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Register a new userMeasurement of the $pi K$ atom lifetime and the $pi K$ scattering length
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After having announced the statistically significant observation (5.6~$sigma$) of the new exotic $pi K$ atom, the DIRAC experiment at the CERN proton synchrotron presents the measurement of the corresponding atom lifetime, based on the full $pi K$ data sample: $tau = (5.5^{+5.0}_{-2.8}) cdot 10^{-15}s$. By means of a precise relation ($<1%$) between atom lifetime and scattering length, the following value for the S-wave isospin-odd $pi K$ scattering length $a_0^{-}~=~frac{1}{3}(a_{1/2}-a_{3/2})$ has been derived: $left|a_0^-right| = (0.072^{+0.031}_{-0.020}) M_{pi}^{-1}$.
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The results of a search for hydrogen-like atoms consisting of $pi^{mp}K^{pm}$ mesons are presented. Evidence for $pi K$ atom production by 24 GeV/c protons from CERN PS interacting with a nickel target has been seen in terms of characteristic $pi K$ pairs from their breakup in the same target ($178 pm 49$) and from Coulomb final state interaction ($653 pm 42$). Using these results the analysis yields a first value for the $pi K$ atom lifetime of $tau=(2.5_{-1.8}^{+3.0})$ fs and a first model-independent measurement of the S-wave isospin-odd $pi K$ scattering length $left|a_0^-right|=frac{1}{3}left|a_{1/2}-a_{3/2}right|= left(0.11_{-0.04}^{+0.09} right)M_{pi}^{-1}$ ($a_I$ for isospin $I$).
The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogen-like $pi^+pi^-$ atoms, produced by protons hitting a beryllium target. A part of these atoms crossed the gap of 96~mm and got broken up in the 2.1~textmu{}m thick platinum foil. Analysing the observed number of atomic pairs, $n_A^L= left.436^{+157}_{-61}right|_mathrm{tot}$, the lifetime of the 2$p$ state is found to be ${tau_{2p}=(left.0.45^{+1.08}_{-0.30}right|_mathrm{tot}) cdot10^{-11}}$s, not contradicting the corresponding QED $2p$ state lifetime ${tau_{2p}^mathrm{QED}=1.17 cdot 10^{-11}}$s. This lifetime value is three orders of magnitude larger than our previously measured value of the $pi^+pi^-$ atom ground state lifetime $tau=(left.3.15^{+0.28}_{-0.26}right|_mathrm{tot})cdot 10^{-15}$s. Further studies of long-lived $pi^+pi^-$ atoms will allow to measure energy differences between $p$ and $s$ atomic states and so to determine $pipi$ scattering lengths with the aim to check QCD predictions.
The branching fractions of the doubly Cabibbo-suppressed decays $D^+rightarrow K^-K^+K^+$, $D^+rightarrow pi^-pi^+K^+$ and $D^+_srightarrowpi^-K^+K^+$ are measured using the decays $D^+rightarrow K^-pi^+pi^+$ and $D^+_srightarrow K^-K^+pi^+$ as normalisation channels. The measurements are performed using proton-proton collision data collected with the LHCb detector at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 2.0 fb$^{-1}$. The results are begin{align} frac {mathcal{B}(D^+rightarrow K^-K^+K^+)} {mathcal{B}(D^+rightarrow K^-pi^+pi^+)}& = (6.541 pm 0.025 pm 0.042) times 10^{-4}, onumber frac {mathcal{B}(D^+rightarrow pi^-pi^+K^+)} {mathcal{B}(D^+rightarrow K^-pi^+pi^+)}& = (5.231 pm 0.009 pm 0.023) times 10^{-3}, onumber frac {mathcal{B}(D^+_srightarrowpi^-K^+K^+)} {mathcal{B}(D^+_srightarrow K^-K^+pi^+)}& = (2.372 pm 0.024 pm 0.025) times 10^{-3}, onumber end{align} where the uncertainties are statistical and systematic, respectively. These are the most precise measurements up to date.
The process $e^+e^- to K^+K^-pi^+pi^-$ has been studied in the center-of-mass energy range from 1500 to 2000,MeV using a data sample of 23 pb$^{-1}$ collected with the CMD-3 detector at the VEPP-2000 $e^+e^-$ collider. Using about 24000 selected events, the $e^+e^- to K^+K^-pi^+pi^-$ cross section has been measured with a systematic uncertainty decreasing from 11.7% at 1500-1600,MeV to 6.1% above 1800,MeV. A preliminary study of $K^+K^-pi^+pi^-$ production dynamics has been performed.
A measurement of the rate for the wrong-sign decay D0 -> K+ pi- pi+ pi- relative to that for the right-sign decay D0 -> K- pi+ pi+ pi- is presented. Using 791 fb-1 of data collected with the Belle detector, we obtain a branching fraction ratio of R_WS = [0.324 +- 0.008 (stat) +- 0.007 (sys)]%. Multiplying this ratio by the world average value for the branching fraction B(D0 -> K- pi+ pi+ pi-) gives a branching fraction B(D0 -> K+ pi- pi+ pi-) = (2.61 +- 0.06 +0.09 -0.08) x 10-4.
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