Analysis of experimental data obtained in relativistic heavy ion collisions at STAR set-up
M. Tokarev
Veksler and Baldin Laboratory of High Energy Physics
Introduction
The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is a world-class scientific research facility that began operation in 2000, following 10 years of development and construction. Hundreds of physicists from around the world use RHIC to study what the universe may have looked like in the first few moments after its creation. RHIC drives two intersecting beams of gold ions head-on, in a subatomic collision. What physicists learn from these collisions may help us understand more about why the physical world works the way it does, from the smallest subatomic particles, to the largest stars.
The Solenoidal Tracker at RHIC (STAR) is a detector which specializes in tracking the thousands of particles produced by each ion collision at RHIC. Weighing 1,200 tons and as large as a house, STAR is a massive detector. It is used to search for signatures of the form of matter that RHIC was designed to create: the quark-gluon plasma. It is also used to investigate the behavior of matter at high energy densities by making measurements over a large area.
STAR's "heart" is the Time Projection Chamber, made of many electronic systems, which tracks and identifies particles emerging from heavy ion collisions. As each collision occurs, STAR measures many parameters simultaneously to look for signs of the quark-gluon plasma. By using powerful computers to reconstruct the sub-atomic interactions which produce the particles emerging from each collision, the detector can, in a sense, run time backwards.This process can be compared to examining the final products which come out of a factory and trying to determine what kinds of machines produced them.
Fig.1 The STAR set-up. Fig.2 AuAu collision from event monitor.
The strong interaction, one of the four fundamental forces, is described by the theory of Quantum Chromodynamics (QCD). STAR is a general-purpose detector, equipped with large acceptance, excellent particle identification and fast data taking, at the Relativistic Heavy Ion Collider for studying the QCD phenomena. The primary physics goals of the experiment are study new form of matter with quark and gluon degrees of freedom, explore the complex and rich QCD phase structures and search for the origin of the spin in proton.
The aim of this work is the acquaintance of the students with some methods of data analysis in high energy physics and the study of the methods of reconstruction of strange particles produced in heavy ion collisions.
Description of the work
1. Equipment and methods
Root framework
ROOT is an object-oriented framework aimed at solving the data analysis challenges of high-energy physics. In a framework, the basic utilities and services, such as Input/Output and graphics, are provided. In addition, ROOT being a HEP analysis framework, it provides a large selection of HEP specific utilities such as histograms and fitting. The more commonly used components of ROOT: Command Line Interpreter, Histograms and Fitting, Writing a Graphical User Interface, 2D Graphics, Input/Output, Collection Classes, Script Processor. (http://root.cern.ch/)
Fig.3 Example of the Root framework use.
STAR data sample
The different samples (pp, dAu, CuCu, AuAu) of events of STAR data at the collisions energy9.2, 130, 200 GeV are used for the analysis. Different hadron species (meson, baryons, hyperons) are produced in these collisions. Root framework is used to reconstruct particle parameters and obtain momentum spectra, invariant mass distributions (Fig.3).
Fig.4 Invariant mass distribution of pairs () registered in the STAR Time Projection Chamber.
Event generator
The Ultrarelativistic Quantum Molecular Dynamics model is a microscopic model used to simulate (ultra)relativistic heavy ion collisons in the energy range from Bevalac and SIS up to AGS, SPS and RHIC. Main goals are to gain understanding about the following physical phenomena within a single transport model (http://th.physik.uni-frankfurt.de/~urqmd/).
Fig.5 Event of nucleus-nucleus collision
generated by UrQMD.
2. Practical works
1. Preparation of data samples of AuAu collisions for analysis.
2. Histogramming of the distributions in the Root framework.
3. Discussion of obtained results.
4. Preparing of the reports.
3. Report content
The students should prepare the report on the works carried out and give a presentation of the results. All stages of data analysis should be shown with illustrations, description of the methods used during the work. Conclusions should be summarized.
4. Level requirements
All students should have basic knowledge of physics, methods of data analysis, methods of particle registration, detectors and computer operating (C++, LINUX, Root).
5. References
1. Investigation of properties of nuclear matter at high temperatures and baryon densities. Eds. A.N.Sissakian et al., Dubna, JINR, 2007.
2. Root, "User's Guide v5.21", http://root.cern.ch/
3. Event generator UrQMD, http://th.physik.uni-frankfurt.de/~urqmd/
1-2 students can work at the Laboratory of High Energy Physics at the same time.
Supervisor of this work: Kechechyan Armen Oganesovich, leading researcher, the Scientific and Experimental Division of Heavy Ion Physics at RHIC, Veksler and Baldin Laboratory of High Energy Physics of JINR.