Murayama Neutrino Mass

7/31/2019 Murayama Neutrino Mass

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P H Y S I C S W O R L D MA Y 2 0 0 2 p hy s i c s w e b . o r g 35

IF WE look deep into the universe, wesee stars and galaxies of all shapes andsizes. What we do not see, however,is that the universe is filled with parti-cles called neutrinos. These particles which have no charge and have littleor no mass were created less thanone second after the Big Bang, andlarge numbers of these primordiallow-energy neutrinos remain in theuniverse today because they interact

very weakly with matter. Indeed,everycubic centimetre of space containsabout 300 of these uncharged relics.

Trillions of neutrinos pass throughour bodies every second almost all ofthese are produced in fusion reactionsin the Suns core. However, neutrinoproduction is not just confined to ourgalaxy. When massive stars die, mostof their energy is released as neutrinos

in violent supernova explosions.Eventhough supernovas can appear asbright as galaxies when viewed withoptical telescopes, this light representsonly a small fraction of the energy re-leased (see figure).

Physicists detected the first neutrinosfrom a supernova in 1987 when a star collapsed some 150000light-years away in the Large Magellanic Cloud, the galaxynearest to the Milky Way. Two huge underground experi-

ments the Kamiokande detector in Japan and the IMBexperiment near Cleveland in Ohio, USA detected neut-rinos from supernova 1987A a full three hours before lightfrom the explosion reached Earth.

The event marked the birth of neutrino astronomy. Newneutrino telescopes were built soon after, including the

AMANDA experiment in Antarctica, and plans are underway to build an even larger experiment called ICECUBE todetect neutrinos from gamma-ray bursters billions of light-

years away.

However, neutrinos are still the least understood of the fun-damental particles. For half a century physicists thought thatneutrinos, like photons, had no mass. But recent data fromthe SuperKamiokande experiment in Japan overturned this

view and confirmed that the Standard Model of particlephysics is incomplete.To extend the Standard Model so that

it incorporates massive neutrinos in anatural way will require far-reachingchanges. For example, some theoristsargue that extra spatial dimensionsare needed to explain neutrino mass,while others argue that the hithertosacred distinction between matter andantimatter will have to be abandoned.The mass of the neutrino may evenexplain our existence.

Birth of neutrinosNeutrinos have been shrouded in mys-tery ever since they were first sugges-ted by Wolfgang Pauli in 1930. At thetime physicists were puzzled becausenuclear beta decay appeared to breakthe law of energy conservation. Inbeta decay, a neutron in an unstablenucleus transforms into a proton andemits an electron at the same time.

After much confusion and debate, theenergy of the radiated electron wasfound to follow a continuous spec-trum. This came as a great surprise tomany physicists because other typesof radioactivity involved gamma raysand -particles with discrete energies.

The finding even led Niels Bohr to speculate that energy maynot be conserved in the mysterious world of nuclei.

Pauli also struggled with this mystery. Unable to attend a

physics meeting in December 1930, he instead sent a letterto the other radioactive ladies and gentlemen in whichhe proposed a desperate remedy to save the law of energyconservation. Paulis remedy was to introduce a new neutralparticle with intrinsic angular momentum or spin ofh/2,where h is Plancks constant divided by 2. Dubbed theneutron by Pauli, the new particle would be emitted to-gether with the electron in beta decay so that the total energywould be conserved.

Two years later, James Chadwick discovered what we now

call the neutron, but it was clear that this particle was tooheavy to be the neutronthat Pauli had predicted.However,Paulis particle played a crucial role in the first theory ofnuclear beta decay formulated by Enrico Fermi in 1933 andwhich later became known as the weak force. Since Chad-wick had taken the name neutronfor something else, Fermi

New experimental data, which show that neutrinos have mass,

are forcing theorists to revise the Standard Model of particle physics

The origin of neutrino massHitoshi Murayama

Ground-based telescopes, like the Anglo-Australian

Observatory, saw the light from supernova 1987A several

hours after the Kamiokande and IMB experiments had

already detected the neutrinos that were emitted.

AAO/DAVID

MALIN


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had to invent a new name. Being Italian,neutrinowas the obvious choice:a lit-tle neutral one.

