More neu­tri­nos, more prob­lems

Cosmos - - Contents -

IT WAS A BALMY sum­mer in 1998 when I first be­came aware of the con­found­ing weird­ness of neu­tri­nos. I have vivid mem­o­ries of that day, as an em­bar­rass­ingly young stu­dent re­searcher, walk­ing along a river in Ja­pan, lis­ten­ing to a grad­u­ate stu­dent tell me about her own re­search project: an at­tempt to solve a frus­trat­ing neu­trino–re­lated mys­tery. We were both vis­it­ing a gi­ant de­tec­tor ex­per­i­ment called Su­per-kamiokande, in the heady days right af­ter it re­leased data that for­ever al­tered the Stan­dard Model of Par­ti­cle Physics. What Su­per-k found was that neu­tri­nos - ghostly, elu­sive par­ti­cles that are pro­duced in the hearts of stars and can pass through the whole Earth with only a minis­cule chance of in­ter­act­ing with any­thing - have mass.

A par­ti­cle having mass might not sound like a big deal, but the orig­i­nal ver­sion of the oth­er­wise fan­tas­ti­cally suc­cess­ful Stan­dard Model de­scribed neu­tri­nos as mass­less – just like pho­tons, the par­ti­cles that carry light and other elec­tro­mag­netic waves. Un­like pho­tons, how­ever, neu­tri­nos come in three ‘flavours’: elec­tron, muon, and tau.

Su­per-k’s dis­cov­ery was that neu­tri­nos could change from one flavour to an­other as they trav­elled, in a process called os­cil­la­tion. This can only hap­pen if the three flavours have dif­fer­ent masses from one an­other, which means they can’t be mass­less.

This dis­cov­ery was a big deal, but it wasn’t the mys­tery the grad stu­dent was work­ing to solve. A few years be­fore, an ex­per­i­ment called the Liq­uid Scin­til­la­tor Neu­trino De­tec­tor (LSND), based in the US, had seen tan­ta­lis­ing ev­i­dence that neu­tri­nos were os­cil­lat­ing in a way that made no sense at all with the re­sults of other ex­per­i­ments, in­clud­ing Su­per-k. The LSND find­ing in­di­rectly sug­gested there had to be a fourth neu­trino in the pic­ture that the other neu­tri­nos were some­times os­cil­lat­ing into. This fourth neu­trino would be in­vis­i­ble in ex­per­i­ments, lack­ing the kind of in­ter­ac­tions that made the oth­ers de­tectable, which gave it the name ‘ster­ile neu­trino’. And it would have to be much more mas­sive than the other three.

As I learned that day by the river, the re­sult had per­sisted, un­ex­plained, for years. Most peo­ple as­sumed some­thing had gone wrong with the ex­per­i­ment, but no one knew what.

In 2007, the plot thick­ened. An ex­per­i­ment called Mini­boone, de­signed pri­mar­ily to fig­ure out what the heck hap­pened with LSND, didn’t find the dis­tri­bu­tion of neu­tri­nos it should have seen to con­firm the LSND re­sult. But some ex­tra neu­tri­nos did show up in Mini­boone in a dif­fer­ent en­ergy range. They were in­con­sis­tent with LSND and ev­ery other ex­per­i­ment, per­haps sug­gest­ing the ex­is­tence of even more flavours of neu­trino.

Mean­while, ex­per­i­ments look­ing at neu­tri­nos pro­duced by nu­clear re­ac­tors were see­ing num­bers that also couldn’t eas­ily be ex­plained with­out a ster­ile neu­trino, though some physi­cists wrote th­ese off as pos­si­bly due to cal­i­bra­tion er­rors.

And now the plot has grown even thicker.

In May, Mini­boone an­nounced new re­sults that seem more con­sis­tent with LSND, but even less palat­able in the con­text of other ex­per­i­ments. Mini­boone works by cre­at­ing a beam of muon neu­tri­nos and shoot­ing them through the dirt at an un­der­ground de­tec­tor 450 m away. The de­tec­tor, mean­while, is mon­i­tor­ing the ar­rival of elec­tron neu­tri­nos, in case any muon neu­tri­nos are shape-shift­ing. More of th­ese elec­tron neu­tri­nos turn up than stan­dard neu­trino mod­els pre­dict, which im­plies that some muon neu­tri­nos trans­form by os­cil­lat­ing into ster­ile neu­tri­nos too. (Tech­ni­cally, all neu­tri­nos would be swap­ping around with all oth­ers, but this beam only makes sense if there’s an ex­tra, mas­sive one in the mix.)

But there are sev­eral rea­sons this ex­pla­na­tion is fac­ing re­sis­tance. One is that ex­per­i­ments just look­ing for muon neu­tri­nos dis­ap­pear­ing (be­com­ing ster­ile neu­tri­nos or any­thing else) don’t find a con­sis­tent pic­ture. Se­condly, if ster­ile neu­tri­nos at the pro­posed mass ex­ist, they should have been around in the very early uni­verse, and mea­sure­ments we have from the cosmic mi­crowave back­ground of the num­ber of neu­trino types kick­ing around then strongly sug­gest it was just the nor­mal three.

So, as usual, there’s more work to be done. A Mini­boone fol­low-up called Mi­cro­boone is cur­rently tak­ing data and might make the pic­ture clearer, and other ex­per­i­ments are on the way. It seems very likely that some­thing strange is hap­pen­ing in the neu­trino sec­tor. It just re­mains to be seen ex­actly what, and how, over the next 20 years of con­stant neu­trino bom­bard­ment, it will change our understanding of ev­ery­thing else.

The find­ing sug­gested there must be a fourth neu­trino, one in­vis­i­ble in ex­per­i­ments.

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