Discussion of anti-DID ( “DIDNT” ? )

1. Discussion of anti-DID ( “DIDNT” ? ) A.Seryi September 27, 2005

2. Motivation • Normal polarity of Detector Integrated Dipole (DID) allows to compensate locally the effect of crossing the solenoid field for the incoming beam, while the field seen by outgoing beam (and low energy pairs) about doubles • Reverse polarity of Detector Integrated Dipole (anti-DID) would effectively zero the crossing angle for outgoing beam (and pairs) but would double it for incoming beam • Doubling the effective crossing angle for incoming beam may create too much synchrotron radiation size growth (which depend as DsSR~q5/2) • Smaller initial crossing angle would ease the use of anti-DID

3. SR in SiD (without DID) • No IP orbit correction • Both Y and Y’ are zeroed by FD quads offsets (very bad case) • Only Y’ at IP is zeroed by QD0 offset Dssr ~0.3nm (20mrad) IP position =0 Dssr ~5nm (20mrad) IP position free Dssr ~ 0.9nm @ 20mrad Pictures correspond to old (before 2005) SiD field

4. Old and 2005 SiD • New SiD field is shorter and SR effect is smaller Old SiD: Dssr =0.3nm (20mrad) 2005 SiD: Dssr =0.22nm (20mrad) at IP y= -20.7um , y’= -97.5 urad

5. with DID or anti-DID • Will now include DID or anti-DID • In addition to bare SiD, will consider three cases: • DID, both Y and Y’ at IP are zeroed (by QD0 offset and DID) • anti-DID, only Y at IP is zeroed (by QD0) • anti-DID, only Y’ at IP is zeroed (by QD0) • Not considered (known will be very bad): • anti-DID, both Y and Y’ at IP are zeroed (by QD0 and QF1) • Will calculate and compare • SR size growth, IP coordinates • Post-IP field and post-IP dY, dY’ for outgoing beam • Will use 20mrad, then recalculate for 14mrad

6. DID, both Y & Y’ at IP are zeroed • SR growth about the same as for bare SiD • Post IP field is about doubled by DID Dssr =0.19nm (20mrad) y‘ and y at IP are zeroed IP IP

7. anti-DID, only Y at IP is zeroed • SR effect for incoming beam increased, but may be tolerable even at 20mrad: • For 5nm beam SR effects reduce Luminosity by 5% ( 5/(5^2+1.6^2)^0.5 = 0.9524 ) • The effective crossing angle for outgoingbeamreducedsignificantly (zeroed?) Dssr =1.6nm (20mrad) y at IP is zeroed, y‘ = -185 urad

8. anti-DID, only Y’ at IP is zeroed • SR effects are too big (L reduced to 67%) and would not be tolerable at 20mrad • Compensation of post-IP field is the same as in previous case (i.e. very good) • Effects of IP offset need to be studied (e+ and e- incoming beamlines are offset (energy dependent), collimation may be affected (especially at low E), etc.) Dssr =5.4nm (20mrad) y‘ at IP is zeroed, y = 860 um

9. 20mrad & 14mrad x-ing • With 14mrad crossing, can use anti-DID and effectively zero the crossing angle for outgoing pairs • can zero Y at IP for ~no Luminosity cost • can zero Y’ for small Luminosity cost • For 20mrad, full strength anti-DID would cause larger Lumi loss, but certainly can use anti-DID with ~50% strength to halve the effective x-ing angle for pairs • If IP y’ does not have to be zeroed, all this is much easier

10. Conclusion • Anti-DID can be used to zero or reduce effective crossing angle for outgoing beam (and pairs) • At 14mrad we have much more flexibility, since SR effects are much smaller • It is also easier if y’ at IP would not need to be zeroed • flexibility of polarization measurements upstream and downstream need to be considered