b-jet slice performances at L2/EF - University of...

b-jet slice performances at L2/EF

Andrea Coccaro, Bianca Osculati, Fabrizio Parodi, Carlo Schiavi

T&P, CERN

20 March 2007

Andrea Coccaro, Bianca Osculati, Fabrizio Parodi, Carlo Schiavi T&P, CERN


Outline

b-jet slice status

b-tagging performance

Status/Outlook



b-jet slice status b-tagging performance Status/Outlook

b-jet slice

The b-tagging selection is an element of flexibility in the ATLAS HLTframework: it might help to increase acceptance for events with multib-jets (e.g. bbA/H → bb or hadronic ttH are not selected by thestandard jet trigger) and to select calibration samples.Efficiency might be recovered by relaxing LVL1 thresholds and applyingb-tagging at HLT to reduce the final rate.

The b-jet slice is essentially so summarized:

I LVL1 jet Roi with ET > 20 GeV and ∆η ×∆φ = 0.4× 0.4

I FEX algo: SiTrack/IDSCAN at LVL2 and EF ID at EF

I Hypo algo: TrigBjetHypo at LVL2/EFI no PV reconstruction in RΦ planeI z PV reconstruction with histogramming tecnique




b-tagging hypothesis algorithm

TrigBjetHypo performs cut on discriminant variables computed with aLikelihood Ratio method:

WRoI =

Ntracks∏i=1

Pb(Si )

Pu(Si )X =

WRoI

WRoI + 1

It implements the following discriminant methods:

I significance of the transverse impact parameterS(d0) = |d0| /σ

I significance of the longitudinal impact parameterS(z0) = |z0 − zvtx | /σ

I 2D combination of the significances S(z0) e S(d0).

Secondary vertex based methods studied partially standalone but not yetincluded in the slice.




b-tagging performance

Intensive b-tagging studies performed with 12.0.0 and already showed(e.g. T&P of October 2006). In this talk will focus on:

1. validation with 12.0.5;

2. comparison LVL2 performance with different tracking algorithms;

3. effect on misaligned sample;

4. discrepancy EF/Offline performance.

Performance results are shown in terms of u-rejection (Ru = 1/εu) vsb-efficiency εb with high statistic. Official datasets for this kind ofstudies:

I signal: 5850 CSC file (WH → bb)

I background: 5851 CSC file (WH → uu)




1. b-tagging performance with 12.0.5

These results are shown with 2D combination of impact parameterssignificances. Performance looks very similar w.r.t 12.0.0.

In order to optimize b-taggingperformance keeping thereconstruction efficiency as high aspossible, track selection studies hasbeen carried on both at LVL2/EF.In particular, χ2 cut has beenoptimized taking into account χ2

probability distribution and a pT cuthas been added.




2. LVL2 b-tagging performance with 12.0.5A first intensive study on the impact on b-tagging of tracking algorithmshas been performed showing quite difference between SiTrack andIDSCAN.

I further specific track selectioncould improve IDSCANperformance.

I intrinsic factors reduce b-taggingperformance with IDSCAN:

I impact parameter pullsuggests understimation onimpact parameter errors

I more fake tracks thanSiTrack (see Fabrizio’s talk)




3. Effect on b-tagging with misaligned sampleThis study has been performed in order to try to understand how muchour knowledge of the detector must be achieved to have reasonableb-tagging performance (Detector Description ATLAS-CSC-01-02-00).LVL2 more sensitive to magnetic field and misaligned geometry withmaterial distortions.




4. Discrepancy between EF/Offline performancePerformance comparison between L2, EF and Offline (with 12.0.0,already shown, e.g. T%P October 2006). Discrepancy betweenEF/Offline arose ⇒ study to understand this behavior.




4. Discrepancy between EF/Offline performanceThe problem has been handled investigating all possible difference:

I EF/Off track quality (see Fabrizio’s talk);I primary vertex reconstruction;I b-tagging method;I between online RoI and offline jet.

In particular, ABSDOS online methodhas been compared with 2D offlinemethod (without rejecting bad tracksand without separate treatment of trackswith shared hits). Moreover, offlineb-tagging weight has been consideredonly in case of a matching online RoI.With these cares same results wereexpected, but...




Summary/Outlook

b-tagging performance studied and understood, missing items:

I investigate further EF/Offline discrepancy, in particular possibledifference between online RoI and offline jet (e.g. possible tuning onRoI size);

I empty RoI need more studies (difference w.r.t. 12.0.0);

I improve primary vertex reconstruction:I create a proper algorithm to get better modularity;I primary vertex with tracks from different RoI in the same event

(possible thanks to new steering implementation)

I impact of the b-jet trigger on the ATLAS physics program studyingdifferent physics channels and calibration sample (see Fabrizio’stalk).

In the meanwhile, our b-jet slice twiki page has changed:https://twiki.cern.ch/twiki/bin/view/Atlas/PesaBjet



b-jet slice performances at L2/EF - University of...

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