COHERENT Elastic Neutrino-Nucleus Scattering Kate Scholberg, Duke University IPA 2016, Orsay, France September 6, 2016
OUTLINE - Coherent elastic neutrino-nucleus scattering - Why measure it? Physics motivations (short and long term) - How to measure it? - stopped pion sources and reactors - Experiments going after CEvNS - The COHERENT Experiment at the Spallation Neutron Source
Coherent elastic neutrino-nucleus scattering (CEvNS) ν ν + A → ν + A ν A neutrino smacks a nucleus Z 0 via exchange of a Z, and the nucleus recoils as a whole; coherent up to E ν ~ 50 MeV A A - Important in SN processes & detection ν ν - Well-calculable cross-section in SM: SM test, probe of neutrino NSI - Dark matter direct detection background - Neutron form factors - Possible applications (reactor monitoring) 4 π 2 k 2 (1 + cos θ )( N − (1 − 4 sin 2 θ W ) Z ) 2 F 2 ( Q 2 ) ∝ N 2 d Ω = G 2 d σ 4
\begin{aside} � Literature has CNS, CNNS, CENNS, ... - I prefer including “E” for “elastic”... otherwise HEP types constantly confuse it with coherent pion production at ~ GeV energies - I’m told “NN” means “nucleon-nucleon” to nuclear types (also CENNS is now a collaboration!) - CE ν NS is a possibility but those internal Greek letters are annoying è CEvNS , pronounced “sevens”... spread the meme! \end{aside} � Scholberg 4
The cross-section is large
Large cross section , but never observed due to tiny nuclear recoil energies: Nuclear recoil energy spectrum in Ge for 30 MeV ν Max recoil 2 /M energy is 2E ν (25 keV for Ge) è but WIMP dark matter detectors developed over the last ~decade are sensitive to ~ keV to 10’s of keV recoils
CEvNS from natural neutrinos creates ultimate background for direct DM search experiments Understand nature of background (& detector response)
Clean SM prediction for the rate è measure sin 2 θ W eff ; � deviation probes � G 2 f E 2 new physics � ( N − (1 − 4 sin 2 θ W ) Z ) 2 σ ∼ 4 π Plot based on arXiv: 1411.4088 Example: hypothetical dark Z mediator CEvNS sensitivity is @ low Q; � (explanation for g-2 need sub-percent precision to compete w/ anomaly) electron scattering & APV, but new channel �
Non-Standard Interactions of Neutrinos: new interaction specific to ν ’s = − G F ν α γ µ (1 − γ 5 ) ν β ] × ( ε qL q γ µ (1 − γ 5 ) q ] + ε qR � L NSI q γ µ (1 + γ 5 ) q ]) [¯ αβ [¯ αβ [¯ √ ν H 2 q = u,d α , β = e,µ, τ Can improve ~order of magnitude beyond CHARM limits with a first-generation experiment (for best sensitivity, want multiple targets ) K. Scholberg, PRD73, 033005 (2006) �
Oscillations to sterile neutrinos w/CEvNS (NC is flavor-blind): a potential new tool; look for deficit and spectral distortion vs L,E Examples: 456 kg Ar 100 kg Ge @ reactor Multi- π DAR sources at B. Dutta et al, arXiv:1511.02834 different baselines (20 & 40 m) Anderson et al., PRD86 (2012) 013004, arXiv:1201.3805
Neutrino magnetic moment Signature is distortion at low recoil energy E dE = πα 2 µ 2 ν Z 2 ✓ 1 − E/k ◆ d σ + E m 2 E 4 k 2 e Ne target � µ B µ B è requires low energy threshold � See also Kosmas et al., arXiv:1505.03202 �
Nuclear physics with coherent elastic scattering If systematics can be reduced to ~ few % level, we can start to explore nuclear form factors P. S. Amanik and G. C. McLaughlin, J. Phys. G 36:015105 K. Patton et al., PRC86 (2012) 024612 Form factor: encodes information about nuclear (primarily neutron) distributions Fit recoil spectral shape to determine the F(Q 2 ) moments (requires very good energy resolution,good systematics control) Ar-C scattering Example: tonne-scale +: model experiment predictions at π DAR source 10% uncertainty on flux
Also note: tonne-scale underground look at astrophysical neutrinos Solar neutrinos Billard et al., arXiv:1409.0050 projected limits if no steriles Rule out sterile oscillations using CEvNS (NC)
Supernova neutrinos in tonne-scale DM detectors 10 kpc L=10 52 erg/s per flavor E avg = (10,14,15) MeV α = (3,3,2.5) for ( ν e , ν e -bar, ν x ) ~ handful of events per tonne @ 10 kpc: sensitive to all flavor components of the flux
A practical application in nuclear safeguards: P. Huber, talk at NA/NT workshop, Manchester, May 2015 Presence of plutonium breeder blanket in a reactor has ν spectral signature Upper: core+blanket Lower: core only ν spectrum is below IBD threshold è accessible with CEvNS, but require low recoil energy threshold
How to detect CEvNS? ν ν è Need low recoil threshold & discrimination (WIMP-style detector) What do you want for your ν source? ü High flux ü Well understood spectrum ü Multiple flavors (physics sensitivity) ü Pulsed source if possible, for background rejection ü Ability to get close ü Practical things: access, control, ...
