Measurement of upsilon production in 7 TeV $pp$ collisions at ATLAS

Using $1.8{\mathrm{fb}}^{-1}$ of $pp$ collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of $\mathit{{\rm Y}}(1\mathrm{S},2\mathrm{S},3\mathrm{S})$ mesons. $\mathit{{\rm Y}}$ mesons are reconstructed using the dimuon decay mode. Total production cross sections for $p_{\mathrm{T}}<70\mathrm{GeV}$ and in the rapidity interval $|y^{\mathit{{\rm Y}}}|<2.25$ are measured to be, $8.01\pm 0.02\pm 0.36\pm 0.31\mathrm{nb}$, $2.05\pm 0.01\pm 0.12\pm 0.08\mathrm{nb}$, and $0.92\pm 0.01\pm 0.07\pm 0.04\mathrm{nb}$, respectively, with uncertainties separated into statistical, systematic, and luminosity measurement effects. In addition, differential cross section times dimuon branching fractions for $\mathit{{\rm Y}}(1\mathrm{S})$, $\mathit{{\rm Y}}(2\mathrm{S})$, and $\mathit{{\rm Y}}(3\mathrm{S})$ as a function of $\mathit{{\rm Y}}$ transverse momentum $p_{\mathrm{T}}$ and rapidity are presented. These cross sections are obtained assuming unpolarized production. If the production polarization is fully transverse or longitudinal with no azimuthal dependence in the helicity frame, the cross section may vary by approximately $\pm 20\%$. If a nontrivial azimuthal dependence is considered, integrated cross sections may be significantly enhanced by a factor of 2 or more. We compare our results to several theoretical models of $\mathit{{\rm Y}}$ meson production, finding that none provide an accurate description of our data over the full range of $\mathit{{\rm Y}}$ transverse momenta accessible with this data set.

Figure 1

The dimuon invariant mass spectum for events used in this analysis. Separate spectra are shown for those events with the dimuon candidate (left) in the central region of the detector ($|y^{\mu \mu }|<1.2$) and (right) in the forward region ($1.2\le |y^{\mu \mu }|<2.25$). Overlaid are individual shapes of the fitted $\mathit{{\rm Y}}(n\mathrm{S})$ signals (shaded regions), background-only fit (dashed curve), and the total signal plus background shape (solid curve). The background shape is modeled here by a fourth-order polynomial and the signal peaks each modeled by a single Gaussian. Also quoted are the fitted mass resolutions of each of the three signal peaks, determined from the fit with a common resolution parameter scaling with invariant mass.

Figure 2

Top: acceptance as a function of $\mathit{{\rm Y}}$ $p_{\mathrm{T}}$ and rapidity for the $\mathit{{\rm Y}}(1\mathrm{S})$ in the unpolarized acceptance scenario. Bottom: the ratio of largest to smallest acceptance correction between extreme spin-alignment scenarios.

Figure 3

Efficiency and acceptance corrections contributing to the overall fiducial and corrected efficiencies (and the inverse of the efficiency, the weight) as a function of the $\mathit{{\rm Y}}(1\mathrm{S})$ $p_{\mathrm{T}}^{\mu \mu }$ in the central (top) and forward (middle) regions of the detector and as a function of $|y^{\mu \mu }|$ (bottom).

Figure 4

Fits to the efficiency-corrected $m_{\mu \mu }$ spectra for candidates in the low (0.5–1.0 GeV; top), mid (20–21 GeV; middle), and high (40–45 GeV; bottom) $p_{\mathrm{T}}^{\mu \mu }$ intervals, for central (left) and forward (right) dimuon rapidities. Fit results are shown for the simplest fit model considered: a single Gaussian signal plus either a second-order polynomial multiplied by an error function (low $p_{\mathrm{T}}$), second-order polynomial (mid $p_{\mathrm{T}}$), or first-order polynomial (high $p_{\mathrm{T}}$) background parametrization.

Figure 5

Sources of statistical and systematic uncertainty on the corrected $\mathit{{\rm Y}}(n\mathrm{S})$ production cross section measurements in the central rapidity region. A common luminosity uncertainty of $\pm 3.9\%$ is not included.

Figure 6

Sources of statistical and systematic uncertainty on the corrected $\mathit{{\rm Y}}(n\mathrm{S})$ production cross section measurements in the forward rapidity region. A common luminosity uncertainty of $\pm 3.9\%$ is not included.

Figure 7

Sources of statistical and systematic uncertainty on the corrected $\mathit{{\rm Y}}(n\mathrm{S})$ production cross section measurements as a function of $\mathit{{\rm Y}}$ rapidity. A common luminosity uncertainty of $\pm 3.9\%$ is not included.

Figure 8

Differential cross sections multiplied by the dimuon branching fraction, $d^2\sigma /d{p}_{\mathrm{T}}dy\times \mathrm{Br}(\mathit{{\rm Y}}\to {\mu }^{+}{\mu }^{-})$, for central (top) and forward (bottom) rapidities, for $\mathit{{\rm Y}}(1\mathrm{S})$, $\mathit{{\rm Y}}(2\mathrm{S})$, $\mathit{{\rm Y}}(3\mathrm{S})$ production within the fiducial acceptance. Points with error bars indicate results of the measurements with statistical uncertainties while shaded areas correspond to total uncertainties on the measurement, including systematic effects.

