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Register a new userEfficient $2^3S$ positronium production by stimulated decay from the $3^3P$ level
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Added by
Ruggero Caravita Dr
Publication date
2019
fields
Physics
and research's language is
English
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We investigate experimentally the possibility of enhancing the production of $2^3S$ positronium atoms by driving the $1^3S$-$3^3P$ and $3^3P$-$2^3S$ transitions, overcoming the natural branching ratio limitation of spontaneous decay from $3^3P$ to $2^3S$. The decay of $3^3P$ positronium atoms towards the $2^3S$ level has been effciently stimulated by a 1312.2nm broadband IR laser pulse. The dependence of the stimulating transition efficiency on the intensity of the IR pulse has been measured to find the optimal enhancement conditions. A maximum relative increase of $ times (3.1 pm 1.0) $ in the $2^3S$ production efficiency, with respect to the case where only spontaneous decay is present, was obtained.
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Positronium in the $2^3S$ metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns) making it a promising candidate for interferometry experiments with a neutral matter-antimatter system. In the present work, $2^3S$ positronium is produced - in absence of electric field - via spontaneous radiative decay from the $3^3P$ level populated with a 205nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full-width at half maximum, different velocity populations of a positronium cloud emitted from a nanochannelled positron/positronium converter were selected by delaying the excitation pulse with respect to the production instant. $ 2^3S $ positronium atoms with velocity tuned between $ 7 cdot 10^4 $ m/s and $ 10 cdot 10^4 $ m/s were thus produced. Depending on the selected velocity, a $2^3S$ production effciency ranging from $sim 0.8 %$ to $sim 1.7%$, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the $3^3P$-$2^3S$ spontaneous decay of $(9.7 pm 2.7) % $. The present velocity selection technique could allow to produce an almost monochromatic beam of $sim 1 cdot 10^3 $ $2^3S$ atoms with a velocity spread $ < 10^4 $ m/s and an angular divergence of $sim$ 50 mrad.
A semi-empirical method is used to characterize the 3s(2)3p(2)-3s3p(3) J=2 transition array in P II. In this method, Slater, spin-orbit, and radial parameters are fitted to experimental energy levels in order to obtain a description of the array in terms of LS-coupling basis vectors. The various IC and CI amplitudes resulting from this model are then used to predict the branching fractions of transitions within the array. Results close to LS-coupling values are presented, and these are compared to branching ratios measured using beam-foil spectroscopy at the THIA laboratory. The work provides support for the hypothesis of Dr. Curtis that transition arrays with little upper state IC but significant upper state CI in atoms of low Z exhibit branching fractions close to LS-coupled values, although the data are inconclusive in this respect.
A general formalism is used to express the long-range potential energies in inverse powers of the separation distance between two like atomic or molecular systems with $P$ symmetries. The long-range molecular interaction coefficients are calculated for the molecular symmetries $Delta$, $Pi$, and $Sigma$, arising from the following interactions: He($2 ^1P$)--He($2 ^1P$), He($2 ^1P$)--He($2 ^3P$), and He($2 ^3P$)--He($2 ^3P$). The electric quadrupole-quadrupole term, $C_{5}$, the van der Waals (dispersion) term $C_{6}$, and higher-order terms, $C_{8}$, and $C_{10}$, are calculated textit{ab initio} using accurate variational wave functions in Hylleraas coordinates with finite nuclear mass effects. A comparison is made with previously published results where available.
We present the results of a search for a hidden mirror sector in positronium decays with a sensitivity comparable with the bounds set by the prediction of the primordial He$^{4}$ abundance from Big Bang Nucleosynthesis. No excess of events compatible with decays into the dark sector is observed resulting in an upper limit for the branching ratio of this process of $4.0times10^{-5}$ ($90%$ C.L.). This is an order of magnitude more stringent than the current existing laboratory bounds and it constraints the mixing strength of ordinary photons to dark mirror photons at a level of $varepsilon<5.8times 10^{-8}$.
In this paper, we carry out the complete $mathcal O(alphaalpha_s^{2})$-order study on the inclusive productions of $Upsilon(nS)$ and $chi_b(nP)$ ($n=1,2,3$) via the Standard Model Higgs boson decay, within the framework of nonrelativistic QCD. The feeddown effects via the higher excited states are found to be substantial. The color-octet $^3S_1^{[8]}$ state related processes consisting of $H^0 to bbar{b}[^3S_1^{[8]}]+g$ and $H^0 to bbar{b}[^3S_1^{[8]}]+Q+bar{Q}$ ($Q=c,b$) play a vital role in the predictions on the decay widths. Moreover, our newly calculated next-to-leading order QCD corrections to $H^0 to bbar{b}[^3S_1^{[8]}]+g$ can enhance its leading-order result by 3-4 times, subsequently magnifying the total $^3S_1^{[8]}$ contributions by about $40%$. Such a remarkable enhancement will to a large extent influence the phenomenological conclusions. For the color-singlet $^3P_J^{[1]}$ state, in addition to $H^{0} to bbar{b}[^3P_J^{[1]}]+b+bar{b}$, the newly introduced light hadrons associated process, $H^{0} to bbar{b}[^3P_J^{[1]}]+g+g$, can also provide non-negligible contributions, especially for $^3P_2^{[1]}$. Summing up all the contributions, we have $mathcal B_{H^0 to chi_b(nP)+X} sim 10^{-6}-10^{-5}$ and $mathcal B_{H^0 to Upsilon(nS)+X} sim 10^{-5}-10^{-4}$, which meets marginally nowadays LHC experimental data and can help in understanding the heavy quarkonium production mechanism as well as the Yukawa couplings.
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