Both cross-section and maximum recoil energy increase with neutrino energy : 30 MeV ν ’s E max = 2 E 2 ν M 3 MeV ν ’s for same flux 40 Ar target Want energy as large as possible while satisfying coherence condition: (<~ 50 MeV for medium A)
Every ~30 years in Supernova burst the Galaxy,~few 10’s neutrinos � of sec burst, all flavors � All flavors, � Supernova relic low flux � neutrinos � Some component � Atmospheric Coherent at low energy � neutrinos � scattering eventually a bg for Most flux below � Solar � DM expts 1 MeV � neutrinos � Very low energy � Geoneutrinos �
Low energy, but very � Reactors � high fluxes possible; ~continuous source, good bg rejection needed � High energy, pulsed beam Stopped pions � possible for good background (decay at rest) � rejection; possible neutron backgrounds � Portable; can get very short Radioactive 51 Cr baseline, monochromatic � sources � Low energy challenging (electron capture) � Relatively compact, � Beam-induced � higher energy than reactor; not radioactive sources � pulsed � (IsoDAR) � Does not exist yet Tunable energy, but � Low-energy � γ =10 not pulsed � boosted beta beams � � 18 Ne ν e Does not exist yet
Reactor vs stopped-pion for CEvNS Source Flux/ Flavor Energy Pros Cons ν ’s per s Reactor 2e20 per nuebar few MeV • huge flux • lower xscn GW • require very low threshold • CW Stopped pion 1e15 numu/ 0-50 MeV • higher xscn • lower flux nue/ • higher • potential nuebar energy fast neutron recoils in-time bg • pulsed beam for bg rejection • multiple flavors
Stopped-Pion ( π DAR) Neutrinos π + → µ + + ν µ 2-body decay: monochromatic 29.9 MeV ν µ PROMPT 3-body decay: range of energies µ + → e + + ¯ ν µ + ν e between 0 and m µ /2 DELAYED (2.2 µ s)
Comparison of pion decay-at-rest ν sources from duty cycle better ∝ ν flux
Oak Ridge National Laboratory, TN Proton beam energy: 0.9-1.3 GeV Total power: 0.9-1.4 MW Pulse duration: 380 ns FWHM Repetition rate: 60 Hz Liquid mercury target
Time structure of the SNS source 60 Hz pulsed source Prompt ν µ from π decay in time with the proton pulse Delayed anti- ν µ, ν e � on µ decay timescale Background rejection factor ~few x 10 -4
The SNS has large, extremely clean DAR ν flux SNS flux (1.4 MW): 430 x 10 5 ν /cm 2 /s @ 20 m BNB off-axis flux (32 kW): 5 x 10 5 ν /cm 2 /s Note that contamination from @ 20 m (CENNS) non π -decay at rest (decay in flight, kaon decay, µ capture...) is down by several orders of magnitude
These are not crummy old cast-off neutrinos...
These are not crummy old cast-off neutrinos... They are of the highest quality!
The COHERENT collaboration arXiv:1509.08702 � • Collaboration: ~65 members, 16 institutions (USA+ Russia) 28
COHERENT Detectors and Status Nuclear Technology Mass Distance Recoil Data-taking start Target (kg) from source threshold date; CEvNS detecBon (m) (keVr) goal CsI[Na] Scin%lla%ng 14 20 6.5 9/2015; 3 σ in 2 yr Crystal Ge HPGe PPC 10 22 5 Fall 2016 LAr Single-phase 35 29 20 Fall 2016 NaI[Tl] Scin%lla%ng 185*/ 28 13 *high-threshold crystal 2000 deployment to start, summer 2016 • CsI installed July 2015; 185 kg of NaI in July 2016 • Two more detectors to be deployed with resources in hand, fall 2016 • For 5 σ discovery, need larger detectors �
Siting for deployment in SNS basement View looking down “Neutrino Alley” (measured neutron backgrounds low) NaI LAr Ge NIN cubes CsI 30
Expected recoil signals Prompt defined as first µ s; note some contamination from ν e and ν µ -bar 31
Neutron Backgrounds Several background measurement campaigns have shown that Neutrino Alley is neutron-quiet 32
Realistic steady-state-bg-subtracted recoil spectra (keVee/MeVee) compared to 1 σ background fluctuations Ge CsI[Na] NaI [Tl] 33
Currently measuring neutrino-induced neutrons in lead, (iron, copper), ... ν e + 208 Pb → 208 Bi* + e - CC 1n, 2n emission ν x + 208 Pb → 208 Pb* + ν x NC 1n, 2n, γ emission - likely a non-negligible background, especially in lead shield - valuable in itself, e.g. HALO SN detector - short-term physics output
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