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Abstract The challenging conditions of the High-Luminosity LHC require tailored hardware designs for the trigger and data acquisition systems. The Apollo platform features a “Service Module” with a powerful system-on-module computer that provides standard ATCA communications and application-specific “Command Modules” with large FPGAs and high speed optical fiber links. The CMS version of Apollo will be used for the track finder and the pixel readout. It features up to two large FPGAs and more than 100 optical links with speeds up to 25 Gb/s. We study carefully the design and performance of the board by using customized firmware to test power consumption, heat dissipation, and optical link integrity. This paper presents the results of these performance tests, design updates, and future plans. 

The Dark Energy Spectroscopic Instrument (DESI) Collaboration has obtained robust measurements of baryon acoustic oscillations in the redshift range $0.1<z<4.2$ , based on the Lyman- $\alpha$ forest and galaxies from data release 2. We combine these measurements with cosmic microwave background (CMB) data from and the Atacama Cosmology Telescope to place our tightest constraints yet on the sum of neutrino masses. Assuming the cosmological $\mathrm{\Lambda }\mathrm{CDM}$ model and three degenerate neutrino states, we find $\sum m_{\nu }<0.0642\mathrm{eV}$ (95%) with a marginalized error of $\sigma (\sum m_{\nu })=0.020\mathrm{eV}$ . We also constrain the effective number of neutrino species, finding $N_{\mathrm{eff}}=3.23_{-0.34}^{+0.35}$ (95%), in line with the Standard Model prediction. When accounting for neutrino oscillation constraints, we find a preference for the normal mass ordering and an upper limit on the lightest neutrino mass of $m_l<0.023\mathrm{eV}$ (95%). However, we determine using frequentist and Bayesian methods that our constraints are in tension with the lower limits derived from neutrino oscillations. Correcting for the physical boundary at zero mass, we report a 95% Feldman-Cousins upper limit of $\sum m_{\nu }<0.053\mathrm{eV}$ , breaching the lower limit from neutrino oscillations. Considering a more general Bayesian analysis with an effective cosmological neutrino mass parameter, $\sum m_{\nu ,\mathrm{eff}}$ , that allows for negative energy densities and removes unsatisfactory prior weight effects, we derive constraints that are in $3\sigma$ tension with the same oscillation limit, while the error rises to $\sigma (\sum m_{\nu ,\mathrm{eff}})=0.053\mathrm{eV}$ . In the absence of unknown systematics, this finding could be interpreted as a hint of new physics not necessarily related to neutrinos. The preference of DESI and CMB data for an evolving dark energy model offers one possible solution. In the $w_0{w}_a\mathrm{CDM}$ model, we find $\sum m_{\nu }<0.163\mathrm{eV}$ (95%), relaxing the neutrino tension. These constraints all rely on the effects of neutrinos on the cosmic expansion history. Using full-shape power spectrum measurements of data release 1 galaxies, we place complementary constraints that rely on neutrino free streaming. Our strongest such limit in $\mathrm{\Lambda }\mathrm{CDM}$ , using selected CMB priors, is $\sum m_{\nu }<0.193\mathrm{eV}$ (95%). 

We conduct an extended analysis of dark energy constraints, in support of the findings of the Dark Energy Spectroscopic Instrument (DESI) second data release cosmology key paper, including DESI data, Planck cosmic microwave background observations, and three different supernova compilations. Using a broad range of parametric and nonparametric methods, we explore the dark energy phenomenology and find consistent trends across all approaches, in good agreement with the $w_0{w}_a\mathrm{CDM}$ (cold dark matter) key paper results. Even with the additional flexibility introduced by nonparametric approaches, such as binning and Gaussian processes, we find that extending $\mathrm{\Lambda }\mathrm{CDM}$ to include a two-parameter $w\left(z\right)$ is sufficient to capture the trends present in the data. Finally, we examine three dark energy classes with distinct dynamics, including quintessence scenarios satisfying $w\ge -1$ , to explore what underlying physics can explain such deviations. The current data indicate a clear preference for models that feature a phantom crossing; although alternatives lacking this feature are disfavored, they cannot yet be ruled out. Our analysis confirms that the evidence for dynamical dark energy, particularly at low redshift ( $z\lesssim 0.3$ ), is robust and stable under different modeling choices. 

