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Probing and More: Beyond the

Professor Liping Gan PROBING MATTER AND MORE: BEYOND THE STANDARD MODEL

Professor Liping Gan and her team at the Thomas Jefferson National Accelerator Facility are working to deepen our understanding of the matter that makes up our . While key theories such as the Standard Model and can provide accurate predictions about the smallest building blocks of matter, there are also notable limitations in the observed phenomena they can explain. Professor Gan continues to push these theoretical boundaries, giving way to a new era of contemporary physics, unrestricted by the Standard Model.

The Standard Model and Quantum describes the strong , which is Chromodynamics transmitted by to tightly bind together. Much evidence has Understanding what makes up the been gathered to support the Standard matter around us has been a unified Model, including the discoveries of goal of scientists around the world predicted by the Standard for decades. Since the 1970s, Model, such as the Higgs . physics has used the Standard Model Nonetheless, physicists have not turned to describe fundamental particles a blind eye to some of the differences and their interactions – a theory that between the theoretical predictions of encompasses the existence of the QCD and real observations. smallest building blocks of matter such as quarks and gluons. In fact, One significant mystery of this theory is quarks are the most elementary matter the confinement of quarks and gluons particles to exist, and when they are within subatomic particles. Although bound together with force-transmitter QCD theory suggests that more than particles called gluons, they form 99% of visible matter is composed heavier particles such as neutrons, of these two important fundamental protons and the lesser-known ‘pions’. In particles, quarks and gluons cannot turn, these heavy particles make up all exist on their own, outside the confines the matter we can see. of subatomic particles such as protons and neutrons. Scientists agree that about the smallest building blocks of The Standard Model names 25 for QCD to be fully accepted, it must matter, scientists hope to explain the fundamental particles in total, from 12 somehow also explain this observation more abstract and complex phenomena force carrier (including eight of and confinement. This that dominate contemporary physics – gluons) and all six types of quarks and problem is one of the most complicated such as dark matter and dark energy – leptons, to the which was challenges facing physicists today. in addition to increasing our discovered in 2012. understanding of QCD confinement. The search for new theories beyond the At the Thomas Jefferson National The theory of Quantum Standard Model motivates continued Accelerator Facility in Virginia, scientists Chromodynamics (QCD) is an important research in the fields of nuclear and are taking part in this endeavour. aspect of the Standard Model, as it . By uncovering more

WWW.SCIENTIA.GLOBAL The Jefferson Lab Experiments accelerators. Scientists at the Jefferson on QCD confinement that is widely Scientists at the Jefferson Lab use the Lab have exploited a particular physical considered as the last frontier within the Continuous Electron Beam Accelerator phenomenon that creates pions in this Standard Model.’ Facility (CEBAF) to probe atomic nuclei way, allowing them to experimentally with continuous beams of high-energy probe these peculiar particles. This research could potentially shed electrons. Professor Liping Gan’s group Remarkably, the lifetime of the neutral some much-needed light on not only of the University of North Carolina, pion is one of only a few quantities can the nuts and bolts of matter, but also Wilmington, has continually been be calculated accurately by QCD at on far more elusive aspects of the making significant strides at this facility, the confinement scale, and as such, its Universe that cannot be explained by including measuring the lifetime of precise measurement provides a robust our current theories. the neutral pion – an unstable particle way to test the theory. made up of a quark and an antiquark. Other experiments at the Jefferson Professor Gan is currently working Lab that are drawing considerable Pions are the lightest particles of all on another experiment to measure interest is the search for a dark gauge the ‘’ – unstable particles that the lifetime of another , called boson. Observational evidence has all contain a quark and an antiquark. eta, which is also made up of pairs shown that dark matter, an unknown To elaborate briefly on what exactly a of quarks. She also aims to extend invisible form of matter, makes up about neutral pion is, it must first be noted these measurements to a similar 85% of all matter in the Universe. As that an antiquark is the anti-matter particle called eta-prime, using the direct detection methods to find out version of a quark, meaning that it same experimental techniques as more about dark matter have been possesses the same mass as a quark, those used to investigate the pion. Her unsuccessful to date, physicists have but has opposite charge. In fact, team’s precision measurements of the started to broaden their searches to try because the neutral pion is composed properties of the neutral pion, eta and and detect force-carrier particles that of a quark and its anti-particle, it has the eta-prime will offer a stringent test of transmit between dark matter potential to annihilate, which is why the some other aspects of QCD theory – particles, and between dark matter and lifetime of this pion is so brief before it more explicitly, to test the symmetry visible matter particles. decays into light energy (or ). structure of QCD at low energy. Because they are so unstable, pions These symmetries are key properties Scientists hypothesise that dark matter are not abundant in nature, and of QCD equations. As Professor possesses a rich symmetry structure mostly exist as short-lived by-products Gan explains, her team’s ‘proposed between its constituent forces and of high energy collisions in particle measurements will have broad impact particles. To elaborate, specific types

