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## Particle Theory in the 21 st Century

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**Particle Theory in the 21st Century**Andreas Karch**The ultimate quest**What are the rules that govern the world at the smallest scales? Why does the universe look the way it looks like at the largest scales? The Theory of Everything Everything else is details; and its messy and complicated (like life).**The Theory of Everything:**The standard model Lagrangian (as of July 3rd, 2012).**The Theory of Everything:**Prediction: Probability to interact two Ws with TeV scale energy > 1 ??????? Solution: Higgs! Is it there?**The Higgs! It’s real!**(or at least something pretty close to it.)**The Theory of Everything:**Problem: Hard to calculate! Solution: Lattice (no real time, no finite density). String Theory via holography**The Theory of Everything:**Problem: That’s it? Aren’t we missing something?!?**Particle Physics in the 21st century:**• Open Problems within the standard model: QCD difficult to calculate. Non-perturbative. Lattice works (sometimes). Sometimes perturbation theory. • Problems beyond SM: We know there is BSM (beyond the standard model) physics: Neutrino masses, Dark Matter, Dark Energy, GRAVITY.**BSM Physics**What physics could be hiding “around the corner”? What novel experimental signatures would we be looking for? Beware the nightmare scenario: “Just the Higgs” Fortunately hints of non-Higgsness. Are they real?**QCD Physics**What happens when baryons melt? Strongly coupled soup of quarks and gluons. What are its properties? We can realize it experimentally (heavy-ion collisions). But can we cacluate? This is the stuff the early universe was made from!**Applied String Theory“Holography”**(and a little applied field theory)**Holography = Solvable Toy Model**Solvable models of strong coupling dynamics. • Study Transport, real time • Study Finite Density • Explore paradigms “beyond Landau” (Challenging in real QCD, experimentally relevant) (Non-Fermi Liquids? Phase Transitions? High Tc superconductors? Topological Insulators?) Gives us qualitative guidance/intuition. Not QCD! Expect errors of order 100% better than extrapolating perturbation theory to αs ~ 1**Challenge for Computers:**We do have methods for strong coupling: e.g. Lattice QCD But: typically relies on importance sampling. weighting in Euclidean path integral. Monte-Carlo techniques. FAILS FOR DYNAMIC PROCESSES OR AT FINITE DENSITY (sign problem)**Holographic Toy models.**Can we at least get a qualitative understanding of how dynamics look like at strong coupling?**Holographic Toy models.**Can we at least get a qualitative understanding of how dynamics looks like at strong coupling?**Holographic Theories:**Holographic toy models have two key properties: “Large N”: theory is essentially classical large separation of scales in the spectrum “Large λ”: mspin-2-meson ~ λ1/4 mspin-1-meson QCD: 1275 MeV 775 MeV**Successes and recent developments**• Viscosity and Hydrodynamics • Energy Loss • Thermalization**Shear Viscosity**Viscosity = Diffusion constant for momentum v Viscosity = [(force/area)] per unit velocity gradient**Viscosity in Heavy Ions.**Au Au low pressure How does the almond shaped fluid expand? high pressure**(Shear) Viscosity η**(1 cp= 10−2p = 10−3Pa·s) Force per unit area per velocity gradient**Measuring Viscosity - an example**(2.3 1011cp)**Measuring Viscosity - an example**Recall: Viscosity of pitch: ~ 2.3 1011cp**Measuring Viscosity - an example**Recall: Viscosity of pitch: ~ 2.3 1011cp RHIC’s measurement of hot QCD (= quark gluon plasma) (from colliding high energy gold nuclei)**Measuring Viscosity - an example**Recall: Viscosity of pitch: ~ 2.3 1011cp RHIC’s measurement of hot QCD (= quark gluon plasma) (from colliding high energy gold nuclei)**Viscosity in Holography:**(KSS; Kovtun - UW grad student; Son – UW faculty, Starinets – UW postdoc) • pinpoints correct observable • gives ball-park figure • large at weak coupling – extrapolation from weak coupling is order of magnitude off!**η/s**• Viscosity to entropy ratio: • close to 1/(4 π) in quark gluon plasma produced at RHIC --- strongly coupled! fluid, not plasma! • 2-3 times that in cold atomic gases • at least factor of 10 times 1/(4 π) in all other substances known to mankind (including superfluid helium, water, …) • 11 orders of magnitude larger that 1/(4 π) in pitch.**Jet quenching.**Sometimes HARD COLLISIONS produce non-thermal particles inside the fire ball = probe of the plasma. See one of two back-to-back created particles. The other one got “stuck” in the fireball Jet quenching is a direct indication of large drag forces on quarks..**Stopping Distance:**Quantify Energy loss in terms of Stopping Distance: How far does quark of energy E travel before it gets thermalized into the plasma? L ~ E1/2 Perturbative QCD or QED:**Energy Loss: Heavy quarks**Constant E - field v (Herzog, Karch, Kozcaz, Kovtun, Yaffe) (all UW: postdoc, faculty, student2,faculty)**Energy Loss: Heavy quarks**Constant E - field SOLVE CLASSICAL EQUATIONS OFMOTION! v (Herzog, Karch, Kozcaz, Kovtun, Yaffe) (all UW: postdoc, faculty, student2,faculty)**Energy Loss, Light Quarks**(Chesler, Jensen, Karch, Yaffe – again all UW)**Stopping Distance:**Perturbative QCD: L ~ E1/2 Holography: Maximal Stopping Distance: L ~ E1/3 others found: Typical Stopping Distance: L ~ E1/4 Experiment: 1/3 preferred over 1/2 ???**Thermalization**Why does the QCD fireball thermalize so rapidly?**Thermalization**Why does the QCD fireball thermalize so rapidly? too hard!**Thermalization**How quickly does the holographic fireball thermalize?**Shockwave-collision to black hole**(Chesler, Yaffe) Energy/area in shock ~ μ3**Shockwave-collision to black hole**(Chesler, Yaffe)**Shockwave-collision to black hole**(Chesler, Yaffe) “RHIC”: μ ~ 2.3 GeV Hydro valid ~ 0.35 fm/c << 1 fm/c But: there is so much more info in this plot! Lots to explore! Strong coupling, non-equilibrium.**HydrolizationvsThermalization**(Chesler, Teaney) Note: Hydro works when transverse and longitudinal pressure differ by a factor of 2. Hydrolization before Thermalization! Hydro works. No well defined temperature.**HydrolizationvsThermalization**(Chesler, Teaney) UV t=0 initial perturbation IR**HydrolizationvsThermalization**(Chesler, Teaney) Asymptotic metric settles to final state plus small peturbations. UV shock follows lightlike geodesic Hydrolization IR**HydrolizationvsThermalization**(Chesler, Teaney) UV Fluctuation Spectrum thermal.. shock reaches near horizon region Thermalization IR**Strong Coupling in CM.**The theory of everything: How hard can it be?**Strong Coupling in CM**Already Helium too difficult to solve analytically. electron/electron Coulomb repulsion not weak! if it is negligible, we have good theory control: Band structure! Insulators and conductors. but what to do when it is not?**Landau’s paradigms:**• Identify physical candidates for • low energy degrees of freedom. • Write down most general allowed interactions • See how interactions scale in low energy limit dominate transport many interactions “irrelevant” = scale to zero**What could they be?**1) weakly coupled fermions. Landau Fermi Liquid at low temperatures resistivity grows as T2 • Fermi Surface • Low energy excitations near • Fermi Surface • Only Cooper Pair Instability • survives at low energies, all • other interactions scale to zero universal!