Direct probes of neutrino mass

0.65 MB·English

by  R. G. Hamish Robertson

Nuclear
Physics B
Proceed-
ings
NuclearPhysicsBProceedingsSupplement00(2015)1–6 Supple-
ment
Direct probes of neutrino mass
R.G.HamishRobertson
CenterforExperimentalNuclearPhysicsandAstrophysics
5 and
1
Dept.ofPhysics,UniversityofWashington,SeattleWA98195,USA
0
2

n
a
J

1 Abstract
3
Thediscoveryofneutrinooscillationshasshownthatneutrinos,incontradictiontoapredictionoftheminimalstandardmodel,

] have mass. Oscillations do not yield a value for the mass, but do set a lower limit of 0.02 eV on the average of the 3 known
x
eigenmasses.Moreover,theymakeitpossibletodetermineorlimitall3massesfrommeasurementsofelectron-flavorneutrinosin
e
- betadecay. Thepresentupperlimitfromsuchmeasurementsis2eV.Wereviewthestatusoflaboratoryworktowardclosingthe
l remainingwindowbetween2and0.02eV,andmeasuringthemass.
c
u
Keywords: Neutrinomass,betadecay,tritium,electronspectroscopy
n
[ 1. Introduction conservationintheweakinteractionin1956[6]andthe

measurementofthehelicityoftheneutrinoin1958[7]
1
Themassoftheneutrinohasbeenatopicofspecula- madeplausibletheideathattheneutrinowasmassless,
v
4 tionandresearchsincethetheoryofbetadecaywasfor- andmasslessneutrinosweresubsequentlybuiltintothe
4 mulatedbyFermi[1]. Neutrinomassaffectstheshape standard model. A decade-long hiatus in searches for
1 of the beta spectrum, and even with the limited data neutrino mass followed. A second kind of neutrino,
0
available at the time, Fermi was able to conclude that the muon neutrino, was discovered in 1962 [8], and
0
themassoftheneutrinomustbe“eitherzero,orinany a third, the tau neutrino, was found to exist in 1975
.
2 caseverysmall,incomparisontothemassoftheelec- [9]. Interest in direct searches for neutrino mass re-
0
tron”[2]. ThediscoverybyAlvarezandCornog[3]in vived once it was realized that neutrinos were not re-
5
1939 that tritium was radioactive and had a small Q- sponsible for parity violation because the neutral cur-
1
: valuewasimportantbecausetheeffectofneutrinomass rent [10] also violated parity [11]. A flurry of excite-
v
is relatively greater in that case. Moreover, tritium has ment pervaded the neutrino-mass community when in
i
X asimpleatomicstructure,auniquelyvaluableproperty 1981Lyubimovetal.[12]reportedthattheelectronan-
r asthesensitivityof neutrinomassexperimentshasad- tineutrino had a mass of 30 eV, a value that closes the
a
vancedovertheyears. Seventy-fiveyearslater, tritium universegravitationallyandwouldexplainitsobserved
remainstheisotopeofchoiceinthecontinuingquestto flatness. Unfortunately this result turned out to be in-
measurethemassoftheneutrino. correct, as was shown by a group at Los Alamos [13].
Thefirstneutrinomassdeterminationsfromtheshape The Los Alamos experiment used a source of gaseous
of the tritium spectrum were carried out with propor- T , because Bergkvist had observed in 1972 [14] that
2
tionalcountersin1948byHannaandPontecorvo[4]at experimentshadreachedasensitivitywhereitbecomes
ChalkRiverandbyCurranetal.[5]inGlasgow. Hanna essential to consider molecular and atomic excitations
and Pontecorvo were able to set a limit of 500 eV on ininterpretingthespectrum. Itwasalmostcertainlythe
the neutrino mass, Curran et al. about 1 keV. Experi- complexities of the tritiated valine molecule that con-
mentalworkcontinueduntilthediscoveryofparitynon- tributedtotheerroneousresultofLyubimovetal.
1
R.G.H.Robertson/NuclearPhysicsBProceedingsSupplement00(2015)1–6 2
Thediscoveryofneutrinooscillationsinatmospheric, minimalstandardmodel. Thestandardmodelisknown
solar, andreactorneutrinos[15,16,17]broughtapro- to be incomplete, lacking things like gravity and dark
foundchange. Neutrinoswereshowntohavemass,and matter, but until neutrinos were found to have mass, it
a lower limit on the average mass of the three eigen- had never produced a disagreement with data. Under-
states(0.02eV)hasbeenestablished. Allthreemasses standingtheoriginandpatternsofneutrinomassisthus
arelinkedbysmalldifferences,whichmeansthatdirect of great interest, because new physics at a very high
searchescanfocusontheexperimentallymostaccessi- mass scale may be responsible. Neutrinos also play a
bleneutrino(theelectronantineutrino)andwillbeable significant role in the formation of the universe. They
to determine the 3 masses from a single measurement areonlyasmallfractionofthedarkmatter,butbecause
(withatwo-foldambiguityfromthepresentlyunknown theycoolfromrelativistictonon-relativisticatarecent
hierarchy). The quantity measured in a beta decay ex- epochinthelastfewbillionyears,theyhaveinfluenced
perimentisaweightedsumofeigenmasses: large-scale structure. Quantifying that influence is de-
(cid:88) sirable, and a laboratory measurement would free cos-
m2 = |U |2m2, (1)
β ei νi mologistsfromtheneedtoincludeneutrinomassinfits
i=1,3
to extract other parameters that can only be obtained
where the m are neutrino eigenmasses and U is
νi fromastronomicalobservation.
the Maki-Nakagawa-Sakata-Pontecorvo mixing matrix
In this review the status of experiments now in
[18]. The absolute lower bound, which neutrino oscil-
progress is considered. There are two experiments on
lation measurements provide, is m ≥ 0.01 eV/c2 for
β tritium. TheKATRINexperimentinKarlsruheisinthe
the normal hierarchy, and m ≥ 0.05 eV/c2 for the in-
β final stages of construction. A new scheme, Project 8,
vertedhierarchy. Whenthemassislargerthanroughly
isunderexploratorydevelopmentinSeattle. Thereare
0.1 eV/c2, the ‘quasi-degenerate’ regime, m (cid:39) m (cid:39)
β ν1 alsoideasaboutusing187Reor163Hoembeddedinmi-
m (cid:39) m . Figure 1 shows a steady ‘Moore’s Law’
ν2 ν3 crocalorimetersbuttheRedecayishindered,callingfor
decrease in the upper limit on neutrino mass from ex-
large amounts of this costly element, and the Ho spec-
perimentsspanningmorethan60years.
trumhasacomplexshape[21].
1000
2. TheKATRINexperiment
100 HDM
W = 1 The KArlsruhe TRItium Neutrino experiment (KA-
mit, eV 10 TetRerI,Na)‘McoAupCle-Es’afiglatesreobuasseTd2osnoMurcaegntoetaiclaArgdeiasbpaeticctrCooml--
ass li 1 liismloactiaotendwoinththEelencotrrtohstcaatmicpruestaordfathtieoKn.aTrlhsreuehxepIenrsitmiteuntet
m
of Technology, contiguous with the prototype tritium-
m b 0.1 IH lower limit handlingfacilitydevelopedfortheITERcontrolledfu-

