APS neutrino study midcourse meeting

1
APS neutrino study midcourse meeting

• Neutrinoless double beta decay
• and direct searches for
• neutrino mass

Co-chairs Steven Elliott and Petr Vogel
Members Craig Aalseth, Henning Back, Loretta
Dauwe, David Dean, Guido Drexlin, Yuri
Efremenko, Hiro Ejiri, Jon Engel, Brian
Fujikawa, Reyco Henning, Jerry Hoff- man, Karol
Lang, Kevin Lesko, Tadafumi Kishimoto, Harry
Miley, Rick Norman, Silvia Pascoli, Serguey
Petcov, Andreas Piepke, Werner Roderjohann,
David Saltzberg, Sean Sutton, Ray Warner, John
Wilkerson, and Lincoln Wolfenstein
2

• The group met at Caltech on Feb. 27-28.
• We agreed on a crude outline of our
• report, heard presentations from the
• main experimental groups as well as
• on several theoretical topics.
• Writing assignment for various sections
• were made and a web page (so far password
• protected) was established.

3
Main points of the presentation (and of the
report)

• Observation of the 0n bb decay would establish
  that neutrinos are massive Majorana particles.
  There are no loopholes.
• It would be a great supporting evidence for
  leptogenesis.
• The value of is constrained by oscillation
  data.
• Its value can distinguish between the normal
  and inverted hierarchies and the degenerate
  spectrum.
• Present experiments probe only the degenerate
  region. To reach the inverted one, much larger
  experiments are needed.
• The matrix elements are uncertain, but not a
  showstopper. The true uncertainty should be
  better understood.
• KATRIN will explore good fraction of the
  degenerate region. It is a well controlled
  laboratory experiment, complementary to the
  cosmological limits.

4

• To confirm or reject the KDHK claim of the 0nbb
  discovery,
• one needs 5-10 kmole-years exposure with low
  background.
• This is in accord with plans of most groups. So,
  no change in strategy is required. It will take
  3 years to accomplish.
• We will argue that R D should be supported and
  funded on many fronts for the next generation of
  bb experiments.
• Eventually, two (or more) large scale experiments
  will be
• needed. This is because of the matrix element
  uncertainty
• and redundancy (exclude false positives and
  negatives). Also, one will have to show that the
  exchange of the light Majorana neutrino is the
  correct mechanism.

5
Importance of Majorana vs. Dirac(attempted for
pedestrians)

• Baryons (e.g. protons and neutrons) and leptons
  (e.g. electrons and neutrinos) are basic
  constituents of matter.
• According to experience (so far) and the
  theoretical
• description (so called Standard Model) in any
  reaction
• involving elementary particles the total
  numbers of
• baryons and leptons are strictly conserved.
• If the neutrinoless double beta decay is
  discovered, i.e.,
• if neutrinos are Majorana particles, the law
  of lepton
• number conservation is violated.
• That would imply that our basic ideas about the
  elementary
• particles and their interactions, and more
  generally about
• the whole Universe around us, must be
  modified.

6
iDiscovery of 0nbb decay would be a great
supporting evidence for leptogenesis.

• There is, at present a net baryon asymmetry in
  the
• Universe, hB 6x10-10. How can one explain
  this value?
• In the see-saw mechanism the heavy right handed
• Majorana neutrinos NR can decay and create
  at temperatures near MN a net lepton asymmetry.
• This happens if CP is violated and
• G(N l H) ? G( N l H)
• The lepton asymmetry is trasferred at later times
• into baryon asymmetry by the sphaleron
  process
• that conserves B-L, but violates B and L.
• Existence of the heavy Majorana neutrinos
• and CP violation in their decay is thus an
• essential ingredient in the explanation of hB .

7

• The value of is constrained by
• oscillation data. Its value can distinguish
  between the normal and inverted
• hierarchies and the degenerate spectrum.

