1. UHE photons and neutrinos at the Pierre Auger Observatory
Enrique Zas
Departamento de Física de Partículas &
Instituto Galego de Física de Altas Enerxías,
Universidade de Santiago de Compostela, SPAIN
Enrique Zas
Departamento de Física de Partículas
Instituto Galego de Física de Altas Enerxías
Universidad de Santiago de Compostela
for the Pierre Auger Collaboration

2. A Hybrid detector Two techniques: Fluorescence (FD)
Particle detector array (SD)
Redundant
~ 10% of events are observed with both: wealth of information about shower development & exploit SD
Fluorescence light FD SD
E. Zas 2

3. Digitised signals: FADC
Communications
antenna
Low consumption
electronics
Solar
panels
Battery box
3 photomultiplier tubes
Rotomolded plastic tank
12 tons of purified water
GPS
SD Units
Calibrated online regularly using signals induced by atmospheric muons
Tank containig 12 tons of purified water… GPS to measure accurately the trigger time
The passage of particles through the tank produces a signal in the PMTs which is digitised with a 25 ns time resolution.
The time structure of the shower front is preserved and this is very important for composition studies as I will show you in a few minutes.
Digitised signals: FADC
Time [ns]
25 ns time bins

4. Rich SD data: Useful observables which can
be correlated with hybrid data
At detector level
Signal: Number of particles
Start time: timing
Rise time: Sperad of particle arrival
Area over Peak: low for single muons
Structure: jumps -> muon counting ....
At shower level
Shower size
Direction
Xmax (FD)
Curvature of particle front

5. “Slow & broad signal” produced by EM component
Signal (VEM)
Time (ns)
“Fast & narrow signal” produced by muonic component
Signal (VEM)
25 ns time resolution allows distinction
between broad and narrow signals
Time (ns)

6. Xmax and curvature are related
Larger Xmax => larger curvature
(smaller radius)
L C, RAV, AAW, EZ (Ap Phys 2004)

7. Apparent for ms in hadronic showers
(ns)
200
150
100 50
400
300
200
100
Time delay of
first muon
(curvature)
& average
600
700
Distance to core
120 80 40 80 60 40 20
870
800
1000 m
1000 m

8. Risetime also related to Xmax
muons travel in straight lines
em component straggles
Two main reasons:
1. Z range (production)
2. m less delayed than e & g Deep showers have more em component
Risetime

9. Photon Search

10. Basis: Xmax discrimination
P. Homola for the Auger Collab., ICRC 2009
g-induced showers reach maximum deeper in the atmosphere than nucleonic ones

11. Use Surface Detector data
Astroparticle Physics 29 (2008)
Two discriminating observables
Radius of curvature of shower front
Time structure of shower front (Risetime)
(both correlated to Xmax)
50% of integrated signal
Rise time is the time it takes to go from 10% to 50% of the total signal
Signal (VEM)
Time (ns)

12. Principal component analysis
Surface Detector
Principal component analysis
Cut: Median of distribution
Deviation of Curvature w.r.t. to mean [s units]
MC photons
Data
5% Data
Cut
MC photons
Deviation of Risetime w.r.t. to mean [s units]

13. Direct Xmax search: Hybrid
Astroparticle Physics 27 (2007) 155 & arXiv v2
Quality cuts
More than 6 PMTs
Shower axis distance to highest signal SD station <1.5 km
Reduced c2 (profile fit) <6 and ratio to c2 (line fit) <0.9
Xmax within field of view
Fiducial volume cuts avoid biasses:
Zenith> 350 +g1(E)
Distance to telescope < 24 km +g2(E)
Viewing to shower axis angle >150 (Cherenkov rejection)
E>2, 3, 5 and 10 EeV
Observed photon candidates within the expectations assuming nucleonic primaries only. 13

14. Full simulations made (Corsika, QGSJET01, FLUKA): Fotons Protons Iron
Hybrid
Observed photon candidates within the expectations assuming nucleonic primaries only.
Quality cuts
Fiducial volume cuts 14

