The electron and positron spectra measured by AMS-02

Claudio Corti on behalf of the AMS-02 collaboration

CosPA 2013 – November 14, 2013

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BackgroundsBackgrounds

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Casolino et al - 10.1016/j.nuclphysbps.2011.03.049

≈ 50 ÷ 500

≈ 103 ÷ 104 ≈ 10 ÷ 20

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The AMS-02 experimentThe AMS-02 experimentTRD

Silicon Tracker

ECAL

TOF

Magnet

RICH

Identify e+, e-

Z, P

E, Identify e+, e-

Z, β

Z, β

±Z

Z, P, E, β are measured independently by the tracker, RICH, TOF and ECAL

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The positron fraction measurementThe positron fraction measurement

f =e +

e ++e−

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Proton rejection– cut on ECAL estimator– fit on TRD estimator

Electron/positron distinction– Sign of rigidity (P)

Charge confusion estimation– fit on E/P

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ECAL estimator: BDTECAL estimator: BDT

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TRD estimator: Log LikelihoodTRD estimator: Log Likelihood

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Charge confusionCharge confusion

The difference between the charge confusion from data and MC is taken as the associated systematic error.

MCISS data

Two sources: Large angle

scattering (spillover) Production of

secondary tracks along the path of the primary track (wrong hits association)

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CC = N wrong sign/ (N wrong sign+N right sign)

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TRD-LL vs E/P templateTRD-LL vs E/P template

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From data

From MC

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2D template fit2D template fit

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2-D fit to TRD estimator and E/P is performed separately on positive and negative rigidity to determine positron or electron signal, proton background and charge confusion simultaneously.

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The positron fractionThe positron fraction

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The e+ and e- flux measurementThe e+ and e- flux measurement

Φ(E , E+Δ E) =N obs (E , E+Δ E )

Δ E ΔT exp Aeff ϵtrig

ΦN obs

ΔT exp

Aeffϵtrig

= Absolute differential flux (m-2 sr -1 s-1 GeV-1)= Number of observed events= Exposure time (s)= Effective acceptance (m2 sr)= Trigger efficiency

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: TRD-LL template fit: TRD-LL template fitN obs

Measurement is performed for the BDT cut that minimizes the overall statistical and systematic uncertainty

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Separate fit for positive and negative rigidity

Number of fitted “e+” and “e-” corrected for change confusion estimated from MC

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: Exposure time: Exposure time Data taking period: May 19, 2011 – May 19, 2013 Total exposure time constant above 25 GeV ≈ 5x107 s Average livetime for primary CRs ≈ 82% (including SAA,

detector dead time, Soyuz docking/undocking, etc)

ΔT exp

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: Acceptance: Acceptance

Aeff (E) = Agenerated×N selected (E)

N generated (E)

The effect of every cut has been checked on ISS data and, if needed, the value of the acceptance has been “corrected” (O(%))

Estimated with MC Geant4

Aeff

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: Trigger efficiency: Trigger efficiency

15

Estimated directly from data: AMS-02 record unbiased trigger events with a pre-scaling factor

ϵtrig

ϵtrig =N phys

N phys+ f ps N unbias

f ps

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Systematic errors: acceptanceSystematic errors: acceptance

16

Tracker reconstruction =# e± with a track

# e± through Trk acceptance

The uncertainty on the MC/Data agreement has been quoted as systematic

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Systematic errors: BDT cut stabilitySystematic errors: BDT cut stability

In each energy interval the cut on the ECAL BDT has been varied around the working point to verify the stability of the measurement.The RMS of the estimated number of e±, over a wide range of BDT cut efficiencies has been quoted as systematic

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Systematic errorsSystematic errors Selection MC/Data comparison 2-3%≈ Normalization Track/No-Track analysis 3%≈ Energy smearing (< 2 GeV) 1%≈ Stability vs BDT cut efficiency 1%≈ TRD-LL reference template for protons 1%≈

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The electron spectraThe electron spectra

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The positron spectraThe positron spectra

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The electron and positron combined spectraThe electron and positron combined spectra

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The electron and positron combined spectraThe electron and positron combined spectra

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(2010)

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Fermi flux from positron fraction paperFermi flux from positron fraction paper

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Ackermann et al - 10.1103/PhysRevLett.108.011103

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AMS – Fermi “comparison”AMS – Fermi “comparison”

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ConclusionsConclusions

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6.5x106 electrons out of 25x10≈ 9 triggers have been used to publish the measurement of the positron fraction up to 350 GeV

9x106 electrons out of 30x10≈ 9 triggers have been used to measure the separated and combined electron and positron spectra up to 700 GeV

This corresponds to 10% of the ≈expected data sample

The analysis is being finalized for the publication

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Backup slidesBackup slides

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OutlineOutline AMS-02 experiment Analysis Systematic errors Results

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: Event selection: Event selection DAQ: efficient data periods (no SAA) Geomagnetic effects: E > 1.25 max cutoff TRD: minimum 8 hits used for e/p identification (TRD-LL) TOF: relativistic down-going particle ECAL: shower axis within fiducial volume and

electromagnetic shape of the shower (BDT estimator) Tracker: Z < 1.5 and track/ECAL matching

N obs

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Charge confusion subtractionCharge confusion subtraction

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CC = NW/(NW + NR) N+ = Npos (1 – CC) + Nele CC : observed positrons N- = Npos CC + Nele (1 – CC) : observed electrons

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BDT cut efficiencyBDT cut efficiency

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An enriched sample in electrons is selected by means of negative sign of the charge and E/P matching. The same fitting procedure as on the full sample is applied in order to estimate the number of electrons as a function of the BDT cut.

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TRD-LL template definitionTRD-LL template definition

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Use 21 different selections varying (E/|P|, BDT) to define the proton template and see the effect on the measurement.

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Bin-to-bin migration (energy smearing)Bin-to-bin migration (energy smearing)

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ECAL resolution = 1.7 % @100 GeVSmearing effects not negligible only below few GeV