Konstantin Belov High Resolution Fly’s Eye (HiRes) Collaboration

1. <Xmax> measurements by fluorescence experiments. GZK-40. INR, Moscow. May 17, 2006. Konstantin Belov High Resolution Fly’s Eye (HiRes) Collaboration • Columbia University • University of Adelaide • University of New Mexico • Rutgers University • University of Montana • Los Alamos National Laboratory • University of Tokyo Konstantin Belov. GZK-40, Moscow.

2. Motivation • Compare the <Xmax> measurements from the three fluorescence experiments: Fly’s Eye, HiRes/MIA, and HiRes Stereo. • Only fluorescence experiment SEE Xmax. • In HiRes energy range (1017.2-1020.5 eV) expect to see a transition between galactic and extragalactic sources: • Expect highest energy galactic cosmic rays to be heavy nuclei (iron); • Expect extragalactic cosmic rays to be light (protons) • Expect transition from mixed to light composition. Konstantin Belov. GZK-40, Moscow.

3. Xmax distribution for p, CNO, Fe • Heavier primaries develop earlier in the atmosphere. • RMS of the Xmax distribution is different. QGSJet Konstantin Belov. GZK-40, Moscow.

4. Elongation rate p, CNO, Fe Konstantin Belov. GZK-40, Moscow.

5. Fly’s Eye detector. • Two fluorescence detectors were positioned 3.5 km apart; • The effective energy range extended from 1017 eV to 31019 eV; • 55 degree pixel size; • Geometry of the event was determined by the intersection of the two event-detector planes; • Elevation angle range from 0 to 90 degrees; • FE II had only partial azimuth coverage Konstantin Belov. GZK-40, Moscow.

6. HiRes prototype/MIA • Hybrid fluorescence - muon array detector; • Fourteen mirror prototype of the current HiRes detector had a 11 degree pixel size; looked from 3 to 71 degrees in elevation. • Was located 3.5 km from the center of the MIA muon array; • The muon array sampled a part of muon lateral distribution. 3 km effective area; • Energy range from 51016 eV to just above 1018 eV; • Event geometry was determined by using the HiRes event-detector plane and timing from both detectors. Konstantin Belov. GZK-40, Moscow.

7. HiRes fluorescence detector. Konstantin Belov. GZK-40, Moscow.

8. Stereo observations. HiRes 1 has 20 operational mirrors in one ring. S/H electronics. FOV 3-17º in elevation, 322º in azimuth HiRes 2 has 42 operational mirrors in two rings. FADC electronics. FOV 3-31º in elevation, 336º in azimuth. Detectors are 12.6 km apart forming a stereo pair. Stereo fixes shower geometry improving resolution. Idealistic view of a stereo event Konstantin Belov. GZK-40, Moscow.

9. An event of the HiRes displays. HiRes1 – one ring detector HiRes2 – two ring detector Konstantin Belov. GZK-40, Moscow.

10. Measured shower profile. Measured shower parameters. Event by event: • Xmax in g/cm2; • Total energy of the primary particle: • Arrival direction Statistically: • p-air inelastic cross-section; • Mass composition. Konstantin Belov. GZK-40, Moscow.

11. HiRes stereo and HiRes/MIA data. • HiRes/MIA - hybrid experiment. Resolution function estimated from simulations ~ 44 gm/cm2 • HiRes stereo - Xmax resolution measured, 30 gm/cm2 Konstantin Belov. GZK-40, Moscow.

12. Fly’s Eye stereo data. • Fly’s Eye Stereo - Xmax resolution ~45 gm/cm2 measured resolution function Konstantin Belov. GZK-40, Moscow.

13. Systematic errors • All three experiments quote systematic errors ~ 25 gm/cm2. • Dominant contributions • Mirror survey • Cherenkov subtraction • Atmospheric profile • Aerosol corrections • Highest energy data is best measurements - most complete profile - minimum Cherenkov subtraction Konstantin Belov. GZK-40, Moscow.

14. Comparison of Fly’s Eye Stereo and HiRes energy scales Energy scale for all three experiments is consistent (location of ankle and second knee). Konstantin Belov. GZK-40, Moscow.

15. Elongation rate Use HiRes stereo average Xmax and Fly’s Eye stereo average Xmax above 1018 eV. Require a 13 gm/cm2 upward shift for Fly’s Eye to bring means into agreement Shift all Fly’s eye Xmax data points by the same amount. Filled circles, HiRes; Triangles, HiRes/MIA, Open circles, Fly’s Eye (shifted). Konstantin Belov. GZK-40, Moscow.

16. Elongation Rate • Simple shift of Fly’s Eye data brings all data into reasonable agreement. • Fly’s Eye and HiRes data are in excellent agreement above 1018 eV. • HiRes/MIA shows earlier transition to “protons”, but point by point discrepancy is small. • HiRes/MIA systematics are better understood, however. Konstantin Belov. GZK-40, Moscow.

17. What about Xmax distributions? • Are Xmax distribution widths consistent? • Two overlap regions - Fly’s Eye and Hires/MIA in 3x1017 to 5 x 1017 eV bin. - Fly’s Eye and HiRes stereo in > 1018 eV bin. • No evidence of discrepancy in distribution widths. Konstantin Belov. GZK-40, Moscow.

18. Fly’s Eye vs HiRes/MIA Xmax at 3-5 x 1017 eV Triangles: Fly’s Eye Stereo; Circles: HiRes/MIA. Konstantin Belov. GZK-40, Moscow.

19. Fly’s Eye vs HiRes Xmax > 1018 eV Triangles: Stereo Fly’s Eye; Circles: HiRes. Konstantin Belov. GZK-40, Moscow.

20. Sequence of Xmax distributions in three increasing energy bins 3-51017, 5-101017 and > 1018eV. Red-Fly’s Eye Blue-HiRes/MIA Purple-HiRes stereo Konstantin Belov. GZK-40, Moscow.

21. Conclusions • A simple Xmax shift brings all three experiments into reasonable agreement. • Widths of Xmax distributions are in agreement. • Normalized Xmax distribution show change to wider distribution above 1018, consistent with change to protons. • Interpretation of elongation rate over limited energy range is problematic - Need large dynamic range in a single experiment! Konstantin Belov. GZK-40, Moscow.