Measurement of the Cosmic-ray Proton Spectrum with the Fermi Large - PowerPoint PPT Presentation

Measurement of the Cosmic-ray Proton Spectrum with the Fermi Large Area Telescope David Green, Liz Hays On Behalf of the Fermi-LAT Collaboration UMD/GSFC ICRC 2017 July 14, 2017

Introduction ATIC (2003)[1] PAMELA (2006-2008)[4] • PAMELA and AMS-02 BESS-TeV (2002)[2] AMS-02 (2011-2013)[5] 16000 ] -1 CREAM-I (2005)[3] observe a spectral break sr at ~400 GeV -2 m 14000 -1 • Have to reconcile with s -1 “standard” theories for J(E) [GeV CR origins, acceleration, 12000 and propagation • Additional 10000 × measurements 2.7 E extending from GeV to LAT Energy Range 8000 TeV can help understand spectral break 3 5 2 4 10 10 10 10 Energy [GeV] 2

The Fermi LAT Tracker (TKR) • 18 x-y layers of silicon strip detectors • Used for direction reconstruction and • The Large Area Telescope (LAT) is one 
 particle identification of two instruments on the 
 Fermi Gamma-ray Space Telescope • The LAT is a pair conversion telescope Anti-coincidence Detector (ACD) • 89 segmented plastic scintillating tiles • Used for particle identification Calorimeter (CAL) • 1536 CsI(Tl) crystals arranged in 8 layers • Hodoscopic, image shower shape and profile • Used for energy measurement 3

Event Selection Based on AMS-02 proton flux Over 8 years of flight • Proton flux is high enough, we don’t need a large acceptance to measure the spectrum to TeV energies Almost 10 6 events above 1 TeV • The proton event selection is defined as: • Event has to trigger and pass onboard filters 3.5 sr] 2 Trigger & Filter Proton Acceptance [m • Require event to have reconstructed 3 Track Found track 2.5 Energy > 20 GeV 2 • Deposited energy >20 GeV in CAL 1.5 • Require a well reconstructed track using Track Reconstruction 1 Pass 8 direction classifier 0.5 PRELIMINARY 0 3 2 4 10 10 10 True Energy [GeV] 4

Charge Measurement • Cosmic-ray helium and nuclei pose large contamination source for this study • We use the TKR and the ACD to independently measure the charge of incoming cosmic ray in the LAT • Define a polygon in ACD-charge vs TKR- charge to select on protons • Developed using flight data and Geant4 Sum proton/electron/nuclei simulations PRELIMINARY Electrons − 1 10 Helium Residual Contamination • Find a residual contamination from CR helium Nuclei and nuclei less than 1% − 2 10 • CR electrons are under 4%, decreasing with energy − 3 10 • We background subtract any residual electron − 4 10 contamination 3 2 10 10 Recon Energy [GeV] 5

Energy Measurement True Energy [GeV] • We use the CAL to measure the energy of the proton induced shower • CAL is up to 2 λ I at off axis angles 3 10 • Develop event selection to select ideal event topologies PRELIMINARY • Does not fall within gaps between CAL modules True Energy = Recon Energy 2 10 • Select events with low ‘backsplash’ into TKR 3 2 4 10 10 10 Recon Energy [GeV] • Require > 0.5 λ I in the CAL • We fit the profile of energy deposition to • We unfold the spectrum in true energy estimate the energy of the incident proton using ROOT’s TUnfold with a Tikhonov • Deposited energy primarily from regularization term electromagnetic component of total shower 6

Systematic Uncertainties Does not include energy uncertainties • This study is dominated by Statistical systematic uncertainties 0.14 Signal Efficiency • We use two methods to estimate Alternative GEANT4 Models 0.12 Systematic Uncertainty Stat + Sys Uncertainties our systematic uncertainties: 0.1 PRELIMINARY • Signal Efficiency 0.08 • Selecting events with different path-lengths 0.06 • Alternative GEANT4 models 0.04 • Response uncertainties via 0.02 alternate hadronic models 0 • Uncertainty in the energy 3 2 10 10 True Energy [GeV] measurement is still being finalized 7

Cosmic-ray Proton Spectrum Does not include energy uncertainties • Using 7 years of LAT flight CREAM-I (2005) ATIC (2003) data, August 4, 2008 to 
 PRELIMINARY AMS-02 (2011-2013) ] -1 July 30, 2015 PAMELA (2006-2008) sr Fermi-LAT (2008-2015) -2 • Extends energy of space- m -1 based measurement to 9.5 s -1 TeV J(E) [GeV • Red markers represent statistical uncertainty 4 • Red shaded region includes 10 × 2.7 systematic uncertainties E • Good agreement with other cosmic-ray measurements 3 2 4 10 10 10 Energy [GeV] 8

