=Paper=
{{Paper
|id=Vol-3041/473-477-paper-87
|storemode=property
|title=Missing Mass Method for Short-Lived Particle Reconstruction in the CBM and STAR Experiments
|pdfUrl=https://ceur-ws.org/Vol-3041/473-477-paper-87.pdf
|volume=Vol-3041
|authors=Yuri Fisyak,Victor Ivanov,Hongwei Ke,Ivan Kisel,Pavel Kisel,Grigory Kozlov, Spyridon Margetis,Aihong Tang,Iouri Vassiliev
}}
==Missing Mass Method for Short-Lived Particle Reconstruction in the CBM and STAR Experiments==
https://ceur-ws.org/Vol-3041/473-477-paper-87.pdf
Proceedings of the 9th International Conference "Distributed Computing and Grid Technologies in Science and
Education" (GRID'2021), Dubna, Russia, July 5-9, 2021
MISSING MASS METHOD FOR SHORT-LIVED PARTICLE
RECONSTRUCTION IN THE CBM AND STAR
EXPERIMENTS
Y. Fisyak 1, V. Ivanov2,3, H. Ke1, I. Kisel4,5,6,7, P. Kisel2,5,a, G. Kozlov2,5,
S. Margetis8, A. Tang1, I. Vassiliev4
1
BNL, Brookhaven National Laboratory, Upton, NY, USA
2
JINR, Joint Institute for Nuclear Research, Dubna, Russian Federation
3
MEPhI, National Research Nuclear University, Moscow, Russian Federation
4
GSI, Helmholtz Center for Heavy Ion Research, Darmstadt, Germany
5
Goethe University, Frankfurt am Main, Germany
6
FIAS, Frankfurt Institute for Advanced Studies, Frankfurt am Main, Germany
7
HFHF, Helmholtz Research Academy Hesse, Frankfurt am Main, Germany
8
KSU, Kent State University, Kent, OH, USA
E-mail: a P.Kisel@gsi.de
The search for short-lived particles is an important part of physics research in experiments with
relativistic heavy ions. To find, select and study the properties of such short-lived particles in real time
in the CBM experiment (FAIR/GSI, Germany), we have developed a package of algorithms, KF
Particle Finder, which contains a search for more than 150 decay channels. The decays of short-lived
charged particles, when one of the daughter particles is neutral and cannot be registered in the detector
system, are also of great physics interest. To find and study such decays, we have extended the KF
Particle Finder package by implementing the missing mass method, which is based on the
conservation of energy and momentum laws.
The method was studied in detail on simulated data of the CBM experiment, showing high efficiency
with a large signal-to-background ratio, as well as high significance. As part of the FAIR Phase-0
program, the KF Particle Finder package of algorithms has been adapted for online and offline
processing in the STAR experiment (BNL, USA).
Here we describe and discuss in detail the missing mass method for finding and analyzing short-lived
particles. The features of the application of the method to both simulated data in the CBM experiment
and in the STAR experiment as part of real-time express data processing, as well as the results of the
real-time reconstruction of short-lived particle decays in the BES-II environment of the STAR
experiment, are given.
Keywords: HPC, parallel programming, heavy-ion collisions, CBM experiment, STAR
experiment
Yuri Fisyak, Victor Ivanov, Hongwei Ke, Ivan Kisel, Pavel Kisel, Grigory Kozlov,
Spyridon Margetis, Aihong Tang, Iouri Vassiliev
Copyright © 2021 for this paper by its authors.
Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
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Proceedings of the 9th International Conference "Distributed Computing and Grid Technologies in Science and
Education" (GRID'2021), Dubna, Russia, July 5-9, 2021
1. Introduction
In modern experiments with heavy ions, such as CBM (FAIR/GSI, Darmstadt, Germany),
ALICE (CERN, Geneva, Switzerland) and STAR (BNL, Upton, USA), the main task is a
comprehensive study of the QCD phase diagram in the quark-gluon plasma (QGP) region and a
possible phase transition from the hadronic to QGP phase.
