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1. First Measurement of νe and νμ Interaction Cross Sections at the LHC with FASER’s Emulsion Detector

   https://doi.org/10.1103/PhysRevLett.133.021802  

   Abraham, Roshan Mammen ; Anders, John ; Antel, Claire ; Ariga, Akitaka ; Ariga, Tomoko ; Atkinson, Jeremy ; Bernlochner, Florian U ; Boeckh, Tobias ; Boyd, Jamie ; Brenner, Lydia ; et al ( July 2024 , Physical Review Letters)

   The first results of the study of high-energy electron neutrino (${\nu }_e$) and muon neutrino (${\nu }_{\mu }$) charged-current interactions in the$\mathrm{FASER}\nu$emulsion-tungsten detector of the FASER experiment at the LHC are presented. A 128.8 kg subset of the$\mathrm{FASER}\nu$volume was analyzed after exposure to$9.5{\mathrm{fb}}^{-1}$of$\sqrt{s}=13.6\mathrm{TeV}$$pp$data. Four (eight)${\nu }_e$(${\nu }_{\mu }$) interaction candidate events are observed with a statistical significance of$5.2\sigma$($5.7\sigma$). This is the first direct observation of${\nu }_e$interactions at a particle collider and includes the highest-energy${\nu }_e$and${\nu }_{\mu }$ever detected from an artificial source. The interaction cross section per nucleon$\sigma /E_{\nu }$is measured over an energy range of 560–1740 GeV (520–1760 GeV) for${\nu }_e$(${\nu }_{\mu }$) to be$({1.2}_{-0.7}^{+0.8})\times {10}^{-38}{\mathrm{cm}}^2{\mathrm{GeV}}^{-1}$[$(0.5\pm 0.2)\times {10}^{-38}{\mathrm{cm}}^2{\mathrm{GeV}}^{-1}$], consistent with standard model predictions. These are the first measurements of neutrino interaction cross sections in those energy ranges.

   Published by the American Physical Society2024 

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2. Search for a heavy neutral lepton that mixes predominantly with the tau neutrino

   https://doi.org/10.1103/PhysRevD.109.L111102  

   Nayak, M ; Dey, S ; Soffer, A ; Adachi, I ; Aihara, H ; Al_Said, S ; Asner, D M ; Atmacan, H ; Ayad, R ; Babu, V ; et al ( June 2024 , Physical Review D)

   We report a search for a heavy neutral lepton (HNL) that mixes predominantly with${\nu }_{\tau }$. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy$e^{+}{e}^{-}$collider. The data sample was collected at and just below the center-of-mass energies of the$\mathrm{\Upsilon }(4S)$and$\mathrm{\Upsilon }(5S)$resonances and has an integrated luminosity of$915{\mathrm{fb}}^{-1}$, corresponding to$(836\pm 12)\times {10}^6$$e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$events. We search for production of the HNL (denoted$N$) in the decay${\tau }^{-}\to {\pi }^{-}N$followed by its decay via$N\to {\mu }^{+}{\mu }^{-}{\nu }_{\tau }$. The search focuses on the parameter-space region in which the HNL is long-lived, so that the${\mu }^{+}{\mu }^{-}$originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the$\tau$neutrino as a function of the HNL mass.

   Published by the American Physical Society2024 

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3. Search for a μ+μ− 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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4. ψ(2S) 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$\psi (2S)$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$\psi (2S)$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$[{\sigma }^{\psi (2S)}/{\sigma }^{J/\psi }{]}_{\mathrm{Pb}\text{−}\mathrm{Pb}}/[{\sigma }^{\psi (2S)}/{\sigma }^{J/\psi }{]}_{pp}$. It is found that in Pb-Pb collisions the$\psi (2S)$is suppressed by a factor of$\sim 2$with respect to the$J/\psi$. The$\psi (2S)$nuclear modification factor$R_{\mathrm{AA}}$was also obtained as a function of both centrality and$p_T$. The results show that the$\psi (2S)$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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5. Photoproduction of K+K− Pairs in Ultraperipheral Collisions

   https://doi.org/10.1103/PhysRevLett.132.222303  

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

   $K^{+}{K}^{-}$pairs may be produced in photonuclear collisions, either from the decays of photoproduced$\varphi (1020)$mesons or directly as nonresonant$K^{+}{K}^{-}$pairs. Measurements of$K^{+}{K}^{-}$photoproduction probe the couplings between the$\varphi (1020)$and charged kaons with photons and nuclear targets. The kaon-proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of$K^{+}{K}^{-}$pairs on lead ions in ultraperipheral collisions using the ALICE detector, including the first investigation of direct$K^{+}{K}^{-}$production. There is significant$K^{+}{K}^{-}$production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range$1.1<M_{KK}<1.4\mathrm{GeV}/c^2$above the$\varphi (1020)$resonance, for rapidity$|y_{KK}|<0.8$and$p_{T,KK}<0.1\mathrm{GeV}/c$, the measured coherent photoproduction cross section is$\mathrm{d}\sigma /\mathrm{d}y=3.37\pm 0.61(\text{stat})\pm 0.15(\mathrm{syst})\mathrm{mb}$. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system$W_{\gamma \mathrm{Pb},n}$ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for$\varphi (1020)$photoproduction alone. The mass spectrum is fit to a cocktail consisting of$\varphi (1020)$decays, direct$K^{+}{K}^{-}$photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct$K^{+}{K}^{-}$photoproduction are presented.

   © 2024 CERN, for the ALICE Collaboration2024CERN 

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