Matthew Strassler      Professor, Theoretical High Energy Physics

Office: 354 Physics-Astronomy Building
Office Phone: 445-5500 x2783 [732 area code]
Rutgers Email: physics dot rutgers dot edu , Username: strassler

    Particle Physics, Quantum Field Theory, String Theory

The Long Trek:
  A.A., Simon's Rock College, 1984
  A.B., Princeton University, 1987
  Ph.D., Stanford University, 1993
  Postdoctoral Fellow, Rutgers University, 1993-1996
  Long-Term Member, Institute for Advanced Study 1996-2000, Spring 2002
  Visiting Assistant Professor, Harvard University, Spring 1997
  Assistant Professor, University of Pennsylvania, 2000-2002
      Fellowship, Alfred E. Sloan Foundation, Spring 2001
      Outstanding Junior Investigator, Department of Energy, Spring 2001
  Associate Professor, University of Washington, 2002-2007
  Professor, Rutgers University , 2007-2013
      Elected Fellow, American Physical Society, 2007
  On Academic Leave, 2011-2013
  Departing Rutgers, July 2013

Public Web Project --- Of Particular Significance: Conversations About Science

Public Talks:
     October 29th, 2012
      Ursinus College

Current Research Focus:
      Searching for New Physics at Colliders (the Fermilab Tevatron and the CERN Large Hadron Collider);
      Effects of Hidden Valleys (sectors of new light particles with their own new forces);
      The Scale Hierarchy (why is the weak nuclear force so vastly stronger than gravity?)

My Research:     Though much of my current effort is concentrated on experiments at the LHC, I have worked in a wide variety of areas. In general I am interested in questions such as these:
What are the basic building blocks of the universe? What are the forces by which they interact with one another?
What impacts do they have on our daily experience and on the universe as a whole?

         Here are explanations of my work for non-physicists;  for physics students ; for experts.  Here is My Bibliography.

Winter 2011: Advanced Topics in Quantum Field Theory   

Important Links:

Hidden Valley Web Page (Updated 11/2008)
Physics at the Large Hadron Collider
String Theory and Gauge Theory
The LHC Olympics: 
Summer 2006, Winter 2006Summer 2005
   The UW Summer 05 Data Challenge Analysis (performed in July 2005 by Matt Bowen, Steve Ellis, and myself) of the Summer 2005 LHC Olympics Data set [pdf format]

LIVE! Real-Time Data From Fermilab Experiments: The DZero Experiment ; The CDF Experiment

[TASI 2001, 2003 notes are now on the archive (; TASI 2010 notes to come soon.]

Some Nontechnical Articles: Keeping Particle Physics in its Proper Place;  

My Research --- for Non-Physicists

I do research in quantum field theory, particle physics, and string theory.  These subjects aren't familiar to most nonphysicists, so I've thought a lot about how to explain them to my family, to my friends, and to random people I meet at parties and on trains.  The good news is that despite the societal myth that physics is impenetrable, it really isn't.  (Well, most of it isn't.)  The bad news is that like any unfamiliar subject, it takes a little while to get it all straight.

So I'll start with a very short explanation; then I'll give a slightly longer one; and finally I'll try to do a decent job for those who really want to know what I do.

The Very Short Version:

We, and all the materials around us, are made from particles, or at least, things that behave like "particles" --- little independent moving objects whose size is too small to measure. At a first pass, all material is made from atoms. Atom means "indivisible" in Greek; and if that were the end of the story, things would have been easy. But there would have been no chemistry, no radioactivity, indeed no sun, if atoms were not themselves built from smaller things.

And if we want to explain how chemical reactions work and how the sun shines, we have to look more closely. When we do, we find the world is much, much more elaborate than we once thought, with a whole menagerie of particles.

As the covers of textbooks remind us, atoms are in fact are made from electrons and atomic nuclei.   The nuclei are made of protons and neutrons; the protons and neutrons from up quarks and down quarks.  To a great extent, that's it for standard material: electrons, up quarks and down quarks.  But that's not all.  The sun turns matter into light (and other forms of electromagnetic radiation) --- light is also made from particles! called photons --- and the sun also makes neutrinos, which are invisible but pass through our bodies in huge numbers.  Meanwhile there are other, rarer objects created when high energy protons hit atoms at the top of our earth's atmosphere.  Some of these are called "muons" --- one passes through your body every second, occasionally causing genetic damage.  Others contain yet another type of quark, dubbed "strange" (it's a long story.) Strange quarks may play a central role in the properties of supernovas, huge explosions of an entire star.  In these explosions, all the silver, gold, uranium, cobalt and other heavy atoms found in the universe were forged.  Muons and strange quarks are rare, and quickly disintegrate into more common particles through "radioactivity", but clearly they play an important role in the universe and on earth!

