Recent developments in tracking and

Transcription

1 Recent developments in tracking and impact on B-tagging Boris Mangano for Tracking POG group page 1

2 Outline B-tagging Enemies Recent developments in iterative tracking and impact on V0 reconstruction efficiency. Impact on Btagging performance? Open issues page 2

3 The enemies Why should a light jet be identified as a b (showing a sizeable IP)? Fake tracks Badly reconstructed tracks (mostly in the vicinity of the IP, in the Pixels) Ks, lambda etc (particles with lifetime) From Tommaso s talk at Tracking meeting: page 3

4 So, twofold track selection strategy Minimize as mush as possible fakes/badly reco tracks Remember, we seek a mistag ~ 10^-4 so even a few bad tracks are a problem Discard decays of longer lifetime particles Ks.) Moreover From Tommaso s talk at Tracking meeting: page 4

5 From Victor Bazterra Currently the track selection used by b-tagging group discards tracks with distance from jet > 0.07 cm. Similar plots and selections for signed decay length and transverse impact parameter : page 5

6 How track reconstruction can help b-tagging 1. Maximize the reconstruction efficiency for both tracks coming from K s, Lambda, Conversions (+ tracks coming from nuclear interactions). 2. Identify the V0s vertices (+ nuclear interaction vertices). 3. Provide to b-tagging a list of tracks which is equal to: tracks for b-tagging = all tracks - tracks from V0s Benefits for b-tagging performance: - lower mis-tagging due to V0s, if the current impact parameter selection is used. - higher b-tagging efficiency, if the impact parameter selection can be loosened. In the next slides, recent developments in tracking that help (mostly, but not only) in the context of V0 reconstruction page 6

7 Red = single sided strips Blue = double sided strips Green = pixels CMS Tracker R-Z view Measurements used for trajectory seeding Measurements used for trajectory building Due to the geometrical acceptance of the pixel detector, some particles don t produce enough pixel measurements to get a trajectory seed. These particles were lost using the original trajectory seeder based only on pixels. The seeding based on both pixel and strips is able to reconstruct them page 7

8 effic ciency [% %] Tracking based on Pixel-less seeder CMSSW_2_1_X Tracking for CMSSW_3_0_0_pre2 η η Starting from CMSSW_3_0_0_pre2, track reconstruction sequence will contain a pure pixel-less seeded tracking step (the 4th) Efficiency drop at eta ~0 finally recovered! page 8

9 Tracking based on Pixel-less seeder TTbar sample No PU Pt > 0.9 GeV CMSSW_ 2_ 1_ X + tags for 4th step by Kevin Stenson Pt [GeV] η Efficiency recovered mostly (but not only) around eta=0 N.B.: Efficiency denominator is # charged particles produced at r <3.5, z<30 page 9

10 Tracking based on Pixel-less seeder TTbar sample No PU Pt > 0.9 GeV CMSSW_ 2_ 1_ X + tags for 4th step by Kevin Stenson Pt [GeV] η Considering a different denominator for the efficiency definition (# charged particles produced at r <20, z<60), the improvement in reconstruction performance is even more striking page 10

11 Tracking based on Pixel-less seeder TTbar sample No PU CMSSW_2_1_X 2 1 X + tags for 4th step by Kevin Stenson Pt [GeV] η The improvement in efficiency is NOT jeopardized by an increase of the fake rate. NB: Selection of tracks from 2nd and 3rd step has been re-tuned as wellby Kevin Stenson. NB 2: Bug in cleaning of pixel-less seeds collection found by I.Tomalin page 11

12 TOB-TEC seeded tracking (5th step) Reconstruction of particles produced at r > 30 cm Measurements used for trajectory seeding Measurements used for trajectory building by K.Stenson, I.Tomalin and B.Drell page 12

13 TOB-TEC seeded tracking (5th step) tags t for 4 steps by Kevin tracking Stenson + tagsfor5steps steps tracking Pt [GeV] η N.B.: Efficiency denominator is # charged particles produced at r <60, z<120 page 13

14 TOB-TEC seeded tracking (5th step) CMSSW_2_1_X Kshort reconstruction efficiency in B 0 -> K s mu mu events + tags for 4 steps tracking + tags for 5 steps tracking Almost a factor 8 improvement in K s reconstruction ti efficiency. i V0 finder has been also retuned to improve timing i performance: factor 5-10X speedup by K.Stenson, B.Drell, K.Ulmer page 14

15 Improvement to seed reconstruction from triplets by C.Saout, F.Sikler, MK M.Konecki Expected improvement of reconstruction efficiency for: - very low pt particles - tracks detached from primary vertex A speed up in general seed reconstruction has already been measured page 15

16 The enemies Why should a light jet be identified as a b (showing a sizeable IP)? Fake tracks Badly reconstructed tracks (mostly in the vicinity of the IP, in the Pixels) Ks, lambda etc (particles with lifetime) From Tommaso s talk at Tracking meeting: page 16

17 Open issue: pixel-less seeding and splitting of merged pixel clusters Code used for splitting merged pixel clusters implemented by Johns Hopkins Univ. group Pixel-less seeding allows to recover particles that actually have pixel hits, but were not reconstructed because of clusters merging. Improvement in efficiency has still to be quantified page 17

18 Conclusion In addition to fake tracks, also particles with a genuine large impact parameter are responsible for (mis)tagging light jets as b-jest These particles are the product of V0s decays, nuclear interactions, conversions. Recent developments in tracking have dramatically increased the efficiency for identifying these particles. TO BE TESTED: try to explicitly remove these particles from the pool of tracks used to tag jets. Tracking performance is improved in general. B-tagging performance should be re-measured starting from CMSSW_ 3_ 0_ 0_pre2. On-going g activities to understand how the splitting of merged pixel clusters can impact on b-tagging of very high pt jets. page 18

19 B-tagging users are welcome to directly contribute to improving track reconstruction. Please contact me and Kevin page 19