Electron Measurements a brief summary HEAD March 3, 2010

Electron Measurementsa brief summaryHEAD March 3, 2010

Martin H. Israel

Washington University in St. Louis

[email protected]

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The spectrum of electrons is steep,so energy precision and energyresolution are important.

Protons are more abundant than electrons at a given energy by a factor ~1000, so proton rejection is vital.

Protons are more abundant than positrons by ~10,000, so proton rejection is even more vital for e+.

PPB-BETS (Torii et al., 2008)

Figure from poster of M. Pesce-Rollins with PPB-BETS points added.

Comparing Instruments

• Original objectives– PPB-BETS: Electrons 10 – 1000 GeV– ATIC: Nuclei 10 GeV – 100 TeV– H.E.S.S.: Gamma rays 100 GeV – 100 TeV– Fermi LAT: Gamma rays 20 MeV – 300 GeV– PAMELA: Positrons 50 MeV – 300 GeV, Electrons to 500 GeV

• Electron spectra reported– PPB-BETS: 10 – 800 GeV ~4 x 104 m2 sr s 13 days balloon

– ATIC: 20 – 3000 GeV ~3 x 105 m2 sr s 21 days balloon

– H.E.S.S.: 600 – 8000 GeV ~8 x 107 m2 sr s 10 days ground

– 300 – 800 GeV ~2 x 107 m2 sr s 3 days ground

– Fermi LAT: 20 – 1000 GeV ~3 x 107 m2 sr s 181 days in orbit

– PAMELA: 10 – 700 GeV ~1.5x105 m2 sr s 850 days in orbit

Comparing Instruments• Calorimeter vertical depth

– PPB-BETS: 9 Xo Pb, sample after each Xo

– ATIC: 18 Xo BGO, all active (Bi4Ge3O12)

– H.E.S.S.: 27 Xo Air, all active

– Fermi LAT: 8.6Xo CsI(Tl), all active

– PAMELA: 16 Xo W, sample after each 0.74 Xo

Deeper calorimeter gives better energy resolution and better ability to distinguish protons from electrons.

• Tracker elements above calorimeter– PPB-BETS: 1 mm scintillating fibers (36 layers)– ATIC: 2cm x 1cm Si pixels (1 layer) & 2 cm scintillator strips (6 layers)– H.E.S.S.:– Fermi LAT: 0.2 mm Si strips (36 layers)– PAMELA: 25 m (67 m) Si strips (6 double layers)

(not used for preliminary total-electron spectrum)

Better imaging of start of shower gives better ability to distinguish protons.

Comparing Instruments• Calorimeter vertical depth

– PPB-BETS: 9 Xo Pb, sample after each Xo

– ATIC: 18 Xo BGO, all active (Bi4Ge3O12)

– H.E.S.S.: 27 Xo Air, all active

– Fermi LAT: 8.6Xo CsI(Tl), all active

– PAMELA: 16 Xo W, sample after each 0.74 Xo

Deeper calorimeter gives better energy resolution and better ability to distinguish protons from electrons.

• Tracker elements above calorimeter– PPB-BETS: 1 mm scintillating fibers (36 layers)– ATIC: 2cm x 1cm Si pixels (1 layer) & 2 cm scintillator strips (6 layers)– H.E.S.S.:– Fermi LAT: 0.2 mm Si strips (36 layers)– PAMELA: 25 m (67 m) Si strips (6 double layers)

(not used for preliminary total-electron spectrum)

Better imaging of start of shower gives better ability to distinguish protons.

(analyzed subset >14 Xo)

ATIC

ATIC 1+2ATIC 1+2+4ATIC 1ATIC 2ATIC 4

ATIC 4 with 22 Xo is consistent with ATIC 1 and ATIC 2 (18 Xo).

Adding ATIC 4 to previously reported data of ATIC 1 + 2sharpens the feature.

PAMELA

Very preliminary PAMELA is consistent with Fermi LAT, and not with ATIC

Comparing Instruments• Proton + gamma fraction after electron selection

– PPB-BETS: ~0.4 at 100 GeV ~0.6 at 400 GeV ~0.7 at 800 GeV– ATIC: ~0.1 at 100 GeV ~0.2 at 400 GeV ~0.3 at 800 GeV– H.E.S.S.: ~0.5 at 340-700 GeV ~0.3 at 1000-4000 GeV– Fermi LAT:0.16 at 100 GeV 0.18 at 400 GeV 0.21 above 600 GeV– PAMELA: I do not have a good estimate of the uncertainty of these subtractions.

• Energy resolution, uncertainty– PPB-BETS: ~ 10% at 100 GeV, varies as 1/√E– ATIC: ~ 2%– H.E.S.S.: ±15%– Fermi LAT: +5%/-15% FW68%/E ~15-20% at 0°, ~5% at 60° – PAMELA:

PPB-BETS (Torii et al., 2008)

Figure from poster of M. Pesce-Rollins with PPB-BETS points added.

If there were a nearby source, some anisotropy might be found in the electron intensity.

Preliminary result from Fermi LAT for E > 60, 120, 240, 480 GeV:

No significant anisotropies have been reported in any of the analyzed energy ranges and angular scales.

New PAMELA data are consistent with previous report.

Rise in positron fraction above ~10 GeV is not consistent with Moskalenko & Strong calculation of positron secondaries, suggesting some source of primary positrons.

But there are explanations that do not involve positron sources, which take account of interactions of primary nuclei in dense regions near the source.(Cowsik & Burch ICRC 2009. Lee & Kamae poster at this conference.)

Conclusions

• Electron (e+ + e-) spectrum– ATIC and FermiLAT still disagree on the feature near 600 GeV.– HESS results do note rule out either.– PPB-BETS agrees better with ATIC, but has poor statistics and large

proton subtraction.– PAMELA spectrum agrees better with Fermi, but is still very

preliminary.

• No report of anisotropy of high-energy electrons.• Positron fraction

– Excess between 10 and 100 GeV compared with M&S secondary production model could be sign of an interesting source.

– Or “excess” could be explained as secondaries in models that take account of interaction of primary nuclei in material around the cosmic-ray sources.

Cowsik & Burch (ICRC 2009, revised)See also poster in this conference Lee & Kamae

Rising positron fraction, could be understood in a propagation model in which the secondary nuclear component has a significant, energy-dependent production in a region around the CR sources.

Cowsik & Burch (ICRC 2009, revised)

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Fermi LAT ATIC PPB-BETS H.E.S.S Protons

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