1

Neutrino Experiment

with a Xenon TPC

M. Sorel, IFIC (CSIC & U. Valencia)

IMFP2011XXXIX International Meeting on Fundamental Physics

February 2011, Canfranc (Huesca)

Majorana Neutrinos

and

Neutrinoless Double Beta

Decay

2

3

Is a neutrino mass eigenstate equal to its antiparticle?YES

MajoranaNeutrino

NO

DiracNeutrino

•Top three reasons why a Majorana neutrino might be interesting:

•Would be unlike any other fundamental fermion•Explain smallness of neutrino masses?•Explain baryon asymmetry in the Universe?

The See-Saw Mechanism

•Most natural mechanism to explain non-zero but small neutrino masses require Majorana neutrinos

•Terms in neutrino mass matrix:•mD: Dirac mass term, ~m(charged leptons), m(quarks) •M: Majorana mass term, can be very large

•Mass eigenvalues:•mD2/M: light neutrino we are familiar with•M: heavy right-handed neutrino

•Both mass eigenstates: Majorana particles

The see-saw mechanismInclude both Dirac and Majorana mass terms.

Terms in neutrino mass matrix:

mD: Dirac mass term, of order of charged lepton and quark masses.

M: Majorana mass term, can be very large.

Mass eigenvalues (both Majorana particles):

mD2/M: light neutrino we are familiar with

M: heavy right-handed neutrino

�0 mD

mD M

�

M

m2D/M

4

ν̅L

ν̅R

νR

νL

The Baryon Asymmetry in the Universe

•BBN and CMB: small level of baryon asymmetry in the early Universe

•Annihilation of matter with antimatter would haveleft us in a matter-dominated Universe

•The baryon asymmetry could have been induced by a lepton asymmetry: leptogenesis

•If netrinos are Majorana particles, the decays of the heavy Majorana neutrinos into leptons in the early Universe provides an ideal scenario for leptogenesis

1,000,000,001 1,000,000,000

MATTER ANTIMATTER

1

US

MATTER ANTIMATTER

5

Double Beta DecayStandard double beta decay Neutrinoless double beta decay

•Second-order weak transition producing two electrons and two antineutrinos (SM-allowed)•Very slow process, but observed in a number of even-even nuclei

•Hypothetical process producing two electrons and no neutrinos•Most promising ∆L=2 process to probe Majorana nature of neutrinos

6

Majorana Neutrino Mass•Light Majorana neutrino exchange expected to be dominant contribution to ββ0ν:

Majorana neutrino mass: mββ = |∑i mi Uei2|

(A,Z) (A,Z+2) (A,Z) (A,Z+2)

e- e- e- e-

Uei Uei

WWNuclear Physics+

∆L=2 Physics Nuclear Physics

7

νiν̅i

Majorana Neutrino Mass

•ββ0ν provides information also on absolute neutrino mass scale!

sin2!13

1

2

3

sin2!12

sin2!23

NORMAL

"e "# "$

%msol2

%matm2

sin2!13

1

2

3

sin2!23

sin2!12

INVERTED

%msol2

%matm2

∆m221 = 7.9 +0.27

−0.28 (+1.1−0.89)× 10−5 eV2,

|∆m231| = 2.6 ± 0.2 (0.6)× 10−3 eV2,

θ12 = 33.7 ± 1.3 (+4.3−3.5),

θ23 = 43.3 +4.3−3.8 (+9.8

−8.8),

θ13 = 0.0 +5.2−0.0 (+11.5

−0.0 ),δCP ∈ [0, 360].

Gonzá

lez-

Garc

ía a

nd

Malto

ni,

arX

iv:0

704.1

800v2

[hep

-ph]

8

•Relationship between Majorana and actual neutrino mass affected by:•Unknown neutrino mass ordering•Unnown phases in PMNS mixing matrix•Uncertainties in measured oscillation parameters and θ13

9

Majorana Neutrino Mass•If one measures ββ0ν, its half-life is inversely proportional to the Majorana mass squared:

(T1/20ν)-1 = mββ2⋅|M0ν|2⋅G0ν

•M0ν(A,Z): nuclear matrix element (uncertain)•G0ν(Qββ,Z): phase space factor (known)

•In last 20 years, field dominated by Ge detectors:•Heidelberg-Moscow, IGEX•Other techniques: NEMO3 and CUORICINO

•H-M experiment:•35 kg⋅yr exposure•(T1/20ν) > 1.9⋅1025 yr•mββ < 0.35 eV (0.3 - 1.24 eV)

•Controversial claim for ββ0ν in 76Ge by part of H-M Collaboration (Klapdor et al.)

