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  1. Abstract

    Einstein’s general theory of relativity from 19151remains the most successful description of gravitation. From the 1919 solar eclipse2to the observation of gravitational waves3, the theory has passed many crucial experimental tests. However, the evolving concepts of dark matter and dark energy illustrate that there is much to be learned about the gravitating content of the universe. Singularities in the general theory of relativity and the lack of a quantum theory of gravity suggest that our picture is incomplete. It is thus prudent to explore gravity in exotic physical systems. Antimatter was unknown to Einstein in 1915. Dirac’s theory4appeared in 1928; the positron was observed5in 1932. There has since been much speculation about gravity and antimatter. The theoretical consensus is that any laboratory mass must be attracted6by the Earth, although some authors have considered the cosmological consequences if antimatter should be repelled by matter7–10. In the general theory of relativity, the weak equivalence principle (WEP) requires that all masses react identically to gravity, independent of their internal structure. Here we show that antihydrogen atoms, released from magnetic confinement in the ALPHA-g apparatus, behave in a way consistent with gravitational attraction to the Earth. Repulsive ‘antigravity’ is ruled out in this case. This experiment paves the way for precision studies of the magnitude of the gravitational acceleration between anti-atoms and the Earth to test the WEP.

     
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  2. Abstract The positron, the antiparticle of the electron, predicted by Dirac in 1931 and discovered by Anderson in 1933, plays a key role in many scientific and everyday endeavours. Notably, the positron is a constituent of antihydrogen, the only long-lived neutral antimatter bound state that can currently be synthesized at low energy, presenting a prominent system for testing fundamental symmetries with high precision. Here, we report on the use of laser cooled Be + ions to sympathetically cool a large and dense plasma of positrons to directly measured temperatures below 7 K in a Penning trap for antihydrogen synthesis. This will likely herald a significant increase in the amount of antihydrogen available for experimentation, thus facilitating further improvements in studies of fundamental symmetries. 
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  3. null (Ed.)
    Abstract The photon—the quantum excitation of the electromagnetic field—is massless but carries momentum. A photon can therefore exert a force on an object upon collision 1 . Slowing the translational motion of atoms and ions by application of such a force 2,3 , known as laser cooling, was first demonstrated 40 years ago 4,5 . It revolutionized atomic physics over the following decades 6–8 , and it is now a workhorse in many fields, including studies on quantum degenerate gases, quantum information, atomic clocks and tests of fundamental physics. However, this technique has not yet been applied to antimatter. Here we demonstrate laser cooling of antihydrogen 9 , the antimatter atom consisting of an antiproton and a positron. By exciting the 1S–2P transition in antihydrogen with pulsed, narrow-linewidth, Lyman-α laser radiation 10,11 , we Doppler-cool a sample of magnetically trapped antihydrogen. Although we apply laser cooling in only one dimension, the trap couples the longitudinal and transverse motions of the anti-atoms, leading to cooling in all three dimensions. We observe a reduction in the median transverse energy by more than an order of magnitude—with a substantial fraction of the anti-atoms attaining submicroelectronvolt transverse kinetic energies. We also report the observation of the laser-driven 1S–2S transition in samples of laser-cooled antihydrogen atoms. The observed spectral line is approximately four times narrower than that obtained without laser cooling. The demonstration of laser cooling and its immediate application has far-reaching implications for antimatter studies. A more localized, denser and colder sample of antihydrogen will drastically improve spectroscopic 11–13 and gravitational 14 studies of antihydrogen in ongoing experiments. Furthermore, the demonstrated ability to manipulate the motion of antimatter atoms by laser light will potentially provide ground-breaking opportunities for future experiments, such as anti-atomic fountains, anti-atom interferometry and the creation of antimatter molecules. 
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  4. A<sc>bstract</sc>

    A comprehensive study of the local and nonlocal amplitudes contributing to the decayB0K*0(→K+π)μ+μis performed by analysing the phase-space distribution of the decay products. The analysis is based onppcollision data corresponding to an integrated luminosity of 8.4 fb1collected by the LHCb experiment. This measurement employs for the first time a model of both one-particle and two-particle nonlocal amplitudes, and utilises the complete dimuon mass spectrum without any veto regions around the narrow charmonium resonances. In this way it is possible to explicitly isolate the local and nonlocal contributions and capture the interference between them. The results show that interference with nonlocal contributions, although larger than predicted, only has a minor impact on the Wilson Coefficients determined from the fit to the data. For the local contributions, the Wilson Coefficient$$ {\mathcal{C}}_9 $$C9, responsible for vector dimuon currents, exhibits a 2.1σdeviation from the Standard Model expectation. The Wilson Coefficients$$ {\mathcal{C}}_{10} $$C10,$$ {\mathcal{C}}_9^{\prime } $$C9and$$ {\mathcal{C}}_{10}^{\prime } $$C10are all in better agreement than$$ {\mathcal{C}}_9 $$C9with the Standard Model and the global significance is at the level of 1.5σ. The model used also accounts for nonlocal contributions fromB0→ K*0[τ+τ→ μ+μ] rescattering, resulting in the first direct measurement of thebsττvector effective-coupling$$ {\mathcal{C}}_{9\tau } $$C9τ.

