COMPASS

1. EN Muon sweeping in the M2 muon beam line for the COMPASS experimentwith a view to its potential for CLIC Lau Gatignon / EN-MEF T6 COMPASS

2. Courtesy: H.Burkhardt A potential problem for CLIC: Muons from beam halo Beam tails are scraped away by a collimation system in the BDS. Below we show simulated profiles of the beam at the BDS entrance (core of beam in red) From I.Agapov et al,2009, to be published From these simulations one estimates that a fraction 2 10-4 hit the collimators,i.e. about 2.4 108 particles per train assuming a total flux of 1.24 1012 per train. Preliminary estimates indicate that out of those ~ 2 105 would reach the detectors. The final rates remain to be studied with BDSIM using the final and detailed geometry Muon sweeping in M2 beam line

3. H.Burkhardt, CLIC09 ‘Typical’ muons at CIC have momenta of a few hundred GeV/c Muon sweeping in M2 beam line

4. DeepaAngal-Kalinin Dipoles??? Muon sweeping in M2 beam line

6. Muon sweeping in M2 beam line

7. Muon sweeping in M2 beam line

8. Basic approach to reduce muon halo:Toroidal fields in Iron Magnetised Iron has three main effects on muons: • Energy loss: dE/dx ~ 1.5 GeV/m • Multiple Coulomb scattering: • Magnetic deflection: For a useful effect, requireqdefl >> qscatt, i.e. 450 L >> 106 √Lor L >> 0.05 m e.g. for qdefl > 10 qscatt one requires L ≥ 5 m In the M2 beam we use two types of toroids: SCRAPERS: adjustable gap, no vacuum (?), smaller outer coverage, expensive MIBS: fixed gap, large coverage, cheap, vacuum tube can go through Muon sweeping in M2 beam line

9. B Magnetic force SCRAPERS(Magnetic Collimators) Have to get polarity right…. Muon sweeping in M2 beam line

10. Muon sweeping in M2 beam line

11. 4 Motors: 2 upstream, 2 downstreamallows to follow beam divergence Have to stop current before moving jaws !!! Muon sweeping in M2 beam line

12. MIB = MagnetisedIron Block

13. Note: • Both SCRAPERS and MIBS are easy to powerTypically operated with 100 to 200 Amps Resistance of one MIB coil (for L=1.6 m) is ~ 100 mOhm Therefore 10 m of MIB consumes only ~150 A x 100 V ~ 15 kW • There is not much reason to push the currentThis would lead to strong saturation of the field in the Iron and quick increase of the stray field on the main beam (but we use the quadrupolar stray field to measure the polarity) • There is (in principle) no effect of the field on the main beamwhereas dipole fields would require compensation • The requirements on precision of machining are very modestSee e.g. the picture of the MIB in he M2 beam Just make sure there is good magnetic contact between blocks • The deflection of a 500 GeV/cmuon by a 5 m long toroid is ~ 5 mradAfter 2 km drift, it is deflected away by 10 m(well outside tunnel & detector) Muon sweeping in M2 beam line

14. The size of the Halo problem Simulations based on the HALO program by C.Iselin(1974) Flux (arbitrary units) ‘Good’ muons Halo after p absorber Halo after m momentumselectionNote: Halo / ‘Good’ m ≈ 6 P (GeV/c) Muon sweeping in M2 beam line

15. The effect of muon sweeping in the M2 muon beam for COMPASS With muon sweeping Without muon sweeping Y (mm) X (mm) X (mm) Muon sweeping in M2 beam line

16. No sweeping Passive iron Magnetic collimation Muon flux (arb.units) Y (mm) X (mm) Muon sweeping in M2 beam line

17. Outside 10 cm Within 10 cm Muon flux (arb.units) P (GeV/c) P (GeV/c) Muon sweeping in M2 beam line

18. Muonsvs Halo Muons within 10 cm Muons with r  [10,200] cm Muon flux (arb.units) P (GeV/c) P (GeV/c) i.e. Halo (R=2 m) / Muons (R = 10 cm) ~ 5%, i.e. Halo/Muons in same surface ~ 1.3 10-4 Excellent agreement With data! Gain > factor 100 Muon sweeping in M2 beam line

19. Alternative possibility, based on MBPL dipole magnet, used in K12 beam: • Fill gap with weak Iron • 40 mm diameterhole for beam passage • Run at 20 Amps(for 75 GeV/c main beam) • Put 3 magnets of 2 m lengthin series

20. Muon sweeping in M2 beam line

21. Correctors needed !

22. Muon sweeping in M2 beam line

23. Advantages: • Do not sweep muons upward, into the fields • In the limited length available (about 100 metres), in the presence ofboth positive and negative muons, avoid the problem of “refocussing” muonswith the second toroid, which were swept away by the first one • Disadvantages: • Covers a smaller surface than e.g. a MIB • Needs high-precision engineering and machining, hence more costly • Stray field on beam axis needs to be compensated by two correction magnets • It takes about 15 minutes for the field to stabilise after a current change.During that time the beam steering shifts continuously

24. Conclusions • The M2 high-energy high-intensity muon beam provides a main beam of4 108muons of 160 GeV/c per SPS cycle to the COMPASS experiment. • These “good” muons are accompanied by a “halo”of about 3 109muons from pion decays along the 600 metres long decay volume. • With a system of 7 magnetic collimators”(5 m long each) and a total of some15 metres of magnetised iron blocks this halo can be reduced to 5% of the beamflux over a surface of about 4x4 metres (the size of the COMPASS chambers).Both types of devices are magnetic toroids. • The halo momentum distribution is comparable to the one of the muons producedin the CLIC BDS collimation system, but in the latter case the lever arm is significantlylonger. Also in the CLIC BDS the main beam are electrons rather than muons, therefore their likelihood to become high-energy muons is reduced w.r.t. M2. • The MIBs combined with some magnetic collimators could therefore be aneconomical solution to reduce the muon background in the CLIC detectors. • This approach merits follow-up in the BDSIM calculations. Muon sweeping in M2 beam line