Dual role could solve the mys­tery

Science Illustrated - - TECHNOLOGY NEUTRINOS -

emit­ted, when a neutrino col­lides with a pro­ton. The col­li­sion pro­duces a neu­tron and a positron, which is im­me­di­ately de­stroyed and con­verted into ra­di­a­tion, be­cause it is an an­tipar­ti­cle.

Neu­tri­nos keep to them­selves

Since Fred­er­ick Reines’ and Clyde Cowan’s ex­per­i­ment, many ex­per­i­ments have been made with neu­tri­nos, but they re­main poorly de­scribed, as they are so ex­tremely dif­fi­cult to mea­sure. Al­though the sci­en­tists’ mea­sur­ing equip­ment by the nu­clear re­ac­tor was struck by 50 bil­lion neu­tri­ons per cm2 per se­cond, a col­li­sion be­tween a neutrino and a neu­tron only hap­pened three times an hour. That is be­cause the par­ti­cles only in­ter­act with other par­ti­cles via grav­ity, which is very weak, and the weak nu­clear force. The weak nu­clear force de­creases so quickly with dis­tance that it is al­most only in­ter­est­ing in­side an atomic nu­cleus and in the close vicin­ity.

This means that the like­li­hood of a neutrino in­flu­enc­ing an­other par­ti­cle is very limited. Al­most all the neu­tri­nos that the Sun is con­stantly emit­ting – some 1038 neu­tri­nos per se­cond – pass freely through Earth at a speed close to that of light, and tens of thou­sands of them pass through your body ev­ery se­cond, with­out you notic­ing. So, the neutrino is known as a ghost par­ti­cle.

Sci­en­tists know that neu­tri­nos are some of the most com­mon par­ti­cles in the uni­verse. They have no charge and ex­ist in a min­i­mum of three dif­fer­ent ver­sions. At least one of them has a mass, which is mil­lions of times smaller than an elec­tron’s. Physi­cists also talk about it that both neu­tri­nos and an­tineu­tri­nos must ex­ist as two dif­fer­ent par­ti­cles. All the par­ti­cles that physi­cists know to­day have a known, par­tic­u­lar an­tipar­ti­cle – ex­cept for the neutrino. So, physi­cists in­creas­ingly sus­pect that the neutrino might be its own an­tipar­ti­cle – and if it is, the tiny par­ti­cle could solve the mys­tery about how a uni­verse of mat­ter could be born in the Big Bang.

Sci­en­tists’ the­ory is that heavy her­maph­ro­dite par­ti­cles, i.e. par­ti­cle and an­tipar­ti­cle in one, formed im­me­di­ately af­ter the Big Bang. Due to their du­al­ity, they could de­cay into much more mat­ter than an­ti­mat­ter, cre­at­ing the uni­verse.

To­day, such par­ti­cles would be long gone – they could only ex­ist in the very early, en­er­gyrich uni­verse. But if the neutrino proves to be its own an­tipar­ti­cle, sci­en­tists know, that the early, heavy par­ti­cles could also be both.

Zero neu­tri­nos is the key

The Cuore de­tec­tor is to try to re­veal the dual role of the neutrino as its own an­tipar­ti­cle. Cuore is short for “Cryo­genic Un­der­ground Ob­ser­va­tory for Rare Events”.

The rare events are a spe­cial type of de­cay of the ra­dioac­tive iso­tope of the tel­lurium el­e­ment, which is known as 130Te. In a high con­cen­tra­tion, the mat­ter is be­tara­dioac­tive, but in an­other way than the beta de­cay that Wolf­gang Pauli stud­ied. In­stead of a neu­tron de­cay­ing into a pro­ton, an elec­tron, and a neutrino, two neu­trons of 130Te de­cay into two pro­tons, two elec­trons, and two neu­tri­nos in dou­ble beta de­cay.

If the neutrino is its own an­tipar­ti­cle, there will some­times be a very spe­cial neu­tri­no­less dou­ble beta de­cay, by which the two neu­tri­nos neu­tral­ize each other, the mo­ment they oc­cur. So, the elec­trons emit­ted in the de­cay must carry the en­ergy that equals the mass dif­fer­ence be­tween the two neu­trons and the two pro­tons. And that is the very en­ergy that Cuore is look­ing for.

Ini­tially, Cuore was ac­tive for two months in 2017 to de­ter­mine the half-life pe­riod of 130Te, so sci­en­tists know how many neu­tri­no­less de­cays to ex­pect from the quan­tity in the de­tec­tor. The re­sult shows that they can­not find any more than one a year or five in the five years that the ex­per­i­ment is ac­tive, mak­ing the ex­per­i­ment the world’s slow­est, but it is worth wait­ing for. If Cuore cap­tures the rare de­cay, it has also re­vealed how the en­tire uni­verse could have been formed.

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