GUINEA-PIG: A tool for beam-beam effect study

1. GUINEA-PIG:A tool for beam-beam effect study Daresbury, 26-27 April 2006 C. Rimbault, LAL Orsay

2. GUINEA-PIG:A tool for beam-beam effect study Beam-Beam effect overview: why a beam-beam simulation tool is needed Examples of backgrounds studies how beam parameters influence detector design how detector design influences beam parameters

3. Beam-Beam effects overview • When beams collide: mixing of classical and quantum effects • Bunches are deformed by electromagnetic attraction: Disruption •  enhancement of luminosity • High beam-beam field (kT for ILC) Energy loss in the form of synchrotron radiation: beamstrahlung (~3%)

4. Disruption & Luminosity • Disruption describes effect of EM field surrounding each bunch during the collision  change in beam trajectory, each beam acting as a thin focusing lens Dx ≈0.162 , Dy≈18.5 for ILC at 250 GeV Dx ≈1.7/0.9 , Dy≈244/127 for SuperB at 4/7 GeV* Angular divergence of the beams q0≈0.35mrad for ILC; q0≈10mrad for SuperB*  Coulomb attraction between electron and positron beams increases the luminosity : pinch effect • Luminosity (1/cm2/s) : enhancement factor HD≈ f(D) ~ 1.7 for ILC at 500 GeV ~ 1.07 for SuperB* geometrical lumi * no more available

5. Beamstrahlung • Beamstrahlung occurs in the EM field of a • charged bunch. • When two charged bunches collide, the EM field • surrounding each bunch bend the trajectories of • the opposite bunch particles •  energetic photon are emitted •  energy and luminosity loss at IP. Characterisation of the beamstrahlung: • Beamstrahlung parameter, U : measure of the field seen by a beam particle in its rest frame • ~0.046 for ILC at 500 GeV; <10-5 for SuperB • Nb of photons radiated during a collision per electron, ng • ~1,25 for ILC at 500 GeV; ~0.3 for SuperB • Fractional beamstrahlung energy loss per bunch, dB • ~0.022 for ILC at 500 GeV; <10-5 for SuperB

6. Beam-Beam effects overview • When beams collide: mixing of classical and quantum effects • Bunches are deformed by electromagnetic attraction: Disruption •  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-→ gg → e+e- … • Coherent pair creation : photon turns into e+e- pair by interacting with collective field of oncoming beam. Dominant process at 0.5 ≤ U ≤ 100 • Incoherent pair creation : a photon of one beam interacts with a photon of the other beam (~60 mb at ILC)

7. Beam-Beam effects overview • When beams collide: mixing of classical and quantum effects • Bunches are deformed by electromagnetic attraction: Disruption •  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-→ gg → e+e- … • Hadronic : e+ + e-→ gg → hadrons • Electromagnetic deflections • Effect on backgrounds (pairs ...) • Effect on luminosity measurements ? (Bhabha scattering) • e+ e- spin depolarisation effects • 2nd order beam-beam effect on background... •  GUINEA-PIG (D. Schulte) & CAIN (K. Yokoya): beam-beam simulation tools

8. GUINEA-PIG and background studies 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...  luminosity, distributions of beam particles beam after collision... GP generates backgrounds (e+e- pairs, hadrons, minijets...) those backgrounds can hit the detectors... Most important background: electromagnetic pairs. ILC detector ECAL HCAL LumiCAL BeamCAL K. Büsser

9. Ex: Impact of beam parameter sets on Vertex Detector background for a first VD layer of 15 mm Nominal Low Power Pairs reaching the VD for an inner layer radius of 15 mm and different magnetic fields: 145 mb 77 mb 50 mb Pt (GeV/c) 88 mb 59 mb 39 mb Pt (GeV/c) for B=3T + + for B=4T + for B=5T q (rad) q (rad) Pairs deflection limit for Nominal option, this limit changes with beam size and charge

10. Example of background study with GUINEA-PIG: incoherent e+ e- pairs 3processes : Breit-WheelerBethe-Heitler Landau-Lifshitz e+ beamstrahlung g e+ virtual g beamstrahlung g e+ e+ e- e- e- virtual g beamstrahlung g e virtual g e- LL process does not depend on beamstrahlung !!!

11. Example of background study with GUINEA-PIG: incoherent e+ e- pairs in Super B Pt vs theta Energy see : 22.5mb

12. Beam-beam effects on pairs • Deflection of low energy pairs due to the field of the opposite beam. e- e+ e- e+ e-(q1> q0 ) e+(q0) After Deflection Before Deflection Pt Pt q q

13. Beam-beam effects on pairsComparison with ILC SuperB ILC Nominal More Deflections in Super B

14. Pairs reaching Vertex Detector in SuperBfor r= 10 mm; B=4T see ~ 2.5mb Pairs reaching VD

15. GuineaPig used to study beam-beam effect on bhabha scattering at low angle at ILC Deflection of Bhabhas due to the field of the opposite beam e+ e+ e- e+ e-(q1< q0 ) e-(q0) Bhabha focusing versus production angle q0 (mrad) • Bhabha angular deflections are about few 10-2 mrad • error on theoritical bhabha cross section • Ds/s  10-4 • Which precision is it possible to obtain on luminosity measurement ?

16. Summary • Interaction Point: most important part of the machine and detector ! • GUINEA-PIG is a nice tool to study backgrounds, beam-beam effects... • GuineaPig improvement at LAL: • C. Rimbault, P. Bambade, G. Le Meur, F. Touze. • Main goals: • Spin depolarization implementation • Web documentation • http://flc.web.lal.in2p3.fr/mdi/BBSIM/bbsim.html • Version manager • http:/svn.lal.in2p3.fr/WebSVN/GuineaPig Code description... In progress