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Posted onOctober 30th,2013





Often times with science, progress is built

GERDA’s Victory in theNeutrinoless Double Beta DecayFailure


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on failure- – not on “eureka!” moments ofpivotal insight (which propel scientists tolegendary status), but on abysmal momentsof failure (which propel scientists to theblackened pages of history books).Ultimately, science, a slow and painstakingfield of study governed by the scientificmethod, insists on both logical hypothesesAND proving the validity of thesehypotheses through careful testing.However, sometimes, failures are not whatthey seem.

As many of you may already know, theNobel prize was recently awarded for theconfirmation of the detection of the HiggsBoson, verifying its corresponding Higgfield this October. Researchers made theofficial announcement of the discovery lastyear. After a culmination of many years ofhard work from thousands of physicistsand collisions from particle accelerators,Peter Higgs and Francois Englert shared thisprestigious award. It is fantastic that wenow know that the Higgs is responsible forimparting mass to subatomic particles;however, there are still a number ofproblems that physicists need to solve. Yousee, according to the triumphant edificeknown as the Standard Model, the universeshould be empty. This is called thematter/antimatter asymmetry problem, andis rather embarrassing.

Ultimately, neutrinos are speculated to holdthe key to solving this problem becausetheir mass is meager, even when comparedto the tiny electron (neutrinos are almostmassless). This could mean that the Higgsfield might not play a role in accruing thistiny portion of mass. This is a interestingprospect which has led to exciting new


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tests (especially as it is a very pressingproblem that is directly related to thequestion of why matter exists and didn’tannihilate itself). Physicists planned to usea process called “double beta decay” in anexpensive detector in Italy, calledthe GERmanium Detector Array (or GERDA,for short), to conduct their tests.

Double beta decay is a rare type of decay. Itis when two neutrons inside an atomicnucleus get transformed into two protons,which subsequently ejects two neutrinosand two electrons. Now, neutrinolessdouble beta decay is exactly how it sounds,two neutrons turn into protons, eject theelectrons but lack the neutrinos. Scientistshypothesized that, if this rare type of decaycould happen, the neutrinos absorbed eachother because the neutrino particle pairemitted are completely identical (thus, theyare their own antiparticle). This idea wasfirst proposed in 1937 by the Italianphysicist Ettore Majorana, and a particlethat exhibited this behavior would beknown as a “Majorana particle.”

To sum the theory: in a certain beta decay, aneutron in a nucleus randomly turns into aproton, electron, and an antineutrino. Andin an inverse beta decay, the neutronabsorbs a neutrino and transforms into aproton and an electron. In this hypotheticalneutrinoless double beta decay, these twoprocesses happen simultaneously. Neutrinos are electrically neutral, so theycan fit the bill for being Majorana particles. If this neutrinoless decay exists, it wouldprove neutrinos are Majorana particles,which could explain the matter-antimatterasymmetry.How exactly does this fit with our model ofthe universe?


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In the physics community, there is a widelysupported hypothesis called the “seesawmechanism“. It posits there are actually twokinds of Majorana particles, light weightones and heavy ones. The light weight onesare the ones we observe today. The heavyones were only able to exist is theextremely high energies of the primordialuniverse. Their mass is intrinsic to oneanother via a seesaw mechanism, arelationship that is inverse to the other one(essentially, one is high and the other islow).

These hypothetical heavy neutrinos, thatcan only exist at high energies that werepresent in the ephemeral fraction of asecond after the big bang, decayedasymmetrically (called Leptogenesis). Thiscreated slightly fewer leptons (electrons,muons and tau particles) than antileptons. By a well known Standard Model process,calculations derived the antilepton surplusthat would have then led to a one-part-per-billion excess of baryons (protons andneutrons) over antibaryons. A processcalled Baryogenesis. This, of course, wouldallow things like stars, black holes, galaxies,and eventually life to emerge. If neutrinosare their own antiparticle, Leptogenesiswould be given a tremendous amount ofcredence.


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If your head has not exploded yet, let usproceed to how GERDA fits in to all of this.

Deep in a mountain in Italy, highlyprotected Germanium discs are suspendedin tanks of water and liquid argon. 15kilograms of pure Germanium crystals arebeing used to (hopefully) detect this ultrarare neutrinoless double beta decay. TheGERDA team await the electrical signal ofGermanium-76transforming into Selenium-76. However,spotting this electrical activity is challengingbecause other false signals such as detector‘noise’, rogue radiation (like that fromcosmic rays), and possibly other types ofdecays. Among this chaos, physicists stillplanned on seeing 2 to 3 spikes, or hits,”from this neutrinoless decay. But that issimply not good enough in the physicscommunity. 8 to 10 spikes are what wasneeded to be certain they were on path fora profound discovery.

After a decade of painstaking work, 60physicists crowded into a conference roomat the Joint Institute for Nuclear Research inDubna, Russia. They eagerly awaited to seethe results. The conclusion: 3 spikesshowed up on the conference screens- theexperiment failed to detect this desiredresult conclusively—and cheers erupted,congratulatory hugs and high fives wererife in the room. The physicist’s camerasfuriously snapped away at the results. Perplexed? You would think this lack ofsignal is a failure. But this negative signal isSTILL a victory for the community.

The results of the GERDA experimentsupport the idea that neutrinos are Diracparticles, not Majorana particles, and ouridea of neutrinos being the source of the


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matter/antimatter asymmetry may bewrong. This experiment showed us that thehalf life of this ultra rare decay in thissample size would occur perhaps once in30 trillion trillion years, if it exists at all.That is two thousand trillion times the ageof the universe. If the half life was shorter,GERDA would have detected it. The natureof the precision of these experiments havenever been before achieved. Previousexperimental results were repudiated bythe community because of uncontrolledbackround noise.

For all senses and purposes, GERDA was asuccess by the fact their experimentworked with the sample they had to startwith (Germanium-76 is not exactlyinexpensive either). The longer the half lifeof a sample , the more atoms must be usedto observe it. The scientists know for surethat now they need a larger sample topossibly observe this decay to see thedesired 8-10 electrical spikes: and 40 kg ofGermanium 76 will be used in theupgraded GERDA Phase II experiment.

With that sample size, the neutrinolessdouble beta decay should be observed in 3years if its half-life is less than 100 trilliontrillion years (which they now know itshould be). Ultimately, neutrino scientistsfully expect to see these results to confirmtheir argument that neutrinos are Majoranaparticles. Confirming this will support ourcurrent cosmological model. Rulingneutrinos out as being Majorana particleswill send the community back to thedrawing board. For physicists, this isalways exciting, regardless of the results.

Additional reading here, at QuantaMagazine.


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alexander_shulgin • 13 days ago

i feel the writer of this article should

know how to use 'there and 'their'

correctly.

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Aron• 6 days ago

> alexander_shulgin

There is no time for spell check!

There is science to play with!!

Also, you should probably

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