FEATURES PIONIC DEUTERIUM Pionic Deuterium l Detlev Gotta - DOI: 10.1051/epn/2012303 l Institut für Kernphysik (IKP) & Jülich Centre for Hadron Physics (JCHP), l Forschungszentrum Jülich, 52425 Jülich, Germany Atoms formed after Coulomb capture of negatively charged pions rapidly develop to nuclear dimensions and eventually decay by strong interaction. The strong force affects energy and line width of X-rays emitted during a de-excitation cascade which allows precise measurements of hadronic quanti- ties by means of crystal spectrometers. Pionic deuterium - involving the lightest composite nucleus - constitutes a bridge between particle and nuclear physics. he close relation between particle and nu- (χPT) - the modern low-energy approach of QCD - ex- clear physics is evident from prediction and ploit chiral symmetry founded on the small masses of discovery of mesons (medium heavy parti- the light quarks u, d and s. In a perturbative approach, cles) mediating the short-range nuclear force. where order parameters are momenta, pion mass, and fine TThe lightest one – the pion – though 270 times heavier structure constant α, strong and electromagnetic interac- than an electron is still light on the typical nuclear scale tions are treated on the same footing [1-4]. of 1 GeV/c2, and its interaction with the nucleon is a corner-stone in understanding the nuclear force. Exotic atoms The description is based on quantum chromodynamics The existence of negatively charged muons (a “heavy“ elec- (QCD), where elementary particles and forces are quarks tron), mesons, and antiprotons suggested immediately the and gluons which, however, are not realised as individual formation of atomic systems with a nucleus A(Z,N) by m Length and time particles in nature (confinement). Nevertheless, sym- eliminating electrons (Box 1). Such exotic atoms were pre- scales of pion capture metries of QCD determine properties of the directly dicted in the late 1940s and verified experimentally soon at a deuterium mole- cule, subsequent de- accessible, but composite, strongly interacting parti- after at accelerator facilities. Later on, systematic and high- excitation cascade, cles (hadrons) – the mesons, nucleons as well as nuclei. statistics precision studies became possible at meson fac- and nuclear decay Effective field theories like chiral perturbation theory tories and dedicated low-energy antiproton storage rings. 28 EPN 43/3 Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn/2012303 PIONIC DEUTERIUM FEATURES Exotic atoms are created as highly excited systems. Dur- aπ-p→π-p - aπ+p→π+p = -√2•aπ-p→πon and ing an atomic cascade, de-excitation occurs among others aπ+p→π+p = aπ-n→π-n . by Auger electron and X-ray emission, which allows a In pionic hydrogen (πH), energy shift ε and level broad- variety of studies [5]: ening Γ of the atomic ground state 1s can be attributed • mass and spin of the captured particle, to the elementary processes elastic and charge exchange • nuclear moments (from muonic atoms), scattering, respectively [6]: • test of bound-state quantum electrodynamics, ε1s (πH) aπ-p→π-p + … 2 • behaviour of excited many-electron systems, Γ1s (πH) (aπ-p→πon) + … • muon catalyzed fusion, and For pionic deuterium, the shift is due to the coherent sum • the strong force at low energies by measuring shift and of proton and neutron scattering broadening of low-lying atomic levels. ε1s (πD) aπ-p→π-p + aπ-n→π-n + … Exotic hydrogen plays an outstanding role as it directly Ellipses stand for the strong (owing to the differ- probes the elementary hadron-proton system and - using ent mass of u and d quark) and the electromagnetic isotopes - additionally the interaction with the neutron. isospin symmetry violation. Isospin and charge-symmetry Pion-nucleon scattering breaking corrections are quan- Neglecting the electric charge, (almost) identical strong tifiable consistently by the interaction properties of proton and neutron led to the methods of χPT [6]. Multiple Exotic atoms allow concept of isospin symmetry. Nowadays, it is identified scattering and nuclear struc- 'scattering experiments' with the (approximate) symmetry of u and d quarks ture effects, as occurring in “ at relative energy 'zero' building up the nucleons. the case of πD, are well under ±,0 In terms of isospin, three pions π (π ) and two nucle- control [9].Hence, ε1s(πD) con- ” ons N (p,n) combine (analogue to spins) to 1/2 and 3/2 stitutes a decisive constraint on the scattering lengths states. Therefore, in the limit of isospin invariance two dif- as obtained from πH, particularly, as chiral symmetry ferent scattering lengths aπN are sufficient to determine all requires one of the isospin amplitudes - the isoscalar + πN→πN reactions. For experimentally important chan- scattering length a aπ-p→π-p+aπ+p→π+p - to vanish in nels, isospin conservation and charge symmetry yield leading order [1,2]. In other words, the π-p and π+p BOX 1: Characteristics OF exotic atoms Slowed down to kinetic en- 3 fm for Z=1 to 82 (µ denotes the the electron cloud of neigh- life time Δt of low-lying atomic ergies of a few eV, negatively reduced mass of particle and nu- bouring atoms, where the nu- states which manifests itself in charged muons, pions, kaons, cleus). Binding energies increase clear Coulomb field induces an additional line broadening or antiprotons (x-) are captured - compared to electronic atoms - an s-state admixture in higher Γ according to the uncertainty in the Coulomb field of a nu- to a few keV for exotic hydrogen l states. Consequently, for relation Γ•Δt ≈ ħ. cleus A(Z,N) at approximately (Z=1) up to MeV for heavy nuclei strongly interacting particles The shift is attributed in lead- the outmost electron (e) shells. like lead (Z=82). the atomic cascade depletes ing order to elastic hadron- Due to the large mass ratio mx / because of inelastic nuclear nucleus scattering and the me , the subsequent quantum De-excitation cascade reactions from the begin- broadening accounts for ine- cascade starts from highly In medium and high Z atoms, ning and X-ray yields depend lastic nuclear reactions. In this excited atomic states. For ex- de-excitation is very fast main- strongly on density. In pionic way, for atomic s-states, εns and ample, in pionic hydrogen and ly due to internal processes. It hydrogen and deuterium, typi- Γns measure real and imaginary antiprotonic xenon, initial prin- proceeds at the fs scale pref- cal yields are a few per cent for part of the effective complex ciple quantum numbers n are erentially by Auger electron Lyman series (Fig. 1). hadron-nucleus scattering distributed around 16 and 200, emission in the upper and X- length aπA [10] respectively, occupying there radiation in the lower part of Strong-interaction effects iΓns 4 En most of the possible angular the cascade. Collisional effects, The small distances enhance εns- — =-–•—•aπA 2 n R momentum states l. e.g., electron refilling may oc- significantly the overlap of the b The mass dependence of cur in dense media. wave function of orbiting par- Atomic binding energies are binding energies En and radii By contrast, the exotic hydro- ticle and nucleus. In hadronic well below the typical hadron- rnl reveals the dimensions of gen cascade, assumed to de- atoms - systems formed with ic scale of 1 GeV. Hence, exotic such atoms: velop within a few 100 ps, is pions, kaons, or antiprotons - atoms constitute a laboratory 2 2 2 2 En = µc α Z /n dominated by collisional de- the nuclear force contributes for "scattering experiments" at 2 rnl = RB•[3n - l(l + 1)]/2. excitation already at low densi- to the binding energy observ- relative energy "zero" without - Exotic-atom Bohr radii RB = ħc/ ties. The electrically neutral x H able as an X-ray line shift ε. the need to extrapolate cross µc2αZ range from about 50 to system penetrates deeply into Nuclear reactions reduce the section data. EPN 43/3 29 FEATURES PIONIC DEUTERIUM outstanding 40% because of the smallness of a+ [11]. Verification via πD and πH precision experiments [7,8] constitutes an important success within the framework of χPT [9]. Pion production and absorption For pion absorption, πNN→NN, energy-momentum con- servation requires at least A=2 nuclei. In πD, as charge exchange is suppressed by isospin conservation, the level broadening Γ1s(πD) is not generated by πN scattering, in contrast to the πH case. Γ1s(πD) is dominated by true absorption π-d→nn. About 25% of the decays are due to radiative capture π-d→nnγ. Exploiting time reversal invariance and charge symmetry, i.e., assuming for matrix elements |Aπ-d→nn| = |Aπ+d→pp|, the broadening measures the s-wave pion production strength α [7]: 2 Γ1s(πD) |App→π+d| α . Given that exchange of virtual pions essentially contrib- utes to nuclear binding, α constitutes an important quan- tity in the description of nuclei in terms of elementary processes [12]. m FIG. 1: Low-lying atomic levels in pionic deuterium and recent experimental results [7]. The shift ε1s is obtained by comparing the measured X-ray energy with the (calculated) pure Experimental approach electromagnetic value. The broadening Γ1s is extracted from the line shape. The small (negative) High-intensity and low-energy pion beams of up to value for ε1s(πD) is a consequence of chiral symmetry resulting in a minor repulsive interaction. 8 For comparison, latest results given for pionic hydrogen are ε1s(πH) = + 7120 ± 11 meV several 10 pions per second are available at the meson - (indicating the strongly attractive π p interaction) and Γ1s(πH) = 823 ± 19 meV [8]. factory of the Paul Scherrer Institut [13]. Exotic atom formation was optimized by means of the cyclotron trap interaction is of identical magnitude but opposite [14], a superconducting split coil magnet, which winds up in sign.