Observation of Coherent Elastic Neutrino-Nucleus Scattering by COHERENT Kate Scholberg, Duke University NuFact 2017 September 28, 2017
OUTLINE - Coherent elastic neutrino-nucleus scattering (CEvNS) - Why measure it? Physics motivations (short and long term) - How to measure CEvNS - The COHERENT experiment at the SNS - First light with CsI[Tl] - Status and prospects for COHERENT 2
OUTLINE - Coherent elastic neutrino-nucleus scattering (CEvNS) - Why measure it? Physics motivations (short and long term) - How to measure CEvNS - The COHERENT experiment at the SNS - First light with CsI[Tl] - Status and prospects for COHERENT 3
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 Nucleon wavefunctions in the target nucleus ν are in phase with each other at low momentum transfer d σ Momentum d Ω ∼ A 2 | f ( k 0 , k ) | 2 Q = k 0 − k transfer For , QR << 1 [total xscn] ~ A 2 * [single constituent xscn] 4
This is not coherent pion production, a strong interaction process (inelastic) A. Higuera et. al, MINERvA collaboration, PRL 2014 113 (26) 2477 ! 5
\begin{aside} Literature has CNS, CNNS, CENNS, ... - I prefer including “E” for “elastic”... otherwise it gets frequently confused with coherent pion production at ~GeV neutrino energies - I’m told “NN” means “nucleon-nucleon” to nuclear types - CE ν NS is a possibility but those internal Greek letters are annoying è CEvNS , pronounced “sevens”... spread the meme! \end{aside} Scholberg 6
First proposed 43 years ago! Also: D. Z. Freedman et al., “The Weak Neutral Current and Its Effect in Stellar Collapse”, Ann. Rev. Nucl. Sci. 1977. 27:167-207 7
The cross section is cleanly predicted in the Standard Model E ν : neutrino energy T: nuclear recoil energy M: nuclear mass Q = √ (2 M T): momentum transfer G V , G A : SM weak parameters dominates vector small for axial most nuclei, zero for spin-zero 8
The cross section is cleanly predicted in the Standard Model E ν : neutrino energy T: nuclear recoil energy M: nuclear mass Q = √ (2 M T): momentum transfer F(Q) : nuclear form factor , <~5% uncertainty on event rate form factor suppresses cross section at large Q 9
For T<<E ν , neglecting axial terms: dT = G 2 Q 2 ✓ ◆ d σ F M 2 − MT 4 F 2 ( Q ) W 2 π E 2 ν Q W = N − (1 − 4 sin 2 θ W ) Z : weak nuclear charge sin 2 θ W = 0 . 231 , so protons unimportant ⇒ d σ dT ∝ N 2 Line: F(Q)=1 Green: Klein-Nystrand FF w/uccty 10
The cross-section is large (per target atom in CsI) 11
Large cross section (by neutrino standards) but hard to observe 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) 12
The only experimental signature: tiny energy deposited by nuclear recoils in the target material deposited energy è WIMP dark matter detectors developed over the last ~decade are sensitive to ~ keV to 10’s of keV recoils 13
OUTLINE - Coherent elastic neutrino-nucleus scattering (CEvNS) - Why measure it? Physics motivations (short and long term) - How to measure CEvNS - The COHERENT experiment at the SNS - First light with CsI[Tl] - Status and prospects for COHERENT 14
CEvNS: what’s it good for? ! (not a complete list!) • Dark matter direct-detection background • Well-calculable cross-section in SM: • sin 2 θ Weff at low Q • Probe of Beyond-the-SM physics • Non-standard interactions of neutrinos • New NC mediators • Neutrino magnetic moment • New tool for sterile neutrino oscillations • Astrophysical signals (solar & SN) • Supernova processes • Nuclear physics: • Neutron form factors • g A quenching • Possible applications (reactor monitoring) 15
The so-called “neutrino floor” for dark-matter searches super atmospheric nova ν ’s ν ’s solar ν ’s Measure CEvNS to understand nature of background (& detector response, DM interaction) 16
