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COHERENT Neutrino Scattering
Available online at www.sciencedirect.com
Nuclear and Particle Physics Proceedings 265–266 (2015) 117–119 www.elsevier.com/locate/nppp
COHERENT Neutrino Scattering P. S. Barbeau for the COHERENT Collaboration Department of Physics, Duke University, and Triangle Universities Nuclear Laboratory (TUNL), Durham North Carolina, USA
Abstract The experimental program to search for the coherent elastic neutrino-nucleus scattering at the Spallation Neutron Source is presented. Keywords: coherent neutrino scattering
1. Introduction The coherent elastic scattering of neutrinos off nuclei was predicted 40 years ago with the realization of the neutral weak currents [1]. This standard model process remains unobserved due to the daunting detector requirements: sub-keV nuclear recoil thresholds, 101000 kg-scale target masses and low backgrounds. Due to the small weak charge of the proton, the coherence results in an enhanced neutrino-nucleon cross-section that is approximately proportional to N2 , the square of the number of neutrons in the nucleus. Coherence is only satisfied when the initial and final states of the nucleus are identical, limiting this enhancement to neutral current scattering. The coherence condition, where the neutrino scatters off all nucleons in a nucleus in phase, is also only maintained when the wavelength of the momentum transfer is larger than the size of the target nucleus. Full coherence for all scatters is only guaranteed for low energy neutrinos (∼30 MeV, depending on the target size). As a result, the experimental signature for the process is a difficult to detect keV to sub-keV recoil for most nuclear targets. A measurement of the coherent scatter cross-section can have many interesting physics implications. To begin with, the process is the largest cross-section in supernovae dynamics [2], and should be measured to validate models of their behavior. A precision measurement can test for extra neutral currents due to new physics above the weak scale [3]. When performed with http://dx.doi.org/10.1016/j.nuclphysbps.2015.06.029 2405-6014/© 2015 Elsevier B.V. All rights reserved.
multiple targets, the measurement can investigate nonstandard neutrino interactions (NSI) that depend on the quark makeup of the nucleus [4][5]. It is also a sensitive test of models with large neutrino magnetic moments [6][5]. Models with large μν can also be simultaneously tested with neutrino-electron scattering by virtue of the extremely low threshold of the detectors, and the characteristic E1rec recoil spectrum. As a suite of neutral-current detectors that are equally sensitive to all known neutrino flavors, and that have the largest crosssections for low energy neutrinos, an experiment with near and far detecters can test for oscillations into sterile neutrinos [7][8]. For neutrino energies greater than 10’s of MeV, full coherence begins to be lost as the shorter wavelength of the momentum transfer to the recoil begins to probe the form factor of the nuclei. By measuring the differential recoil rate at a stopped pion beam (Eν < 60 MeV) the neutron distribution function can be measured with neutrinos [9]. The observation of this Standard Model process, though difficult to detect and long overdue, will not come as a surprise. Instead, the coherent elastic scattering of neutrinos off nuclei should be viewed as a tool to search for extensions to the Standard Model. 2. COHERENT ν Scattering Experiment at the SNS The COHERENT collaboration has recently initiated a program [10] to search for the coherent elastic scattering of neutrinos off nuclei at the Spallation Neutron
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P.S. Barbeau / Nuclear and Particle Physics Proceedings 265–266 (2015) 117–119
3. Backgrounds
Figure 1: The ionization spectrum expected in a PPC germanium detector due to coherent elastic neutrino-nucleus scattering at the SNS, 20 m from the target.
Source (SNS), a stopped pion decay at rest neutrino source at the Oak Ridge National Laboratory. The collaboration aims to bring together several individually developed detector technologies with the goal of measuring the coherent cross-section for several target nuclei and for several detector technologies. These include a suite of P-type Point Contact (PPC) High Purity Germanium detectors [11] such as those developed in the CoGeNT collaboration [12] and utilized in the Majorana Demonstrator [13]; a low-background CsI(Na) detector developed by the CosI collaboration [14]; and a 2-phase liquid xenon detector developed by the RED collaboration [15]. While the SNS is the world’s premier facility for neutron scattering research, the spallation reactions produced from the ∼1 GeV photons in mercury target result in the worlds most intense (∼2×107 cm−2 s−1 for all flavors), pulsed source of neutrinos with energies 1060 MeV. The so-called “prompt” νμ ’s from pion decay arrive within a 700 ns window, while the “delayed” neutrinos that result from the decay of the resulting muon arrive later and share the characteristic 2.2 μs decay time of the muon. The expected ionization signal observed in germanium PPC detectors can be seen in figure 1, for which the detector response is rapid enough to enable the separation of these populations. The pulsed nature of the beam also provides for a dramatic reduction in background by more than a factor of 2000.
