CMS-FSQ-16-007 ; CERN-EP-2018-285 | ||
Measurement of exclusive $\rho(770)^{0}$ photoproduction in ultraperipheral pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | ||
CMS Collaboration | ||
5 February 2019 | ||
Eur. Phys. J. C 79 (2019) 702 | ||
Abstract: Exclusive $\rho(770)^{0}$ photoproduction is measured for the first time in ultraperipheral pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV with the CMS detector. The cross section $\sigma (\gamma {\mathrm{p}}\to \rho(770)^{0}{\mathrm{p}})$ is 11.2 $\pm$ 1.4 (stat) $\pm$ 1.0 (syst) $\mu$b at $\langle W_{\gamma{\mathrm{p}}}\rangle = $ 92.6 GeV for photon-proton centre-of-mass energies $W_{\gamma{\mathrm{p}}}$ between 29 and 213 GeV. The differential cross section ${\mathrm{d}}\sigma/{\mathrm{d}}|t|$ is measured in the interval 0.025 $ < |t| < $ 1 GeV$^{2}$ as a function of $W_{\gamma{\mathrm{p}}}$, where $t$ is the squared four-momentum transfer at the proton vertex. The results are compared with previous measurements and theoretical predictions. The measured cross section $\sigma (\gamma {\mathrm{p}}\to \rho(770)^{0}{\mathrm{p}})$ has a power-law dependence on the photon-proton centre-of-mass, consistent with electron-proton collision measurements performed at HERA. The $W_{\gamma{\mathrm{p}}}$ dependence of the exponential slope of the differential cross section ${\mathrm{d}}\sigma/{\mathrm{d}}|t|$ is also measured and is consistent with the previous measurements. | ||
Links: e-print arXiv:1902.01339 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
Comparison between the $ {p_{\mathrm {T}}} ^{{\pi ^+} {\pi ^-}}$ distributions of the reconstructed $ {\rho \mathrm {(1700)}}$ mesons in the data (full symbols) and the STARLIGHT simulation (histogram) when only two oppositely charged pions are selected. The triangles correspond to same-sign two-track events (either $ {\pi ^+} {\pi ^+}$ or $ {\pi ^-} {\pi ^-}$) in the data; they mostly come from $ {\rho \mathrm {(1700)}}$ decays with two undetected pions. The integrals of all three distributions are normalized to unity. Vertical bars correspond to the statistical uncertainties. The region to the left of the dashed vertical line is not included in the analysis (see Section 5 for details). |
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Figure 2:
The measured distribution of the reconstructed $ {\pi ^+} {\pi ^-}$ transverse momentum (full circles) together with the fitted sum of signal and backgrounds described in the text (red solid histogram). The STARLIGHT direct $ {\pi ^+} {\pi ^-}$ contribution (pink dotted histogram), the $ {\rho \mathrm {(1700)}}$ background (blue dotted-short-dashed histogram), and the proton-dissociative contribution (green dotted-long-dashed histogram) are also shown. The shaded areas represent the systematic uncertainties. The region to the left of the dashed vertical line is not included in the analysis (see Section 5 for details). |
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Figure 3:
Unfolded $ {\pi ^+} {\pi ^-}$ invariant mass distribution in the pion pair rapidity interval $ {| y_{{\pi ^+} {\pi ^-}} |} < 2.0$ (full circles) fitted with the S{o}ding model, the results of the fit are also given (see text for details). The green dashed line indicates resonant $ {\rho \mathrm {(770)}}^{0}$ production, the red dotted line the interference term, the magenta dash-dotted line corresponds to the background from $ {\omega \mathrm {(783)}}\to {\pi ^0} {\pi ^+} {\pi ^-}$, the black dash-dotted line the non-resonant contribution, and the blue solid line represents the sum of all these contributions. |
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Figure 4:
Unfolded $ {\pi ^+} {\pi ^-}$ invariant mass distributions in the pion pair rapidity interval $ {| y_{{\pi ^+} {\pi ^-}} |} < 2.0$ (full circles) fitted with the S{o}ding model in different $ {| t |}$ bins. The green dashed lines indicate resonant $ {\rho \mathrm {(770)}}^{0}$ production, the red dotted lines the interference term, the magenta dash-dotted lines correspond to the background from $ {\omega \mathrm {(783)}}\to {\pi ^0} {\pi ^+} {\pi ^-}$, the black dash-dotted lines to the nonresonant contribution, and the blue solid lines represent the sum of all these contributions. |
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Figure 5:
Differential cross section $ {\mathrm {d}}\sigma / {\mathrm {d}} {| t |}$ for exclusive $ {\rho \mathrm {(770)}}^{0}$ photoproduction in four different rapidity bins together with the STARLIGHT prediction. The error bars show the statistical uncertainty, whereas the shaded areas represent the statistical and systematic uncertainties added in quadrature. The dashed lines show the unweighted STARLIGHT predictions. |
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Figure 5-a:
Differential cross section $ {\mathrm {d}}\sigma / {\mathrm {d}} {| t |}$ for exclusive $ {\rho \mathrm {(770)}}^{0}$ photoproduction in the $-2.0 < y_{{\pi ^+} {\pi ^-}} < -1.2$ rapidity bin, together with the STARLIGHT prediction. The error bars show the statistical uncertainty, whereas the shaded areas represent the statistical and systematic uncertainties added in quadrature. The dashed lines show the unweighted STARLIGHT predictions. |
