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1. Inference of the Mass Composition of Cosmic Rays with Energies from ${10}^{18.5}$ to ${10}^{20}\mathrm{eV}$ Using the Pierre Auger Observatory and Deep Learning

   https://doi.org/10.1103/PhysRevLett.134.021001  

   Abdul_Halim, A; Abreu, P; Aglietta, M; Allekotte, I; Almeida_Cheminant, K; Almela, A; Aloisio, R; Alvarez-Muñiz, J; Ammerman_Yebra, J; Anastasi, G A; et al (January 2025, Physical Review Letters)

   We present measurements of the atmospheric depth of the shower maximum $X_{\max }$, inferred for the first time on an event-by-event level using the surface detector of the Pierre Auger Observatory. Using deep learning, we were able to extend measurements of the $X_{\max }$distributions up to energies of 100 EeV ( ${10}^{20}\mathrm{eV}$), not yet revealed by current measurements, providing new insights into the mass composition of cosmic rays at extreme energies. Gaining a 10-fold increase in statistics compared to the fluorescence detector data, we find evidence that the rate of change of the average $X_{\max }$with the logarithm of energy features three breaks at $6.5\pm 0.6\left(\mathrm{stat}\right)\pm 1\left(\mathrm{syst}\right)\mathrm{EeV}$, $11\pm 2\left(\mathrm{stat}\right)\pm 1\left(\mathrm{syst}\right)\mathrm{EeV}$, and $31\pm 5\left(\mathrm{stat}\right)\pm 3\left(\mathrm{syst}\right)\mathrm{EeV}$, in the vicinity to the three prominent features (ankle, instep, suppression) of the cosmic-ray flux. The energy evolution of the mean and standard deviation of the measured $X_{\max }$distributions indicates that the mass composition becomes increasingly heavier and purer, thus being incompatible with a large fraction of light nuclei between 50 and 100 EeV. Published by the American Physical Society2025 

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2. Search for photons above ${10}^{18}\mathrm{eV}$ by simultaneously measuring the atmospheric depth and the muon content of air showers at the Pierre Auger Observatory

   https://doi.org/10.1103/PhysRevD.110.062005  

   Abdul_Halim, A; Abreu, P; Aglietta, M; Allekotte, I; Almeida_Cheminant, K; Almela, A; Aloisio, R; Alvarez-Muñiz, J; Ammerman_Yebra, J; Anastasi, G A; et al (September 2024, Physical Review D)

   The Pierre Auger Observatory is the most sensitive instrument to detect photons with energies above $10^{17}\mathrm{eV}$. It measures extensive air showers generated by ultrahigh energy cosmic rays using a hybrid technique that exploits the combination of a fluorescence detector with a ground array of particle detectors. The signatures of a photon-induced air shower are a larger atmospheric depth of the shower maximum ( $X_{\max }$) and a steeper lateral distribution function, along with a lower number of muons with respect to the bulk of hadron-induced cascades. In this work, a new analysis technique in the energy interval between 1 and 30 EeV ( $1\mathrm{EeV}=10^{18}\mathrm{eV}$) has been developed by combining the fluorescence detector-based measurement of $X_{\max }$with the specific features of the surface detector signal through a parameter related to the air shower muon content, derived from the universality of the air shower development. No evidence of a statistically significant signal due to photon primaries was found using data collected in about 12 years of operation. Thus, upper bounds to the integral photon flux have been set using a detailed calculation of the detector exposure, in combination with a data-driven background estimation. The derived 95% confidence level upper limits are 0.0403, 0.01113, 0.0035, 0.0023, and $0.0021{\mathrm{km}}^{-2}{\mathrm{sr}}^{-1}{\mathrm{yr}}^{-1}$above 1, 2, 3, 5, and 10 EeV, respectively, leading to the most stringent upper limits on the photon flux in the EeV range. Compared with past results, the upper limits were improved by about 40% for the lowest energy threshold and by a factor 3 above 3 EeV, where no candidates were found and the expected background is negligible. The presented limits can be used to probe the assumptions on chemical composition of ultrahigh energy cosmic rays and allow for the constraint of the mass and lifetime phase space of super-heavy dark matter particles. Published by the American Physical Society2024 

