The UHECR Spectrum observed with HiRes in monocular mode

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

2. 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

3. Ultra-High Energy Cosmic Ray Physics

4. knee ankle second knee 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 ~1015.5 eV • ‘ankle’ at ~1018.5 eV

5. Propagation Effects • magnetic fields (galactic, extragalactic) • red-shifting • e+e-- pair production with CMBR (at ~ 1017.8 eV) • photo-spallation of cosmic ray nuclei • GZK effect with CMBR (at ~ 1019.8 eV)  (2.7 K) + p (1232) + + n  (2.7 K) + p (1232) o + p Strong flux suppression expected for extra-galactic sources.

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

7. AGASA 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.

8. Fly’s Eye Air Fluorescence Detectors • Detection of longitudinal shower profile via UV fluorescence light. • Reconstruction of geometry from recorded shower ‘track’. • Using the atmosphere as a calorimeter. • 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

9. 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.

10. The HiRes (High Resolution Fly’s Eye) Experiment

11. 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 The HiRes Collaboration

12. 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.

13. Mirror area ~ 5 m2 . • 256 (16x16) PMT per mirror. • One PMT sees ~ 1 degree of the sky.

14. 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. Measuring the Energy Spectrum with HiRes

15. 1. Reconstruction of the shower-detector plane • 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

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

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

18. 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. Phototube Calibration

19. Rayleigh contribution is quite stable and well known. Aerosol profile of the atmosphere has to be monitored during the run. =><VAOD> = 0.04 +/- 0.02 => +/- 15 % in J(E) Detailed monitoring with steerable lasers at both sites. Additional vertical laser outside of Dugway (Terra). “Shoot the Shower” Atmospheric Calibration

20. Unfolding the Cosmic Ray Spectrum

21. We observe the spectrum convoluted with detector acceptance and limited resolution. Deconvolution with help of a correction factor: D(Ei)= Rij T(Ej) T(Ei)= [Gmc(Ei)/Rmc(Ei)] D(Ei) 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. Deconvolution of the UHECR Spectrum

22. HiRes Monte Carlo Simulation

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

24. 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

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

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

27. Energy Distribution & Resolution  =18%

28. HiRes-1: distance to shower core

29. HiRes-1: Energy Resolution

30. HiRes-1 HiRes-2 Instant Apertures

31. The HiRes-2 UHECR Spectrum

32. HiRes and Fly’s Eye

33. HiRes and Haverah Park

34. HiRes and Yakutsk

35. HiRes and AGASA

36. 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 %

37. 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

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

39. Fits to the Spectrum

40. 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 & 19.79+/-0.09eV =3.32+/-0.04 & 2.86+/-0.04 & 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

41. Extragalactic Galactic 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 (g) and m • uniform source density evolving with (1+z) m • Extragalactic component • 45% protons at 1017 eV • 80% protons at 1017.85 eV • 100% protons at 1020 eV • Use binned maximum likelihood method

42. Interpretation • Pion-production pileup causes the bump at 1019.5 eV. • e+e- pair production excavates the ankle. • Fractionation in distance and energy; e.g., z=1 dominates at second knee.

43. The Future of HiRes: TA / TALE

44. TA - the “Telescope Array” • SD: 576 scintillation counters, each 3 m2 area, 1.2 km spacing. • 3 fluorescence stations, each covering 108o in azimuth, looking inward. • Central laser facility. • Millard County, Utah, flat valley floor for SD, hills for fluorescence, low aerosols. • A 1020 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.

45. Ideas for Recyling HiRes • Two HiRes detectors, moved to Millard Co. • 6 km stereo with TA fluorescence detectors. • Each HiRes detector has two rings, 270o azimuthal coverage. • Aperture of 16000 km2 ster. • Increase fluorescence aperture from 500 to 1,780 km2 ster, including 10% duty cycle. (TA SD=1400). • Increase in fluorescence aperture of x 3.6

46. TALow energy Extension:“Tower of Power” & Infill Array • 15 mirrors, 3xHiRes area, in rings 3,4,5 ( 3o - 71o ) => 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

47. for more information:www.cosmic-ray.orgwww.physics.rutgers.edu/~aszech

48. Extragalactic Galactic 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 (g) and m • uniform source density evolving with (1+z)3 • Extragalactic component • 45% protons at 1017 eV • 80% protons at 1017.85 eV • 100% protons at 1020 eV • Use binned maximum likelihood method  = 2.32+/-0.01

49. Summary

50. We have measured the UHECR spectrum from 1017.2 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 1018.5 eV. • The combined monocular HiRes spectra show evidence for a break above 1019.8 eV. The Poisson probability for continuation of the spectrum with unchanged slope from the HiRes monocular data is 2.4 * 10-4 .