     
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    Free, publicly-accessible full text available September 1, 2025
  5. A search for hidden-charm pentaquark states decaying to a range ofΣcD¯andΛc+D¯final states, as well as doubly charmed pentaquark states toΣcDandΛc+D, is made using samples of proton-proton collision data corresponding to an integrated luminosity of5.7fb1recorded by the LHCb detector ats=13TeV. Since no significant signals are found, upper limits are set on the pentaquark yields relative to that of theΛc+baryon in theΛc+pKπ+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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    Free, publicly-accessible full text available August 1, 2025
  6. A<sc>bstract</sc>

    A search for the fully reconstructed$$ {B}_s^0 $$Bs0→ μ+μγdecay is performed at the LHCb experiment using proton-proton collisions at$$ \sqrt{s} $$s= 13 TeV corresponding to an integrated luminosity of 5.4 fb1. No significant signal is found and upper limits on the branching fraction in intervals of the dimuon mass are set$$ {\displaystyle \begin{array}{cc}\mathcal{B}\left({B}_s^0\to {\mu}^{+}{\mu}^{-}\gamma \right)<4.2\times {10}^{-8},& m\left({\mu}^{+}{\mu}^{-}\right)\in \left[2{m}_{\mu },1.70\right]\textrm{GeV}/{c}^2,\\ {}\mathcal{B}\left({B}_s^0\to {\mu}^{+}{\mu}^{-}\gamma \right)<7.7\times {10}^{-8},&\ m\left({\mu}^{+}{\mu}^{-}\right)\in \left[\textrm{1.70,2.88}\right]\textrm{GeV}/{c}^2,\\ {}\mathcal{B}\left({B}_s^0\to {\mu}^{+}{\mu}^{-}\gamma \right)<4.2\times {10}^{-8},& m\left({\mu}^{+}{\mu}^{-}\right)\in \left[3.92,{m}_{B_s^0}\right]\textrm{GeV}/{c}^2,\end{array}} $$BBs0μ+μγ<4.2×108,mμ+μ2mμ1.70GeV/c2,BBs0μ+μγ<7.7×108,mμ+μ1.70, 2.88GeV/c2,BBs0μ+μγ<4.2×108,mμ+μ3.92mBs0GeV/c2,

    at 95% confidence level. Additionally, upper limits are set on the branching fraction in the [2mμ,1.70] GeV/c2dimuon mass region excluding the contribution from the intermediateϕ(1020) meson, and in the region combining all dimuon-mass intervals.

     
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    Free, publicly-accessible full text available July 1, 2025
  7. The LHCb upgrade represents a major change of the experiment. The detectors have been almost completely renewed to allow running at an instantaneous luminosity five times larger than that of the previous running periods. Readout of all detectors into an all-software trigger is central to the new design, facilitating the reconstruction of events at the maximum LHC interaction rate, and their selection in real time. The experiment's tracking system has been completely upgraded with a new pixel vertex detector, a silicon tracker upstream of the dipole magnet and three scintillating fibre tracking stations downstream of the magnet. The whole photon detection system of the RICH detectors has been renewed and the readout electronics of the calorimeter and muon systems have been fully overhauled. The first stage of the all-software trigger is implemented on a GPU farm. The output of the trigger provides a combination of totally reconstructed physics objects, such as tracks and vertices, ready for final analysis, and of entire events which need further offline reprocessing. This scheme required a complete revision of the computing model and rewriting of the experiment's software. 
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    Free, publicly-accessible full text available May 1, 2025
  8. The production of 𝜂 and 𝜂′ mesons is studied in proton-proton and proton-lead collisions collected with the LHCb detector. Proton-proton collisions are studied at center-of-mass energies of 5.02 and 13TeV and proton-lead collisions are studied at a center-of-mass energy per nucleon of 8.16TeV. The studies are performed in center-of-mass (c.m.) rapidity regions 2.5<𝑦c.m.<3.5 (forward rapidity) and −4.0<𝑦c.m.<−3.0 (backward rapidity) defined relative to the proton beam direction. The 𝜂 and 𝜂′ production cross sections are measured differentially as a function of transverse momentum for 1.5<𝑝T<10GeV and 3<𝑝T<10GeV, respectively. The differential cross sections are used to calculate nuclear modification factors. The nuclear modification factors for 𝜂 and 𝜂′ mesons agree at both forward and backward rapidity, showing no significant evidence of mass dependence. The differential cross sections of 𝜂 mesons are also used to calculate 𝜂/𝜋0 cross-section ratios, which show evidence of a deviation from the world average. These studies offer new constraints on mass-dependent nuclear effects in heavy-ion collisions, as well as 𝜂 and 𝜂′ meson fragmentation. 
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  9. Garisto, R (Ed.)
    The ratios of branching fractions R(D*)= B(B0 --> D*+tau- nu(bar))/ B(B0--> D*+mu- nu(bar)) and R(D)= B(B0 --> D0tau- nu(bar))/ B(B0 --> D0mu- nu(bar)) are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to 3.0 fb−1 of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode τ− → μ−ντν¯μ. The measured values are R*D*)= 0.281+/- 0.018+/- 0.024 and R(D0)=0.441+/- 0.060+/- 0.066, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is ρ= −0.43. The results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the standard model 
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