But where did all these particles come from?  Why do they have such different properties from one another?  Is there some organization underlying them?

Out of this chaotic array of particles and phenomena was born the field of particle physics.  Through the study of these particles and their properties, we have indeed found an underlying order, and techniques for studying it. We now know that there are six types of quarks, three electron-like particles, and three neutrinos, along with one photon, three photon-like heavy particles which cause radioactivity, and eight photon-like particles which hold quarks together inside of a proton or a neutron.  With the mathematics we have now developed, we can predict the forces between the various particles with astonishing accuracy.  But we still don't understand why some of the particles are light and some are heavy.  And we don't know where the order that we've found comes from.  Perhaps most important, we also know that gravity should fit in with the picture somehow --- and we're still totally confused as to how to bring it in properly.

So there you have it. And yes, I study these questions for a living.

The Short Version:

What do we know about nature?  At first sight it appears terribly capricious --- and some aspects of it, such as the weather, or turbulence, or human behavior, still seem that way --- but remarkably it turns out to be much more predictable than we might have expected.  There are some signs of this in ordinary experience; the sun rises every day; a pendulum clock keeps reliable time; rocks fall down, never up; etc.  Well, if some aspects of the world show signs of reliability, it's a natural human urge to want to learn which parts of the world are predictable, how to predict them, and how to do so more and more precisely.  From this curiosity was born "natural philosophy," a branch of which became what we now call "physics."

"Physics" is the general study of the behavior of objects without minds of their own.  As such it is enormously broad, and individual physicists have to specialize rather dramatically.  In my own case, I am interested in the smallest known objects.  Why?  Well, large objects are made from small objects, and smaller objects from still tinier ones; and it seems unlikely that a full understanding of any object at all can be achieved if nothing is known about its substructure.  Now, one should take the previous sentence with a lot of skepticism; as we have learned in particle physics itself, and in other branches of physics, there are huge loopholes in such a philosophy.  But often it is true; a full understanding of biology requires chemistry, which requires atomic physics, which requires an understanding of subatomic particles.  Remarkably, the early universe also cannot be understood without knowing a great deal about subatomic particles.  It is these subatomic particles which my colleagues and I study; we want to know more about them and to develop mathematics which allows us to predict their behavior.

The last century has seen an astonishingly rapid growth in our ability to predict.  The development of new technologies allowed many new experiments to be done; these led to surprising new mathematical developments such as quantum mechanics, which in turn revolutionized our conceptual thinking.  Compressing this history into a sentence: we now have a mathematical framework, called rather unpoetically the "Standard Model of Particle Physics", which we can use to predict the properties of all the known classes of subatomic particles.  The predictive power of this mathematical machinery --- a "quantum field theory" --- is spectacular. The behavior of an electron near a magnet, for example, is calculated, through turgid and tedious methods, to one part in a million million; it has also been measured, by an entirely independent group of people doing a tremendously difficult experiment, to one part in a million million; and astonishingly the theory and the experiment agree.  While this is an exceptional case, the theory succeeds in predicting experimental results again and again.  Maybe we are done?  Maybe the theory, and our predictive power, is complete?

It can't be.  The Standard Model has a basic mathematical inconsistency hidden in it (as well as a number of seemingly arbitrary features) and so at some point it must show a disagreement with experiment.  We are confident that it will happen in the next decade --- because by 2015, experiments will be done for which the theory predicts nonsense.  The results of those experiments will give us hints as to how to fix the theory.  The disagreement with experiment could show up earlier, but it hasn't happened yet.

There's another big problem, also: the Standard Model does not treat gravity consistently.  For one thing, it predicts that the universe should never have been able to expand to anything near its present immense size.  These are huge flaws, and we know we need a theory which is qualitatively different from the Standard Model to solve them.  String theory is the first mathematically-consistent approach to solving this problem (there may be another -- loop quantum gravity -- but its viability is not clear yet.)  In this theory, the subatomic particles are all little bits of string, too short for us to have yet seen that they are more than particles.  It is precisely the stringiness, the fact that the particles are now smoothed out a bit, which solves some (though as yet not all) of the profound problems of putting together gravity and the Standard Model of particle physics.

So --- I mainly do three classes of things.  I try to use the Standard Model, and variants of it, to predict what my friends will see in their experiments, or in their telescopes.  I try to fix the Standard Model's inconsistencies and explain the parts of it which seem, at the moment, arbitrary.  And I try to understand more thoroughly the underlying mathematical structure of quantum field theories and of string theory.

