We report evidence for a narrow structure, $X(5568)$, in the decay sequence $X(5568)\to B_s^0{\pi }^{\pm }$, $B_s^0\to J/\psi \varphi$, $J/\psi \to {\mu }^{+}{\mu }^{-}$, $\varphi \to K^{+}{K}^{-}$. This is evidence for the first instance of a hadronic state with valence quarks of four different flavors. The mass and natural width of this state are measured to be $m=5567.8\pm 2.9{(\mathrm{stat})}_{-1.9}^{+0.9}(\mathrm{syst})\mathrm{MeV}/c^2$ and $\mathrm{\Gamma }=21.9\pm 6.4{(\mathrm{stat})}_{-2.5}^{+5.0}(\mathrm{syst})\mathrm{MeV}/c^2$. If the decay is $X(5568)\to B_s^{*}{\pi }^{\pm }\to B_s^0\gamma {\pi }^{\pm }$ with an unseen $\gamma$, $m\mathbf{(}X(5568)\mathbf{)}$ will be shifted up by $m(B_s^{*})-m(B_s^0)\sim 49\mathrm{MeV}/c^2$. This measurement is based on $10.4{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collision data at $\sqrt{s}=1.96\mathrm{TeV}$ collected by the D0 experiment at the Fermilab Tevatron collider.

On 23rd November 2009, during the early commissioning of the CERN Large Hadron Collider (LHC), two counter-rotating proton bunches were circulated for the first time concurrently in the machine, at...

Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools/reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. The content and structure of this report are guided by the scope and priorities defined in the mandate of the FCC Feasibility Study. It is therefore not intended to serve as an exhaustive review of the full physics potential of FCC. Several topics, already covered in earlier reports such as the FCC CDR, are not reiterated here or are addressed only briefly, in alignment with the study’s focus. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

The ${\Lambda }_b(udb)$ baryon is observed in the decay ${\Lambda }_b\to J/\psi \Lambda$ using $6.1{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions collected with the D0 detector at $\sqrt{s}=1.96\mathrm{TeV}$. The production fraction multiplied by the branching fraction for this decay relative to that for the decay $B^0\to J/\psi K_s^0$ is measured to be $0.345\pm 0.034(\mathrm{stat})\pm 0.033(\mathrm{syst})\pm 0.003(\mathrm{PDG})$. Using the world average value of $f(b\to B^0)·\mathcal{B}(B^0\to J/\psi K_s^0)=(1.74\pm 0.08)\times {10}^{-5}$, we obtain $f(b\to {\Lambda }_b)·\mathcal{B}({\Lambda }_b\to J/\psi \Lambda )=(6.01\pm 0.60(\mathrm{stat})\pm 0.58(\mathrm{syst})\pm 0.28(\mathrm{PDG}))\times {10}^{-5}$. This measurement represents an improvement in precision by about a factor of 3 with respect to the current world average.

We measure the ${\Lambda }_b^0$ lifetime in the fully reconstructed decay ${\Lambda }_b^0\to J/\psi {\Lambda }^0$ using $10.4{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions collected with the D0 detector at $\sqrt{s}=1.96\mathrm{TeV}$. The lifetime of the topologically similar decay channel $B^0\to J/\psi K_S^0$ is also measured. We obtain $\tau ({\Lambda }_b^0)=1.303\pm 0.075(\mathrm{stat})\pm 0.035(\mathrm{syst})\mathrm{ps}$ and $\tau (B^0)=1.508\pm 0.025(\mathrm{stat})\pm 0.043(\mathrm{syst})\mathrm{ps}$. Using these measurements, we determine the lifetime ratio of $\tau ({\Lambda }_b^0)/\tau (B^0)=0.864\pm 0.052(\mathrm{stat})\pm 0.033(\mathrm{syst})$.

We perform a search for the rare decay $B_s^0\to {\mu }^{+}{\mu }^{-}$ using data collected by the D0 experiment at the Fermilab Tevatron Collider. This result is based on the full D0 Run II data set corresponding to $10.4{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$. We use a multivariate analysis to increase the sensitivity of the search. In the absence of an observed number of events above the expected background, we set an upper limit on the decay branching fraction of $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})<15\times {10}^{-9}(12\times {10}^{-9})$ at the 95% (90%) C.L.