In this regard, strange particles, which are sensitive probes of the medium formed in heavy-ion
collisions, play an important role. Since strange particles have decay modes with at least one neutral
daughter particle, their decay products cannot be registered in a tracking detector system. To identify
such decays, when one of the daughter particles is neutral, we proposed and implemented the missing
mass method.
The missing-mass method was originally developed by us within the FLES (First Level Event
Selection) package [1] for the CBM experiment. In the process of developing and investigating the
method in CBM, the method has shown its high efficiency on simulated data. However, for the first
phase of the CBM experiment, it is important to prove the concept of the developed approach and its
current implementation on real data. The FAIR Phase-0 program [2], which includes participation in
the STAR experiment at BNL (USA), gives us such an opportunity.
2. Missing mass method
The missing mass method is used to reconstruct decays of short-lived charged particles, when
one of the daughter particles is neutral and not registered in the detector system. The method is based
on the laws of conservation of energy and momentum.
As an illustration of the method, let us consider the well-known decay
Σ− → nπ−. (1)
In this decay, the daughter pion is registered in the tracking system and identified in one of the
PID detectors, while the neutron is not detected. Although the sigma particle has a short lifetime cτ Σ− =
4.4 cm, in fixed-target experiments due to relativistic boost, the average length of its trajectory in the
laboratory system can reach 15–20 cm. In this case, the Σ− particle will produce measurements in
several tracking stations and can therefore be reconstructed. Thus, in the given decay, two charged
tracks out of total three tracks can be reconstructed.
A specific feature of this type of decays is that the short-lived mother track usually cannot be
identified, making it impossible to directly calculate the missing neutron mass. However, the
momentum of the mother particle Σ−, unlike its mass, can be determined from the curvature of the
trajectory. Therefore, we can assume and apply the hypothesis about the mass of the mother track, and
then look at the resulting neutron mass spectrum.
Figure 1. Three stages of the reconstruction of a short-lived charged particle using the missing
mass method
Thus, the reconstruction of a short-lived charged particle using the missing mass method is
performed in three steps [Fig. 1]:
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Proceedings of the 9th International Conference "Distributed Computing and Grid Technologies in Science and
Education" (GRID'2021), Dubna, Russia, July 5-9, 2021
1. reconstruction of tracks of the charged mother particle (Σ−) and its charged daughter
particle (π−) in the tracking system;
2. reconstruction of the parameters of the neutral daughter particle (n) by the parameters of
the mother particle and the charged daughter;
3. reconstruction of the Σ− mass spectrum using the charged particle and the obtained neutral
daughter.
The method is implemented as an extension of the KF Particle Finder [1, 3] package, which is
now actively used to search and reconstruct short-lived particles, both online and offline, in major
heavy ion experiments such as CBM (FAIR/GSI), ALICE (CERN) and STAR (BNL). A search for 18
decays with a neutral daughter particle has now been added to the KF Particle Finder package.
Once the particle candidates are found in the search for short-lived particles in the KF Particle
Finder, there is competition among them to ensure that the daughter particles participate in only one,
the most probable, decay channel. As an example, the decays Ks0 → π+π− and Λ → pπ− can be given,
when in both decays one of the daughter particles is π −.
Within the KF Particle Finder package, two approaches have been implemented to perform
competition between particle candidates with possible common daughter particles: deterministic
approach and artificial neural network approach. In the deterministic approach, the reconstruction
quality of such competing particle candidates is compared, and on the basis of certain criteria, such as
χ2-value and closeness to the table mass value, the best particle is chosen. In the artificial neural
network approach, similar to its other applications [4, 5] to optimize and select data, the network is
first trained on the simulated data and then selects the best particles on its own. Both approaches show
almost identical results [6], so the choice of one for data processing is purely practical.
