6-2016 Conf slides with notes (final)
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1 Possibili)es for terrestrial detec)on of kevmass ‘sterile’ neutrinos Peter F Smith (UCLA, RAL) Brief review of Direct detec)on of sterile ν dark maGer • Inverse beta decay • electron scaGering Detec)ng rare sterile ν events in beta decay • End point of Tri)um beta spectrum • ν mass reconstruc)on in Kcapture events
Transcript of 6-2016 Conf slides with notes (final)
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Possibili)es for terrestrial detec)on of kev-‐mass ‘sterile’ neutrinos
Peter F Smith (UCLA, RAL)
Brief review of Direct detec)on of sterile ν dark maGer
• Inverse beta decay • electron scaGering Detec)ng rare sterile ν events in beta decay
• End point of Tri)um beta spectrum • ν mass reconstruc)on in K-‐capture events
(3) Possible galac)c 3.5 X-‐ray signal could be decaying 7 keV νs
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(1) Exis)ng neutrino family looks incomplete
(2) Theore)cal studies for > 20 years show keV-‐mass νs is a convincing DM candidate
1994: Dodelson &Widrow, propose ~ 30 – 300 eV sterile ν as dark maGer
1999 : Shi & Fuller propose 0.1 – 10 keV sterile ν as dark maGer
2000 – 2010: > 40 papers & reviews confirming that a keV-‐mass sterile ν matches DM requirements
Galac)c dark maGer may be keV-‐mass sterile neutrinos
text book list of weak couplings to Z boson show missing RH ν current (and LH ν) -‐
leptons quarks
100 10
1
10-‐12 10-‐11 10-‐10 10-‐9 10-‐8 10-‐7 10-‐6
mνs keV
mixing (sin2 2θ)
Excluded X-‐ray region (100% DM = sterile ν)
sin2 2θ ~ 10-‐10 if 100% DM ~ 10 -‐9 if 10% DM
2014 Boyarsky et al , Bulbul et al
Direct detec)on of dark maGer sterile neutrinos by inverse beta decay ?
For 7 keV, Li & Xing PLB2011 find 10 events/year for 10 kg 3H (t1/2 = 12y) or 1 ton 106Ru (t1/2 = 1y ) needs replenishment !
Must be in the form of gas or surface layers to detect the electrons. Not feasible in near future. Li & Xing: “may not be hopeless in the long run”
Example: normal β decay 3H -‐> 3He+ + e-‐ + νimplies inverse νs + 3H -‐> 3He + e-‐
Would give events beyond the β decay end point separated by an energy mνs
Proposed 30 years ago to detect relic neutrino background, but needed large tri)um target (now aim of PTOLEMY) Same principle would detect sterile ν dark maGer
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Direct detec)on of dark maGer sterile neutrinos by electron scaGering ?
νs"νs"
νs"νs"
e$"
e$""
e$"
e$""
Z"
N"
N"
W"
Atom"
Atom"
Sufficient energy transfer to produce ioniza)on
Rate ~ 1 event/year/kiloton (inc Z2 coherence)
Ando & Kusenko PRD 2010
But for ms ~ 7 keV, coupling ~ 10-‐9
1 km
1 km
1 km Also νs decay to X-‐ray
In an enclosure Rate ~ 1 event/year/km3
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Distor)on of beta spectrum by sterile neutrino admixture Basis of aGempts over 30 years to measure ν mass* by its effect on the end point of a beta spectrum.
(*nowadays νe ≡ sum over three mass eigenstates)
from F Bezrukov Conf: New Instrumenta)on for Par)cle Physics (CERN 2008)
Exis)ng direct limits on mass and coupling (mainly spectrum kink searches)
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KATRIN high resolu)on spectrometer 70m x 10m Aims to measure sub-‐eV ν mass from end-‐point
KATRIN also analyzing spectrum for admixture of sterile ν events.
Susanne Mertens et al arXiv:1409.0920v2 (2014):
(a) 3-‐year mixing limit 10-‐7-‐10-‐8 for design T source.
(b) Future limit 10-‐9 (3y) achievable with 10x source.
(c) Thus 10-‐10 (3y) achievable with 100x source.
(a) (b) (c)
Possible future limits from KATRIN
Reconstructed mν2
keV2 0 50 100
No of events
keV sterile νs seen as separated popula)on
High precision )me of flight measurements needed to achieve 6σ separa)on from zero mass peak
Detec)on of sterile ν in beta decay by total energy-‐momentum reconstruc)on
mν2 = [Q -‐ Ee -‐ Eγ -‐ EN]2 -‐ [ pγ + pe + pN ]2
electron pe
recoil ion pN neutrino
β-‐decay atom in trap
trigger gamma pγ
Beta decay with electron emission
Beta decay by K-‐capture
recoil ion pN neutrino
K-‐capture atom in trap
trigger X-‐ray pγ low energy Augers pea
mν2 = [Q -‐ Ea -‐ Eγ -‐ EN]2 -‐ [ pγ + pea + pN ]2
Recent studies show this may now be feasible
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Not a new idea -‐ suggested for over 20 years. but reaching masses < 100 keV and < 10-‐4 mixing is very difficult (‘challenging’)
Proposed 3H decay reconstrucKon for 17 keV neutrino with mixing ~ 10-‐2
1992
Used 37Ar K-‐capture reconstrucKon to set limit ~ 10-‐2 at 300-‐600 keV
1998
Used 38mK decay reconstrucKon to set limit ~ 10-‐2 at 0.7-‐3.5 MeV
2003
Discussion of 3H decay reconstrucKon for keV masses & mixing < 10-‐6
2007
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Used extensively for 20 years for 3-‐D studies of atom-‐atom and photon atom collisions
COLTRIMS – COLd Target Recoil Ion Mass Spectroscopy
MOT -‐ Magneto-‐Op)cal Trap
Developed for over 20 years for Cooling and suspension of neutral atoms
No of trapped atoms: 108-‐ 1012 effec)ve temperature: 10 -‐ 100 µK
)me of flight precision 200 ps spa)al precision (MCP) 40 µm
(supplied by Roentdek, Germany)
Trapped atoms
Laser beam 3
MCP array for recoil ion Auger electrons .
