暗物質(zhì)粒子探測衛(wèi)星的能量重建和宇宙線質(zhì)子能譜的分析

岳川

(中國科學(xué)院紫金山天文臺南京210023)

宇宙線的觀測研究和暗物質(zhì)粒子的間接探測是高能天體物理領(lǐng)域兩個重大研究課題. 自1912年V.Hess發(fā)現(xiàn)宇宙線開始, 人類對宇宙線的觀測歷史已經(jīng)超過了一個世紀(jì), 傳統(tǒng)理論模型預(yù)言“膝”區(qū)以下能段的宇宙線能譜應(yīng)服從單一冪率分布, 而近些年的空間和高空氣球?qū)嶒?yàn)表明10 GeV–100 TeV的宇宙線質(zhì)子能譜可能存在偏離單一冪律譜分布的重要結(jié)構(gòu), 這對研究銀河系內(nèi)宇宙線的起源、傳播和加速機(jī)制具有重要意義. 另一方面, 得益于宇宙線和伽馬射線觀測精度的提高和觀測能段的拓寬, 暗物質(zhì)粒子的間接探測在國際上受到越來越多的關(guān)注, 暗物質(zhì)粒子可能會發(fā)生湮滅或衰變產(chǎn)生穩(wěn)定的普通高能粒子, 包括正負(fù)電子對、正反質(zhì)子對、伽馬射線和中微子等, 進(jìn)而在宇宙線或伽馬射線留下可探測的信號.

暗物質(zhì)粒子探測衛(wèi)星悟空號(DAMPE)是中國發(fā)射的第1顆空間天文衛(wèi)星, 也是目前國際上最為先進(jìn)的空間高能粒子探測器之一, 可以實(shí)現(xiàn)對高能宇宙線和伽馬射線的精確探測, 并在此基礎(chǔ)上開展暗物質(zhì)信號搜尋和其他高能天體物理課題的研究. 伴隨悟空號的發(fā)射和在軌運(yùn)行, 作者參與了衛(wèi)星在軌標(biāo)定、科學(xué)數(shù)據(jù)分析軟件和探測器模擬程序的開發(fā)與測試、物理事例重建和宇宙線粒子能譜分析等諸多方面的工作.

探測器模擬不僅對研究數(shù)據(jù)重建方法和理解實(shí)驗(yàn)結(jié)果具有重要意義, 同時也是獲取探測器對不同入射粒子的儀器響應(yīng)(如能量分辨、角度分辨)的重要途徑. 本文第2章介紹了DAMPE探測模擬程序開發(fā)過程中的相關(guān)問題, 包括探測器的幾何建模、探測單元響應(yīng)信號的讀出、模擬信號數(shù)字化(電子學(xué)噪聲添加)過程等. 此外, 本文利用衛(wèi)星在軌數(shù)據(jù)對探測器模擬程序進(jìn)行了對比驗(yàn)證.

在完成探測器模擬程序的基礎(chǔ)之上, 本文利用大量的模擬數(shù)據(jù)研究了針對不同粒子的能量重建方法. 粒子在量能器中的簇射發(fā)展可分為電磁簇射和強(qiáng)子簇射兩種, 兩種簇射在量能器中的發(fā)展過程和能量沉積行為有很大不同. 針對電磁簇射型粒子(電子、光子), 本文研究了一種基于簇射發(fā)展形狀的參數(shù)化能量修正算法, 能夠逐事例對入射電子或光子的泄露能量進(jìn)行補(bǔ)償修正, 并可以有效提高能量分辨率; 針對強(qiáng)子簇射型粒子(質(zhì)子、重核), 本文研究了能譜反卷積的算法, 利用量能器的沉積能量分布重建出入射粒子的原初能量分布. 我們利用電子和質(zhì)子束流實(shí)驗(yàn)數(shù)據(jù), 分別對兩種算法進(jìn)行了檢驗(yàn). 這些工作分別在第3、4章進(jìn)行了介紹.

