The UHECR Spectrum observed with HiRes in monocular mode Andreas Zech (LPNHE, Paris) Seminar at UNM Albuquerque, 03/29/05

Outline Ultra-High Energy Cosmic Ray Physics The HiRes Experiment Unfolding the Cosmic Ray Spectrum Fits to the Spectrum Summary The Future of HiRes: TA & TALE

Ultra-High Energy Cosmic Ray Physics

Energy Spectrum differential flux: dN / (dE A  dt) follows roughly E -3 power law direct observation not possible above 1 PeV two widely observed features: –‘knee’ at ~ eV –‘ankle’ at ~ eV knee ankle second knee

Propagation Effects magnetic fields (galactic, extragalactic) red-shifting e + e - - pair production with CMBR (at ~ eV) photo-spallation of cosmic ray nuclei GZK effect with CMBR (at ~ eV)  (2.7 K) + p  (1232)  + + n  (2.7 K) + p  (1232)  o + p Strong flux suppression expected for extra-galactic sources.

Extensive Air Showers main channels:  +(-)  +(-) +  (  )  o 2  K +(-)  +(-) +  (  )  2  main e.m. processes: bremsstrahlung pair production ionization

Ground Arrays (Surface Detectors) Detection of lateral particle profile on ground. Reconstruction of geometry from pulse & time information. Reconstruction of energy by model comparisons. Pro: 100 % duty cycle, low cost, low maintenance, good geometry reconstr., nearly constant aperture Contra: Energy reconstr. is model dependent, uncertainties due to fluctuations in lateral profile. AGASA

Detection of longitudinal shower profile via UV fluorescence light. Reconstruction of geometry from recorded shower ‘track’. Using the atmosphere as a calorimeter. Air Fluorescence Detectors Pro: Direct measurement of cosmic ray energy and shower maximum, good geometry & energy reconstruction. Contra: 10 % duty cycle, higher cost & maintenance, energy dependent aperture, atmospheric uncertainties Fly’s Eye

UHECR Composition depth of shower maximum ( Xmax ) depends on energy & cosmic ray species => indirect composition measurement comparison of Xmax with simulation allows bi-modal determination of c.r. composition in a statistical way.

The HiRes (High Resolution Fly’s Eye) Experiment

The HiRes Collaboration J.A. Bellido, R.W. Clay, B.R. Dawson, K.M. Simpson University of Adelaide J. Boyer, S. Benzvi, B. Connolly, C. Finley, B. Knapp, E.J. Mannel, A. O’Neil, M. Seman, S. Westerhoff Columbia University J. Belz, M. Munro, M. Schindel Montana State University G. Martin, J.A.J. Matthews, M. Roberts University of New Mexico D. Bergman, L. Perera, G. Hughes, S. Stratton, D. Ivanov, S. Schnetzer, G.B. Thomson, A. Zech Rutgers University N. Manago, M. Sasaki University of Tokyo T. Abu-Zayyad, J. Albretson, G. Archbold, J. Balling, K. Belov, Z. Cao, M. Dalton, A. Everett, J. Girard, R. Gray, W. Hanlon, P. Hüntemeyer, C.C.H. Jui, D. Kieda, K. Kim, E.C. Loh, K. Martens, J.N. Matthews, A. McAllister, J. Meyer, S.A. Moore, P. Morrison, J.R. Mumford, K. Reil,R. Riehle, P. Shen, J. Smith, P. Sokolsky, R.W. Springer, J. Steck, B.T. Stokes, S.B. Thomas, T.D. Vanderveen, L. Wiencke University of Utah J. Amann, C. Hoffman, M. Holzscheiter, L. Marek, C. Painter, J. Sarracino, G. Sinnis, N. Thompson, D. Tupa Los Alamos National Laboratory

HiRes-1 consists of one ring of 22 mirrors. Coverage in elevation is from 3 to 17 deg. Sample & Hold Electronics are used to record pulses. (5.6 µs window) HiRes-2 has two rings of 21 mirrors each. Coverage in elevation from 3 to 31 deg. Flash ADC electronics record signals at a frequency of 10 MHz.

Mirror area ~ 5 m (16x16) PMT per mirror. One PMT sees ~ 1 degree of the sky.

Measuring the Energy Spectrum with HiRes Stereo observation of the cosmic ray flux yields a better resolution in geometry and energy than monocular. Analyzing our data in monocular mode has also some advantages, though: 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.

project signal tubes onto sky fit tube positions to a plane through the center of the detector reject tubes that are off-track (and off in time) as noise => shower axis lies in the fitted shower-detector plane 1. Reconstruction of the shower-detector plane

2. Reconstruction of the geometry within the shower-detector-plane

3. Shower Profile & Energy Reconstruction Reconstruct charged particle profile from recorded p.e.’s. Fit profile to G.H. function. Subtract Č erenkov light. Multiply by mean energy loss rate  => calorimetric energy Add ‘missing energy’ (muons, neutrinos, nuclear excitations; ~10%) => total energy

Phototube Calibration Relative calibration at the beginning and end of each nightly run. –using YAG laser –optical fibers distribute the laser signal to all mirrors. Absolute calibration using a portable light- source (“RXF”), that is carried to both sites about once a month. –calibration of RXF in the lab using HPDs. => +/- 10% uncertainty in energy scale.

