Physics reach of a low threshold scintillating argon bubble chamber in coherent elastic neutrino-nucleus scattering reactor experiments
L. J. Flores, Eduardo Peinado, E. Alfonso-Pita, K. Allen, M. Baker, E. Behnke, M. Bressler, K. Clark, R. Coppejans, C. Cripe, M. Crisler, C. E. Dahl, A. de St. Croix, D. Durnford, P. Giampa, O. Harris, P. Hatch, H. Hawley, C. M. Jackson, Y. Ko, C. Krauss, N. Lamb, M. Laurin, I. Levine, W. H. Lippincott, R. Neilson, S. Pal, M.-C. Piro, Z. Sheng, E. Vázquez-Jáuregui, T. J. Whitis, S. Windle, R. Zhang, A. Zuñiga-Reyes
FFERMILAB-PUB-21-016-AE-E-LDRD-ND
Physics reach of a low threshold scintillating argon bubble chamber in coherent elasticneutrino-nucleus scattering reactor experiments
L. J. Flores ∗ and Eduardo Peinado † (CE ν NS Theory Group at IF-UNAM)E. Alfonso-Pita, ‡ K. Allen, M. Baker, E. Behnke, M. Bressler, K. Clark, R. Coppejans,
6, 7
C. Cripe, M. Crisler, C. E. Dahl,
6, 8
A. de St. Croix, D. Durnford, P. Giampa, O. Harris, P. Hatch, H. Hawley, C. M. Jackson, Y. Ko, C. Krauss, N. Lamb, M. Laurin, I. Levine, W. H. Lippincott, R. Neilson, S. Pal, M.-C. Piro, Z. Sheng, E. Vázquez-Jáuregui,
1, 14, § T. J. Whitis, S. Windle, R. Zhang, and A. Zuñiga-Reyes (SBC Collaboration) Instituto de Física, Universidad Nacional Autónoma de México, A.P. 20-364, Ciudad de México 01000, México. Department of Physics, Indiana University South Bend, South Bend, Indiana 46634, USA Department of Physics, University of Alberta, Edmonton, T6G 2E1, Canada Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104, USA Department of Physics, Queen’s University, Kingston, K7L 3N6, Canada Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA Colegio de Física Fundamental e Interdiciplinaria de las Américas (COFI), San Juan, Puerto Rico 00901, USA Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA SNOLAB, Lively, Ontario, P3Y 1N2, Canada Northeastern Illinois University, Chicago, Illinois 60625, USA Pacific Northwest National Laboratory, Richland, Washington 99354, USA Département de Physique, Université de Montréal, Montréal, H3T 1J4, Canada Department of Physics, University of California Santa Barbara, Santa Barbara, California 93106, USA Department of Physics, Laurentian University, Sudbury, P3E 2C6, Canada
The physics reach of a low threshold (100 eV) scintillating argon bubble chamber sensitive to Coher-ent Elastic neutrino-Nucleus Scattering (CE ν NS) from reactor neutrinos is studied. The sensitivityto the weak mixing angle, neutrino magnetic moment, and a light Z (cid:48) gauge boson mediator areanalyzed. A Monte Carlo simulation of the backgrounds is performed to assess their contribution tothe signal. The analysis shows that world-leading sensitivities are achieved with a one-year exposurefor a 10 kg chamber at 3 m from a 1 MW th research reactor or a 100 kg chamber at 30 m from a2000 MW th power reactor. Such a detector has the potential to become the leading technology tostudy CE ν NS using nuclear reactors.
