CMS-PAS-FSQ-13-009

CMS-PAS-FSQ-13-009
Measurement of exclusive $\Upsilon$ photoproduction in pPb collisions at $\sqrt{s_{_\mathrm{NN}}} =$ 5.02 TeV
CMS Collaboration
April 2016

Abstract: A measurement of exclusive photoproduction of $\Upsilon ( \mathrm{1S} )$, $\Upsilon ( \mathrm{2S} ) $ and $\Upsilon ( \mathrm{3S} ) $ mesons, in their $\mu^+\mu^-$ decay modes, in ultraperipheral pPb collisions at a center-of-mass energy per nucleon pair of 5.02 TeV is presented. The measurement is based on data collected by the CMS experiment corresponding to an integrated luminosity of 32.6 nb$^{-1}$. The $ \Upsilon ( \mathrm{1S} )$ photoproduction cross section is measured, over the rapidity range $\|y\|<$ 2.2, as a function of the photon-proton center-of-mass energy, $W_{\gamma\mathrm{p}}$, in the range 91 $ < W_{\gamma\mathrm{p}} < $ 826 GeV. The differential cross section $\mathrm{d}\sigma/\mathrm{d}\|t\|$, where $\|t\| \approx p_\mathrm{T}^2$ is the squared transverse momentum of the $ \Upsilon ( n \mathrm{S} )$ mesons is measured in the range 0.01 $ <\|t\|< $ 1 GeV/$c^2$. The results are compared to other measurements and to theoretical predictions.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, EPJC 79 (2019) 277. The superseded preliminary plots can be found here.

Figures
png pdf	Figure 1-a: Diagram representing exclusive $\Upsilon $ photoproduction in pPb collisions.
png pdf	Figure 1-b: Diagrams representing exclusive dimuon QED continuum in pPb collisions.
png pdf	Figure 2: Invariant mass distribution for the $\mu ^{+}\mu ^{-}$ pairs in the range 8 $< M_{\mu ^{+}\mu ^{-}}<$ 12 GeV. The three peaks correspond to the $ \Upsilon ( 1 \mathrm{S} ) $, $ \Upsilon ( 2 \mathrm{S} ) $ and $ \Upsilon ( 3 \mathrm{S} ) $ mesons. The fit to the data is performed with RooFit. The blue line corresponds to the polynomial fit to the two-photon QED continuum, the red dashed line to the Gaussian fit of the resonances. The corresponding number of events are indicated in the legend.
png pdf	Figure 3-a: Distributions of the transverse momentum squared $p^2_{\rm {T}}$ and rapidity $y$ of the muon pairs with invariant mass 9.12 $ < m_{\mu ^{+}\mu ^{-}} < $ 10.64 GeV in the combined pPb and Pbp samples.
png pdf	Figure 3-b: Distributions of the transverse momentum squared $p^2_{\rm {T}}$ and rapidity $y$ of the muon pairs with invariant mass 9.12 $ < m_{\mu ^{+}\mu ^{-}} < $ 10.64 GeV in the combined pPb and Pbp samples.
png pdf	Figure 4: Differential $\Upsilon $ photoproduction cross section as a function of $\|t\|$ measured in pPb collisions at $\sqrt s_{\rm NN} = $ 5.02 TeV in the dimuon rapidity region $\|y\|<$ 2.2. The solid line represents the result of a fit with an exponential function $Ne^{-b\|t\|}$.
png pdf	Figure 5: Cross section for exclusive $ \Upsilon ( 1 \mathrm{S} ) $ photoproduction, $\gamma \mathrm{p} \rightarrow \Upsilon (1\mathrm{S}) \mathrm{p} $ as a function of photon-proton center-of-mass energy.

Tables
png pdf	Table 1: Fraction of $ \Upsilon ( 1 \mathrm{S} ) $ to all $ \Upsilon ( n \mathrm{S} ) $, $f_{\Upsilon ({\rm 1S})}$, estimated in this analysis compared with previous measurements.
png pdf	Table 2: Systematic uncertainties on the measurements of the $b$ of the exponential $\|t\|$ dependence and the ${\rm d}\sigma /{\rm d}y$ cross section; individual contributions, as well as the total systematic uncertainty are shown.
png pdf	Table 3: Relevant parameters for the extraction of the $ \Upsilon ( 1 \mathrm{S} ) $ photoproduction cross section in four rapidity bins (corresponding to four different ranges in $\gamma $-p center-of-mass energies) in pPb collisions at $\sqrt s_{NN}=$ 5.02 TeV. The symbols, $N^{\Upsilon {\rm (nS)}}_{{\rm back-sub}}$, $N^{\Upsilon {\rm (nS)}}_{{\rm unfol}}$ and $N^{\Upsilon {\rm (nS)}}_{{\rm corr}} $ give the number of $ \Upsilon ( n \mathrm{S} ) $ after the background subtraction, unfolding and extrapolation with $A_{{\rm corr}}$ factor, respectively. The factors, $f_{\Upsilon {\rm (1S)}}$ and $f_{{\rm FD}}$, are discussed in Section 6. The first (second, if given) uncertainty quoted corresponds to the statistical (systematical) uncertainty. $\Phi $ is the effective photon flux used to evaluate $\sigma _{\gamma \rm {p}}$ cross section from the $\mathrm{d}\sigma _{\Upsilon {\rm {(1S)}}}/\mathrm{d}y$ at each average ($W_0$) photon-proton center-of-mass energy, given with theoretical uncertainty (discussed in text).

Summary

We reported the first measurement of the exclusive photoproduction of $\Upsilon (\mathrm{1S,2S,3S}) $ mesons in the $\mu^{+}\mu^{-}$ decay modes in ultraperipheral pPb collisions at $\sqrt s_{\rm NN} =$ 5.02 TeV, using data collected with the CMS detector in 2013, corresponding to an integrated luminosity of 32.6 nb$^{-1}$. The exclusive photoproduction cross sections have been measured as a function of the photon-proton center of mass energy in the range 91 $ < W_{\gamma \mathrm{p}}< $ 826 GeV, probing the region of parton fractional momenta in the proton $x\approx 10^{-4}$--$10^{-2}$, bridging a previously unexplored region between HERA and LHCb measurements. Our data are compatible with a power law dependence of $\sigma(W_{\gamma \mathrm{p}})$, disfavouring faster rising extrapolations of HERA results (starlight) and LO pQCD predictions. The differential cross section $\mathrm{d}\sigma/\mathrm{d}|t|$, has been also measured in the range $|t|<$ 1.0 GeV$^2$, and the exponential spectral slope $b=$ 4.5 $\pm$ 1.7 (stat) $\pm$ 0.6 (syst) GeV$^{-2}$ has been extracted, in agreement with earlier measurements and consistent with predictions based on pQCD models. The data presented here provide new constraints for the evolution of the gluon density at low values of fractional momenta $x$ in the proton.

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