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

2. Depolarization
Spin Precession induced by the collective EM field of the oncoming beam, described by T-BMT equation (dominant effect at ILC):
Where a= 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++)

3. 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=
After 5 collisions, DP=
After 10 collisions, DP=

4. 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

5. 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.

6. 2nd Preliminary estimations of depolarisation due to Beam-Beam INTeraction at SuperB - 1
After 1 collision, DP=
After 5 collisions, DP= (x1.7)
2 orders of magnitude less than previous study !
Nominal

7. 2nd Preliminary estimations of depolarisation due to Beam-Beam INTeraction at SuperB - 2
After 1 collision, DP=
After 5 collisions, DP= (x1.8)
x 1.4 compare to nominal
Low Emittance

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

9. 2nd Preliminary estimations of depolarisation due to Beam-Beam INTeraction at SuperB - 4
After 1 collision, DP=
After 5 collisions, DP= (x4.3 )
2 orders of magnitude less than Nominal
Tau/Charm

10. 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