The Many Facets of Quantum Chromo Dynamics · Quantum Chromo Dynamics Thomas Mannel Theoretische...

Peculiar properties of QCDSome of the facets of QCD

Summary: (Future) QCD Machines

The Many Facets ofQuantum Chromo Dynamics

Thomas Mannel

Theoretische Physik I

Universität Siegen

Public Lecture, Part II, Oct 8th, 2012

T. Mannel, Siegen University The many facets of QCD



Contents

1 Peculiar properties of QCD

2 Some of the facets of QCDQuarks become Jets: perturbative QCDQuarks become Hadrons: Nonperturbative QCDHot and Dense QCD

3 Summary: (Future) QCD Machines




Peculiar Properties of QCD




Peculiarities I: Running Coupling

For massless quarks:QCD does not have any intrinsic length scale(remember:[mass]=[Energy]=[Temperature]=[lenght]−1=[time]−1)This is in fact the same for Maxwell’s theory ofelectromagnetismConsider the potential between two static quarks(or static charges) at a distance r :for dimensional resons:

V (r) =α

rα : dimensionless constant




This is only true on Maxwell’s theory:Coulomb PotentialBy renormalization QCD generates a scale ΛQCD

(“Dimensional Transmutation” (Coleman))

ΛQCDr is dimensionless:

V (r) =1rα(ΛQCDr)

Concept of “running constants” in quantum theoriesUsually the fourier transformed is studied 1/r → Q




In almost all quantum field theories the couplingincreases as the distance decreases... except in a special class of field theories with

Asyptotic Freedom

α(ΛQCDr)→ 0 as r → 0α(ΛQCD/Q)→ 0 as Q →∞




Peculiarities II: The States in QCD

QCD is the theory of quarks and gluonsObservation: There are no free quarks (and gluons)QCD has the property of ConfinementThis is compatible with a growing coupling as r →∞Numerical Simulations of QCD (“Lattice QCD”)indicates a linear Potential between static quarks atlarge distances:

V (r) = σ · r σ : String Tension

There are many states which can be built out of3, 4 or 5 different types (“flavors”) of quarks




Buliding Principle for the hadron states

The color of quarks:The binding force is due to the “color charge”The force is mediated by “gluons”Allowed are only colorless states: ConfinementBound states of a quark and an antiquark: Mesons... and of three quarks: Baryons

The flavor of quarks

There are six flavors of quarks

Allows for a large “zoo” of hadrons

Except for top hadrons: The toplives too short to form hadrons




Lattice QCD predicts the masses of these states:




Quarks become Jets: perturbative QCDQuarks become Hadrons: Nonperturbative QCDHot and Dense QCD

Some of the Facets of QCD





Quarks become Jets: Perturbative QCD

At large momentum transfers = shot distancesApproximation by perturbation theoryUse of “Feynman Diagrams”,which also serve as an illustration:





Quarks are confined in hadrons:Must know the probability to find a quark in a hadron,when it is probed with large momentum transfer Q

Red Blob: Non-perturbative, usually input from ameasurement.





Qutgoing Quarks turn into “Jets”Jet = bundle of HadronsTotal momentum of the hadrons = momentum of theinitial quarkProbability that a quark turns into hadrons = 100%





Summary of perturbative QCD

QCD is well tested at short distances... even at the quantum levelNonperturbative input is well constrainedPrecision measurement and tests possible

αs(Mz) = 0.1184± 0.0007 (PDG 2012)





Processes of Hadrons

At long Distances the coupling becomes strong:Perturbation Theory becomes useless!What about “soft processes”= processes which are sensitive to long distances?Various possibilities:

Use quark modelsUse approximate symmetries of QCDEffective Field TheoriesUse analytic properties of Quantum Field Theories:QCD Sum rulesTackle non-perturbative QCD numerically:Lattice QCD simulations





Effective Field Theories

Usually Effective Theories replace the Quarks andGluons by the the degrees of freedom which are“relevant” at this scale.Effective Theories are systematic expansion of QCDHigh Energies (Q →∞):Quarks and Gluons are relevant→ perturbative QCD, Expansion in 1/QVery slow hadrons (Q → 0):Pions and Kaons are relevant→ approximate symmetries, Expansion in QHeavy Quarks (mQ →∞):Light Quarks and Gluons are relevant→ Use approximate symmetries, Expansion in 1/mQ





This machinery works very well:Detailed understanding of pion-pion scattering atsmall energiesDetailed understanding of pion-nucleon scattering atsmall energiesPrecision calculations for processes with heavyquarks

However:Predictions are often not in terms of αs and of ΛQCD

Predictions are often only relations between hadronicquantities (symmetries)





QCD Sum Rules

Make use of the fact that all quarks eventually have toturn into hadronsDuality between quarks and hadronsDual description:

There are ways to project out the lowest statesCan be used to make reliable QCD based estimates





This machinery works also very well:Estimates for static properties (masses ...)Estimates for decay amplitudes of particles... in particular for hadrons with heavy quarksPredictions in terms of QCD parameters

However:Limited precisionUncertainties are hard to estimate





Numerical Simulations: Lattice QCD

The only known nonperturbative approach from firstprinciplesUses a discrete space-time grid for numericalevaluations of QCDExtensive and expensive Computers needed

Forschungszentrum Jülich Rechenzentrum TU MünchenT. Mannel, Siegen University The many facets of QCD




This has become an almost separate field of research:Calculation of static properties (masses ...)Processes can be calculated, presently only forspecial kinematicsCan be combined with effective theories and alsowith perturbation theoryPredictions in terms of QCD parameters

However:Limited by computing resources





Hot and Dense QCD

High temperatures also correspond to high energiesDoes hadronic matter at high temperatures and highpressures become Quark-and-Gluon matter?Lattice simulations indicate a phase transition:





The Phase Diagram of QCD




Summary




SummaryQCD will remain one of the hottest topics

in particle and hadron physicsAs a complicated Quantum Field Theory it will remainan attractive research subject for theorists




QCD Machines 0: Hera at DESY

H1 ZEUSInner Structure of the Proton




QCD Machines I: LHC

ATLAS CMS LHCbProton Proton Collisions, Higgs Search (Discovery?)




Dedicated Heavy Ion experiment at LHC: ALICE

Hot and Dense QCD




QCD Machines II: GSI at Darmstadt




Heavy Ion Research, Nuclear Matter, Hot and Dense QCD




QCD Machines III: RHIC at Brookhaven

High Energy Heavy Ion CollisionsT. Mannel, Siegen University The many facets of QCD



The Future QCD Machine: FAIR at GSI




The future Workhorse for QCD Studies ...




The final Outlook

QCD is a very rich theory with many facets:It is an important part of what we call the “StandardModel pf Particle Physics”It has a rich phenomenology, such as the “chemistryof the various nuclei”It is important also for astrophysical objects:Neutron Stars, Dwarf Stars, our own sun ...

QCD will be with us for many more years, not only asa topic in fundamental research




The final Outlook

QCD is a very rich theory with many facets:It is an important part of what we call the “StandardModel pf Particle Physics”It has a rich phenomenology, such as the “chemistryof the various nuclei”It is important also for astrophysical objects:Neutron Stars, Dwarf Stars, our own sun ...

QCD will be with us for many more years, not only asa topic in fundamental research

Thank You


The Many Facets of Quantum Chromo Dynamics · Quantum Chromo Dynamics Thomas Mannel Theoretische...

Documents

Transcript of The Many Facets of Quantum Chromo Dynamics · Quantum Chromo Dynamics Thomas Mannel Theoretische...