1. Jonathan Bouchet, For the STAR collaboration
Charm reconstruction using a micro-vertexing technique with the STAR Silicon Vertex Detectors
Motivation
Heavy Flavor measurement in STAR
STAR detector and microvertexing technique
Some initial results
Summary and future measurements
Jonathan Bouchet,
For the STAR collaboration
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

2. Charmed mesons in Heavy Ion collisions
Heavy flavor is produced at the earlier stages of the collision via gluon fusion :
not affected by chiral symmetry restoration.
production cross section are found to binary scale[1].
ideal to probe the medium created in HI collision.
Theoretical models predicted gluon radiative energy loss for heavy quarks to be smaller than of light quarks[2], which is not experimentally observed [3].
Measuring collective motion (v2) of charm mesons will also indicate whether thermalization in light quark sector is reached in the earlier steps of the collision.
[1]Phys. Rev. Lett. 94 (2005)
[2]Phys. Lett. B519 (2001)
[3]Phys. Rev. Lett. 98 (2007)
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

3. Measurement via semi leptonic decays
indirect method
Electrons from semi-leptonic decays of B,D mesons.
D0  e++X BR:6.9 %
D+/- e+/-+X BR:17.2%
Measurement include electrons from B and D decays.
Use of specific triggers.
Large pT range.
At pT≥6GeV/c
,RAA(npe)~RAA(h+/-)[5].
Azimuthal correlation of electrons with open charm meson would disentangle between the charm and bottom contribution[6].
[4]:STAR
Preliminary
To undersrtnad this aspect , a measurement of B and D separately is needed.
[4] nucl-ex/ ; Y Zhang QM2008
[5] Phys. Rev. Lett 98, (2007)
[6] Phys. Lett. B671(2008) 361 and
Phys. Rev. Lett. 105 (2010)
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

4. Measurement via hadronic decays
Combinatorial method
Measurement of hadronic decay modes via invariant mass analysis.
D0 (D0)K-+(K+-) BR : 3.8 %
D+/-K BR : 9.2%
Obtained by pairing identified kaons and pions.
No triggers, no decay vertex reconstruction.
Typically limited to low momentum (pT<3GeV/c).
[7]dAu : Phys. Rev. Lett. 94 (2005) 62301
[8]AuAu : arXiv:
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

5. XLIV International Winter Meeting on Nuclear Physics, Bormio
STAR detector (in 2007)
SSD
TPC
SVT
The tracking system consisted of :
TPC : provides track momentum, particle identification.
2 silicon detectors :
1 layer of silicon strip detectors (SSD) and 3 layers of silicon drift detectors (SVT).
high spatial resolution : pointing resolution of 280µm in transverse direction was achieved with Cu+Cu data in run 5 (y2005)[9].
[9] Fisyak Y V et al J. Phys. Conf. Ser
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011 5 5

6. Distance of Closest Approach resolution
RHIC run 7 (MinBias trigger).
DCA resolution as a function of inverse momentum.
Reflect the (detector+alignment) resolution and Multiple Coulomb Scattering (MCS).
STAR preliminary
Including the silicon detectors in the tracking improves the pointing resolution.
with 4 silicon hits, the pointing resolution to the interaction point ~ 220µm at P = 1GeV/c.(order of magnitude of charmed particles decays. For example, c(D0) = 123µm)
 Note : the Silicon Vertex detectors were not designed (thickness, geometry) for charm measurement.
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

7. XLIV International Winter Meeting on Nuclear Physics, Bormio
Decay fitting
Full reconstruction/fit of the decay vertex [10].
Introduction/Use of full track error matrix for best error estimates.
Optimization of cuts based on MC studies. K- π+
3d path length from primary vertex
to decay particle vertex
[10] Decay Chain Fitting with a Kalman Filter,W. D. Hulsbergen (arxiv:physics, )
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011 7

8. Secondary vertex fit (simulation studies)
Correlation between the reconstructed decay and MC
Mean of the difference reconstructed -MC
Rms of the difference reconstructed -MC
Reconstructed decay[cm]
Reconstructed decay - GEANT decay[cm]
There is no systematic shift in reconstructed quantities.
The standard deviation of the distribution is flat at ~ 250 m , which is
of the order of the resolution of (SSD+SVT).
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011 8 8