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Figure 5-b:
Differential cross section $ {\mathrm {d}}\sigma / {\mathrm {d}} {| t |}$ for exclusive $ {\rho \mathrm {(770)}}^{0}$ photoproduction in the $-1.2 < y_{{\pi ^+} {\pi ^-}} < 0$ rapidity bin, together with the STARLIGHT prediction. The error bars show the statistical uncertainty, whereas the shaded areas represent the statistical and systematic uncertainties added in quadrature. The dashed lines show the unweighted STARLIGHT predictions. |
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Figure 5-c:
Differential cross section $ {\mathrm {d}}\sigma / {\mathrm {d}} {| t |}$ for exclusive $ {\rho \mathrm {(770)}}^{0}$ photoproduction in the $0 < y_{{\pi ^+} {\pi ^-}} < 1.2$ rapidity bin, together with the STARLIGHT prediction. The error bars show the statistical uncertainty, whereas the shaded areas represent the statistical and systematic uncertainties added in quadrature. The dashed lines show the unweighted STARLIGHT predictions. |
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Figure 5-d:
Differential cross section $ {\mathrm {d}}\sigma / {\mathrm {d}} {| t |}$ for exclusive $ {\rho \mathrm {(770)}}^{0}$ photoproduction in the $1.2 < y_{{\pi ^+} {\pi ^-}} < 2.0$ rapidity bin, together with the STARLIGHT prediction. The error bars show the statistical uncertainty, whereas the shaded areas represent the statistical and systematic uncertainties added in quadrature. The dashed lines show the unweighted STARLIGHT predictions. |
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Figure 6:
Differential cross section $ {\mathrm {d}}\sigma / {\mathrm {d}} {| t |}$ (full circles) for exclusive $ {\rho \mathrm {(770)}}^{0}$ photoproduction in the rapidity interval $-1.2 < y_{{\pi ^+} {\pi ^-}} < 0$. The square symbols indicate the H1 results, and the triangles the ZEUS results. The error bars show the statistical uncertainty, while the shaded areas represent the statistical and systematic uncertainties added in quadrature. For the H1 data [15], the error bars represent the statistical and systematic uncertainties added in quadrature, and for the ZEUS data [14] the reported uncertainties are negligible. |
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Figure 7:
The slope parameter $b$ extracted from the exponential fits of the differential cross sections $ {\mathrm {d}}\sigma / {\mathrm {d}} {| t |}$ shown as a function of $W_{{\gamma} {\mathrm {p}}}$. The inner error bars show the statistical uncertainty, while the outer error bars indicate the statistical and systematic uncertainties added in quadrature. The dashed line shows the result of the Regge fit discussed in the text. |
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Figure 8:
Exclusive $ {\rho \mathrm {(770)}}^{0}$ photoproduction cross section as a function of $W_{{\gamma} {\mathrm {p}}}$. The inner bars show the statistical uncertainty, while the outer bars represent the statistical and systematic uncertainties added in quadrature. Fixed-target [41,42,43,44] and HERA [15,14] data are also shown. The dashed lines indicate the results of the fits described in the text. |
Tables | |
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Table 1:
Integrated luminosity and number of events after each of the selection requirements for the two data samples. The leading tower is the tower with the largest energy deposition in the calorimeter. |
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Table 2:
Summary of the systematic uncertainties in the $ {\rho \mathrm {(770)}}^{0}$ photoproduction cross section. The numbers are given in percent. The total uncertainty is calculated by adding the individual uncertainties in quadrature. |
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Table 3:
Differential cross section for exclusive $ {\rho \mathrm {(770)}}^{0}$ photoproduction, $\sigma ({\gamma} {\mathrm {p}}\to {\rho \mathrm {(770)}}^{0} {\mathrm {p}})$, with statistical and systematic uncertainties, for $ {| t |} < $ 0.5 GeV $^{2}$. The differential cross section $ {\mathrm {d}}\sigma / {\mathrm {d}} {| t |}$ is also shown, along with the rapidity range, the average value of $W_{{\gamma} {\mathrm {p}}}$, $< W_{{\gamma} {\mathrm {p}}}> $, and $ k \frac {{\mathrm {d}}n}{{\mathrm {d}}k}$. |
Summary |
The CMS Collaboration has made the first measurement of exclusive $\rho(770)^{0}$ photoproduction off protons in ultraperipheral pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The cross section for this process is measured in the photon-proton centre-of-mass energy interval 29 $ < W_{\gamma{\mathrm{p}}} < $ 213 GeV. The results are consistent with those of the H1 and ZEUS Collaborations at HERA, indicating that ion-proton collisions can be used in the same way as electron-proton ones, with ions acting as a source of quasi-real photons. The combination of the present data and the earlier, lower energy results agrees with theory-inspired fits. The differential cross section ${\mathrm{d}}\sigma/{\mathrm{d}}|t|$ for $\rho(770)^{0}$ photoproduction is measured as a function of $W_{\gamma{\mathrm{p}}}$. The STARLIGHT prediction is systematically higher than the data in the high-$|t|$ region. |
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Compact Muon Solenoid LHC, CERN |