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## PowerPoint Slideshow about 'What is QCD' - gittel

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What is QCD?

- Quantum ChromoDynamics is the theory of the strong force
- the strong force describes the binding of quarks by gluons to make particles such as neutrons and protons
- The strong force is one of the four fundamental forces in the Standard Model of Physics– the others are:
- Gravity
- Electromagnetism
- The Weak force
- The 2004 Physics Nobel prize was awarded to David J. Gross, H. David Politzer, and Frank Wilczekfor their work leading to QCD

What is Lattice QCD?

- LatticeQCD is the numerical simulation of QCD
- The QCD action, which expresses the strong interaction between quarks mediated by gluons:where the Dirac operator (“dslash”) is given by
- LatticeQCD uses discretized space and time
- A very simple discretized form of the Dirac operator iswhere a is the lattice spacing

A quark, (x), depends upon (x + a) and the local gluon fields U

- (x)is complex 3x1 vector, and the Uare complex 3x3 matrices. Interactions are computed via matrix algebra
- On a supercomputer, the space-time lattice is distributed across all of the nodes

Why do Lattice QCD?

- Many QCD problems can only be simulated numerically, using Lattice QCD
- To test the Standard Model, physicists take experimental data and compare to the theoretical predictions of QCD
- Differences may indicate new physics, for example, explaining why the universe is dominated by matter and has so little anti-matter

Testing the Standard Model

- and are two parameters of the Standard Model
- Theory and experiments constrain the possible values of and
- The regions of different colors correspond to different constraints
- The allowed values of and from the Standard Model would lie in the intersection of all of the colored regions
- This figure shows the current constraints

Testing the Standard Model

- New B-factory experiments, such as BaBar at SLAC and Belle at KEK, will reduce the experimental errors, shrinking the areas of each region

Testing the Standard Model

- Even tighter constraints will result from better theoretical calculations from LatticeQCD
- If the regions don’t overlap, there must be new physics!

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