Napneutrínó-probléma

A napneutrínó-probléma a részecskefizika egyik problémája volt. A problémát az jelentette, hogy a neutrínódetektorok kevesebb neutrínót észleltek, mint amit a Nap működési modelljéből várni lehetett volna. A napneutrínók számában tapasztalt hiányt elsőként Raymond Davis és John Bahcall amerikai fizikusok publikálták 1968-ban. Nem sokkal ezután a részecskefizikusok arra a feltételezésre jutottak, hogy hibás az általánosan elfogadott napmodell. Ezt a feltételezést támasztotta alá az a tény, hogy nem ismerjük pontosan a Nap magjának kémiai összetételét, amely összefüggésben van a neutrínóképződés mértékével. A modell bemenő paraméterein való változtatások azonban nem bizonyultak megoldásnak, mivel a modell így ellentmondásba került a Nap megfigyelhető tulajdonságaival.

1969-ben Bruno Pontecorvo és Grimov felvetett egy elméletet, mely szerint az alacsony energiájú neutrínók átalakulhatnak egymásba (oszcillálhatnak), abban az esetben, ha eltér a nyugalmi tömegük (tehát az oszcilláció feltétele, hogy legalább az egyik rendelkezzen nyugalmi tömeggel). A Napból érkező elektronneutrínók így a Földig megtett útjukon átalakulhatnak más típusú, müon- vagy tau-neutrínókká, amit a detektorok nem érzékelnek.

Csak a 21. század elejére állt rendelkezésünkre a technika, hogy olyan detektorokat építsünk, amelyek érzékenyek az elektronneutrínókra és ugyanakkor a neutrínók mindhárom típusára is. Ilyen detektor a Super-Kamiokande és a Sudbury Neutrínó Obszervatórium, melyek mérései egyeztek a napmodell által előre jelzett neutrínófluxussal, bizonyítva ezzel a neutrínóoszcillációt.

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