WWW.SCIENTIA.GLOBAL to solve exactly at a low energy scale. Search for New Physics Beyond the While it is supported by plentiful Standard Model evidence, some aspects of QCD are not completely understood. For instance, In recent years, Professor Gan is the problem of quark confinement still leading a group of physicists who requires quantitative analysis to be successfully developed the JLab Eta fully explained, as does a second major Factory (JEF) experiment. This is a problem with QCD theory – known as new experiment aimed to search for chiral symmetry breaking. dark gauge bosons and a new type of symmetry-breaking force, by measuring The equations of QCD encompass some very rare decays of eta. With certain crucial symmetries, referred to JLab’s 12 GeV energy beam, available as ‘chiral symmetry’. Chiral symmetry is GlueX experimental apparatus, and present when two separate forms of an a future upgraded calorimeter, this of symmetries (referred to as ‘gauge’ object are mirror images of each other, experiment will offer the best data in and ‘Lorentz’ symmetries) of the such as a left hand and a right hand. The the world of neutral rare meson decays. Standard Model impose certain theory suggests that chiral symmetry The team hopes to reveal new physics restrictions on the ways that a force- spontaneously ‘breaks’ because of the that could exist beyond the scope of the carrier particle of dark matter can so-called ‘QCD vacuum’. This symmetry Standard Model. behave. This is where the search for breaking effect gives birth to particles a dark , a force-carrier known as ‘Goldstone bosons’. The As mentioned earlier, despite its particle, could provide a key extension neutral pion is one of such Goldstone many successes, the Standard Model of the Standard Model – potentially boson particles – whereby the predicted still cannot explain issues such as providing a way to expand this basic symmetry somehow ends as soon as the the existence of dark matter and theory to include an explanation of the particles are measured experimentally. dark energy. It also does not explain ever-elusive dark matter. Therefore, a precise value of the lifetime why there is far more matter than of the pion could significantly help antimatter in the Universe – another key The Lifetime of the Neutral Pion scientists to learn more about the unanswered question in this . puzzling mechanisms responsible for Professor Liping Gan and her colleagues this symmetry breaking. These limitations have given rise to obtained ultra-precise measurements the search for new theories beyond that tested vital aspects of QCD theory, At the Jefferson Lab, Professor Gan the boundaries of what the Standard by conducting the so-called ‘Primakoff and her team worked on the Primakoff Model can offer. Professor Gan explains Experiment’, an experimental program Experiment to find the value of this how the ‘results from this experiment at the Jefferson Lab. From their first important quantity by aiming a beam will have the potential to shed light experiment in 2004, the researchers of gamma-rays at atomic nuclei. This on the … mystery of dark matter, and determined the lifetime of the set up was implemented to induce the the asymmetry of matter-antimatter neutral pion with more than double ‘Primakoff effect’, a phenomenon in in the Universe.’ Her continued the precision ever recorded before. which the incident particles exchange research in these fields could drive The team then carried out a second a single with the target nuclei contemporary physics rapidly forwards experiment in 2010 to challenge their to produce neutral pions. As explained to provide explanations of the abstract own previous results, and further earlier, the neutral pion rapidly decays phenomena that current theories improved their precision by a factor of into two daughter photons. By taking cannot yet describe. two. The final results from the team’s detailed measurements of the decay second experiment is expected to be processes, Professor Gan and her team In addition to her ground-breaking published in 2018. acquired key lifetime measurements for work in nuclear and particle physics, these particles. Professor Gan is also actively engaged It may not immediately be apparent in STEM outreach work. Not only why the precise measurement of this Effectively, with their measurements of does her research aim to advance our quantity is so important to furthering the so-called Primakoff effect on various understanding of the Universe, but she the aims of particle and nuclear mesons (such as neutral pions), the also emphasises the importance of physicists alike. However, it must be scientists are working to vigorously test undergraduate research. Professor Gan understood first that QCD, despite the predictions of QCD confinement also enthusiastically encourages the being a successful theory that entails theories as discussed above, with the role of women in the field, and generally a complete description of the strong hope of finally unveiling the true origins aims to promote the integration of force, is almost impossibly complex of QCD confinement. research and education.

WWW.SCIENTIA.GLOBAL Meet the researcher Professor Liping Gan Department of Physics and Physical Oceanography University of North Carolina Wilmington Wilmington, NC, 28403 USA

Professor Liping Gan obtained her BSc in physics at Peking CONTACT University in China in 1985, and her MSc at the same institution in 1988. She then achieved her PhD in physics at the University E: [email protected] of Manitoba, in Canada, after which she completed two post- T: (+1) 910 962 3583 doctoral research fellowships. She moved to the University of W: people.uncw.edu/ganl/ North Carolina, Wilmington (UNCW) in 2001, where she initially worked as an Assistant Professor, then an Associate Professor, KEY COLLABORATORS and was ultimately promoted to her current position as a full Professor of Physics. Professor Gan joined the Executive The PrimEx collaboration and GlueX collaboration at Committee for the Southeastern Section of the American Jefferson Lab Physical Society as a Vice Chair in 2016, then Chair-Elect in 2017, and became Chair in 2018.She also served as a nuclear FUNDING physics panel member for the American National Science Foundation (NSF). Her current research interests focus on QCD US National Science Foundation confinement and new physics beyond the Standard Model, which she explores by testing fundamental symmetries in REFERENCES precision measurements of light meson decays at Jefferson Lab (JLab). She is a spokesperson or a co-spokesperson for L Gan, Probes for Fundamental QCD Symmetries and a Dark three high rated Jlab experiments (the PrimEx, PrimEx-eta Gauge Boson via Light Meson Decays, Proceedings of Science, and JEF experiments). In addition to her research, Professor 2015, CD15. Gan is a motivated STEM promoter, and has been a long- running member of various UNCW undergraduate research organisations and ‘Women in Science and Engineering’ boards.

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