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 If these ε ’s are ~unity, there is a new interaction of ~Standard-model size... many not currently well constrained J. Barranco et al., JHEP 0512 (2005), K. Scholberg, PRD73, 033005 (2006), 021 Can improve ~order of magnitude beyond CHARM limits with a first-generation experiment (for best sensitivity, want multiple targets ) 17 More studies: see https://sites.duke.edu/nueclipse/files/2017/04/Dent-James-NuEclipse-August-2017.pdf
OUTLINE - Neutrinos and neutrino interactions - Coherent elastic neutrino-nucleus scattering (CEvNS) - Why measure it? Physics motivations (short and long term) - How to measure CEvNS - The COHERENT experiment at the SNS - First light with CsI[Tl] - Status and prospects for COHERENT 18
How to detect CEvNS? ν You need a neutrino source and a 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, ... 19
Both cross-section and maximum recoil energy increase with neutrino energy : 30 MeV ν ’s stopped π E max = 2 E 2 ν M 3 MeV ν ’s reactor for same flux 40 Ar target Want energy as large as possible while satisfying coherence condition: (<~ 50 MeV for medium A) 20
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) 21
Comparison of pion decay-at-rest ν sources from duty cycle better ∝ ν flux 22
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 The neutrinos are free! 23
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 24
The SNS has large, extremely clean DAR ν flux 0.08 neutrinos per flavor per proton on target Note that contamination from non π -decay at rest (decay in flight, kaon decay, µ capture...) SNS flux (1.4 MW): is down by several 430 x 10 5 ν /cm 2 /s orders of magnitude @ 20 m 25
Backgrounds • cosmogenics Usual suspects: • ambient and intrinsic radioactivity • detector-specific noise and dark rate Neutrons are especially not your friends* Steady-state backgrounds can be measured off-beam-pulse ... in-time backgrounds must be carefully characterized 26 *Thanks to Robert Cooper for the “mean neutron”
A “friendly fire” in-time background: Neutrino Induced Neutrons (NINs) lead shielding ν e + 208 Pb → 208 Bi* + e - recoil-sensitive CC detector 1n, 2n emission ν x + 208 Pb → 208 Pb* + ν x NC 1n, 2n, γ emission • potentially non-negligible background from shielding • requires careful shielding design relatively • large uncertainties (factor of few) large xscn in xscn calculation wrt CEvNS • [Also: a signal in itself, e.g, HALO SN detector] 27
OUTLINE - Neutrinos and neutrino interactions - Coherent elastic neutrino-nucleus scattering (CEvNS) - Why measure it? Physics motivations (short and long term) - How to measure CEvNS - The COHERENT experiment at the SNS - First light with CsI[Tl] - Status and prospects for COHERENT 28
The COHERENT collaboration http://sites.duke.edu/coherent ~80 members, 19 institutions 4 countries arXiv:1509.08702 29
COHERENT CEvNS Detectors Nuclear Technology Mass Distance Recoil Target (kg) from source threshold (m) (keVr) CsI[Na] Scintillating 14.6 19.3 6.5 flash crystal Ge HPGe PPC 10 22 5 zap LAr Single-phase 22 29 20 flash NaI[Tl] Scintillating 185*/ 28 13 flash crystal 2000 Multiple detectors for N 2 dependence of the cross section CsI[Na] 30
Siting for deployment in SNS basement View looking down “Neutrino Alley” (measured neutron backgrounds low, ~ 8 mwe overburden) NaI LAr Ge NIN cubes CsI Isotropic ν glow from Hg SNS target 31
Expected recoil energy distribution Prompt defined as first µ s; note some contamination from ν e and ν µ -bar 32
The CsI Detector in Shielding in Neutrino Alley at the SNS Almost wrapped up... A hand-held detector! 33
COHERENT data taking 1.76 x10 23 POT delivered to CsI (7.48 GWhr) Neutron background data- CsI data-taking taking for ~2 years starting summer 2015 before first CEvNS detectors 34
The First COHERENT Result: CsI[Na] Led by U. Chicago group Scintillating crystal • high light yield • low intrinsic bg • rugged and stable • room temperature • inexpensive Sodium-doped CsI is favorable, due to suppressed afterglow 2 kg test crystal @U. Chicago. Amcrys-H, Ukraine J.I. Collar et al., NIM A773 (2016) 56-67 35
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