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1. $\psi \left(2S\right)$ Suppression in Pb-Pb Collisions at the LHC

   https://doi.org/10.1103/PhysRevLett.132.042301  

   Acharya, S; Adamová, D; Adler, A; Aglieri_Rinella, G; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Ahuja, I; et al (January 2024, Physical Review Letters)

   The production of the $\psi \left(2S\right)$ charmonium state was measured with ALICE in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ , in the dimuon decay channel. A significant signal was observed for the first time at LHC energies down to zero transverse momentum, at forward rapidity ( $2.5<y<4$ ). The measurement of the ratio of the inclusive production cross sections of the $\psi \left(2S\right)$ and $\mathrm{J}/\psi$ resonances is reported as a function of the centrality of the collisions and of transverse momentum, in the region $p_T<12\mathrm{GeV}/c$ . The results are compared with the corresponding measurements in $pp$ collisions, by forming the double ratio $[{\sigma }^{\psi \left(2S\right)}/{\sigma }^{J/\psi }{]}_{\mathrm{Pb}\text{−}\mathrm{Pb}}/[{\sigma }^{\psi \left(2S\right)}/{\sigma }^{J/\psi }{]}_{pp}$ . It is found that in Pb-Pb collisions the $\psi \left(2S\right)$ is suppressed by a factor of $\sim 2$ with respect to the $J/\psi$ . The $\psi \left(2S\right)$ nuclear modification factor $R_{\mathrm{AA}}$ was also obtained as a function of both centrality and $p_T$ . The results show that the $\psi \left(2S\right)$ resonance yield is strongly suppressed in Pb-Pb collisions, by a factor of up to $\sim 3$ with respect to $pp$ . Comparisons of cross section ratios with previous Super Proton Synchrotron findings by the NA50 experiment and of $R_{\mathrm{AA}}$ with higher- $p_T$ results at LHC energy are also reported. These results and the corresponding comparisons with calculations of transport and statistical models address questions on the presence and properties of charmonium states in the quark-gluon plasma formed in nuclear collisions at the LHC. © 2024 CERN, for the ALICE Collaboration2024CERN 

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2. Observation of double $J/\psi$ meson production in $p\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=8.16\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevD.110.092002  

   Hayrapetyan, A; Tumasyan, A; Adam, W; Andrejkovic, J W; Bergauer, T; Chatterjee, S; Damanakis, K; Dragicevic, M; Hussain, P S; Jeitler, M; et al (November 2024, Physical Review D)

   The first observation of the concurrent production of two $J/\psi$ mesons in proton-nucleus collisions is presented. The analysis is based on a proton-lead ( $p\mathrm{Pb}$ ) data sample recorded at a nucleon-nucleon center-of-mass energy of 8.16 TeV by the CMS experiment at the CERN LHC and corresponding to an integrated luminosity of $174.6{\mathrm{nb}}^{-1}$ . The two $J/\psi$ mesons are reconstructed in their ${\mu }^{+}{\mu }^{-}$ decay channels with transverse momenta $p_{\mathrm{T}}>6.5\mathrm{GeV}$ and rapidity $\left|y\right|<2.4$ . Events where one of the $J/\psi$ mesons is reconstructed in the dielectron channel are also considered in the search. The $p\mathrm{Pb}\to J/\psi J/\psi +X$ process is observed with a significance of 5.3 standard deviations. The measured inclusive fiducial cross section, using the four-muon channel alone, is $\sigma (p\mathrm{Pb}\to J/\psi J/\psi +X)=22.0\pm 8.9\left(\mathrm{stat}\right)\pm 1.5\left(\mathrm{syst}\right)\mathrm{nb}$ . A fit of the data to the expected rapidity separation for pairs of $J/\psi$ mesons produced in single (SPS) and double (DPS) parton scatterings yields ${\sigma }_{\mathrm{SPS}}^{p\mathrm{Pb}\to J/\psi J/\psi +X}=16.5\pm 10.8\left(\mathrm{stat}\right)\pm 0.1\left(\mathrm{syst}\right)\mathrm{nb}$ and ${\sigma }_{\mathrm{DPS}}^{p\mathrm{Pb}\to J/\psi J/\psi +X}=5.4\pm 6.2\left(\mathrm{stat}\right)\pm 0.4\left(\mathrm{syst}\right)\mathrm{nb}$ , respectively. This latter result can be transformed into a lower bound on the effective DPS cross section, closely related to the squared average interparton transverse separation in the collision, of ${\sigma }_{\mathrm{eff}}>1.0\mathrm{mb}$ at 95% confidence level. © 2024 CERN, for the CMS Collaboration2024CERN 

