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| CMS-FSQ-13-009 ; CERN-EP-2018-225 | ||
| Measurement of exclusive $\Upsilon$ photoproduction from protons in pPb collisions at $\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 28 September 2018 | ||
| Eur. Phys. J. C 79 (2019) 277 [Erratum] | ||
| Abstract: The exclusive photoproduction of ${\Upsilon\mathrm{(nS)}} $ meson states from protons, ${\gamma{\mathrm{p}}} \to {\Upsilon\mathrm{(nS)}} \,{\mathrm{p}}$ (with $\mathrm{n}=$ 1, 2, 3), is studied in ultraperipheral pPb collisions at a centre-of-mass energy per nucleon pair of $\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV. The measurement is performed using the ${\Upsilon\mathrm{(nS)}} \to \mu^+\mu^-$ decay mode, with data collected by the CMS experiment corresponding to an integrated luminosity of 32.6 nb$^{-1}$. Differential cross sections as functions of the ${\Upsilon\mathrm{(nS)}} $ transverse momentum squared ${p_{\mathrm{T}}}^2$, and rapidity $y$, are presented. The $\Upsilon\text{(1S)}$ photoproduction cross section is extracted in the rapidity range $|y| < $ 2.2, which corresponds to photon-proton centre-of-mass energies in the range 91 $ < {W_{\gamma{\mathrm{p}}}} < $ 826 GeV. The data are compared to theoretical predictions based on perturbative quantum chromodynamics and to previous measurements. | ||
| Links: e-print arXiv:1809.11080 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Diagrams representing (a) exclusive $ {\Upsilon}$ photoproduction, (b) proton dissociative, or "semiexclusive'', $ {\Upsilon}$ photoproduction, and (c) exclusive dimuon QED continuum production in pPb collisions. |
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Figure 1-a:
Diagram representing exclusive $ {\Upsilon}$ photoproduction. |
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Figure 1-b:
Diagram representing proton dissociative, or "semiexclusive'', $ {\Upsilon}$ photoproduction. |
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Figure 1-c:
Diagram representing exclusive dimuon QED continuum production in pPb collisions. |
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Figure 2:
Invariant mass distribution of the exclusive muon pair candidates in the range 8 $ < m_{\mu ^{+}\mu ^{-}} < $ 12 GeV that pass all the selection criteria, fitted to a linear function for the two-photon QED continuum (blue dashed line) plus three Gaussian distributions corresponding to the $ {\Upsilon \mathrm {(1S)}}$, $ {\Upsilon \mathrm {(2S)}}$, and $ {\Upsilon \mathrm {(3S)}}$ mesons (dashed-dotted-red curves). |
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Figure 3:
Distributions of the (a) transverse momentum squared $ {p_{\mathrm {T}}} ^2$, and (b) rapidity $y$ of exclusive muon pairs with invariant mass 9.1 $ < m_{\mu ^{+}\mu ^{-}} < $ 10.6 GeV after all selection criteria have been applied. Both distributions are compared to the expectations of signal and background contributions discussed in the text. |
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Figure 3-a:
Distribution of the transverse momentum squared $ {p_{\mathrm {T}}} ^2$ of exclusive muon pairs with invariant mass 9.1 $ < m_{\mu ^{+}\mu ^{-}} < $ 10.6 GeV after all selection criteria have been applied. The distribution is compared to the expectation of signal and background contributions discussed in the text. |
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Figure 3-b:
Distribution of the rapidity $y$ of exclusive muon pairs with invariant mass 9.1 $ < m_{\mu ^{+}\mu ^{-}} < $ 10.6 GeV after all selection criteria have been applied. The distribution is compared to the expectation of signal and background contributions discussed in the text. |
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Figure 4:
Differential $ {{\Upsilon}\mathrm {(nS)}} \to \mu ^+\mu ^-$ photoproduction cross section as a function of (a) $ {p_{\mathrm {T}}} ^2$ and (b) rapidity $y$, measured in pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. In the left plot, the data points are placed along the abscissa following the prescription of [57], and the solid line is an exponential fit of the form $e^{-b {p_{\mathrm {T}}} ^2}$. In the right plot, the horizontal bars are shown to indicate the width of each $y$ bin. In both plots, the vertical bars represent the statistical uncertainties and the boxes represent the systematic uncertainties. |
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Figure 4-a:
Differential $ {{\Upsilon}\mathrm {(nS)}} \to \mu ^+\mu ^-$ photoproduction cross section as a function of $ {p_{\mathrm {T}}} ^2$, measured in pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The data points are placed along the abscissa following the prescription of [57], and the solid line is an exponential fit of the form $e^{-b {p_{\mathrm {T}}} ^2}$. The vertical bars represent the statistical uncertainties and the boxes represent the systematic uncertainties. |
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Figure 4-b:
Differential $ {{\Upsilon}\mathrm {(nS)}} \to \mu ^+\mu ^-$ photoproduction cross section as a function of rapidity $y$, measured in pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The horizontal bars are shown to indicate the width of each $y$ bin. The vertical bars represent the statistical uncertainties and the boxes represent the systematic uncertainties. |
