Fun with QCD
Jiunn-Wei Chen, National Taiwan U.
Credit:
CERN Courier Credit: CERN Courier
An Ultimate Question in Science
An Ultimate Question in Science
Life = Physical Laws ?
An Ultimate Question in Science
Life = Physical Laws ?
Or more specifically,
Life = known Physical Laws?
An Ultimate Question in Science
Life = Physical Laws ?
Or more specifically,
Life = known Physical Laws?
The answer has profound implications in science, philosophy and even theology.
An Ultimate Question in Science
Life = Physical Laws ?
Or more specifically,
Life = known Physical Laws?
The answer has profound implications in science, philosophy and even theology.
A computational problem!
An Ultimate Question in Science
Life = Physical Laws ?
Or more specifically,
Life = known Physical Laws?
The answer has profound implications in science, philosophy and even theology.
A computational problem!
• Condensed matter
• Nuclear physics, large number of nucleons in a nucleus
Complexity: superfluidity from nucleon
pairing; complicated spectra, e.g. the Hoyle state
Emergence of complexity from
simple rules
An excited state of C12 predicted by Hoyle with energy close to 3 alpha threshold so C12 (and
life) can be formed
Hoyle State
• QCD (Quantum Chromodynamics)
While the theory, QCD,
is very simple!
• Interaction is strong at long distance (>1fm).
Non-perturbative
• Systematic methods:
Lattice QCD
Effective Field Theory (EFT) (Wilson ‘71) …
• Even the structure of proton is complicated already…
Computations in QCD is
challenging
Feynman’s Parton Model
The momentum distributions of partons (quarks, antiquarks and gluons) become one dimensional distributions in the infinite
momentum frame.
Measuring Parton Distributions Using DIS experiments
Credit:T.Ichihara
x: momentum fraction
Current Status of Proton PDFs
How do momentum and spin distribute among partons?
• Exp: 1d mom. dist. largely mapped out (up to parameterizations of the functional forms);
largest sys. uncertainty in Higgs production.
improve 1d(spin)+3d: BNL, JLab, J-PARC, COMPASS, GSI, EIC, LHeC, ...
• Theory: Only first few moments could be
computed directly from QCD until recent years
Negele’s colloquium @ Maryland
Detmold, Melnitchouk, Negele, Renner, Thomas Phys. Rev. Lett. 87 (2001) 172001
Meson Cloud Model
A Eureka Moment
--- I can do it by
Chiral Perturbation theory!
Chiral Perturbation Theory: an Effective Field Theory of QCD
• QCD with three light flavors: “a theoretical paradise” (Leutwyler)
• Exhibits spontaneous and explicit chiral symmetry breaking
• Can be analyzed systematically in quark mass and momentum double expansions (Weinberg (1979) Gasser, Leutwyler (1984,1985))
• A model independent approach
The floodgate was open
JWC, Ji, Phys. Lett. B523 (2001) 107
Phys.Rev.Lett. 87 (2001) 152002 Phys.Rev.Lett. 88 (2002) 052003 JWC, Stewart, Phys.Rev.Lett. 92 (2004) 202001
Got a call from MIT…
2004 “Effective Summer” at Berkeley Lab:
do it for nuclear systems
EMC effect (‘83): nuclear modification
of the nucleon parton distributions
Jerry Miller:
EMC =
Everyone’s Model is Cool
Using ChPT again to nuclear systems
Credit: U-G Meissner
Hoyle State Obtained
Epelbaum, Krebs, Lee, Meißner, Phys. Rev. Lett. 106, 192501 (2011)
Factorization
implies symmetries!
Using large Nc counting
1-body op. 2-body op. determined by deuteron
EFT predicts :
EFT predicts :
Very Satisfying!
• Finished in one week and gave a talk while at LBL
• Had a hard time with PRL… JWC, Detmold, Phys. Lett. B625 (2005) 165
• Nobody cared..., until 2014
A New Twist
--- x > 1 results learned in Adelaide, 2014
Weinstein et al., Phys. Rev. Lett.106, 052301 (2011) A highlight in 2015 US NSAC Long Range Plan
Another Eureka Moment!
Indept of scheme
& scale!
a
2: scheme and scale independent
• EMC-SRC linear relation reproduced
• Some a2 reproduced ab initioly
• Remaining problem: EMC slope from LQCD (only need deuteron)
Summary and Outlook
Credit: CERN Courier
Outlook
• Applications: ν-A scattering for long baseline exp., MiniBooNe, NuTeV
• LQCD might get the EMC-SRC slope in 5 years to complete the picture
• 3D imagining of nuclear PDFs