INTRODUCTION
The detection of neutrinos produced at nuclear re-actors via Coherent Elastic neutrino-Nucleus Scattering(CE ν NS) presents both an experimental challenge and ahost of new opportunities in neutrino physics. Measure-ments of CE ν NS to date have relied on pion decay-at-restneutrino sources [1, 2], measuring O (10) -keV nuclear re-coils and taking advantage of the ∼ − duty cycle of theSpallation Neutron Source at Oak Ridge National Lab-oratory. By contrast, the few-MeV neutrinos produced by nuclear reactors give a continuous rate of sub-keV nu-clear recoils, requiring an order-of-magnitude reductionin threshold and many-order-of-magnitude reduction inbackgrounds. The payoff, if these challenges are met,includes precision measurements of neutrino propertiesenabled by the up to × -higher neutrino flux, fully co-herent scattering of low-energy neutrinos and pure anti-electron neutrino flavor. A variety of detector technolo-gies are now in an experimental race to make the firstreactor CE ν NS observation [3–13].This paper explores the potential neutrino physics a r X i v : . [ h e p - e x ] J a n reach of a new enabling technology for reactor CE ν NSdetection, the liquid-noble (scintillating) bubble cham-ber. As in dark matter direct detection, this techniqueachieves the necessary background reduction by distin-guishing between nuclear recoils (signal) and electron re-coils (backgrounds from γ -rays and beta decays), butwhere existing detection techniques lose discrimination atnuclear recoil energies below ∼ Z (cid:48) gaugeboson mediator. We conclude that reactor CE ν NS pro-vides both a realistic and powerful opportunity to con-strain and discover neutrino physics beyond the StandardModel (SM).
EXPERIMENT DESCRIPTION
Superheated liquids have been used for over a decadeby dark matter direct detection experiments searchingfor Weakly Interacting Massive Particles (WIMPs), mostrecently in the PICO Collaboration’s fluorocarbon bub-ble chambers [20–23]. Nuclear recoils in the superheatedtargets of these devices create a single bubble, which,if the nuclear recoil energy is above a threshold set bythe temperature and pressure of the target fluid, growswithin a few milliseconds to macroscopic size . These de-tectors are completely insensitive to electron recoils (nu-cleation efficiency < − ) when operated with nuclearrecoil thresholds above a few keV [18], since the bubblenucleation depends not only on the energy deposited bythe incoming particle but also on its stopping power. This process is described by the Seitz model of bubble nucle-ation [24].
Work by the SBC Collaboration has shown thatliquid-noble bubble chambers are able to operate atmuch higher degrees of superheat (lower thresholds)than fluorocarbon-based detectors [25]. Most recently, axenon bubble chamber was operated at thresholds downto 500 eV while remaining insensitive to electron re-coil backgrounds, proving the feasibility of reducing thethreshold with noble liquids and demonstrating simulta-neous bubble nucleation and scintillation by nuclear re-coils. The SBC Collaboration is currently designing andbuilding a 10-kg liquid argon (LAr) bubble chamber witha target energy threshold of 100 eV. This detector will beequipped with Silicon Photomultipliers (SiPMs) to col-lect scintillation light generated in the target fluid, usedto veto high-energy events ( (cid:39) -keV nuclear recoil equiv-alent). These experimental techniques and developmentsopen a new window of opportunity to study CE ν NS innuclear reactors using noble liquids operated at very lowthresholds and free of electron recoil backgrounds.Two main detector configurations are considered inthis work: a 10-kg LAr chamber operated at a 100-eVenergy threshold and located 3 m from a 1-MW th reactor(setup A) , where ∼ neutrino events/day above thresh-old are expected; and a 100-kg LAr chamber operatedat the same threshold and located 30 m from a 2000-MW th power reactor (setup B) , where ∼ neutrinoevents/day above threshold are expected. These config-urations assume a 2.4% uncertainty in the anti-neutrinoflux and 5% systematic uncertainty in the energy thresh-old. A third configuration named setup B(1.5) is alsoconsidered, with the same parameters as setup B butwith a 1.5% uncertainty in the anti-neutrino flux and a2% systematic uncertainty in the energy threshold. Backgrounds