A<sc>bstract</sc> A search for Higgs boson (H) production at high transverse momentum (p_T) in the WW decay channel is presented. The analysis uses proton-proton collisions at$$ \sqrt{s}=13 $$ $\sqrt{s}=13$ TeV recorded by the CMS experiment in 2016–2018, corresponding to an integrated luminosity of 138 fb⁻¹. The visible decay products of the Higgs boson are reconstructed as a single large-radius jet with one isolated lepton or none (1ℓand 0ℓ, respectively;ℓ= e,μ). The H-candidate jets are identified using an advanced transformer-based algorithm and are calibrated with the Lund jet plane reweighting technique. The 1ℓchannel is further split into gluon fusion, vector boson fusion, and associated production with hadronically decaying vector boson categories, while the 0ℓchannel considers all production processes inclusively. The measured cross section times the H→WW branching fraction relative to the standard model expectation is$$ \mu =-{0.19}_{-0.46}^{+0.48} $$ $\mu =-{0.19}_{-0.46}^{+0.48}$ , indicating no evidence of a signal above the background. This measurement represents the first dedicated study of highly Lorentz-boosted H→WW decays, complementing earlier searches for high-p_THiggs boson in other decay channels. 

A search for the pair production of heavy spin- $1/2$ or spin- $3/2$ resonances ( $t^{*}$ ) in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ is presented. Data collected with the CMS detector at the CERN LHC from 2016 to 2018 corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ are used. The analysis targets benchmark signal scenarios where one $t^{*}$ decays into a top quark ( $t$ ) and a photon ( $\gamma$ ), and the other into a $t$ quark and a gluon ( $g$ ), i.e., $pp\to t^{*}\overline{t^{*}}\to tt\gamma g$ . All-hadronic final states from the $t$ pair decay chain are selected using jet substructure techniques. The signal is probed as a function of the $t^{*}$ candidate mass, which is reconstructed using the photon and a top quark candidate jet. No significant deviation from the background-only hypothesis is found. Observed (expected) upper limits on the signal cross section at 95% confidence level are set, excluding masses of spin- $1/2$ $t^{*}$ particles below 930 (930) GeV and spin- $3/2$ $t^{*}$ particles below 1330 (1390) GeV. This analysis marks the first search for heavy resonances in the $t\overline{t}\gamma g$ channel. Exploiting the high-energy photon to reduce the backgrounds, this search achieves sensitivity competitive with $pp\to t^{*}\overline{t^{*}}\to t\overline{t}gg$ searches for spin- $1/2$ $t^{*}$ despite the small expected $t^{*}\to t\gamma$ branching fraction. 

A<sc>bstract</sc> A measurement of the angular structure of inclusive jets and those containing a prompt D⁰meson in proton-proton collisions at the LHC at a center-of-mass energy of 5.02 TeV is presented. The data corresponding to an integrated luminosity of 301 pb⁻¹were collected by the CMS experiment in 2017. Two jet grooming algorithms, late-k_Tand soft drop, are used to study the intrajet radiation pattern using iterative Cambridge-Aachen declustering. The splitting-angle distributions of jets with transverse momentum (p_T) of around 100 GeV, obtained with these two algorithms, show that there is a shift of the distribution for jets containing a prompt D⁰meson with respect to inclusive jets. The suppression of emissions at small angles observed in the late-k_Tgrooming approach is consistent with the dead-cone effect, whereas the similar suppression for splittings selected with the soft-drop algorithm appears to be induced by gluon splitting to charm quark-antiquark pairs at large angles. The measured distributions are corrected to the particle level and can be used to constrain model predictions for the substructure of high-p_Tcharm quark jets. 

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 $\sqrt{s_{\mathrm{NN}}}=5.36\mathrm{TeV}$ correspond to integrated luminosities of $6.1{\mathrm{nb}}^{-1}$ and $1.02{\mathrm{pb}}^{-1}$ , respectively. The $R_{\mathrm{AA}}$ is below unity with a minimum of $0.69\pm 0.04$ around $p_{\mathrm{T}}=6\mathrm{GeV}$ . The data exhibit better agreement with theoretical models incorporating parton energy loss as compared to baseline models without energy loss.