[1] R. Oerter (2006). The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics (Kindle ed.). Penguin Group. p. 2.
[2] Alde, D., Binon, F. G., Boutemeur, M., Bricman, C., Donskov, S. V., Gouanere, M., ... & Knapp, E. A. (1988). Evidence for a 1−+ exotic meson. Physics Letters B, 205(2-3), 397-400.
[3] Sakurai, J. J. (1960). Theory of strong interactions. Annals of Physics, 11(1), 1-48.
[4] GlueX Collaboration. (2006). Mapping the Spectrum of Light Mesons and Gluonic Excitations with Linearly Polarized Photons. Proposal to Jefferson Laboratory PAC30.
[5] The dE/dx vs. total energy technique for charged particle identification. (1997, September 5). Retrieved April 12, 2019, from Space Radiation Lab website: http://www.srl.caltech.edu/ACE/CRIS_SIS/dedx.html
[6] File:Standard Model of Elementary Particles.svg (2019, March 3). Retrieved April 25, 2019 from Wikipedia Commons website: https://commons.m.wikimedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg
[7] Szczepaniak, A. P., & Swat, M. (2001). Role of photoproduction in exotic meson searches. Physics Letters B, 516(1-2), 72-76.
The Standard Model
The standard model is the currently accepted set of particles that compose all observed matter in the universe. The protons and neutrons in your body consist of fundamental particles called quarks. Quarks have twin particles that are called anti-quarks. These anti-quarks are exactly the same as normal quarks, but have an opposite charge. If a quark and an antiquark meet, then annihilate each other[1].A meson is a quark and an antiquark bound together, and their instability is explained by this relationship between quarks and antiquarks.
OBJECTIVES
Detector
The detector of the GlueX experiment has two main types of detectors: calorimeters and drift chambers. Calorimeters, like those shown in gray and yellow in Figure 2, measure the energy of a shower of particles. Drift chambers, like those shown in Figure 2 in red and blue, on the other hand, measure the momentum and charge of a particle. No particle can be identified by this information alone. Therefore, the drift chambers can also obtain a value of energy loss, shortened to dE/dx (change in energy over change in distance). This value has been found to be proportional to the mass and velocity of a particle. The momentum of a particle happens to be its mass times its velocity, so by plotting the dE/dx value against the momentum of a particle, a particle can be identified[5].
METHODS
CURRENT STATUS
Currently, the data have been obtained, and the dE/dx plotting has begun. Upon the completion of this step, the hadron spectroscopy will begin. The methods have been rehearsed, and therefore this final phase of identifying particles can be quite brief, but can also be extended as long as needed. The initial quest upon completion of the detected particle identification will be to find the signals coming from known unstable particles that have been seen in prior experiments. After that, the quest will be extended to higher mass, which is the terra incognita of the search for exotic mesons.
References
Researching anything as small as fundamental particles creates difficulties in theoretical and experimental research. The current leading theory, the standard model, tries to explain what matter is comprised of and how the particles described interact with each other. Theory, however, must be able to explain observed phenomena, and without experimentation to attempt explain predictions made by such theory, the theory remains just that- a theory. The GlueX Collaboration seeks to study exotic mesons in order to see if their properties align with those predicted by the standard model. If they do, then the standard proves itself to be more accurate in describing the world around us than expected. In the more interesting case, however, where the properties do not nicely align with those predicted by the standard model, the standard model would be shown to be incomplete at the very least, which thus opens the door for further theory to be developed. In this project in particular, the particles that can be identified as currently understood unstable particles will be identified, so that any unusual signals that may be an exotic meson can be identified.
Richard Dube, Richard Jones
Department of Physics, University of Connecticut in Storrs, Connecticut
Identification of Unstable Mesons in GlueX Data
BACKGROUND
Photoproduction
In other previous experiments, attempts have been made to find favorable conditions for the production and detection of these exotic mesons. Most previous research has been based on hadroproduction, the process of creating exotic mesons using beams of high energy hadrons. A hadron is any particle that experiences the strong nuclear force. Recently, a major new experiment, called GlueX, based on photoproduction has been launched, in which exotic mesons are created using high energy polarized photons. It is argued that photoproduction should be significantly more effective in creating such states than hadroproduction[7].
dE/dx Identification
When the momentum is plotted against the dE/dx value for each track detected, a pattern, as shown in Figure 3, will appear. There are distinct curves where there is a higher density of points. These bands each represent a particle[5]. The only prominent particle in the plot to the right is a proton, labeled p, but if the plot was more focused on the more dense area of the plot, several other curves, representing other particles like would be apparent. By grouping together all those points that are considered to be a part of the curve, a set of identified particles can be obtained.
Hadron Spectroscopy
In the search for unstable mesons, the decay products must be used to search for signals of unstable mesons. In order to do so, several conservation laws are used. For example, the law of conservation of energy states that the total energy of a parent particle must be equal to the sum of the energies of all the decay products. The same can be done for momentum. Using the momentum and energy, a mass value can be calculated. Therefore, in order to search for signals, every possible pair or trio of particles have their momenta and energies summed and their mass calculated. All these values are finally plotted on a histogram, as shown in Figure 4.1 and 4.1, and if an unstable mesons decayed into the pair or trios of particles, a sharp peak will appear at the mass of the unstable particle.
“Exotic” Mesons
Every particle has a set of properties including charge q, particle spin J, isospin I, intrinsic parity P, charge parity C, and helicity λ, all of which constitute conserved quantum numbers that can be measured in an experiment. Early nuclear physics experiments showed that certain combinations of J, P, C were missing in the particle spectrum, e.g. J=0,C=-1 and J=1,P=-1,C=+1 are among the forbidden combinations[2]. These observations were later explained neatly as a prediction of the quark model. However, this turned out to not be the full story. Nearly a decade later, when a full theory of strong interactions appeared, which introduced a new fundamental field, the gluon[3]. When these gluons got added to the mix, it soon became clear that the quark model censorship of these forbidden J,P,C combinations could be overridden by excitations of the gluonic field[4]. It is a longstanding quest of experimental nuclear physics to observe these so-called exotic mesons and to measure their masses and decay patterns.
Acknowledgements
I would like to thank my mentor, Dr. Richard Jones, for providing me with the knowledge, guidance, and the data needed for my analysis. I would also like to thank Ms. Diane Pintavalle for assisting me in developing the skills necessary to conduct such involved research.
Figure 3 A sample dE/dx plot of particles in the Central Drift Chamber
Figure 1 A visual of the elementary particles of the standard model, courtesy of MissMJ on Wikipedia Commons
Figure 2 A cross-section of the GlueX detector
Figure 4.2 A sample histogram of a two pion neutral decay mode with a peak at about 500 MeV, representing a K-short.
Figure 4.1 A sample histogram of a two photon decay mode with a peak appearing at about 140 MeV, representing a neutral pion
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