Recent Results on Galactic Cosmic Rays with the DAMPE Experiment

Recent results on galactic cosmic rays with the DAMPE experiment

Xin Wu

Department of Nuclear and Particle Physics University of Geneva, Switzerland

XSCRC2019, CERN 14 November, 2019 Outline

• Introduction to the DAMPE experiment

• Description and performance of the detector

• Recent cosmic ray results

Xin Wu XSCRC2019, 14/11/2019 2 DAMPE Launched ~4 years ago (17/12/2015)

High energy particle physics experiment in space

Xin Wu XSCRC2019, 14/11/2019 3 The Detector Plastic Scintillator Detector (PSD)

Silicon-Tungsten Tracker (STK)

BGO Calorimeter (BGO)

Neutron Detector (NUD)

ü Charge measurements (PSD and STK) ü Precise tracking with Si strip detectors (STK) high energy g-ray, ü Tungsten photon converters in tracker (STK) electron and cosmic ray ü Thick imaging calorimeter (BGO of 32 X0, 1.6 l) nuclei telescope ü Extra hadron rejection (NUD)

Xin Wu XSCRC2019, 14/11/2019 4 The Collaboration • China – Purple Mountain Observatory, CAS, Nanjing – Institute of High Energy Physics, CAS, Beijing – National Space Science Center, CAS, Beijing – University of Science and Technology of China, Hefei – Institute of Modern Physics, CAS, Lanzhou • Switzerland – University of Geneva, Switzerland • Italy – INFN Perugia and University of Perugia – INFN Bari and University of Bari – INFN Lecce and University of Salento – INFN LNGS and Gran Sasso Science Institute

Xin Wu XSCRC2019, 14/11/2019 5 The DAMPE Satellite

EQM, Oct. 2014, CERN Integrated satellite, Sept. 2015, Shanghai Weight : 1450/1850 kg (payload/satellite) Power: 300/500 W (payload/satellite) Readout channels: 75,916 (STK 73,728) Size: 1.2m x 1.2 m x 1.0 m

Xin Wu XSCRC2019, 14/11/2019 6 The Orbit • Altitude: 500 km • Inclination: 97.4065 • Period: 95 minutes • Orbit: sun-synchronous

Launched Dec. 17 2015 • Dec. 20: all detectors powered on, except the HV for PMTs • Dec. 24: HV on! • Dec. 30: stable trigger condition Xin Wu XSCRC2019,• Very 14/11/2019 smooth operation since! 7 Particle hit counts vs orbit

• 15 orbits/day, 96 minutes/orbit • ~50 Hz average trigger rate, ~5 M events/day – Main high energy trigger and prescaled low energy and MIP triggers Xin Wu XSCRC2019, 14/11/2019 8 Plastic Scintillator Detector (PSD) 2 layers (x, y) of strips 1 cm thick, 2.8 cm wide and 88.4 cm long Sensitive area 82.5 cm x 82.5 cm, no dead zone • Strip staggered by 0.8 cm

Readout both ends with PMT, each uses 2 dynode signals (factor ~40) to extend the Xin Wu XSCRC2019,dynamic 14/11/2019 range to cover Z = 1, 26 9 Silicon-Tungsten Tracker (STK)

• Outer envelop 1.12m x 1.12m x 25.2 cm • Detection area 76 x 76 cm2 • ~7 m2 of silicon • Total weight: 154.8 Kg • Total power consumption: ~85W

Xin Wu XSCRC2019, 14/11/2019 10 The STK structure • 12 layers (6x, 6y) of single-sided Si strip detector mounted on 7 support trays • Tungsten plates (1mm thick) integrated in trays 2, 3, 4 (from the top)

– Total 0.85 X 0 for photon conversion 192 ladders 768 silicon sensors 95 x 95 x 0.32 mm3

1,152 ASICs

73,728 channels

Xin Wu XSCRC2019, 14/11/2019 11 BGO Calorimeter (BGO) • 14-layer BGO, 7 x-layers + 7 y-layers – BGO bar 2.5 cm2.5 cm x 60 cm, readout both ends with PMT • Use 3 dynode (2, 5, 8) signals to extend the dynamic range – Charge readout/Trigger: ASIC with dynamic range up to 12 pC Detection area 60cm60cm X Layer (22 BGO bars)

Y Layer

14 Layers

Xin Wu XSCRC2019, 14/11/2019 Total thickness 32 X0/1.6 l 12 Neutron Detector (NUD) • 4 large area boron-doped plastic scintillators ( 30 cm30 cm1 cm) – Detect the delayed thermal neutron capture signal to help e/h separation – Gating circuit to detect delayed signal with a settable delay (0-20µs) after the trigger from the BGO

n + 10B → α + 7Li + g

Xin Wu XSCRC2019, 14/11/2019 13 On-ground calibration • Several weeks at CERN PS and SPS beams from Oct. 2012 – Nov. 2015 (EQM) – Plus many cosmic muon data (FM)