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Figure 5:
Differential $ {\Upsilon \mathrm {(1S)}}$ photoproduction cross section as a function of rapidity measured in pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV in the dimuon rapidity region $ |y| < $ 2.2, compared to various theoretical predictions [9,16,17,14,15,18]. The horizontal bars are plotted to indicate the width of each $y$ bin. The vertical bars represent the statistical uncertainties and the boxes represent the systematic uncertainties. |
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Figure 6:
Cross section for exclusive $ {\Upsilon \mathrm {(1S)}}$ photoproduction, $ {{\gamma} {\mathrm {p}}} \to {\Upsilon \mathrm {(1S)}} {\mathrm {p}}$, as a function of photon-proton centre-of-mass energy, $ {W_{{\gamma} {\mathrm {p}}}} $, compared to previous HERA [19,21,20] and LHCb [30] data as well as to various theoretical predictions [9,16,17,14,15,18]. The vertical bars represent the statistical uncertainties and the boxes represent the systematic uncertainties. |
| Tables | |
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Table 1:
Relative systematic uncertainties in percent in the measurements of $\mathcal {B}_{{{\Upsilon}\mathrm {(nS)}} \to \mu ^+\mu ^-} {\mathrm {d}}\sigma / {\mathrm {d}} {p_{\mathrm {T}}} ^2$, the exponential $b$ slope of the $ {p_{\mathrm {T}}} ^2$ spectrum, $\mathcal {B}_{{{\Upsilon}\mathrm {(nS)}} \to \mu ^+\mu ^-} {\mathrm {d}}\sigma /{{\mathrm {d}}}y$, $ {\mathrm {d}}\sigma _{{\Upsilon \mathrm {(1S)}}}/{{\mathrm {d}}}y$, and $\sigma _{{{\gamma} {\mathrm {p}}} \to {\Upsilon \mathrm {(1S)}} {\mathrm {p}}}$. Individual contributions, as well as total systematic uncertainties added in quadrature are presented. For the $ {p_{\mathrm {T}}} ^2$- and $y$-differential cross sections, the values averaged over all bins are shown. |
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Table 2:
Differential exclusive $ {{\Upsilon}\mathrm {(nS)}} \to \mu ^+\mu ^-$ photoproduction cross sections in four $ {p_{\mathrm {T}}} ^2$ and $y$ bins. The first and second uncertainties correspond to statistical and systematic components, respectively. |
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Table 3:
Values of the $ {\Upsilon \mathrm {(1S)}}$ photoproduction cross section in four rapidity $y$ bins, corresponding to four photon-proton $ {W_{{\gamma} {\mathrm {p}}}} $ centre-of-mass energy ranges (with central $W_{0}$ value obtained following the procedure outlined in Ref. [57]), in pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The symbols $N_{{{\Upsilon}\mathrm {(nS)}}}^{\text {back-sub}}$, $N_{{{\Upsilon}\mathrm {(nS)}}}^{\text {unfol}}$, and $N_{{{\Upsilon}\mathrm {(nS)}}}^{\text {corr}} $ represent the numbers of $ {{\Upsilon}\mathrm {(nS)}} $ candidates after background subtraction, unfolding, and extrapolation with the $A^{\text {corr}}$ factor, respectively; $N_{{\Upsilon \mathrm {(1S)}}}$ is the extracted number of $ {\Upsilon \mathrm {(1S)}}$ mesons, and $\mathrm{h}i $ is the theoretical effective photon flux (see text). The first (second, if given) uncertainty quoted corresponds to the statistical (systematic) component. |
| Summary |
| The first study of the exclusive photoproduction of $\Upsilon \mathrm{(1S,2S,3S)}$ mesons, in the $\mu^{+}\mu^{-}$ decay mode, from protons in ultraperipheral pPb collisions at $\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV, has been reported using data collected with the CMS detector corresponding to an integrated luminosity of 32.6 nb$^{-1}$. The differential cross section ${\mathrm{d}}\sigma/{\mathrm{d}}{p_{\mathrm{T}}}^2$ and associated exponential slope $b$ have been measured in the squared transverse momentum range ${p_{\mathrm{T}}}^2 < 1.0 GeV^2$. The extracted value of $b=$ 6.0 $\pm$ 2.1 (stat) $\pm$ 0.3 (syst) GeV$^{-2}$ is consistent with the slope measurement at other centre-of-mass energies. The exclusive $\Upsilon\text{1S}$ photoproduction cross sections, differential in rapidity $y$ and as a function of the photon-proton centre-of-mass energy ${W_{\gamma{\mathrm{p}}}} $, have been measured in the range 91 $ < {W_{\gamma{\mathrm{p}}}} < $ 826 GeV. Such measurements probe the region of parton fractional momenta $x\approx 10^{-4}$-$10^{-2}$ in the proton, bridging a previously unexplored region between the HERA and LHCb measurements. The dependence of $\sigma_{{\gamma{\mathrm{p}}} \to\Upsilon\text{1S}{\mathrm{p}}}$ on ${W_{\gamma{\mathrm{p}}}} $ is well described by a power law with an exponent smaller than that predicted by leading order perturbative quantum chromodynamics (pQCD) approaches. The exponent is, however, consistent with that extracted from a fit to the HERA and LHCb data, and with that predicted by next-to-leading-order pQCD calculations. The data, within their currently large statistical uncertainties, are consistent with various pQCD approaches that model in different ways the behaviour of the low-$x$ gluon density. |
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