Because neutrinos interact so weaklywith matter, Pauli bet a case of cham-

pagne that nobody would ever detectone. Indeed this was the case until 1956,when Clyde Cowan and Fred Reinesdetected antineutrinos emitted from anuclear reactor at Savannah River inSouth Carolina,USA.When their resultwas announced, Pauli kept his promise.

Two years later, Maurice Goldhaber,Lee Grodzins and Andrew Sunyarmeasured the handedness of neut-

rinos in an ingenious experiment atthe Brookhaven National Laboratoryin the US. The handedness of a parti-cle describes the direction of its spinalong the direction of motion the spinof a left-handed particle, for example,always points in the opposite directionto its momentum.

Goldhaber and co-workers studiedwhat happened when a europium-152nucleus captured an atomic electron.The europium-152 underwent inversebeta decay to produce an unstablesamarium-152 nucleus and a neutrino.The samarium-152 nucleus then decayed by emitting agamma ray. When the neutrino and the gamma ray wereemitted back-to-back, the handedness of the two particleshad to be the same in order to conserve angular momentum.By measuring the handedness of the gamma ray using a po-larized filter made of iron, the Brookhaven team showed thatneutrinos are always left-handed.

This important result implies that neutrinos have to beexactly massless.To see why this is,suppose that neutrinos dohave mass and that they are always left-handed. Accordingto special relativity,a massive particle can never travel at thespeed of light. In principle,an observer moving at the speedof light could therefore overtake the spinning massive neut-rino and would see it moving in the opposite direction. Tothe observer, the massive neutrino would therefore appearright-handed. Since right-handed neutrinos have never beendetected, particle physicists concluded that neutrinos had to

be massless.

The Standard ModelWe now know that all the elementary particles six quarksand six leptons are grouped into three families or genera-tions. Indeed, precision experiments at the Large ElectronPositron (LEP) collider at CERN in Switzerland have demon-strated that there are exactly three generations. Everydaymatter is built from members of the lightest generation: theup and down quarks that make up protons and neutrons; the

electron; and the electron neutrino involved in beta decay.The second and third generations comprise heavier versionsof these particles with the same quantum numbers.The anal-ogues of the electron are called the muon and the tau, whilethe muon neutrino and tau neutrino are equivalent to theelectron neutrino.Each particle also has a corresponding anti-

particle with opposite electric charge.Inthe case of neutrinos, the antineutrino isneutral but right-handed.

The Standard Model also includesa set of particles that carry the forces

between these elementary particles.Photons mediate the electromagneticforce; the massive W+ and Wparticlescarry the weak force, which only acts onleft-handed particles and right-handedantiparticles;and eight gluons carry thestrong force.

All the particles that make up matterhave mass from the lightest, the elec-tron, to the heaviest, the top quark and

can be left- or right-handed. Althoughthe Standard Model cannot predict theirmasses, it does provide a mechanismwhereby elementary particles acquiremass.This mechanism requires us to ac-cept that the universe is filled with parti-cles that we have not seen yet.

No matter how empty the vacuumlooks, it is packed with particles calledHiggs bosons that have zero spin (and aretherefore neither left- or right-handed).Quantum field theory and Lorentz in-

variance show that when a particle isinjected into the vacuum, its handed-

ness changes when it interacts with a Higgs boson (figure 2a).For example,a left-handed electron will become right-handedafter the first collision, then left-handed following a secondcollision, and so on. Put simply, the electron cannot travelthrough the vacuum at the speed of light; it has to becomemassive. Similarly, muons collide with Higgs bosons morefrequently than electrons, making them 200 times heavier

than the electron,while the top quark interacts with the Higgsboson almost all the time.

This picture also explains why neutrinos are massless. If aleft-handed neutrino tried to collide with the Higgs boson,it would have to become right-handed. Since no such stateexists, the left-handed neutrino is unable to interact with theHiggs boson and therefore does not acquire any mass. In thisway, massless neutrinos go hand in hand with the absence ofright-handed neutrinos in the Standard Model.

