Evidence For The Production Of Slow Antiprotonic Hydrogen In Vacuum

We present evidence showing how antiprotonic hydrogen, the quasistable antiproton ($\overline{p}$)-proton bound system, has been synthesized following the interaction of antiprotons with the molecular ion ${\mathrm{H}}_2^{+}$ in a nested Penning trap environment. From a careful analysis of the spatial distributions of antiproton annihilation events, evidence is presented for antiprotonic hydrogen production with sub-eV kinetic energies in states around $n=70$, and with low angular momenta. The slow antiprotonic hydrogen may be studied using laser spectroscopic techniques.

Figure 1

$r\mathrm{\text{−}}z$ scatter plot and radial densities $(\frac{1}{r}\frac{dN}{dr})$ of the annihilation vertices for: (a) hot mixing; (b) cold mixing. The dashed black line indicates the position of the trap wall; the red semiellipse shows the section of the $e^{+}$ plasma. The green radial densities are for the corresponding central $z$-region events (inside the green lines) while the data in blue are for the 2 lateral $z$-regions, normalized for $r>1.25\mathrm{cm}$.

Figure 2

Experimental radial distribution of $\overline{p}$ annihilation vertices (black histogram) for HM ($-1.5\mathrm{cm}<z<1.5\mathrm{cm}$) with a Monte Carlo simulation $T_e=8000\mathrm{K}$, $v_{\mathrm{th}}=5600{\mathrm{ms}}^{-1}$, generation on the surface of a spheroid with $z_p=16\mathrm{mm}$ and $r_p=1\mathrm{mm}$ rotating with a frequency of 300 kHz, i.e., $v_{\mathrm{tang}}=2000{\mathrm{ms}}^{-1}$); see text for details. Results of simulations with different mean lifetimes are shown: green, $\tau =0.8\mu \mathrm{s}$ (${\chi }_{\mathrm{red}}^2=2.78$); red, $\tau =1.1\mu \mathrm{s}$ (${\chi }_{\mathrm{red}}^2=1.48$); blue, $\tau =1.4\mu \mathrm{s}$ (${\chi }_{\mathrm{red}}^2=2.14$).

Figure 3

Experimental axial distributions: hot mixing for events near the trap wall (a), and near the trap axis (b) and for cold mixing for events near the trap axis (c). In (a,b): the red line is the simulation with the parameters of Fig. 2 and with lifetime $\tau =1.1\mu \mathrm{s}$. In (c): the red line is the simulation with the parameters of Fig. 4.

Figure 4

Experimental radial distribution for cold mixing 2003 (a) and cold mixing 2002 (b), obtained by subtracting the $\overline{\mathrm{H}}$ contribution (see text). The red lines are the simulation results. In (a): $T=15\mathrm{K}$, $v_{\mathrm{th}}=250{\mathrm{ms}}^{-1}$, generation from a spheroid with $z_p=16\mathrm{mm}$ and $r_p=1\mathrm{mm}$ rotating with a frequency of 300 kHz, i.e., $v_{\mathrm{tang}}=2000{\mathrm{ms}}^{-1}$; mean lifetime is $\tau =1.1\mu \mathrm{s}$. In (b): same parameters as in (a) except $r_p=2.5\mathrm{mm}$ and the rotation frequency is 80 kHz, i.e., $v_{\mathrm{tang}}=1300{\mathrm{ms}}^{-1}$. In (b), the green line corresponding to the parameters of the red line in (a) is shown for comparison.