The adapted DIRAC experiment at the CERN PS accelerator observed for the first time long-lived hydrogenlike π^{+}π^{-} atoms, produced by protons hitting a beryllium target. A part of these atoms ...crossed the gap of 96 mm between the target and a 2.1 μm thick platinum foil, in which most of them dissociated. Analyzing the observed number of atomic pairs, n_{A}^{L}=436_{-61}^{+157}|_{tot}, the lifetime of the 2p state is found to be τ_{2p}=(0.45_{-0.30}^{+1.08}|_{tot})×10^{-11} s, not contradicting the corresponding QED 2p state lifetime τ_{2p}^{QED}=1.17×10^{-11} s. This lifetime value is three orders of magnitude larger than our previously measured value of the π^{+}π^{-} atom ground state lifetime τ=(3.15_{-0.26}^{+0.28}|_{tot})×10^{-15} s. Further studies of long-lived π^{+}π^{-} atoms will allow us to measure energy differences between p and s atomic states and so to discriminate between the isoscalar and isotensor ππ scattering lengths with the aim to check QCD predictions.
The observation of hydrogenlike πK atoms, consisting of π^{-}K^{+} or π^{+}K^{-} mesons, is presented. The atoms are produced by 24 GeV/c protons from the CERN PS accelerator, interacting with ...platinum or nickel foil targets. The breakup (ionization) of πK atoms in the same targets yields characteristic πK pairs, called "atomic pairs," with small relative momenta Q in the pair center-of-mass system. The upgraded DIRAC experiment observed 349±62 such atomic πK pairs, corresponding to a signal of 5.6 standard deviations. This is the first statistically significant observation of the strange dimesonic πK atom.
The results of a search for hydrogen-like atoms consisting of π∓K± mesons are presented. Evidence for πK atom production by 24 GeV/c protons from CERN PS interacting with a nickel target has been ...seen in terms of characteristic πK pairs from their breakup in the same target (178±49) as well as in terms of produced πK atoms (653±42). Using these results, the analysis yields a first value for the πK atom lifetime of τ=(2.5−1.8+3.0) fs and a first measurement of the S-wave isospin-odd πK scattering length |a0−|=13|a1/2−a3/2|=(0.11−0.04+0.09)Mπ−1 (aI for isospin I).
The DIRAC spectrometer installed at CERN PS was upgraded in order to study simultaneously A2π and AπK atoms, namely the bound states of two π mesons, and of π and K mesons, respectively. The detector ...system can now accept a high intensity beam of 2–6×1011 primary protons per second. The electronics and the data-acquisition system can handle a very large amount of data to identify π, K, p, e and µ, allowing the selection of ππ and πK pairs in the offline analysis. The resolution of the longitudinal and transverse components of the relative momentum Q of each meson pair in their center-of-mass system with respect to the direction of the pair was substantially improved. The analysis of their distributions allowed an reliable separation between the meson pairs originating from hadronic atoms and the backgrounds permitting the measurement of the lifetimes of hadronic atoms in the ground state and π–π, π–K s-wave scattering lengths. The upgraded setup also allowed the study of the long-lived excited states of ππ atoms.
After observing and investigating the double-exotic (a double-exotic atom is a bound system, in which both oppositely charged components are unstable particles like μ,π,K,…) π+π− atom with the ground ...state lifetime τ of about 3×10−15 s, the upgraded DIRAC experiment at the CERN PS accelerator observes for the first time long-lived states of the same atom with lifetimes of about 10−11 s and more. The number of characteristic pion pairs resulting from the breakup (ionisation) of long-lived π+π− atoms amounts to 436±61, corresponding to a signal-to-error ratio of better than 7 standard deviations. This observation opens a new possibility to measure energy differences between p and s atomic states and so to determine ππ scattering lengths.
The DIRAC experiment at CERN has achieved a sizeable production of Ie super(+Ie) super(-) atoms and has significantly improved the precision on its lifetime determination. From a sample of 21 227 ...atomic pairs, a 4% measurement of the S-wave IeIe scattering length difference inline image has been attained, providing an important test of Chiral Perturbation Theory.
Phys. Rev. Lett. 122, 082003 (2019) 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 $\pi\pi$
scattering lengths with the aim to check QCD predictions.
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 \(\pi\pi\) scattering lengths with the aim to check QCD predictions.
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 $I$).
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