1. Beam-Beam Background and the Forward Region at a CLIC Detector André Sailer (CERN PH-LCD) LC Physics School, Ambleside 22st August, 2009

2. Outline Detector for CLIC and my work on it Forward Region Design Backscatters Conclusions & Outlook

3. 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

4. Software Beam-Beam Background simulated using Guinea-Pig –Perfectly aligned Beams Full Detector Simulation using Mokka (Geant4) Analysis with Marlin

5. 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 LumiCal Quad LHCal BeamCal (including 10cm graphite)

6. 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

7. 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 …

8. Back-up Slides

9. 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 SourceStep 1Step 2Step 3 LCal901000 BCal10400119007300 QD078001000 Pipe438003940074800 Step 3 Step 1

10. 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.