Unfortunately, the SNS also produces large fluxes of high energy neutrons (>100 MeV) that are difficult to shield and are pulsed in time with the beam, making them impossible to reject by using the timing of the signal. As a result of this, the COHERENT collaboration has explored the SNS facility for areas devoid of these neutrons. At least one suitable location has been identified in the basement of the facility 20 m from the target, where there is also the benefit of an 8 m.w.e. overburden. This location has recently been put to use to study a newly realized and intriguing source of background. The higher energy neutrinos from pion decay-at-rest have energies above the neutron separation threshold in 208 Pb and can produce copious numbers of neutrons in the Pb shield that are also pulsed in time with the beam and have the right energy (few MeV) to produce nuclear recoils that mimic the signal from coherent scattering. The offending interactions are: νe +208 Pb ⇒208 Bi∗ +e− ⇒208−y Bi+ xγ+yn+e− (1) ν x +208 Pb ⇒208 Pb∗ +νx ⇒208−y Pb+xγ+yn+νx (2) where the number of neutrons depends on the neutrino energy and the type of interaction. While the exact cross-section is unknown (100-200% uncertainty [16]), it may prohibit the use of Pb in the shield, or at least complicate the design. It is believed that steel can be used as the gamma shielding material instead, as this cross-section is expected to be reduced by over a factor of 30; however, the Fe cross-section has never been measured. Importantly the COHERENT collaboration, has recently endeavored to measure this neutron production cross-section in both natural Pb and Steel at the SNS so that the optimal radiological shields can be designed. The lead radiological shielding that was designed for the CsI(Na) detector (figure 2) has been deployed to the basement location, and is currently being operated with liquid scintillator cells capable of neutron/gamma pulse shape discrimination. The deployment should result in a first-measurement of the neutrino-induced neutron cross-section on natural lead, as well as provide an in-situ determination of the impact of this background on the future coherent scattering measurement with the CsI(Na) detector. The measurement of this neutrino-induced-neutron cross-section on lead is not just academic: it provides an important validation for the HALO experiment [17]
P.S. Barbeau / Nuclear and Particle Physics Proceedings 265–266 (2015) 117–119
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form the first measurement of coherent elastic neutrinonucleus scattering using decay-at-rest neutrinos produced at the Spallation Neutron Source. The main goal is to observe the coherent cross-section on several target nuclei and with several detector technologies in order to pursue searches for new physics. Within the last year, extraordinarily rapid progress has occurred. It is expected that the first observation of neutrinos at the SNS will occur in 2015 in the form of the neutrino-induced neutron measurements on lead. Figure 2: Left: The assembly of the 2 kg CsI(Na) prototype (with low-background PMT and OFHC Cu parts) in the radiological shielding. Right: The full model of the shield with the 15 kg CsI(Na) detector. The shield, without the steel container pipe and also filled with liquid scintillator neutron detectors, was deployed to a basement location in late 2014 in order to perform an in-situ measurement of the neutrino-induced neutrons produced in Pb that could be a background for the coherent neutrino scattering experiment.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Figure 3: Schematic drawing of the dedicated detector to measure the neutron producing neutrino cross-section on lead. ∼1000 kg of natural lead are instrumented with liquid scintillator detectors capable of neutron/gamma pulse-shape discrimination. This is surrounded by a muon veto, 25 cm of neutron-moderating water, and is secured to a pallet.