Evidence for neutrino massI was at the conference in Takayama, near Kamioka, in 1998when the SuperKamiokande collaboration announced thefirst evidence for neutrino mass. It was a moving moment.Uncharacteristically for a physics conference,people gave thespeaker a standing ovation. I stood up too. Having survivedevery experimental challenge since the late 1970s, the Stan-dard Model had finally fallen. The results showed that at the

very least the theory is incomplete.The SuperKamiokande collaboration looked for neutrinos

that were produced when cosmic rays bombarded oxygen ornitrogen nuclei in the atmosphere. These atmospheric neut-rinos are mostly muon neutrinos and interact very weaklywith matter. Filled with 50000 tonnes of water, however, theSuperKamiokande detector located deep in the Kamiokamine in Japan is so large that it can detect atmospheric neut-

1 Rounding up neutrinos

A view of the SNO detector located 2000 metres

underground in the Creighton mine near Sudbury,

Canada. The vessel is 12 metres across and is

filled with 1000 tonnes of heavy water. A few of

the neutrinos that pass through the detector

interact to produce electrons that travel faster

than the speed of light in the heavy water. These

electrons create flashes of Cerenkov light that are

detected by the 9600 photomultiplier tubes

surrounding the vessel.


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rinos. These neutrinos interact with atomic nuclei in thewater to produce electrons, muons or tau leptons that travelfaster than the speed of light in water to produce a shock waveof light called Cerenkov radiation. This radiation can bedetected by sensitive photomultiplier tubes surrounding the

water tank.From these signals, the SuperKamiokande team could also

determine the directions from which the neutrinos came.Since the Earth is essentially transparent to neutrinos, thoseproduced high in the atmosphere on the opposite side of theplanet can reach the detector without any problems. Theteam discovered that about half of the atmospheric neutrinosfrom the other side of the Earth were lost, while those fromabove were not.The most likely interpretation of this result isthat the muon neutrinos converted or oscillatedto tau neut-

rinos as they passed through the Earth.SuperKamiokande isunable to identify tau neutrinos. The particles coming fromthe other side of the Earth have more opportunity to oscillatethan those coming from above. Moreover, if neutrinos con-

vert to something else by their own accord,we conclude thatthey must be travelling slower than the speed of light andtherefore must have a mass.

SuperKamiokande was also used to monitor solar neut-rinos. The fusion reactions that take place in the Sun onlyproduce electron neutrinos, but these can subsequently oscil-late into both muon and tau neutrinos. Though the experi-ment was able to detect the solar neutrinos, it was unableto distinguish between the different neutrino types. In con-trast, the Sudbury Neutrino Observatory (SNO) in Canadacan identify the electron neutrinos because it is filled withheavy water,which contains hydrogen nuclei with an extraneutron. Small numbers of electron neutrinos react with theheavy-hydrogen nuclei to produce fast electrons that createCerenkov radiation (figure 1).

By combining the data from SuperKamiokande and its ownexperiment, the SNO collaboration determined how many

muon neutrinos or tau neutrinos were incident at the Japan-ese detector. The SNO results also provided further evidencefor neutrino mass and confirmed that the total number ofneutrinos from the Sun agreed with theoretical calculations.

The implications of neutrino mass are so great that it isnot surprising that particle physicists had been searchingfor direct evidence of its existence for over four decades. Inretrospect, it is easy to understand why these searches wereunsuccessful (figure 3). Since neutrinos travel at relativisticspeeds, the effect of their mass is so tiny that it cannot be

determined kinematically. Rather than search for neutrinomass directly, experiments such as SuperKamiokande andSNO have searched for effects that depend on the differenceinmass between one type of neutrino and another.

In some respects these experiments are analogous to inter-ferometers, which are sensitive to tiny differences in frequencybetween two interfering waves.Since a quantum particle canbe thought of as a wave with a frequency given by its energydivided by Plancks constant, interferometry can detect tinymass differences because the energy and frequency of the

particles depend on their mass.Interferometry works in the case of neutrinos thanks to thefact that the neutrinos created in nuclear reactions are actu-ally mixtures of two different mass eigenstates.This means,for example, that electron neutrinos slowly transform intotau neutrinos and back again.The amount of this mixingis

quantified by a mixing angle, . We can only detect interfer-ence between two eigenstates with small mass differences ifthe mixing angle is large enough. Although current experi-ments have been unable to pin down the mass difference andmixing angle, they have narrowed down the range of possi-bilities (figure 4).

