Direct neutrino mass experiments are complementary to searches for neutrinoless double β-decay and to analyses of cosmological data. Up to recently all direct neutrino mass experiments have been performed with tritium. Starting with the pioneering experiments by Curran, Angus and Cockroft as well as by Hanna and Pontecorvo the last tritium beta decay experiments at Mainz and at Troitsk have achieved upper limits on the neutrino mass of about 2 eV/c2. The KATRIN experiment under construction will improve the neutrino mass sensitivity down to 200 meV/c2 by increasing strongly the statistics and – at the same time – reducing the systematic uncertainties. Commissioning measurements with half of the KATRIN experiment, the huge main spectrometer and the detector, have been performed just recently. As an alternative to tritium β-decay experiments cryobolometers

investigating the endpoint region of 187Re β-decay or of the electron capture of 163Ho are being developed

Weinheimer, C.

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DOI 10.1393/ncc/i2014-11768-4Colloquia: Pontecorvo100IL NUOVO CIMENTO Vol. 37 C, N. 3 Maggio-Giugno 2014Direct measurements of neutrino massC. Weinheimer(∗)Westfa¨lische Wilhelms-Universita¨t Mu¨nster, Institut fu¨r KernphysikWilhelm-Klemm-Str. 9, D-49149 Mu¨nster, GermanySummary. — Direct neutrino mass experiments are complementary to searches forneutrinoless double β-decay and to analyses of cosmological data. Up to recentlyall direct neutrino mass experiments have been performed with tritium. Start-ing with the pioneering experiments by Curran, Angus and Cockroft as well as byHanna and Pontecorvo the last tritium beta decay experiments at Mainz and atTroitsk have achieved upper limits on the neutrino mass of about 2 eV/c2. TheKATRIN experiment under construction will improve the neutrino mass sensitivitydown to 200 meV/c2 by increasing strongly the statistics and – at the same time– reducing the systematic uncertainties. Commissioning measurements with half ofthe KATRIN experiment, the huge main spectrometer and the detector, have beenperformed just recently. As an alternative to tritium β-decay experiments cryo-bolometers investigating the endpoint region of 187Re β-decay or of the electroncapture of 163Ho are being developed.1. – IntroductionTritium β-decay experiments started in the late forties with the pioneering exper-iments by Curran, Angus and Cockroft [1] as well as by Hanna and Pontecorvo [2].The tritium β-spectrum was measured with proportional counters, in which tritium wasmixed to the counting gas. Hanna and Pontecorvo already excluded a neutrino mass of500 eV/c2. Large progress in the experimental techniques has been made in the followingdecades. Firstly, different types of magnetic spectrometers were used starting with theexperiment by Langer and Moffat [3] and ending with the modern Tretyakov-type spec-trometers of the Los Alamos [4], the Zurich [5] and the Livermore [6] groups. Togetherwith the improved experimental resolution the systematic corrections of the measuredβ-spectrum became more and more important, especially the electronic final states [7]and the inelastic scattering in the tritium source.In the late eighties a new experimental technique, the so-called MAC-E-Filter [8,9], anelectrostatic retarding spectrometer with a magnetic field for guiding the β-electrons and(∗) E-mail: weinheimer@uni-muenster.dec© Societa` Italiana di Fisica 8990 C. WEINHEIMERcollimating their momenta, was applied to tritium β-spectroscopy by the experiments atMainz and Troitsk. Finally, the Mainz and the Troitsk experiment obtained neutrinomass limits of about 2 eV/c2 [10, 11]. Recent reviews describing the history and thecurrent situation of direct neutrino mass searches are references [12,13].With the discovery of the oscillation of atmospheric neutrinos by the Super-Kamiokande experiment in 1998 [14] and the further strong evidence for the oscillation ofsolar, atmospheric, accelerator and reactor neutrinos [15], it become clear that neutrinosmix and that their masses are non-zero. Neutrino oscillation experiments can only deter-mine the neutrino mixing angles and the differences between squared neutrino masses,but not the absolute value of the neutrino masses. The value of the neutrino masses arevery important for astrophysics and cosmology on one hand to describe the role of neu-trinos in the evolution of the universe. Although neutrinos are very light they contributeto the mass density of the universe: with 336 neutrinos per cm3 left over from the bigbang they are about a billion times more abundant than atoms. On the other hand thevalues and the pattern of the neutrino masses are very important for nuclear and particlephysics, since they