[13] [14] [15] [16] [17] [18] [19]
as well as helping to understanding supernovae dynamics and r-process nucleosynthesis [16] [18] [19]. As such, a dedicated detector that is designed to obtain a more precise, and perhaps more information-rich, measurement of this process was also deployed to the same location. The detector system, which can be seen in figure 3, is palletized and easily movable. The system will also perform a measurement of this neutron producing cross-section on steel in order to inform the radiological shielding designs for the coherent neutrino scattering experiments. 4. Outlook The COHERENT collaboration is proceeding with a phased and robust approach that is designed to per-
D. Z. Freedman, Phys. Rev D, 9(5):1389-1392 (1974). J. R. Wilson, Phys. Rev. Lett 32:849-852 (1974). L. M. Krauss, Phys. Lett. B269:407-411 (1991) . K. Scholberg, Phys. Rev. D73(3):033005 (2006). J. Barranco, O. G. Miranda and T. I. Rashba, JHEP 12:021 (2005). A. C. Dodd, E. Papageorgiu, and S. Ranfone, Phys. Lett. B266:434-438 (1991). A. Drukier and L. Stodolsky, Phys. Rev. D, 30(11): 2295-2309 (1984). C. Giunti, M. Laveder, Phys. Rev. D, 84(9):093006 (2011). K. Patton, J. Engel, G. C. McLaughlin, N. Schunk, Phys. Rev. C 86(2):024612 (2012). COHERENT, C13-07-29.02 and arXiv:1210.0125. P. S. Barbeau, J. I. Collar, and O. Tench, JCAP, 2007(09):009 (2007) C. E. Aalseth, P. S. Barbeau, J. Colariesi, J. I. Collar, J. Diaz Leon, J. E. Fast, N. E. Fields, T. W. Hossback, A. Knecht, M. S. Kos, et al., Phys. Rev. D, 88(1):012002 (2013). Majorana Collaboration, AIP Conf.Proc. 1604:413-420 (2014). J. I. Collar, N. E. Fields, M. Hai, T. W. Hossbach, J. L. Orrell, et. al., Nucl. Instrum. Meth. A, 773:56 (2014). RED Collaboration, arXiv:1212.1938 G. C. McLaughlin, Phys. Rev. C, 70(4):045804 (2004). C. Duba, F. Duncan, J. Farine, A. Habig, A. Hime, et. al., J. Phys. Conf. Ser. 136:042077 (2008). W. C. Haxton, Phys. Rev. Lett. 60(9):768–771 (1988). Y-Z. Qian, W. C. Haxton, K. Langanke, P. Vogel, et. al, Phys. Rev. C 55(3):1532–1544 (1997).
Nuclear and Particle Physics Proceedings 265–266 (2015) 117–119 www.elsevier.com/locate/nppp
COHERENT Neutrino Scattering P. S. Barbeau for the COHERENT Collaboration Department of Physics, Duke University, and Triangle Universities Nuclear Laboratory (TUNL), Durham North Carolina, USA
Abstract The experimental program to search for the coherent elastic neutrino-nucleus scattering at the Spallation Neutron Source is presented. Keywords: coherent neutrino scattering
1. Introduction The coherent elastic scattering of neutrinos off nuclei was predicted 40 years ago with the realization of the neutral weak currents [1]. This standard model process remains unobserved due to the daunting detector requirements: sub-keV nuclear recoil thresholds, 101000 kg-scale target masses and low backgrounds. Due to the small weak charge of the proton, the coherence results in an enhanced neutrino-nucleon cross-section that is approximately proportional to N2 , the square of the number of neutrons in the nucleus. Coherence is only satisfied when the initial and final states of the nucleus are identical, limiting this enhancement to neutral current scattering. The coherence condition, where the neutrino scatters off all nucleons in a nucleus in phase, is also only maintained when the wavelength of the momentum transfer is larger than the size of the target nucleus. Full coherence for all scatters is only guaranteed for low energy neutrinos (∼30 MeV, depending on the target size). As a result, the experimental signature for the process is a difficult to detect keV to sub-keV recoil for most nuclear targets. A measurement of the coherent scatter cross-section can have many interesting physics implications. To begin with, the process is the largest cross-section in supernovae dynamics [2], and should be measured to validate models of their behavior. A precision measurement can test for extra neutral currents due to new physics above the weak scale [3]. When performed with http://dx.doi.org/10.1016/j.nuclphysbps.2015.06.029 2405-6014/© 2015 Elsevier B.V. All rights reserved.