NH lower limit sionexperimentnowunderconstructioninFrance. The
0.01 apparatus concept is shown in Fig. 2. The basic op-
1950 1960 1970 1980 1990 2000 2010 2020 eration of the experiment begins with gaseous molec-
Year ular tritium recirculated through the WGTS at a tem-
peratureof27Kmaintainedbyatwo-phaseneoncool-
ing loop. A solenoidal field guides electrons from the
Figure1. Upperlimitsonneutrinomassobtainedfromtritiumbeta
sourcethroughseveralstagesofpumpingandintoapre-
decayexperimentsvs.year.Thepointwitherrorbarsisthenon-zero
valuereportedbyLyubimovetal.[12]. AlsoindicatedbyHDMis spectrometerthatrejectsallbutthelast100eVorsoof
themassthatwouldclosetheuniverseintheabsenceofothercontri- thespectrum. Electronsthatsurmounttheprespectrom-
butions,andthelowerboundssetbyoscillationsfortheinvertedand
eterpotentialenterthemainspectrometer,whichhasan
normalhierarchies.
energy resolution of 0.93 eV base width. If they also
Thecurrentupperlimitonneutrinomasscomesfrom surmountthemainspectrometerpotential,theelectrons
two tritium experiments, one at Mainz [19], and the aretransmittedtoamultipixelSidetectorforcounting.
other at Troitsk [20]. The experiments lead to a con- Aspectrumcanbebuiltpointbypoint,bysteppingei-
cordantlimitofabout2eV[18]. therthemainspectrometerpotentialorthepotentialof
Neutrinomassisanimportantquestioninphysicsfor theWGTS.Acomprehensivedescriptionoftheconcept
tworeasons. Itisthefirstdefinitivecontradictiontothe ofKATRINcanbefoundinRef.[22].
2
KATRIN monthly report: Executive Summary

