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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. First Measurement of R(Xτ/ℓ) as an Inclusive Test of the b→cτν Anomaly

   https://doi.org/10.1103/PhysRevLett.132.211804  

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

   We measure the tau-to-light-lepton ratio of inclusive$B$-meson branching fractions$R(X_{\tau /\ell })\equiv \mathcal{B}(B\to X\tau \nu )/\mathcal{B}(B\to X\ell \nu )$, where$\ell$indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed$B$meson and a charged lepton candidate from$189{\mathrm{fb}}^{-1}$of electron-positron collision data collected with the Belle II detector. We find$R(X_{\tau /\ell })=0.228\pm 0.016(\mathrm{stat})\pm 0.036(\mathrm{syst})$, in agreement with standard-model expectations. This is the first direct measurement of$R(X_{\tau /\ell })$.

   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. Search for prompt production of pentaquarks in charm hadron final states

   https://doi.org/10.1103/PhysRevD.110.032001  

   Aaij, R ; Abdelmotteleb, A_S W ; Abellan_Beteta, C ; Abudinén, F ; Ackernley, T ; Adeva, B ; Adinolfi, M ; Adlarson, P ; Afsharnia, H ; Agapopoulou, C ; et al ( August 2024 , Physical Review D)

   A search for hidden-charm pentaquark states decaying to a range of${\mathrm{\Sigma }}_c\overline{D}$and${\mathrm{\Lambda }}_c^{+}\overline{D}$final states, as well as doubly charmed pentaquark states to${\mathrm{\Sigma }}_{c}D$and${\mathrm{\Lambda }}_c^{+}D$, is made using samples of proton-proton collision data corresponding to an integrated luminosity of$5.7{\mathrm{fb}}^{-1}$recorded by the LHCb detector at$\sqrt{s}=13\mathrm{TeV}$. Since no significant signals are found, upper limits are set on the pentaquark yields relative to that of the${\mathrm{\Lambda }}_c^{+}$baryon in the${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$decay mode. The known pentaquark states are also investigated, and their signal yields are found to be consistent with zero in all cases.

   © 2024 CERN, for the LHCb Collaboration2024CERN 

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5. Evidence for B+→K+νν¯ decays

   https://doi.org/10.1103/PhysRevD.109.112006  

   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 search for the rare decay$B^{+}\to K^{+}\nu \overline{\nu }$in a$362{\mathrm{fb}}^{-1}$sample of electron-positron collisions at the$\mathrm{\Upsilon }(4S)$resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying$B$meson in$\mathrm{\Upsilon }(4S)\to B\overline{B}$events to suppress background from other decays of the signal$B$candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying$B$meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the$B^{+}\to K^{+}\nu \overline{\nu }$branching fraction of$[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst})]\times {10}^{-5}$and$[{1.1}_{-0.8}^{+0.9}(\mathrm{stat}{)}_{-0.5}^{+0.8}(\mathrm{syst})]\times {10}^{-5}$, respectively. Combining the results, we determine the branching fraction of the decay$B^{+}\to K^{+}\nu \overline{\nu }$to be$[2.3\pm 0.5(\mathrm{stat}{)}_{-0.4}^{+0.5}(\mathrm{syst})]\times {10}^{-5}$, providing the first evidence for this decay at 3.5 standard deviations. The combined result is 2.7 standard deviations above the standard model expectation.

   Published by the American Physical Society2024 

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