. Beam lifetimes and halo repopulation times were found to be sufficiently long to allow for several periods with acceptable background levels between scraping integrating to several hours of data taking in a single LHC fill.