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Probing the mechanism of neutrinoless double beta decay with SuperNEMO
Progress in Particle and Nuclear Physics 64 (2010) 278–280
Contents lists available at ScienceDirect
Progress in Particle and Nuclear Physics journal homepage: www.elsevier.com/locate/ppnp
Review
Probing the mechanism of neutrinoless double beta decay with SuperNEMO Frank Deppisch, Chris Jackson, Irina Nasteva ∗,1 , Stefan Söldner-Rembold University of Manchester, Manchester, United Kingdom
article
info
Keywords: Neutrinoless double beta decay SuperNEMO Neutrino mass
abstract The SuperNEMO experiment is being designed to search for neutrinoless double beta decay. Its experimental technique of tracking and calorimetry provides the means to discriminate different underlying mechanisms for neutrinoless double beta decay by measuring the angular and energy distributions of electrons. The results of a study by the SuperNEMO Collaboration and F. Deppisch (in preparation) [7] for identifying light Majorana neutrino exchange and right-handed currents are presented. © 2010 Elsevier B.V. All rights reserved.
1. Neutrinoless double beta decay Neutrinoless double beta decay (0νββ ) is the process in which two neutrons in a nucleus undergo simultaneous beta decays with the emission of two electrons. It violates lepton number and therefore represents physics beyond the Standard Model. The most commonly considered underlying mechanism of 0νββ is light neutrino exchange, or the mass m mechanism (MM). In it the 0νββ half-life can be expressed as [T1/ν2 ]−1 = (hmν i/me )2 G01 |M mν |2 , where hmν i is the effective neutrino P mass with contributions of individual neutrino mass eigenstates weighted by mixing matrix elements squared, hmν i = | Uei2 mi |, G01 is a calculable phase-space factor, and Mmν is the nuclear matrix element (NME). 1.1. Right-handed currents Other lepton number violating models could also contribute to the 0νββ decay rate. The example model considered m here is right-handed currents (RHC), which arise in left–right symmetric models. The general decay rate is [T1/ν2 ]−1 =
Cmm (hmν i/me )2 + Cλλ hλi2 + Cmλ (hmν i/me )hλi, where λ is the coupling of V + A interactions at hadronic and leptonic vertices, and the coefficients C contain phase-space factors and NMEs. The angular and energy difference distributions of the decay electrons, predicted by RHC models, differ from these of the MM model. This feature allows us to distinguish the two by measuring the electrons’ kinematical correlations [1]. 2. Expected sensitivity of SuperNEMO SuperNEMO is a next-generation experiment which will employ a technique of tracking and calorimetry [2] to search for 0νββ decay in ∼100 kg of enriched isotopes. It will comprise 20 modules, each containing ∼5 kg of isotope source (82 Se or 150 Nd) as a thin foil, surrounded by a tracking chamber with drift cells in Geiger mode, and calorimeter walls with plastic scintillators and PMTs. The experiment is currently in an R&D phase, expecting to start the first module construction in 2010.
∗
Corresponding author. E-mail address: [email protected] (I. Nasteva).
1 Speaker. 0146-6410/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ppnp.2009.12.028
F. Deppisch et al. / Progress in Particle and Nuclear Physics 64 (2010) 278–280
279
Fig. 1. Reconstructed slope factor krecon as a function of admixture of the true ktrue from angular and energy distributions in 82 Se (left) and 150 Nd (right).