multiple targets, the measurement can investigate nonstandard neutrino interactions (NSI) that depend on the quark makeup of the nucleus [4][5]. It is also a sensitive test of models with large neutrino magnetic moments [6][5]. Models with large μν can also be simultaneously tested with neutrino-electron scattering by virtue of the extremely low threshold of the detectors, and the characteristic E1rec recoil spectrum. As a suite of neutral-current detectors that are equally sensitive to all known neutrino flavors, and that have the largest crosssections for low energy neutrinos, an experiment with near and far detecters can test for oscillations into sterile neutrinos [7][8]. For neutrino energies greater than 10’s of MeV, full coherence begins to be lost as the shorter wavelength of the momentum transfer to the recoil begins to probe the form factor of the nuclei. By measuring the differential recoil rate at a stopped pion beam (Eν < 60 MeV) the neutron distribution function can be measured with neutrinos [9]. The observation of this Standard Model process, though difficult to detect and long overdue, will not come as a surprise. Instead, the coherent elastic scattering of neutrinos off nuclei should be viewed as a tool to search for extensions to the Standard Model. 2. COHERENT ν Scattering Experiment at the SNS The COHERENT collaboration has recently initiated a program [10] to search for the coherent elastic scattering of neutrinos off nuclei at the Spallation Neutron
118
P.S. Barbeau / Nuclear and Particle Physics Proceedings 265–266 (2015) 117–119
3. Backgrounds
Figure 1: The ionization spectrum expected in a PPC germanium detector due to coherent elastic neutrino-nucleus scattering at the SNS, 20 m from the target.
Source (SNS), a stopped pion decay at rest neutrino source at the Oak Ridge National Laboratory. The collaboration aims to bring together several individually developed detector technologies with the goal of measuring the coherent cross-section for several target nuclei and for several detector technologies. These include a suite of P-type Point Contact (PPC) High Purity Germanium detectors [11] such as those developed in the CoGeNT collaboration [12] and utilized in the Majorana Demonstrator [13]; a low-background CsI(Na) detector developed by the CosI collaboration [14]; and a 2-phase liquid xenon detector developed by the RED collaboration [15]. While the SNS is the world’s premier facility for neutron scattering research, the spallation reactions produced from the ∼1 GeV photons in mercury target result in the worlds most intense (∼2×107 cm−2 s−1 for all flavors), pulsed source of neutrinos with energies 1060 MeV. The so-called “prompt” νμ ’s from pion decay arrive within a 700 ns window, while the “delayed” neutrinos that result from the decay of the resulting muon arrive later and share the characteristic 2.2 μs decay time of the muon. The expected ionization signal observed in germanium PPC detectors can be seen in figure 1, for which the detector response is rapid enough to enable the separation of these populations. The pulsed nature of the beam also provides for a dramatic reduction in background by more than a factor of 2000.
Unfortunately, the SNS also produces large fluxes of high energy neutrons (>100 MeV) that are difficult to shield and are pulsed in time with the beam, making them impossible to reject by using the timing of the signal. As a result of this, the COHERENT collaboration has explored the SNS facility for areas devoid of these neutrons. At least one suitable location has been identified in the basement of the facility 20 m from the target, where there is also the benefit of an 8 m.w.e. overburden. This location has recently been put to use to study a newly realized and intriguing source of background. The higher energy neutrinos from pion decay-at-rest have energies above the neutron separation threshold in 208 Pb and can produce copious numbers of neutrons in the Pb shield that are also pulsed in time with the beam and have the right energy (few MeV) to produce nuclear recoils that mimic the signal from coherent scattering. The offending interactions are: νe +208 Pb ⇒208 Bi∗ +e− ⇒208−y Bi+ xγ+yn+e− (1) ν x +208 Pb ⇒208 Pb∗ +νx ⇒208−y Pb+xγ+yn+νx (2) where the number of neutrons depends on the neutrino energy and the type of interaction. While the exact cross-section is unknown (100-200% uncertainty [16]), it may prohibit the use of Pb in the shield, or at least complicate the design. It is believed that steel can be used as the gamma shielding material instead, as