Klapdor’s claim

Ingredients for the ultimate neutrinoless double beta

decay experiment

10

11

NEXT and the race toward the ultimate ββ0ν experiment 25

Ge diodes(GERDA, MAJORANA)

Race Towards the Ultimate Experiment

Cryogenic bolometers(CUORE)

LXe TPC w/scint(EXO)

Liquid scintillators(SNO+, KamLAND)

Foils + tracking(SuperNEMO, MOON)

HPXe TPC w/scint(NEXT, EXO?)

Scintillating crystals(CANDLES)

CZT detectors(COBRA)

Scintillating bolometers(BOLUX)

ULT

IMAT

E U

LTIM

ATE

ββ0ν

ββ0ν

EXP

ERIM

ENT

EXP

ERIM

ENT

Huge Isotope Mass

12

•First and foremost, one needs a large ββ-emitting isotope mass

Next-generation

Next-to-next

Mass O(100 kg) O(multi-ton)

T1/20ν sensitivity O(1026-1027 yr) O(1028-1029 yr)

GoalKlapdor’s claim & degenerate

spectrum

Inverted hierarchy

•Best proposals for mass:•Liquid scintillators (SNO+, KamLAND)•Xe TPCs (EXO, NEXT)

•High ββ detection efficiency also crucial

Future of 0!ββ

69

Current experimental programs have a long way to go to reach interesting level of sensitivity

Emphasis on increasing mass, reducing backgrounds

New approaches are to dilute isotopes of interest in water (150Nd in SNO+,

136Xe in KamLAND-ZEN)

The next round of experiments has a good chance of reaching mββ ~50-100 meV (factor of >10 better in τ1/2)

KamLAND: balloon filled with 136Xe loaded LS

Good ββ Isotope

13

•Large ββ0ν phase space and nuclear matrix element (eg, 150Nd, 82Se, 130Te)

•High isotope enrichment (eg, 136Xe)

•Relatively slow ββ2ν mode (eg, 130Te)

•Well understood nuclear physics (no clearly preferred nucleus)

48 76 82 96 100 124 128 130 136 150

A

0

1

2

3

4

5

6

7

8

GCM

IBM

ISM

HFB

QRPA(J)

QRPA(T)

PMA

M0!

Figure 1. Recent NME calculations from the different techniques (GCM [17], IBM [15], ISM [9, 10],HFB [16], QRPA(J) [11, 12], QRPA(T) [13, 14]) with UCOM short range correlations. QRPA valuesare calculations with gA = 1.25, and IBM-2 and HFB results are multiplied by 1.18 to account for thedifference between Jastrow and UCOM. The intervals used to define the physics-motivated averagevalues are defined as [1.25× ISM, 0.75×min(GCM, IBM,QRPA)].

approaches treat the correlations in an approximate way, and, because the correlations tendto diminish the NMEs, are bound to overestimate them [9, 21]. Therefore, a 25% reductionof the predictions appears reasonable.

Consequently, we propose here the possibility that the experimental collaborations usea physics-motivated average (PMA) between the different calculations. The lower limit of thePMA is obtained by increasing the ISM predictions by 25%, and the upper limit of the PMAfollows from decreasing the largest NME for each nuclei by 25%1. The resulting intervalscan be seen in Figure 1. We believe that the uncertainty reflected by this choice of the PMAis more realistic than simply take the differences between the NMEs. Consequently, for theremainder of this paper, we take the PMA set as our NMEs, using the center of the intervalin our calculations.

In this paper we will consider 5 different isotopes: 76Ge, 82Se, 130Te, 136Xe and 150Nd.Their properties, including our proposed values for the NMEs, are shown in Table 1.