Nov. 2014 Oct. 2012

Nov. 2015 XinMarch Wu 2015 XSCRC2019, 14/11/2019 14 Electron energy linearity and resolution

1-20 GeV 50-243 GeV

Linearity DE/E < 1.5% for > 100 GeV

Good linearity and resolution, good agreement between test beam and simulation Xin Wu Energy correction:XSCRC2019, ~6-7% 14/11/2019 for 100 GeV – 1 TeV 15 Proton energy resolution

Good agreement between test beam and simulation

Proton energy cannot be easily corrected. Need unfolding!

Xin Wu XSCRC2019, 14/11/2019 16 BGO in-flight MIP calibration • “MIP” calibration: ADC → MeV and equalization, use events near the equator, 20

MIP spectrum of a BGO bar Geomagnetic MIP MPV shift % cut-off effect

MPV of a BGO bar After “temperature correction” MeV ADC

BGO Helium MIP

Xin WuProton “MIP” MPV vs temperatureXSCRC2019, (time) 14/11/2019Helium “MIP” mean vs time, stable17 <1% Absolute energy scale validation • Overall energy scale can be checked with geomagnetic cut-off effects – Charge particles detected in a geomagnetic zone have specific cut-off in the flux (deflection by the magnetic shield) • Use L in 1 – 1.14, cut-off ~ 13 GeV • Measured cut-off compared to MC simulation with IGRF-12 model and back-tracing code

Cdata/Cpred = 1.0120.017(stat.) 0.013(sys.) McIlwain L shells Energy scale agrees with expectation within 2%

Stable with time (small decrease in cut-off due to solar modulation)

Xin Wu XSCRC2019, 14/11/2019 18 PSD in-orbit charge measurement

Astropart. Phys., 105, 31 (2019) Element sZ H 0.07 He 0.10 Li 0.14 Be 0.21 B 0.17 C 0.18 N 0.21 O 0.20

Excellent for cosmic-ray measurements

Xin Wu XSCRC2019, 14/11/2019 19 STK noise very stable since launch

1 year

Average noise 2.84-2.88 ADC Very small temperature variation

Xin Wu ~4 year in orbit, >99.5%XSCRC2019, channels 14/11/2019 are still working perfectly! 20 STK in-flight alignment • Good thermal stability guaranteed a good short term mechanical stability 300 x2 x1 Re-align every 2 weeks to x3 x6 200 x4 y1 correct for long term shift x5 y6 y2 aligned m) y3 non-aligned µ y4 y5 100 90 80 70 60 Outside layers with larger extrapolation errors

Effective resolution ( 50

40 Intrinsic position resolution 40 -50 µm

10 20 30 40 50

θx(θy) (deg) Unbiased hit residual of 12 layers before/after (re)alignment, as function of incidence angle

Xin Wu XSCRC2019, 14/11/2019 21 X Residual width variation: aligned

1.6 0° < Q < 10° 1.6 10° < Q < 20° 1.5 x x1 x2 1.5 x 1.4 x3 x4 1.4 1.3 1.3 1.2 x5 x6 1.2 1.1 1.1 1 1

2015 2016 2016 2017 2017 2018 2018 2019 2015 2016 2016 2017 2017 2018 2018 2019 1.6 Dec-3120°Jul-01 < QDec-31 < 30Jul-01° Dec-31 Jul-02 Dec-31 Jul-02 1.6 Dec-3130° Jul-01< Q Dec-31 < 40Jul-01° Dec-31 Jul-02 Dec-31 Jul-02 1.5 x 1.5 x 1.4 1.4

(January,1st) 1.3 1.3

1 1.2 1.2

s 1.1 1.1

/ 1 1 1

s 2015 2016 2016 2017 2017 2018 2018 2019 2015 2016 2016 2017 2017 2018 2018 2019

1.6 Dec-31 Jul-01 Dec-31 Jul-01 Dec-31 Jul-02 Dec-31 Jul-02 1.6 Dec-31 Jul-01 Dec-31 Jul-01 Dec-31 Jul-02 Dec-31 Jul-02 40° < Q < 50° Q > 50° 1.5 x 1.5 x 1.4 1.4 1.3 1.3 1.2 1.2 1.1 1.1 1 1

