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Recent Charm Measurements through Hadronic Decay Channels with STAR at RHIC
in 200 GeV Cu+Cu Collisions
Stephen Baumgart for the STAR Collaboration,Yale University
1August 18, 2008
Outline
1) Motivation2) Analysis Procedure3) D0 Analysis4) Charm Cross-Section5) Ds Analysis
6) Outlook
August 18, 2008 2
Charm Production• Prediction of Charm Cross-Section in p+p from perturbative
Quantum Chromodynamics (pQCD) (NLO/FONLL) Ref: M. Cacciari, P. Nason, R. Vogt, Phys. Rev. Lett. 95, 122001 (2005)
• Charm produced during initial gluon fusion.– Therefore, we expect the cross-section to scale with the number
of binary collisions– Charm produced before thermalization. – Charm is therefore a good probe of the quark-gluon plasma
medium.
Charm Cross-Section from Theory
b244 381134
NLO 1n1f
cc
Charm Cross Section Predicted for 200 GeV Collisions:
b301 1000210
NLOn1f cc
Ref: R. Vogt, arXiv:0709.2531v1 [hep-ph]
Method 1:• use dpt slices, then integrate final result• treat charm as active flavor•FONLL Calculation
Charm Cross Section Predicted for 200 GeV Collisions:
b256 400146
FONLL 1n1f
ccMethod 2:• calculate on full pt range in one step• treat charm as NOT an active flavor (heavy quark considered massive)•NLO Calculation
4August 18, 2008
Measuring Heavy Flavor
Secondary vertices are located by using the Silicon Vertex Tracker (SVT). These can be used for open charm measurements.
5
Charm or beauty is created early in the evolution of the Quark Gluon Plasma, generally from gluon fusion.
Full hadronic reconstruction doneby using the Time Projection Chamber (TPC) The STAR DetectorAugust 18, 2008
D0 to K Analysis Method in Cu+Cu Minimum Bias Collisions
Combinatorial Technique
Rotational Background Subtractionor Event Mixing Background Subtraction
Kaon TracksPion TracksUnused Tracks
Momentum and dE/dx cuts used
py
px
5 degree rotations
13 rotations
D0 Mass
6August 18, 2008
D0 + D0 in Minimum Bias Cu+Cu
STAR Preliminary
2
00
GeV/c 0.00031.8645 mass PDG
%07.080.3..
,,
RB
KDKD
)B1
1(S
S
4.3 ceSignifican
.rotn
For Kpairs:|y| < 1.0pt < 4.0 GeV/c
7August 18, 2008
Double Counting Systematic Error
• Double Counting may cause yields to be exaggerated• Simulations suggest that the effect is negligible when using certain cut sets.• Results from simulation can be used to determine degree of systematic error in
real data.• Check will be done using embedded D0s.
August 18, 2008 8
D0 Reconstruction in Simulation
The D0 peak can be successfully recreated in simulation.
9August 18, 2008
Embedding Simulation
(stat.) 0.078 0.360/
)(22
1
0
0
00
/)(2
dydN
TmT
e
dy
dN
dydpp
Nd
N
D
D
Tmm
D
ttevts
Dt
( D0 + D0)/2 Spectra in Minimum Bias Cu+Cu
Fitting Function:
|y| < 1.0
10August 18, 2008
Conversion from dN/dy to Cross-Section
mbstat
NNR
f
mb
N
dydN
NNcc
ccD
ppinel
CuCubinary
D
.)( 36.064.1
05.054.0/
7.07.4
42
5.6-5.94.08
(stat.) 0.078 0.360/
0
0
RfNdydN CuCubin
ppinel
CuCu
D
NNcc ///0
p+p inelastic cross section
conversion to full rapidity(using PYTHIA simulation, ver. 6.152)
ratio from e+e- collider data
number of binary collisions
mb 18.017.0 conversion dy todN fromerror sys. *Systematic error evaluation for dN/dy in progress.
11August 18, 2008
0 - 60% Centrality
Charm Cross-Section Comparison at 200 GeV
*Systematic error evaluation for STAR Cu+Cu in progress.NLO Ref: R. Vogt, arXiv:0709.2531v1 [hep-ph]
PHENIX:S. Adler, et al. Phys. Rev. Lett. 94 082301 (2005)S. Adler, et al. Phys. Rev. Lett. 97 252002 (2006)
STAR:J. Adams et al. Phys. Rev. Lett 94, 062301 (2005)S. Baumgart, arXiv:nucl-ex/0709.4223Y. Zhang, arXiv:nucl-ex/0805.0364 12August 18, 2008
Charm Cross-Section Conclusions
August 18, 2008 13
• STAR has measured the charm cross-section to be near the pQCD upper limit.
• STAR results from multiple systems are consistent with binary scaling.
• Global results are generally higher than the NLO prediction.
Secondary Vertex Finding for Ds
mesons using the SVTFinding the secondary vertex using the Silicon Vertex Tracker (SVT) allows one to use geometric cuts to identify particles with decay lengthsfrom ~100 m to 10s of centimeters.
August 18, 2008 14
(negligible decay length)
K+
K-
Ds+ (c = 149.9 m)
Primary Vertex
Secondary Vertex
To Detectors
To find Ds mesons, I take only decays with decay lengths between 100 and 400 m.
Looking for Ds through use of SVT Secondary Vertexing: Resonance
August 18, 2008 15
These are s with displaced vertices.
STAR Preliminary
Ds from using the SVT in Cu+Cu
16
Weak ~3 sigma Ds+ peak
found from reconstruction of Ds
K+K-
No Ds- found.
August 18, 2008
STAR Preliminary
STAR Preliminary
Ds Charge Asymmetry?
Cu+Cu Ds+
Cu+Cu Ds-
d+Au Ds-
d+Au Ds+
August 18, 2008 17
STAR Preliminary
STAR Preliminary
STAR Preliminary
STAR Preliminary
Summary/Outlook• Charm cross-section in Cu+Cu measured to be near
upper pQCD limit and consistent with binary scaling• Systematic error evaluation and propagation will be
completed for D0 in Cu+Cu• Hadronic D0 results can be combined and compared
with non-photonic electron results in Cu+Cu (Anders Knospe)
• Ds charge asymmetry in Cu+Cu and d+Au collisions, measurement in new Run 8 d+Au data should illuminate issue
August 18, 2008 18
Calibration of TPC dE/dx Measurements (Backup)
The TPC dE/dx energy loss can be calibration by identifying resonance decays. Since the decay daughters are known, they can be used for calibration.
August 18, 2008 19
In Progress!
Using the Time Projection Chamber (Backup)
August 18, 2008 20
The STAR Time Projection Chamber (TPC) measures both rigidity and energy loss (dE/dx). From these, the masses of particles traversing the detector can be found.
Kaon TracksPion TracksUnused Tracks
Systematic Error Evaluation (Peak Fitting) (Backup)
21
•Different methods and possible cuts are checked in order to evaluate the systematic errors.•Different binnings checked (5,15,25 MeV/bin)•Different fit ranges (200 MeV range for start and finish of Gaussian fit.•Peaks are STABLE under different fit ranges + binnings! But subtraction method introduces systematic error.
August 18, 2008
Yields Gaussian Widths
Gaussian Centroids
STAR Preliminary
