Preliminary results on depolarization due to beam-beam interaction at SuperB Cecile Rimbault LAL - Orsay

Depolarization Spin Precession induced by the collective EM field of the oncoming beam, described by T-BMT equation (dominant effect at ILC): Where a=0.0011596 is the coeff of anomalous magnetic moment of electron Precession angle = ga x deflection angle Spin-Flip effect during synchrotron radiation: Sokolov-Ternov effect, tends to depolarize in linear collider. Probability for the spin to flip (s-s) at the moment of photon emission, proportional to the photon energy. Very small at SuperB Those 2 effects are implemented in GUINEA-PIG++ (GP++)

Very Preliminary estimation of depolarisation due to Beam-Beam INTeraction at SuperB - 1 (Feb 2010) Start with totally polarised beam: sz=1, Nmacro=50000 GP++ simulations only: beam is almost not disrupted  reloaded in GP++ (E loss + DP due to BBINT) After 1 collision, DP=2.4 10-7 After 5 collisions, DP=6.6 10-6 After 10 collisions, DP=1.7 10-5

Very Preliminary estimation of depolarisation due to Beam-Beam INTeraction at SuperB - 2 (Feb 2010) Depolarisation as a function of the particle position within the bunch (after 5 collisions) Transverse distance from the bunch center longitudinal distance from the bunch center

Remarks from those very preliminary estimations 1- To improve the model: during ring transport particles are mixing: they don’t keep the same position within the bunch  the effect will be reduced 2- Simulations were performed with "old" beam parameters.  Run of 5 GP++ collisions on 5 different seeds, reinitializing particle distribution and keeping E and Pz coming from the collision, still start with initial Pz=1 (just to see)  Test of the 4 parameter sets (version v11? from 25feb2010): Nominal, Low Emittance, High Current, Tau/Charm.

2nd Preliminary estimations of depolarisation due to Beam-Beam INTeraction at SuperB - 1 After 1 collision, DP=9.5 10-9 After 5 collisions, DP=1.6 10-8 (x1.7) 2 orders of magnitude less than previous study ! Nominal

2nd Preliminary estimations of depolarisation due to Beam-Beam INTeraction at SuperB - 2 After 1 collision, DP=1.2 10-8 After 5 collisions, DP=2.2 10-8 (x1.8) x 1.4 compare to nominal Low Emittance

2nd Preliminary estimations of depolarisation due to Beam-Beam INTeraction at SuperB - 3 After 1 collision, DP=3.4 10-8 After 5 collisions, DP=9.6 10-8 (x2.8) x 6 compare to nominal High Current

2nd Preliminary estimations of depolarisation due to Beam-Beam INTeraction at SuperB - 4 After 1 collision, DP=6 10-11 After 5 collisions, DP=2.6 10-10 (x4.3 ) 2 orders of magnitude less than Nominal Tau/Charm

Conclusion Seems that depolarisation due to beam-beam effect very low. (See also BBdiffusion talk tomorrow) But need more accurate simulations:  Improve GP++ and combine with spin tracking code (ZGOUBI) - Add Crab waist - Add longitudinal field - Check propagation direction - Interface between I/O files of the 2 codes is under development with N. Monseu Find alternative way to use GP++ as it is : (Center of mass frame studies + boost) ? Use larger number of macro-particles A light version of GP++ is being tested: Suppress very high energy background options Main problem: “large” crossing angle  large grid is required to compute particles displacement. Can we reduce the grid to the dimensions of the “effective” interaction region? Discussions and suggestions are welcome