high level triggers at cms with the tracker

Tommaso BoccaliTommaso Boccali

SNS and INFN PisaSNS and INFN Pisa

Vertex 2002Vertex 2002

Kailua-Kona, HawaiiKailua-Kona, Hawaii

3-8 November 20023-8 November 2002

High Level High Level Triggers at Triggers at CMS with the TrackerCMS with the Tracker

Vertex 2002Kailua-Kona, Hawaii

High Level Triggers at CMS with the Tracker Tommaso Boccali

The ChallengeThe ChallengeLot of b’s and tau’s from

interesting Physics– Supersymmetry – Higgs decays– Top, B physics

Large QCD backgrounds (factor ~1000 or more)

The ChallengeThe ChallengeL1: 40 MHz input

L1:100 KHz outputWrite to offline:100 Hz



How to select events?How to select events?

• General idea: we would like to benefit from full offline analyses to discriminate events.

• Since all the data is available after Lvl-1, we would need “only” ~infinite CPU power.

• What can be done What can be done with a reasonable with a reasonable number of number of commercial CPUs?commercial CPUs?

CMSHLT functionality depends on data rate and CPU resources available

TraditionalL2 functionalityis the most challenging parameter. Does notbenefits of full-granularity



Timings?Timings?•We have ~100 kHz of input rate

•We realistically can have a few thousands of (2007) CPUs

•This means ~50 ms per decision

•Use Moore’s Law:– 50 ms in 2007 ~ 500 ms on a 1

GHz CPU todayTiming for a 1 GHz CPU today ~500 ms



What about the Tk?What about the Tk?• The Tracker is the most precise subsystem, but is

(thought to be) slow not used at all for Lvl-1not used at all for Lvl-1• Extensive studies have be performed to answer the

question: what can we do in ~500 ms?

• The CMS Tracker has been already described by S. Schael (Strips) and L. Cremaldi (Pixels)

6,136 Thin 300 m (1 sensor)9,096 Thick 500 m (2 sensors)99xx101066 strips stripsOccupancy ~10Occupancy ~10-2-2

14192 chips39x39x101066 pixelspixelsOccupancy ~10Occupancy ~10-4-4



BaselineBaseline

TK Inner Barrel (TIB)

TK Outer Barrel (TOB)

TK Inner Disk (TID)

TK Outer disks (TEC)

pixels

One charged track traversing the CMS Tracker interacts with a maximum of ~14 hits in the silicon (+3 in the Pixels)



General recipesGeneral recipes•Do something only when needed!

•reconstruction on demand: never do anything until it is requested

•Usually, one is not really interested in reconstructing “the event” in the trigger decision, a smaller region is sufficient

•regional tracking

•How close we have to be to full offline performance?

•conditional tracking

•In many cases, it is simple enough to recognize the trigger event from the pile-ups

•pixel reconstruction



Regional trackingRegional tracking•Reconstruct only clusters and tracks in a certain region of the Tracker (i.e., around a jet)

•In principle, complexity (time!)

scales with (/4

•The region searched for is defined in terms of R

typical cases R~0.4

22 R

region around a Lvl-1calo jet

-2.5<<2.5; 0<<2



Conditional trackingConditional tracking•After the complete solution of the combinatorial problem, the time spent for each track is proportional to the number of hits. What about performances?What about performances?

if at least 7 hits are used, efficiencies and fake rates are close to optimal;

5 hits are enough if we can accept a

higher ghost rate.

@ 2x10 33 lumi



Conditional trackingConditional trackingNot only eff/fake rates are fine, but also resolutionsresolutions

Good resolution even with only 5 hits

Full Full reconstructionreconstruction


High Level Triggers at CMS with the Tracker Tommaso BoccaliPixel Resolution in z (cm)

Pixels @ HLTPixels @ HLT•@ 2x1033, at LHC we have ~3.5 minimum bias events per bunch crossing, @1034, ~17.5

•moreover, we integrate signals from bunch crossings [-5,+3].

•we have up to 200 events contribute; it is essential to isolate the trigger trigger primary vertexprimary vertex and the tracks associated to it.