2015 2016 2016 2017 2017 2018 2018 2019 2015 2016 2016 2017 2017 2018 2018 2019 Xin Wu Dec-31 Jul-01 Dec-31 Jul-01 Dec-31 Jul-02 9thDec-31 DAMPEJul-02 Workshop,Dec-31 15/06/2019Jul-01 Dec-31 Jul-01 Dec-31 Jul-02 Dec-31 Jul-02 22 X Residual width variation: not aligned

1.6 0° < Q < 10° 1.6 10° < Q < 20° 1.5 y y1 y2 1.5 y 1.4 y3 y4 1.4 1.3 1.3 1.2 y5 y6 1.2 1.1 1.1 1 1

(January,1st) 1.3 1.3

1 1.2 1.2

s 1.1 1.1

/ 1 1 1

s 2015 2016 2016 2017 2017 2018 2018 2019 2015 2016 2016 2017 2017 2018 2018 2019

1.6 Dec-31 Jul-01 Dec-31 Jul-01 Dec-31 Jul-02 Dec-31 Jul-02 1.6 Dec-31 Jul-01 Dec-31 Jul-01 Dec-31 Jul-02 Dec-31 Jul-02 40° < Q < 50° Q > 50° 1.5 y 1.5 y 1.4 1.4 1.3 1.3 1.2 1.2 1.1 1.1 1 1

Energy bins

DAC calibration

STK threshold update HV reset

~5.2 M events recoded per day

More than 7 billion events collected Skimmed datasets: number of events per energy interval very stable Xin Wu XSCRC2019, 14/11/2019 24 Proton flux • Published in 17 September 2019 in Science Advances (Sci. Adv., 5, 9 (2019)) • ) Measurement of the cosmic ray proton spectrum from 40 GeV to 100 TeV with the DAMPE satellite

Cross-checked with 3 independent analyses

Xin Wu XSCRC2019, 14/11/2019 25 Proton flux measurement

• Key steps of the analysis – Preselection • High Energy Trigger • STK track selection: crucial for linking BGO shower to the correct PSD hit – Charge selection • Proton selection based on the charged measured in PSD – STK charge used for validation • Helium background: template fit of the PSD charge – Helium contamination ≲1% for deposited energies at <10 TeV and up to ∼5% around 50 TeV • Electron rejected based on BGO shower shape variables, residual < 0.05% – Energy unfolding • Bayesian unfolding (D'Agostini, NIM A362(1995), 487) – Systematic uncertainties • Three independent analyses gave consistent final proton flux results

Xin Wu XSCRC2019, 14/11/2019 26 Trigger efficiency

• Trigger efficiency is validated with data using the unbiased trigger

1 MC Proton Flight Data 0.8 HE Trigger Efficiency

0.6

0.4

0.2

1.2 102 103 104

MC/Data 1.1

0.9

0.8 102 103 104 BGO Energy [GeV] Good data-MC agreement, use 2.5% as systematic error

Xin Wu XSCRC2019, 14/11/2019 27 Tracking efficiency

• Trigger efficiency is validated with data with a proton selection using “BGO tracks” 1

0.8

0.6 Track Rconstruction Efficiency

0.4 MC Proton Flight Data 0.2

3 1.2 10 102 10 104

MC/Data 1.1

0.9

0.8 10 102 103 104 BGO Energy [GeV] Data-MC consistent within 3.5%, use as systematic error

Xin Wu XSCRC2019, 14/11/2019 28 Charge selection efficiency • PSD charge templates from proton and Helium MC (corrected for back scattering) 447 – 562 GeV 4.47 – 5.62 TeV 20 – 63 TeV

• Cut: • Efficiency validated by data using first layer of STK

1 1 Charge Efficiency Charge Efficiency 0.8 0.8

0.6 0.6 Good data-MC agreement, MC Proton MC Proton Flight Data Flight Data 1.8% systematic error 0.4 0.4 PSD layer-y PSD layer-x

1.1 10 102 103 104 1.1 10 102 103 104

MC/Data 1.05 MC/Data 1.05

1 1

0.95 0.95 1%Xin Wu 1.5%XSCRC2019, 14/11/2019 29 0.9 3 0.9 3 10 102 10 104 10 102 10 104 BGO Energy [GeV] BGO Energy [GeV] Background fraction and effective acceptance

0.05 sr] 2 Helium Contamination 10 G4 FTFP_BERT Electron Contamination 0.04 Contamination [%] 1 0.03 Effective Acceptance [m

10-1 0.02

-2 10 0.01

10-3 0 102 103 104 102 103 104 105 Deposited Energy [GeV] Incident Energy [GeV] • Helium background varies between ≲1% • Effective Acceptance (GF x Eff) evaluated for <10 TeV to ∼5% around 50 TeV with G4 with the FTFP_BERT physics list – Electron negligible