第5章系統(tǒng)介紹了宇宙線質(zhì)子能譜的分析工作, 主要包括在軌質(zhì)子事例的挑選、選擇效率的標(biāo)定、本底的估算、能譜反卷積、質(zhì)子流量的計(jì)算和誤差分析. 基于悟空號兩年的在軌飛行數(shù)據(jù), 本文得到了50 GeV–20 TeV的宇宙線質(zhì)子能譜的初步測量結(jié)果, 結(jié)果顯示在該能量范圍內(nèi)質(zhì)子能譜明顯偏離單一冪律分布, 能譜在約400 GeV能量出現(xiàn)變硬的結(jié)構(gòu), 這與空間磁譜儀實(shí)驗(yàn)PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics)和阿爾法磁譜儀2號(AMS-02)得到的觀測結(jié)果一致.

The observation of cosmic rays and indirect detection of dark matter particles are two important topics in high-energy astrophysics. In 1912, V. Hess discovered the cosmic rays.Since then, the observation of cosmic rays has lasted for more than a century. Traditional theories suggest that the cosmic-ray flux follows a single power-law below the so called“knee”at a few PeV energies. Recent measurements of cosmic-ray proton flux from space-based observatories and balloon-borne experiments have revealed some unexpected structures in the energy range of 10 GeV–100 TeV, which provided important clues for the study of the origin,propagation,and acceleration mechanism of galactic cosmic rays. On the other hand,thanks to the improvement of observational accuracy and the extending of observational energy range for cosmic-rays and gamma-rays,the indirect detection of dark matter particles(DM)has attracted more and more attentions in the past few years. DM annihilation/decay may producee±,protons,and anti-protons,gamma-rays,or neutrinos,which can potentially lead to observable signals in cosmic-rays or gamma-rays.

The DArk Matter Particle Detector Explorer(DAMPE)is the first astronomical satellite of China. DAMPE is one of the most advanced space-based particle detectors for the observation of high-energy cosmic-rays/gamma-rays and the detection of DM signals. Since the launch of DAMPE, I have been deeply involved in the calibration, simulation, and scientific data analysis for this project.

Detector simulation plays a crucial role not only for the studies of analysis methods,but also for the understanding of the detector response (e.g. energy resolution and angular resolution) for different particles. In the second chapter, several issues about the detector simulation of DAMPE are introduced,including the geometric modelling of the detector,the readout of the response signals from sensitive units,and the digitization process. The detector simulation program is then verified by comprehensive comparisons between simulation data and flight data.

Based on the simulation data,we studied the energy reconstruction method for different kinds of incident particles. In the calorimeter, there are two kinds of showers related to different interaction mechanisms between incident particles and the detector. One is the electromagnetic shower induced by an electron/positron or a gamma-ray photon, the other is the hadronic shower induced by a proton or heavy ion. For the electromagnetic shower produced bye±/γ, we proposed a parameterized correction method using the lateral and longitudinal information of shower development to estimate the primary energy event by event. This method is verified with data of electron beam test at CERN, which shows significant improvements for both the energy linearity and resolution. For the hadronic shower produced by proton/heavy ion, we studied an unfolding method based on the Bayes’theorem to directly estimate the primary energy distribution instead of the event-by-event level correction of the energy.

The final part is the analysis of the cosmic-ray proton spectrum. The whole procedure is introduced, including event selection, particle identification, background estimation, efficiency validation, spectral deconvolution, acceptance calculation, and uncertainty analysis.Based on two years of data recorded by DAMPE, the preliminary cosmic-ray proton spectrum from 50 GeV to 20 TeV is obtained. The cosmic-ray proton spectrum of DAMPE is not consistent with a featureless power-law model,but exhibits a spectral hardening at hundreds of GeV, which confirms the results reported by PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) and AMS-02 (Alpha Magnetic Spectrometer-2).