**Office Location:** 249 Physics Bldg, Durham, NC 27708**Office Phone:** +1 919 660 2555**Email Address:** mehen@phy.duke.edu**Web Page:** http://www.phy.duke.edu/~mehen

**Specialties:**

Theoretical particle physics and string theory

Theoretical nuclear physics

**Education:**

M.A., Johns Hopkins University, 1997

Ph.D., Johns Hopkins University, 1998

Ph.D., Johns Hopkins University, 1997

B.S., University of Virginia, 1992

**Research Categories:** *Theoretical Nuclear and Particle Physics*

**Research Description:** Prof. Thomas Mehen works primarily on Quantum Chromodynamics (QCD) and
the application of effective field theory to problems in hadronic physics.
Effective field theories exploit the symmetries of hadrons to make model
independent predictions when the dynamics of these hadrons are too hard to
solve explicitly. For example, the properties of a hadron containing a
very heavy quark are insensitive to the orientation of the heavy quark
spin. Prof. Mehen has used this heavy quark spin symmetry to make
predictions for the production and decay of heavy mesons and quarkonia at
collider experiments. Another example is the chiral symmetry of QCD which
is a consequence of the lightness of the up and down quarks. The
implications of this symmetry for the force between nucleons is a subject
of Prof. Mehen's research. Prof. Mehen has also worked on effective field
theory for nonrelativistic particles whose short range interactions are
characterized by a large scattering length. This theory has been
successfully applied to low energy two- and three-body nuclear processes.
Some of Prof. Mehen's work is interdisciplinary. For example, techniques
developed for nuclear physics have been used to calculate three-body
corrections to the energy density of a Bose-Einstein condensate whose
atoms have large scattering lengths. Prof. Mehen has also worked on novel
field theories which arise from unusual limits of string theory. Examples
include noncommutative field theories and theories of tachyonic modes on
non-BPS branes.

**Recent Publications**
(More Publications)
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- Copeland, M; Fleming, S; Gupta, R; Hodges, R; Mehen, T,
*Polarized J/ψ production in semi-inclusive DIS at large Q2: Comparing quark fragmentation and photon-gluon fusion*, Physical Review D, vol. 109 no. 7 (April, 2024) [2713399], [doi] [abs]. - Copeland, M; Fleming, S; Gupta, R; Hodges, R; Mehen, T,
*Polarized TMD fragmentation functions for J/ψ production*, Physical Review D, vol. 109 no. 5 (March, 2024) [2689297], [doi] [abs]. - Dai, L; Fleming, S; Hodges, R; Mehen, T,
*Strong decays of Tcc+ at NLO in an effective field theory*, Physical Review D, vol. 107 no. 7 (April, 2023) [doi] [abs]. - Liu, Q; Low, I; Mehen, T,
*Minimal entanglement and emergent symmetries in low-energy QCD*, Physical Review C, vol. 107 no. 2 (February, 2023), American Physical Society (APS) [doi] [abs]. - R. Boussarie, et.al.,
*TMD Handbook*(2023) [2304.03302] .

**Highlight:**

Prof.~Thomas Mehen works primarily on Quantum Chromodynamics (QCD) and the application of effective field theory (EFT) to problems in hadronic physics. EFTs rely on three key ideas: i) identifying the relevant degrees of freedom for a specific physical process; ii) using symmetries of QCD to simplify or constrain the form of interactions; and iii) finding small parameters, either small coupling constants or ratios of disparate mass scales, which can be used to formulate systematic perturbative calculations of observables.

Much of Mehen's research involves hadrons containing one or more heavy quarks. A long standing problem in QCD is understanding the production mechanism of heavy quarkonium (bound states of heavy quarks and antiquarks) in collider experiments. Mehen has worked on this problem throughout his career. Recently he and his collaborators proposed new tests of quarkonium production theory involving quarkonium produced within jets which motivated experimental measurements by colleagues at the Large Hadron Collider (LHC). Another important area of research is exotic quarkonium. Since 2003, particle and nuclear physics experiments around the world have discovered dozens of unusual particles that do not fit in to the conventional picture of quarkonium as a nonrelativistic bound state of heavy quark and antiquark. Some of these so-called XYZ mesons are thought to be molecular bound states of heavy mesons. Mehen has invented EFTs for these particles that are widely used to predict the properties of these mesons and interpret experimental data.

Mehen played a role in developing the Soft Collinear Effective Theory (SCET) which is an EFT suitable for processes in which there are one or more highly energetic quarks or gluons. This theory is now widely used in the study of energetic jets of particles produced at the LHC and other collider experiments. Mehen has been a leader in applying SCET to quarkonium production in colliders. He is also an expert on EFTs for strongly interacting systems with shallow two-body bound states and/or large scattering lengths, and has applied these EFTs to a wide variety of physical systems: low energy two- and three-body nuclear physics, the XYZ mesons, cold trapped atoms, and strongly interacting nuclei embedded in a QED plasma, which is potentially relevant for astrophysics and cosmology. In his research, Mehen frequently applies heavy hadron chiral perturbation theory (HH$\chi$PT), which combines the heavy quark and chiral symmetries of QCD and is used to study the low energy behavior of heavy hadrons. Mehen and his student invented a version of HH$\chi$PT suitable for baryons and tetraquarks with two heavy quarks for the purpose of studying the doubly charm baryon recently discovered at the LHC and related states.

Some Prof. Mehen's work crosses over into other areas of physics. For example, techniques developed for nuclear physics have been used to calculate three-body corrections to the energy density of a Bose-Einstein condensate whose atoms have large scattering lengths. Prof. Mehen has also worked on novel field theories which arise from unusual limits of string theory. Examples include noncommutative field theories and theories of tachyonic modes on non-BPS branes.

**Current Ph.D. Students**(Former Students)- Reggie Bain
- Yiannis Makris
- Dilun Yang
- Jie Hu

**Postdocs Mentored**- Jared Vanasse (2012- present)
- Chul Kim (September 2007 -December 2009)
- Ahmad Idilbi (September, 2006 - February, 2010)
- Brian Tiburzi (September, 2004 - August, 2006)
- Carlos Schat (2002/09-2004/09)