Accelerator Generated Backgrounds for e + e - B-Factories

1. Accelerator Generated Backgrounds for e+e- B-Factories M. Sullivan for the Super-B Factory Workshop Hawaii January 19-22, 2004

2. Types of Accelerator Backgrounds • Synchrotron Radiation (SR) • Lost beam particles (BGB) • Touschek scattering • Luminosity backgrounds

3. Synchrotron Radiation • Fan radiation 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 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).

4. Quadrupole radiation 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.

5. Beam Tails 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

6. Lost beam particles 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.

7. Touschek scattering • Touschek scattering is a transfer of a small amount of transverse momentum to longitudinal momentum space between two beam particles in the same beam bunch • The result is one beam particle has too much energy and the other has too little beam energy. Somewhat similar to coulomb scattering except in energy space. • This can happen all around the ring and can be more or less collimated out much like BGB events. • However, there may be a significant contribution to backgrounds from the IP

8. Luminosity backgrounds • Radiative Bhabhas 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 at the IP? 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 ~1107 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.

12. Summary 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.