18 december 2002, NIKHEF Jamboree Tracking and Physics Studies, Jeroen van Tilburg 1 Tracking and Physics Studies in LHCb Jeroen van Tilburg NIKHEF Jaarvergadering

18 december 2002,NIKHEF Jamboree

Tracking and Physics Studies, Jeroen van Tilburg

1

Tracking and Physics Studiesin LHCb

Jeroen van TilburgNIKHEF Jaarvergadering

Outline:• LHCb light• OT simulation• Track finding/fitting• Trigger• Physics analysis



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LHCb classic setupLHCb light setup

~65 m2

21 stationsR and φ sensors

VELO

~1.41.2 m2



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LHCb in a new light

Why this re-optimization:• Material budget was too high in a realistic setup.• Trigger performance dropped with new tuning of Pythia.

Classic setup (Techical Proposal):• 11 tracking stations.• Tracking through magnet.• Magnetic shield for RICH1.• Realistic design was too heavy: 60% X0 in front of RICH2.

Light setup (now): • 3 tracking stations plus Trigger Tracker.• Also VELO, beampipe & RICH1 lighter.• Shield removed: B-field in RICH1.• New: all silicon TT stations.• Much lighter: 40% X0 in front of RICH2.

Tracking & reconstruction improved: Less interactions in detector. Better trigger performance: add pT information using TT.



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LHCb Software

Transition from Fortran C++Geant3 Geant 4 in progress.Now completely C++:• Digitization• Reconstruction• Physics analysis

Data production last summer3.6 M events generated at various centers

First physics studies done with all new software in place:Similar performance compared to Technical Proposal.

Next data challenge Feb-April 2003 with 10x more statisticsneeded for the Trigger and Light TDR (submission Sept 2003).

More realistic simulation• Material budget based on TDR’s. • Detailed detector responses; tuned to test beam data.• No use of MC Truth: Full pattern recognition (realistic ghost rate).



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OT Software

Geant3 LHCb event display

OT Software divided in three steps:• Detector Simulation:

Currently in Geant3 (Fortran). Stores entry- and exitpoints of sensitive layers.• Digitization:

Finds which OT wires are hit. Detailed simulation of the detector response.

• T0-calibration:Correct for the time-of-flight. Input to

the tracking.

Outer Tracker simulated in LHCb software



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OT Digitization

OT geometry:• 3 stations with 4 double layers of straws per station • These 4 layers arranged in 0°,+5°,-5°,0°.

OT double layer cross section• Find closest distance to wire.• Smear with 200 μm Gaussian.• Calculate drift time.

Maximum drift time 32.5 ns.

Tuned to test beam results

pitch 5.25 mm

5mm cellsTrack

e-

e-

e-

Track less efficient near edge of straw



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Time spectrum of single bunch for interaction at t=0.

But LHC bunch spacing is 25 ns. So time spectrum of previous and next bunches overlap spillover....

Time spectrum with spillover:Two previous spills and next spill.Dead time of 50 ns included.Cross talk of 5% included.

But electronics read-out window is only 50 ns....

Time spectrum as it is measured with read-out gating.

But particles need time to travel to OT stations Time of flight correction....

Time spectrum after time of flight correction:For each hit assume that the particle traveled in a straight line from IP with speed of light.

OT Digitization



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Efficiency = 98 %(at least one hit in a double layer for p>2GeV; inefficiency due to dead regions and dead time)

On average 20 % of the hits are due to cross-talk and spillover.

Average occupancy in OT ~ 4 %(hottest region ~ 7 %)

OT PerformanceCore resolution = 200 micron

Long Tails:Time of flight correction not a good approximation for particles with p < 2 GeV. No problem for reconstruction of “physics” tracks.



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Track finding algorithms

Many track types, many algorithms

Velo tracks: used to find primary vertex.Forward tracks: used for most physics studies: B decay products.Seed tracks: improve RICH2 performance.Matched tracks: additional to Forward tracks.

Upstream (T TT): enhance KS finding.Velo TT (VTT): improve RICH1 performance, slow pions, kaon tagging.



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Efficiency for tracks p>5 GeV ~ 95%

Track finding

Ghost rate for tracks p>5 GeV ~ 10%



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The Kalman Fit properties:• Adds measurements recursively: easy to use in the pattern recognition.• Mathematically equivalent to least χ2 method. But faster.• Needs as input initial track estimate.• Easy to include multiple scattering and energy loss.

Track Fit

The tracks are fitted using the Kalman Filter.

The Kalman Fit:1. The prediction step.2. The filter step. Adds

measurements one-by-one.3. The smoother step.

direction of the filter track predictionfiltered track



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Kalman fit provides an excellent momentum estimate at the vertex.

N.B. Fitted with single Gaussian in each bin.

Track Fit

Δp/p

LHCb lightLHCb classic (TP)

Momentum resolutioncore σ = 0.35 %2nd σ = 1.0% (fraction 0.1)



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Trigger performance

Magnetic shield removed B-field between Velo and TT.Trigger (Level 1) reconstructs tracks in Velo to cut on large impact parameters.Add TT clusters to these tracks for rough estimate on momentum (~30%).

Trigger performance improved by combining impact parameter and pT information.

Minimum bias

Feasibility study to include OT information in Level-1.

LightClassic



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This autumn first (large) physics study after TP. With new software.Many decay modes are studied.

Large contribution from NIKHEF:• Bs Ds

-( K+K-π-)π+ and c.c. Bs oscillations, measures ΔmS • Bs Ds

-( K+K-π-)K+ and c.c. measures sin(γ-2δγ)• Bs J/ψ ( l+l-) φ ( K+K-) measures sin(2δγ)

Physics studies

LHC provides huge statistics for B-physics: 1012 bb pairs produced per yearσbb/σinel = 1/160



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Bs Dsπ and Bs DsK

Bs Dsπ: Annual yield = 50 kB/S < 2.7 @ 90% CL

Bs DsK:Annual yield = 6.7 kB/S < 12.4 @ 90% CL

Estimates after trigger and offline selection (before tagging):

After applying selection cuts on 1 M bb background event 0 events are selected.



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ΔmS sensitivity

After one year:>5σ measurement of ΔmS up to 48 ps-1

95% CL excl. of ΔmS up to 58 ps-1

Good time resolution = precise measurement of ΔmS

A few months statistics.(Tagging not yet tuned: use tag from MC.)

Bs DsK: σγ =[6º,13º] per year



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Bs J/ψ ( μ+μ-) φ( KK)

BS massσ = 13.9 ± 0.6 MeV/c2

Time resolutionσ = 37.2 ± 1.5 fs

Annual yield = 80 k B/S < 1.3 @ 90% CLσδγ ~ 0.6° per year

Estimates after trigger and offline selection (before tagging):

After applying selection cuts on 1 M bb background event 0 events are selected.



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Conclusions

Light status• Tracking:

High efficiency for “physics” tracks.Good resolutions.

• Trigger: Much improved. More robustness.Fine tuning for Trigger TDR with coming MC production.

• Overall reconstruction & Physics analysis:Final decay channel efficiencies similar to TP.Current estimates on background are inconclusive.Winter production will generate much more statistics.

Last year a lot of studies done with LHCb-light. Also much more realism all parts of software.

Documents

18 december 2002, NIKHEF Jamboree Tracking and Physics Studies, Jeroen van Tilburg 1 Tracking and Physics Studies in LHCb Jeroen van Tilburg NIKHEF Jaarvergadering