= 26 = 26 mm

Using only the PixelsUsing only the Pixels: :

fast fast ~ 50 ms/1GHz CPU~ 50 ms/1GHz CPU

good resolutiongood resolution ~ 30~ 30 mm in zin z

efficientefficient ~100% ~100%



Physics @ HLTPhysics @ HLT•With these ingredients, we can do

•inclusive tagging

•inclusive b tagging

•exclusive b tagging

•Moreover, we must take care about detector configuration: we (yet) don’t know what will be available at the startup, particularly about pixel configuration:

•2 or 3 barrel layers?2 or 3 barrel layers?

•1 or two forward disks?1 or two forward disks?

•All combinations checked!All combinations checked!

!!



taggingtagging

Regional TrackingRegional Tracking: Look only in: Look only inJet-track matching coneJet-track matching cone

Loose Primary Vertex associationLoose Primary Vertex association

Conditional TrackingConditional Tracking: Stop track as soon as: Stop track as soon asPixel seed found (PXL)Pixel seed found (PXL) / / 6 hits found (Trk)6 hits found (Trk)

If Pt<1 GeV with high C.L.If Pt<1 GeV with high C.L.

Reject event if no “leading track” foundReject event if no “leading track” found

Regional TrackingRegional Tracking: Look only inside: Look only insideIsolation coneIsolation cone


Conditional TrackingConditional Tracking: Stop track as soon as: Stop track as soon asPixel seed found (PXL)Pixel seed found (PXL) / / 6 hits found (Trk)6 hits found (Trk)


Reject event as soon as additional track foundReject event as soon as additional track found

a region R=0.5 around the calo jet is searched



The time spent to reject QCD background is ~300 ms @ low lumi and less than 400 ms @ high lumi

taggingtaggingA0/H022-jet selection with Tracker: various configurations studied; heavy use of conditional tracking (6 hits) and regional tracking

Efficiency in various situations and two

different Higgs masses. The working point is

chosen at a QCD background rejection of

~103

@ 2x1033

sec



inclusive b tagginginclusive b tagging

Performance of Performance of HLTHLT tagging tagging close to close to offlineoffline!!

Regional Tracking: Look only inRegional Tracking: Look only inJet-track matching coneJet-track matching cone


Conditional Tracking: Stop track as soon asConditional Tracking: Stop track as soon asPixel seed found (PXL)Pixel seed found (PXL) / / 6 hits found (Trk)6 hits found (Trk)


timings:timings:~300ms low lumi~300ms low lumi~1s high lumi~1s high lumi

bbuu



exclusive b taggingexclusive b tagging•Specific decay channels: we have too many b’s in non interesting decay channels!

•Use dilepton triggers to start the selection, and apply tracker selection on invariant masses

•Use large tail in Lvl-1 resolution: we are mostly interested in muons with low pT wrongly selected (Lvl-1

thresholds are 95%)

•Most important: low rates on QCD backgrounds to tape.

2 Lvl-1

Primary vertex to associate tracks

Reg & Cond tracking around the two jets

(only tracks pT>2 GeV, max 5 hits)

Hard cuts on mass peaks

Tape

900 Hz

pT>3 GeV @ 95%

<2 Hz bkg

eff ~ 10%



exclusive b taggingexclusive b taggingExclusive b-tagging: Use Level 1 events with 2 muons, together with regional/conditional tracking

•BS

•BSJ/KK

•bBSDS (KK)

BS OfflineHLT

BSJ/KK

Everything feasible in HLT These are events in year 1 (~10 fb-

1)

eff~10%rejection~100

CDF RunIIa 0CDF RunIIb maybe

CDF RunIIa~ 14 for now



ConclusionsConclusions

•The CMS Tracker can be used in HLT on the full Lvl-1 output (100 kHz) with nearly offline performance now (startup –5y–5y)

•b and inclusive physics in good shape and ready for discovery

•b exclusive physics possible in parasitic mode

•These results are solid even if the detector will be staged, at and both 2x1033 and 1034 Lumi

high level triggers at cms with the tracker

Documents

cms tracker

cpu time

offline performance

offline precisionwhat

offline analyses

smaller region

certain region

w4pkthe region