this cross-section is expected to be reduced by over a factor of 30; however, the Fe cross-section has never been measured. Importantly the COHERENT collaboration, has recently endeavored to measure this neutron production cross-section in both natural Pb and Steel at the SNS so that the optimal radiological shields can be designed. The lead radiological shielding that was designed for the CsI(Na) detector (figure 2) has been deployed to the basement location, and is currently being operated with liquid scintillator cells capable of neutron/gamma pulse shape discrimination. The deployment should result in a first-measurement of the neutrino-induced neutron cross-section on natural lead, as well as provide an in-situ determination of the impact of this background on the future coherent scattering measurement with the CsI(Na) detector. The measurement of this neutrino-induced-neutron cross-section on lead is not just academic: it provides an important validation for the HALO experiment [17]
P.S. Barbeau / Nuclear and Particle Physics Proceedings 265–266 (2015) 117–119
119
form the first measurement of coherent elastic neutrinonucleus scattering using decay-at-rest neutrinos produced at the Spallation Neutron Source. The main goal is to observe the coherent cross-section on several target nuclei and with several detector technologies in order to pursue searches for new physics. Within the last year, extraordinarily rapid progress has occurred. It is expected that the first observation of neutrinos at the SNS will occur in 2015 in the form of the neutrino-induced neutron measurements on lead. Figure 2: Left: The assembly of the 2 kg CsI(Na) prototype (with low-background PMT and OFHC Cu parts) in the radiological shielding. Right: The full model of the shield with the 15 kg CsI(Na) detector. The shield, without the steel container pipe and also filled with liquid scintillator neutron detectors, was deployed to a basement location in late 2014 in order to perform an in-situ measurement of the neutrino-induced neutrons produced in Pb that could be a background for the coherent neutrino scattering experiment.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Figure 3: Schematic drawing of the dedicated detector to measure the neutron producing neutrino cross-section on lead. ∼1000 kg of natural lead are instrumented with liquid scintillator detectors capable of neutron/gamma pulse-shape discrimination. This is surrounded by a muon veto, 25 cm of neutron-moderating water, and is secured to a pallet.
[13] [14] [15] [16] [17] [18] [19]
as well as helping to understanding supernovae dynamics and r-process nucleosynthesis [16] [18] [19]. As such, a dedicated detector that is designed to obtain a more precise, and perhaps more information-rich, measurement of this process was also deployed to the same location. The detector system, which can be seen in figure 3, is palletized and easily movable. The system will also perform a measurement of this neutron producing cross-section on steel in order to inform the radiological shielding designs for the coherent neutrino scattering experiments. 4. Outlook The COHERENT collaboration is proceeding with a phased and robust approach that is designed to per-
D. Z. Freedman, Phys. Rev D, 9(5):1389-1392 (1974). J. R. Wilson, Phys. Rev. Lett 32:849-852 (1974). L. M. Krauss, Phys. Lett. B269:407-411 (1991) . K. Scholberg, Phys. Rev. D73(3):033005 (2006). J. Barranco, O. G. Miranda and T. I. Rashba, JHEP 12:021 (2005). A. C. Dodd, E. Papageorgiu, and S. Ranfone, Phys. Lett. B266:434-438 (1991). A. Drukier and L. Stodolsky, Phys. Rev. D, 30(11): 2295-2309 (1984). C. Giunti, M. Laveder, Phys. Rev. D, 84(9):093006 (2011). K. Patton, J. Engel, G. C. McLaughlin, N. Schunk, Phys. Rev. C 86(2):024612 (2012). COHERENT, C13-07-29.02 and arXiv:1210.0125. P. S. Barbeau, J. I. Collar, and O. Tench, JCAP, 2007(09):009 (2007) C. E. Aalseth, P. S. Barbeau, J. Colariesi, J. I. Collar, J. Diaz Leon, J. E. Fast, N. E. Fields, T. W. Hossback, A. Knecht, M. S. Kos, et al., Phys. Rev. D, 88(1):012002 (2013). Majorana Collaboration, AIP Conf.Proc. 1604:413-420 (2014). J. I. Collar, N. E. Fields, M. Hai, T. W. Hossbach, J. L. Orrell, et. al., Nucl. Instrum. Meth. A, 773:56 (2014). RED Collaboration, arXiv:1212.1938 G. C. McLaughlin, Phys. Rev. C, 70(4):045804 (2004). C. Duba, F. Duncan, J. Farine, A. Habig, A. Hime, et. al., J. Phys. Conf. Ser. 136:042077 (2008). W. C. Haxton, Phys. Rev. Lett. 60(9):768–771 (1988). Y-Z. Qian, W. C. Haxton, K. Langanke, P. Vogel, et. al, Phys. Rev. C 55(3):1532–1544 (1997).