Implications of neutrino massNow that neutrinos do appear to have mass,we have to solvetwo problems. The first is to overcome the contradiction be-

tween left-handedness and mass.The second is to understandwhy the neutrino mass is so small compared with other parti-cle masses indeed, direct measurements indicate that elec-trons are at least 500000 times more massive than neutrinos.When we thought that neutrinos did not have mass, theseproblems were not an issue.But the tiny mass is a puzzle, andthere must be some deep reason why this is the case.

Basically, there are two ways to extend the Standard Modelin order to make neutrinos massive. One approach involvesnew particles called Dirac neutrinos, while the other ap-

proach involves a completely different type of particle calledthe Majorana neutrino.The Dirac neutrino is a simple idea with a serious flaw. Ac-

cording to this approach, the reason that right-handed neut-rinos have escaped detection so far is that their interactions areat least 26 orders of magnitude weaker than ordinary neut-

2 Neutrinos meet the Higgs boson

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(a) According to the Higgs mechanism in the Standard Model, par ticles in the

vacuum acquire mass as they collide with the Higgs boson. Photons () are

massless because they do not interact with the Higgs boson. All particles,

including electrons (e), muons () and top quarks (t), change handednesswhen they collide with the Higgs boson; left-handed particles become

right-handed and vice versa. Experiments have shown that neutrinos () are

always left-handed. Since right-handed neutrinos do not exist in the Standard

Model, the theory predicts that neutrinos can never acquire mass. (b) In one

extension to the Standard Model, left- and right-handed neutrinos exist.

These Dirac neutrinos acquire mass via the Higgs mechanism but

right-handed neutrinos interact much more weakly than any other particles.

(c) According to another extension of the Standard Model, extremely heavy

right-handed neutrinos are created for a brief moment before they collide with

the Higgs boson to produce light left-handed Majorana neutrinos.

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rinos. The idea of the Dirac neutrino works in the sense thatwe can generate neutrino masses via the Higgs mechanism(figure 2b).However, it also suggests that neutrinos should havesimilar masses to the other particles in the Standard Model. Toavoid this problem, we have to make the strength of neutrinointeractions with the Higgs boson at least 1012 times weakerthan that of the top quark. Few physicists accept such a tinynumber as a fundamental constant of nature.

An alternative way to make right-handed neutrinos ex-tremely weakly interacting was proposed in 1998 by Nima

Arkani-Hamed at the Stanford Linear Accelerator Center,Savas Dimopoulous of Stanford University, Gia Dvali of theInternational Centre for Theoretical Physics in Trieste and

John March-Russell of CERN. They exploited an idea fromsuperstring theory in which the three dimensions of spacewith which we are familiar are embedded in 10- or 11-dimen-sional spacetime. Like us, all the particles of the StandardModel electrons, quarks, left-handed neutrinos, the Higgsboson and so on are stuck on a three-dimensional sheetcalled a three-brane.

One special property of right-handed neutrinos is that theydo not feel the electromagnetic force,or the strong and weakforces. Arkani-Hamed and collaborators argued that right-handed neutrinos are not trapped on the three-brane in thesame way that we are, rather they can move in the extradimensions. This mechanism explains why we have neverobserved a right-handed neutrino and why their interactionswith other particles in the Standard Model are extremelyweak. The upshot of this approach is that neutrino massescan be very small.

The second way to extend the Standard Model involvesparticles that are called Majorana neutrinos. One advantageof this approach is that we no longer have to invoke right-handed neutrinos with extremely weak interactions. How-ever, we do have to give up the fundamental distinctionbetween matter and antimatter.Although this sounds bizarre,neutrinos and antineutrinos can be identical because theyhave no electric charge.

Massive neutrinos sit naturally within this framework.Recall the observer travelling at the speed of light who over-

takes a left-handed neutrino and sees a right-handed neut-rino. Earlier we argued that the absence of right-handedneutrinos means that neutrinos are massless.But if neutrinosand antineutrinos are the same particle, then we can arguethat the observer really sees a right-handed antineutrino andthat the massive-neutrino hypothesis is therefore sound.

