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Recent Progress in Measuring Charm Through Hadronic Decays: D s in d+Au and D 0 in Cu+Cu

Recent Progress in Measuring Charm Through Hadronic Decays: D s in d+Au and D 0 in Cu+Cu. Stephen Baumgart. Charmed Motivation. Charm quarks are produced in relativistic collisions mainly through initial gluon fusion.

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Recent Progress in Measuring Charm Through Hadronic Decays: D s in d+Au and D 0 in Cu+Cu

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  1. Recent Progress in Measuring Charm Through Hadronic Decays: Ds in d+Au and D0 in Cu+Cu Stephen Baumgart

  2. Charmed Motivation • Charm quarks are produced in relativistic collisions mainly through initial gluon fusion. • Open charm is a good probe of the early stages of the collision fireball. • The Ds and D0 contribute to the total charm cross-section. • The production of charm-strange (Ds) mesons is expected to be enhanced in a Quark Gluon Plasma relative to a hadronic system because their production is more kinematically favored. Therefore, a d+Au study can provide a baseline measurement for a future Au+Au measurement of Ds mesons. A D0 measurement is a good way to find the total charm cross-section.

  3. Previous Charm Results in STAR (non-Leptonic Decays) • The D0 signal detected in d+Au and Au+Au collisions has already been published. • Preliminary D+/- and D* results for d+Au exist. However, the signals are weak. D0 Peak Reconstructed from Kp,STAR Collaboration, J. Adams et al., Phys. Rev. Lett. 94, 062301 (2005). Various Charmed Mesons in STAR J. Phys. G Nucl. Part. Phys. 31 (2005) S267-S272

  4. Channel Branching Ratio Q-Value (GeV) Comments The low efficiency of K0 identification ( <~ 10%) will make this channel difficult. The f decays to a K+K- with a branching ratio of 49.2 +0.6 -0.7% The large width of the K* makes a resonance cut on its mass not very effective in reducing background. Ds Creation According to Pythia As a 0th order approximation one can look at the ratios of the particles generated by Pythia. The decay channels of the Ds+ at which one could plausibly look are: The fp channel looks like it’s the best bet out of the set.

  5. The STAR Time Projection Chamber • In this analysis, only data from the STAR Time Projection Chamber (TPC) was used. The TPC measures particles through their ionization of the gas inside the TPC while the particles are under the influence of a magnetic field. • Minimum bias d+Au collisions at a collision energy of 200 GeV per nucleon were used in this analysis.

  6. Particle Identification for Ds Reconstruction Identification of Positively Charged Tracks in the TPC K+ in blue p+ in red Unused in black • Three Daughters •  Low Momentum Tracks •  Good Particle ID. • Electrons Contaminate Kaons • Approximate Positive/Negative Symmetry

  7. f Invariant Mass Peak from K+K- Photon Conversion Electrons (mis-ID’ed as K+K-) STAR Preliminary 10.8 Million d+Au MinBias Events f (K+K-) peak Mass cuts at 1 sigma greatly reduce background.

  8. Rotational Background p+ Momentum Vector px py 60 degrees Rotations destroy correlations to create a background. f Momentum Vector (arbitrary direction)

  9. f + p Meson Spectra Reconstructed fp+ Reconstructed fp- Signal + Background Rotational Background Signal + Background Rotational Background STAR Preliminary STAR Preliminary

  10. After Background Subtraction Rotational Background Subtracted STAR Preliminary Ds mass • Possible signal around the Ds mass of 1.968 GeV. • The residual background at m < mDs is not yet well understood.

  11. A Gaussian Fit? Rotational Background Subtracted STAR Preliminary This fitted Gaussian has a statistical significance of 3.1s *. The centroid of the Gaussian is at 1.960 +/- 0.007 GeV compared to the PDG value of 1.968 GeV  Agrees within errors!

  12. Negative Species STAR Preliminary Nothing? The background structure can still be seen but there does not seem to be any spike. It is unknown why combining the negative species would get a different result.

  13. Outlook for the Future for Ds • Au+Au, Cu+Cu, and p+p datasets must be carefully studied to see if there are any hints of the Ds. This analysis is in progress. • The residual background must be understood. This will be done through simulation. • More d+Au statistics can be used to confirm or reject the evidence for a Ds peak. • Efficiency and acceptance corrections must be obtained from simulation.

  14. D0 measurement in Cu+Cu • A measurement of D0 in Cu+Cu will allow a comparison to the Au+Au and d+Au results (shown). • Charm yields from hadronic decays can be compared to charm yields from semi-leptonic decays in Cu+Cu (Anders Knospe’s work) D0 Peak Reconstructed from Kp in d+Au,STAR Collaboration, J. Adams et al., Phys. Rev. Lett. 94, 062301 (2005).

  15. Particle Identification for D0 Reconstruction K+ in blue p+ in red Unused in black • Two Daughters & High-Q Value •  Must Take High-p Tracks • Major Contamination • Approximate Positive/Negative Symmetry

  16. D0 Background Subtraction STAR Preliminary Possible peak seen with centroid of 1.878 +/- 0.002 GeV; compare with D0 mass of 1.8645 GeV. Unknown population around 1.78 GeV.

  17. D0 Outlook • Discovery of a 6 sigma D0 signal in d+Au and a 4 sigma D0 signal in Au+Au means that a D0 signal should be seen in Cu+Cu. • Initial scan of data shows potential peak consistent with D0. • Measuring the yield and spectrum seems to be doable!

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