Electron Observations from ATIC and HESS A Brief Overview of the Experimental Results Called Pre-FERMI Experiments ATIC – Advanced Thin Ionization Calorimeter HESS – High Energy Spectroscopic System plus PPB-BETS – see K. Yoshida presentation later in session John P. Wefel Department of Physics and Astronomy Louisiana State University Baton Rouge, LA 70803 AAS HEAD Meeting, March 2010, Hawaii

Very Different Experimental Techniques ATIC is a balloon experiment flown from Antarctica; measure electrons directly in deep calorimeters (18-22 Xo) plus tracking and charge measurement; excellent energy resolution; accelerator calibration; background determined from data; gamma ray events track electron response; limited exposure time; statistics limited results HESS is a ground based atmospheric Cherenkov telescope array for gamma-ray astronomy; for electrons observes off-source regions of sky; huge collecting area; separates proton background by ‘random forest’ approach using simulations; systematic uncertainty from simulation model; electron energy scale uncertainty of 15%; high statistics results.

ATIC Instrument Antarctic Flights: 12/28/00 - 1/13/01 12/29/02 – 1/18-03 12/27/07 – 1/15/08

The ATIC Instrument was calibrated at CERN Determine instrument response. Investigate energy resolution. Check accuracy of simulations to allow extrapolation to higher energy. Used 150 GeV electrons and 375 GeV protons to validate electron analysis and evaluate the proton contamination (i.e. 1 in 5000).

The ATIC electron results exhibit a “feature” Sum of data from both ATIC 1 and ATIC 2 flights Curves are from GALPROP diffusion propagation simulation Solid curve is local interstellar space Dashed curve is with solar modulation Spectral index is -3.23 below ~ 100 GeV “Feature” at about 300 – 800 GeV Significance is about 3.8 sigma Also seen by recent PPB-BETS Emulsion chamber data is currently being re-analyzed  ATIC 1+2,  Alpha Magnetic Spectrometer,  HEAT magnetic spectrometer,  BETS,  PPB-BETS,  Emulsion chambers Chang et al., Nature, 456, 362-365, 20 November 2008

All three ATIC flights are consistent “Source on/source off” significance of bump for ATIC1+2 is about 3.8 sigma ATIC4 ATIC-4 with 10 BGO layers has improved e , p separation. “Bump” is seen in all three flights. Significance for ATIC1+2+4 is 5.1 sigma

HESS Analysis for Electrons Observe away from known gamma ray sources Analyze images from four telescopes using standard techniques Divide data into energy ranges Fit each shower using a Random Forest approach to determine electron-like character CORSIKA Simulation code using both QGSJET and SIBYLL Cut on fit parameter – assign each remaining shower to electron (gamma) or hadronic background. Correct for possible gamma ray contamination Check with Xmax distribution Use simulations to determine effective collection area (energy dependent).

High Energy ( > TeV) HESS Analysis (Phys Rev Letters, 101, 261104, 31 December 2008)

High Energy HESS Results are in agreement with ATIC data and confirm a rapid decrease in the electron intensity beyond about a TeV in energy. Red ATIC- 1 + 2 Blue ATIC-4 (prelim,) Black HESS Is something ‘interesting’ happening at the highest energy?

Low Energy (0.34 – 0.7 TeV) HESS Results (Astronomy and Astrophysics, 508, 561, December 2009)

Threshold may affect the data interpretation The ATIC threshold is ~20 GeV and the electron data appears to show a “bump” If the threshold was moved to ~300 GeV then only the peak values with the high energy cutoff are apparent Now the ATIC electron data looks like a flat spectrum between 300 GeV and ~700 GeV with a high energy cutoff (qualitatively, like the HESS results).

Summary and Conclusions The newest ATIC flight, with a 20% deeper calorimeter, shows superior proton rejection and a spectrum in agreement with the previous results from ATIC-1+2. The HESS and ATIC results are in substantial agreement, considering both the systematic and statistical uncertainties, and show a rapid fall off in the spectrum beyond TeV energies. Is this the ‘end’ of the electron spectrum? That question is still open. The region beyond a few TeV still requires exploration. We may well be living in a very special, non-standard region of the Galaxy.

Secondary -rays guide electron selection ATIC responds to gamma rays and electrons in an identical fashion Flight -ray events guide cuts in the shower lateral width and energy fraction which are then applied to all electron-like flight data Method allows proton contamination to be evaluated using flight data Avoid relying exclusively on simulations for event selection and background evaluation

ATIC, PPB-BETS, HESS and Fermi Results

Cross Plot of Cuts

e,p shower development by calorimeter layer Plot fraction of energy deposited in layer versus shower lateral width (R.M.S.) distribution CERN data Simulations