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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2026). Search for KS(L)0 → π+ π- μ+ μ- decays at LHCb. Phys. Rev. D, 113(7), 072001–13pp.
Abstract: A search for K-S(L)(0) -> pi(+) pi(-) mu(+) mu(-) decays is performed using proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb(-1). No K-S(L)(0) -> pi(+) pi(-) mu(+) mu(-) signals are found and upper limits are set for the first time on the branching fractions B(K-S(0) -> pi(+) pi(-) mu(+) mu(-)) < 1.4 x 10(-9) and B(K-L(0) -> pi(+) pi(-) mu(+) mu(-)) < 6.6 x 10(-7), at the 90% confidence level.
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Pellumaj, J. et al, Valiente-Dobon, J. J., & Perez-Vidal, R. M. (2026). Development of deformed intruder structures along the N=50 magic number: Insights from 83Se. Phys. Lett. B, 876, 140432–7pp.
Abstract: The 83Se nucleus was studied in the context of shape coexistence employing a 82Se(d,p)83Se reaction performed in inverse kinematics with the GALILEO-II + SPIDER setup to reach the desired selectivity on the reaction channel of interest via y-particle coincidences. The lifetimes of the 1/2+ 540-keV and 3/2+3 1100-keV low-lying intruder states of 83Se were measured for the first time by employing the Recoil Distance Doppler Shift and the Doppler Shift Attenuation Method. The deduced transition probabilities, together with the mixing ratios obtained from a separate neutron-capture experiment performed at ILL with the FIPPS array, are compared and discussed in the framework of shell-model calculations performed with the PFSDG-U Hamiltonian. Large transition probabilities of hundreds of e2fm4 were found for the E2 decays of the states built on top of the intruders, pointing to the development of quadrupole collectivity and large deformation in the proximity of N = 50 shell closure.
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Miernik, K. et al, & Algora, A. (2026). γ spectroscopy of fission fragments from the 12C+182W fusion-fission reaction. Phys. Rev. C, 113(4), 044624–12pp.
Abstract: Background: The fission character changes along the mercury isotopic chain, from asymmetric (180Hg) through slightly asymmetric (190Hg) to symmetric (198Hg). Mercury isotopes have been studied using various techniques including 9-delayed fission, Coulomb-induced fission, and fusion-fission, but the isotope 194Hg has not been investigated until now. Purpose: To experimentally study the fusion-fission reaction of 194Hg at moderate excitation energy and to determine previously unknown independent fission yields and properties of emitted neutrons and y rays. Method: The compound nucleus was formed in the reaction 12C + 182W. Prompt gamma rays emitted during the reaction were measured with the v-ball2 spectrometer. Independent fission yields of even-even nuclei were determined by detecting gamma-gamma cascades in the fission fragments and performing a maximum-likelihood analysis. The number of emitted neutrons was determined from the fragment distribution, as well as from the study of fission partners. Average momentum carried by the y rays was determined by the Manchester method. Additionally, the long-term 9 activity of fragment residues was measured as a complementary method. Results: The measured fission-fragment mass distribution is consistent with that of neighboring mercury isotopes, confirming the gradual change along the isotopic chain and influence of the Z = 36 deformed proton shell in the light fragment. The independent fragment yield and distribution of fission partners is well described by the GEF model. However, there are discrepancies concerning the angular momentum of the fragments and the width of the mass distribution, confirmed by the independent residual activity measurements. Conclusions: Our results demonstrate that the method based on fusion-fission and y spectroscopy is a valuable alternative to direct mass or charge measurements. Independent fission yields, correlation of fission partners, and the study of angular momentum of fragments open the possibility of a comprehensive description of the process. This is important both from a theoretical and experimental point of view and is needed to achieve further refinement of our understanding of the fission process.
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ATLAS Collaboration(Aad, G. et al), Ahuja, S., Aikot, A., Amos, K. R., Bouchhar, N., Cabrera Urban, S., et al. (2026). Measurement of coherent exclusive J/ψ → μ+ μ- production in ultraperipheral Pb plus Pb collisions at √sNN=5.36 TeV with the ATLAS detector. J. High Energy Phys., 04(4), 020–47pp.
Abstract: The ATLAS experiment has performed a measurement of coherent exclusive J/psi -> mu(+) mu(-) production in ultraperipheral Pb+Pb collisions at root s(NN) = 5.36 TeV. The data was recorded at the Large Hadron Collider (LHC) during 2023, and corresponds to an integrated luminosity of 79 μb(-1). Exclusive J/psi candidates were selected with a dedicated track-sensitive trigger based on the ATLAS transition radiation tracker. The analysis involves reconstruction of the dimuon invariant mass based on muon tracks from the inner detector, as the muon transverse momentum range of interest precludes the use of the standard muon reconstruction and identification algorithms. Differential cross sections are measured as a function of J/psi rapidity and are compared with theoretical predictions. After extrapolation to root s(NN) = 5.02 TeV, they are also compared with previous measurements performed by other experiments using data from LHC Run 2. While the results agree reasonably well with theoretical predictions, they are in tension with previous Run-2 results for the central rapidity region.
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Berbig, M. (2026). Type II Seesaw Leptogenesis in a Majoron background. J. High Energy Phys., 04(4), 116–65pp.
Abstract: We discuss spontaneous Leptogenesis in the Type II Seesaw model of neutrino masses featuring an electroweak triplet scalar T in a coherent pseudo Nambu-Goldstone boson (pNGB) background. In the “wash-in” scenario the inverse decays of Higgs bosons to T generate a chemical potential for the triplet, that is then transmitted to the lepton sector via the leptonic decays of T. Our mechanism works with a single triplet, that can be as light as 1 TeV, and has a vacuum expectation value VT in the window O (1 keV) < vT < O (1 MeV). This range of VT can lead to appreciable decays of the triplet's doubly charged component into both same sign di-leptons and same sign pairs of W-bosons, which could potentially allow for an experimental distinction from a recently proposed inflationary Type II Seesaw Affleck-Dine scenario preferring the leptonic mode. In the “singlet-doublet-triplet Majoron” UV-completion of the Type II Seesaw model, the required pNGB is automatically included in the form of the Majoron, that originates from the phase of the lepton number breaking singlet scalar. The coherent motion of the Majoron can furthermore explain the dark matter relic abundance via the kinetic misalignment mechanism. Cogenesis of dark matter and the baryon asymmetry can work for a lepton number breaking scale of O(10(5) GeV) < v(sigma) < O(10(8) GeV) and a Majoron mass of O(1 eV) > m(j )> O (1 & micro;eV).
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