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 l Askaryan (1962) äA 10-30% excess of electrons over positrons develops in a high energy shower. äThe electrons emit Cherenkov radiation in a material äNormally P CR  N e (optical) äHowever, if bunch size << (microwave): E  N e  P CR  E 2  N e 2. (N e  E shower (eV) /(0.2*10 9 )) l Radio allows the largest target volumes to be “instrumented”. l Modern predictions using simulations: Halzen, Zas, Stanev, Alvarez-Muniz

3. Goals of Accelerator Measurements l 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 ? ¹Do antennas respond to picosecond pulses as we expect? l Experience with the signals we are trying to detect. l & Prepare for runs with higher energy beams l (Previous work has been published on TR+CR w/ mm waves in air: Takahashi, Shibata...)

4. The Argonne Runs l Attempted to address all but the first question at the Argonne Wakefield Accelerator ó 15.2 MeV e - from L-band linac ó 10 10-11 electrons/pulse (0.5 to 25 nC/pulse) Equivalent to excess at shower max of 10 21 eV shower. ó 40 ps pulses,  T ~1 cm Sept 23-24, 1999 (three days prep time)

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

6. Shower Simulation (EGS4)

7. Horn Antenna l Pyramidal Standard Gain Horn l S-band: 1.7 -- 2.4 GHz l Directivity: 15.3 dBi l Linear Polarization l Moved in 15 0 increments around target: 0-135 0 l S 11 measurement shows >90% efficient in bandpass. But what about short pulses? Plan a calibration with a 2nd antenna.

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

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

10. Datasets l Pure TR runs ä8.5 0 and 16.6 0, pointing at vacuum window, d=183 cm äSimple geometry: useful for calibration l Empty-target runs äFor background to CR measurement d=107cm äAlso gives extra TR points l Full target runs äd=107 cm, various polarizations. Find CR ? l Diagnostic runs äuse 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 l Oscilloscope recorded E-field intensity as voltage-- 100 psec resolution, interpolated in to 40 psec by scope l P=4V 2 /(50  ) (account for power dividing with horn) over 3 nsec window (75 bins) l Account for losses: horn efficiency & acceptance, cables, attenuators... l Measure pulse in  J/sr

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

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

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

16. A complication due to geometry l Forward lensing of TR until about 50 0 l We had not expected TR to be such a large contribution (clear in hindsight)

17. Polarization Measurements l Expect TR/CR to be radially polarized. In beam-horn plane this is (0 0 ) linear polarization. l Extract polarization via 3 Stokes parameters l Beyond 50 0 appears purely polarized. l Consistent with lensing not being a problem at higher angles.

18. Data vs. CR ``prediction’’ l Cannot use Frank-Tamm for short tracks <. Expect very wide “Cherenkov cone” l Expect no lensed TR beyond 50 deg. l Difficult to calculate, very sensitive to track length. l No good TR prediction for air  medium (air-sand interface) l Coherence at 60 0 seen

19. Discussion-I l Transition radiation, under some conditions, produces comparable energy to Cherenkov äTR is traditionally neglected in studies of the sensitivities of these UHE neutrino experiments - (considered by Markov/Zheleznykh) - threshold for TR detection alone ~ 5 x 10 20 eV (albeit with smaller volume) - should be examined as background to terrestrial experiments from downward-going extensive air showers äIn future accelerator work, using photon beams will simplify interpretation. l Our experiment was not as well suited to separating CR and TR as we hoped.

20. Discussion - II l Coherence obtains in S-band for TR and most likely CR. äWe 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. äCalculations such as HZS do not include this effect even though RF would exceed total energy at 10 24 eV -- only ~5 orders of magnitude beyond our Goldstone energy threshold. We might be subject to some effect. äHowever, scaling our results (even with x35 discrepancy)...if deviation is due to physics, Goldstone theshold is still ~5x10 20 eV, only about 5 times higher than simulations. l We have run up against gaps in theory: max currents, poles in predicted power, formation zone effects.

21. Discussion-III l 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. l Still want to address question #1...Does an excess of electrons over positrons develop in a high energy shower. l Would like to address questions #2-4 in an environment better emulating what is induced by a UHE neutrino.