Astroparticle Physics

336 16 GlossaryXX bosonXMM-NewtonX-ray astronomyX-ray bursterYY bosonylemYukawa particleZThe Heisenberg uncertainty relation does not allow a finite quan-tum-mechanical system to have a definite position and definite momentumat the same time. Thus any system even in the lowest energystate must have a non-zero energy. Zero energy would lead tozero momentum and thereby to an infinite position uncertainty.Z bosonzero-point energySee Y boson.European X-ray space observatory mission, with its X-ray Multi-Mirror design using three telescopes. Named in honour of Sir IsaacNewton.Astronomy in the X-ray range (0.1 keV–100 keV).X-ray source with irregular sudden outbursts of X rays.The observed baryon–antibaryon asymmetry in the universe requires,among others, a baryon-number-violating process. Hypotheticalheavy X and Y bosons, existing at around the GUT scalewith masses comparable to the GUT energy scale (≈ 10 16 GeV),could have produced this matter–antimatter asymmetry in their decayto quarks and antiquarks, resp. baryons and antibaryons. Thecouplings of the X and Y bosons to fermion species is presently thesimplest idea for which baryon-number violation appears possible.Name for the state of matter before the Big Bang. Gamow proposedthat the matter of the universe originally existed in a promordialstate, which he coined ‘ylem’ (from the Greek υ´λη ↩ which standsfor ‘matter’, ‘wood’ via the medieval latin hylem meaning the primordialelements of life). According to his idea all elements wereformed shortly after the Big Bang from this primary substance.Yukawa predicted a particle which should mediate the binding betweenprotons and neutrons. After its discovery the muon was initiallymistaken to be this particle. The situation was resolved withthe discovery of the pion as Yukawa particle.Neutral boson of weak interactions. It mediates weak interactionsin all those processes, where electric charge and flavour do notchange.