Atmospheric Calibration Rayleigh contribution is quite stable and well known. Aerosol profile of the atmosphere has to be monitored during the run. => = / => +/- 15 % in J(E) Detailed monitoring with steerable lasers at both sites. Additional vertical laser outside of Dugway (Terra). “Shoot the Shower”

Unfolding the Cosmic Ray Spectrum

Deconvolution of the UHECR Spectrum We observe the spectrum convoluted with detector acceptance and limited resolution. Deconvolution with help of a correction factor: D(E i )=  R ij T(E j ) T(E i )= [G mc (E i )/R mc (E i )] D(E i ) We need M.C. to simulate acceptance (& resolution) of our detectors for the flux measurement: This requires a simulation program that describes the shower development and detector response as realistically as possible.

HiRes Monte Carlo Simulation

CORSIKA Shower Library (proton & iron) Gaisser-Hillas fit to the shower profile: Fit parameters scale with primary energy:

Data / Monte Carlo Comparisons Testing how well we understand and simulate our experiment... HiRes-1: – data shown from 06/1997 to 02/2003. – 6920 events in final event sample HiRes-2: – data shown from 12/1999 until 09/2001. – 2685 events in final event sample Measurement of average atmosphere used M.C. : ~ 5 x data statistics

HiRes-2: light (# p.e. / deg of track)

HiRes 2:  2 /d.o.f. of time vs. angle fit

Energy Distribution & Resolution  =18%

HiRes-1: distance to shower core

HiRes-1: Energy Resolution

Instant Apertures HiRes-1 HiRes-2

The HiRes-2 UHECR Spectrum

HiRes and Fly’s Eye

HiRes and Haverah Park

HiRes and Yakutsk

HiRes and AGASA

Systematic Uncertainties Systematic uncertainties in the energy scale: absolute calibration of phototubes: +/- 10 % fluorescence yield: +/- 10 % correction for ‘misssing’ energy: +/- 5 % aerosol concentration: ~ 9 % => uncertainty in energy scale: +/- 17 % + atmospheric uncertainty in aperture => total uncertainty in the flux: +/- 31 %

Systematics due to MC Input Composition Detector acceptance at low energies depends on c.r. composition. MC uses HiRes/MIA measurement as input composition. Relevant uncertainties : –detector calibration –atmosphere –fit to HiRes/MIA data => +/-5 % uncertainty in proton fraction

Systematics due to Atmospheric Variations Repeated HiRes-2 analysis using the atmospheric database. Regular Analysis: – =25 km, =0.04 –in MC generation –in data & MC reconstr. Systematics Check: –HAL & VAOD from database (hourly entries) –in MC generation –in data & MC reconstr. spectrum with database spectrum with average

Fits to the Spectrum

Power Law Fits: Observation of Ankle and Evidence for High Energy Break fit without break points:  2 / d.o.f = 114 / 37 fit with one break point:  2 / d.o.f. = 46.0 /35, logE=18.45+/-0.03 eV fit with two break points:  2 / d.o.f. = 30.1 / 33, logE=18.47+/-0.06 eV & /-0.09eV  =3.32+/-0.04 & 2.86+/ & 5.2+/-1.3 In case of unchanged spectrum above 2nd break point, we’d expect 28.0 events where we see 11 => Poisson prob.: 2.4 E-4

Fit with Toy Model Fit to the HiRes monocular spectra assuming –galactic & extragalactic components –all propagation effects (e+e-, red-shift, GZK) Details of the fit procedure –Float normalization, input spectral slope (  ) and m –uniform source density evolving with (1+z) m –Extragalactic component 45% protons at eV 80% protons at eV 100% protons at eV –Use binned maximum likelihood method Galactic Extragalactic

Interpretation Pion-production pileup causes the bump at eV. e + e - pair production excavates the ankle. Fractionation in distance and energy; e.g., z=1 dominates at second knee.

The Future of HiRes: TA / TALE

TA - the “Telescope Array” SD: 576 scintillation counters, each 3 m 2 area, 1.2 km spacing. 3 fluorescence stations, each covering 108 o in azimuth, looking inward. Central laser facility. Millard County, Utah, flat valley floor for SD, hills for fluorescence, low aerosols. A eV event (on a night when the moon is down) will be seen by SD and all three fluorescence detectors. A powerful detector for hybrid and stereo cross correlation with SD.

