1. GUINEA-PIG++ Workshop on Spin Simulation Tools, 9-11 Nov 2010, DESY
Cécile Rimbault, LAL-IN2P3/CNRS

2. History
GUINEA-PIG: Generator of Unwanted Interactions for Numerical Experiment Analysis -- Program Interfaced to Geant
Beam-beam simulation code in C created by Daniel Schulte (CERN) in 1999
Comparable to CAIN
GUINEA-PIG++ (or guineapig++): C++ version of GUINEA-PIG by LAL (Guy Le Meur, Francois Touze and CR)
Since 2005, work started with EUROTeV.
Mainly use for future linear collider studies (ILC, CLIC) but also for SuperB

3. Beam-beam effects overview
When beams collide: mixing of classical and quantum effects
Bunches are deformed by electromagnetic attraction: Disruption + pinch effect  enhancement of luminosity
High beam-beam field (kT for ILC) Energy loss in the form of synchrotron radiation: beamstrahlung (~3%)
Secondary backgrounds
Electromagnetic : e+ + e- → g(*) g(*) → e+e- ...
Hadronic : e+ + e- → gg → hadrons
Electromagnetic deflections
Effect on backgrounds (pairs ...)
Effect on luminosity measurements (Bhabha scattering)
e+ e- spin depolarisation effects...

4. Beam-Beam interaction simulation
Macro-particles replace particles (can be 105 Macro  part)
Bunches are cut into slices: a slice of one bunch interacts with a slice of the other bunch when they occupy the same transverse plan.
Slices are moved longitudinally on a 3D grid
For each slice-slice interaction:
Macro-part are distributed on a 3Dim. grid
Fields calculation
Macro-part are moved according to fields + photons are produced.
if photon treatment is asked:
photons are distributed and moved on the grid
(if asked) pairs are generated and moved.
for each e-e+ interaction: spin, luminosity calculation, bhabha production...

5. To run GP++
GP++ simulates the collision of two bunches (e-e+ or e-e-) for a given set of input parameters: bunches sizes, emittances, energy, offset + computation parameters... defined in an input file named acc.dat
> guineapig.exe accelarator_name parameters_name file.out
ex:
$ACCELERATOR:: ILCnominal
{energy=250.0;
particles=2;
charge_sign=-1;
beta_x=21.0;
beta_y=0.4;
sigma_x=655;
sigma_y=5.7;
sigma_z=300.0;}
$PARAMETERS:: ILCtest
{n_x=32;
n_y=64;
n_z=32;
n_t =5 ;
cut_x=3.0*sigma_x.1 ;
cut_y=6.0*sigma_y.1 ;
cut_z=3.0*sigma_z.1 ;
n_m=50000;
store_beam=1;}
doc from C version:

6. Output files from GP++
A main output file is provided, given the main results:
luminosities,
beamstrahlung Umax parameter,
average angles of the particles of each beam after the collision,
backgrounds if asked (pairs, hadrons, minijets), amount and corresponding total energy
average energy loss...
Specific output files can be produced in Ascii format:
beam files, photon files, pair files, luminosity files, others backgrounds files...
Example: Macro-electron output file beam1.dat
E(GeV) x(mm) y(mm) z(mm) vx(mrad) vy(mrad) sx sy sz
doc from C version:

7. Distribution and management of GP++
GP++ use configuration management environment CMT  easy compilation
GP++ versioning, updating and releasing achieved with SVN
GP++ is distributed on the web software development tool TRAC:

8. New physics and running options in GP++
GP++ code can be run both on 32-bit and 64-bit computers, which is automatically recognized.
Keyword rndm_seed allows to choose the random generation seed  enable independent simultaneous runs on a GRID
All results are now in the main output file, with units
Automatic GRID size option
Physics simulation improvement: easy interface to apply beam-beam effects on Bhabha event input files + photons treatment.
Depolarization effects.  tests performed for ILC

9. 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  567xdeflection angle for ILC nominal
Spin-Flip effect during synchrotron radiation: Sokolov-Ternov effect, tends to depolarize spins in linear collider. Probability for the spin to flip (s-s) at the moment of photon emission, proportional to the photon energy.
At very hight energy ST effect becomes more important

10. Depolarization & beam-beam simulations
In beam-beam simulation, particles are replaced by macro-particles  statistical representation.
In GP++, access to the state of the interacting particles in the luminosity file.
In Cain, luminosity file contains luminosity values as function of energy bins for all the polarization couples
At a first implementation, Yokoya’s formulae are used for ST process in GP++.
Depolarization comparison between GP++ and CAIN can be direct for beam, not so direct for luminosity.
For the moment: only beam particles have a spin treatment in GP++, CAIN as a more complete one (photons, pairs...)
GP++ and CAIN works with totally or partially initial polarized beams
Comparisons are shown for the 4 ILC beam parameter sets at 500GeV cms (Nominal, lowN, largeY, lowP) with 100% polarized beams

11. Comparison of GP++ & CAIN total depolarization for e- beam after interaction: DP = 1 -<P>
Nominal
LowN
LargeY
LowP
(GP-CAIN)/CAIN (%)
-2.70 ~0
-3.27
-1.59

12. GP++ for SuperB ?
SuperB: assymetric circular collider, with beams comparable to ILC, 66mrad crossing angle
Aim: Use GP++ to predict backgrounds and depolarisation due to beam-beam interaction
First studies with GP++ have predicted a small depolarisation of ~10-7 for one bunch crossing.
Questions: what is the “reality” of this prediction? (CAIN seems to predict no depolarisation)
Needs: interface GP++ with a ring tracking code (Zgoubi... Slicktrack)
Two problems: computation time and longitudinal displacement approximation

13. GP++ for SuperB ?
SuperB: assymetric circular collider, with beams comparable to ILC, 66mrad crossing angle
Aim: Use GP++ to predict backgrounds and depolarisation due to beam-beam interaction
First studies with GP++ have predicted a small depolarisation of ~10-7 for one bunch crossing.
Questions: what is the “reality” of this prediction? (CAIN seems to predict no depolarisation)
Needs: interface GP++ with a ring tracking code (Zgoubi, Slicktrack...)
Two problems: computation time and longitudinal displacement approximation
From this WS: Understand CAIN/GP++ predictions for SuperB.
Is there a tracking code easy to interface with GP++?