Abstract

The STAR Heavy Flavor Tracker (HFT) has enabled a rich physics program, providing important insights into heavy quark behavior in heavy ion collisions. Acquiring data during the 2014 through 2016 runs at the Relativistic Heavy Ion Collider (RHIC), the HFT consisted of four layers of precision silicon sensors. Used in concert with the Time Projection Chamber (TPC), the HFT enables the reconstruction and topological identification of tracks arising from charmed hadron decays. The ultimate understanding of the detector efficiency and resolution demands large quantities of high quality simulations, accounting for the precise alignment of sensors, and the detailed response of the detectors and electronics to the incident tracks. The background environment presented additional challenges, as simulating the significant rates from pileup events accumulated during the long integration times of the tracking detectors could have quickly exceeded the available computational resources, and the relative contributions from different sources was unknown. STAR has long addressed these issues by embedding simulations into background events directly sampled during data taking at the experiment. This technique has the advantage of providing a completely realistic picture of the dynamic background environment while introducing minimal additional computational overhead compared to simulation of the primary collision alone, thus scaling to any luminosity. We will discuss how STAR has applied this technique to the simulation of the HFT, and will show how the careful consideration of misalignment of precision detectors and calibration uncertainties results in the detailed reproduction of basic observables, such as track projection to the primary vertex. We will further summarize the experience and lessons learned in applying these techniques to heavy-flavor simulations and discuss recent results.

Highlights

  • Monte-Carlo simulations are central to high-energy and nuclear physics experiments, providing the detailed modeling of detector response, signal production and underlying background distributions necessary to produce physics results comparable with theory

  • As noted in table 2, the Time Projection Chamber (TPC) and pixel detector https://doi.org/10.1051/epjconf/202024502007 (PXL) detectors integration times are significant compared to the ∼50 kHz interaction rate typical during the 2014 and 2016 AuAu runs

  • The left panel shows the Heavy Flavor Tracker (HFT) matching ratio. This is the number of tracks reconstructed in the event with HFT hits divided by the total number of tracks found in the event by the TPC

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Summary

Introduction

Monte-Carlo simulations are central to high-energy and nuclear physics experiments, providing the detailed modeling of detector response, signal production and underlying background distributions necessary to produce physics results comparable with theory. Backgrounds pose particular challenges for simulations of experiments, especially when interaction rates exceed detector integration times, and events “pile-up” during read-out. Cavern backgrounds, beam-gas interactions and (in heavyion collisions) low-energy particles from ultra-peripheral collisions (UPC) create additional complications, requiring additional effort to tune models and understand relative yields for input into the simulation chain. The STAR experiment[1] at the Relativistic Heavy Ion Collider (RHIC) has long addressed these challenges by embedding simulations into appropriate background events measured in-situ during data taking. We will show how careful treatment of these effects in embedding simulations provide excellent agreement in observables, such as the distanceof-closest approach to the vertex and the relative efficiencies of the trackers, which give confidence that the extracted efficiencies from simulation are correct

The Heavy Flavor Program
Simulation Strategies
Pure Monte Carlo
Embedding Simulation
Integrating the HFT into Embedding
Conclusion
Full Text
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