Xin Wu XSCRC2019, 14/11/2019 30 Energy unfolding

• With a homogenous detector, the energy resolution is relatively good. • Unfolding performed with the Bayesian method (D'Agostini, NIM A362(1995), 487)

G4 FTFP_BERT

Xin Wu XSCRC2019, 14/11/2019 31 Systematic Uncertainties

• Selection efficiency: 4.7%

– Charge selection became important above 50 TeV

• Background only relevant at highest energy: 1-4% from 40 -100 TeV

• Statistical error 2-6% from 10 -100 TeV

• Hadronic model: 7-10%

Xin Wu XSCRC2019, 14/11/2019 32 Hadronic model uncertainty • The largest systematic uncertainty is the hadronic model – Low energy (<1 TeV): ~7% by comparing 400 GeV test beam data and GEANT4 FTFP_BERT (baseline model for <100 TeV) simulation. – High energy (>1 TeV): 7-10% by comparing FTFP_BERT and FLUKA • FLUKA is baseline model for >100 TeV, used only for unfolding – Further check with comparing FLUKA and CRMC

DAMPE simulation preliminary • G4-CRMC interface implemented (A. Tykhonov) 0.5 – CRMC (DPMJET) and GEANT4 (FTFP_BERT) has good agreement 0.4 – No significant differences between DPMJET and 0.3 EPOS in CRMC – FLUKA energy deposited softer than G4-CRMC 0.2 GEANT4 (FTFP_BERT) • Likely due to low-energy hadronic models

0.1 GEANT4--CRMC (DPMJET_FTFP_BERT) • Also the geometry implementations of G4

Mean Deposited / True Energy and FLUKA is different FLUKA (Fluka-DPM & DPMJET) 0 103 104 105 106 Primary Energy (GeV) A. Tykhonov et. al. ICRC2019, https://pos.sissa.it/358/122/ Xin Wu XSCRC2019, 14/11/2019 33 Comparison of proton flux measurements ] 1.6 DAMPE

GeV ATIC-2 (2009) -1 s

-1 PAMELA (2011) sr

-2 AMS-02 (2015) [m 104 CREAM-I+III (2017) 2.6 NUCLEON-KLEM (2018) E × CALET (2019) Flux

DAMPE Uncertainties (Stat.+Syst.)

10 102 103 104 105 Kinetic Energy [GeV]

Xin Wu XSCRC2019, 14/11/2019 34 Preliminary Helium flux

• Analysis in progress, final result will extend to higher energy (~25-50 TeV/n)

39 months of data

M. Di Santo et. al. ICRC2019, https://pos.sissa.it/358/058/

Xin Wu XSCRC2019, 14/11/2019 35 Electron + positron flux • Published in 29 November 2017 in Nature, 552, 63 (2017) 10−3 broken power law DAMPE (2017) single power law 10−4

10−5 ) -1

GeV −6

-1 10 sr -1 s -2 10−7 A break at ~0.9 TeV is observed Flux (m −8 10 17 months of data

10−9 To be updated soon with more than 2 times data 10−10 103 Xin Wu Energy (GeV) XSCRC2019, 14/11/2019 36 Electron + positron flux • Published in 29 November 2017 in Nature, 552, 63 (2017) 10−3 broken power law DAMPE (2017) single power law 10−4 H.E.S.S. (2008) H.E.S.S. (2009)

10−5 AMS-02 (2014) ) -1 Fermi-LAT (2017)

GeV −6

-1 10 CALET (2017) sr -1 s -2 10−7 A break at ~0.9 TeV is observed Flux (m −8 10 17 months of data

10−9 To be updated soon with more than 2 times data 10−10 103 Xin Wu Energy (GeV) XSCRC2019, 14/11/2019 37 Conclusions • DAMPE is working extremely well since its launch about 4 years ago – A precise electron + positron flux in the TeV region has been measured • A clear spectral break has been observed at ~ 0.9 TeV • Update with more data, extend the measurement to 10 TeV soon – Direct high statistics proton flux up to 100 TeV has been published • A spectral softening at ~13.6 TeV is observed – Direct high statistics Helium flux up to at least 25 TeV/n is coming soon • DAMPE has provided several new pieces of puzzle to understand many mysteries in cosmic ray physics • Direct detection of TeV – PeV cosmic rays is entering a precision era – Validation of hadronic interaction models in the TeV – PeV range will be key to reduce systematic errors

Xin Wu XSCRC2019, 14/11/2019 38 Thank You!

Xin Wu XSCRC2019, 14/11/2019 39