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Register a new userFirst measurement of a long-lived $pi^+ pi^-$ atom lifetime
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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.
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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: $left|a_0^-right| = (0.072^{+0.031}_{-0.020}) M_{pi}^{-1}$.
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 COMPASS collaboration at CERN has investigated the pi^- gamma -> pi^- pi^- pi^+ reaction at center-of-momentum energy below five pion masses, sqrt(s) < 5 m(pi), embedded in the Primakoff reaction of 190 GeV pions impinging on a lead target. Exchange of quasi-real photons is selected by isolating the sharp Coulomb peak observed at smallest momentum transfers, t < 0.001 (GeV/c)^2. Using partial-wave analysis techniques, the scattering intensity of Coulomb production described in terms of chiral dynamics and its dependence on the 3pi-invariant mass m(3pi) = sqrt(s) were extracted. The absolute cross section was determined in seven bins of $sqrt{s}$ with an overall precision of 20%. At leading order, the result is found to be in good agreement with the prediction of chiral perturbation theory over the whole energy range investigated.
Dalitz plot distribution of the $etatopi^+pi^-pi^0$ decay is determined using a data sample of $1.2cdot 10^7$ $eta$ mesons from $pdto ^3textrm{He}eta$ reaction at 1 GeV collected by the WASA detector at COSY.
Using $1.6$ fb$^{-1}$ of $e^+ e^-tophitoetagamma$ data collected with the KLOE detector at DA$Phi$NE, the Dalitz plot distribution for the $eta to pi^+ pi^- pi^0$ decay is studied with the worlds largest sample of $sim 4.7 cdot 10^6$ events. The Dalitz plot density is parametrized as a polynomial expansion up to cubic terms in the normalized dimensionless variables $X$ and $Y$. The experiment is sensitive to all charge conjugation conserving terms of the expansion, including a $gX^2Y$ term. The statistical uncertainty of all parameters is improved by a factor two with respect to earlier measurements.
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