Protonium, also known asantiprotonic hydrogen, is a type ofexotic atom in which aproton (symbol: p) and anantiproton (symbol:p) are bound to each other.^[1]

Since protonium is abound system of aparticle and its correspondingantiparticle, it is an example of a type ofexotic atom called anonium.

Protonium has amean lifetime of approximately 1.0 μs and a binding energy of −0.75 keV.^[2]

Like all onia, protonium is aboson with all quantum numbers (baryon number,flavour quantum numbers, etc.) andelectrical charge equal to 0.

There are two known methods to generate protonium. One method involves violent particle collisions. The other method involves putting antiprotons andprotons into the same magnetic cage. The latter method was first used during the experimentATHENA (ApparaTus for High precision Experiment on Neutral Antimatter) at theCERN laboratory inGeneva in 2002, but it was not until 2006 that scientists realized protonium was also generated during the experiment.^[3]

Reactions involving a proton and an antiproton at high energies give rise to multiparticle final states. In fact, such reactions are the basis ofparticle colliders such as theTevatron atFermilab. Indirect searches for protonium atLEAR (Low Energy Antiproton Ring atCERN) have used antiprotons impinging on nuclei such ashelium, with unclear results. Very low energy collisions in the range of 10 eV to 1 keV may lead to the formation of protonium.

Planned experiments will use traps as the source of low-energy antiprotons. Such a beam would be allowed to impinge on atomichydrogen targets in the field of a laser, which is meant to excite the bound proton–antiproton pairs into an excited state of protonium with some efficiency (whose computation is an open theoretical problem). Unbound particles are rejected by bending them in a magnetic field. Since the protonium is uncharged, it will not be deflected by such a field. This undeflected protonium, if formed, would be allowed to traverse a meter of high vacuum, within which it is expected to decay via annihilation of the proton and antiproton. The decay products would give unmistakable signatures of the formation of protonium.^{[citation needed]}

Theoretical studies of protonium have mainly used non-relativisticquantum mechanics. These give predictions for thebinding energy andlifetime of the states. Computed lifetimes are in the range of 0.1 to 10microseconds. Unlike thehydrogen atom, in which the dominant interactions are due to theCoulomb attraction of the electron and the proton, the constituents of protonium interact predominantly through thestrong interaction. Thus multiparticle interactions involvingmesons in intermediate states may be important. Hence the production and study of protonium would be of interest also for the understanding ofinternucleon forces.^{[citation needed]}

• Positronium
• Antiprotonic helium
• Antiproton Collector
• Antiproton Accumulator

• Battersby, S. (13 October 2006)."Antimatter and matter combine in chemical reaction".New Scientist. Retrieved2015-06-26.
• Klempt, E.; Bradamante, F.; Martin, A.; Richard, J.-M. (2002)."Antinucleon-nucleon interaction at low energy: scattering and protonium"(PDF).Physics Reports.368 (2–3):119–316.Bibcode:2002PhR...368..119K.doi:10.1016/S0370-1573(02)00144-8.
• Zurlo, N.; et al. (2006). "Evidence For The Production Of Slow Antiprotonic Hydrogen In Vacuum".Physical Review Letters.97 (15) 153401.arXiv:0708.3717.Bibcode:2006PhRvL..97o3401Z.doi:10.1103/PhysRevLett.97.153401.PMID 17155325.S2CID 36091971.

• Onia
• Proton

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