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

C. Rimbault, LAL Orsay

Daresbury, 26-27 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

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

0≈0.35mrad for ILC; 0≈10mrad for SuperB*

Coulomb attraction between electron and positron beams increases the luminosity : pinch effect

• Luminosity (1/cm2/s) :

Dbryx

HnfN

4

2

L

)(, ,

2yxyx

zeyx

NrD

enhancement factor HD ≈ 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

)(6

5 2

yxz

eave

Nr

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 • 2nd 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=3Tfor B=4T

for B=5T

Pairs reaching the VD for an inner

layer radius of 15 mm and different magnetic fields:

Pt

(Ge

V/c

)

(rad)

Pt

(Ge

V/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

beamstrahlung

e+

e-

beamstrahlung

virtual

e+

e-

e

e+

e-

e+

e-

virtual

virtual

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 DeflectionPt

After Deflection

e-

e+(0)

e- e+e+

e-(1> 0 )

Beam-beam effects on pairsComparison with ILC

ILC Nominal SuperB

More Deflections in Super B

Pairs reaching Vertex Detector in SuperBfor 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-(0)

e- e+e+

e-(1< 0 )

Bhabha focusing versus production angle (mrad)

• Bhabha angular deflections are about few 10-2 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

http://flc.web.lal.in2p3.fr/mdi/BBSIM/bbsim.html

Version manager

http:/svn.lal.in2p3.fr/WebSVN/GuineaPig Code description...

In p

rogr

ess