gg IR background issues and plans for Snowmass

1. gg IR background issuesand plans for Snowmass Jeff Gronberg/LLNL Linear Collider Workshop October 25, 2000

2. gg Backgrounds • All the same backgrounds as in the e+e- case must be evaluated for gg • Machine backgrounds (muons,synchrotron) - no difference • Disrupted beam - extraction line must accommodate • Neutrons from the beam dump • Charged hits and neutrons from pairs • Additionally • Physics processes from ee, eg and gg interactions ( ex. gg -> qq) • Backgrounds from focusing lens materials

3. Compton Backscattering • After backscattering the bunch contains both high energy photons and electrons. • Angular spread photons ~ 1/g • Micro radian at 250 GeV • 63% Conversion efficiency • Low energy tail due to electrons scattering more than once. Photons Electrons

4. Disrupted Beam • The first priority is to ensure that the disrupted beam can be transported away from the IR. • Easy for e+e-, minimal angular disruption. • Difficult for gamma-gamma • Energy spread in incoming pulse leads to larger angles. • Requires extraction line aperture +/- 5 milliradians • Zero field extraction line, no optics. • Physics impact of no diagnostics? e+e- gg e+e- gg

5. At Z=4m the extraction line aperture must have 2cm radius. Incoming SC quad must stay clear. Maybe need larger crossing angle to accommodate quad. Neutrons from dump: In the standard IR L1-SVX does not see the beam dump. With 2cm radius aperture both L1 and L2 can see the dump. Need Rad hard SVX? Redesign dumpline to reduce neutrons? Hard, significant fraction of beam energy in photons. Cannot steer. Still need to simulate IR impact

6. ZDR Optics for the laser pulses • A complete optics design completed for Snowmass 96 • No serious IR or accelerator constraints • Requires a mirror to be placed in the beam path and in the region where the pairs are traveling

7. Focusing mirrors - tight fit • Current IR beam pipe doesn’t accommodate but there is room for the mirrors with modification. • Only the final mirror is in the path of the pairs. • Standard mirror design 4.3 cm thick. • Reduce material in the path of the pairs to 1cm thick. LCD - Large with ZDR mirrors

8. Pairs are produced with the same angular distribution as in the e+e- case. The number of pairs is dependent on the charged-charged luminosity which can be tuned More on that later For test use pairs from NLC500-B with the central mirror included in the simulation. No extra charged hits in the SVX, still dominated by direct hits Photons in the tracker goes from 120000/train -> 600000/train An increase of a factor of 5. Need to reduce material or add shielding Pair backgrounds • Material reduction: • 1cm radius holes for ingoing and outgoing beam • Thickness of area between beams reduced to 1cm

9. Physics backgrounds • We are currently using pandora-pythia as our Monte Carlo generator. • Interface of CAIN luminosity calculation to pandora-pythia still to be written • Important ee, eg and gg processes need to be identified and implemented • Other generators available? • Theorist input? • Evaluate processes both for machine background and for backgrounds to physics analysis channels

10. Collision options • Polarization increases the high energy gg luminosity. • e- 80%, e+ 0% or 50% • e-e- operation requires change over of one linac. ~ 1 month? • Reduced charged luminosity due to defocusing, reduced number of pairs • e+e- no operational issues but lower polarization gives reduced high energy gg luminosity 80 % 50 % 0 %

11. Optimization of peak vs. tail • The peak region of high energy luminosity is defined as 65% of the ee energy. 65% of max Low High

12. Low vs. High Energy gg • High Energy Luminosity: • Compton spectrum • Beam shape • Low Energy Luminosity • Compton spectrum • Beam Shape • Beamstrahlung at IP • Is low energy a physics background? • Is it a machine background? • Flat vs. Round Beams: • Much higher beamstrahlung • Order of magnitude more low energy luminosity • Double high energy luminosity • Offset beams in y • If gamma spot size at IP is larger than electron spot can suppress ee, eg at the cost of gg luminosity • Conversion point position. • Energy-Angle correlation • Low Energy luminosity can be suppressed at cost of high energy luminosity

13. Conclusions • Extraction line aperture is a bigger problem. • Accommodating outgoing particles exposes the SVX to neutrons from the dump. • May need bigger crossing angle to avoid final quad • Pairs can be mitigated by reducing the ee luminosity • Central focusing mirror must be optimized or shielded to reduce photons into the tracker • No difference in machine backgrounds • Backgrounds from ee, eg and gg physics still to be simulated. • Must be optimized both for machine backgrounds and for physics

14. Goals for Snowmass 2001 • We are embarking on a “crash program” to do a complete pre-conceptual design of a gg IR + laser system • To demonstrate; • Manageable backgrounds • Acceptable impact on machine • Integrated design of laser optics + machine final focus • Realistic physics reach calculation • Realistic laser and optics technologies and a roadmap for the remaining R&D • We are building a multidisciplinary team of physics, engineering, accelerator physics and lasers and optics with the beginnings of active collaboration with the Asian gg effort