Atmospheric n

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Atmospheric ns in alarge LAr Detector
G.Battistoni, A.Ferrari, C.Rubbia, P.R.Sala
F.Vissani
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Motivations to continue the study of atmospheric
neutrinos

• There is still interest in continuing the study
  of atmospheric neutrinos
• the confirmation of SK results with a technology
  having a large reduction of experimental
  systematics with respect to water Cerenkov
• the search for subleading contributions in the
  mixing matrix
• a possible (in principle) precision measurement
  of q23
• a possible discrimination of Normal vs Inverted
  Hierarchy of masses

Tiny effects!!

• Can a very large LAr detector be the tool to
  perform these new investigations (Precision
  Physics)? How does it compare to SK?

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This work
FLUKA NUX with 3-f oscillations with matter
effects
Atmospheric neutrino Fluxes (2002) _at_LNGS

• Dm223 2.5 x 10-3eV2 (positive)
• Dm212 8.x10-5eV2
• q12 34o
• q23 40o, 45o , 50o
• q13 0o, 3o , 5o , 10o
• dCP 0o

A.Strumia F.V. hep-ph/0503246
1000 Kton year exposure
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Event selection and definition
LAr Super-Kamiokande
Thresh. for e event 10 MeV 100 MeV (single prong)
Thresh. for muon event 50 MeV 200 MeV (single prong) 600 MeV (Multi-prong)
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ne CC Simulated Event Gallery
How SubGeV ne events will appear in ICARUS in one
of its projective views (full detector desponse
simulation using FLUKA)
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ne CC Simulated Event Gallery
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The standard nm analysis
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A slightly less standard opportunity
Direction reconstruction using leptonrecoiling
proton

• In general
• a superior capability
• in pointing
• a better resolution
• in L/E

Minimum Goal 50-100 kton yr
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The Precision Physics case

• Solar n and KamLAND experiments contributed to
  determine with relatively high precision Dm212
  and q12
• At present the only determination of q23 come
  from atmospheric neutrinos and has a large
  uncertainty. How close is q23 to ?/4? Is it
  larger or lower than ?/4? (octant ambiguity)

q23 lt ?/4 ? ltnmn3gt gt ltntn3gt
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The determination of q23 in atm. neutrino exp.
DIscussion previously proposed by P.Lipari
Essentially the best determination of 2 q23 comes
from the analysis of Multi-GeV muon-like events
At present 36 lt q23 lt 54
The solar (12) sector generates significant
effects on Sub-GeV neutrinos which might help
resolve the octant ambiguity. This is true even
in case q13 0
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Oscillation effects in e-like events in the q13
0 approximation
Fosce F0e P(ne ? ne) F0m P(nm ? ne) F0e
,F0m n flux w/o osc. F0e P(ne ?
ne) r P(nm ? ne) r F0m / F0e m/e flux
ratio F0e 1 P12 r cos2
q23 P12 P12 Aem2 2n transition
probability ne ? nmt in matter driven
by Dm212 (Fosce / F0e) 1 P12 (r cos2 q23
1) screening factor for low energy n
(r 2) 0 if cos2 q23 0.5 (sin2 q23
0.5) lt 0 if cos2 q23 lt 0.5 (sin2 q23 gt
0.5) gt 0 if cos2 q23 gt 0.5 (sin2 q23 lt 0.5)
Important only in SubGeV region where Dm212L/E is
sufficiently large
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A new measurement of q23
SubGeV r2
Also the nm rate is affected but this would be an
extra term which adds to the standard 2-flavor
oscillations
However, the general case of non vanishing q13
(and possibly dCP) plus matter effects is more
complex
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To give an idea
osc. web calculator based on the code of F.V.
(thanks to V.Vlachoudis CERN) http//pceet075.cern
.ch/neutrino/oscil/
ns from nadir
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Implications
The knowledge of the absolute level of SubGeV ne
can provide the best possible measurement of q23
and of its octant.
The unique features of a large LAr detector (gt50
kton?) can provide an important measurement of of
SubGeV ne with null or largely reduced
experimental systematics. The ICARUS tecnology
can explore for the first time the region with
Pelt100 MeV/c (to be demonstrated by T600)
Of course, from the point of view of statistical
significance, this requires a very high exposure.
How large?
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Other possibilities

• There are q13 induced oscillations which instead
  affect the MultiGeV region these could be used
  to discriminate the hierarchy of masses (sign of
  Dm223) if n and anti-n could be distinguished
  (MSW resonance is present for n when Dm223gt0 or
  for anti-n (when Dm223lt0)
• This measurement, which requires n/anti-n
  separation, might be more problematic for a LAr
  detector (magnet)

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ne ne SubGeVCC interaction rates (kton yr)-1
No Osc. 51.3 (62.8)
40o 45o 50o
0o 52.2 (63.9) 51.3 (62.8) 50.2 (61.7)
3o 51.7 (63.3) 50.9 (62.5) 49.7 (61.2)
5o 51.4 (63.0) 50.6 (62.2) 49.6 (61.1)
10o 50.8 (62.00) 50.4 (61.9) 49.3 (60.8)
q23
q13
Enlt1 GeV
Pleptonlt1 GeV/c
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In graphic form...
q13 0o q13 3o q13 5o q13 10o
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Results for q13 0
q23 40o
q23 50o
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Results for q13 0
Ratio Ne/Ne0
q23 40o
q23 50o
0.037 /- 0.006
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Results for q13 gt 0
q13 5o
q13 10o
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The problem of systematics
Leaving aside for a moment the question if such
an extremely large exposure can be achieved The
proposed measurement requires an absolute
no-oscillation prediction affected by a
systematic uncertainty not exceeding 1. Is this
achievable? (absolute level, ne/nm ratio)

• Primary c.r. fluxes (maybe we can take this
  under control)
• Neutrino-nucleus cross sections
• Hadronic interactions and atm. shower development
  
• is exactly 2 at low energy only
  if just p are there!
• 
  K/p?

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A less naive method...

• Of course it is hard to believe that one could
  rely on the absolute level of Ne prediction...
  (the c.r. flux normalization remains one of the
  most important uncertainties)
• A better analysis is the ratio
• so that many common
• systematics cancel out
• The important topic remains the
  uncertainty as a function of energy

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For example (q13 0)
it could be possible to achieve a 3 s
separation even for 500 kton yr
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Considerations from SK

• This topic has been debated at the end of 2004 in
  the context of a dedicated workshop
• http//www-rccn.icrr.u-tokyo.
  ac.jp/rccnws04/
• Requirements for SK the measurement of q23
  octant can be done with an exposure of at least
  20 years of SK (depending on q13) to distinguish
  (Dc22) between the 2 mirror values of
  corresponding to sin2q23 0.96 with the present
  level of systematics

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Conclusions

• A very large LAr TPC, in principle, can give new
  important contributions to neutrino physics, also
  with atmospheric neutrinos
• It allows to detect low energy neutrinos with
  null or negligible experimental systematic error.
  An exposure of 50-100 kton yr would allow be the
  minimum goal for this topic.
• the sector of SubGeV ne, in particular, offers
  the possibility of performing new interesting
  measurements.
• To perform new precision measurements a very
  large exposure (gt500 kton yr) is anyway needed
• Such a large exposure might be in part useless
  without an effort to reduce the existing
  systematic uncertainties (n fluxes, cross
  sections,...).