Topological D-meson Reconstruction with STAR Using the Silicon Vertex

Tracker (SVT)

Sarah LaPointe Wayne State University

SVT review, BNL, July 7th/8th, 2006

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Overview• Method

• Strategy

• Results from simulation of pure D0 events• Results from simulation of D0 embedded in HIJING• Results from p06id & p06ib 200 GeV CuCu productions

• Estimates for AuAu minbias running

• Summary

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Detecting Heavy Mesons

Hadronic ChannelsBranching Ratio • D0 K- + 3.8%

• D+/- K 9.2%

Semileptonic Channels• D0 e+ + anything 6.9%• D+/- e+/- + anything 17.2%• B0 e+ + anything 10.9%

Techniques used to measure open charm?

• Event Mixing• Single Electrons• Direct Hadron Reconstruction

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Strategy• Determine the reconstruction efficiency by optimizing a

topological cut set in pure D0 events

• Test the cut set on D0’s embedded in HIJING

• Determine the background suppression factor achievable using such cuts in a realistic event sample (200 GeV CuCu production)

• Project the significance of the method to measure D-mesons in 200 GeV AuAu.

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Assumptions

• In simulations the SVT and SSD are 100% active. The SVT has a (realistic) position resolution of 80 m (fast simulator) plus TPC slow simulator and GEANT. Vertex smearing of 150 micron.

• We only attempted reconstruction of D0 to K-+

• In real data the detector inefficiency has to be taken into account. The hardware efficiency during the CuCu run varied between 65-85%. For the runs used here the efficiency was closer to 80%.

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Results from pure D0 events without cuts• 350 K single D0 events (PYTHIA). 16,500 trackable D0 (rapidity & primary vertex distr.)

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Results from pure D0 events with cuts• 706 reconstructed D0, reconstruction efficiency: 4.3%

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Final cuts from cut optimization• Number of TPC points on track: 25• Number of SVT points on track: > 1• Number of SSD points on track: > 0• Decay length: > 300 m, < 7 mm• DCA V0 to PV: < 300 m• DCA pos daughter to PV: > 100 m, < 400 m• DCA neg daughter to PV: > 100 m, < 400 m• DCA neg to pos: < 500 m• dE/dx: nsig()<3, nsig(K)<2, nsig(K)<1 if p>0.7 GeV• PVz cut: +- 30 cm• Rapidity cut: no

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Results from D0 in HIJING events• Based on 220 K HIJING minbias

200 GeV CuCu events. One D per event but not all trackable (f=20)

• Background almost realistic: 7/event (should be 10/event with no cuts)

• Signal is a factor 2.5 too high (factor 20 more because of non-trackable events, factor 50 less because of branching ratio, acceptance and primary tracking efficiency)

• Ideal detector: we see a signal !!

• S/B = 1/14 (realistically 1/35), significance = S/sqrt(S+2B) = 1.7 (in 220 K events)

before cuts

after cuts

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Results: 200 GeV CuCu p06id (no cuts)• 300,000 events: Many zero SVT hit candidates (cut parameter)

Plots with more stats will follow

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Results: 200 GeV CuCu p06id (cuts)• No signal in 300,000 events, background suppression factor = 11,000

Plots with more stats will follow

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Caveats and projections for 200 GeV CuCu production

• Detector is not 100% alive (somewhere between 65-85%)

• 2 hit requirement in SVT more stringent than in simulation

• Inefficiency in global tracker

• With a 100% efficient the significance in a 3 Million event CuCu analysis is about 6.

• Assuming a factor 2 reduction in the signal due to detector and tracking in efficiencies gives me a significance of S/sqrt(S+2B) = 3 for 3 Million event and S/sqrt(S+2B) = 4 for 5 Million events. Signal in 5 Million events = 4,500 counts

• Conclusion: 5 Million minbias CuCu events yield a good signal

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Comparison p06ib (TPC only) to p06id• No signal in 300,000 events, significantly wider DCA distributions

p06ib

p06id

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Reference for 200 GeV AuAu: Haibin’s mixing analysis with TPC only

• Significance = S/sqrt(S+2B) = 4 Year-4 data P05ia production, minbias trigger0-80% centrality, |VertexZ|<30 cm~13.3M

|NSigmaPion|<3||NSigmaKaon|<2 |NSigmaKaon|<1 (p>0.7 GeV/c)

p>0.3 GeV/c, pT>0.2 GeV/cnFitPnts>15, |eta|<1, gDCA<1.5 cm|PairRapidity|<1　

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The Background consistency check (CuCu to AuAu)

• background in Haibin’s study (no cuts, but some dE/dx, rapidity and fitpoints restrictions): 220/event in AuAu

• scaled down to CuCu via Nbin2 = 11/event

• my CuCu background from p06id with no cuts: 44/event

• with Haibin’s cuts: 10/event

• with topological cuts: 0.004/event

• Conclusion: background is consistent.

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Projection for D-mesons in 200 GeV AuAu• raw signal (based on Haibin’s analysis) using 3.8% branching ratio,

50% acceptance plus primary tracking efficiency: 1 Million D in 60 Million minbias AuAu events

• 60 Million is based on a 11 week run in 2007.• Reconstruction efficiency: 4.3% = 43,000 D0 reconstructed• Signal per event: 43,000/60 Million = 0.007/event• Background: scale CuCu background by Nbin

2 (f=20).– CuCu bkgd, no cuts: 44/event, with cuts: 0.004/event (f=10,000)– AuAu bkgd: 0.004*20=0.08/event– S/B in 60 Million AuAu events: 0.0875– Significance: S/sqrt(S+2B) = 14 (compared to 4.5 in Haibin’s analysis

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Summary

• Based on our analysis STAR should be able to topologically reconstruct D0 in the K channel using the SVT and SSD in:

– 5 Million minbias CuCu events

– 60 Million minbias AuAu events