Detector Backgrounds in a Muon Collider

1. Detector Backgrounds in a Muon Collider Steve Kahn Muons Inc. LEMC Workshop S. Kahn -- Muon Collider Detector Backgrounds

2. Introduction • This talk is a review of previous presentations on muon collider detector backgrounds. Nothing presented here is new. A large fraction of the the detector background studies was performed by Iuliu Stumer. • I will try to convince you that you can do physics at a Muon Collider. • The backgrounds encountered are certainly worse than an ee– collider, but they are no worse and probably better than that expected at the LHC and the LHC will produce physics in that environment! • References: • Snowmas 1996 Feasibility Study • Status Report published in Phys. Rev. AB(1999) • Highest Energy Muon Collider Workshop (Montauk, 1999) • Rosario Muon Collider Workshop (May 1997) • UCLA Workshop (July 1997) •  Collider Conference, San Francisco (Dec 1997) S. Kahn -- Muon Collider Detector Backgrounds

3. Parameters Used For Various Muon Collider Scenarios S. Kahn -- Muon Collider Detector Backgrounds

4. Background Sources • Muon Decay Background • Electron Showers from high energy electrons. • Lepto-production of hadrons not included in studies. • Not important for 22 TeV or smaller colliders. • Bremsstrahlung Radiation for decay electrons in magnetic fields. • Photonuclear Interactions • Source of hadrons background. • Bethe-Heitler muon production. • Beam Halo • Beam Scraping at 180° from IP to reduce halo. Could it cause some? • Collider sources such as magnet misalignments. • Beam-Beam Interactions. • Believed to be small. S. Kahn -- Muon Collider Detector Backgrounds

5. Muon Decay Backgrounds • Muon decay backgrounds are expected to be high (see table) • The effort to minimize the backgrounds will have strong influence on • Design of the Detector • Design of the Final Focus for the IR • The IR design itself • If the  per bunch can be reduced as we believe can be done for the LEMC, the detector backgrounds will also be reduced. • An order of magnitude reduction is a blessing. • Most of the numbers presented in this talk will refer to the earlier designs with larger numbers of muons per bunch. The results should be scaleable. S. Kahn -- Muon Collider Detector Backgrounds

6. Muon Decay Background • Upper figure shows electron energy spectrum from decay of 2 TeV muons. • 2×1012 Muons/bunch in each beam • 2.6×105 decays/meter • Mean Decay Electron energy = 700 GeV • Lower figure shows trajectories of decay electrons. • Electron decay angles are of the order of ~10 microradians. • In the final focus section, the decay electrons tend to stay in the beam pipe until they see the final focus quad fields. S. Kahn -- Muon Collider Detector Backgrounds

7. Strawman Detector Concept for a Muon Collider S. Kahn -- Muon Collider Detector Backgrounds

8. The Intersection Region as Modeled in Geant for 2×2 TeV Muon Collider 130 m Region from IP Final Focus Quadrupoles 5 m High Field Dipole Magnets to Sweep Upstream Decay Electrons 20 m S. Kahn -- Muon Collider Detector Backgrounds

9. IP Region for 2×2 TeV(Similar Diagrams for other Energies) Tracker Region Vertex Detector Borated Polyethelene for neutron capture 20º Tungsten Cone For electromagnetic shielding Last final focus quadrupole The figure represents 10 meters around the IP S. Kahn -- Muon Collider Detector Backgrounds

10. Interior Design of the Tungsten Shielding • The tungsten shielding is designed so that the detector is not connected by a straight line with any surface surface hit by a decay electron in forward or backward direction. 50×50 GeV case 250×250 GeV case Borated Polyethelene W Cu S. Kahn -- Muon Collider Detector Backgrounds

11. Summarizing Shielding Configuration to Reduce Backgrounds • 20 degree conical tungsten shield in forward/backward direction. • Expanding inner cone from minimum aperture point is set at 4  beam size. • Inverse cone between IP and minimum aperture point is set to 4  beam divergence. • Designed so detector does not see surfaces struck by incident electrons. • Inner surface of each shield shaped into collimating steps and slopes to maximize absorption of electron showers. • Reduces low energy electrons in beam pipe. • High field sweeping dipole magnets placed upstream of first quadrupole. These dipoles have collimators inside to sweep decay electrons in advance of final collimation. S. Kahn -- Muon Collider Detector Backgrounds

