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

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

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%)

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 D x ≈0.162, D y ≈18.5 for ILC at 250 GeV D x ≈1.7/0.9, D y ≈244/127 for SuperB at 4/7 GeV*  Angular divergence of the beams  0 ≈0.35mrad for ILC;  0 ≈10mrad for SuperB*  Coulomb attraction between electron and positron beams increases the luminosity : pinch effect Luminosity (1/cm 2 /s) : enhancement factor H D ≈ f(D) ~ 1.7 for ILC at 500 GeV ~ 1.07 for SuperB* geometrical lumi * no more available

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,  : 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, n  ~1,25 for ILC at 500 GeV; ~0.3 for SuperB Fractional beamstrahlung energy loss per bunch,  B ~0.022 for ILC at 500 GeV; <10 -5 for SuperB

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 - →  → e + e - …  Coherent pair creation : photon turns into e + e - pair by interacting with collective field of oncoming beam. Dominant process at 0.5 ≤  ≤ 100  Incoherent pair creation : a photon of one beam interacts with a photon of the other beam (~60 mb at ILC)

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 - →  → e + e - … Hadronic : e + + e - →  → hadrons Electromagnetic deflections Effect on backgrounds (pairs...) Effect on luminosity measurements ? (Bhabha scattering) e + e - spin depolarisation effects 2 nd order beam-beam effect on background...  GUINEA-PIG (D. Schulte) & CAIN (K. Yokoya): beam-beam simulation tools

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. ECAL LumiCAL BeamCAL HCAL K. Büsser ILC detector

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

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

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

Beam-beam effects on pairs Deflection of low energy pairs due to the field of the opposite beam. Pt   Before Deflection Pt After Deflection e-e- e+(0)e+(0) e-e- e+e+ e+e+ e - (  1 >  0 )

Beam-beam effects on pairs Comparison with ILC ILC Nominal SuperB More Deflections in Super B

Pairs reaching Vertex Detector in SuperB for r = 10 mm; B=4T  ee ~ 2.5mb Pairs reaching VD

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-(0)e-(0) e-e- e+e+ e+e+ e - (  1 <  0 ) Bhabha focusing versus production angle   (mrad) Bhabha angular deflections are about few mrad error on theoritical bhabha cross section   Which precision is it possible to obtain on luminosity measurement ?

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 Version manager Code description... In progress