My Research --- for undergraduate and graduate physics students interested in my work:

    Here is a sample --- by no means complete --- of topics among my research papers:

       Hidden Valleys:  In addition to the particles and forces that we know so far, it is very likely that there are other particles and forces yet to discover. Some of those new particles might not interact with ordinary matter via any of the known forces except gravity. Even if these particles are very light, we would not have observed them yet. So new sectors of lightweight particles --- which may interact with each other via unknown forces --- are very possibly out there waiting for us to discover, like a hidden valley that lies secluded behind a mountain ridge. Hidden valley sectors of particles and forces, under the right circumstances, can produce unusual signatures in particle physics experiments, providing new and (until recently) little-studied opportunities and challenges at hadron colliders such as the Tevatron and the LHC. The dark matter of the universe may well be a particle from a hidden valley.

       Top Quark Production:  Long before the top quark was actually discovered, physicists were confident that it would be found.  With M. Peskin, I calculated the rate at which top quarks would be produced in collisions of electrons and positrons.  The goal was to understand whether the measurement of this physical process would lead to better constraints on important parameters in the standard model: the top quark's mass and decay rate, its coupling to the Higgs boson, and the Higgs boson mass.  All of these parameters are predicted or constrained in the standard model, and if they violate those predictions or constraints they could serve as windows into new physical phenomena.  As it turns out, however, the appropriate electron-positron collider has been delayed by decades, and many of these parameters will be measured already at the Large Hadron Collider.   I also carried out work, to calculate production at Fermilab of one top quark at a time.  This was one of the most important and challenging measurements at Fermilab that was carried out over the past few years.   And right now my focus is on understanding why top quarks, in proton-antiproton collisions at Fermilab, are produced more often going in the direction roughly of the proton (with the top antiquark going in the direction of the antiproton.)   This observed ``asymmetry'' is several times larger than predicted in the Standard Model.   Why?   Is it an experimental mismeasurement, an incomplete theoretical calculation, or a new discovery?   Time (and hard work) will tell.

        Duality in Quantum Field Theory: Quantum mechanics never ceases to amaze.  It is often the case the theories which classically appear to be completely different from one another are exactly the same when treated quantum mechanically.   Another way to say this is that quantum theories often have more than one classical limit.  Sometimes a problem formulated in one classical limit looks very complicated, but when restated in terms of a second classical limit, it becomes easy!  We've seen this before: for example, it is very difficult to understand how protons and neutrons interact by looking at them as complicated strongly-interacting clusters of quarks and gluons.  However, it is much easier to do so using the so-called "chiral Lagrangian", where we understand the protons and neutrons as interacting weakly by exchanging pions.  What we have discovered in the last decade is that such "duality transformations" between one classical limit and another are much more widespread than previously realized.  It is even possible that the particles we know and love --- quarks, photons, muons, --- though simple at the energy range of present experiments, may turn out to be very complicated objects at higher energy scales.  If we are lucky, a duality transformation may allow us to re-express these complications in a simple way.  I have worked extensively finding examples of and trying to more completely understand these transformations.

        Stringy Representations of Quantum Field Theory Physics:  Even more astounding, some quantum field theories in 3+1 dimensions also have a classical limit in which they look like string theory in 9+1 dimensions!! This means that many well-known phenomena in gauge theories, such as confinement, spontaneous symmetry breaking, and duality transformations can themselves be represented in string theory.  I have worked on two papers, one with Joe Polchinski and another with Igor Klebanov, which demonstrated these effects, in fully 3+1 dimensional contexts, for the first time.  Additional papers with Polchinski and with my students have explored the detailed properties of the bound states of quarks and gluons that arise in these theories.

        Models of Fermion Masses: One of the great puzzles of the standard model is that the fermions (electrons, muons, taus, their neutrinos, and the six quarks --- up, down, strange, charm, bottom and top) have a wide variety of masses, with few discernible patterns.  Where do these masses come from?  One of the ideas that I have explored is the possibility that quantum mechanical effects, which can violate our usual intuition for dimensional analysis, can take a naive pattern for fermion masses and distort it in a complex way.  Ann Nelson and I showed that the distortion which results typically leads to a pattern of fermion masses qualitatively similar to that seen in nature.  It is difficult to verify this idea in the near future, but we showed that certain variants of our idea lead to predictions which could be tested in the next decade.

       Nonperturbative Properties of Nonsupersymmetric Theories: Although supersymmetric theories have become much better understood over the last decade, because of new techniques which are now available, such as the dualities mentioned above, nonsupersymmetric theories have remained largely mysterious.  This is most unfortunate, since all so-far discovered particle physics, and all condensed matter physics, is nonsupersymmetric.  Our one nonperturbative theory in particle physics --- quantum chromodynamics --- remains largely mysterious, as only a few calculations can be done reliably.  Under these circumstances, any progress in this area would be welcome.  I have written a couple of papers recently which attempt to extract some predictions for certain interesting classes of nonsupersymmetric gauge theories. 

Matt Strassler

July 18, 2011