大型强子对撞机上的大型重离子碰撞实验

探索物质微观结构和新的物质形态始终是人类认识物质世界的基本规律的一个重要组成部分。随着物理学的发展,人类对物质的微观结构的认识逐步深入,先后认识到：物质世界是由原子组成的,     

Track matching study from TPC to PHOS in ALICE

We present a study for matching charged tracks reconstructed from the ALICE tracking detectors with the clusters measured in the photon spectrometer. Matching efficiency and contaminations due to wrong matches have been deduced for charged pions and muons. For electrons, the effect of the material in front of PHOS is discussed.     

ALICE实验内径迹系统探测器升级

大型重离子对撞实验(A Large heavy-Ion Collision Experiment，ALICE)按计划在大型强子对撞机(Large Hadron Collider，LHC)的第二次停机(2019—2021)期间进行探测器升级工作。为了对强相互作用物质——夸克胶子等离子体(Quark-Gluon Plasma，QGP)的性质进行更细致的研究，作为升级计划中重要的一个内容是把ALICE实验现有的内径迹系统探测器(Inner Tracking System，ITS)全面升级为基于单片有源像素传感器(Monolithic Active Pixel Sensor，MAPS)技术的硅像素探测器(习惯称之为ITS2)，并在Run 3和Run 4期间采集更多的铅核-铅核碰撞数据。新的ITS2共由7层(内3层，中间2层和外2层)探测桶面组成，共由24 000余片尺寸为3 cm × 1.5 cm的MAPS硅像素芯片(该芯片称之为ALPIDE)构成，有效探测面积达10 m2，共约120亿像素。ALPIDE芯片厚度为50 μm，单个像素的尺寸是27 μm × 29 μm，该芯片具有低功耗、高空间分辨率和高速读出等特点。ITS2将使ALICE探测器在测量极低横动量粒子时具备优异的探测效率和碰撞参数分辨率，同时也使ALICE探测器适应于LHC高束流亮度环境。目前ITS2的探测器模块量产和测试已于2019年完成，并在欧洲核子中心(CERN)洁净室完成了7层桶面的组装与安装，于2020年完成试运行测试。2021年1月启动ITS2在ALICE探测器中的安装与试运行工作，计划于2021年5月底完成ITS2的安装与测试。本工作将对ALICE/ITS2的探测器结构、ALPIDE芯片和升级进展等方面进行介绍。     

相对论重离子碰撞手征电荷分离效应的研究

相对论重离子碰撞的拓扑荷作用破坏作用平面两侧不同手征性的夸克数目的平衡,从而引起P和CP破坏特征。本工作主要分析RHIC和LHC能区的手征电荷分离作用,分析手征电荷分离随碰撞中心度、碰撞能量和核屏蔽等依赖关系。并未发现手征电荷分离效应与碰撞能量大小有非常紧密的依赖关系,但发现核屏蔽效应对手征电荷分离有重要的影响,与不考虑屏蔽效应相比,考虑屏蔽效应会使手征分离效应明显压低,由于屏蔽效应出现,使得a₊₊（a_--）的分布与a_+-（a_-+）分布并不对称。并且还发现手征分离效应主要发生在碰撞参量较大的周边碰撞,越接近中心碰撞,手征分离效应几乎可以忽略。The topological charge interactions in relativistic heavy ion collisions cause quark chirality imbalance, resulting in charge separation under the strong magnetic field and local P and CP violation. In this paper, the chiral charge separation at RHIC and LHC energies is systematically analyzed as functions of the collision centrality, the collision energy and the nuclear shielding factor. It is found that there is not a very close dependence of the chiral charge separation effect on the collision energies, but that has an important dependence on nuclear shielding factor. Compared with the non-shielding effect, the shielding effect can reduce the chiral separation effect obviously. Due to the shielding effect, the distribution of a₊₊(a_--) and the distribution of a_+-(a_-+) are asymmetric. One also finds that chiral separation effect, which is almost negligible when more close to the central collision, occurs mainly in the peripheral collision for larger impact parameter.     

Energy Loss of Charm Quark Observed via Dimuon Production in Pb-Pb Collisions at √snn = 5.5 TeV

Based on the HVQMNR model of heavy flavour production and the BDMPS approach on parton energy loss, we have investigated the energy loss effect of charm quark on （di）muon spectra in forward rapidity region covered by ALICE forward muon spectrometer in Pb＋Pb collisions at c.m. energy √snn= 5.5 TeV. Results show that （di）muon spectra are very sensitive to the charm quark energy loss and can provide valuable information on the energy loss of heavy flavours.     

Estimation of the chiral magnetic effect considering the magnetic field response of the QGP medium

The magnetic field plays a major role in searching for the chiral magnetic effect in relativistic heavy-ion collisions. If the lifetime of the magnetic field is too short, as predicted by simulations of the field in vacuum, the chiral magnetic effect will be largely suppressed. However, the lifetime of the magnetic field will become longer when the QGP medium response is considered. We give an estimate of the effect, especially considering the magnetic field response of the QGP medium, and compare it with the experimental results for the background-subtracted correlator H at RHIC and LHC energies. The results show that our method explains the experimental results better at the top RHIC energy than at the LHC energy.     

Study of the photon identification efficiency with ALICE photon spectrometer

The efficiency for the detection and identification of photons with the ALICE PHOton Spectrometer PHOS has been studied with the Monte-Carlo generated data. In particular, the influence on the efficiency of the PHOS-module edge-effect and of the material in front of PHOS have been examined.     

Rapidity Losses in Heavy-Ion Collisions from AGS to RHIC Energies

We study the rapidity losses in central heavy-ion collisions from ACS to RHIC encrgolcs with the mean rapidity determined from the projectile net-baryon distribution after collisions. The projectile net-baryon distribution in the full rapidity range is obtained by phenomenologically removing the target contribution at the forward rapidity region from the experimental net-baryon measurements and taking into account the projectile contribution in the backward rapidity region. Based on the full projectile net-baryon distributions, calculations show that the rapidity loss stops increasing from the SPS top energy to RHIC energies, indicating that baryon transport does not depend strongly on energy at high energies.     

High pt and photon physics with ALICE at LHC

ALICE,A Large Ion Collider Experiment,is dedicated to study the QCD matter at extreme high temperature and density to understand the Quark Gluon Plasma （QGP） and phase transition.High-transversemomentum photons and neutral mesons from the initial hard scattering of partons can be measured with ALICE calorimeters,PHOS （PHOton Spectrometer） and EMCAL （ElectroMagnetic CALorimeter）.Combing the additional central tracking detectors,the γ-jet and π 0-jet measurements thus can be accessed.These measurements offer us a sensitive tomography probe of the hot-dense medium generated in the heavy ion collisions.In this paper,high p T and photon physics is discussed and the ALICE calorimeters capabilities of high-transverse-momentum neutral mesons and γ-jet measurements are presented.     

ALICE光子谱仪前端电子学系统的性能研究

首先讨论了ALICE光子谱仪前端电子学的物理需求, 介绍了光子谱仪前端电子学测试系统. 采用该系统对光子谱仪前端电子学系统板(FEE)进行了检测, 检测表明现行设计的前端电子学系统板的物理性能已达到了ALICE实验的预期物理目标.