November 2014
INTECH GmbH, Geitnerweg 21, 12209 Berlin.
KIT Project Management
KIT contractor for support in ProRje.cGt m.Han.aRgoebmeretnsto n/NuclearPhysicsBProceedingsSupplement00(2015)1–6 3
FigKNuIarTet io–2n Ua.nl OiRveevrsseeirtayvr coihef twCheen oStfetart tohef e othfK eB aAHdeTelmnR-hWIoNultze raAttpsesmpoabceiraartgito uanns d. Functionalunits(fromleft):Rearsystemwithcalibrationdevices(RS);differentiawlpuwmwpin.kgiste.cetiodnu
(DPS1R);windowlessgaseoustritiumsource(WGTS);differentialpumpingsection(DPS1F);differentialpumpingsectionwithmagneticchicane
(DPS2F);cryogenicpumpingsectionwithargonfrostat3K(CPS);prespectrometer;mainspectrometer;detector.
On site and complete are the prespectrometer, main upperlimitof0.2eVat90%CLafter5calendaryears,
spectrometer,anddetector.Withanelectrongunsource, oradiscoveryat5σof0.35eV.
themainspectrometeranddetectorhavebeenundergo-
ingcommissioningtests. ThecryogenicsfortheWGTS
3. Project8
have been built and tested: the system performs very
well, with temperatures controlled to 4 mK at 27 K
AsBergkvistpointedoutin1972,thefinal-statespec-
[23]. The RS is nearing completion in Santa Barbara.
trum in the decay of tritium bound in a molecule im-
TheDPS1R,DPS1F,andWGTSmagnetsarebuiltand
poses a line-broadening that must be folded with the
ready for integration with the cryogenic system. The
theoretical beta spectrum. The line-broadening exacts
monolithicDPS2Fsysteminitiallyconstructedsuffered
both statistical and systematic penalties when small
afailureofquenchprotectiondiodesthatwereinacces-
neutrino mass distortions are being searched for. The
sibleandcouldnotberepaired. Anewsystemisbeing
statistical demands are already severe: only 2×10−13
builtinKarlsruhewith5separatemagnets. TheCPSis
ofdecayspopulatethelast∆E = 1eVofthespectrum,
underconstructioninGenoa.
andthisfractionscalesasthecubeof∆E.TheKATRIN
The main spectrometer and detector operate largely experimentreachesitsultimatesensitivitywithapprox-
as expected and required for a successful experiment. imatelyequalcontributionsofstatisticalandsystematic
Thespectrometerhasrunatupto35kV,hasdelivered uncertainty.
sub-eV energy resolution, and has shown background A new approach (dubbed ‘Project 8’) was proposed
ratesofafractionof1c/s. Onesurprisewasthediscov- by Monreal and Formaggio in 2009 [25]. The statisti-
eryof219Rndecayingintheactivevolumeofthespec- cal limit mentioned might be circumvented by not ex-
trometers [24]. This unusual isotope in the 235U series tractingandanalyzingelectronsfromthetritiumsource
isemanatingfromtheZr-richgetterstripsusedtopump gas, but instead detecting the cyclotron radiation they
hydrogenisotopes,water,andair. Installationofliquid- emitwhilemovinginauniformmagneticfield B. The
nitrogen-cooled baffles has greatly reduced this back- electronsfromtritiumbetadecayaremildlyrelativistic
ground but necessitated a challenging design for high- (γ=1.035)andthecyclotronfrequencydependsonthe
voltageinsulatorscarryingtheLNsupplyfromground kineticenergyK oftheelectron:
potential. Another difficulty has been the appearance,
f eB
duringbakeout, ofshortcircuitsintheconnectionsbe- f ≡ c = , (2)
tween grid layers that line the shell of the spectrome- γ γ γme
ter.Thegridssuppresselectronbackgroundsatthewall,
where e(m ) is the electron charge (mass), c is the
and function best when operated at potentials differing e (cid:16) (cid:17)
speed of light in vacuum, and γ = 1+K/m c2 is
byafewhundredvolts. Progresshasbeenmadeinre- e
the Lorentz factor. The nonrelativistic frequency f is
pairingtheshorts,andworkcontinuesapace. c
2.799249110(6)×1010HzT−1. The orbiting electron
Itisexpectedthatinitialoperationwithtritiumcould emits coherent electromagnetic radiation with a power