12. Electrons in the Intersection Region • Top figure shows the expanded view of the region near the IP. • The lines represent electrons from a random sample of muon decays. • Electrons are removed by interior collimation surfaces. • The bottom figure shows a detailed view of the IR. • Electrons from a random set of muon decays. • Electrons do not make it into the detector region. S. Kahn -- Muon Collider Detector Backgrounds

13. IP Configuration Parameters S. Kahn -- Muon Collider Detector Backgrounds

14. Synchrotron Radiation • The decay electrons radiate synchrotron photons as they propagate through the fields in the final focus region, losing on the average about 20% of their energy. • Each electron radiates on the average 300 synchrotron photons. • The synchrotron photons carry small energy and do not point to small opening at the intersection region. • The resulting background, however, in the detector region is small compared to the other backgrounds because of the design of the shielding as previously described. <E>=500 MeV Log( ) S. Kahn -- Muon Collider Detector Backgrounds

15. Incoherent Pair Production • Incoherent pair production from ee can be significant for high energy muon colliders. • Estimated cross section of 10 mb giving 3×104 electron pairs per bunch crossing. • The electron pairs have small transverse momentum, but the on-coming beam can deflect them towards the detector. • Figures show examples of electron pairs tracked near the detector in the presence of the detector solenoid field. • With a 2 Tesla field, only 10% of electrons make it 10 cm into the detector. With 4 Tesla field no electrons reach 10 cm. S. Kahn -- Muon Collider Detector Backgrounds

16. Photonuclear Interactions • This is the primary source of hadron background. • The probability for photo production is small relative to other processes. • Large numbers of photons released per crossing make this an important background. • Different mechanisms in different energy bands: • Giant Dipole Resonance Region • 5<E<30 MeV • Produce ~1 neutron • Quasi-Deuteron Region • 30<E< 150 MeV • Produce ~1 neutron • Baryon Resonance Region • 150 MeV<E<2 GeV • Produce  and nucleons • Vector Dominance Region • E>2 GeV • Produce 0 that decay to . • GEANT 3.2.1 had to be modified to include photonuclear production. (I think that GEANT 4 includes these.) S. Kahn -- Muon Collider Detector Backgrounds

17. Gamma Nuclear Interaction Models S. Kahn -- Muon Collider Detector Backgrounds

18. Neutron Background Generated Neutron Spectrum Neutron Spectrum Seen in Detector Log( ) Log( ) ) S. Kahn -- Muon Collider Detector Backgrounds

19. Time Distribution of Neutron Background • The top distribution shows the time distribution of the neutron background generated. • The lower distribution shows the time distribution of the neutron background that is seen in the tracker. • The neutron flux has fallen by two orders of magnitude before the next bunch crossing (10 s later). S. Kahn -- Muon Collider Detector Backgrounds

20. Pion Background in the Detector S. Kahn -- Muon Collider Detector Backgrounds

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25. Photon and Neutron Fluxes at Radial Planes S. Kahn -- Muon Collider Detector Backgrounds

26. Silicon Pad Occupancy as a Function of Radial Position S. Kahn -- Muon Collider Detector Backgrounds

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28. Bethe-Heitler Muons • Electrons interacting with the beam pipe wall or tungsten shielding can produce muon pairs. We call these muon pairs Bethe-Heitler Muons. • These ’s can penetrate the shielding to reach the detector. • Some Bethe-Heitler ’s will cross the calorimeter and produce catastrophic bremsstrahlung losses that could put spikes in the energy distribution. • Time-of-Flight information: • Fast timing can remove B-H ’s in the central calorimeter. • Significant number of B-H ’s in for forward calorimeter are likely to be in time with the signal. • Fine Segmentation in both longitudinal and transverse directions will be necessary to distinguish B-H background from signal. S. Kahn -- Muon Collider Detector Backgrounds

29. Bethe-Heitler Muon Spectrum S. Kahn -- Muon Collider Detector Backgrounds

30. Bethe-Heitler Muon Trajectories for the 2×2 TeV Collider S. Kahn -- Muon Collider Detector Backgrounds

31. Effect of Timing on Bethe-Heitler Muons S. Kahn -- Muon Collider Detector Backgrounds

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35. Conclusions • A carefully optimized design of the detector and the final focus system of the collider ring can significantly reduce the detector backgrounds. • The reduction of detector backgrounds will be an important consideration in the design of the collider ring. • With the proper design the background levels are likely to be les than those expected at the LHC. S. Kahn -- Muon Collider Detector Backgrounds