Upsilon cross section in p+p collisions at sqrt(s) = 200 GeV

TL;DR

This study reports the first STAR measurement of the Upsilon(1S+2S+3S) to e+e− cross section at midrapidity in p+p collisions at 200 GeV, establishing a crucial baseline for quarkonium production at RHIC energies. Utilizing a two-stage BEMC-based trigger (L0/L2) and a detailed offline analysis, the authors extract the Υ(1S+2S+3S) yield while disentangling Drell-Yan and bb̄ continuum contributions via Crystal-Ball fits and continuum parameterizations. The results favor NLO pQCD calculations in the Color Evaporation Model and show that the Color Singlet Model underpredicts the cross section by about 2σ, aligning with the energy trend seen in world data. The work also quantifies the Drell-Yan plus bb̄ continuum cross section and outlines how increased luminosity will enable separation of the Υ states and more precise tests of QCD in both p+p and heavy-ion collisions.

Abstract

We report on a measurement of the Upsilon(1S+2S+3S) -> e+e- cross section at midrapidity in p+p collisions at sqrt(s)=200 GeV. We find the cross section to be 114 +/- 38 (stat.) +23,-24 (syst.) pb. Perturbative QCD calculations at next-to-leading order in the Color Evaporation Model are in agreement with our measurement, while calculations in the Color Singlet Model underestimate it by 2 sigma. Our result is consistent with the trend seen in world data as a function of the center-of-mass energy of the collision and extends the availability of Upsilon data to RHIC energies. The dielectron continuum in the invariant mass range near the Upsilon is also studied to obtain a combined cross section of Drell-Yan plus (b b-bar) -> e+e-.

Figures (14)

• Figure 1: The $E_\perp$ distribution for simulated $\Upsilon$ daughter electron (filled circle) or positron (open square) with the highest $E_\perp$. We show only daughters that fall in the STAR BEMC geometrical acceptance. The histogram is the sum of the two distributions. The L0 HT Trigger II threshold of 16 counts corresponds to $\hbox{$E_\perp$}\approx 3.5$$\mathrm{GeV}$.
• Figure 2: The L0 DSM-ADC distribution for the electron (filled circle) or positron (open square) with the highest ADC. The histogram is the sum. The simulated $\Upsilon$ is triggered when one of its daughter electrons is above the dashed line, which indicates the L0 HT Trigger II threshold of 16 counts.
• Figure 3: The L0 DSM-ADC distribution ($\propto \hbox{$E_\perp$}$) for the highest EMC tower of a candidate pair. We show Trigger I data after applying Trigger II thresholds (red circles) and Trigger II data (black squares). The yields are normalized by the integrated luminosity. The histograms are from simulation of $\Upsilon(\textrm{1S})$, showing the corresponding distribution for electron daughters satisfying acceptance (solid line) and trigger (dotted line) requirements. The simulation histograms are normalized assuming $\mathcal{B}\times d\sigma/dy$= 100 pb, times a factor of 500 for clarity. The vertical line is the Trigger II threshold of 16 counts. The histograms have the bin centers set at integer values to match the integer nature of ADC counts.
• Figure 4: The distribution of L2 Cluster-1 energy $E_1^{L2}$ for all towers above the HT threshold in a trigger patch above the TP threshold. The L2 trigger requires $E_1^{L2}> 4.0$$\mathrm{GeV}$ (vertical line). The line histograms show the $\Upsilon$ (from simulation) after acceptance requirements (solid line), and after all trigger requirements (dashed line). The normalization and scaling factors are the same as in Fig. \ref{['fig:HighTowerAdc']}.
• Figure 5: The distribution of cosine of L2 opening angle $\theta_{12}^{L2}$ for accepted events. The line histograms show the $\Upsilon$ distribution after acceptance requirements (solid line), and after all trigger requirements (dashed line). The vertical line shows the location of the trigger threshold. Normalization and scaling factors are the same as in Fig. \ref{['fig:HighTowerAdc']}.