Zisis Papandreou

Exotic Hybrid Mesons (Nuclear Glue)

Subatomic physics seeks to understand nature - and all its laws - at the most fundamental level, down to its elementary constituents and their interactions. Tremendous strides have been made in the last several decades, using accelerators and astrophysical observations, which have strengthened our conviction that the basic model, known as the Standard Model, is essentially a valid description of the subatomic world but not necessarily a complete one. Among the four forces of the Standard Model, the strong force inside an atomic nucleus is described by a theory called Quantum Chromodynamics (QCD). 

QCD describes the physics underlying the interaction of quarks and gluons and it predicts a virtual “zoo” of particles and resonances. To date, only colorless “quark model” states have been observed — such as quark-antiquark pairs (mesons) and quark triplets (baryons) — while gluonic degrees of freedom are difficult to observe or suppressed. But QCD predicts more types of states than just mesons and baryons.

The goal of the GlueX Experiment is to produce and identify unseen, exotic forms of matter that, by their very nature, exhibit unique signatures among all other particles that will allow QCD to be tested in a most constraining way possible. Specifically, hybrid mesons result from the addition of gluon quantum numbers to those of the quarks and a subset of hybrids is predicted to have exotic quantum number combinations that are not allowed in the simple quark model. Our plan is to map out hadron spectrum patterns and test the features of theoretical models. To achieve this goal, we must systematically study all possible decay modes of conventional and hybrid mesons by conducting an amplitude analysis of many different hadronic final states. The expected masses for the lightest hybrids are well-matched to the energy and kinematics accessible to the GlueX experiment.

This is a difficult task requiring experimental conditions that GlueX was designed to possess: a unique, high-intensity, polarized photon beam and a hermetic detector.  The latter has benefited from the recent development of large silicon-based photo sensors that are completely immune to magnetic fields.  Our group played a defining role in guiding industry towards the development of large-area SiPMs (see IEEE article) and these devices are now available commercially to the subatomic, medical and nuclear safety fields. 

At the "heart" of the GlueX detector lies the barrel calorimeter (BCAL). This device is responsible for the detection, identification and total energy measurement of both neutral (photons, neutrons) and charged (protons, pions) particles. The BCAL consists of 48 optically isolated calorimeter modules, each having a trapezoidal cross-section and forming a 390 cm long (hollow) cylinder having inner and outer radii of 65 cm and 90 cm, respectively. This detector is an electromagnetic sampling calorimeter, composed of layers of lead sheets and scintillating fibers, the latter oriented parallel to the module's cylindrical axis. The design and construction of the BCAL took place at the University of Regina. For more information, please refer to our BCAL web site.

A description of the experiment can be found at arXiv.org. Our first physics publication is available through Physical Review C and arXiv.org.

Rare Eta Decays 

I am a co-spokesman of the JEF (Jefferson Lab Eta Factory) program in Hall D.  The JEF program aims to perform precision measurements of various η decays with emphasis on rare neutral modes.  The physics goals included, but are not limited to: a) The measurement of η → 3π to improve the understanding of the light quark mass ratio, b)  A search for a leptophobic dark boson B′ coupled to baryon number towards a dark photon or invisible decay searches for dark sector particles. c) A low-background measurement of the rare decay η → π0γγ to test O(p6) in Chiral Perturbation Theory (ChPT).

The experiment is at the design stage and is expected to run around 2022.

Nuclear Imaging of Plants

I collaborate with Dr. Teymurazyan on this project. Please visit his site for more information or visit the following links:

• Explanation of the project.
• Our group is part of a large effort in Saskatchewan in molecular imaging.  See video.

Publications