DAMPE Iron Spectrum Analysis And Research In Cosmic Ray Physics

The discovery of cosmic rays has a history of more than 110 years.However,there are still some questions that have not been fully explained regarding the origin,acceleration,and propagation of cosmic rays.Since the launch of the Alpha Magnetic Spectrometer(AMS-02),cosmic ray detection has entered the era of precise measurement,and some fine structures of the galactic cosmic ray spectrum have been revealed.Observations have found that the energy spectra of primary cosmic rays such as protons,helium,carbon,oxygen,neon,magnesium,and silicon become harder beyond～200 GV,while the proton and helium spectra become softer again at about～10 TV.DAMPE has measured the high-precision energy spectra of the boron-carbon(B/C)and boron-oxygen(B/O)ratios with energies up to 5 TeV/n,and found that both ratios become significantly harder at about～100 GeV/n.However,as an important member of primary cosmic rays,the iron nucleus spectrum has not yet shown a similar trend of becoming harder or softer.Therefore,the main problem of this paper is to measure the energy spectra of iron nucleus cosmic rays at higher energies.The accurate measurement of cosmic ray energy spectrum relies on the particle identification capability of the detector.We studied how to improve DAMPE’s particle identification capability based on principal component analysis(PCA).We extracted information such as the charge of the Plastic Scintillator strip Detector(PSD),the energy deposition ratio and lateral spread of each layer of the Bismuth Germanium Oxide(BGO)calorimeter,and performed data dimensionality reduction using PCA.The results show that PCA can effectively improve the particle identification capability of iron nuclei in the PSD detector.In addition,we found that PCA has a strong particle identification capability for particles with small charge numbers.Iron nuclei cosmic rays,which have the highest abundance among heavy nuclei(Z>14),are believed to be mainly produced and accelerated in astrophysical sources.The interaction cross-section between iron nuclei and the interstellar medium(H,He)is much larger than that of light nuclei(He,C,O,Ne,Mg,and Si),so the interaction between iron nuclei and the interstellar medium is stronger during propagation.The precise measurement of the spectrum of iron nuclei provides important information about the origin,acceleration,and propagation of cosmic rays in the Milky Way.Currently,the precise measurement of the spectrum of iron nuclei is mainly performed by AMS02 and CALET(the CALorimetric Electron Telescope),and the highest energy is lower than 100 TeV.Compared with the previous two detectors,DAMPE has a larger geometric acceptance.Our detector can accumulate about 24 iron nuclei events with energies above 1 TeV per day.Based on seven years of DAMPE data,we have provided a preliminary energy spectrum of iron nuclei.The current results are consistent with those of AMS-02 in the energy range of 1-100 TeV.We further studied the systematic errors that may be introduced in the spectrum analysis process.The current analysis shows that the error in the energy spectrum of iron nuclei may be mainly due to model problems.FTFP and FLUKA are commonly used physical models in heavy nuclei cosmic ray spectrum measurements.We found that the fragmentation cross-section of iron nuclei in the FTFP model is larger.In the energy range of 100 GeV to several TeV,the deflection angle of secondary particles after the fragmentation of iron in the FTFP model is larger,which makes it difficult to reconstruct the trajectory of iron nuclei particles and affects the charge measurement,reducing the acceptance of this model,leading to a systematic high bias in the spectrum at this energy range.In addition,the current analysis shows that the energy deposition ratio is the main difference between the FTFP and FLUKA models.The iron nuclei spectra of ATIC(Advanced Thin Ionization Calorimeter)and CALET were obtained using the FLUKA model,and their results were about 20%lower than those of AMS-02.Our results obtained using the FLUKA model are actually closer to those of CALET.We need to further study how to avoid the uncertainty introduced by the model in data analysis as much as possible.Additionally,based on the recent measurements of B/C and B/O energy spectra by DAMPE,we have conducted research on cosmic ray propagation theories.At kinetic energies of around 100 GeV/n,DAMPE observed that both ratios became significantly harder,which cannot be explained by a simple diffusion-reacceleration model,thus requiring a bend in the diffusion coefficient at high energies.Furthermore,we also attempted to use other propagation models to explain DAMPE’s observations.However,the propagation parameters obtained from current experimental data have relatively large errors.Using China’s next-generation high-energy cosmic ray detector,HERD,we predicted the constraints on cosmic ray propagation parameters from 10 years of data.Comparing the results,we found that HERD’s data will effectively limit the errors in propagation parameters.