can decide between different models or theories accounting for thenon-zero but very tiny neutrino masses beyond our present Standard Model of particlephysics.Therefore, other ways to determine the absolute value of the neutrino masses ar-erequired. Three methods are sensitive to the values of the neutrino mass eigenstatesand their mixing angles in different ways: The analysis of cosmological data [16], thesearch for neutrinoless double β-decay [17] and the direct neutrino mass search.The direct neutrino mass determination is based purely on kinematics or energy andmomentum conservation of β-decay or electron capture (EC). The neutrino mass canbe determined from the shape of the β-spectrum or the EC spectrum at their endpoint.When the different neutrino mass states cannot be resolved, the “average electron neu-trino mass squared” m2(νe) is determined [12,13]:m2(νe) :=∑|U2ei|m2(νi)(1)This incoherent sum is not sensitive to phases of the neutrino mixing matrix in contrastto neutrinoless double β-decay.This paper is structured as follows: The upcoming tritium β-decay experiment KA-TRIN is described in section 2. An alternative way to tritium β-decay spectroscopyinvestigating the 187Re β-decay or the electron capture of 163Ho with cryogenic bolome-ters is presented in section 3. Our conclusions are given in section 4.2. – The KATRIN experimentThe KArlsruhe TRItium Neutrino experiment KATRIN is currently being set up atthe Karlsruhe Institute of Technology KIT. The aim of KATRIN is to improve the sen-sitivity on the neutrino mass down to 200 meV/c2. Since m2(νe) is the observable, thisrequires an improvement by two orders magnitude compared to the previous tritiumβ-decay experiments at Mainz and Troitsk. The KATRIN design is based on the suc-cessful MAC-E-Filter spectrometer technique combined with a very strong windowlessgaseous molecular tritium source [20].The windowless gaseous molecular tritium source (WGTS) essentially consists of a10 m long tube of 9 cm diameter kept at 30 K. Molecular tritium gas injected in theDIRECT MEASUREMENTS OF NEUTRINO MASS 91middle of this tube is freely streaming to both ends of the beam tube. The tritiumgas is pumped back by huge turbo-molecular pumps placed at pump ports intersectedwith straight sections. The β-electrons are guided by superconducting solenoids housingthe beam tubes. A so-called WGTS demonstrator has been set up to prove that thenew concept of the ultra-stable beam-pipe cooling works: Gaseous and liquid neon issent through two tubes welded onto the beam tube. By stabilizing the pressure ofthis two-phase neon the temperature of the beam tube can be stabilized well below therequirement of 10−3 [21]. The input pressure is chosen to obtain a total column densityof 5 · 1017 molecules/cm2 allowing a near maximum count rate at moderate systematicuncertainties. Currently the WGTS demonstrator is being upgraded into the full WGTS.The electron guiding and tritium retention system consists of a differential and acryogenic pumping unit. It has been demonstrated that the tritium flow reduction bythe differential pumping is about as large as expected by Monte Carlo simulations [22].Because of a not fully working quench protection system the differential pumping sectionis being rebuilt in 2013-2014. Inside the differential pumping sections Fourier transformion cyclotron resonance Penning traps will be installed to measure the ion flux from thetritium source [23]. Ions will be ejected from the beam by a transverse electric field. Theprinciple of the cryogenic pumping section based on argon frost at 3 − 4.5 K has beendemonstrated in a test experiment [24]. The overall tritium reduction amounts to 10−14.A pre-spectrometer will transmit only the interesting high energy part of the β-spec-trum close to the endpoint into the main spectrometer [25], in order to reduce the rate ofbackground-producing ionization events therein. The big main spectrometer is of MAC-E-Filter type as the pre spectrometer. It is essentially an electric retarding spectrometerwith a magnetic guiding and collimating field [8]. In order to achieve the strong energyresolution of 1:20,000 the magnetic field in the analyzing plane in the centre of thespectrometer has to be 20,000 times smaller than the maximum magnetic field of 6 Tprovided by the pinch magnet. Due to conservation of the magnetic flux from the WGTSto the spectrometer it needs to have a diameter of 10 m in the analyzing plane. To avoidbackground by scattering of β-electrons inside the spectrometer extreme requirementsfor the vacuum pressure in the 10−11 mbar regime are necessary [26]. The β-electronswhich have enough energy to pass the MAC-E-Filter are counted with a state-of-the-art segmented PIN detector [27]. The