Ideas for Recyling HiRes Two HiRes detectors, moved to Millard Co. 6 km stereo with TA fluorescence detectors. Each HiRes detector has two rings, 270 o azimuthal coverage. Aperture of km 2 ster. Increase fluorescence aperture from 500 to 1,780 km 2 ster, including 10% duty cycle. (TA SD=1400). Increase in fluorescence aperture of x 3.6

TA Low energy Extension: “Tower of Power” & Infill Array 15 mirrors, 3xHiRes area, in rings 3,4,5 ( 3 o - 71 o ) => good coverage down to logE = 16.5 eV 111 AGASA counters, spacing of 400m, shown in red. 10 x HiRes/MIA hybrid aperture. => observation of spectrum & composition around second knee

for more information:

Fit with Toy Model Galactic Extragalactic  = 2.32+/-0.01 Fit to the HiRes monocular spectra assuming –galactic & extragalactic components –all propagation effects (e+e-, red-shift, GZK) Details of the fit procedure –Float normalization, input spectral slope (  ) and m –uniform source density evolving with (1+z) 3 –Extragalactic component 45% protons at eV 80% protons at eV 100% protons at eV –Use binned maximum likelihood method

Summary

We have measured the UHECR spectrum from eV to the highest energies with the HiRes detectors in monocular mode. A simulation of the exact data taking conditions was used to determine the acceptance and resolution of the detector, and tested in detail against data. We observe the ‘ankle’ in the HiRes-2 spectrum at eV. The combined monocular HiRes spectra show evidence for a break above eV. The Poisson probability for continuation of the spectrum with unchanged slope from the HiRes monocular data is 2.4 *

Cosmology with TA/TALE ? Adjust evolution to match QSO’s: m=2.6, z<1.6 Lower m, z>1.6 Must extend spectrum measurement lower by an order of magnitude.

Mono versus Stereo Energy Measurements The HiRes monocular energy is in excellent agreement with stereoscopic measurements ! HiRes-1 mono vs. stereo

Calibration Correction Problems with the HiRes-2 calibration due to limited access to Dugway. We adopted HiRes-1 calibration for the absolute energy scale. Correction factors for each dataset were determined from comparisons of stereo events. -22 % -11 % -5 %

Varying Detection Parameters Trigger logic => data divided into 3 datasets Trigger gains Dead mirrors Live-time => Nightly Database Atmospheric Density => Seasonal variations Weather => strict cuts based on hourly observation Aerosols => atmospheric database from laser shots => average values were used for this analysis Light pollution => Average for each data set

Noise assisted triggering Track angle distribution shows a deficit in the MC for nearly vertical tracks.

Noise assisted triggering Additional sky noise (high amplitude) is added to the M.C. to get agreement with data of a certain period. Ambient noise (low amplitude) is added to each channel in the MC. It is measured from the variances taken from “snapshots”. Adding noise to the MC increases the number of nearly vertical tracks. This effect is caused by an inefficiency in the HiRes-2 trigger.

Fits to the HiRes-2 Spectrum J  E /-0.01 J  E /-0.02

Atmospheric Database Atmospheric data of the selected nights in this analysis: = 27 km = 0.035

Acceptances & Aperture R mc (E i ) / G mc (E i ) Acceptances from simulations broken up into 3 datasets. A*  * R mc (E i ) / G mc (E i ) Average instant aperture (in km 2 sr) for all 3 datasets.

Exposure A*  * t * R mc (E i ) / G mc (E i ) Exposure (in 10 4 km 2 sr s) with fit. A*  * t * R mc (E i ) / G mc (E i ) ‘Smoothed’ exposure (in 10 4 km 2 sr s).

We observe the ‘ankle’ in the HiRes-2 spectrum at eV. The HiRes-2 result is in close agreement with HiRes-1 and Fly’s Eye. The HiRes-2 spectrum is consistent with the ‘second knee’ and GZK flux suppression. The combined monocular HiRes spectra show evidence for a break above eV. The Poisson probability for continuation of the spectrum with unchanged slope from the HiRes monocular data is 2.4 *

HiRes-2 Composition Measurement We can extend composition analysis down to about eV with HiRes-2 data. Preliminary HiRes-2 Composition HiRes/MIA & HR stereo Composition.

HiRes vs. Auger FD 2 eyes, 22 / 42 spherical mirrors azimuth ~360, elevation / 3-31 mirror radius 1.3 m 16x16 PMT per mir. Pixel size: 1 x 1 UV filter Sample&Hold / 10 MHz 2 eyes (so far), 6 spherical mirrors each azim. 180, el Schmidt optics mirror radius 3.4 m 20 x 22 PMT per mir. pixel size: 1.5 x 1.5 UV filter, Winston cones 10 MHz

Phototube Calibration pe = qe * ce * A *   = G * pe  = G * √(  *pe) pe =  * (  /  ) 2 Relative calibration at the beginning and end of each nightly run. –using YAG laser –optical fibers distribute the laser signal to all mirrors. Absolute calibration using a portable light-source (“RXF”), that is carried to both sites. –calibration of RXF in the lab using HPDs. => +/- 10% uncertainty in energy scale.