Andreas Zech Rutgers University for the HiRes Collaboration CRIS ‘04 (May 31st , 2004)

A Measurement of the Ultra-High Energy Cosmic Ray Spectrum with the HiRes FADC

Detector

Andreas Zech

Rutgers University

for the HiRes CollaborationCRIS ‘04 (May 31st , 2004)

Outline

• Monocular vs. Stereoscopic Observation

• HiRes FADC Event Reconstruction

• Monte Carlo Simulation Programs

• Data / Monte Carlo Comparisons

• The HiRes-2 Energy Spectrum

• Studies of Systematic Effects on the Aperture

The two HiRes Detectors

HiRes-1:

• taking data since 1997

• 1 ring with 21 mirrors ( elev. 3o to 17o)

• Sample & Hold Electronics ( 5.6 s )

HiRes-2:

• started data taking in 1999

• 2 rings with 42 mirrors (elev. 3o to 31o)

• FADC electronics recording at 10 MHz.

Seeing more with one eye closed ?!?

Measuring the Energy Spectrum with HiRes

Stereo observation of the cosmic ray flux yields a better resolution in geometry and energy than monocular.

=> HiRes is a stereoscopic detector. The analysis of stereo events is currently under way.

Analyzing our data in monocular mode has some advantages:

• better statistics at the high energy end due to longer lifetime of HiRes-1.

• extension of the spectrum to lower energies due to greater elevation coverage and better time resolution of HiRes-2.

Mono versus Stereo Energy Measurements

The HiRes monocular energy is in excellent agreement with stereoscopic measurements !

HiRes-1 mono vs. stereo

HiRes FADC Event Reconstruction

1. Reconstruction of the shower-detector-plane

• project signal tubes onto the sky• fit tube positions to a line• reject tubes that are off-track (and off in time) as noise

=> the detector position and fitted line define the shower-detector-plane.

2. Reconstruction of the geometry within the

s-d-plane

Shower Profile & Energy Reconstruction

• Reconstruct charged particle profile from recorded p.e.

• Subtract Čerenkov light.• Fit G.H. function to the profile.• Multiply by mean energy loss rate

=> calorimetric energy• Add ‘missing energy’ (muons,

neutrinos, nuclear excitations; ~10%) => total energy

Monte Carlo Simulation Programs

The Role of Monte Carlo Simulations in the HiRes Experiment

We need M.C. to calculate the acceptance of our detectors for the flux measurement:

M.C. is also a powerful tool for resolution studies.

This requires a simulation program that describes the shower development and detector response as realistically as possible.

HiRes Monte Carlo Simulation

Varying Run Parameters

• Trigger gains

• Dead mirrors

• Livetime

=> Nightly Database

• Light pollution

=> Average for each data set

• Atmospheric Density

=> Seasonal variations

• Weather

=> strict cuts based on hourly observation

• Aerosols

=> atmospheric database from laser shots

=> currently, we use average values

Data / Monte Carlo Comparisons&

Resolution

Photoelectrons per degree of track

black: HiRes-2 data

red: Monte Carlo

(5 x data statistics)

data

Monte Carlo

Distance to the shower axis (Rp)

m

- Angle

Energy Resolution

(Erec - Etrue)

Etrue

~ 16 %

Resolution

deg

~ 5 deg

rec. - true

The HiRes-2 Energy Spectrum

Flux:

fit to the exposure

HiRes-2 Exposure

)]}logexp(1[exp{ Ecba

HiRes-2 Energy Spectrum

statistics:

123 good nights,

536 hours live time,

6320 events with reconstructed geometry, 2685 events after final cuts

The HiRes Mono Spectra

• HiRes-1

‘97 - ‘04

• HiRes-2

‘99 - ‘01

HiRes Mono and Fly’s Eye Stereo

• HiRes-1

• HiRes-2

• Fly’s Eye stereo

Systematic Uncertainties

Systematic Uncertainties

Systematic uncertainties in the energy scale:• absolute calibration of phototubes: +/- 10 %

• fluorescence yield: +/- 10 %

• correction for unobserved energy: +/- 5 %

• aerosol concentration: < 9 %

+ atmospheric uncertainty in aperture

=> total uncertainty in the flux: +/- 31 %

What uncertainties in the aperture are introduced with our inputs to the Monte Carlo ? (i.e. input spectrum, composition, atmosphere)

Systematics due to the Input Energy Spectrum

A fit to the Fly’s Eye Stereo spectrum is used as an

input to the Monte Carlo.

Fly’s Eye vs. E-3 input spectrum

red: MC with Fly’s Eye input spectrum black: data set 2

red: MC with E-3 input spectrum black: data set 2

A bias that we are avoiding...

Assuming a wrong ( E-3 ) input spectrum would cause us a bias of ~ 20 % in the aperture.

aperture using E-3 input spectrum

aperture using Fly’s Eye input spectrum

Systematics due to the Input Composition

The input composition ( = fraction of proton and iron showers)is chosen from HiRes Stereo and HiRes/MIA measurements.

Exposures for pure proton / pure iron

• lower acceptance for iron at low energies (< 10 18.5 eV )

• agreement at higher energies.

red: proton exposure blue: iron exposure

log E (eV)

Systematic Uncertainty due to Input Composition

• We assume a +/- 20 % uncertainty in the proton

fraction from HiRes / MIA & HiRes Stereo

measurements. • This is a conservative

estimate of the uncertainties in the composition.

• A new composition measurement is needed !

=> HiRes , TA/TALE

black: stat. errors red: sys.

uncertainty

Systematics due to Aerosol

• We are currently using a measurement of the average aerosol content of the atmosphere for our analysis.

• What is the systematic effect on the energy resolution and aperture due to this assumption?

• ( This is work in progress ... )

• Aerosol VAOD measurement using vertical laser tracks.

• Aerosol Horizontal Extinction Length from horizontal laser shots.

Atmospheric Database

09/00 - 03/01 clear nights

09/00 - 03/01 clear nights

PreliminaryPreliminary

<VAOD> ~ 0.034

<1/hxl> -1 ~ 20.8 km

Systematic Effect on Reconstructed Energies (MC study)

Energy Resolution for

MC with atmos. database,

reconstructed with database

Energy Resolution for

MC with atmos. database,

reconstructed with average

~ 15.9 % ~ 17.5 %

Systematic Effects on the Aperture

Ratio of Apertures:

• numerator: using MC with atmos. db. , reconstructed with atmos. db.

• denominator: using MC with atmos. db. , reconstructed with averagelog (E)

Conclusions

• Measurements of the Cosmic Ray Flux in monocular mode cover a wider energy range than in stereoscopic mode while providing very good energy resolution.

• Our Monte Carlo Programs simulate all aspects of our experiment in a realistic way.

• We have investigated systematic uncertainties related to the input spectrum, input composition and the aerosol content of the atmosphere. Further studies of atmospherics are under way.