3 Experimental aspects

The choice among ββ isotopes discussed in Sec. 2 is not the only important factor to beconsidered when optimizing future ββ0ν proposals. According to Table 1, one should prefer,everything else being the same, experiments based on 150Nd, 82Se, 130Te, or even 136Xe,rather than experiments based on 76Ge. However, 76Ge-based experiments have dominatedthe field so far.

1For 150Nd, in the absence of an ISM calculation, we adopt the largest and smallest numbers among theother descriptions.

– 4 –

100 1000 10000

10

100Ge-76Se-82Te-130Xe-136Nd-150

exposure (kg year)

mββ

(m

eV)

Figure 3. Sensitivity of ideal experiments at 90% CL for different ββ isotopes. Since the yields arevery similar, the sensitivities of 82Se, 130Te and 150Nd overlap.

ideal experiments need a large exposure (>103 kg · y) to fully explore the inverse hierarchy.To start exploring the normal hierarchy, one would need of the order of 104 kg · y, that is,perfect detectors of 1 ton running for 10 years: a truly formidable experimental challenge.

4.2 Experiments with background and the collapse of the classical limit

Consider now the more realistic case of an experiment with background. Call b the expectedvalue of the background, and assume (unrealistically) that it is known with no uncertainty.The relevant pdf will then be:

Po(n;µ+ b) =(µ+ b)n

n!e−(µ+b) , (4.7)

where µ is the (unknown) mean signal expectation and b is the known mean backgroundexpectation. The Poisson variable n is such that n = ns + nb, where the signal and back-ground Poisson variables ns and nb have mean expectation values E[ns] = µ, and E[nb] = b,respectively.

A priori, it appears as if we could treat the problem in exactly the same way as for thecase without background. A CI can be constructed using the following equations:

α =�∞

nobs

(µlo+b)n

n! e−(µlo+b) (4.8)

β =�nobs

0(µup+b)n

n! e−(µup+b) (4.9)

Let us work out an explicit example. Suppose that we run an experiment in whichthe predicted background is b = 5, and we observe nobs = 5 events. Then, Equation (4.9)becomes:

β =5�

n=0

(µup + 5)n

n!e−(µup+5). (4.10)

– 8 –

Sensitivity of ideal experiments Nuclear matrix element calculations

Low Radioactivity and Cosmogenics

14

•Low cosmogenics•Underground laboratories•Self-shielding

•Very low detector radioactivity•208Tl and 214Bi decays from Th and U decay chains particularly pernicious•May require activities for some detector components at μBq/kg level

Th decay chain Underground physics laboratories

Excellent Energy Resolution

15

•Signature of a ββ0ν decay is an energy deposition consistent with the decay reaction’s Q-value

•Energy resolution needed to discriminate against:

•ββ2ν events, exponentially decreasing with energy

•other backgrounds, ~flat in energy region of interest ββ0νββ2ν

•Techniques with best energy resolution:

•Ge diodes: GERDA, Majorana•Te bolometers: CUORE•0.2% FWHM at Qbb or better!

GERDA measured energy spectrum

Additional Background Handles

16

•Information on event topology

•Tag two connected electron-like tracks•Possible in foil+tracker designs, HPXe TPCs

•Daughter ion tagging

•eg, 136Ba++ in 136Xe -> 136Ba++ + 2e-

•Proposed for EXO. Technically very challenging, but huge payoff if successful

NEMO3 ββ0ν candidate event

Neutrino Experiment

with a Xenon TPC

17

18

•TPC filled with >90% enriched 136Xe gas at ~10 bar pressure•Building on experience from Gotthard TPC

•Baseline detector with ~100 kg fiducial mass: NEXT-100•To be installed in Canfranc Underground Laboratory (LSC, Spain)

Neutrino Experiment

with a Xenon TPC

NEXT Collaboration

19

CIEMAT (Madrid) ● U. Girona ● IFAE (Barcelona) ● IFIC (Valencia) ● U. Santiago ● U.P. Valencia ● U. Zaragoza

LBNL ● Texas A&M

CEA (Saclay)

U. Aveiro ● U. Coimbra

JINR (Dubna)

UAN (Bogota)

•Spain provides:

•Most collaborators•Most of secured funding (~5 M€)•Host laboratory (LSC)

•Smaller but key contributions from experienced groups in other countries:

•TPC detector design•Gaseous detectors•Tracking with MicroMegas•Xe supply and enrichment•Software

