CMS-HIN-24-009

CMS-HIN-24-009 ; CERN-EP-2025-051
Observation of coherent $\phi(1020)$ meson photoproduction in ultraperipheral PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} =$ 5.36 TeV
CMS Collaboration
6 April 2025
Submitted to Phys. Rev. Lett.
Abstract: The first observation of coherent $\phi(1020)$ meson photoproduction off heavy nuclei is presented using ultraperipheral lead-lead collisions at a center-of-mass energy per nucleon pair of 5.36 TeV. The data were collected by the CMS experiment and correspond to an integrated luminosity of 1.68 $\mu$ b $^{-1}$ . The $\phi(1020)$ meson signals are reconstructed via the $\mathrm{K^+} \mathrm{K^-}$ decay channel. The production cross section is presented as a function of the $\phi(1020)$ meson rapidity in the range 0.3 $< \|y\| <$ 1.0, probing gluons that carry a fraction of the nucleon momentum ( $x$ ) around $10^{-4}$ . The observed cross section exhibits little dependence on rapidity and is significantly suppressed, by a factor of ${\sim}$ 5, compared to a baseline model that treats a nucleus as a collection of free nucleons. Theoretical models that incorporate either nuclear shadowing or gluon saturation predict suppression of the $\phi(1020)$ meson cross section with only a small dependence on rapidity, but the magnitude of the predicted suppression varies greatly. Models considering only nuclear shadowing effects result in the best agreement with the experimental data. This study establishes a powerful new tool for exploring nuclear effects and nuclear gluonic structure in the small- $x$ regime at a unique energy scale bridging the perturbative and nonperturbative quantum chromodynamics domains.
Links: e-print arXiv:2504.05193 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Examples of the invariant mass ( $m_{\mathrm{K^+} \mathrm{K^-}}$ ) distribution in the coherent process dominant transverse momentum region of pair $p_{\mathrm{T}} <$ 0.2 GeV (left) and the pair $p_{\mathrm{T}}$ distribution in the $\phi$ meson mass window 0.99 $< m_{\mathrm{K^+} \mathrm{K^-}} <$ 1.05 GeV (right). The results of the template fit, as described in the text, are represented by various curves. These include the coherent (\textCoh.) $\phi$ , incoherent (\textIncoh.) $\phi$ for both elastic (el.) and dissociative (dis.) processes, and the direct $\mathrm{K^+} \mathrm{K^-}$ continuum (cont.). The fitted numbers of raw $\phi$ meson candidates and the $\mathrm{K^+} \mathrm{K^-}$ continuum are indicated in the legend of the left panel. The vertical bars on the data points represent statistical uncertainties ( $\sigma_{\text{Data}}$ ). The bottom panels show the pull distribution of the fit, defined as $\text{Pull} =$ (Data-Fit) $/\sigma_{\text{Data}}$ , with the red lines indicating the zero value.
png pdf	Figure 1-a: Examples of the invariant mass ( $m_{\mathrm{K^+} \mathrm{K^-}}$ ) distribution in the coherent process dominant transverse momentum region of pair $p_{\mathrm{T}} <$ 0.2 GeV (left) and the pair $p_{\mathrm{T}}$ distribution in the $\phi$ meson mass window 0.99 $< m_{\mathrm{K^+} \mathrm{K^-}} <$ 1.05 GeV (right). The results of the template fit, as described in the text, are represented by various curves. These include the coherent (\textCoh.) $\phi$ , incoherent (\textIncoh.) $\phi$ for both elastic (el.) and dissociative (dis.) processes, and the direct $\mathrm{K^+} \mathrm{K^-}$ continuum (cont.). The fitted numbers of raw $\phi$ meson candidates and the $\mathrm{K^+} \mathrm{K^-}$ continuum are indicated in the legend of the left panel. The vertical bars on the data points represent statistical uncertainties ( $\sigma_{\text{Data}}$ ). The bottom panels show the pull distribution of the fit, defined as $\text{Pull} =$ (Data-Fit) $/\sigma_{\text{Data}}$ , with the red lines indicating the zero value.
png pdf	Figure 1-b: Examples of the invariant mass ( $m_{\mathrm{K^+} \mathrm{K^-}}$ ) distribution in the coherent process dominant transverse momentum region of pair $p_{\mathrm{T}} <$ 0.2 GeV (left) and the pair $p_{\mathrm{T}}$ distribution in the $\phi$ meson mass window 0.99 $< m_{\mathrm{K^+} \mathrm{K^-}} <$ 1.05 GeV (right). The results of the template fit, as described in the text, are represented by various curves. These include the coherent (\textCoh.) $\phi$ , incoherent (\textIncoh.) $\phi$ for both elastic (el.) and dissociative (dis.) processes, and the direct $\mathrm{K^+} \mathrm{K^-}$ continuum (cont.). The fitted numbers of raw $\phi$ meson candidates and the $\mathrm{K^+} \mathrm{K^-}$ continuum are indicated in the legend of the left panel. The vertical bars on the data points represent statistical uncertainties ( $\sigma_{\text{Data}}$ ). The bottom panels show the pull distribution of the fit, defined as $\text{Pull} =$ (Data-Fit) $/\sigma_{\text{Data}}$ , with the red lines indicating the zero value.
png pdf	Figure 2: The coherent $\phi$ meson photoproduction differential cross section as a function of the $\phi$ meson rapidity in the range 0.3 $< \|y\| <$ 1.0 is shown in the upper panel. Vertical bars and shaded bands around data points indicate the statistical and systematic uncertainties, respectively. The theoretical predictions, including the impulse approximation (IA) [39], the color dipole model based approaches of IIM [53], bCGC [54], IP-SAT [55,56], GBW and GBW $\text{f}_\text{c}$ [57,58], the reggeometric pomeron (RP) approach using the classical Glauber (CG) and Gribov--Glauber (GG) formalisms [59,23], the modified vector meson dominance (mVMD) model incorporating GG formalism with strong shadowing (SS) and weak shadowing (WS) scenarios [27], the STARLIGHT model incorporating the VMD with CG formalism [39], are represented by the colored lines. The ratio between data and IA prediction is shown in the bottom panel.

Summary

In summary, the first measurement of coherent

$\phi(1020)$ meson photoproduction off a heavy nucleus has been performed by the CMS experiment in ultraperipheral lead-lead collisions at a center-of-mass energy per nucleon pair of 5.36 TeV. The production cross section is reported in the

$\phi$ meson rapidity range 0.3

$< |y| <$ 1.0, probing gluons within the lead nucleus having

$x$ values around 10

$^{-4}$ . This corresponds to a unique energy scale between perturbative and nonperturbative quantum chromodynamics regimes. The cross section is found to be significantly suppressed by a factor of

${\sim}$ 5 compared to an impulse approximation model that treats a nucleus as a collection of free nucleons. Models that account for nuclear shadowing effects generally provide better predictions of the data compared to those that incorporate gluon saturation effects. The measurement of coherent

$\phi$ meson photoproduction off a heavy nucleus establishes a powerful new tool for exploring nuclear effects and nuclear gluonic structure in the small-

$x$ regime.

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