9. Proof of principle with K0s
Signed decay length :
an excess can be observed on the positive side of the decay length distribution, indicating the presence of long-lived decays.
use the decay length significance SL = L/L to improve the signal.
more appropriate because of the momentum dependence of the decay length.
Test with K0s decay reconstruction :
K0S π+ π- (BR = 69.2%) ; c = 2.68 cm ; Mass = MeV/c2
background
Signal+background
Before cut
After cut
 After using a cut SL > 10, a clear peak at the K0S mass is observed.
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011 9 9

10. Invariant mass of D0 and D0bar combined
Uncorrected raw yield
Au+Au at s= 200GeV
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

11. Invariant mass of D0 and D0bar separately
Uncorrected raw yield
Au+Au at s=200GeV
Uncorrected raw yield
Au+Au at s=200GeV
Ratio of D0bar/D0 = /- .19(stat.)
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

12. Summary and perspectives
We presented a method (and preliminary results) using full track information plus a secondary vertex fit using Silicon Vertex information to obtain higher precision data.
Ongoing efforts :
To tune cuts to maximize S/N.
Evaluate background using different methods (track rotation, event mixing etc).
Estimate efficiency correction to obtain pT spectra.
Estimate v2 of D0s.
Silicon upgrades (under construction) in STAR will perform detailed exclusive charm and bottom studies.
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

13. Future measurements at STAR
future upgrade in STAR : Heavy Flavor Tracker[11]
Low mass detector designed to identify mid-rapidity Charm and Beauty mesons and baryons through direct reconstruction and measurement of the displaced vertex with unprecedented pointing resolution.
CMOS sensors will provide single track resolution ~ µm.
[11]: E. Anderssen et al., A Heavy Flavor Tracker for STAR,
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

14. Key measurements of the HFT
v2 and Rcp of D0
Charm baryon Λc
3) Bottom cross sections
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

15. XLIV International Winter Meeting on Nuclear Physics, Bormio
End
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

16. XLIV International Winter Meeting on Nuclear Physics, Bormio
Inner tracker system
Number of layer
(radius)
technology
Sensor size
(mm2)
Intrinsic resolution
(design)
Radiation length
SSD 1
(23 cm)
Double sided silicon strips
42 x 73
r/ ~ 20 µm
Z ~ 700 µm
~1% X0
SVT 3
(6.8 cm ; 10.8 cm ; 14.8 cm)
Silicon drift
60 x 60
Z ~ 20 µm
~1.5% X0 per layer
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

17. Performance example on the D0  Kπ reconstruction (HFT)
Simulation of Hijing events with STAR tracking software including pixel pileup (RHIC-II luminosity) extrapolated to 500 M events.
Identification done via topological cuts and PID using Time Of Flight.
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

18. Strategy of reconstruction/ Datasets
Apply cuts to reduce the combinatorial background and select good quality tracks and pairs.
EVENTS level :
Primary vertex position and its error (ensured by trigger detectors).
TRACKS level
Number of hits in the vertex detectors : Silicon Hits > 2 (tracks with sufficient DCA resolution).
Number of fitted TPC hits > 20 (avoid splitting tracks).
Particle identification : ndEdx :|nK|<2, |nπ|<2 (select kaon and pion candidates).
Pseudo-rapidity :||<1 (Silicon detector acceptance).
DCA to Primary vertex (transverse) DCAxy< .1 cm (remove tracks compatibles with strange particles decays).
PAIRS level
Momentum of pairs.
results given by the secondary vertex fit.
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011 18 18

19. Charm cross-section at RHIC
Charm total cross section follows roughly Nbin scaling from d+Au to minbias Au+Au to central Au+Au considering errors.
confirms charm quarks made via hard process at initial impact.
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

20. D0 candidates kinematic
<P> and <pT> of D0 are ~ 1GeV/c.
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011

21. XLIV International Winter Meeting on Nuclear Physics, Bormio
e-D0 correlation
unlike sign
Same sign
Same sign :
Bottom on near side
charm and a fraction on the away side
Unlike sign : bottom on the away side
XLIV International Winter Meeting on Nuclear Physics, Bormio
23-29 January 2011