The long-range collective flow of particles produced in oxygen-oxygen (OO) and neon-neon (NeNe) collisions is measured with the CMS detector at the CERN LHC. The data samples were collected at a center-of-mass energy per nucleon pair of 5.36 TeV, with integrated luminosities of 7 nb$^{-1}$ and 0.8 nb$^{-1}$ for OO and NeNe collisions, respectively. Two- and four-particle azimuthal correlations are measured over nearly five units of pseudorapidity. Significant elliptic ($v_2$) and triangular ($v_3$) flow harmonics are observed in both systems. The ratios of $v_n$ coefficients between NeNe and OO collisions reveal sensitivity to quadrupole correlations in the nuclear wave functions. Hydrodynamic models with $\textit{ab initio}$ nuclear structure inputs qualitatively reproduce the collision-overlap dependence of both the $v_n$ values and the NeNe to OO ratios. These measurements provide new constraints on hydrodynamic models for small collision systems and offer valuable input on the nuclear structure of $^{16}$O and $^{20}$Ne.
The $v_{2}\{2,\lvert\Delta\eta\rvert>2\}$, $v_{3}\{2,\lvert\Delta\eta\rvert>2\}$ and $v_{2}\{4\}$ values for charged particles as functions of centrality in OO collisions at 5.36 TeV.
The $v_{2}\{2,\lvert\Delta\eta\rvert>2\}$, $v_{3}\{2,\lvert\Delta\eta\rvert>2\}$ and $v_{2}\{4\}$ values for charged particles as functions of centrality in NeNe collisions at 5.36 TeV.
The $v_{2}\{2,\lvert\Delta\eta\rvert>2\}$ and $v_{2}\{4\}$ ratios for charged particles as functions of centrality in NeNe to OO collisions at 5.36 TeV.
A hot and dense state of nuclear matter, known as the quark-gluon plasma, is created in collisions of ultrarelativistic heavy nuclei. Highly energetic quarks and gluons, collectively referred to as partons, lose energy as they travel through this matter, leading to suppressed production of particles with large transverse momenta ($p_\mathrm{T}$). Conversely, high-$p_\mathrm{T}$ particle suppression has not been seen in proton-lead collisions, raising questions regarding the minimum system size required to observe parton energy loss. Oxygen-oxygen (OO) collisions examine a region of effective system size that lies between these two extreme cases. The CMS detector at the CERN LHC has been used to quantify charged-particle production in inclusive OO collisions for the first time via measurements of the nuclear modification factor ($R_\mathrm{AA}$). The $R_\mathrm{AA}$ is derived by comparing particle production to expectations based on proton-proton (pp) data and has a value of unity in the absence of nuclear effects. The data for OO and pp collisions at a nucleon-nucleon center-of-mass energy $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV correspond to integrated luminosities of 6.1 nb$^{-1}$ and 1.02 pb$^{-1}$, respectively. The $R_\mathrm{AA}$ is below unity with a minimum of 0.69 $\pm$ 0.04 around $p_\mathrm{T}$ = 6 GeV. The data exhibit better agreement with theoretical models incorporating parton energy loss as compared to baseline models without energy loss.
Inclusive charged particle spectra for pp collisions at 5.36 TeV for $3 < p_{T} (GeV) <103.6$. The errors represent statistical, systematics and normalization uncertainties.
Inclusive charged particle spectra for OO collisions at 5.36 TeV for $3 < p_{T} (GeV) <103.6$. The errors represent statistical, systematics and normalization uncertainties.
Inclusive charged particle R_{AA} for 5.36 TeV OO collisions for $3 < p_{T} (GeV) <103.6$. The errors represent statistical, systematics and normalization uncertainties.
The ratio between the B$_\mathrm{c}^+$$\to$ J/$ψ$$τ^+ν_τ$ and B$_\mathrm{c}^+$$\to$ J/$ψ$$μ^+ν_μ$ branching fractions is measured using a data sample of proton-proton collisions collected by CMS at a center-of-mass energy of 13 TeV in the years 2016$-$2018 and corresponding to an integrated luminosity of 138 fb$^{-1}$. The J$/ψ$ meson is identified through its J$/ψ$$\to$$μ^+μ^-$ decay and the tau lepton is reconstructed in the hadronic three-prong final state. The measured ratio of branching fractions in this tau decay mode, $\mathcal{R}^\text{had}_{\mathrm{J}/ψ}$ = 1.04$_{-0.44}^{+0.50}$, is combined with the previous analysis based on the $τ^+$$\to$$μ^+ν_μ\barν_τ$ leptonic decay channel, leading to $\mathcal{R}_{\mathrm{J}/ψ}$ = 0.49 $\pm$ 0.26. As this result is consistent with the standard model prediction of 0.258 $\pm$ 0.004, no evidence of lepton flavor universality violation is found.
Measured R(J/psi) ratio
A search is presented for narrow resonances, with a mass between 0.6 and 1.8 TeV, decaying to pairs of jets, in proton-proton collisions at $\sqrt{s}$ = 13 TeV. The search is performed using dijets that are reconstructed, selected, and recorded in a compact form by the high-level trigger in a technique referred to as "data scouting", from data collected in 2016$-$2018 corresponding to an integrated luminosity of 177 fb$^{-1}$. The dijet mass spectra are well described by a smooth parameterization, and no significant evidence for the production of new particles is observed. Model-independent upper limits are presented on the product of the cross section, branching fraction, and acceptance for the individual cases of narrow quark-quark, quark-gluon, and gluon-gluon resonances, and are compared to the predictions from a variety of models of narrow dijet resonance production. The upper limit on the coupling of a dark matter mediator to quarks is presented as a function of the mediator mass. The sensitivity of this search goes beyond what is expected from statistical scaling with the integrated luminosity alone, as a consequence of the use of fewer parameters in the background function within a more robust statistical procedure.
Observed differential dijet spectrum using the 2016 data.
Observed differential dijet spectrum using the 2017 data.
Observed differential dijet spectrum using the 2018 data.
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