LUNACEE ( LUN ar C herenkov E mission E xperiment)

1. LUNACEE(LUNar Cherenkov Emission Experiment) • Radio-Frequency Measurements of Coherent Transition and Cherenkov Radiation: • (hep-ex/0004007) • Implications for High Energy Neutrino Detection • D. Saltzberg • (5/23/00) • for • Peter Gorham, D.S., Paul Shoessow, Wei Gai, John Power, Dick Konecny, Manuel Conde

2. The Basic Ideas of Radio Detection of UHE cosmic rays • Askaryan (1962) • A 10-30% excess of electrons over positrons develops in a high energy shower. • The electrons emit Cherenkov radiation in a material • Normally PCR Ne (optical) • However, if bunch size <<  (microwave): E  NePCR E2 Ne2. (Ne Eshower (eV) /(0.2*109)) Radio allows the largest target volumes to be “instrumented”. Modern predictions using simulations: Halzen, Zas, Stanev, Alvarez-Muniz

3. Goals of Accelerator Measurements • Basic Questions -- checking simulations • Does the 10-30% electron/positron excess really develop in a high energy shower? Need SLAC run • Do the electrons in the shower emit as much Cherenkov radiation as expected? • Is the radiation coherent in S-band, as expected ? • Do antennas respond to picosecond pulses as we expect? • Experience with the signals we are trying to detect. • & Prepare for runs with higher energy beams • (Previous work has been published on TR+CR w/ mm waves in air: Takahashi, Shibata...)

4. The Argonne Runs • Attempted to address all but the first question at the Argonne Wakefield Accelerator • 15.2 MeV e- from L-band linac • 1010-11 electrons/pulse (0.5 to 25 nC/pulse) Equivalent to excess at shower max of 1021 eV shower. • 40 ps pulses, T~1 cm Sept 23-24, 1999 (three days prep time)

5. The Target 800 lbs. dry silica sand = 1.6 g/cc, n=1.55 (@2GHz) thin polyethylene walls, RF absorber top & bottom mounted on hydraulic table (dust free)

6. Shower Simulation (EGS4)

7. Horn Antenna Pyramidal Standard Gain Horn S-band: 1.7 -- 2.4 GHz Directivity: 15.3 dBi Linear Polarization Moved in 150 increments around target: 0-1350 S11 measurement shows >90% efficient in bandpass. But what about short pulses? Plan a calibration with a 2nd antenna.

8. Trigger Antenna • Used fixed dipole antenna (balanced half-wave) • Provided stable trigger (<40 psec)

9. Data Acquisition • Tektronix TDS694C (real-time, 4ch. 3 GHz, 10 GSa/s) • w/o beam (RF only) absolutely no measurable noise • No amplification of antenna signals! Needed attenuators. • Heliax cable to control room • Measured E-field intensity (V/m) directly, not a power measurement.

10. Datasets • Pure TR runs • 8.50 and 16.60, pointing at vacuum window, d=183 cm • Simple geometry: useful for calibration • Empty-target runs • For background to CR measurement d=107cm • Also gives extra TR points • Full target runs • d=107 cm, various polarizations. Find CR ? • Diagnostic runs • use absorber & timing to identify sources of reflection etc.

11. Is coincident RF from accelerator a problem? Some people have worried that the image charge of beam in beampipe could contribute to radiation. Solid: TR in horn looking at vacuum window Dotted: TR in horn with aluminum sheet (>>skin depth) sheilding vacuum window No difference.

12. Extracting pulse energy • Oscilloscope recorded E-field intensity as voltage--100 psec resolution, interpolated in to 40 psec by scope • P=4V2/(50 ) (account for power dividing with horn) over 3 nsec window (75 bins) • Account for losses: horn efficiency & acceptance, cables, attenuators... • Measure pulse in J/sr

13. TR (+ Empty Target) Runs • Unattenuated pulses would be up to 30 volt peaks. • fits shape of TR formula (e.g., Ginzberg) • TR does not depend much on material, esp. at forward angles. • meas./calc.= 0.037 • Solid:pure TR run • Open:empty target • Calc. includes beam form-factors

14. TR Coherence Runs • Range of pulse intensities (ICT) • Line is slope=2 (full coherence) • Coherence in S-band observed • At high currents is coherence breaking down? (Must happen some point.)

15. Full vs. Empty Target: Timing Information Timing shows power is coming from center of target. Advantage of direct sampling. solid:full dashed: empty

16. A complication due to geometry Forward lensing of TR until about 500 We had not expected TR to be such a large contribution (clear in hindsight)

17. Polarization Measurements Expect TR/CR to be radially polarized. In beam-horn plane this is (00) linear polarization. Extract polarization via 3 Stokes parameters Beyond 500 appears purely polarized. Consistent with lensing not being a problem at higher angles.

18. Data vs. CR ``prediction’’ • Cannot use Frank-Tamm for short tracks < . Expect very wide “Cherenkov cone” • Expect no lensed TR beyond 50 deg. • Difficult to calculate, very sensitive to track length. • No good TR prediction for air medium (air-sand interface) • Coherence at 600 seen

19. Discussion-I • Transition radiation, under some conditions, produces comparable energy to Cherenkov • TR is traditionally neglected in studies of the sensitivities of these UHE neutrino experiments • (considered by Markov/Zheleznykh) • threshold for TR detection alone ~ 5 x 1020 eV (albeit with smaller volume) • should be examined as background to terrestrial experiments from downward-going extensive air showers • In future accelerator work, using photon beams will simplify interpretation. • Our experiment was not as well suited to separating CR and TR as we hoped.

20. Discussion - II • Coherence obtains in S-band for TR and most likely CR. • We worry about loss of coherence due to quenching at high energy. It would be nice to go up 2 or more orders of magnitudes in beam currents to see. • Calculations such as HZS do not include this effect even though RF would exceed total energy at 1024 eV -- only ~5 orders of magnitude beyond our Goldstone energy threshold. We might be subject to some effect. • However, scaling our results (even with x35 discrepancy)...if deviation is due to physics, Goldstone theshold is still ~5x1020 eV, only about 5 times higher than simulations. • We have run up against gaps in theory: max currents, poles in predicted power, formation zone effects.

21. Discussion-III • We have gained some valuable experience working with the pulses produced by electrons (emulating shower max) in the lab. Phenomenon behaves basically as we expect, where we could test it. • Still want to address question #1...Does an excess of electrons over positrons develop in a high energy shower. • Would like to address questions #2-4 in an environment better emulating what is induced by a UHE neutrino.