Accelerator Generated Backgrounds for e + e - B-Factories. M. Sullivan for the Super-B Factory Workshop Hawaii January 19-22, 2004. Types of Accelerator Backgrounds. Synchrotron Radiation (SR) Lost beam particles (BGB) Touschek scattering Luminosity backgrounds.
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Super-B Factory Workshop
January 19-22, 2004
Fan radiation or dipole radiation results from bending the entire beam. Bending magnets like the main bends in the ring have dipole fields but quadrupole magnets can also bend the entire beam if the beam orbit through the quad is not on axis
Focusing radiation or quadrupole radiation is synchrotron radiation generated when a beam passes through a quadrupole magnet on axis. Since the beam is on axis, most of the particles are barely bent and hence produce very little SR. The main part of the SR comes from beam particles that are several sigmas out in the x or y plane (> ~3-4).
Backgrounds from quadrupole radiation are dominated by the SR generated from the final 2 focusing magnets. The beam is usually largest in these magnets making it difficult to completely protect the detector beam pipe from SR. For flat beams, the Y plane (a) is much easier to shield than the x plane (b). As one can see, the beam must be over-focused in the horizontal plane which permits the SR to cross over the collision axis and strike the opposite side of the beam pipe.
The quadrupole generated SR primarily comes from the off–axis beam particles. A single stored beam has a gaussian beam profile. However, in a colliding accelerator each beam develops a beam tail that tends to fill the available aperture at high luminosity. The tail particle density is very much lower than the core density; the integral of the tail being about 1-2% of the core.
Assumed beam-tails for SR background calculations for PEP-II
Beam particles can get lost (mostly in the detector) when they scatter off of a gas molecule. There are 2 primary modes of scattering, elastic (Coulomb) and inelastic (Beam-gas Bremsstrahlung) where a high energy gamma is created. Lost particle scatters occur all around each ring. BGB events generally do not contribute background unless they are between the last bend magnet and the IP. Coulomb events can come from nearly anywhere in the ring and end up in the detector because the beams are largest right around the IP.
These are initial state radiation events. In single ring colliders, these events were buried in the beam envelope and were eventually lost when the beam went through a bending magnet, many meters from the detector.
The 2 ring B-factories, with shared quadrupoles and bending magnets closer to the IP, these events come out of the beam envelope much sooner and can be a source of detector background.
Touschek scattering ~1/sxsy . For PEP-II, given the Touschek lifetime to be about 400 min and the average beta x, y around the ring is ~30 m and that the IP betas are 0.3 and 0.012 we find a Touschek scattering enhancement at the IP of 1580. Normalized by length (1cm vs 2200 m) we get the IP rate to be ~7e-3 of the ring. For a 2A beam then there are ~1107 Touschek events per second at the IP from the LER. This is a very crude calculation and we need to check this guess with a full blown simulation.
Accelerator generated backgrounds come in many forms. Each type must be carefully studied in order to maximally shield the detector while not degrading the accelerator luminosity performance.
Collision generated backgrounds are somewhat new and in general this effect has not been included at the design level of the IR as a source of detector background. Future designs will need to take this background source into account, especially since the new designs call for higher luminosity.