15. Hybrid search: g candidates
Cut:
Median of the simulated photon Xmax distribution
5% of protons simulated with QGSJET01 above this line E
cand
p(Fe)
2 EeV 8
30 (0.3) 3 1
12 (0.2) 5
4 (0.1) 10
1 (0)
Observed photon candidates within the expectations assuming nucleonic primaries only.
Uncertainties:
s(Xmax) ~ 16 g cm-2
s(E)/E ~ 22 % 15

16. Deepest event observed
Observed photon candidates within the expectations assuming nucleonic primaries only. 16

17. Limits on g fractions: SD & Hybrid
3.8%
2.4%
3.5%
2.0%
5.1%
31 %
P. Homola for the Auger Collab., ICRC 2009
A1, A2 = AGASA
HP = Haverah Park
Y = Yakutsk
g fraction constrained in Energy - range
2 EeV → 40 EeV
Strong constraints on:
Super-Heavy DM & Topological Defect models

18. Neutrino Search

19. Cosmogenic ns Cosmic rays at ultra high energy (neutrino?)
V.S. Berezinsky, G.T. Zatsepin
Academy of Sciences of the USSR, Physical Institute, Moscow, Russia
Physics Letters B Vol. 28, Issue 6, pp (1969)
Received: 8 November 1968
Published: 6 January 1969
Abstract:
The neutrino spectrum produced by protons on microwave photons is calculated. A spectrum of extensive air shower primaries can have no cut-off at an energy E> eV, if the neutrino-nucleon total cross-section rises up to the geometrical one of a nucleon.

20. Selected developments in neutrino search with EAS:
1969 Inclined showers for neutrino detection Berezinsky, Zatsepin
1987 n bound with Fly’s Eye Fly’s Eye
n bounds with Tokyo data Halzen, EZ, ...
1996 Auger UHE n possibilities shown Capelle, Cronin, Parente, EZ
1999 Earth skimming nt effect Fargion / Lettessier-Selvon & Bertou, Billoir
2007 First earth skimming experimental bound Auger / HiRes

21. Inclined showers Protons, nuclei, g: Shower g’s e+’s and e-’s
do not reach ground level
Only muons

22. Inclined hadron Air Showers
vertical atmospheric depth g
e+e- m
Depth (g/cm2)

23. Case 2: Earth-skimming nt nt t
Case 1: down-going n n
Air shower
Detection (deep)=>inclined
Earth
Case 2: Earth-skimming nt
Air shower nt t
Earth
Upgoing: detection=>inclined)
Complex three stage process
Attenuation through Earth and regeneration: NC
CC & t CC
CC & t decay
CC interaction, t energy loss and no decay
Exit and t decay in the atmosphere

24. “Earth skimming” nt Low t loss => large target volume
Auger results:
PRL 100 (2008) Jan 04- Aug 07
PRD 79 (2009) Jan 04- Apr 08
Low t loss => large target volume
Large density: Earth’s crust
Only sensitivity to nt CC channel
Small zenith angle range (50) (solid angle) 24

25. “Down-going” n Low density target Zenith angle range 750(600?)-900
All channels and flavors. Relative contributions:
Channel
CC ne interactions
NC n interactions
CC nm nt interactions
CC nt the t decay
Resonant ne interact qq
ene
mnm
tnt
s x flv
3 x 2
1 x 6
3 x 4
6 x 1
1 x 1
Shower
EM + Hadronic
Hadronic
t decay EM
NO shower
Energy Transfer
100%
25%
40%
50%

26. Searching for n in data: general criteria
D. Gora for the Auger Collab., ICRC 2009
(1) Search for Inclined Showers
Footprint of the shower on ground compatible with that of an inclined shower:
Elongated pattern (large Length over Width).
“Speed of propagation of signal” along Length, close to speed of light.
Angular reconstruction. 26

27. (2) Search for showers with large electromg component
Inclined proton/nuclei showers induced high in the atmosphere: (mainly) of muons at ground.
“Fast & narrow signal” produced by muonic component
Neutrinos: inclined showers with broad traces
“Slow & broad signal” produced by EM component
Examining inclined showers enhances the differences between deep young showers and those produced early in the atmosphere.