A GEANT4 [26–28] Monte Carlo simulation wasdeveloped to estimate the main background contribu-tions, primarily neutrons from cosmic rays and thereactor itself. While backgrounds from cosmic rays canbe statistically subtracted with a reactor-off dataset, Low-threshold performance from private communication, publi-cation in preparation. A TRIGA Mark III research reactor located at the National In-stitute for Nuclear Research (ININ) near Mexico City is beingexplored as a possible location. The Laguna Verde (LV) power reactor consisting of two BRW-5(Boiling Water Reactors) units located in the east coast of Mex-ico in the Gulf of Mexico is also explored as a possible location. reactor-induced backgrounds must be estimated with in-situ measurements and simulations. Backgroundswere studied in the explored sites at the NationalInstitute for Nuclear Research (ININ) near Mexico City,for the 1 MW th reactor configuration, and at LagunaVerde (only from cosmic rays and not from the reactor)on the east coast of Mexico, for a 2000 MW th reactor.For setup A, the model includes the experimentalhall at ININ, which is surrounded by approximately3 m of high-density borated concrete that will act as ashield for cosmogenic neutrons. Moreover, the shieldingmodel features 25 cm of water and 5 cm of polyethylenesurrounding the detector, a 30 cm thick Pb-wall betweenthe water pool and the shielding, and another 20 cmthick Pb-wall next to the bubble chamber. The distancebetween the reactor core centre and the bubble chamberis 3 m, including 1.6 m of water shielding provided bythe reactor pool.Neutrons produced by the reactor core are estimatedusing a measurement at ININ performed as part of theradiation programme [29, 30]. Nuclear recoils producedby ( γ , n) reactions and Thomson ( γ -nucleus elastic)scattering [31] from γ -rays produced by the reactor areestimated using a gamma flux simulation for a TRIGAMark III reactor, obtained with an MCNP model of thecore [6].Cosmogenic neutrons are estimated with a simulationof the neutron flux using the code CRY [32] and neu-trons induced by muons interacting with materials in thedeployment site are estimated using the parametrizationfrom [33] in water and concrete.The simulations predict . events/day abovethreshold (3.1% of the signal) from backgrounds pro-duced by the reactor. Of these, . events/day (0.4%of the signal) are from reactor neutrons, . events/day(2.0% of the signal) are from H( γ ,n) reactions in thewater, and . events/day (0.7% of the signal) arefrom Pb( γ ,n) and Pb( γ ,n). The shielding conceptproposed reduces the gamma flux from the reactor coreto ∼ Hz in the LAr target volume. At this rate electronrecoil backgrounds are negligible given the expectedinsensitivity to these events. Thomson scattering isexpected to contribute . events/day ( < . events/day abovethreshold (10.8% of the signal) from backgroundsproduced by cosmic rays, including . events/dayfrom cosmogenic neutrons (4.8% of the signal) and . events/day (6.0% of the signal) from muon-inducedneutrons in water and concrete.For setups B and B(1.5), only simulations for cosmogenic and muon-induced neutrons were consid-ered, since at 30 m (usually outside of the reactorbuilding) the backgrounds produced from the core arenegligible. Shielding consisting of 3 m of water and50 cm of polyethylene is included in this simulation,which reduces the backgrounds from cosmic rays to180 events/day above threshold (11.5% of the signal),including 125 events/day from cosmogenic neutrons and55 events/day from muon-induced neutrons in the watershield.Backgrounds from internal radioactivity are negligi-ble for all configurations, accounting for approximately . events/day above threshold (<1% of the signal),where the purity of the components assumed is similarto the materials used in bubble chambers built by thePICO Collaboration [20, 22].Overall, the background contribution to the signal isestimated to be on the order of 5% (from the reactor)and 11% (from cosmic rays) for setup A, and 12% (fromcosmic rays) for setups B and B(1.5). The physics reachreported in this manuscript assumes these backgroundlevels, which do not consider the ability to veto (cid:39) -keV recoils by their scintillation light. A systematicuncertainty of 10% is assumed for reactor backgrounds,which can be characterized in-situ from non-signalregions (multiply-scattering neutron events and bubblescoincident with scintillation signals). Backgroundsfrom cosmic rays are statistically subtracted with nosystematic uncertainty. Calibration
The response of a bubble chamber to nuclear recoils isdescribed by a nucleation efficiency function, represent-ing the probability of a recoil with energy T to nucleatea bubble, rising from 0 to 100% in the vicinity of an en-ergy threshold E T . For the physics reach reported here,a Normal Cumulative Distribution Function (GaussianCDF) is assumed, P r ( T ) = 12 (cid:18) (cid:18) T − E T σ √ (cid:19)(cid:19) , (1)where E T is set to 100 eV and the width σ is set to10 eV, a comparably sharp turn-on to that observed inC F [22]. This functional form is chosen for convenience;the exact shape will need to be experimentally measured.A 5% (setups A and B) or 2% (setup B(1.5)) systematicuncertainty in E T is assumed, intended to encompassboth threshold and general shape uncertainties followinga calibration program.Low energy, nearly mono-energetic, neutrons can beproduced by ( γ ,n) reactions in beryllium. Three photo-neutron sources, each producing different recoil energyspectra in the detector, are proposed to calibrate low-energy nuclear recoils. Bi-Be (94 keV neutrons),
