User:YuriVSol/sandbox


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Introduction
Synchrotron Radiation background is emitted by beam particles in magnetic field at a tangent to the orbit and can be produce a high occupancy in the vertex detectors PXD and SPD which are very close to the beam orbit. The limit of occupancy from all sources of background in PXD is 3%.The occupancy from all other sources of background in PXD is estimated as ~1%. The estimation of occupancy in PXD from synchrotron radiation background is an important task. The task consists of three parts :
 * Ideal alignment of detector (central beam pipe) to the orbit.
 * Maximal possible misalignment of detector to the orbit.
 * Contribution of the beam halo to the PXD occupancy.

Approach
Simulation utilizes the latest optical lattice for LER and HER and 2D phase space (transverse emittance only) assiming uncoupled beam matrix and gaussian beam shape.The 2D beam phase space ellipse can be parametrized by Courant-Snyder invariant: γx2 + 2αxp + βp2 = ε ,where γ,α,β,ε - parameters of the lattice. The particle position (x,y) and direction (px,py) are generated using Gaussian distributions in normalized coordinates : (χ,χ') : x = χ√β, px = -α$1/undefined$√β - χ'$1/undefined$√β where the phase space is a circle with radius √ε and then transformed to physical coordinates using the inverse 2D beam matrix. The initial values for position and direction of generated particle (e±) determined by shooting the particle from IP downstream to the point of generation (in front of QC2) with the step size of 0.5mm. Events with photons that hit the central beam pipe (both Ti and Be parts) are selected.The power of background is estimated by the value of occupancy in the PXD. The occupancy can be estimated either by the hit rate of photons that penetrate the beam pipe into PXD or by using hit rate in the beam pipe and stopping power of materials of beam pipe.

Misalignment
The installation of the central part of the BELLE2 detector (±482mm from IP) doesn't exclude misalignment which can reach ±0.5mm. The misalignment in vertical plane can be neglected due to very narrow beam size. Two possible cases:rotation and displacement. In case of rotation the shift of 0.5mm would propagate to 0.1mm at the "dangerous" part of the central beam pipe,the maximal angle of rotation 0.06°. In case of displacement the shift of 0.5mm at bellow position would give the same value of shift for the whole beam pipe. The displacement is concidered as a most dangerous misalignment.To estimate the changes in synchrotron radiation background due to misalignment in horizontal plane the data generated for ideal alignment are used. The photon's track parameters are used to extrapolate the photon's track in the geometry with misalignment and the changes in occupancy are observed.

Beam halo
The tails of the beam (halo) can produce a sizeable fraction of photons and increase background. The estimation of its contribution is done in assumption of a gaussian shape of the halo with σhalo = 10σcore. The normalization factor of gaussian for the halo is estimated using beam-beam simulation.