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

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

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

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

“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)

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

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

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

Photon Search

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

Use Surface Detector data Astroparticle Physics 29 (2008) 243-256 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)

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]

Direct Xmax search: Hybrid Astroparticle Physics 27 (2007) 155 & arXiv 0903.1127v2 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) [35+7 @ 1019.7 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

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

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

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

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

Neutrino Search

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. 423-424 (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>3 1019 eV, if the neutrino-nucleon total cross-section rises up to the geometrical one of a nucleon.

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 1991-97 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

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

Inclined hadron Air Showers vertical atmospheric depth g e+e- m 0 2000 4000 6000 8000 10000 12000 Depth (g/cm2)

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

“Earth skimming” nt Low t loss => large target volume Auger results: PRL 100 (2008) 211101 Jan 04- Aug 07 PRD 79 (2009) 102001 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

“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%

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

(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.

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)

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)

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

Down-going neutrino channels

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

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

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]”.

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

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

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

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

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