Transcript Document

The Auger Observatory
and UHE neutrinos
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Why UHE neutrinos ?
What is the Auger Observatory ?
How can it see UHE neutrinos ?
How to discriminate them ?
What sensitivity ?
NO-VE 2006
Pierre Billoir, LPNHE Paris, CNRS/univ. Paris 6 and 7
Auger Collaboration
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UHE neutrinos
• expected from interaction of accelerated particles with photons
in the source region or with the CMBR (GZK effect):
relatively soft spectrum
• decay of ultra massive objects: harder spectrum expected:
UHE photons and neutrinos are a signature of top-down scenarii
propagation in straight line: point to the source
differences with photons :
- propagation over cosmological distances
- low probability to produce an observable atmospheric shower
Photons and neutrinos:
possible interesting byproducts of the Auger Observatory
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general framework
• n oscillations : ~ equal fluxes of the 3 flavours
• assume neutrinos weakly interacting, even at UHE
• probability of interaction in atmosphere <~ 10-4
better sensitivity to nt t in earth skimming scenario
(t emerging within a few degrees from horizontal)
This study: based on Astrop. Phys. 17 (2002) 183
(X. Bertou, P.B., O. Deligny, C. Lachaud, A. Letessier-Selvon)
+ work on first Auger data (2004-05)
(special contribution of Oscar Blanch Bigas)
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Water Cherenkov tanks
Communications
antenna
Electronics
enclosure
GPS antenna
Solar panels
Battery box
3 nine inch
photomultiplier
tubes
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Plastic tank
with 12 tons of
water
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Optical system (fluorescence telescopes)
corrector lens
(aperture x2)
440 PMT camera
1.5° per pixel
segmented
spherical
mirror
aperture box
shutter
filter UV pass
safety curtain
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Hybrid detection
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“normal” (nucleic) showers
almost vertical:
thick curved front
muons + electromagnetic
very inclined:
thin flat front
High energy muons
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a real vertical event (20 deg)
Noise !
doublet
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a real horizontal event (80 deg)
“single” peaks : fast rise + exp. light decay (t ~ 70 ns)
accidental background signals are similar
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neutrino showers
(distinguishable if almost horizontal)
downgoing
(direct n interaction
in atmosphere)
upgoing
(n
t in earth
t decay in flight )
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Simulation chain
• inject nt at 0.1, 0.3, 1, 3, …, 100 EeV into earth crust
• generate c.c. and n.c interactions (CTEQ4-DIS) , t decay and energy loss
• if a t emerges: generate decay in atmosphere
(modes e, p, pp0, ppp0 , pp0p0, ppp , pppp0 , pp0p0p0 + neutrinos)
• inject the products of decay into AIRES (shower simulation package)
• regenerate particles entering the tank from the “ground” output file
• simulate the Cherenkov response and FADC traces
• apply a specific analysis (trigger + selection)
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ground spot
decay of an horizontal t of 1 EeV
enn (almost pure e.m. cascade)
pn (hadronic+e.m. cascade)
injected t
average level of trigger
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Simulated t
p+ (0.27 EeV) n
400 m above ground
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Simulated t
p+ (5.1) p0(16.1) n
1800 m above ground
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n candidate selection
1. “young” showers
online local triggers (one tank):
• threshold: one slot above Th
(detection of peaks)
• time over threshold: N slots within
3 ms above th
(detection of long signals)
Global condition:
at least 3 t. o. th. stations satisfying
area/peak > 1.4 * “single”
one “central” + one within 1500 m + one within 3000 m
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Trigger efficiency
Fraction of decaying t (excluding mnn channel)
giving a trigger
En = 0.1 EeV
En = 1 EeV
En = 100 EeV
En = 10 EeV
1 km
2 km
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footprint analysis
Variables defined from the footprint
(in any configuration, even aligned)
• length L and width W
(major and minor axis of the ellipsoid of inertia)
• “speed” for each pair of stations
(distance/difference of time)
ti d tj
ij
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n candidate selection
2. Discriminating variables
Search for long shaped configurations, compatible with a front
moving horizontally at speed c, well contained inside the array
(background: vertical or inclined showers, d/Dt > c )
cuts:
L/W > 5
0.29 < av. Speed < 0.31
r.m.s. < 0.08
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from years 2004-2005:
real
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What can be measured ?
• direction: precision better than 2deg
(improving with Nstat)
• energy: possible lower bound for a given event
- unknown energy losses in interaction/decay chain
- estimation of Eshower depends on altitude
possible strategy:
inject in the simulation chain a spectrum with a given shape
deduce from the selected data a level (or an upper bound)
model dependent result
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main sources of systematic errors
• detection: triggering/selection efficiency, effective
integrated aperture: to be evaluated (not dominant)
• cross section of neutrinos
• energy loss of t in earth: big uncertainty !
- bremsstrahlung + pair production: well defined
- deep inelastic scattering in photonuclear process:
“pessimistic” hypothesis from Dutta et al, Phys.Rev. D63 (2001)
factor of ~5 between low and high estimation of the acceptance
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uncertainty
range
Auger sensitivity
TD
GRB
AGN
GZK
Points: 1 event / year / decade of energy
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upper bounds for 1 year of full Auger
(if no candidate)
(“pessimistic” hypothesis for t energy loss)
Solid:
various models from
Protheroe (astro-ph/9809144)
Dashed:
upper bounds at 95 %
C.L. for each shape
if no candidate
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summary and perspectives
• the surface array of Auger is sensitive to UHE neutrinos
most promising: earth skimming (decay of t in air)
• real data are clean
• simple criteria allow to reject the background
still room for refinement of criteria…
• constraining upper bounds expected within a few years
Ongoing studies:
• other criteria to select neutrino candidates
• specific trigger to enhance sensitivity at low energy
• acceptance calculations
• shower energy evaluation
• observation with the fluorescence detector
• atmospheric n interactions (less horizontal)
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