1. E-165 (FLASH) Experiment Status Report Johnny S.T. Ng SLAC EPAC Meeting, Nov. 15, 2003

2. Fluorescence from Air in Showers (FLASH) J. Belz 1, Z. Cao 2, F.Y. Chang 4, P. Chen 3*, C.C. Chen 4, C.W. Chen 4, C. Field 3, P. Huentemeyer 2, W-Y. P. Hwang 4, R. Iverson 3, C.C.H. Jui 2, G.-L. Lin 4, E.C. Loh 2, K. Martens 2, J.N. Matthews 2, J.S.T. Ng 3, A. Odian 3, K. Reil 3, J.D. Smith 2, P. Sokolsky 2*, R.W. Springer 2, S.B. Thomas 2, G.B. Thomson 5, D. Walz 3 1 University of Montana, Missoula, Montana 2 University of Utah, Salt Lake City, Utah 3 Stanford Linear Accelerator Center, Stanford University, CA 4 Center for Cosmology and Particle Astrophysics (CosPA), Taiwan 5 Rutgers University, Piscataway, New Jersey * Collaboration Spokespersons SLAC E-165 Experiment

3. Outline Introduction: Motivation and Goals Report on the Sep.2003 Run –Thin-target results –Spectrograph results Plans and outlook –Thick-target stage

4. Motivation Recent observations of ultra-high energy cosmic ray events extend out to 10 20 eV However, there is apparent discrepancy between AGASA and HiRes > ~10 19 eV

5. Cosmic Rays have been observed with energies beyond 10 20 eV The flux (events per unit area per unit time) follows roughly a power law: ~E -3 Changes of power-law index at “knee” and “ankle”. Onset of different origins/compositions? Where does the spectrum stop? Ultra High Energy Cosmic Rays

6. AGASA HiRes Discrepancy Between Two UHECR Experiments

7. Motivation (cont.) Recent observations of ultra-high energy cosmic ray events extend out to 10 20 eV However, there is apparent discrepancy between AGASA and HiRes > ~10 19 eV Systematics in energy measurement being investigated by both experiments. One possible contribution to the discrepancy is the air fluorescence yield.

8. Current Understanding of the Air Fluorescence Spectrum Bunner (1967), Kakimoto et al. (1995), Nagano et al. ( 2002) indicates ~15% systematic errors in overall yield and larger errors in individual spectral lines. Ground based experiments – non-linear effects possible due to λ -4 dependence of atmospheric attenuation. At 30 km, event energy can change by 25% if 390 nm line intensity changes by 40%.

9. Experimental Program The T-461 Test (June 2002) showed that we can measure the pressure dependence and fluorescence lifetime integrated over 300-400 nm with FFTB beams  Spectrally resolved pressure and lifetime measurements  Energy dependence over realistic shower energies The E-165 program: Thin target Thick target T-461

10. Objectives Of E-165 Spectrally resolved measurement of fluorescence yield to better than 10%. Study effects of atmospheric impurities. Observe showering of electron pulses in air equivalent substance (Al 2 O 3 ) with energy equivalents around 10 18 eV. Investigate dependence on electron energy.

11. Sep. 2003 Run: Thin Target FFTB Beam: 28.5 GeV, 5E8 – 2E9 e/pulse Main target vessel: –Dry air, N2, “SLAC Air”, 10-760 Torr –Narrow-band filters (10nm BW, 300-420 nm) –Calibrated HiRes PMTs Spectrograph system –Dry air, N2, “SLAC Air”, 15-760 Torr –Hamamatsu linear-array MPMT’s –Grating spectrograph system: 6 nm and 4 nm per pixel

12. Thin-Target Run: Main Vessel Layout e Pressure vessel NB Filter wheel Calibration LED housing e Filter wheel PMT Pressure vessel

13. OTR Beam Spot Monitor e  Monitor beam spot size and position  Provide energy profile diagnostics as well Wonderful contribution from the CosPA (Taiwan) group 45-deg OTR foil

14. E-165: Air Fluorescence Spectrum Narrow Band filters Dry-Air 750 Torr Arbitrary normalization N1A: Corrected for filter transmission and quantum efficiency Preliminary Bunner

15. Air Fluorescence vs. Pressure E-165 Preliminary

16. Effects of Contaminants “SLAC Air”: H 2 O, and traces of CO 2 and Ar Possible slight difference, but within our present measurement accuracy. E-165 Preliminary

17. Beam-related Background Various measurements as cross-checks: “Black”-filter, “Blind”-PMT, and Ethylene (non-fluorescent) gas.

18. Narrow-Band Data Analysis Data have been corrected for  Filter transmission efficiency  PMT Quantum efficiency  Background contribution Further systematic issues (some examples):  Absolute calibration: PMT + Mirror + Filter  Absolute calibration of low-current toroid  Stability (gain variation over time, etc.)  Uncertainty in background subtraction  Detector acceptance (ray-tracing + modeling)

19. Grating Spectrograph e Light pipe and mirror Grating Spectrograph

20. Spectrograph Measurement: Air E-165 Preliminary!

21. Spectrograph: Fluorescence vs. Pressure E-165 Preliminary At low pressure (<100 Torr):  Fluorescence mechanisms extremely complex!  Sensitive to non-linear effects. Above ~150 Torr (for HiRes,eg):  Small pressure variation  Allows altitude spectral correction

22. Spectrograph Data Analysis Data have been corrected for:  PMT quantum efficiency vs wavelength  Anode - anode difference in response  Wavelength dependence of transmission to PMT  Spectral dispersion at the PMT face (measured using a mercury lamp with 4 useful lines.) Further systematic issues (some examples):  Independently measure transmission and associated uncertainties.  Calibrate dispersion for the wider-range grating set-up  Quantify the non-linear effects seen at higher beam intensities and low pressures.

23. Summary on Thin-Target Run Over-all successful thin-target run Measured spectrally resolved fluorescence for various pressures and gas mixtures Took reference data at low beam charge (1E7 e/pulse) for the thick-target stage Beam-related background (most likely soft photons from shower) was a major problem Detailed data analysis underway – plan to present results at the upcoming “International Workshop on Air Fluorescence – Air Light 03” in December 2003.

24. The Next Step: Thick-Target For a 10 18 eV cosmic-ray proton, or a 30 GeV electron, the cascade consists of electrons with energies between 100 keV and a few GeV at shower-max. Pass electron beam through varying amounts of approx. air equivalent showering material (Al 2 O 3 ). Measure light yield as a function of depth in the shower –Is fluorescence proportional to dE/dx? –What are the contributions of low-energy (<1 MeV) electrons? –Can existing shower models (EGS, GEANT, CORSIKA) correctly predict fluorescence light?

25. Thick Target Requirements Low beam intensity ~ 10 7 e/pulse –Beam monitor and control Showering beam spreads out at target exit – Careful calculation/measurement of optical acceptance necessary. Need shower profile measurements to constraint Monte Carlo simulations Beam-related background issue –Beam test scheduled for Jan. 5-7, 2004 to optimize layout and shielding

26. Thick Target Setup  Design to be finalized pending beam test and further simulation studies.

27. Summary E-165 is an experiment with the aim to contribute and support astrophysics investigations using high energy beams. We’re making good progress thanks to the strong support from SLAC.