A simulation study was performed in the SuperNEMO software framework, using the DECAY0 [3] event generator for 0νββ signals and 2νββ , 214 Bi and 208 Tl backgrounds. The generated particles undergo a full simulation of the detector response, track√reconstruction and realistic event selection, with the SuperNEMO design parameters of 40 mg/cm2 foil thickness, 7%/ E (FWHM) energy resolution, and 500 kg y exposure. Limits were set at 90%CL as a function of RHC m admixture, by analysing the electron energy sum. The resulting bounds are T1/ν2 > 1.15 · 1026 y (1.15 · 1026 y) for 82 Se (150 Nd) in the case of pure MM, with a slight decrease for higher RHC admixtures due to acceptance effects. 2.1. Angular and energy distributions In addition to the energy sum, SuperNEMO can measure the individual electron energies and tracks, allowing the use of kinematical correlations to distinguish between new physics scenarios. The distribution of the angle between the decay dΓ electrons can be written in general as d cos = Γ2 (1 − kθ cos θ12 ) [4], where the slope kθ depends on the new physics θ12 mechanism. An angular asymmetry Aθ = kθ /2 is defined with respect to cos θ12 = 0. Similarly, an energy difference asymmetry AE = kE /2 is defined with respect to Qββ /2, giving the measured energy parameter kE . The reconstructed k parameters as a function of the theoretical values are given in Fig. 1. 3. Identifying the 0νββ decay mechanism Fig. 2 (left) shows the SuperNEMO exclusion plot in hmν i–λ parameter space. The plot was obtained from the 0νββ half-life sensitivity by using the NME values from [5] and a 2.7 correction factor for the deformed nucleus of 150 Nd [6]. SuperNEMO can achieve sensitivity of about hmν i ≤ 70 meV for the effective neutrino mass, and hλi ≤ 1.2 · 10−7 for the right-handed coupling parameter. In the case of discovery of 0νββ decay, SuperNEMO can combine the measured half-life with the angular and energy difference distributions to pinpoint the underlying physics mechanism. Fig. 2 (right) shows the allowed regions in (mν , λ) parameter space from the observed half-life, the angular and energy correlations, and a combined statistical analysis of both. The plot includes NME errors of 30% and shows the case of 25% RHC admixture. For a half-life of 1026 y, SuperNEMO can identify pure RHC at 2σ , and for the more favourable half-life value of 1025 y, it can measure the RHC admixture to 20%.
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F. Deppisch et al. / Progress in Particle and Nuclear Physics 64 (2010) 278–280 300
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Nd
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〈 mv 〉 [meV]
〈 mv 〉 [meV]
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100 20 50
0
0 -2
-1
0
λ [107]
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2
-4
-2
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λλ⋅ 107
Fig. 2. Left: Expected SuperNEMO sensitivity on the model parameters (mν , λ) for the isotopes 82 Se (light blue) and 150 Nd (dark blue). Right: Constraints for k = 0.3 (25% RHC admixture) at 1σ CL on (mν , λ) from: observation of 0νββ decay half-life of 82 Se at T1/2 = 1025 and 1026 y (outer and inner blue elliptical areas); reconstruction of the angular (outer, lighter green) and energy (inner, darker green) distribution shape; combined analysis of the decay rate and energy distribution shape (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusion SuperNEMO has the unique potential to measure the decay electrons’ kinematical distributions, and thus probe the underlying new physics mechanism of neutrinoless double beta decay. It can greatly improve the current bounds on the effective neutrino mass and the RHC λ parameter, or, in the case of 0νββ discovery, identify the RHC admixture. The angular and energy correlations provide a powerful tool for discriminating between underlying physics mechanisms. The results of this study will be published in [7], and the method could be extended to other new physics models. References [1] M. Doi, T. Kotani, H. Nishiura, E. Takasugi, Prog. Theor. Phys. 69 (1983) 602; A. Ali, A.V. Borisov, D.V. Zhuridov, hep-ph/0606072 ; 0801.2512 [hep-ph]; Phys. Rev. D 76 (2007) 093009. 0706.4165 [hep-ph]. [2] R. Arnold, et al., Nucl. Instrum. Meth. A536 (2005) 79. physics/0402115; Phys. Rev. Lett. 95 (2005) 182302. hep-ex/0507083. [3] O.A. Ponkratenko, V.I. Tretyak, Yu.G. Zdesenko, Phys. Atom. Nucl. 63 (2000) 1282. nucl-ex/0104018. [4] A. Ali, A.V. Borisov, D.V. Zhuridov, hep-ph/0606072. [5] K. Muto, E. Bender, H.V. Klapdor, Z. Phys. A 334 (1989) 187. [6] F. Simkovic, Private communication. [7] The SuperNEMO Collaboration and F. Deppisch (in preparation).