20

A Novel Detection Concept

cathode Photo-detectors

cathode

anode

- - - - -

cathode

EL region

Photo-detectors

-

cathode Photo-detectors

-

EL region

•Baseline concept:•Primary scintillation light for t0•Electroluminescent (EL) light for tracking, “PID” and calorimetry•Single drift volume, photosensors at endcaps

•Alternatives being explored:•Charge avalanche gain via Micromegas •Dual drift volume, instrumented barrel

ββ electrons Primary scintillation light Ionization drift

EL light for tracking

EL light for calorimetry

E1

E2

NEXT Readouts

21

•PMTs for calorimetry and start-of-event:•Excellent charge resolution and linearity, >10% coverage, single-PE detection, large dynamic range, fast time response

•SiPMs for tracking:•Fine (~1 cm) pixelization, low cost per channel, some charge information•Alternatives: APDs, Micromegas

•Technical challenges to be overcome:•Operation under high (~10 bar) pressure and in pure Xe•Radiopurity•Optical readouts: VUV light sensitivity

22

NEXT Strengths•Isotope mass: TPC easily scalable

•Isotope enrichment: noble elements easy toenrich (and purify)

•Slow ββ2ν mode: not yet measured!

•Radiopurity: no surfaces for contaminants toattach to

•Energy resolution: goal is 1% FWHM with EL

•Event topology: visible electron tracks

Simulation of anode readout including diffusion and EL light production

Energy resolution in Xe (ionization-only)Bolotnikov and Ramsey, NIM A396 (1997)

HPGXe

LXe

Backgrounds to ββ0ν

23

•Main backgrounds around Qββ = 2.458 MeV from de-excitation gammas of 208Tl and 214Bi decay daughters in the TPC vessel

•214Bi (2.447 MeV photons)•Photoelectric events

•208Tl (2.614 MeV photons)•Photoelectric events where electron looses energy via escaping bresstrahlung photons•Compton events where scattered gamma reinteracts

NEXT-1005 years

1% FWHM Energy Res.

24

NEXT Physics Case

•Conservative scenario:•0.1 mBq/kg vessel

•100 mBq from readouts

•Minimum set of cuts on ββ topology

•δE/E = 1% (FWHM)

•Sensitivity (500 kg⋅yr, 90% CL):

•Aggressive scenario:•More radiopure vessel and readouts•Full exploitation of topological signature•Better energy resolution

mββ < 100 meV

T1/20ν = 1.8⋅1026 yr

mββ = 110 meV

25

NEXT Physics Case

26

NEXT At

Work

NEXT R&D Phases

27

•NEXT-0 and NEXT-1 (2008-2011):

•Show technological feasibility•Acquire know-how•Finalize detector conceptual design•Proliferation of small-scale (≤1kg Xe) TPC prototypes at various NEXT institutions

•NEXT-10 (2011-2013):

•Show control of backgrounds•Radiopure prototype operating at LSC

•NEXT-100 (2013-):

•ββ2ν and ββ0ν searches•Large-scale (100 kg Xe) detector at LSC

NEXT Prototypes

28

•Small-scale (<1 kg Xe) prototypes operating at Coimbra, IFAE, IFIC, LBNL, U. Zaragoza

•Example: NEXT1-EL at IFIC, now being commissioned:

Field cagePressure vessel

SiPM plane

PMT plane

R&D Goals

29

•Two main goals:

•Energy reconstruction performance•Tracking performance

•Insight for NEXT-100 final design

•Some examples of energy reconstruction performance in pure Xenon in various NEXT prototypes

•NEXT goal of ≤1% FWHM at Qββ seems achievable

•More details at this meeting in two NEXT posters, by:•L. Segui (NEXT1-MM)•P. Ferrario and L. Serra (NEXT1-EL)

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Summary

30

•NEXT is a new ββ0ν experiment

•Led by Spanish groups and to be installed here, at LSC, in 2013

•Marries two “old” instrumental concepts (TPCsand electroluminescence) in a novel approach, providing excellent background rejection thanks to good energy resolution and tracking

•Ongoing R&D to demonstrate main issues and choose technological solutions

•For further details: arXiv:0907.4054v1 [hep-ex]

The NEXT experimentat the LSC