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Final Report Summary - BESIII-XYZ (Understanding the Nature of Exotic Hadrons - Charmonium Spectroscopy at BESIII)

The research project “BESIII-XYZ” at the Johannes Gutenberg University of Mainz has focused on the spectroscopy of charmonium and charmonium-like particles at the BESIII experiment in Beijing, China. The project has made significant contributions to the final goal of fully understanding the so-called XYZ particles, of which some were discovered about ten years ago at electron-positron particle accelerators (BABAR experiment, Stanford, USA and Belle experiment, KEK, Japan). With the advent of the BESIII experiment at the BEPCII electron-positron collider in Beijing, China, which is directly running in the relevant XYZ mass region, these particles can be studied with unprecedented accuracy.

The expression “XYZ particles” is a synonym for the field of hadron spectroscopy of those hadrons containing a charm and an anti-charm quark. In the past 10 years, XYZ physics has become one of the most exciting fields of particle physics, as many discoveries of new hadrons have been made, however without identifying a clear explanation about the underlying quark structure of these particles. There are several theoretical scenarios proposed, such as four-quark states (dubbed tetraquarks), charmed hadronic molecules, as well as hybrid states, which consist of a bound system of two charm quarks and a gluon (gluon is the binding particle of strong interactions). Still it remains an open question how quarks – as the fundamental building blocks of nature – are precisely arranged in bound hadronic states (such as the proton or neutron). Exotic combinations, as the ones mentioned above, give a chance to study this quark binding quantitatively.

This project has contributed to answer one of the most pressing questions in hadron and particle physics: Are there new forms of matter beyond the conventional baryon or meson states? There is overwhelming evidence that some of the XYZ particles indeed are of exotic nature. In that context it is extremely interesting and important to note that within the project, a new particle called psi(1^3D2) was found, for which it lies exactly in the XYZ mass range, and which shows properties consistent with a conventional charmonium state. This can be taken as a proof that the theory describing the conventional charmonium states seems to work well. This in turn also shows that the unexpected behavior of many exotic XYZ particles is not due to the limitations in the theoretical prediction. The work of Dr. Zhiqing Liu in that sense has led to high-profile and most visible results in basic research, which have helped to strengthen the research profile of the European hadron physics community.

Prior to the start of the project, Dr. Zhiqing Liu during his PhD thesis had made a discovery of a new particle, dubbed Zc(3900). This discovery came as a major surprise as there is clear evidence that this particle cannot be a conventional charmonium state, i.e. a bound system of charm and anti-charm quarks. Therefore the Zc(3900) represents a new and exotic state of matter, possibly made of four quarks (tetraquark). The finding of the Zc(3900) represented a major milestone. Now that the new particle is firmly established, within the research project of Dr. Liu a detailed measurement of the spin-parity of the Zc(3900) is carried out, as well as a more precise mass and width determination of this particle is achieved, based on a larger statistics data sample. The refinement parameters of the Zc(3900) serve an important input to distinguish different theoretical scenarios of this particle.

By analyzing data taken at the beginning of the year 2014, the particle X(3872), which so-far was only measured in decays of B-mesons, was also found as a decay product of another potentially exotic XYZ particle, the Y(4260). It is therefore the first time that three XYZ particles could be related to each other: The Zc(3900) particle was found in the decay of the Y(4260), while also the particle X(3872) is seen most likely as a decay product of Y(4260). In a CERN COURIER article, the BESIII result which was achieved by Dr. Liu, was emphasized as a ’surprising observation’.

As mentioned before, a conventional charmonium state, the psi(1^3D2) state was also found in the BESIII data. The numbers in the name of the particle represent its quantum numbers and hence define its production as well as decay pattern. While the spectrum of charmonium states below the open-charm threshold (the threshold to produce open charm particles consisting of one (anti-)charm quark only) is well established by now, the spectrum above the threshold is still far from being understood. While new states were discovered, which do not fit into the predicted spectrum from QCD-inspired potential models, many of the predicted slots for ordinary charmonium states are still open. The measured features of the particle, which is called X(3823), are consistent with the theoretical predictions. Thus, the X(3823) is a perfect candidate for the missing psi(1^3D2) charmonium state.

Within the project, furthermore, contributions were made to the observations of the neutral partners of the Zc(3900). This is exciting as the observation of those neutral states tells us about the pattern of XYZ particles. Indeed, it is for the first time that electrically charged and neutral partners of an exotic XYZ particle were found.

Finally, Dr. Liu has searched also for a new production process at electron-positron colliders for the X(3872) particle. Due to its quantum numbers, the X(3872) cannot be produced directly in electron-positron annihilation at leading order. Indeed, so far no particle with the quantum numbers of the X(3872) has been observed in direct production. They only appear in decays of other particles, which do have the right assignment of quantum numbers. A direct production is in principle only possible in a highly suppressed production mechanism. The investigations of Dr. Liu have led to a proposal for a dedicated beam time of 3 weeks at the BESIII experiment, which was accepted by the BESIII collaboration. The goal is to search for yet another charmonium state, the chi_c1 particle, with the same quantum numbers of the X(3872) and which hence can also not be produced at leading order. These investigations open the avenue to new experimental techniques in electron-positron physics and influence the physics program of future e+e- colliders.

All of the results of the project were not only published in highly appreciated scientific journals, but also presented at various conferences, which resulted also in a number of conference proceedings. Dr. Liu received in fact not only invitations to high-profile conferences in the field of nuclear and particle physics, as for instance the Flavor Physics and CP violation or the Charm Conferences, but also was invited to a number of institute seminars throughout Europe. Like this, he could further strengthen the collaborations with the leading theory groups in the field of hadron spectroscopy. For his discovery of the Zc(3900) particle he was awarded the prize for the best PhD work in the field of physics in the year 2014 in China.

Within the project, Dr. Liu obtained experiences in teaching, for instance by tutoring undergraduate students at the Johannes Gutenberg University of Mainz. He also worked out innovative concepts, as for instance a data mining campaign as a part of the advanced lab course. Two students have been trained in all aspects of modern data analysis and the handling of data analysis tools, like ROOT. Finally, Dr. Liu was active also in various outreach activities. He participated in the science fair, which explains the interested public the wonders of the subatomic world. Moreover, two press releases appeared which were triggered by the sensational findings of Dr. Liu.

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