Contents lists available at ScienceDirect
Progress in Particle and Nuclear Physics journal homepage: www.elsevier.com/locate/ppnp
Review
Probing the mechanism of neutrinoless double beta decay with SuperNEMO Frank Deppisch, Chris Jackson, Irina Nasteva ∗,1 , Stefan Söldner-Rembold University of Manchester, Manchester, United Kingdom
article
info
Keywords: Neutrinoless double beta decay SuperNEMO Neutrino mass
abstract The SuperNEMO experiment is being designed to search for neutrinoless double beta decay. Its experimental technique of tracking and calorimetry provides the means to discriminate different underlying mechanisms for neutrinoless double beta decay by measuring the angular and energy distributions of electrons. The results of a study by the SuperNEMO Collaboration and F. Deppisch (in preparation) [7] for identifying light Majorana neutrino exchange and right-handed currents are presented. © 2010 Elsevier B.V. All rights reserved.
1. Neutrinoless double beta decay Neutrinoless double beta decay (0νββ ) is the process in which two neutrons in a nucleus undergo simultaneous beta decays with the emission of two electrons. It violates lepton number and therefore represents physics beyond the Standard Model. The most commonly considered underlying mechanism of 0νββ is light neutrino exchange, or the mass m mechanism (MM). In it the 0νββ half-life can be expressed as [T1/ν2 ]−1 = (hmν i/me )2 G01 |M mν |2 , where hmν i is the effective neutrino P mass with contributions of individual neutrino mass eigenstates weighted by mixing matrix elements squared, hmν i = | Uei2 mi |, G01 is a calculable phase-space factor, and Mmν is the nuclear matrix element (NME). 1.1. Right-handed currents Other lepton number violating models could also contribute to the 0νββ decay rate. The example model considered m here is right-handed currents (RHC), which arise in left–right symmetric models. The general decay rate is [T1/ν2 ]−1 =
Cmm (hmν i/me )2 + Cλλ hλi2 + Cmλ (hmν i/me )hλi, where λ is the coupling of V + A interactions at hadronic and leptonic vertices, and the coefficients C contain phase-space factors and NMEs. The angular and energy difference distributions of the decay electrons, predicted by RHC models, differ from these of the MM model. This feature allows us to distinguish the two by measuring the electrons’ kinematical correlations [1]. 2. Expected sensitivity of SuperNEMO SuperNEMO is a next-generation experiment which will employ a technique of tracking and calorimetry [2] to search for 0νββ decay in ∼100 kg of enriched isotopes. It will comprise 20 modules, each containing ∼5 kg of isotope source (82 Se or 150 Nd) as a thin foil, surrounded by a tracking chamber with drift cells in Geiger mode, and calorimeter walls with plastic scintillators and PMTs. The experiment is currently in an R&D phase, expecting to start the first module construction in 2010.
∗
Corresponding author. E-mail address: [email protected] (I. Nasteva).
1 Speaker. 0146-6410/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ppnp.2009.12.028
F. Deppisch et al. / Progress in Particle and Nuclear Physics 64 (2010) 278–280
279
Fig. 1. Reconstructed slope factor krecon as a function of admixture of the true ktrue from angular and energy distributions in 82 Se (left) and 150 Nd (right).
