${\rm Y}$ cross section in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$

We report on a measurement of the ${\rm Y}(1\mathrm{S}+2\mathrm{S}+3\mathrm{S})\to e^{+}{e}^{-}$ cross section at midrapidity in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$. We find the cross section to be $114\pm 38(\mathrm{stat}+\mathrm{\text{fit}}{)}_{-24}^{+23}(\mathrm{syst})\mathrm{pb}$. 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 ${\rm Y}$ data to RHIC energies. The dielectron continuum in the invariant-mass range near the ${\rm Y}$ is also studied to obtain a combined yield of $e^{+}{e}^{-}$ pairs from the sum of the Drell-Yan process and $b\mathrm{\text{−}}\overline{b}$ production.

Figure 1

The $E_{\perp }$ distribution for simulated ${\rm Y}$ 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 $E_{\perp }\approx 3.5\mathrm{GeV}$.

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 ${\rm Y}$ 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 ($\propto E_{\perp }$) 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 ${\rm Y}(1\mathrm{S})$, showing the corresponding distribution for electron daughters satisfying acceptance (solid line) and trigger (dotted line) requirements. The simulation histograms are normalized assuming $\mathcal{B}\times d\sigma /dy=100\mathrm{pb}$, 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 $E_1^{\mathrm{L}2}$ for all towers above the HT threshold in a trigger patch above the TP threshold. The L2 trigger requires $E_1^{\mathrm{L}2}>4.0\mathrm{GeV}$ (vertical line). The line histograms show the ${\rm Y}$ (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. 3.

Figure 5

The distribution of cosine of L2 opening angle ${\theta }_{12}^{\mathrm{L}2}$ for accepted events. The line histograms show the ${\rm Y}$ 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. 3.

Figure 6

The L2 invariant-mass $M_{12}^{\mathrm{L}2}$ distribution for accepted events. The line histograms show the ${\rm Y}$ $M_{12}^{\mathrm{L}2}$ distribution after acceptance requirements (solid line), and after all trigger requirements (L0 HTTP trigger, L2 cluster, L2 opening angle, and L2 invariant mass) of the $e^{+}{e}^{-}$ daughters (dashed line). Vertical lines show location of the trigger thresholds. Normalization and scaling factors are the same as in Fig. 3.

Figure 7

Combined $\mathrm{\text{acceptance}}\times \mathrm{\text{efficiency}}$ as a function of rapidity $y_{{\rm Y}}$ for the decay ${\rm Y}\to e^{+}{e}^{-}$ in STAR. Down triangles: BEMC acceptance only; open squares: adding L0 requirement; upright triangles: adding L2 requirement; open circles: adding $\mathrm{TPC}\mathrm{\text{acceptance}}+\mathrm{TPC}\mathrm{\text{tracking}}$ cuts and TPC-BEMC matching.

Figure 8

The radial distance $R=\sqrt{\Delta {\eta }^2+\Delta {\phi }^2}$ between the TPC track and the EMC electron cluster for simulated ${\rm Y}$ daughters (line histogram, arbitrary scale), and the $R$ distribution from data (circles). We reject candidate tracks with $R>0.04$ (vertical line).

Figure 9

$E/p$ distribution from ${\rm Y}$-triggered data events, selecting a high-purity sample of positrons via $dE/dx$ (filled circles). We compare this to simulations from reconstructed $e^{+}$ and $e^{-}$ from ${\rm Y}$ decays (solid line) and from ${\pi }^0$ events where the decay photons produce a conversion $e^{+}{e}^{-}$ pair (dashed line). The scaled sum of these two contributions is shown as the dot-dashed line.

Figure 10

Unlike-sign raw yield $N_{+-}$ (filled circles) in the region $|y_{ee}|<0.5$, where $y_{ee}$ is the pairwise rapidity, and like-sign combinatorial background $2\sqrt{N_{++}{N}_{--}}$ (line histogram). The background is fit to an exponential multiplied with an error function representing the turn-on of the STAR ${\rm Y}$ trigger (dashed line).

Figure 11

The $e^{+}{e}^{-}$ signal after subtracting the like-sign combinatorial background. The solid line is the function used to fit the data, composed of (a) three Crystal Ball functions [Eq. (2)] that represent the combined ${\rm Y}(1\mathrm{S}+2\mathrm{S}+3\mathrm{S})$ line shape, (b) the Drell-Yan, and (c) the $b\mathrm{\text{−}}\overline{b}$ contributions. We fit the data using the integral of the fit function in each mass bin, shown by the dashed-line histogram. The sum of the two continuum contributions is shown by the dot-dashed line. The integral of the Crystal Ball functions provides the net ${\rm Y}$ yield.

Figure 12

The $1\mathrm{\text{−}}\sigma$ (horizontal lines) and $2\mathrm{\text{−}}\sigma$ (diagonal lines) contours obtained by fitting the data, showing the favored regions for the Drell-Yan and $b\mathrm{\text{−}}\overline{b}$ cross sections. The dashed line shows the prediction for the Drell-Yan cross section from a pQCD NLO calculation [32].

Figure 13

The STAR measurement of the midrapidity ${\rm Y}(1\mathrm{S}+2\mathrm{S}+3\mathrm{S})$ cross section times branching ratio into electrons (star). Error bars are statistical, the box shows the systematic uncertainty, and the scale is given by the left axis. The raw yield vs $y$ is shown by the histogram at the bottom (diagonal-line fill pattern), with the scale on the right axis. The cross section was calculated from the yield between the vertical dot-dashed lines, $|y_{ee}|<0.5$. The open squares are from an NLO CEM calculation, and the two dotted lines give the limits for the prediction from a NLO CSM calculation of the ${\rm Y}$ cross section (see text).

Figure 14

Evolution of the ${\rm Y}(1\mathrm{S}+2\mathrm{S}+3\mathrm{S})$ cross section with center-of-mass energy for the world data and an NLO CEM calculation. The error bars on the STAR datum point are statistical and systematic as in Fig. 13.