Measurement of absolute energy scale of ECAL of DAMPE with geomagne;c rigidity cutoff Jingjing Zang *(PMO,CAS), Chuan Yue, Xiang Li (On behalf of DAMPE collabora;on) *Speaker, zangjj@pmo.ac.cn 35th ICRC, BUSON KOREA 2017 1
Outline • Introduc;on of DAMPE detectors • Mo;va;on and method reseach • CRE flux measurement – Pre-selec;on – Background contamina;on 1. hadron background 2. Secondary background • CRE flux and cutoff • Systema;c Uncertainty Launched on Dec. 17 th 2015 • Conclusion 35th ICRC, BUSON KOREA 2017 2
DAMPE Detectors Plas;c Scin;llator Detector(PSD) Silicon Tungsten Tracker(STK) Ø γ an;coincidence Ø γ convertor, par;cle track Ø Z-measurement Ø Z-measurement BGO Calorimeter(BGO) Ø Energy measurement(32X 0 &1.6 λ I ) Neutron Detector(NUD) Ø e/p separa;on Ø e/p separa;on Ø Trigger primi;ves More details and performances can be found at arXiv:1706.08453 35th ICRC, BUSON KOREA 2017 3
Mo;va;on and Method research Mo;va;on: • Energy scale of BGO-ECAL is given by on-orbit simulated energy deposi;on of cosmic ray proton “MIP” events, the real absolute energy scale remains unknown. • Absolute energy scale is a systema;c uncertainty on energy measurement Method research: • Geomagne;c cutoff on cosmic ray electron and positron spectrum provides a strong spectral feature. 35th ICRC, BUSON KOREA 2017 4
Procedure of absolute energy scale measurement 1. Measure low energy CRE flux with 1<L<1.14 – Primary cosmic ray electron + positron flux from 8GeV to 100GeV – Filng flux in 1<L<1.14 to extract cutoff value 2. Calculate Geomagne;c cutoff – Trace CRE in the Earth’s magne;c field – Calculate geomagne;c cutoff within L bin range 35th ICRC, BUSON KOREA 2017 5
Flight Data Sample Jan 2016 – Feb 2017 (425days, 2.15B events in total ) • Pre-selec;on ： • 1. High energy Trigger 2. 400MeV<Total deposited Energy<150GeV 3. Reject side and upward events 4. Reject heavy ion events by charge measurement Aqer Pre-selec;on, 40M events leq • First sight on G-cutoff Energy distribu;on 35th ICRC, BUSON KOREA 2017 6
Acceptance The pre-selec;on efficiency is evaluated by MC Simula;on and corrected by • flight data. Acceptance is calculated in 30 energy intervals • Due to strict pre-selecCon criteria, acceptance is limited within the absolute energy scale analysis 35th ICRC, BUSON KOREA 2017 7
Hadron Background contamina;on electron-hadron discrimina;on electron candidates sta;s;c e-candi selec;on criteria are kept at lowest valley between e-peak and H- peak. [14.4, 15.7]GeV e-candidate Electron pid efficiency is 90%@8GeV, rapidly increase to 98-99% above 20GeV. [36.4, 39.6]GeV e-candidate Background contaminaCon 20%@8GeV, 1%@12GeV,5%@100GeV 35th ICRC, BUSON KOREA 2017 8
How to es;mate Secondary Background The selected CRE candidates have two sources • 1. Primary electrons originate from interstellar space (primaries) 2. Secondary electrons generated in the atmosphere (secondaries) Primaries and secondaries have different features on azimuth distribu;on • Back Tracing for primary template • IGRF-12 for geomagne;c field • Trace par;cle trajectory in geomagne;c field • Data-Driven method of making secondary template • – Geomagne;c field blocks charged par;cles with low rigidity – CREs with energy far less than cutoff should be secondary dominant. Geomagne;c cutoff Secondary Template 1<L<1.14 Sub-cutoff events are selected to extract secondary template 35th ICRC, BUSON KOREA 2017 9
Secondary Background Contamina;on Ø Ra;o of Secondary: Secondary background ra;o change with Energy ü 8-13GeV:56% to 0% ü >13GeV :0% 10-11GeV 13-14GeV 10 35th ICRC, BUSON KOREA 2017
CRE Flux & G-cutoff CRE flux in L bin can be calculated by formula • ( ) 1 − δ s ( ) N 1 − δ h Flux = A ⋅ ε pid ⋅ T ⋅Δ E 8 p − Cutoff filng func;on y p x / 1 x p / • ( ) = ∗ + 1 0 2 p 0 : normaliza;on constant, p 1 : spectrum index, p 2 : G-cutoff • Cutoff energy = 13.20GeV C data /C tracer = 1.0125 ± 0.0175(stat) Flight data G-cutoff have a 1.25% exceeding comparing with back tracing result. 35th ICRC, BUSON KOREA 2017 11
