Beam-Beam Background and the Forward Region at a CLIC

DetectorAndré Sailer (CERN PH-LCD)

LC Physics School, Ambleside

22st August, 2009

Outline

• Detector for CLIC and my work on it

• Forward Region Design

• Backscatters

• Conclusions & Outlook

Linear Collider Detector for CLIC

• As discussed on Thursday morning (and today)– Some Changes are necessary for the Current ILC

Detectors to work properly at 3 TeV CLIC

• My work is concerned with the Beam-Beam Background, i.e. Incoherent and Coherent Pairs going towards the Forward Region– Looking at Backscatters into the Main Detector,

focused mostly on the Vertex Detector and reducing backscatters

Software

• Beam-Beam Background simulated using Guinea-Pig– Perfectly aligned Beams

• Full Detector Simulation using Mokka (Geant4)

• Analysis with Marlin

Forward Region• ILD Forward Region• CLIC Forward Region

– Incoherent Pairs out to ~80 mm (at 2.5m)

• Don’t destroy LumiCal• Larger Inner Radius (From

80(85?) to 100mm)– Let out the Coherent Pairs ~10^8

Particles• Coherent Pairs Out to ~6mrad• Larger Inner Radius for BeamCal

(From 15 to 25mm)– Make LumiCal and BeamCal

Longer (40 radiation lengths) to contain 1.5 TeV electron showers

– FF Quad at L*=3.5m• Remove LHCal

– Beam pipe as large as possible• ~1 cm distance to QD0

LumiCalQuad

LHCal

BeamCal (including 10cm graphite)

Backscatters

• Main source of Backscatters (that hit the VXD) is the inner wall of the beam pipe (Cyan colored hits in the picture)

• Can get rid of 50% them by using racetrack formed beam pipe (in horizontal (y) direction)

• Rate is still 9 Hits per mm² per Bunch Train

• Supposedly OK for B-Tagging, study is ongoing

• Predicted Rate from simpler simulation factor ~5 less

Conclusions & Outlook

• Backscattering is more of a Challenge at CLIC– No AntiDID field, larger number of particles, higher bunch

repetition rate• Full Detector Simulation needed to get a realistic picture

of backscattering• Still trying to reduce Backscatters into the VXD

– Check how much useful BeamCal is for SM Veto of Electrons– Change Geometry for better masking– Make Sure Coherent Pairs really leave through BeamCal

• Look at background in other sub detectors– Forward Tracking, TPC, Calorimeters

• Fluctuations from non perfect bunch collisions• …

Back-up Slides

Hits in the VXD depending on Forward Region Geometry

• From Step 1 Step 2– Smaller QD0, less

Backscatters (larger one was not shown before)

• From Step 2 Step 3– Larger inner Radii for LCal

and BCal– Less Backscatters from

BeamCal, none from LumiCal

– More Backscatters from the Beam pipe hit VXD

Source Step 1 Step 2 Step 3

LCal 90 100 0

BCal 10400 11900 7300

QD0 7800 1000 1000

Pipe 43800 39400 74800

Step 3Step 1

Backscatters from Beam pipe• Origin of the Backscattered

Particle not uniformly distributed around Beam pipe

• Somewhat Higher Concentration for larger |Y|

• Radial Beam pipe size limited by QD0

• Use a Beam pipe with an Elliptical/Racetrack cross section behind BeamCal– Backscatters are blocked by

BeamCal– Reduce Background from

Backscatters by ~40%– Still 9 Hits per mm² per Bunch

Train

Origin of the Particles causing a Hit in one of the Tracking Detectors, with the Origin behind one BeamCal (Z>3.34m)Centered on the Outgoing Beam axis.