1. Depolarization due to beam-beam interaction at SuperB
Cecile Rimbault
LAL - Orsay, IN2P3/CNRS
XIV SuperB General Meeting, INFN-LNF, 27 Sept - 1 Oct. 2010
C. Rimbault

2. Depolarization due to beam-beam interaction at SuperB
Purpose: to evaluate the depolarisation arising from beam-beam interaction
We use for this a strong-strong simulation code GUINEA-PIG++
1- Depolarisation effects
2- Main results presented at Annecy
3- New results
4- Next to do
C. Rimbault

3. Depolarization effects
Spin Precession induced by the collective EM field of the oncoming beam, described by T-BMT equation:
Where a= is the coeff of anomalous magnetic moment of electron
Precession angle = ga x deflection angle
Spin-Flip effect during synchrotron radiation: 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++)
C. Rimbault

4. Annecy meeting estimations of depolarisation due to Beam-Beam INTeraction at SuperB
No crab waist
Nominal
Low Emitt
High Current
Tau/Charm
DPbbint
Maybe too optimistic
C. Rimbault

5. New studies
Large number of GP++ tests (A. Mohammed, internship 05-06/2010) find optimal reliable computing parameters for SuperB:
Stable results
“reasonable” computing time
Changes compare to the parameters used previously.
Crab waist transformation : y  y + y’x/2q
Simulation of collision in centre of mass frame:
calculations done (G. Le Meur)
implementation not finished nm nt nx ny nz Dx Dy Dz
160000 5
128
100
0.1sz
6sy
3sz
C. Rimbault

6. New results including crab waist - 1
Luminosity
Nominal
Low Emitt
High Current
L NoCrabW (1031m-2)
4.17
4.10
1.98
L CrabW(1031m-2)
4.40
4.33
2.08 DL
+5.5%
+4.9%
+5.1%
C. Rimbault

7. New results including crab waist - 2
LEB
Nominal
Low Emitt
High Current
DPbbint NoCrabW
DPbbint CrabW
DPbbint gain
-19.8%
-24.5%
-19.5%
Crab waist seems to induce a reduction of depolarisation of ~20% for LEB
C. Rimbault

8. New results including crab waist - 3
LEB
Nominal
Low Emitt
High Current
DPbbint NoCrabW
DPbbint CrabW
DPbbint gain
-19.8%
-24.5%
-19.5%
HEB
Nominal
Low Emitt
High Current
DPbbint NoCrabW
DPbbint CrabW
DPbbint gain
-4.7%
-8.5%
-4.9%
Crab waist seems to induce a reduction of depolarisation of ~5% for HEB
C. Rimbault

9. Conclusion More accurate simulations have been performed
Beam-beam depolarisation is of the order of several 10-7 per crossing,
larger for HEB (~x2)
Crab Waist seems reduce depolarisation of ~20% for LEB and ~5% for HEB
But need to be cross-check
GP++ is going to be improve, several ways are under studies.
Computing time is a major problem.
C. Rimbault