1. Neutrino Magnetic Moments:
Status and Prospects
Physics Overview
Astrophysics Bounds
Recent Results on Direct Experiments
Future Projects
Summary
Henry T. Wong
Academia Sinica, Taiwan
@ Neutrino 2004, Paris

2. Motivations
e.g.
fundamental neutrino properties & interactions ; a channel to differentiate Dirac/Majorana neutrinos (nD/nM)
look for surprises and/or constrain parameter space with future precision oscillation data
explore new neutrino sources ; understand known ones
explore new detection channels & techniques, low energy
explore roles of neutrinos in astrophysics

3. Particle Physics
※ (ni)L – (nj )R – g vertices described in general by:
eij and bij are the electric & magnetic dipole moments coupling ni & nj
constrained by symmetry principles, nD/nM, diagonal/transition moments (ni =/ nj ?) [ e.g. nM & i=j  b=e=0 ]
※ Experimental measureable “neutrino magnetic moment” depends on |nl > after oscillation distance L
Subtleties/Complications:
measureable is an effective/convoluted parameter (c.f. 0nbb)
physics information depends on exact |n> compositions at the detector

4. Experimental Manifestations
Minimally-Extended Standard Model with nD : mn VERY small
 many ways to significantly enhance it (nM, WR …..)
study consequences from the change of neutrino spin states in a (astrophysical) medium
1/T spectral shape in n-e scattering, T is electron recoil energy
Neutrino radiative decays
…………

5. Astrophysics Bounds/Indications
From:
Big Bang Nucleosynthesis degree of freedom
Stellar Cooling via
Cooling of SN1987a via nactive  nsterile
Absence of solar (KamLAND-03 < 2.8x10-4)
ranges of mn(astro) < – mB
Complications/Assumptions :
astrophysics modeling (e.g. Solar B-field)
Neutrino properties (e.g nD / nM ; no other anomalous interactions)
Global treatment (e.g. effects from matter, oscillations ; interference/competitions among channels ……)

6. Magnetic Moments & Solar Neutrinos
For completeness/historical footnote ……
Magnetic Moments & Solar Neutrinos
ne produced in the Sun interact with B to become nx(xe), via spin-flavor precession(SFP), with or without resonance effects in solar medium.
used to explain anti-correlation of Cl-n data with Sun-spot cycles in late 80’s
scenario compatible with all solar neutrino data
KamLAND data fixes LMA as the solution for solar neutrino problem  SFP cannot be the dominant contribution
[n data + LMA allowed region + no ]
 constrain  mn B dr : [Akhmedov et al., Miranda et al. …]
ranges of mn(solar) < – mB

7. Direct Experiments
using sources understood by independent means : reactor n , accelerator n , n at detector , n-sources (future)
look for 1/T excess due to n-e scattering via mn channel over background and Standard Model processes
reactor n : reactor ON/OFF comparison to filter out background uncertainties
n : account for background spectra by assumptions/other constraints
limits independent of |n>final : valid for nD/nM & diag./tran. moments -- no modeling involved
interpretation of results : need to take into account difference in |n>initial

8. Solar n : SK & Borexino
※ SK spectral distortion over oscillation “bkg” [hep-ex/ ]
8B n
SK alone : mn(n ) < 3.6 X mB (90% CL)
+ all n data : mn(n-LMA ) < 1.3 X mB (90% CL)
+ KamLAND : mn(n-LMA ) < 1.1 X mB (90% CL)
※ Borexino/CTF spectral analysis [PLB 563,2003]
mainly 7Be n
bkg=0 : mn(n ) < 1.2 X 10-9 mB (90% CL)
Assume linear bkg + best fit :
mn(n) < 5.5 X mB (90% CL)

9. Accelerator n : LSND & DONUT
※ LSND [PRD 63, 2001]:
select “single electron” events
taking SM s(nm-e), measured s(ne-e)
 agrees with SM  set limit
※ DONUT [PLB 513, 2001]:
observed nt from Ds decays at expected level
look for “single electron” events at level >> SM s(n-e)
observed 1 event with 2.3 expected e=9%
limit: mn(nt) < 3.9 X 10-7 mB (90% CL)

10. Reactor Bugey : MUNU
CF4 TPC (total mass 11.4 kg; containment e~0.5 at 1 MeV)
excellent “single electron” event selection via 4p liquid scintillator & tracking
distinguish start/end of track & measure scattering angle w.r.t. reactor direction  measure neutrino energy
Reactor ON/OFF comparison  forward/background events comparison