So how is neutrino mass generated? In this scheme, it ispossible for right-handed neutrinos to have a mass of theirown without relying on the Higgs boson. Unlike other quarksand leptons, the mass of the right-handed neutrino,M, is nottied to the mass scale of the Higgs boson. Rather, it can be

much heavier than other particles.When a left-handed neutrino collides with the Higgs boson,

it acquires a mass, m, which is comparable to the mass ofother quarks and leptons.At the same time it transforms intoa right-handed neutrino, which is much heavier than energyconservation would normally allow (figure 2c). However, theHeisenberg uncertainty principle allows this state to exist for ashort time interval, t, given by t~ h/Mc

2, after which theparticle transforms back into a left-handed neutrino withmass m by colliding with the Higgs boson again. Put simply,

we can think of the neutrino as having an average mass ofm2/Mover time.This so-called seesaw mechanism can naturally give rise to

light neutrinos with normal-strength interactions. Normallywe would worry that neutrinos with a mass, m, that is similarto the masses of quarks and leptons would be too heavy. How-ever, we can still obtain light neutrinos if Mis much largerthan the typical masses of quarks and leptons.Right-handedneutrinos must therefore be very heavy, as predicted by grand-unified theories that aim to combine electromagnetism withthe strong and weak interactions.

Current experiments suggest that these forces were unifiedwhen the universe was about 1032 m across. Due to the un-certainty principle, the particles that were produced in suchsmall confines had a high momentum and thus a large mass.It turns out that the distance scale of unification gives right-handed neutrinos sufficient mass to produce light neutrinos

via the seesaw mechanism.In this way, the light neutrinos thatwe observe in experiments can therefore probe new physics atextremely short distances.Among the physics that neutrinoscould put on a firm footing is the theory of supersymmetry,

which theorists believe is needed to make unification happenand to make the Higgs mechanism consistent down to suchshort distance scales.

Why do we exist?Abandoning the fundamental distinction between matter andantimatter means that the two states can convert to eachother. It may also solve one of the biggest mysteries of our uni-

verse: where has all the antimatter gone? After the Big Bang,the universe was filled with equal amounts of matter and anti-

matter, which annihilated as the universe cooled. However,roughly one in every 10 billion particles of matter survivedand went on to create stars, galaxies and life on Earth.Whatcreated this tiny excess of matter over antimatter so that wecan exist?

With Majorana neutrinos it is possible to explain whatcaused the excess matter. The hot Big Bang produced heavyright-handed neutrinos that eventually decayed into theirlighter left-handed counterparts. As the universe cooled,therewas insufficient energy to produce further massive neutrinos.

Being an antiparticle in its own right, these Majorana neut-rinos decayed into left-handed neutrinos or right-handedantineutrinos together with Higgs bosons, which underwentfurther decays into heavy quarks.Even slight differences in theprobabilities of the decays into matter and antimatter wouldhave left the universe with an excess of matter.

3 Fermions weigh in

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fermion masses

A comparison of the masses of all the fundamental fermions, particles with

spin h/2. Other than the neutrino, the lightest fermion is the electron, with amass of 0.5 MeVc2. Neutrino-oscillation experiments do not measure the

mass of neutrinos directly, rather the mass difference between the different

types of neutrino. But by assuming that neutrino masses are similar to this

mass difference, we can place upper limits on the mass of a few hundred

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It is encouraging that we have seensuch a phenomenon recently. In thepast three years, the KTeV experimentat Fermilab near Chicago and theNA48 experiment at CERN have es-

tablished that the neutral kaon abound state of a down quark and anti-strange quark and its antiparticledecay in a slightly different manner. Atonly one part in a million, this differ-ence is very small. However, we onlyneed one part in 10 billion for us toexist. If a similar difference in the decayprobabilities exist in right-handed neut-rinos, which is quite likely, it could have

produced a small excess of primordialmatter from which all the other parti-cles have been formed.

OutlookIt is an exciting time for neutrino phys-ics. Many experiments are currentlyunder way or are being constructedor planned to put the evidence forneutrino mass on a more solid footing.Physicists prefer to use man-madeneutrinos produced by accelerators orin nuclear reactors because these neut-rinos can be controlled, unlike atmo-spheric or solar neutrinos.