begin in late 2016. The sensitivity of KATRIN is an spectrum centered at f . Due to the K dependence of
γ
3
R.G.H.Robertson/NuclearPhysicsBProceedingsSupplement00(2015)1–6 4
f ,afrequencymeasurementofthecyclotronradiation K. A uniform magnetic field of 0.9467(1) T was pro-
γ
is sensitive to the energy of the electron, and thus pro- vided by a warm-bore superconducting magnet, and a
videsanewformofspectroscopy,CyclotronRadiation weakharmonictrappingfieldwassuperimposedonthe
Emission Spectroscopy (CRES). The statistical advan- uniformfieldbymeansofacircularcoppercoilwound
tageistwo-fold:becausethesourcegasistransparentto onthecell.Thistrappingfieldwasnecessaryinorderto
theradiation, alarge, high-activitysourcecanbeused, keepelectronsinthecelllongenoughtomakeaprecise
andbecausearangeoffrequenciescanbecollectedand measurementoftheirfrequencies. Signalsweredown-
analyzedatonce, thetimetoaccumulateaspectrumis converted through two stages of double-balanced mix-
shorterthanwithaspectrometerthatrecordsdatapoint- ersandamplifierstoa125-MHzwideslicethatwasdig-
by-point. itizedbyafree-running250-MSPSdigitizer.
Anexperimentalverificationofthisconceptbeingde- Spectrogramswereformedfromthedigitizeddataby
sirable,apparatuswassetupattheUniversityofWash- successive Fourier transforms every 32.8 µs. The fre-
ingtontosearchforcyclotronemissionfromsingleelec- quencyaxiswasbinnedin30.52-kHzbins.Oneexpects
trons orbiting in a uniform magnetic field. Somewhat asignalfromanelectrontoshowasuddenonsetofRF
surprisingly, no observation of this has been reported. poweratacertainfrequencyatthemomentofthedecay
Cyclotronmotionandradiationis,ofcourse,wellestab- followed by steady emission that drifts monotonically
lished, but the radiation detected was either from large upinfrequencyastheelectronradiates. Eventuallythe
numbersofelectronsor,wheresingleelectronswerebe- electronwillcollidewithabackgroundgasatomandbe
ingstudied,involveddetectionthroughcouplingtotheir ejectedfromthetrap.
axialmotioninaPenningtrap[26]. After a number of attempts success was achieved in
ThepowerradiatedbyanelectronisgivenbytheLar- June2014. Aspectrogramfromthefirstsecondofdata-
morformula, takingisshowninFig.3.Thetracksvisibleinthisspec-
1 2 e4 (cid:16) (cid:17)
P(γ,θ)= B2 γ2−1 sin2θ, (3)
4π(cid:15) 3m2c
0 e
where (cid:15) is the permittivity of free space and θ is the
0
pitchangleoftheelectron,definedastheanglebetween
themomentumvectoroftheelectronandthemagnetic
field. An electron with energy near the 18.6keV end-
point of tritium and a pitch angle near 90o radiates ap-
proximately1.2fWina1Tmagneticfield.Morepower
is available at higher fields, but the electron is losing
energymorequicklybyradiationandthereceivertech-
nology is more difficult. At lower fields, on the other
hand, the signal-to-noise ratio with respect to back-
groundthermalradiationbecomesmoredifficult.
The experiment [27] was carried out not with tri-
tium but with 83mKr, a 1.8-hr isomeric activity that
is the daughter of a much longer-lived parent, 86-d
83Rb. Not only is the Kr non-contaminating, but it
Figure3. Spectrogramofcyclotronradiationfromatrapped30-keV
produces a spectrum of nearly monoenergetic conver-
electron.
sion electrons at kinetic energies of 17830.0(5)eV,
30227(1)eV,30424(1)eVand30477(1)eV[28]. trogramhavethepredictedcharacteristics:asuddenon-
The Kr was introduced into a cell made of a section set of power followed by a slow, quasi-linear increase
ofWR42waveguideenclosedwith25-µmKaptonwin- in frequency. What was not expected was the persis-
dows. A vacuum of order 10 µPa was maintained by tenceoftheelectroninthetrap. Severalcollisionswith
