Primary Populations of Metastable AntiprotonicH4eandH

Masaki Hori

¹

,

J. Eades

²

,

R. S. Hayano

³

et al.

Abstract

Initial distributions of metastable antiprotonic 4 He and 3 He atoms over principal (n) and angular momentum (') quantum numbers have been deduced using laser spectroscopy experiments. The regions n 37-40 and n 35-38 in the two atoms account for almost all of the observed fractions [ 3:0 0:1 % and 2:4 0:1 %] of antiprotons captured into metastable states. DOI: 10.1103/PhysRevLett.89.093401 PACS numbers: 36.10.-k, 25.43.+t, 34.90.+q A negatively charged particle X ÿ (such as ÿ , ÿ , or p) approaching an atom at electron-volt energies can displace one of its electrons, thereby forming an exotic atom [1]. Little is known about the principal (n) and angular momentum (') quantum numbers of the atom's primordial (i.e., initially occupied) states. It has been widely believed since the 1950s [2-4] that in exotic hydrogen and helium atoms, n will be distributed around the value(M being the reduced mass of the X ÿ -nucleus system, and m e the electron mass), which corresponds to the X ÿ orbital with the same radius and binding energy as that of the displaced 1s electron. In this paper, we describe measurements of the n; ' distributions of antiprotons captured into atomic states in metastable antiprotonic helium [5][6][7][8], including the isotopes p 4 He for which n 0 38:3 and p 3 He for which n 0 37:1. These pHe states (see Fig. 1) have microsecond-scale lifetimes against annihilation, because their large ' values (' n ÿ 1) do not permit them to deexcite by Auger emission of the electron. Instead, the atoms slowly undergo a series of constant v n ÿ ' ÿ 1 radiative transitions (i.e., n ' ÿ1). They finally reach states which proceed rapidly to pHe 2 ions, which are immediately destroyed by collisional Stark effects. So far, the numbers of antiprotons populating two metastable states in p 4 He [namely, n; ' 39; 35 and 37; 34 ] have been measured [9,10] using laser spectroscopic methods [11] at the former LEAR facility at CERN. The two states represent only a small fraction of the totality of metastable states shown in Fig. 1; moreover, since for technical reasons [9] the populations could be measured only after a minimum delay of t 1:8 s following atomic formation, the primary populations (those at t 0) could not be reliably estimated. We study here a total of 20 transitions in p 4 He and p 3 He atoms starting at t 300 ns, and estimate the primary populations in nearly all the metastable states.These experiments were made at the Antiproton Decelerator at CERN, the setup [12] being an extension of our former one. Metastable pHe atoms were produced

Populations and lifetimes in thev=n−ℓ−1=2and 3 metastable cascades ofp¯Hori1, Eades2, Widmann3 et al. 2004Phys. Rev. ASelf Cite352230View full textAdd to dashboardCite

“…The measurements were made using pulsed and continuous types of antiproton beam [9,10] supplied by the Low Energy Antiproton Ring ͑LEAR͒ of CERN. Although the experiments reported here ended in 1996 with the closure of LEAR, there is renewed interest in these unpublished data, in light of recent results [11,12] produced at CERN's Antiproton Decelerator ͑AD͒.…”

“…Experimental [12][13][14][15][16][17][18][19] and theoretical studies have shown that five phases can be discerned in the life history of metastable pHe + . (i) Capture: when an antiproton collides with a helium atom at electron-volt energies, it may replace one of the electrons orbiting the helium nucleus, and become captured into a pHe + state.…”

“…(i) Capture: when an antiproton collides with a helium atom at electron-volt energies, it may replace one of the electrons orbiting the helium nucleus, and become captured into a pHe + state. This occurs [12,[20][21][22][23][24][25][26][27][28] with the highest probability into states with n-values of about n ϳ n 0 ϵ ͱ M * /m e = 38.3…”

Precise laser spectroscopy of antiprotonic helium at CERN’s Antiproton Decelerator

“…2). Some of the antiprotons replaced the electrons in normal He and were captured [14] into states with principal quantum number n 0 = √ M * /m e , wherein M * denotes the reduced We induced laser transitions [18] from these metastable states, to states with nanosecond-scale lifetimes (wavy lines) against Auger electron emission [19,20]. Singly-charged, two-body pHe 2+ ions [21] remained after Auger decay.…”

Development of precise nanosecond and picosecond dye lasers for spectroscopy of antiprotonic atoms

“…Some ∼ 20 transition frequencies of the p 4 He + and p 3 He + isotopes [2,3], and the populations [11,12] and Auger rates [13] of the associated states were systematically measured using the laser.…”