Online Spacepoints in EC fixed

1. Online Spacepoints in EC fixed After the fixes were applied, the IDScan inefficiency (with respect to EF) has decreased by 90%, ie. out of an example set of 30 previously failing electron tracks, 27 are now reconstructed. • The fixes went into release 13. Additionally, for those who would like to use it in 12.0.6, they have been backported. Please check out TrigOnlineSpacePointTool-00-00-23-bf01. • The fix will effect all slices at varying degrees and should help both IDScan and SiTrack.

2. Fixing the Spacepoints – Some Details E. Özcan University College London

3. Symptoms • Efficiency of IDScan with respect to EF is found to be less in EC than in barrel. • Problem tracked to spacepoints. • Comparison with offline SPs yielded two issues: • Smaller problem : Online SCT clusters a slightly off compared to offline SCT clusters. • Dominant problem : Even if offline clusters are used, online SP formation creates different SPs than offline algorithm does. • r and f coordinates are quite off. • Occasionally no online SP corresponding to offline.

4. Before the Fix Crosses: Offline SPs Colored points: SPs created by the online algorithm from offline clusters To see the effect of the fixes easily, follow, for example the pink points.

5. Step 1 – Fix f • Lines along the strips do not intersect on the z-axis. (Strips are not really radial.) • Result can be modeled by a small rotation around the center of the module (MC). • Fix: Create SP as usual. Then find the 2D local vector connecting MC to SP. Rotate that vector by the rotation angle. Revert back to global coordinates. double xsp = rsp*cos(phisp), ysp = rsp*sin(phisp); xsp -= posPhi.x(); ysp -= posPhi.y(); double tlsp = atan2(ysp,xsp); double rlsp = sqrt(xsp*xsp+ysp*ysp); xsp = rlsp*cos(tlsp+rotAng); ysp = rlsp*sin(tlsp+rotAng); xsp += posPhi.x(); ysp += posPhi.y(); sp->phi(atan2(ysp,xsp)); posPhi: Position vector of the center of the f module. rotAng: Angle between posPhi and longitudinal local axis on the f module.

6. After the f Fix Crosses: Offline SPs Colored points: SPs created by the online algorithm from offline clusters Note the orange points at the top, which were off the figure before. All SPs have roughly aligned at the correct f.

7. Step 2 – Fix r • r coordinate of SP calculated with : • Here a is the stereo angle, obtained from: sina = uv_element->sinStereo(); alpha=asin(sina); • Unfortunately, when there is misalignment, this is incorrect! Fix by finding the angle between the transverse axes of phi and uv elements. const Hep3Vector& dirT = phi_element->phiAxis(); const Hep3Vector& dirT2 = uv_element->phiAxis(); double rotAng = -asin((dirT.getX()*posPhi.x()+ dirT.getY()*posPhi.y())/posPhi.perp()); alpha = asin(dirT.getY()*dirT2.getX()- dirT.getX()*dirT2.getY()); 10-20% effect in a causes r to be miscalculated by centimeters!

8. After the f and r Fixes Online SPs (colored points) in very good agreement with offline SPs (crosses). Gray crosses right in the middle (indicated in red circle) => One online SP is missing. Track (red V) has changed. Previously no L2 track, now there is one.

9. Step 3 – Find missing SPs • Disc 8 : Inner ring is missing and “middle” ring is populated with short-middle modules. • SiDetectorElement::center() returns center of active area (red cross), not the point around which the stereo rotation is done (black/white circle, Crot). • Major effect => Essentially no SPs from disc 8 middle ring. • Fix: Reduce rF (r of MC) Crot MCF MCS correction rsp=Rphi*B; if (maxLocL<35 && Rphi>400) { // short-mid strips double posDiff = sqrt(pow(posPhi.x()-posStereo.x(),2)+ pow(posPhi.y()-posStereo.y(),2)); rsp=(Rphi-posDiff/2./sin(fabs(alpha)/2.))*B; } Crot

10. After Algorithm Fixes Online SPs (colored points, created from offline clusters) in very good agreement with offline SPs (crosses). Gray crosses right in the middle => Now accompanied with cyan points!

11. Back to Clusters • Problem in clusters=>SPs solved. • So go back to investigate differences in clusters. • Online and offline clusters have different coordinates even when they are made of exactly the same strips. This difference is usually on the order of 0.02-0.05 mm (less than 1 pitch-length, surprising, but not significant). • For strips on disk 8, difference much more significant: Can be as high as 2.2mm or more!!! • Source of problem: Disk 8 “middle” ring is short – module centers at higher radius compared to other rings. => Different pitch length needed (instead of default one used for all middle-ring modules). • Fix: 2D array of pitch lengths filled at initialization: for (int disk=0; disk<s_ndisk; disk++) for (int ieta=0; ieta<s_neta; ieta++) { InDetDD::SiDetectorElement* element = man->getDetectorElement(2, disk,0, ieta, 0); if (0==element) continue; m_forward_pitch[disk][ieta]=element->phiPitch();}

12. Fixing the clusters Left: Disk-8 SPs created from online clusters (squares) are significantly off from offline SPs (crosses), before the cluster fix. Right: All online SPs, created from online clusters, are in perfect agreement with offline SPs, after the cluster fix.

13. Conclusion • Tested on 30 events from Monika, where IDScan failed, but EF found a track. • After fixes, 27 have tracks reconstructed. • Likewise, studies by Mike Flowerdew with Z->ee events immediately shows the improvement. • The fix is in release 13, there also exists a backport that can be used with release 12. • IDScan to be tuned to make best use of the fix. – Preliminary studies show a 50% decrease in fake rates for the same efficiency.