getterpumps. Thecellwasconnectedbya1-mlength backgroundgasatomsareseen,leadingtojumpsinthe
of waveguide to a pair of cascaded low-noise HEMT frequency, but the electron is not ejected until the sev-
amplifiersmaintainedataphysicaltemperatureof50K enthorperhapseighthcollision. Evidentlythechanges
by a Gifford-McMahon cryocooler. The cell tempera- inpitchangleinthecollisionofa30-keVelectronwith
turewasheldat130KtopreventKrfromfreezingout. agasmoleculearetypicallyverysmall. Ahistogramof
Thesystemnoisetemperaturewasmeasuredtobe145 thefrequencyjumpsfrommanysucheventsshowsthat
4
R.G.H.Robertson/NuclearPhysicsBProceedingsSupplement00(2015)1–6 5
themostprobableenergylossis14eV,consistentwith tosub-Ktemperatures. Ifthiscanbeaccomplished,the
theexpectedbackgroundgasbeinghydrogen. molecular component present as a contaminant will be
An energy spectrum was formed by analyzing the frozenout.Verypureatomictritiumisrequiredbecause
tracks to find the lowest frequency in each event, the the endpoint of the molecular spectrum is 8 eV higher
frequency at the instant the electron was emitted. The thantheatomicspectrum.
spectrumisshowninFig.4.
4. Conclusions
Frequency (GHz)
25.6 25.4 25.2 25 24.8 24.6
0.3 A direct measurement of the mass of the neutrino
from a study of the phase space near the endpoint of
eV0.25 0.3 a beta spectrum is very attractive for a number of rea-
0
per 4 0.2 00.2.25 saoDnsir.aTchoerreMisajnooradneappenardteicnlcee.oTnhwerheeitshnerotnheeendeutotrkinnoowis
d
on 0.15 a nuclear matrix element precisely, it serves only as a
c0.15
se 0.1 normalization constant for the count rate. No complex
per 0.1 0.05 phasescloudtheinterpretationofEq.1. Nocosmologi-
s caldegreesoffreedomarecorrelatedwithit.
nt 0 30 30.1 30.2 30.3 30.4 30.5 30.6
ou0.05 It is nevertheless an extremely difficult endeavor to
C
study and quantify a region of the beta spectrum that
0 ispopulatedonlywithafractionalintensityof10−13 −
16 18 20 22 24 26 28 30 32 34
Reconstructed energy (keV) 10−19. Two experimental campaigns are in progress
now to reach below the presently known 2 eV level of
Figure4. Spectrumofconversionlinesfrom83mKr. Theinsetisan mass sensitivity toward the lower limit set by oscilla-
expandedviewofthe30-keVline.Theenergyresolutioninthisspec- tions, 0.01 eV. If 0.05 eV is reached without seeing a
trumisabout160eV.(The30-keVlineisinfacta53-eVdoublet.)
signal,thenthehierarchymustbenormal.TheKATRIN
Theshadedregionwasnotincludedinthefrequencyrangeexplored.
project,byfarthelargestandmostcomplextritiumbeta
decayexperimenteverconceived,ismovingsteadilyto
It is clear from these results that the basic scheme
firsttritiumoperationin2016andwilleventuallyeither
outlinedbyMonrealandFormaggioisaviableonefor
see the mass or limit it to 0.2 eV. A new idea, Project
measuringthebetaspectrumoftritium. Aplanforfur-
8, has just passed its proof-of-concept milestone, and
ther development of this technique includes a small-
designworkonthenextphasesisbeginning.
scaletritiumexperimentwithapparatusnotgreatlydif-
ferentfromtheoneusedfortheconceptualverification,
andthenlarger-scaleexperimentswithincreasinglysen- 5. Acknowledgments
sitivecapabilities. Whilethestatisticaladvantagesofa
largesourceandspectralefficiencymaybewithinreach, Preparation of this report has been supported by the
the fundamental limitation to experiments with molec- U.S. Department of Energy Office of Science, Office
ular T is the final-state distribution, which has a stan- of Nuclear Physics under Award Number DE-FG02-
2
dard deviation of about 0.4 eV. The distribution is cal- 97ER41020.
culabletohighaccuracy(oforder1%,althoughdifficult
to quantify) but it nevertheless comes at a high price
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