The missing mass method has been repeatedly and successfully tested within the KF Particle
Finder package on simulated data of the CBM experiment, demonstrating its value to the physics
program of the experiment [7].
Figure 2. Performance of the missing mass method in the CBM experiment on 5M central AuAu
UrQMD events at 10 AGeV (left: standard version, right: improved version)
After the intensive investigation of the method, we have made numerous improvements to its
standard realization. Thus, in the modified realization of the method, the decay point of a charged
particle, or its secondary vertex, is in two steps at once at the initial stage and does not require any
further correction and recalculation, which is mathematically more accurate. In addition, we have
significantly improved the mathematics of the implementation of the method: a more accurate and
faster mathematical apparatus has been used; all basic calculations are performed with 6x6 matrices
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Proceedings of the 9th International Conference "Distributed Computing and Grid Technologies in Science and
Education" (GRID'2021), Dubna, Russia, July 5-9, 2021
instead of the former 7x7; the energy has been removed from the state vector and the covariance
matrix; the energy and mass calculations are done separately in the final step; the parameters of all
decaying particles are estimated in one iteration of the Kalman filter instead of the former two. This
has allowed one to avoid the very resource-intensive method of the mass constraint, which had to
balance the parameters of the particles in some parts of the calculations.
The modification of the method resulted in a 2-fold increase in efficiency and a 25-30%
increase in significance [Fig. 2].
As part of the FAIR Phase-0 program [2], the missing mass method has been successfully
tested on pion and kaon decay channels offline on real STAR data from 2016 and 2020 data sets.
Figure 3. Results of the standard version of the missing mass method on 2M AuAu events of 2020
year data at (5.75+7.3+9.8+26.5) AGeV
Figure 4. Results of the improved version of the missing mass method on 2M AuAu events of 2020
year data at (5.75+7.3+9.8+26.5) AGeV
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Proceedings of the 9th International Conference "Distributed Computing and Grid Technologies in Science and
Education" (GRID'2021), Dubna, Russia, July 5-9, 2021
Here we present the results of applying the missing mass method to search for pion (π ± → μ±ν)
and kaon (K± → μ±ν and K± → π±π0) decays to real data 2M AuAu events of STAR at
(5.75+7.3+9.8+26.5) AGeV collected in 2020 within the Beam Energy Scan program BES-II and
reconstructed online running the express data production and analysis chain on the STAR HLT farm.
The standard version of the missing mass method provides the online reconstruction of short-
lived particles of 2020 year data with a high signal-to-background ratio and significance [Fig. 3].
In the improved version, in the π ± → μ±ν channels, the signal increased by a factor of 40, and
the significance increased by a factor of 7. In the K± → μ±ν and K± → π±π0 channels, the signal
increased by a factor of 2, and the significance increased by a factor of 1.5 [Fig. 4].
3. Conclusion
The missing mass method enables a significant expansion of the physics programs of high-
energy and heavy-ion physics experiments. Within the FAIR Phase-0 program, the KF Particle Finder
package, initially developed for the CBM experiment, has been adapted to the STAR experiment and
applied to real data of BES-II. The missing mass method provides the online reconstruction of real
data with a high signal-to-background ratio and significance.
4. Acknowledgement
The work is supported by the German Federal Ministry of Education and Research BMBF and
the Helmholtz Research Academy Hesse HFHF.
References
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CBM Experiment”, IEEE Trans. on Nucl. Science, 60, 5, (2013) 3703.
[2] I. Kisel (for CBM and STAR Collaborations), “Real-Time Event Reconstruction and Analysis in
CBM and STAR Experiments”, Journal of Physics: Conference Series 1602 (2020) 012006.
[3] M. Zyzak, “Online Selection of Short-Lived Particles on Many-Core Computer Architectures in
the CBM Experiment at FAIR”, Dissertation thesis, Goethe university, Frankfurt am Main
(2016).
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