<-‐-‐-‐Recoil ion
Trigger X ray
Posi)on sensi)ve X-‐ray & Auger detector arrays
Plane of laser tbeams 1 and 2
3m
Am)-‐Helmholz coils Anri-‐Helmholz coils
mu metal magne)c screen
neutrino.
BaF2 SiPM
plas)c grid
phoswitch + mul)-‐anode SiPM array
grid
Basic geometry for 3-‐D )me-‐of flight measurements
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Some backgrounds
residual gas scaGers Radioac)vity & untrapped atoms cosmic ray muons
Reject by real )me tests -‐ most self-‐vetoing: • Signals not in correct )me window • Signal pulses wrong size or shape • X-‐Ray triggers not followed by Augers in 200ns • Mass reconstruc)on mν
2=[Q-‐ Ee-‐ Eγ -‐EN]2 – [pγ+pe+pN]2 nega)ve or wrong magnitude
Backgrounds found tolerable in published MOT atomic experiments
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Requirements to reach mass and coupling levels of the X-‐ray signal ?
• need > 6 sigma separa)on of signal from zero mass peak
• kinema)c simula)ons show possible with ± 0.2 ns )ming and ± 0.1 degrees X-‐ray direc)on
100 10 1
10-‐12 10-‐11 10-‐10 10-‐9 10-‐8 10-‐7 10-‐6
mνs keV
mixing (sin2 2θ)
Excluded by astronomical X-‐ray observa)ons (100% DM = sterile ν)
• needs 1011 reconstructed decays/y for 5-‐10 events/y
• analysis of 3000 events/s – achieved in COLTRIMS expts
• fast decaying isotope: eg Cs131 (t1/2=10 days) good for MOT
feasible in principle
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EsKmate from X-‐ray peak
10-‐12 10-‐11 10-‐10 10-‐9 10-‐8 10-‐7 10-‐6
mνs keV
mixing (sin2 2θ)
Excluded by astronomical X-‐ray observa)ons (100% DM = sterile ν)
100 10 1
mνs keV
mixing (sin2 2θ)
100 10 1
Possible coverage of Cs131 mass reconstrucKon search
10-‐12 10-‐11 10-‐10 10-‐9 10-‐8 10-‐7 10-‐6
131Cs K-‐capture has Q = 352 keV -‐ allows sterile mass range 5 -‐350 keV
all masses searched at the same )me – several peaks could be seen)
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Event )melines – no overlapping
X-‐ray trigger
Auger electrons
Recoil ion
0.2µs
200µs
15µs average trigger interval
• find trigger with matching Auger and ion signals • wait for comple)on of event before searching for next matching set
(shown )mescales not linear)
Decays 1.5µs average decay interval )me
)me
)me
)me average 300µs
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Faster processing with overlapping events
.• search for overlapping events by matching corresponding Auger and ion signals • BUT majority of events are incomplete (Auger and/or ion signals missing) • thus overlap allows more analyzed events/year, but so�ware analysis complex
X-‐ray trigger
Auger electrons
Recoil ion
0.2µs
200µs
Decays
(shown )mescales not linear)
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CACHE (Cesium Atomic-‐electron Capture with Heavy neutrino Emission)
C J Martoff, J Napolitano (Temple) E R Hudson, H Wang, P F Smith (UCLA) A Renshaw (Houston), G M Fuller (UCSD, theory), E Grohs (Michigan, theory)
Proposal submiGed based on Cs131:
Smaller-‐scale phase 1 experiment confirming mass reconstruc)on & background rejec)on
Ini)al objec)ves • 100 processed events/h • coupling sensi)vity ~ 10-‐6 in 400d running • mass sensi)vity ~ 30-‐300 keV Could be upgraded to a fully instrumented phase 2 reaching sensi)vity < 10-‐10 and 7 keV mass
recoil ion electron neutrino
νe = c11ν1 + c12ν2 + c13ν3 + c14νs + ?
recoil ion wave components neutrino wave packet components
Detec)on of recoil collapses combined wave func)on
100cm, 1 ms
Wave func)on collapse to sterile ν
300 km
K-‐capture provides example of wave packet collapse and ‘EPR paradox’
different masses -‐> different momenta and speed
mass eigenstates
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Summary
• keV mass sterile neutrinos are a strong dark maGer candidate
• Detec)ng keV sterile neutrinos in the laboratory will be feasible
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• X-‐ray leaves another vacancy, producing a cascade of electron transi)ons, including Auger electron emission • Choice of N-‐shell X-‐ray as trigger gives only 1 or 2 Augers of 60-‐120 eV, whose momentum and direc)on must be measured.
pν
recoil ion 131Xe
neutrino
Trigger X-‐ray 30 keV Auger electrons ~ 60-‐120 eV
pγ
pe
pΝ3
pΝ2 pΝ1
θα131Cs
Auger electrons
O N M L K
nucleus
K-‐capture
X-‐ray
Internal electron transi)ons with Auger electron emission
Auger electrons
Typical atomic level diagram
(vacancy)