Sb-Be (23 and 380 keV neutrons) and Co-Be (9 keVneutrons) sources were simulated in the GEANT4 geom-etry developed for the 10-kg chamber. The simulationsindicate that with sources of 1 to 100 µ Ci activities, high-statistics recoil energy spectra below 8 keV, 3 keV, and1 keV can be achieved with the
Bi-Be,
Sb-Be, and Co-Be sources, respectively. These sources would al-low constraint of the nucleation efficiency function fordifferent thermodynamic conditions. A similar techniquehas previously been implemented by the PICO Collabo-ration [22].Blindness to electron recoils allows for a novel ad-ditional calibration with nuclear recoils from Thomsonscattering. For example, 1.33, 1.41 and 1.46 MeV γ -raysfrom Co,
Eu and K produce nuclear recoil spectrawith sharp cut-offs at 95, 107 and 115 eV respectively,and would provide strong constraints on the nucleationefficiency for recoils ∼
100 eV. Finally, a tagged recoilcalibration may be possible with thermal neutrons. De-excitation γ -rays from neutron capture on Ar result ina recoiling Ar nucleus with energy peaked ∼
320 eV.
PHYSICS REACH
The physics reach of the setups described above is in-vestigated for a one-year exposure. The SM cross-sectionfor CE ν NS, after neglecting the axial contribution, is: dσdT = G F π M N Q w (cid:18) − M N TE ν (cid:19) F ( q ) , (2)where T is the nuclear recoil energy, E ν the incomingneutrino energy, F ( q ) the nuclear form factor, Q w = Zg Vp + N g Vn is the weak nuclear charge and M N , Z , N are the nuclear mass, proton, and neutron numberof the detector material, respectively. The cross-sectionis convolved with the reactor anti-neutrino spectrumand the detector efficiency to compute the number ofevents. The theoretical prediction of the Huber + Muellermodel [34, 35], which gives a 2.4% uncertainty in the to-tal flux, is considered for setups A and B for neutrinoenergies between 2 and 8 MeV (Ref. [36] is used for neu-trinos below 2 MeV). On the other hand, the Daya Bayexperiment measured the anti-neutrino flux from theirreactors with an uncertainty of . [37]. Setup B(1.5) considers this uncertainty. It is worth mentioning thatat reactor energies, the uncertainties in the form factorsare negligible compared to the uncertainty in the anti-neutrino spectrum [38].The sensitivity of this experiment is fitted with thefollowing χ function: χ = min α,β,γ (cid:20)(cid:16) N meas − (1+ α ) N th ( X,γ ) − (1+ β ) B reac σ stat (cid:17) + (cid:16) ασ α (cid:17) + (cid:16) βσ β (cid:17) + (cid:16) γσ γ (cid:17) (cid:21) , (3)where N meas is the measured number of events aftersubtracting the background from cosmogenic and muon-induced neutrons ( B cosm ), N th ( X, γ ) is the theoreti-cal prediction with the nuclear recoil threshold set to(1+ γ ) ·
100 eV, B reac is the background coming from thereactor, σ stat = (cid:112) N meas + ( R + 1) B cosm is the statisti-cal uncertainty, where R is the ratio of reactor-on timeto reactor-off time , and σ α,β,γ are the systematic un-certainties on the signal, background, and threshold, re-spectively. The variable X refers to the parameter tobe fitted (weak mixing angle, NSI parameters, or neu-trino magnetic moment). The χ function is minimizedover the nuisance parameters α , β and γ . The system-atic uncertainties have the values σ α = 0 . , σ β = 0 . ,and σ γ = 0 . for setups A and B, coming from the un-certainty on the anti-neutrino flux, the reactor neutronbackground, and the energy threshold, respectively. Theparameters β and σ β are absent in setups B and B(1.5)since the reactor component of the background reachingthe detector is negligible. The systematic uncertaintiesfor setup B(1.5) are σ α = 0 . and σ γ = 0 . . In thefollowing analyses, N meas is assumed to be the SM pre-dicted signal. The Weak Mixing Angle
Assuming that the experiment measures only the SMsignal, a fit is performed and the value of the weak mixingangle at low energies is extracted with its correspondinguncertainty. The weak mixing angle can be extractedfrom the CE ν NS differential cross-section through theSM weak coupling g Vp = 1 / − θ W . A fit usingEq. (3) is performed where X = sin θ W . In Fig. 1 theRenormalization Group Equation (RGE) running of theweak mixing angle as a function of the energy scale is Four months off time is assumed at ININ (R=3) and one monthoff time at LV (R=12).