Conclusions and Future Courtesy of Matt Meehan • Space-based spectral measurement to from 54 GeV to 9.5 TeV • Additional cosmic-ray proton studies with the LAT • Cosmic-ray Proton Anisotropy with LAT by Matt Meehan - CRD092 • Testing methods to estimate energy uncertainty 9

References 1.A. D. Panov et al. Energy spectra of abundant nuclei of primary cosmic rays from the data of ATIC-2 experiment: Final results. Bulletin of the Russian Academy of Sciences: Physics, 73(5): 564–567, 2009. 2.Y. Shikaze et al. Measurements of 0.2–20 GeV/n cosmic-ray proton and helium spectra from 1997 through 2002 with the BESS spectrometer. Astroparticle Physics, 28(1):154 – 167, 2007. 3.Y. S. Yoon et al. Cosmic-ray proton and helium spectra from the first CREAM flight. The Astrophysical Journal, 728(2):122, 2011. 4.O. Adriani et al. PAMELA measurements of cosmic-ray proton and helium spectra. Science, 332(6025):69–72, 2011. 5.M. Aguilar et al. Precision measurement of the proton flux in primary cosmic rays from rigidity 1 GV to 1.8 TV with the alpha magnetic spectrometer on the international space station. Phys. Rev. Lett., 114:171103, Apr 2015. 10

Backup Slides

Anti-Coincidence Detector (ACD) • ACD’s main purpose is to detect CRs • Consists of 89 plastic scintillating tiles and 8 plastic scintillating ribbons that cover the TKR • Top tiles arranged in a 5 x 5 grid • Side tiles arranged in 5 x 3 grid with single large tile on the bottom row • Signal in each tile read by two PMTs • Each PMT has a dual range, linear low range and non-linear high range • Energy deposition in ACD described by ionization arXiv:0902.1089v1 ACD Base Electronics Assembly • Can use this to identify charge of incident particle 12

The Tracker (TKR) Astropart. Phys., 28, 422 Photon • 16 layers of high Z tungsten foil • Convert photon to e + e − pair • Last 4 conversion layers about 6 times thicker Tungsten Layer 1 than previous 12 x silicon strips y silicon strips • 18 layers of silicon strip detectors • Measure position of charged particle Structural Tray Material • TKR is 1.5 radiation lengths thick Tungsten • TKR is used to measure direction of incident x silicon strips Layer 2 cosmic-ray y silicon strips • Direction used to path-length correct signal and in reconstruction of several variables • Additionally, energy deposited via ionization Tungsten • Can use TKR as independent measure of CR x silicon strips Layer 3 y silicon strips charge 13

Calorimeter (CAL) • Use CAL to measure CR energy and direction CDE: CsI Detectors + • Composed of 16 modules; each module has 96 Al Cell Closeout Carbon Cell Array PIN diodes (both ends) CsI(Tl) crystals • Arranged in 8 layers in alternating x-y directions • This allows for not only measuring energy deposition but also imaging of shower shape and direction • Shower shape can be used for particle identification • 8.6 radiation lengths deep (0.5 nuclear Readout Electronics interactions) at normal incidence Al EMI Shield • 2.5 nuclear interactions lengths for maximum off angle axis Atwood 2009 arXiv:0902.1089v1 • At higher energies shower leakage crystal saturation needs to be corrected and accounted 14

Hadronic Showers in the LAT 100 GeV • We can estimate how proton induced shower 1 TeV look like in the CAL 100 GeV 1 TeV • Same can be seen for radial profile, EM core with hadronic extension • EM component dominates early longitudinal profile and radial core 15

Unfolding The Spectrum True Energy [GeV] − Event Rate 1 10 − 2 10 ] -1 Unfold via 
 Total event rate [s − 3 10 3 10 ROOT’s TUnfold − 4 10 PRELIMINARY PRELIMINARY − 5 10 − 6 Response Matrix 10 2 10 PRELIMINARY − 1 10 − 7 10 ] -1 Unfolded event rate [s 3 2 4 3 2 4 10 10 10 10 10 10 Recon Energy [GeV] Recon Energy [GeV] − 2 10 Unfolded − 1 10 PRELIMINARY − 3 10 Event Rate ] − -1 2 10 sr 2 3 10 10 -2 True Energy [GeV] m − 3 10 -1 0.3 s -1 J(E) [GeV 0.25 − 4 10 sr] 0.2 2 Acceptance [m Divide by 
 − 5 10 0.15 PRELIMINARY acceptance 
 Proton Spectrum 0.1 − 6 10 Acceptance and bin width 0.05 3 2 10 10 True Energy [GeV] 0 2 3 10 10 16 True Energy [GeV]

Signal Efficiency • Primary measure of systematic uncertainty in acceptance • Test stability of spectral measurement over different path-lengths through LAT • Probes shower development through different geometric cross-sections of LAT • Find energy dependent quantiles of path-length and produces cuts for 90% - 30% quantiles • Produce different IRF for each quantile cut and reconstruct the spectrum • The maximum variation of all spectra determines the uncertainty 17