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 K$^+$K$^-$ decay channel. The production cross section is presented as a function of the $\phi$(1020) meson rapidity in the range 0.3 $\lt$$\lvert y\rvert$$\lt$ 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.
The differential coherent $\phi$ photoproduction cross section and the nuclear suppression factor as a function of rapidity.
Three rare decay processes of the Higgs boson to a $\rho$(770)$^0$, $\phi$(1020), or K$^{*}$(892)$^0$ meson and a photon are searched for using $\sqrt{s}$ = 13 TeV proton-proton collision data collected by the CMS experiment at the LHC. Events are selected assuming the mesons decay into a pair of charged pions, a pair of charged kaons, or a charged kaon and pion, respectively. Depending on the Higgs boson production mode, different triggering and reconstruction techniques are adopted. The analyzed data sets correspond to integrated luminosities up to 138 fb$^{-1}$, depending on the reconstructed final state. After combining various data sets and categories, no significant excess above the background expectations is observed. Upper limits at 95% confidence level on the Higgs boson branching fractions into $\rho$(770)$^0$$\gamma$, $\phi$(1020)$\gamma$, and K$^{*}$(892)$^0\gamma$ are determined to be 3.7 $\times$ 10$^{-4}$, 3.0 $\times$ 10$^{-4}$, and 3.0 $\times$ 10$^{-4}$, respectively. In case of the $\rho$(770)$^0$$\gamma$ and $\phi$(1020)$\gamma$ channels, these are the most stringent experimental limits to date.
Expected and observed UL on $\mathcal{B}(H\rightarrow\rho\gamma)$ split by analysis categories and combined. Green and yellow bands correspond to 68\% and 95\% confidence intervals on the expected upper limits.
Expected and observed UL on $\mathcal{B}(H\rightarrow\phi\gamma)$ split by analysis categories and combined. Green and yellow bands correspond to 68\% and 95\% confidence intervals on the expected upper limits.
Expected and observed UL on $\mathcal{B}(H\rightarrow K^{*0}\gamma)$ split by analysis categories and combined. Green and yellow bands correspond to 68\% and 95\% confidence intervals on the expected upper limits.
Forum