# Graduate Student Summer Fellows Named by Leinweber Center for Theoretical Physics

The Leinweber Center for Theoretical Physics (LCTP), formerly the Michigan Center for Theoretical Physics, was created in 2017 following an endowment from the Leinweber Foundation. LCTP scholars focus on fundamental topics in theoretical particle physics and cosmology, studying dark matter, black holes, string theory, and more. One aspect of support provided by the Leinweber Foundation’s endowment is sponsorship of graduate students studying questions in these areas. For summer 2019, four students - Christopher Dessert, Shruti Paranjape, Marios Hadjiantonis, and Yangwenxiao Zeng - received support as Summer Fellows.

**Mr. Christopher Dessert** works with Professor Ben Safdi to determine the nature of dark matter using astrophysical observations. Dark matter, although it makes up 85% of the matter in the universe, has never been observed by researchers except via its gravitational interactions. Some candidates for dark matter can decay into X-rays. The XMM-Newton and Chandra telescopes have detected X-rays of unknown origin from galaxy clusters, which some thought could be due to such dark matter decays. Using careful analysis of the data from these telescopes, Mr. Dessert was able to conclude that these X-rays do not in fact come from dark matter. Mr. Dessert also studies axions, a new type of particle that might make up dark matter. If present in nature, axions are expected to be produced in the cores of white dwarf and neutron stars. Then, as they move through the magnetic field of the star, they could transform into photons, the particles that comprise light. These photons could be detected by telescopes on Earth. In the future, Mr. Dessert plans to continue searching for signatures of dark matter candidates in astrophysical data.

**Ms. Shruti Paranjape** studies scattering amplitudes in effective field theories with Professor Henriette Elvang. Fundamental particles, like quarks or electrons, and their interactions can be described by quantum field theories (QFTs). A particular class of QFTs is known as effective field theories (EFTs): these are theories that describe the low-energy physics of a system without necessarily knowing the details at high-energies. Particle scattering processes allow the predictions of quantum field theories to be tested, and the probability that a given process will occur is described mathematically by an object known as a “scattering amplitude.” Usually, scattering amplitudes are determined using Feynman diagrams, a pictorial method of representing complex mathematical quantities. However, when the number of particles or the need for high precision results grows, the complexity of the computations utilizing this diagrammatic method grows enormously. Ms. Paranjape is interested in finding simpler, alternative methods. She uses the fundamental symmetries of effective field theories to constrain the calculations. This allows scattering amplitudes to be calculated much more easily. In addition, Ms. Paranjape is interested in the reverse, examining scattering amplitudes to understand the properties of the effective field theory from which they originated.

**Mr. Marios Hadjiantonis** is also a student of Professor Henriette Elvang. Mr. Hadjiantonis has studied a wide range of topics in particle theory during his time as a graduate student at Michigan, including scattering amplitudes and effective field theories and their relationship to fundamental symmetries. An example of a symmetry is rotational symmetry, which is present when systems look the same before and after they have been rotated, like a round ball. However, the way in which underlying symmetries reveal themselves in nature can be subtle. For instance, the equations of magnetism respect rotations, but since a rotating a magnet 180 degrees exchanges the north and south poles, rotational symmetry is broken by the magnet. This phenomenon -- when the equations underlying a system possess a symmetry but the system itself does not -- is called “spontaneous symmetry breaking”. Similar phenomena occur in theories of particle physics as well. The recently discovered Higgs boson is an example. Mr. Hadjiantonis is working on understanding scattering amplitudes in models with spontaneously broken symmetry. In addition, he investigates whether these models are compatible with other symmetries and has worked to classify and test models that combine different types of spontaneous symmetry breaking. Mr. Hadjiantonis defended his PhD thesis this summer and is continuing his work as a postdoc at Nordita in Stockholm, Sweden.

**Mr. Yangwenxiao Zeng** is a student of Professor Finn Larsen. He works on understanding quantum mechanical properties of gravity by studying black holes. A theory of quantum gravity is a central goal of high-energy physics. Such a theory would unite the two main theories of modern physics: quantum mechanics and general relativity (gravity). However, it is difficult to study both of these at the same time. Quantum mechanics works on a microscopic scale, describing interactions of elementary particles, such as behaviors of electrons in semiconductors and particle colliders. On the other hand, general relativity is a highly successful theory at longer distance scales, but its effects also make our GPS systems run accurately, for example. Mr. Zeng studies black holes, which are ideal objects for examining the interplay of quantum effects and general relativity. Black holes are regions of spacetime from which nothing can escape classically. But quantum mechanically they can evaporate thermodynamically, as famously described by Stephen Hawking. By doing calculations to elucidate the properties of Hawking radiation of black holes, Mr. Zeng hopes to put constraints on proposed theories of quantum gravity, in particular in the context of string theory.

Professor Aaron Pierce, Director of LCTP, says, “I am delighted to have such a fantastic group of Leinweber Summer Fellows and am excited to see the advances that come from their research.”

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