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3. First Measurement of the $\left|t\right|$ Dependence of Incoherent $J/\psi$ Photonuclear Production

   https://doi.org/10.1103/PhysRevLett.132.162302  

   Acharya, S; Adamová, D; Adler, A; Aglieri_Rinella, G; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Ahuja, I; et al (April 2024, Physical Review Letters)

   The first measurement of the cross section for incoherent photonuclear production of $J/\psi$ vector mesons as a function of the Mandelstam $\left|t\right|$ variable is presented. The measurement was carried out with the ALICE detector at midrapidity, $\left|y\right|<0.8$ , using ultraperipheral collisions of Pb nuclei at a center-of-mass energy per nucleon pair of $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$ . This rapidity interval corresponds to a Bjorken- $x$ range $\left(0.3–1.4\right)\times {10}^{-3}$ . Cross sections are given in five $\left|t\right|$ intervals in the range $0.04<\left|t\right|<1{\mathrm{GeV}}^2$ and compared to the predictions by different models. Models that ignore quantum fluctuations of the gluon density in the colliding hadron predict a $\left|t\right|$ dependence of the cross section much steeper than in data. The inclusion of such fluctuations in the same models provides a better description of the data. © 2024 CERN, for the ALICE Collaboration2024CERN 

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4. Observation of $CP$ Violation in $B^0\to J/\psi \rho (770{)}^0$ Decays

   https://doi.org/10.1103/6f6r-51br  

   Aaij, R; Abdelmotteleb, A_S W; Beteta, C Abellan; Abudinén, F; Ackernley, T; Adefisoye, A A; Adeva, B; Adinolfi, M; Adlarson, P; Agapopoulou, C; et al (July 2026, Physical Review Letters)

   The time-dependent $CP$ asymmetry in $B^0\to J/\psi \rho (770{)}^0$ decays is measured using proton-proton collision data corresponding to an integrated luminosity of $6{\mathrm{fb}}^{-1}$ , collected with the LHCb detector at a center-of-mass energy of 13 TeV during the years 2015–2018. The $CP$ -violation parameters for this process are determined to be $2{\beta }_{c\overline{c}d}^{\mathrm{eff}}=0.710\pm 0.084\pm 0.051\mathrm{rad}$ and $\left|\lambda \right|=1.019\pm 0.034\pm 0.024$ , where the first uncertainty is statistical and the second systematic. This constitutes the first observation of time-dependent $CP$ violation in $B^0\to J/\psi \rho (770{)}^0$ decays. Assuming approximate SU(3) flavor symmetry, these results are combined with the previous consistent LHCb measurement to set the most stringent constraint on the penguin contribution, $\mathrm{\Delta }{\varphi }_s$ , to the $CP$ -violating phase ${\varphi }_s$ in $B_s^0\to J/\psi \varphi \left(1020\right)$ decays, yielding $\mathrm{\Delta }{\varphi }_s=5.0\pm 4.6\mathrm{mrad}$ . 

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5. Observation of the decay ${\chi }_{c1}\left(3872\right)\to J/\psi {\mu }^{+}{\mu }^{-}$

   https://doi.org/10.1103/jqvq-sbp7  

   Aaij, R; Abdelmotteleb, A_S W; Abellan_Beteta, C; Abudinén, F; Ackernley, T; Adefisoye, A A; Adeva, B; Adinolfi, M; Adlarson, P; Agapopoulou, C; et al (April 2026, Physical Review D)

   The first observation of the ${\chi }_{c1}\left(3872\right)\to J/\psi {\mu }^{+}{\mu }^{-}$ decay is reported using proton-proton collision data recorded with the LHCb detector corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . The decay mode is observed for the first time, with a significance of $6.5\sigma$ . Its branching fraction is measured relative to the ${\chi }_{c1}\left(3872\right)\to J/\psi {\pi }^{+}{\pi }^{-}$ decay mode $\frac{\mathcal{B}({\chi }_{c1}\left(3872\right)\to J/\psi {\mu }^{+}{\mu }^{-})}{\mathcal{B}({\chi }_{c1}\left(3872\right)\to J/\psi {\pi }^{+}{\pi }^{-})}=(1.68\pm 0.32\pm 0.05)\times {10}^{-3}$ , where the first uncertainty includes both statistical contributions and systematic contributions which are uncorrelated between data-taking periods, and the second represents the systematic contributions that are correlated between data-taking periods. 

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