Theoretical Elementary Particle Physics

The aim of research in elementary particle theory is to discover the laws of physics which have governed the Universe since the time of the Big Bang. While the standard model of the strong and electroweak interactions has been very successful in explaining a large amount of data up to the highest energies reached so far, it has several known limitations. Specifically, the model contains many arbitrary parameters. It suffers from the so-called gauge hierarchy problem-there are mass scales in the theory differing by factors of 10¹⁰ and more which cannot be explained by the theory. Finally, it does not allow for unification with gravity without extensions (supergravity, string theory), which is a main focus of current research.

Supersymmetry (SUSY), and more specifically its local extension supergravity (SUGRA), provides a framework for unification with gravity, which may also explain the large hierarchy of mass scales. Combined with the ideas of grand unification, SUSY leads to supersymmetric grand unification (SUSYGUT) and supergravity grand unification (SUGRA). Currently SUGRA models, including its minimal version (mSUGRA), are among the leading candidates for testing new physics beyond the standard model at high energy colliders. Important support for such models has already emerged from the high precision LEP data which shows that gauge coupling constants for the electroweak and the strong forces unify at a high scale as expected. Further, such models predict many new particles (sparticles) some of which should become visible at the Fermilab Tevatron and at the Large Hadron Collider (LHC) under construction at CERN in Geneva, Switzerland.

Superstring theory, and its more recent formulation known as M-theory, are ambitious attempts to construct a 'theory of everything', which can provide a specific context in which SUGRA models can be derived.

Beyond the physics at very small distance scales explored by high-energy accelerators, astronomical observations using telescopes, satellites, and balloons, have revealed some startling and mysterious information about the universe we live in. For example,

1. Only about 1 or 2% of the matter in the universe is of the sort familiar to us-protons, neutrons, electrons and photons. The rest is something we have named 'dark matter', but that only hides our ignorance.

2. All of the matter (including the invisible dark matter), only constitutes about 30% of the energy content of the universe. The balance, which we label 'dark energy', establishes a gravitational repulsion in space causing the universe to expand even more rapidly than expected after the Big Bang.

3. As deeply as we probe the universe, we see only matter, with no evidence of antimatter.

What has become clear in recent years is that any understanding of the reasons for these phenomena which occur in the universe in the large will be based on our understanding of elementary particle physics at the smallest level. We find that the leading candidates for the 'dark matter' are the hypothesized heavy 'superpartners' of the photon (or its equivalent from the weak interactions), which can be discovered in the coming decade of accelerator experiments. The most plausible candidate for the 'dark energy' is a field whose role may also be to decide various important things like the number of ordinary spatial dimensions, or the strength of the electromagnetic charge. The asymmetry between matter and antimatter in the universe may originate in the same elementary particle physics which underlies radioactive decay, when aided by the 'superpartners' just mentioned.

Faculty and students in our elementary particle theory group are actively exploring these questions, with a view to understanding the connection between the universe at very large and very small. This leads to the study of possible extra dimensions, beyond our usual four, which can cause measurable deviations from Newton's Law of Gravitation at laboratory scales of micrometers or nanometers. Relateded exotic phenomena are mini-black holes, which may be produced at accelerators, or by ultra high energy cosmic rays. Our formal investigations in superstring theory and M-theory are also conducted with the purpose of making connection between fundamental theory and experiment.

International Conferences at NU

To mark twenty years since the formulation of supergravity (sugra) models at NU, the Physics Department held an international conference (SUGRA20) at NU during the period March 17-20, 2003.

The elementary particle theory group initiated the PASCOS and SUSY series of conferences, which have become major conferences in high energy physics. The first two conferences on PArticles, Strings and COSmology (PASCOS) were held at NU in 1990 and 1991; Stephen Hawking attended these conferences and delivered public lectures in addition to his contributions to the conference. Since then, the PASCOS conference has been held at Berkeley, Syracuse, Johns Hopkins,UC Davis, U North Carolina (Chapel Hill), Tata Institute of Fundamental Research, Bombay. The conference on SUperSYmmetry and Unification of Fundamental Interactions (SUSY) was started at NU in 1993, and has subsequently been held at U Michigan (Ann Arbor), École Polytechnique (Paris), Maryland, U Pennsylvania, Fermilab, Oxford (UK), CERN (Geneva), Dubna (Russia), DESY (Hamburg), KEK (Japan), Arizona (USA), Durham (UK), and next year (2006) it will be held in Irvine (USA). Link to PASCOS 2005 at http://ctp.snu.ac.kr/pascos05.

The PASCOS conference returned to Northeastern in August 2004. The conference included a special program (the Pran Nath Fest) honoring Matthews Distinguished University Professor Pran Nath for his pioneering contributions over four decades in the fields of high energy theory, supersymmetry, supergravity, and unification. The proceedings of the conference and the Fest are published by World Scientific edited by George Alverson, Emanuela Barberis and Michael T Vaugh (http://www.worldscibooks.com/physics/5873.html). The PASCOS-2005 was held in Gyeongju (Korea), and the PASCOS-2006 will be held at the Ohio State University (USA).

The Elementary Particle Theory faculty includes Professors Haim Goldberg, Pran Nath, Brent Nelson, Yogendra Srivastava, Tomasz Taylor,Michael Vaughn.