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3. Search for New Resonances Decaying to Pairs of Merged Diphotons in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevLett.134.041801  

   Hayrapetyan, A; Tumasyan, A; Adam, W; Andrejkovic, J W; Bergauer, T; Chatterjee, S; Damanakis, K; Dragicevic, M; Hussain, P S; Jeitler, M; et al (January 2025, Physical Review Letters)

   A search is presented for an extended Higgs sector with two new particles, $X$and $\varphi$, in the process $X\to \varphi \varphi \to \left(\gamma \gamma \right)\left(\gamma \gamma \right)$. Novel neural networks classify events with diphotons that are merged and determine the diphoton masses. The search uses LHC proton-proton collision data at $\sqrt{s}=13\mathrm{TeV}$collected with the CMS detector, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$. No evidence of such resonances is seen. Upper limits are set on the production cross section for $m_X$between 300 and 3000 GeV and $m_{\varphi }/m_X$between 0.5% and 2.5%, representing the most sensitive search in this channel. © 2025 CERN, for the CMS Collaboration2025CERN 

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4. Search for high-mass exclusive diphoton production with tagged protons in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevD.110.012010  

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

   A search is presented for high-mass exclusive diphoton production via photon-photon fusion in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$in events where both protons survive the interaction. The analysis utilizes data corresponding to an integrated luminosity of $103{\mathrm{fb}}^{-1}$collected in 2016–2018 with the central CMS detector and the CMS and TOTEM precision proton spectrometer (PPS). Events that have two photons with high transverse momenta ( $p_{\mathrm{T}}^{\gamma }>100\mathrm{GeV}$), back-to-back in azimuth, and with a large diphoton invariant mass ( $m_{\gamma \gamma }>350\mathrm{GeV}$) are selected. To remove the dominant inclusive diphoton backgrounds, the kinematic properties of the protons detected in PPS are required to match those of the central diphoton system. Only events having opposite-side forward protons detected with a fractional momentum loss between 0.035 and 0.15 (0.18) for the detectors on the negative (positive) side of CMS are considered. One exclusive diphoton candidate is observed for an expected background of 1.1 events. Limits at 95% confidence level are derived for the four-photon anomalous coupling parameters $\left|{\zeta }_1\right|<0.073{\mathrm{TeV}}^{-4}$and $\left|{\zeta }_2\right|<0.15{\mathrm{TeV}}^{-4}$, using an effective field theory. Additionally, upper limits are placed on the production of axionlike particles with coupling strength to photons $f^{-1}$that varies from $0.03{\mathrm{TeV}}^{-1}$to $1{\mathrm{TeV}}^{-1}$over the mass range from 500 to 2000 GeV. © 2024 CERN, for the CMS and TOTEMs Collaboration2024CERN 

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5. Search for a ${\mu }^{+}{\mu }^{-}$ resonance in four-muon final states at Belle II

   https://doi.org/10.1103/PhysRevD.109.112015  

   Adachi, I; Adamczyk, K; Aggarwal, L; Ahmed, H; Aihara, H; Akopov, N; Aloisio, A; Anh_Ky, N; Asner, D M; Atmacan, H; et al (June 2024, Physical Review D)

   We report on a search for a resonance $X$decaying to a pair of muons in $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}X$events in the $0.212–9.000\mathrm{GeV}/c^2$mass range, using $178{\mathrm{fb}}^{-1}$of data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of $X$beyond the standard model: a $Z^{\prime }$vector boson in the $L_{\mu }-L_{\tau }$model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the $L_{\mu }-L_{\tau }$model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below $6\mathrm{GeV}/c^2$, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the $L_{\mu }-L_{\tau }$model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search. Published by the American Physical Society2024 

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