Tevatron
Setup B(1.5)Setup BSetup A
APV Q weak
SLAC-E158eDIS NuTeV SLCLEP LHC -4 -3 -2 -1 µ (GeV) s i n ˆ θ W ( µ ) FIG. 1: RGE running of the weak mixing angle in the MS renormalization scheme [39, 40], as a function of theenergy scale µ . The expected measurements and σ un-certainties for setups A, B and B(1.5) are shown in solidpurple, solid orange, and dashed orange, respectively.Measurements from other experiments are also presented.Figure adapted from [39].shown, in the Minimal Subtraction ( MS ) renormaliza-tion scheme [39, 40], as well as the projections of thedetectors for the setups described, and their estimated σ uncertainties.The projection obtained for the configuration assum-ing 1.5% uncertainty in the reactor spectrum is notonly complementary to the low-energy measurement fromAtomic Parity Violation (APV) [41], but is also the mostsensitive among projections for several CE ν NS experi-ments [42] that assume . to . systematic uncer-tainty in the reactor spectrum. Light Gauge Boson Mediator
Extra U (1) gauge symmetries are common extensionsof the SM [43–45]. Many phenomenological studies sen-sitive to both heavy and light Z (cid:48) mediators have beencompleted combining beam dump experiments and di-rect searches in colliders [46–48], and even to explain theanomalous magnetic moment of the muon [49–51]. Inthis work, a gauged B − L symmetry is studied, namelythat the extra gauge boson couples to quarks and leptons.In this scenario, quarks have U (1) B − L charge Q q = 1 / , Setup ASetup BSetup B(1.5)CONNIE (Lindhard) C O H E R E N T - C s I COHERENT-LArBeam dump A T L A S BaBar LHCb -7 -6 -5 -4 -3 -3 -2 -1 M Z (GeV) g FIG. 2: Exclusion limits (95% C.L.) in the g (cid:48) - M Z (cid:48) plane.The solid purple, solid orange and dashed orange linesrepresent the limits for setups A, B and B(1.5), respec-tively. The dash-dotted gray curve is the exclusion setby CONNIE [55]. The shaded brown and yellow regionscorrespond to the exclusions set by COHERENT, usingCsI [1] and LAr [2, 56] detectors, respectively. Exclusionregions for dark photon searches from BaBar [57] andLHCb [58] are shown in light gray, and from beam dumpexperiments [59–68] are shown in blue. These limits wereobtained in the framework of Ref. [69]. The exclusion re-gion from an ATLAS search for dilepton resonances [70]is also shown in light gray, using the software developedin Ref. [71].while leptons have Q l = − . This will induce the fol-lowing Beyond the SM interaction between neutrinos andquarks: L eff = − g (cid:48) Q l Q q q + M Z (cid:48) (cid:34)(cid:88) α ¯ ν α γ µ P L ν α (cid:35) (cid:34)(cid:88) q ¯ qγ µ q (cid:35) , (4)where q is the transferred momentum. This interactionwill give rise to interference with the SM cross-section.In Fig. 2 the expected sensitivities from the detec-tors are shown for all setups in the g (cid:48) − M Z (cid:48) plane. Thelimits for a one-year exposure are better than other cur-rent CE ν NS experiments for all setups. The scintillatingbubble chamber would be the leading technology in newvector boson searches from MeV to ∼ GeV and from to GeV. These constraints are similar to scenarios of gauged B − L e [52, 53], B − L e − L µ,τ and B − L e − L µ,τ [54]. C O H E R E N T - C s I C O H E R E N T - L A r Setup ASetup BSetup B(1.5) -11 -10 -9 -8 µ ν ( µ B ) ∆ χ
90% C . L . FIG. 3: Limits for the neutrino magnetic moment. Thesolid purple, solid orange and dashed orange lines rep-resent the limits for setups A, B and B(1.5), respec-tively. The shaded brown and yellow regions correspondto the exclusions set by COHERENT, using CsI [1] andLAr [2, 56] detectors, respectively.
