Surprises of strongly interacting particles Seok Kim Seoul

Surprises of strongly interacting particles Seok Kim (Seoul National University) POSTECH colloquium 12 Oct 2016

Triumph of particle physics The standard model works: Explains the microscopic world with elementary particles. 2

The subtlety Very difficult when particles strongly interact, such as in QCD. The successes, complications, and even failures of particle physics are features of quantum field theory, the basic formulation of (relativistic) quantum particles. Curious aspects come from the fact that “particles” are derived concepts, not built-in. Today, I’ll discuss the far-reaching implications of the emergent nature of particles. 3

Particles in quantum mechanics Particles vs. Waves? electron diffraction: matter waves photoelectric effect: photons We were forced to accept a “duality” of two notions But, more precisely, the fact is: We are only given waves. Particles are emergent. This will be one of my key messages. So let me explain it in some detail. 4

“Quantum” is derived in quantum mechanics • 5

Particles are also derived • new quantum fluctuations can change particle number 6

Field 마당 場 7

Forces are fields “Fields” were used physics, from many centuries ago, to describe forces. Newton Einstein 8

Matters are also fields Matter fields: wave-functions Schrodinger eqn. Dirac eqn. Unification: matters & forces ~ fields (waves) Quantizing fields ~ waves yields particles as solutions. Particles/fields mean much more than “elementary particles. ” (E. g. vibration of solid lattice yields “phonon” particles. ) QFT is the universal framework of ALL many-body systems, having notion of waves at long distance. Particles are solutions of quantized fields. But only in certain situations, not always. 9

Extreme conditions • 10

Extreme conditions: Yang-Mills theory & QCD It is a problem related to QCD. Gluons give very strong force. Basic fields (gluons, quarks) are no longer particles themselves. They give way to bound states at a new mass scale ~ a few 100 Me. V: “mass gap generation” (Find analytic understanding & earn 1 M dollars…!) Where is your mass? gluonic force binds quarks into a proton I don’t know… Wolfgang Pauli Chen Ning Yang However, the basic concept of particles is still present. It is simply that the elementary fields aren’t directly particles in any sense.

Extreme conditions: topological fields Example: Chern-Simons QFT in d=2+1. Trivial, no particles…! Describes certain gapped systems, e. g. quantum hall system. Even if “trivial” in a sense, this QFT describes very interesting physics. - Gapless boundary matters are forced to exist, from d=2+1 gauge symmetry - Partition functions of heavy external particles w/ non-Abelian statistics (“braid” “knot invariant”) Edward Witten In a sense trivialized the system, still finding there are interesting physics. 12

Beyond what we see From now on, I’ll tell you about what we cannot see (so far). They are “strings” in string theory. Nobody can say they see them nowadays. I mention it since string theory highlights our ignorance on QFT & provides insights. Strings in 21 st century play roles similar to atoms in 19 th century - Some physicists used the concept of “atoms” to understand physical phenomena. Much before we could see them. - Concrete evidences for atoms came later (e. g. Brownian motion) - However, pioneers used atoms not because they saw it, but since it solved their problems. - invisible but useful: I think it is an occasional feature of great physics while it is developing. 13

• String theory 14

Quantum fields for strings Some QFT’s are exactly same as strings in certain curved background. [Maldacena] (1997) idea: holography (‘t Hooft, Susskind) n dim. QFT = n+1 dimensional gravity in “anti de Sitter (Ad. S)” spacetime Precisely in the regime without particles. Very delicate rules to see gravity & strings: makes a closed string Strongly interacting particles can disappear to yield strings. 15

Quantum fields for strings • 5 -branes open membrane 16

“Applications” or insights from string theory A key reason for studying string theory is still as the fundamental law of our Nature, including quantum gravity. It is still a very hard journey. But string theory also provides crucial insights to strongly interacting QFTs. Guidance to d=2+1 Chern-Simons + matter QFTs, from holographic duals - Exact solutions of certain large N theories [Minwalla et. al. ] (2011 - ) - Discovery of new non-perturbative operators (monopole operators) [SK et. al. ] (2009 - ) - These QFT’s, or their variations, may be useful for studying table-top materials New insights on d=3+1 QFTs from higher dimensional QFT’s or holography - dualities in QFT: compactification of higher dimensional QFTs [Witten] (1995) - strong-coupling phenomena like confinement, mass gap generation, etc. from holography Holographic toy models of QCD-like or condensed-matter systems 17

Conclusions • My talk started with particles, but will conclude with strings. • Lack of traditional QFT tools: drastic examples and clues from string theory. • We are steadily re-interpreting 20 th century physics in 21 st century language. • What will string theory be like in the future? Currently provides insights to QFT. Concrete insights/guidance to particle physics and condensed matters physics? Ernst Mach Ludwig Boltzmann Assuming the existence of strings, one atoms, canone deduce can derive many properties many of properties quantum(e. g. gravity thermodynamics) and QFT. of gases. I don’t believe that atoms strings exist. 18