spatial information provided by the 148 pixelsallow to correct for the residual inhomogeneities of the electric retarding potential andthe magnetic fields in the analyzing plane. Active and passive shields minimize thebackground rate at the detector.After baking of the KATRIN main spectrometer at 300 ◦C commissioning measure-ments of the main spectrometer and the detector were performed in 2013. The requestedultra-high vacuum conditions were achieved with a residual gas pressure of 4·10−11 mbar.Switching on the retarding high voltage and the magnetic field exhibited no problem,demonstrating that there are no large Penning traps in the spectrometer, which wasone major concern. To our knowledge this was the first time a MAC-E-Filter could beswitched on without major problems. Obviously this is due to the good preparation byextended electromagnetic design calculations and the intense test measurements at thepre spectrometer.During the main spectrometer and detector test measurements the background ratewas well below 1 cps, but did not reach yet the KATRIN design value. The active(zeroing the magnetic field by a pulsed reversing of the air coil field, applying an electricdipole field) and passive (LN2 baffle to freeze out radon coming from the NEG pumps,wire electrode to screen secondary electrons from the spectrometer wall) background92 C. WEINHEIMERreduction methods worked well. Unfortunately the wire electrode cannot be operatedin dual layer mode yet. Some electric shorts in the supply lines need to be removedfirst. By shooting with the e-gun to selected pixels of the detector transmission functionshave been measured and the detector alignment been tested. Some upgrades of thespectrometer and detector system in 2014–2015 will further improve the performance.The tritium source as well as the electron transport and tritium elimination sectionwill be put into operation in 2014–2015. First tritium data with the full KATRIN setupare expected for 2016.3. – Cryo-bolometer experimentsCompared to tritium the isotope 187Re has a 7 times lower endpoint energy of 2.47 keVresulting in a 350 times higher relative fraction of the β-spectrum in the interestingendpoint region. Unfortunately 187Re exhibits a very complicated electronic structureand a very long half life of 4.3 ·1010 y. This disadvantage can be compensated by using itas β-emitter in cryo-bolometers, which measure the entirely released energy, except thatof the neutrino.A cryo-bolometer is not an integral spectrometer like the MAC-E-Filter but measuresalways the entire β-spectrum. Pile-up of two random events may pollute the endpointregion of a β-decay on which the neutrino mass is imprinted. Therefore cryo-bolometerwith mg masses are required to suppress pile-up by 4 or more orders of magnitude.Unfortunately large arrays of cryo-bolometers are then required to reach a the necessarysensitivity on the neutrino mass. Another technical challenge is the energy resolution ofthe cryo-bolometers. Although cryo-bolometers with an energy resolution of a few eVhave been produced with other absorbers, this resolution has not yet been achieved withrhenium.Two groups have started the field of 187Re β-decay experiments: The MANU exper-iment at Genoa was using one metallic rhenium crystal of 1.6 mg working at a temper-ature of 100 mK and read out by Germanium doped thermistor. The β environmentalfine structure was observed for the first time giving rise to a modulation of the shape ofthe β-spectrum by the interference of the out-going β-electron wave with the rheniumcrystal [28]. The spectrum near the endpoint allowed to set an upper limit on the neu-trino mass of m(νe) < 26 eV [29]. The MiBeta collaboration at Milano was using 10crystals of AgReO4 with a mass of about 0.25 mg each [30]. The energy resolution of asingle bolometer was about 30 eV. One year of data taking resulted in an upper limit ofm(νe) < 15 eV [30].Both groups have started with additional groups the MARE project [31] to furtherexplore the development of sensitive micro-calorimeters investigating the 187Re β-decay.MARE consists of two phases [32]: MARE-1 aims to investigate alternative micro-calorimeter concepts to improve the energy resolution, to shorten the rise time of thesignals and to develop possibly a multi-plexing read-out. Among these technologies aretransition edge and neutron-doped thermistors for the temperature read-out, but alsonew technologies based on magnetic micro-calorimeters [33]. These new dectectors arebeing tested in medium-size arrays with up to 300 cryo-bolometers enabling MARE-1to reach a sensitivity on the neutrino mass of a few eV/c2. After selection of the mostsuccessful technique a full scale experiment with sub-eV/c2 sensitivity to the neutrinomass