[Paper Review] Measurement of the Open Charm Cross-Section in $\sqrt{s_{NN}}$ = 200 GeV Cu+Cu Collisions at STAR

Stephen Baumgart|arXiv (Cornell University)|Sep 26, 2007

High-Energy Particle Collisions Research4 citations

TL;DR

This study measures the mid-rapidity open charm cross-section in $√{s_{NN}}$ = 200 GeV Cu+Cu collisions at RHIC using direct reconstruction of $D^0$ and $\overline{D^0}$ via the $K\pi$ decay channel. The measured charm cross-section per binary collision is $1.30 \pm 0.25$ mb, confirming binary scaling and indicating production via initial hard scattering processes, with implications for FONLL model under-prediction.

ABSTRACT

We present the mid-rapidity $(D^{0} + \bar{D^{0}})/2$ yield measured in $\sqrt{s_{NN}}$ = 200 GeV Cu+Cu collisions at RHIC via direct reconstruction through the $Kπ$ decay channel. A charm cross-section is reported and compared to theoretical predictions and to previous RHIC experimental results.

Motivation & Objective

• To measure the open charm cross-section in $√{s_{NN}}$ = 200 GeV Cu+Cu collisions at RHIC using direct reconstruction of $D^0$ and $\overline{D^0}$ via the $K\pi$ decay channel.
• To test whether charm production scales with the number of binary nucleon-nucleon collisions, as expected for initial hard scattering processes.
• To compare the measured charm cross-section with theoretical FONLL predictions and previous RHIC measurements from STAR and PHENIX.
• To investigate discrepancies between experimental data and FONLL calculations, which may indicate missing physics in current models.

Proposed method

• Reconstruct $D^0$ and $\overline{D^0}$ via the $K\pi$ decay channel using invariant mass reconstruction with the formula $m_{D^0} = \sqrt{m_\pi^2 + m_K^2 + 2(E_\pi E_K - |p_\pi||p_K|\cos\theta)}$.
• Apply rotational background subtraction by rotating one kaon-pion pair in momentum space every 5° from 150° to 210° to estimate and subtract uncorrelated background.
• Use a Gaussian function plus a first-order polynomial to fit the invariant mass spectrum, achieving a 4.6σ significance for the $D^0+\overline{D^0}$ peak.
• Correct for detector efficiency and acceptance using detailed GEANT simulations of the STAR TPC and apply a branching ratio correction of $3.80 \pm 0.07\%$.
• Fit the transverse momentum spectrum with an $m_t - m_0$ exponential function to extract the $dN_{D^0}/dy$ yield.
• Convert the $dN_{D^0}/dy$ yield to a cross-section per binary collision using the formula $\sigma_{c\overline{c}}^{NN} = (dN_{D^0}/dy) \times (\sigma_{pp}^{inelastic}/N_{bin}^{CuCu}) \times (f/R)$, with $\sigma_{pp}^{inelastic} = 42$ mb, $N_{bin}^{CuCu} = 51.5^{+1.0}_{-2.9}$, $f = 4.7 \pm 0.7$, and $R = 0.54 \pm 0.05$.

Experimental results

Research questions

• RQ1 Does charm production in Cu+Cu collisions at $\sqrt{s_{NN}}$ = 200 GeV scale with the number of binary nucleon-nucleon collisions, as expected for initial hard scattering processes?
• RQ2 How does the measured open charm cross-section in Cu+Cu collisions compare to theoretical FONLL predictions at this energy?
• RQ3 Are the discrepancies between previous STAR and PHENIX charm cross-section measurements statistically significant or consistent with systematic uncertainties?
• RQ4 Does the observed charm cross-section in Cu+Cu collisions support the hypothesis of charm production via gluon fusion in early-stage collisions?

Key findings

• The $D^0 + \overline{D^0}$ invariant mass peak was observed with a statistical significance of 4.6σ after background subtraction using the rotational method.
• The mid-rapidity $dN_{D^0}/dy$ yield was measured to be $0.184 \pm 0.035$ (statistical uncertainty) from the $m_t - m_0$ exponential fit to the transverse momentum spectrum.
• The preliminary charm cross-section per binary collision in Cu+Cu collisions is $1.30 \pm 0.25$ mb, consistent with binary scaling observed in previous STAR data from p+p to central Au+Au.
• The measured cross-section exceeds the FONLL prediction of $0.256^{+0.400}_{-0.146}$ mb, indicating a potential under-prediction by current theoretical models.
• The result supports the interpretation that charm quarks are produced in initial hard scattering processes, consistent with perturbative QCD expectations.