PhD Thesis Research

In high-energy nuclear physics, the primary goal is to develop a thorough understanding of the phase diagram of nuclear matter. This nuclear phase diagram (see diagram below) defines how matter transforms with a change in temperature and/or chemical potential. Unfortunately, due to the complexity of studying the phase diagram, little is known of its features and the form that we use today is derived mainly from conjecture.

The phase diagram of matter. At very high temperatures amd matter density, ordinary nuclear matter transforms to an exotic type of matter known as a quark-gluon plasma.

Courtesy of the Institute of Space Sciences


To probe the nuclear phase diagram, we collide heavy ions, such as those of lead and gold, in very large and sophisticated machines known as particle colliders. When these ions are first accelerated close to the speed of light, and then suddenly crashed together, the kinetic energy they carry is transformed into heat thereby melting their nucleus. For that brief moment right after collision, the conditions of temperature and matter density experienced by the colliding particles closely resembles the conditions that existed at the beginning of the Universe or what is commonly known as the Big Bang. Recently, scientists have been able to show that in the first few microseconds after collision (which mirrors the first few microseconds of the birth of the Universe), a very short-lived phase of matter consisting of unbound quarks and gluons, the very building blocks of nuclear matter, is formed. This phase of matter is known as a quark-gluon plasma or QGP. Eventually, as the matter expands and cools down, it coalesces to form new particles that are then measured by particle detectors such as the PHENIX detector pictured below.

The PHENIX detector used in experiments in high-energy nuclear physics.

Courtesy of the PHENIX collaboration


Studying the physical properties of these particles can reveal a lot about the initial conditions when the matter was still hot and dense, thereby giving as a vital tool to understanding the nuclear phase diagram.

My Thesis

One question that has been of great interest to scientists is the location of the critical point of nuclear matter. In thermodynamics, a critical point represents a point in a phase diagram where the different phases exist in equilibrium. It is also the end point of the sharp demarcation line from one phase to another. Matter tends to behave differently in the vicinity of a critical point and therefore one of the strategies been employed by physicists in trying to identify the critical point is to look for sudden changes in behavior as you probe matter at different locations on the phase diagram.

When heavy ions are collided at very high speeds, the matter initially produced is at extremely high temperatures (measured in MeV). As the matter expands and cools, similar to what happened in the first few microseconds after the Big Bang, it follows a trajectory depicted by the different yellow lines in the diagram. Reaction trajectories that are close to the location of the critical point will be modified and it is the signals associated with these modifications that would help physicists determine the location of the critical point.

Courtesy of Brookhaven National Laboratory


For my PhD research, I utilized the very large datasets produced from particle collisions to study how the different systems evolved on colliding particles at different energies compare in size. The idea was that in the vicinity of a critical point, we would expect to see a sudden change in the monotonous increase in the sizes at different energies, i.e., in the case where you would expect to observe a gradual increase in the size of the system produced as you probe at higher and higher energies, a sudden increase in the size at a particular energy (or narrow range in energy) would suggest a change in the underlying dynamics of the system, as would be expected for a system whose trajectory traverses a critical point or phase transition line. The analysis involved collecting large amounts of data, writing and running software programs to select for events and subsequently particles of interest, and background suppression so as to isolate the signal and look for interesting correlations. One of the results from this work was that I was able to narrow down the range in energy (and therefore the location on the phase diagram) that one would expect to find the critical point.

The PHENIX Collaboration

Particle detectors and the whole business of high-energy nuclear physics is an expensive undertaking, both in terms of infrastructure as well as man-power. For this reason, experiments such as those conducted at the Relativistic Heavy Ion Collider require the pooling together of resources from institutions and governments around the world. One such collaboration is the PHENIX collaboration comprising of over 400 scientists and engineers drawn from 12 countries.

You can read more about the experiment here. You can also follow on the work that is being done by different physicists working in particle and nuclear physics at this amazing blog.

Relevant Publications

Mwai, A. Beam-Energy and System-Size Dependence of the Space-Time Extent of the Pion Emission Source Produced in Heavy Ion Collisions. Ph.D. Dissertation, Stony Brook University, Stony Brook, NY, 2014.

Adare, A.; et al. Comparison of the Space-Time Extent of the Emission Source in d+Au and Au+Au Collisions at $\sqrt{s_{NN}}$ = 200 GeV. Nucl.Phys. 2014, A931:1082-1087.

Papers Under Review

Adare, A.; et al. Beam-Energy and System-Size Dependence of the Space-Time Extent of the Pion Emission Source Produced in Heavy Ion Collisions. ArXiv:1410.2559, under review.

Conference Proceedings

Mwai, A. A Comparison of HBT Measurements for d+Au and Au+Au Collision Systems at $\sqrt{s_{NN}}$ = 200 GeV. J.Phys.Conf.Ser. 2014, 535:012025.

Selected Conference Presentations

Mwai, A. Recent Results from the PHENIX Beam Energy Scan Program. Paper presented at the Twelfth Conference on the Intersections of Particle and Nuclear Physics, Vail, Colorado, May 19-24, 2015.

Mwai, A. Au+Au Beam Energy Dependence of HBT Measurements for Charged Pions at RHIC-PHENIX. Paper presented at the Fall Meeting of the American Physical Society Division of Nuclear Physics, Newport News, Virginia, October 23-26, 2013.

Mwai, A.; Lacey, R. A. HBT Measurements for Charged Pions in Au+Au Collisions at √sNN = 39, 62.4, an 200 GeV. Paper presented at the American Physical Society April Meeting, Atlanta, Georgia, March 31-April 3, 2012.

Media

The Owl Shift at STAR and PHENIX: https://www.bnl.gov/rhic/news2/news.asp?a=3118&t=today
PHENIX profile: https://www.phenix.bnl.gov/WWW/pub/awards/html/mwai_alex.html

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