Experimental Study of the Cascade Time of Negative Mesons in a Liquid Helium Bubble Chamber

Abstract

We wish to report preliminary results of a measurement of the cascade time of n~ mesons in liquid helium, and to discuss some possible implications of the result. We mean by cascade time the period between initial atomic capture of the meson and its absorption by the nucleus. It is important to understand the mechanism of the cascade since it determines not only the cascade time, but other important effects such as the angular momentum states from which mesons are absorbed by the nucleus.If channel k is one of r channels through which the meson cascades in average time T^ from initial atomic capture to nuclear absorption, and if Nfe is the number of mesons cascading through channel k, then we define the average cascade time to be This is equivalent towhere T^ is the mean life against decay of the meson, N^ is the number of mesons observed to decay at rest, from a total sample, N s , of mesons observed to stop in liquid helium.The technique used to determine the cascade time, T^, is exactly the same as that used by Fields et _al. x in liquid hydrogen. In the present case, TT~ mesons were stopped in a 1.3-liter liq-9 -uid helium bubble chamber, 2 and the range and angle of all backward 7T-JUL decays were measured to determine the velocity of the pion at the instant of decay. We observe 11 decays at rest with 0^ > |ir for a sample of 2255 7r~ stops in the chamber. Because of range straggling, we cannot distinguish pions slower than 0^ a 0.01 from stopped pions; i, however, the time taken by the pion to go from Py = 0. 01 to atomic capture has been estimated 3 to be much shorter than the cascade time measured here. From ordinary stopping power theory we calculate that the number of events between E^ = 0.175 MeV and E n = 1 MeV with 0^ > | TT should be 0.8, We observe none. From the observed number of backward decays at rest we calculate the cascade time to be T =[2N(6 >\*)/N ]r =(2.5±1.0)xl0~1 0 sec.Day 4 has made theoretical estimates of the magnitudes of several effects which might be of importance in determining the history of a typical K~ meson during its cascade. Day assumes that the meson is initially captured from the continuum into a bound orbit with principal quantum number n a 30, ejecting one of the atom's electrons in the process. Because of the strong binding of electrons in helium, the (K~ He ++ ) ion cannot capture electrons from neighboring atoms, and therefore only one ordinary Auger transition of the K~ meson is possible. In addition to ordinary radiative transitions, Day considers three other mecha-

A possible way to promote antihydrogen formation via metastable antiprotonic helium atoms

“…When compared with the K + lifetime of 12.4 ns, we came to the conclusion that the K's of this fraction were trapped somewhere by metastable states formed in liquid helium, having an overall lifetime of about 50 ns. This finding supports the trapping hypothesis of Condo [8], which was proposed for explaining the known puzzling phenomenon of free-decay fractions of 1-2% of ~ [9] and K- [10] stopped in helium bubble chambers; this phenomenon had been ascribed to slow atomic cascade times without theoretical justification.…”

Search for free decay of negative pions in water and light materials

“…The free decay fraction of π − 's was measured to be 8.7 × 10 −5 [4] in Hydrogen and is expected to decrease with the atomic number. However, there is an exception in the π − He atom system [5,6], where the π − is trapped in a meta-stable state that has little wave-function overlap with the nucleus and the Auger processes are suppressed [7]. It is therefore important to confirm the absence of other mechanisms that may cause free decays of π − 's.…”

Density Shift and Broadening of Transition Lines in Antiprotonic Helium

“…Most of the antiprotons are captured in shortlived highly excited states of thepHe 1 atom that within nanoseconds deexcite via fast Auger transitions down to states in which the annihilation takes place immediately. In 1964 Condo [1] explained experimental observations [2] of the anomalous free-decay fractions of negative pions in helium by pointing out the metastability of the circularorbit hydrogenlike states with a high principal quantum number. In the 1990s, anomalous longevity of antiprotons was discovered [3][4][5][6] (see also Ref.…”

“…(2) and (3). The classical trajectories of the incoming He atom were assumed to be determined by the interaction potential V nl ͑R͒ corresponding to the initial state of thepHe 1 atom, i.e., the state ofpHe 1 prior to absorbing the photon.…”

“…A comparison of the contributions from various regions of R to the integral in Eqs. (2) and (3) shows that more than 90% of the values of ≠v͞≠N and ≠G͞≠N come from V nl ͑R͒ in the range 4 , R , 6.5 bohr. The contribution from larger R is small due to the smallness of V in this region, and alternating sign contributions from smaller R tend to cancel each other.…”