8
vs. the mass of the lightest neutrino
Blue lines LMA I (best fit only) Red lines
errors in oscillation parameters included.
9
Region explored by KATRIN
10
bb cannot distinguish between degenerate normal
and degenerate inverted while long baseline
cannot distinguish between hierarchical
and degenerate.
11

• The matrix elements are uncertain, but not a
  showstopper. The true uncertainty should be
  better understood.

12
From Rodin et al, PRC68,044302(2003) Average and
s of 9 variants each (different potentials and


different number of s.p. states.) The 2 most
impor- tant parameters adjusted always so that
the gap and 2n decay have the correct value.
13
From Engel Vogel, PRC69,034304(2004) Comparison
of exact (full) and QRPA (dashed) m.e.
Two shells, e apart, schematic force, but similar
to the treatment of real nuclei.
14

• KATRIN will explore good fraction of the
  degenerate region. It is a well controlled
  laboratory experiment, complementary to
• the cosmological limits.

15
Illustration of the complementarity between
different
methods, for two flavors for
simplicity.
S sum of the masses (cosmology), dm2 constraint
from oscillations, Ellipses constraints from b
and bb decay. Upper panel Correct choice
of Majorana phases for bb decay Lower panel
Incorrect choice of Majorana phases
(thanks to S. Elliott)
16
Limits (or values) for Smi from cosmology and
astrophysics

• 0.560.30-0.26 eV Allen et al., astro-ph/0306386
• These limits use similar but not identical input
  data.
• Large number of parameters is fitted there are
• correlations, and typically some dependence on
• prior assumptions.
• We will argue that determining the neutrino mass
  in the
• lab will teach us about cosmology it will be
  harder to do
• it the other way around.

17
Claim by KDHKT1/2 1.19-0.502.99 x 1025 y (3s
range)
From Klapdor-Kleingrothaus, Dietz, Krivosheina,
Chkvorets, hep-hp/0403018. 71.7 kg-y
exposure, spectrum without and with the 0nbb
line. Additional lines at 2010, 2017, 2022, 2053
are assigned to the 214Bi decay, line at
2030, previously not seen, is of an unknown
origin. Unidentified previously fitted lines at
2066 and 2075 keV are not shown now.
18
Exposure in kmole-years needed to identify the
halflife in 1025 years labeling the curves, to 5
s, vs. total bckg.
Thanks to S. Elliott
19

• To confirm or reject the KDHK claim of the 0nbb
  discovery, one needs 5-10 kmole-years exposure
  with low background.
• This is in accord with plans of most groups. So,
  no change in strategy is required. It might take
  3 years to accomplish.
• Independently of the KDHK claim R D
• should be supported and funded on many fronts
  for the next generation of bb
• decay experiments.

20
There is no shortage of ideas for the next
generation of bb decay experiments
The table is from Elliott Vogel,
Ann.Rev.Nuc.Part.Sci.52. It reflects the
situation two years ago several proposals have
changed since, others are no longer viable.
21

• Eventually, two (or more) large scale experiments
  will be needed. This is
• because of the matrix element uncertainty
• and redundancy (exclude false positives and
  negatives). Also, one will have to show that the
  exchange of the light Majorana neutrino is the
  correct mechanism.

22
Determining T1/2 in several nuclei wouldeasily
distinguish between differentcalculations of
matrix elements

• Assume that 0nbb decay in 76Ge has T1/2 1x1026y.
  What
• halflives in 130Te and 136Xe will different
  calculations predict?
• 
  130Te 136Xe
• Haxton Stephenson (SM,1984) 8.8x1024
  --
• Caurier et al.(SM,1996)
  3.3x1025 6.8x1025
• Engel et al.(QRPA,1988)
  5.0x1024 2.3x1025
• Staudt et al. (QRPA,1990) 2.1x1025
  9.5x1025
• Rodin et al.(RQRPA,2003) 4.8x1025
  (8.3-13.9)x1025
• Thus 25 measurement in either nucleus would
  eliminate
• most of the wrong entries.