will then be set up in MARE phase 2 comprising about 50000 detectors.Due to the persisting difficulties with superconducting metallic rhenium absorberscoupled to the sensors [34] the main activities of the cryo-bolometer groups goes now intoDIRECT MEASUREMENTS OF NEUTRINO MASS 93the investigation of the electron capture of 163Ho. The isotope 163Ho could be implantedinto well-suited cryo-bolometers. The very upper end of the electromagnetic de-excitationspectrum of the 163Ho daughter 163Dy looks similar to the endpoint spectrum of a β-decayand is sensitive to the neutrino mass. The ECHO collaboration will perform the directneutrino mass search with 163Ho implanted in magnetic micro-calorimeters. A first 163Hospectrum has been presented by ECHO [33]. A second group is pursuing the similarHOLMES project [35].4. – ConclusionsThe direct neutrino mass measurements are complementary to searches for neutrino-less double β-decay and to cosmological analyses. A major improvement in sensitivity onthe neutrino mass by one order of magnitude will be achieved by the KATRIN experimentcurrently under construction. The commissioning of the KATRIN main spectrometer anddetector demonstrated that these major components operate nicely. It is important thatthe methods to avoid Penning traps in the spectrometer were really successful. Still someimprovements at the spectrometer and detector will be done before the start of the fullKATRIN experiment in 2016.An alternative to tritium β spectroscopy is the use of cryo-bolometers investigating the187Re β-decay or 163Ho electron capture. If a breakthrough in cryo-bolometer technologywill be achieved allowing to set up arrays with tens of thousands of cryo-bolometers withan energy resolution of O(eV) and a rise time of O(100 μs), experiments like MARE,ECHO or HOLMES may reach KATRIN’s sensitivity or go even beyond in the long-term.There is also R&D on rather different approaches, like Project-8, which aims to measurethe endpoint spectrum of tritium β-decay by detecting the radio emission of coherentcyclotron radiation from a KATRIN-like tritium source [36].∗ ∗ ∗The work of the author is supported by the German Ministery of Education andResearch BMBF.REFERENCES[1] S. C. Curran, J. Angus, A. L. Cockroft Phys. Rev. 76 (1949) 853[2] G. C. Hanna and B. Pontecorvo Phys. Rev. 75 (1949) 983[3] L. M. Langer and R. J. D. Moffat Phys. Rev. 88 (1952) 689[4] R.G.H. Robertson et al., Phys. Rev. Lett. 67 (1991) 957[5] E. Holzschuh et al., Phys. Lett. B 287 (1992) 381[6] W. Stoeffl and D.J. Decman, Phys. Rev. Lett. 75 (1995) 3237[7] A. Saenz, S. Jonsell and P. Froehlich, Phys. Rev. Lett. 84 (2000) 242[8] A. Picard et al., Nucl. Instrum. Meth. B 63 (1992) 345[9] V.M. Lobashev, P. E. Spivak, Nucl. Instr. Meth. A 240 (1985) 305[10] C. Kraus et al., Eur. Phys. Jour. C 40 (2005) 447-468[11] V.N. Aseev et al., Phys. Rev. D 84 (2011) 112003[12] E.W. Otten and C. Weinheimer, Rep. Prog. Phys.71 (2008) 086201[13] G. Drexlin, V. Hannen, S. Mertens, C. Weinheimer, Advances in High Energy Physics 2013(2013) 293986[14] Y. Fukuda et al. (Super-Kamiokande Coll.), Phys. Rev. Lett. 81 (1998) 1562[15] L. Fogli et al., Phys. Rev. D 86 (2012) 013012[16] J. Lesgourgues and S. Pastor Advances in High Energy Physics 2012 (2012) 60851594 C. WEINHEIMER[17] A. Giuliani and A. Poves Advances in High Energy Physics 2012 (2012) 857016[18] T.J. Loredo and D.Q. Lamb, Phys. Rev. D65 (2002) 063002[19] G. Pagliarolia, F. Rossi-Torresa and F. Vissania, Astropart. Phys.33 (2010) 287[20] J. Angrik et al., (KATRIN Collaboration), Wissenschaftliche Berichte, FZ Karlsruhe 7090.[21] S. Grohmann et al., Cryogenics 51 (2011) 438-445[22] S. Lukic et al., Vacuum 86(2012) 1126–1133.[23] M. Ubieto-Dı´az et al., Int. J. of Mass Spect. 288 (2009) 1–5[24] O. Kazachenko et al., Nucl. Instrum. Meth.A 587 (2008) 136[25] M. Prall et al., New J. Phys. 14 (2012) 073054[26] X. Luo, L. Bornschein, C. Day, J. Wolf, Vacuum 81 (2007) 777[27] B. L. Wall et al., accept. for publ. in Nucl. Instr. Meth. A (2014)[28] F. Gatti et al., Nature 397 (1999) 137[29] F. Gatti, Nucl. Phys. B - Proc. Supp. 91 (2001) 293[30] M. Sisti et al., Nucl. Inst. and Meth. A 520 (2004) 125-131[31] A. Monfardini et al., Nucl. Instr. Meth. A 559 (2006) 346[32] A. Nucciotti, J. of Low Temp. Phys. 151 (2008) 597–602[33] P.C.-O. Ranitzsch et al., J. of Low Temp. Phys. 167 (2012) 1004[34] E. Ferri, J. of Low Temp. Phys. 167 (2012) 1035[35] The HOLMES project, https://artico.mib.infn.it/nucriomib/experiments/holmes[36] B. Monreal and J. Formaggio, Phys. Rev. D 80 (2009) 051301(R)

Direct measurements of neutrino mass

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Direct measurements of neutrino mass

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