Figure 9

Differential cross sections multiplied by the dimuon branching fraction, $d\sigma /dy\times \mathrm{Br}(\mathit{{\rm Y}}\to {\mu }^{+}{\mu }^{-})$ ($p_{\mathrm{T}}$-integrated) $d\sigma /dy\times \mathrm{Br}(\mathit{{\rm Y}}\to {\mu }^{+}{\mu }^{-})$ for $\mathit{{\rm Y}}(1\mathrm{S})$, $\mathit{{\rm Y}}(2\mathrm{S})$, $\mathit{{\rm Y}}(3\mathrm{S})$ production within the fiducial acceptance. Points with error bars indicate results of the measurements with statistical uncertainties while shaded areas correspond to total uncertainties on the measurement, including systematic effects.

Figure 10

Differential cross sections multiplied by the dimuon branching fraction, for $\mathit{{\rm Y}}(1\mathrm{S})$, $\mathit{{\rm Y}}(2\mathrm{S})$, and $\mathit{{\rm Y}}(3\mathrm{S})$ production extrapolated to the full phase space for the (top) $|y^{\mathit{{\rm Y}}}|<1.2$, (bottom) $1.2\le |y^{\mathit{{\rm Y}}}|<2.25$ rapidity intervals. Points with error bars indicate the results of the measurements with total statistical and systematic errors. Results are shown assuming an isotropic spin-alignment scenario.

Figure 11

Differential cross sections multiplied by the dimuon branching fraction, for $\mathit{{\rm Y}}(1\mathrm{S})$, $\mathit{{\rm Y}}(2\mathrm{S})$, and $\mathit{{\rm Y}}(3\mathrm{S})$ production extrapolated to the full phase space, $p_{\mathrm{T}}$-integrated as a function of absolute $\mathit{{\rm Y}}$ rapidity. Points with error bars indicate the results of the measurements with total statistical and systematic errors. Results are shown assuming an isotropic spin-alignment scenario.

Figure 12

Differential cross sections multiplied by the dimuon branching fraction, $d^2\sigma /d{p}_{\mathrm{T}}dy\times \mathrm{Br}(\mathit{{\rm Y}}\to {\mu }^{+}{\mu }^{-})$, for $\mathit{{\rm Y}}(1\mathrm{S})$ production extrapolated to the full phase space for (top) central and (bottom) forward rapidities. Points with error bars indicate results of the measurements with total (statistical and systematic) uncertainties. The maximal envelope of variation of the result due to spin-alignment uncertainty is indicated by the solid band. Also shown are predictions of direct production with the NNLO* color-singlet mechanism (CSM) and inclusive predictions from the color evaporation model (CEM). These theory predictions are shown as a ratio to the data in the lower panes for CEM (middle) and CSM (bottom), along with detail of the variations of the cross section measurement under the four anisotropic spin-alignment scenarios as a ratio to the nominal data.

Figure 13

As for Fig. 12, but for $\mathit{{\rm Y}}(2\mathrm{S})$ production.

Figure 14

As for Fig. 12, but for $\mathit{{\rm Y}}(3\mathrm{S})$ production.

Figure 15

Differential cross sections multiplied by the dimuon branching fraction, $d\sigma /dy\times \mathrm{Br}(\mathit{{\rm Y}}\to {\mu }^{+}{\mu }^{-})$, for $\mathit{{\rm Y}}(1\mathrm{S})$ production extrapolated to the full phase space. Points with error bars indicate results of the measurements with total (statistical and systematic) uncertainties. The maximal envelope of variation of the result due to spin-alignment uncertainty is indicated by the solid band. The variations of the cross section measurement under the four anisotropic spin-alignment scenarios are shown in the lower pane as ratios to the unpolarized scenario.

Figure 16

As for Fig. 15, but for $\mathit{{\rm Y}}(2\mathrm{S})$ production.

Figure 17

As for Fig. 15, but for $\mathit{{\rm Y}}(3\mathrm{S})$ production.

Figure 18

Ratios of differential $\mathit{{\rm Y}}(2\mathrm{S})/\mathit{{\rm Y}}(1\mathrm{S})$ and $\mathit{{\rm Y}}(3\mathrm{S})/\mathit{{\rm Y}}(1\mathrm{S})$ cross sections multiplied by the dimuon branching fractions as a function of dimuon rapidity. Points with error bars indicate results of the measurements with statistical uncertainties while shaded areas correspond to total uncertainties on each point, including systematic effects, but excluding spin-alignment effects.

Figure 19

Ratios of differential $\mathit{{\rm Y}}(2\mathrm{S})/\mathit{{\rm Y}}(1\mathrm{S})$ and $\mathit{{\rm Y}}(3\mathrm{S})/\mathit{{\rm Y}}(1\mathrm{S})$ cross sections multiplied by the dimuon branching fractions versus $\mathit{{\rm Y}}$ $p_{\mathrm{T}}$ in the (top) central and (bottom) forward rapidity regions. Points with error bars indicate results of the measurements with statistical uncertainties while shaded areas correspond to total uncertainties on each point, including systematic effects, but excluding spin-alignment effects.