Medium Energy Experiment-Dept of Physics - Carnegie Mellon University

Experimental Medium Energy Particle Physics

Faculty: Gregg Franklin
Michael McCracken
Curtis Meyer
Brian Quinn
Reinhard Schumacher

Group Overview

The vast majority of the visible matter in the universe is made up of protons and neutrons. These particles are in turn, made of of quarks and gluons whose interactions are described by Quantum Chromodynamics (QCD). In fact, these interactions are responsible for most of the observable properties of the proton and neutron, including their mass, spin and how they bind together to form the nucleus of an atom. QCD is also responsible for the phenomena of confinement, which appears to forever trap the quarks inside the protons and neutrons. Understanding how QCD works in the energy regime that describes the nature of the proton and neutron is one of the main goals of Medium Energy Physics.

The Carnegie Mellon Medium Energy Group is focussed on studying QCD through a large number of experiments that can probe the nature of nuclear matter. The group's efforts are now focussed on experiments at the Thomas Jefferson National Accelerator Facility (JLab) and is heavily involved in an on-going upgrade to double the beam energy of the facility and build a new experimental hall (GlueX). We are also involved in the largest experiment currently running at JLab (CLAS), where we are studying QCD by looking at how photons can excite protons and neutrons into their excited states.

Another major thrust involves studies of how the quarks are distributed throughout the proton by looking at charge and current distributions in protons and neutrons.  This includes a recently completed effort to look at the distribution of strange quarks within the proton as well as the development of a new facility, dubbed the SuperBigbite Spectrometer, which will be the heart of a series of experiments designed to illuminate the underlying quark-structure of the proton and the neutron.

Although our experiments are carried out at JLab the development, construction and analysis takes place at Carnegie Mellon. The group maintains its own on-campus shop for the development and construction of detectors, and two labs for testing equipment and electronics. The group also maintains, in conjunction with Medium Energy Theory, the largest computer cluster in the department with 700 modern compute cores and nearly 300 TB of data storage.

Because of this infrastructure, our graduate students and post docs are exposed to a wide variety of experiments in various stages of development. Our group also enjoys fruitful interactions with colleagues at many universities around the world. In addition to the four faculty members, the group has support for at least three postdocs and six to seven graduate students. We also involve a large number of undergraduate students directly in our research activities.

Learn more. Click here to download the Medium Energy Group's Annual Report.

Member Research Thrusts

Gregg Franklin’s research probes the proton structure using experiments, based at the Jefferson Lab electron accelerator, which take advantage of recent developments in polarized target and polarized beam technologies. For example, the Jefferson Lab G0 experiment directly measured the distribution of “strange” quarks and anti-quarks in the proton using spin-dependent (parity violating) elastic scattering as a signature. It required an accuracy significantly better than one part in a million. The group has also probed the electric form-factor of the neutron using a polarized 3He target. As the design goals of future experiments put increasing demand on obtaining precise measurements of the actual beam polarization, the group will be implementing a new beam polarimeter based on helicity-dependent Compton scattering of beam electrons off of photons in a resonant laser cavity. On a longer time-frame, the group is involved in a suite of newly approved experiments which will utilize a new spectrometer dubbed “Super Bigbite”. Dr. Franklin will be overseeing the development of a hadron calorimeter and is co-spokesperson of an experiment which will user Super Bigbite to measure transverse quark-spin distributions.

Curtis Meyer is the spokesperson of the GlueX experiment which is currently being built JLab. The primary goal of the experiment is to search for a theoretically expected exotic form of matter in which the gluonic fields of QCD directly impact the observables properties of matter. It is believed that the spectrum of these exotic hybrid mesons will help to elucidate the nature of confinement in QCD. In order to carry out this analysis, large-scale, sophisticated analyses need to be performed on big data sets. We have developed a significant set of software tools that are currently being applied to data on excited protons from the CLAS at Jefferson. These analsyses hope to resolve a long standing issue in QCD known as the missing baryon problem by uncovering new exited states. Results from this program are just starting to come out, where we are seeing some exciting hints of new states.

Quinn is involved in several experiments which probe the structure of the nucleon (neutron and proton) by using the electron beam available at Jefferson Lab.  Recently he led the effort to design and propose an experiment to measure the magnetic form factor of the neutron at high momentum transfer.  That experiment has been approved and will run after the 12 GeV upgrade  The apparatus for that experiment with be constructed by reconfiguring pieces of the Super BigBite spectrometer, leading to his involvement in that project.  He led CMU's development of specialized very high rate electronics for the G-zero experiment which used parity-violating electron scattering as a probe of the weak structure of the nucleon, which can then serve to measure the contribution of strange quarks to the electromagnetic structure of the nucleon.  This line of research continues with the HAPPEX III experiment which uses a very different technique to make the same measurements.  The PREX experiment aims to measure quite different physics, the neutron radius of lead, but makes cleaver use of the parity-violation technique to probe the neutron distribution.

Schumacher's main area of research is in the electromagnetic production of strange particles (kaons and hyperons). The production of strange-quark pairs via the well-known electromagnetic interaction (via photon and electron-induced reactions) gives us an avenue for refining our knowledge of baryon resonances and coupling constants; the results are compared with modern quark model calculations. This work is being done at JLab. His research uses the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at Jefferson Lab. He is a former Chairman of the CLAS Collaboration.

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