Studies of the FASER Calorimeter and Searches for New Physics with LHC Run 3 Data

FASER是大型强子对撞机的一项新实验，旨在探测由粒子物理标准模型的几个扩展所预测的光和弱相互作用粒子。它的目标是识别属于黑暗区域的新粒子，称为暗光子，这可能是在ATLAS相互作用点[2]产生的长寿命粒子（llp）。这些暗光子被认为衰变成e+e-(45%)、（45%）或（10%）。这种光的衰变产物，弱相互作用的粒子是高度准直的，并且在非常向前的区域，只需要一个小的探测器。FASER探测器位于TI12 -一个以前用于连接超级质子同步加速器（SPS）和LEP[2]的未使用隧道。安装将在目前的长时间停机期间完成，准备在计划于2022年进行的第3次运行期间进行数据采集。FASER探测器的主要组成部分是闪烁体、跟踪站、偶极磁体和量热计。在数据采集之前，这个项目的最初重点是FASER的采样电磁量热计（ECAL）[3]的研究和模拟。这是建立在先前的工作基础上的，该工作使用Geant4[4]中的例子建立了一个简单的模拟设置，并评估了角度倾斜的影响，并执行角度扫描来研究量热计间隙中的能量损失。还开展了旨在使用更复杂的设置[5]实现快速模拟的工作。在寻找新物理学的过程中，数据分析的发展需要对ECAL进行模拟。目前正在进行的工作是对探测器进行全面模拟，这需要仔细评估材料和几何形状对ECAL中沉积能量的影响。随后将进行额外的工作以实现快速模拟，以减少计算时间并使未来的分析更高效。一旦这项工作完成，分析数据所需的软件将被开发出来，并用于科学结果。最初的研究证实了之前使用简单的Geant4模拟的结果，即沉积能量的比例和行为作为注入能量的函数。使用FASER软件框架Calypso[6]的进一步模拟研究正在更精确地评估高动量粒子的能量依赖。目的是用第一手数据验证电磁量热计的模拟。最终目标是继续分析FASER收集的数据，以寻找暗光子或其他长寿命粒子形式的新物理现象。如果没有这样的发现，目标将转向在目前尚未探索的参数空间范围内设置世界上最好的约束。参考文献：[1]FASER合作（2019）FASER的物理到达长寿命粒子，物理学。Fabjan， C. W., Gianotti， F.（2003）粒子物理量热法，CERN， [4] Geant4量热法https://geant4-userdoc.web.cern.ch/geant4-userdoc/Doxygen/examples_doc/html/ExampleB4.html[5] Fellers， D. (2020) FASER快速量热计模拟，FASER离线会议25/05/2020 https://indico.cern.ch/event/918531/ [6] https://gitlab.cern.ch/dcasper/calypso/-/tree/master-ecal-2020Oct28

FASER is a new experiment at the LHC designed to detect light and weakly-interacting particles predicted by several extensions of the Standard Model of particle physics [1]. It aims to identify new particles belonging to a dark sector, called dark photons, which are possibly long-lived particles (LLPs) produced at the ATLAS interaction point [2]. These dark photons are thought to decay to e+e- (45%), (45%) or (10%). The decay products of such light, weakly-interacting particles are highly collimated and in the very forward region, requiring only a small detector. The FASER detector is located in TI12 - an unused tunnel previously used to connect the Super Proton Synchrotron (SPS) to LEP [2]. Installation will be completed during the current long shutdown ready for data acquisition during Run 3, which is scheduled for 2022. The main components of the FASER detector are the scintillators, the tracking stations, the dipole magnets, and the calorimeter [1]. The initial focus of this project, prior to data acquisition, is the study and simulation of FASER's sampling electromagnetic calorimeter (ECAL) [3]. This is building on previous work that used examples in Geant4 [4] to build a simple simulation setup and evaluated the impact of angular tilt and performing angular scans to investigate the loss of energy in the gap in the calorimeter. Work was also carried out which aimed to implement fast simulations using a more complex setup [5]. Simulation of the ECAL is required for the development of data analysis in the search for new physics. Current work is being performed towards the implementation of a full simulation of the detector, which requires careful evaluation of the impact of materials and geometry on the deposited energies in the ECAL. Additional work will subsequently be carried out to realise a fast simulation to reduce the computational times and make future analysis efficient. Once this is completed, the software needed to analyse the data will be developed and used for scientific results.Initial studies confirm the results previously shown using simple Geant4 simulations in terms of the fraction of deposited energy and behaviour as a function of injected energy. Further simulation studies using FASER's software framework, Calypso [6], are on-going to evaluate the energy dependence more precisely for high momentum particles. The goal is to validate the electromagnetic calorimeter simulation with first data. The ultimate aim is to go on to analyse data taken at FASER in the search for new physics in the form of dark photons or other long-lived particles. In the event that no such discoveries are made, the goal moves towards setting world-best constraints over a range of parameter space that is currently unexplored.References:[1] FASER Collaboration (2019) FASER's Physics Reach for Long-Lived Particles, Phys. Rev. D 10.1103/PhysRevD.99.095011[2] FASER Collaboration (2018) FASER: FORWARD SEARCH EXPERIMENT AT THE LHC, Technical Report arXiv:1812.09139v1[3] Fabjan, C. W., Gianotti, F. (2003) Calorimetry for Particle Physics, CERN[4] Geant4 Calorimetry Example: https://geant4-userdoc.web.cern.ch/geant4-userdoc/Doxygen/examples_doc/html/ExampleB4.html[5] Fellers, D. (2020) FASER Fast Calorimeter Simulation, FASER Offline Meeting 25/05/2020 https://indico.cern.ch/event/918531/ [6] https://gitlab.cern.ch/dcasper/calypso/-/tree/master-ecal-2020Oct28