The Neutrino Magnetic Moment
Neutrino magnetic moments can arise from their in-teraction with the electromagnetic field, either for Ma-jorana or Dirac neutrinos [72, 73]. This new interactioncontributes to the CE ν NS cross-section without interfer-ence, with the following expression: dσdT = π α Z µ ν m e (cid:18) T − E ν + T E ν (cid:19) F ( q ) , (5)where α EM is the electromagnetic coupling and m e is theelectron mass. The neutrino magnetic moment, µ ν , isnormalized by the Bohr magneton µ B .The resulting limits from the χ analysis for thethree setups are presented in Fig. 3. The bounds onthe neutrino magnetic moment are of the same order ofmagnitude as the current GEMMA and Borexino bounds[74, 75]. CONCLUSIONS
The physics reach of a low threshold LAr scintillat-ing bubble chamber for CE ν NS in a reactor has beeninvestigated. A Monte Carlo simulation has shown thatit is possible to reach a background level approximately of the signal ( in-situ measurements would constrainthe associated systematic uncertainties). A plan to de- termine the nuclear recoil efficiency at a 100 eV energythreshold has been evaluated with the Monte Carlo modeldeveloped, showing that it is possible to calibrate to sub-keV energy thresholds using photo-neutron and Thom-son scattering sources. The sensitivity for an electroweakprecision test, a new vector mediator, and the neutrinomagnetic moment is very competitive under realistic as-sumptions for backgrounds and systematic uncertainties.A precision as good as is obtained in the case of theweak mixing angle, a value of the same order as the uncer-tainty from APV. The setups considered here would setthe most stringent bounds for new gauge vector bosonsin the MeV to ∼ GeV and to GeV massranges. For the neutrino magnetic moment, the best sce-nario gives a bound of . × − µ B ( C.L.), of thesame order of magnitude as the current GEMMA andBorexino limits. This detector technology has the poten-tial to lead different physics scenarios for coherent elas-tic neutrino-nucleus scattering experiments and a worldleading physics programme can be achieved not only ina power reactor facility (2000 MW th ), but also in a lowpower research reactor (1 MW th ) with only a one-yearexposure. ACKNOWLEDGEMENTS ∗ luisjf89@fisica.unam.mx † epeinado@fisica.unam.mx ‡ ernestoalfonso@estudiantes.fisica.unam.mx § ericvj@fisica.unam.mx[1] D. Akimov et al. (COHERENT), Science , 1123(2017), arXiv:1708.01294.[2] D. Akimov et al. (COHERENT), Phys. Rev. Lett. ,012002 (2021), arXiv:2003.10630.[3] Y. Abreu et al. (SoLid), (2020), arXiv:2002.05914.[4] J. Hakenmüller et al., Eur. Phys. J. C , 699 (2019),arXiv:1903.09269.[5] V. Belov et al., JINST , P12011 (2015).[6] G. Agnolet et al. (MINER), Nucl. Instrum. Meth. A ,53 (2017), arXiv:1609.02066.[7] J. Billard et al., J. Phys. G , 105101 (2017),arXiv:1612.09035.[8] G. Fernandez Moroni et al., Phys. Rev. D , 072001(2015), arXiv:1405.5761.[9] D. Akimov et al., JINST , C06018 (2017).[10] G. Fernandez-Moroni et al., (2020), arXiv:2009.10741.[11] J. J. Choi, PoS NuFact2019 , 047 (2020).[12] H. T.-K. Wong, The Universe , 22 (2015),arXiv:1608.00306.[13] R. Strauss et al., Eur. Phys. J. C77 , 506 (2017),arXiv:1704.04320.[14] R. Agnese et al. (SuperCDMS), Phys. Rev.
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