A simulation study was performed in the SuperNEMO software framework, using the DECAY0 [3] event generator for 0νββ signals and 2νββ , 214 Bi and 208 Tl backgrounds. The generated particles undergo a full simulation of the detector response, track√reconstruction and realistic event selection, with the SuperNEMO design parameters of 40 mg/cm2 foil thickness, 7%/ E (FWHM) energy resolution, and 500 kg y exposure. Limits were set at 90%CL as a function of RHC m admixture, by analysing the electron energy sum. The resulting bounds are T1/ν2 > 1.15 · 1026 y (1.15 · 1026 y) for 82 Se (150 Nd) in the case of pure MM, with a slight decrease for higher RHC admixtures due to acceptance effects. 2.1. Angular and energy distributions In addition to the energy sum, SuperNEMO can measure the individual electron energies and tracks, allowing the use of kinematical correlations to distinguish between new physics scenarios. The distribution of the angle between the decay dΓ electrons can be written in general as d cos = Γ2 (1 − kθ cos θ12 ) [4], where the slope kθ depends on the new physics θ12 mechanism. An angular asymmetry Aθ = kθ /2 is defined with respect to cos θ12 = 0. Similarly, an energy difference asymmetry AE = kE /2 is defined with respect to Qββ /2, giving the measured energy parameter kE . The reconstructed k parameters as a function of the theoretical values are given in Fig. 1. 3. Identifying the 0νββ decay mechanism Fig. 2 (left) shows the SuperNEMO exclusion plot in hmν i–λ parameter space. The plot was obtained from the 0νββ half-life sensitivity by using the NME values from [5] and a 2.7 correction factor for the deformed nucleus of 150 Nd [6]. SuperNEMO can achieve sensitivity of about hmν i ≤ 70 meV for the effective neutrino mass, and hλi ≤ 1.2 · 10−7 for the right-handed coupling parameter. In the case of discovery of 0νββ decay, SuperNEMO can combine the measured half-life with the angular and energy difference distributions to pinpoint the underlying physics mechanism. Fig. 2 (right) shows the allowed regions in (mν , λ) parameter space from the observed half-life, the angular and energy correlations, and a combined statistical analysis of both. The plot includes NME errors of 30% and shows the case of 25% RHC admixture. For a half-life of 1026 y, SuperNEMO can identify pure RHC at 2σ , and for the more favourable half-life value of 1025 y, it can measure the RHC admixture to 20%.
280
F. Deppisch et al. / Progress in Particle and Nuclear Physics 64 (2010) 278–280 300
80
82
Se
150
250
Nd
60
〈 mv 〉 [meV]
〈 mv 〉 [meV]
200
40
150
100 20 50
0
0 -2
-1
0
λ [107]
1
2
-4
-2
0
2
4
λλ⋅ 107
Fig. 2. Left: Expected SuperNEMO sensitivity on the model parameters (mν , λ) for the isotopes 82 Se (light blue) and 150 Nd (dark blue). Right: Constraints for k = 0.3 (25% RHC admixture) at 1σ CL on (mν , λ) from: observation of 0νββ decay half-life of 82 Se at T1/2 = 1025 and 1026 y (outer and inner blue elliptical areas); reconstruction of the angular (outer, lighter green) and energy (inner, darker green) distribution shape; combined analysis of the decay rate and energy distribution shape (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusion SuperNEMO has the unique potential to measure the decay electrons’ kinematical distributions, and thus probe the underlying new physics mechanism of neutrinoless double beta decay. It can greatly improve the current bounds on the effective neutrino mass and the RHC λ parameter, or, in the case of 0νββ discovery, identify the RHC admixture. The angular and energy correlations provide a powerful tool for discriminating between underlying physics mechanisms. The results of this study will be published in [7], and the method could be extended to other new physics models. References [1] M. Doi, T. Kotani, H. Nishiura, E. Takasugi, Prog. Theor. Phys. 69 (1983) 602; A. Ali, A.V. Borisov, D.V. Zhuridov, hep-ph/0606072 ; 0801.2512 [hep-ph]; Phys. Rev. D 76 (2007) 093009. 0706.4165 [hep-ph]. [2] R. Arnold, et al., Nucl. Instrum. Meth. A536 (2005) 79. physics/0402115; Phys. Rev. Lett. 95 (2005) 182302. hep-ex/0507083. [3] O.A. Ponkratenko, V.I. Tretyak, Yu.G. Zdesenko, Phys. Atom. Nucl. 63 (2000) 1282. nucl-ex/0104018. [4] A. Ali, A.V. Borisov, D.V. Zhuridov, hep-ph/0606072. [5] K. Muto, E. Bender, H.V. Klapdor, Z. Phys. A 334 (1989) 187. [6] F. Simkovic, Private communication. [7] The SuperNEMO Collaboration and F. Deppisch (in preparation).