Systema;c uncertainty (1.29%) Five major systema;c uncertainty sources • Filng range I. Binning migra;on rms/mean = 1.1% cutoff aqer unfolding = 13.22GeV, varia;on = 0.15% Ø II. Choice of filng range change range wider or narrower, uncertainty = 1.1% Ø III. IGRF-12 model Difference between DAMPE and Fermi-LAT Ø uncertainty level = 0.5% Fermi-LAT result IV. e/p Template from Astropar;cle Physics 35(6), 2012, 346-353 change tail longer or shorter, uncertainty = 0.4% Ø V. Choice of energy interval for secondary template Change energy interval wider or narrower, closer or farther to the cutoff, Ø uncertainty = 0.15% e/p template rms/mean = 0.4% 35th ICRC, BUSON KOREA 2017 12
Conclusion • We measured absolute energy scale of BGO calorimeter of DAMPE with geomagne;c cutoff on CRE spectrum using 2.15B events collected from Jan2016 to Feb 2017. • By comparing measured geomagne;c cutoff by flight data with predicted one by back tracing, we found DAMPE’s absolute energy scale is higher by 1.0125 ± 0.0175(stat) ± 0.0134(sys) in ~13GeV range. 35th ICRC, BUSON KOREA 2017 13
Thanks for your a}en;on We’re very appreciated for your ques;ons and comments! 35th ICRC, BUSON KOREA 2017 14
BACK UP SLIDES 35th ICRC, BUSON KOREA 2017 15
BGO energy scale in the official DAMPESW see PDG chapter 32 Energy scale determina;on procedure • in DAMPESW – “MIP” calibra;on run within la;tude -20 ° ~ Avoid cliffy <2GeV +20 ° , “MIP” calibra;on run range flat – Reconstruct proton “MIP” energy spectrum of each BGO crystal, – DAMPESW simulate CR-proton, and give verCcal rigidity cutoff @ DAMPE Orbit simulated proton “MIP” spectrum, – Simu & flight spectra are in good agreement Energy scale of BGO-ECAL is given by • on-orbit simula;on of CR-proton “MIPs” Same method has been verified with • beam test “MIP” spectrum is the reference of energy reconstrucCon. 35th ICRC, BUSON KOREA 2017 16
Systema;c uncertainty - Binning G-Cutoff obtained by filng differen;al flux, the migra;on from bin to bin • will change flux and affect filng result. Es;ma;on Method • – Based on MC Reco vs Ekin matrix, do unfolding Results • – cutoff Aqer unfolding = 13.22GeV, varia;on = 0.15% 35th ICRC, BUSON KOREA 2017 17
Systema;c uncertainty – Filng range Choice of filng range will affect filng result. • Method • – Modify filng range wider or narrower while keep range cover cutoff energy – Fit elow = Gaus(11,1),Fit ehigh = Gaus(80,10), do 500 ;mes filng – The rms of filng results is systema;c uncertainty Results • – Uncertainty = 0.1464/13.21 = 1.1% 35th ICRC, BUSON KOREA 2017 18
Systema;c uncertainty – IGRF IGRF-12 model was used to do backtracing and primary template. • Method • – It’s difficult to directly es;mate the uncertainty caused by IGRF – We can use Fermi-LAT result to assess uncertainty level, since Fermi-LAT and DAMPE are two independent experiment. Results • – Fermi-LAT, 1<L<1.14, G-cutoff = 13.27GeV – DAMPE, 1<L<1.14, G-cutoff = 13.20GeV – Rela;ve error = 0.5% Fermi-LAT result 35th ICRC, BUSON KOREA 2017 19
Systema;c uncertainty – e/p template The Monte Carlo template we used to es;mate hadron background contamina;on • could induce uncertainty on cutoff rigidity. Method • Background contamina;on is ~1% at 12GeV – Verify leq tail of MC template longer or shorter – and calculate background ra;o and flux to perform filng rigidity cutoff. Result • Background contaminaCon 1%@12GeV,13%@100GeV, 20%@8GeV Filng result changed within ~0.5% 35th ICRC, BUSON KOREA 2017 20
Systema;c uncertainty – secondary template • Secondary template was obtained from low energy flight data sample (E<<E_cutoff) where the CRE is secondary dominant. Choice of energy interval (2-4GeV) will affect shape of template and thus flux and final value of rigidity cutoff Method • Change energy interval wider or narrower, closer or farther to the – cutoff to extract template Flux measurement and filng to get final value of rigidity cutoff – Template change slightly with energy The spread of cutoffs reflect systema;c uncertainty caused by interval. – secondary template Energy Width Energy center Result • Max DeviaCon: (13.18-13.20)/13.20=0.15% – 35th ICRC, BUSON KOREA 2017 21
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