The difficulty is that neutrinos onlyappear to oscillate over long distances,thereby motivating a series of so-calledlong-baseline experiments. The K2Kexperiment in Japan has already beenrunning for a few years. It involves

firing a beam of muon neutrinos pro-duced in an accelerator at the KEKlaboratory towards the SuperKamio-kande detector, some 250 km away. Sofar the experiment has detected thedisappearance of muon neutrinos dueto neutrino oscillations, which is com-pletely consistent with what we havelearned from atmospheric neutrinos. An even better ex-periment called MINOS will extend the search for neutrino

oscillations.Currently under construction,the neutrinos pro-duced at Fermilab will be sent a distance of 750 km to theSoudan mine in Minnesota,and there are similar plans to firemuon neutrinos produced at CERN towards detectors at theGran Sasso Laboratory in Italy. Particle physicists there arealso hoping to detect tau leptons produced by the oscillationof muon neutrinos into tau neutrinos.

Last year the SNO collaboration upgraded its detector in aneffort to detect muon neutrinos or tau neutrinos directly. Onthe rare occasions when these neutrinos interact in the detec-

tor, they break up the deuterium nuclei in the heavy water torelease neutrons. In order to count the muon neutrinos andtau neutrinos,the SNO team added purified sodium chloride,which captures the neutrons. And another experiment calledKamLAND in Japan is studying antineutrinos from commer-cial nuclear-power plants some 175 km away. Researchers

there are hoping to establish that elec-tron neutrinos do indeed convert toother types of neutrinos.

In the longer term, there are seriousdiscussions about sending neutrinos

thousands of kilometres. Beams pro-duced at Fermilab or Brookhaven, forexample,could be fired towards experi-ments in Japan or Europe.Also, a seri-ous effort is being made to observethe conversion of matter and antimat-ter using a rare process in nuclei calledneutrinoless double beta decay. In thisreaction, which is forbidden by theStandard Model, two neutrons decay

into two protons and two electronswithout emitting any antineutrinos.Re-cently Hans Klapdor-Kleingrothausand co-workers at the Max Planck In-stitute for Nuclear Physics in Heidel-berg claimed to have observed such aprocess, but the evidence is far fromconclusive (seePhysics WorldMarch p5).

ConclusionWe are at an amazing moment in thehistory of particle physics. The Higgsboson, the mysterious object that fillsour universe and disturbs particles,willbe found sometime this decade, andevidence for neutrino mass appears

very strong. The Standard Model,which was established in late 1970s andhas withstood all experimental tests,has finally been found to be incom-plete. To incorporate neutrino mass

into the theory and to explain why itis so small requires major changesto the Standard Model. We may needto invoke extra dimensions or we mayneed to abandon the sacred distinctionbetween matter and antimatter. If thelatter is the case, neutrino mass mayreveal the very origins of our existence.

One thing is certain, we are sure to learn a lot more aboutneutrinos in the coming years.

Further readingS Abel and J March-Russell 2000 The search for extra dimensionsPhysics World

November pp3944

Q R Ahmad et al. 2001 Measurement of the rate ofe + d p+p+ e

interactions produced by 8B solar neutrinos at the Sudbury Neutrino

Observatory Phys. Rev. Lett. 87 071301

Y Fukudaet al. 1998 Evidence for oscillation of atmospheric neutrinosPhys. Rev.

Lett.8115621567

H Quinn and J Hewett 1999 CP and T violation: new results leave open questions

Physics World May pp3742The ultimate neutrino page cupp.oulu.fi/neutrino

The history of the neutrino wwwlapp.in2p3.fr/neutrinos/aneut.html

Hitoshi Murayama is in the Department of Physics, University of California,

Berkeley, CA 94720, USA, e-mail [email protected]

4 Limits on neutrino properties

Previous experiments have failed to detect neutrino

oscillations due to a lack of sensitivity. The lack of a

signal, however, can be interpreted as a limit on the

mass difference m2 between types of neutrinos and

the mixing angle,. This plot ofm2 as a function of

tan2 shows the regions inside the lines that are

excluded. The grey region is excluded by

SuperKamiokande. The solid lines are from searches

for electron neutrinos (e) transforming into any other

type of neutrino. The limits on oscillations specifically

between muon neutrinos () and tau neutrinos ()

are indicated by the dotted line, while the dashed lineshows the results for eto oscillations. The

dot-dashed line highlights the limits onetooscillations. For experiments that are able to detect

neutrino oscillations, the blue and yellow areas

highlight the preferred values ofm2 and tan2 with

90% and 99% confidence. The LSND experiment at

the Los Alamos National Lab also reported evidence

for neutrino oscillations, but this is unconfirmed.

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Murayama Neutrino Mass