11. MUNU data (66.6 d ON/16.7 d OFF) [PLB 564, 2003]
excess of counts < 900 keV ; NO explanations yet [fn(reactor) below 2 MeV not well-known]
limits depends on energy range taken :
Visual scan T > 700 keV
mn(ne) < 1.4 X mB (90% CL)
Visual scan T > 900 keV
mn(ne) < 1.0 X mB (90% CL)
Auto scan T > 300 keV
mn(ne) < 1.7 X mB (90% CL)
studies of hidden n-sources and/or low energy background necessary

12. Reactor n @ Kuo-Sheng : TEXONO
simple compact all-solid design : HPGe (mass 1 kg) enclosed by active NaI/CsI anti-Compton, further by passive shieldings & cosmic veto
focus on keV range for high signal rate & robustness:
mn >> SM [decouple irreducible bkg  unknown sources make searches more conservative ]
T<<En  ds/dT depends on total fn flux but NOT spectral shape [well known : ~6 fission-n  ~ U capture-n per fission ]
selection: single-event after veto, anti-Comp., PSD
Inner Target Volume

13. TEXONO data (4712/1250 hours ON/OFF) [PRL 90, 2003]
comparable bkg level to underground CDM experiment at keV :
~ 1 day-1keV-1kg-1 (cpd)
analysis threshold 12 keV
No excess of counts ON/OFF comparison
Limit:
mn(ne) < 1.3 X mB (90% CL)
more data/improvement to get to sensitivity range
mn(ne) 1.0 X mB

15. Combined Analysis [Grimus et al., …. ]
use all available information, incl. LMA global fit & error contour
take nM  only transition moments ni  nj & m†=m
adopt mn direct limits from SK spectral shape & reactor expts
“Total” Magnetic Moment :
Reactor n:
n at SK:

16. Sensitivity Improvement
Scales as:
Nn : signal events
B : background level
m : target mass
t : measurement time
Nn  fn (neutrino flux) & related to T-threshold
T-threshold : e.g. Nn increase X~3 from 10 keV to 10 eV in Ge (1/T  atomic energy level threshold)
BIG statistical boost in mn comes from enhancement in fn by, e.g. artificial n-sources, b-beams etc.
BUT: for systematics control, coupled with
low threshold to keep mn >> SM rates
maintain low background level

17. GEMMA Project : Reactor n (Russia)
at Kalininskaya Power Plant
Improvement over TEXONO-HPGe parameters :
f~2X1013 cm-2s-1 at 15 m
2 kg HPGe target size
2 keV threshold
Sensitivity :
mn(ne)  3 X mB

18. MAMONT Project : 3H-source (Russia, USA, Germany)
TRITIUM SOURCE of 40 MCi activity (4 kg 3H) with flux of 61014 cm-2s-1 (!)
ULTRA-LOW-THRESHOLD DETECTORS Eth~10 eV (!):
SILICON CRYODETECTOR 15100cc M=3kg, ionization-into-heat conversion effect (CWRU-Stanford-JINR)
HIGH-PURITY-GERMANIUM DETECTOR 6150cc, M=4.8kg, internal amplification by avalanche multiplication (ITEP)
SENSITIVITY (95% C.L.):  2.5 10-12B
50 cm
Conceptual layout of the -e scattering experiment with 40 MCi tritium source

19. Reactor Neutrino Interaction Cross-Sections
TEXONO - ULEGe
Reactor Neutrino Interaction Cross-Sections
On-Going Data Taking (200 kg CsI) :
SM s(ne)
T > 2 MeV
R&D (ULEGe Prototype):
Coh. (nN)
T < 1 keV
Results & More Data (HPGe) :
mn(ne)
T ~ keV

20. “Ultra-Low-Energy” HPGe Prototype
modular mass 5 g  can be constructed in multi-array form
threshold <100 eV after modest PSD
R&D on sub-keV background/calibration/threshold
applications in nN coherent scattering & Dark Matter searches
T>500 eV can be used for mn(ne)  3 X mB for a 1 kg detector at ~1 cpd background level
Threshold ~ 100 eV

21. Summary & Outlook Surprises Need Not be Surprising in
Neutrino Physics ….
a conceptually rich subject ; much neutrino physics & astrophysics can be explored n-osc. : Dmn , Uij
0nbb : mn , Uij , nD/nM
mn : mn , Uij , nD/nM , n  g
practical consequences remain to be seen ; NO indications yet
experimental studies push on new neutrino sources & detection techniques/ranges/channels
potential importance in other areas