28. Selection for earth skimming neutrinos
Trace cleaning (remove random muons)
Inclined
signal pattern length/width>5 (elongated)
0.31 m/ns > ground speed > 0.29 m/ns (horizontal)
r.m.s. (ground speed) < 0.08 m/ns (compatible)
Electromagnetic
>60% of stations satisfy “Offline ToT” (Time over threshod: 13 bins above 0.2 VEM)
Signal over peak>1.4
Central trigger condition only to Off Tot stations
Quality trigger (T5)

29. Selection for down going neutrinos Only events of 4 or more stations
Trace cleaning (remove random muons)
Inclined
signal pattern length/width>3 (elongated)
0.313 m/ns > ground speed > 0.29 m/ns (horizontal)
r.m.s. (ground speed) < 0.08 m/ns (compatible)
Zenith reconstructed < 750
Electromagnetic
Fisher discriminant analysis on ten variables (related)

30. Acceptance (Monte Carlo) Earth skimming Earth conversion of nt to t
t decay in the atmosphere
extensive air shower
Trigger and identification efficiency (Et, h10km)
detector exposure (integration over running array)
Down-going
Atmospheric interactions

31. Down-going neutrino channels

32. Fisher discriminant analysis
Maximise discrimination power using multivariate analysis (Fisher discriminant).
Very simple idea:
Find “projection line” for maximal hadrons & n separation
var2
var1
Simple example in 2D
Neutrinos
HAS
F is a linear combination
F = a1·var1 +a2·var2 32

33. Fisher discriminant analysis
F is the linear combination of discriminating variables used maximising the ratio:
“mean” of F for HAS and neutrinos maximally SEPARATED relative to
Variance of F for HAS
Variance of F for neutrinos 33

34. Variables for Fisher method
Exploit that neutrino showers have:
(1) Broad signals in the early part
(2) Asymmetry in time spread of signals between early and late parts.
Useful variable: AOP = integrated signal over peak signal
Broad signal Large AOP
Narrow signal Small AOP
Signal (VEM)
Training data 01Jan04-31Oct07 (black) and Nu showers (red)
Area Over Peak of the first T2 tank
AOP Product of the first four T2 tanks
Time (ns)
Time (ns)
Ten discriminating variables:
First 4 AOPs
First 4 (AOPs)2.
Product of the first 4 AOPs.
An asymmetry parameter: “Mean[early AOP] - Mean[late AOP]”.

35. Asymmetry in time spread:
Neutrinos interacting deep in the atmosphere
“Early” region
“Late” region 35

36. Spread in time of the signal [ns] Spread in time of the signal [ns]
Spread in time of the FADC trace of each station in an event
Real inclined event
Simulated down-going neutrino
Each dot represents a station in the event
early (broad signals)
Spread in time of the signal [ns]
Spread in time of the signal [ns]
late (narrow signals)
early
late
(μs)
(μs)
Attenuation of the EM component of the shower from the earliest to the latest station

37. Example distributions:
Inclined real events (black) Simulated nu showers (red)
AOP of the 1st tank in the event
early – late asymmetry parameter of the event

38. Blind search for neutrinos:
Data from 01 Jan 04 to 31 Oct 07 used to “train” Fisher method:
Select the best discriminating observables.
Set cuts in Fisher variable above which an event is a n candidate.
Data from 01Nov07 to 28Feb09 to do a blind search for neutrinos
No neutrino candidates in the search period

39. Flux limits for a E-2 neutrino spectrum
COSMOGENIC ns
AUGER limits
Down 01Nov07- 28Feb09
Up 01Jan04-28Feb09
K [GeV cm-2 s-1 sr-1]
3.2 x 10-7
4.7 x 10-8
J. Tiffenberg for the Auger Collab., ICRC 2009