Exclusive TwoPhoton Processes in QCD
Abstract
Hadron pair production from twophoton annihilation plays an important role in unraveling the perturbative and nonperturbative structure of QCD, first by testing the validity and empirical applicability of leadingtwist factorization theorems, second by verifying the structure of the underlying perturbative QCD subprocesses, and third, through measurements of angular distributions and ratios which are sensitive to the shape of the distribution amplitudes. In effect, photonphoton collisions provide a microscope for testing fundamental scaling laws of PQCD and for measuring distribution amplitudes. The determination of the shape and normalization of the distribution amplitude is particularly important in view of their importance in the analysis of exclusive semileptonic and twobody hadronic Bdecays. The data from the Belle and CLEO collaborations on single and double meson production are in excellent agreement with the QCD predictions. In contrast, the normalization of the nominal leadingorder predictions of PQCD for proton pair production appears to be significantly below recent Belle measurements. I also review issues relating to renormalization scale setting.
 Authors:
 Publication Date:
 Research Org.:
 Stanford Linear Accelerator Center (SLAC)
 Sponsoring Org.:
 USDOE
 OSTI Identifier:
 877525
 Report Number(s):
 SLACPUB11577
TRN: US0601549
 DOE Contract Number:
 AC0276SF00515
 Resource Type:
 Conference
 Resource Relation:
 Conference: Invited talk at Photon: Its First Hundred Years and the Future. Includes PHOTON2005 and PLC2005, Warsaw and Kazimierz, Poland, 30 Aug  8 Sep 2005
 Country of Publication:
 United States
 Language:
 English
 Subject:
 72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; 71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; 73 NUCLEAR PHYSICS AND RADIATION PHYSICS; AMPLITUDES; ANGULAR DISTRIBUTION; ANNIHILATION; FACTORIZATION; HADRONS; MESONS; MICROSCOPES; PAIR PRODUCTION; PHOTONPHOTON INTERACTIONS; PROTONS; QUANTUM CHROMODYNAMICS; RENORMALIZATION; SCALING LAWS; SHAPE; TESTING; PhenomenologyHEP,HEPPH, HEPTH
Citation Formats
Brodsky, Stanley J., and /SLAC. Exclusive TwoPhoton Processes in QCD. United States: N. p., 2005.
Web.
Brodsky, Stanley J., & /SLAC. Exclusive TwoPhoton Processes in QCD. United States.
Brodsky, Stanley J., and /SLAC. Fri .
"Exclusive TwoPhoton Processes in QCD". United States.
doi:. https://www.osti.gov/servlets/purl/877525.
@article{osti_877525,
title = {Exclusive TwoPhoton Processes in QCD},
author = {Brodsky, Stanley J. and /SLAC},
abstractNote = {Hadron pair production from twophoton annihilation plays an important role in unraveling the perturbative and nonperturbative structure of QCD, first by testing the validity and empirical applicability of leadingtwist factorization theorems, second by verifying the structure of the underlying perturbative QCD subprocesses, and third, through measurements of angular distributions and ratios which are sensitive to the shape of the distribution amplitudes. In effect, photonphoton collisions provide a microscope for testing fundamental scaling laws of PQCD and for measuring distribution amplitudes. The determination of the shape and normalization of the distribution amplitude is particularly important in view of their importance in the analysis of exclusive semileptonic and twobody hadronic Bdecays. The data from the Belle and CLEO collaborations on single and double meson production are in excellent agreement with the QCD predictions. In contrast, the normalization of the nominal leadingorder predictions of PQCD for proton pair production appears to be significantly below recent Belle measurements. I also review issues relating to renormalization scale setting.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Fri Dec 02 00:00:00 EST 2005},
month = {Fri Dec 02 00:00:00 EST 2005}
}

Exclusive twophoton processes at large momentum transfer, particularly Compton scattering {gamma}p{yields}{gamma}p and its crossedchannel reactions {gamma}{gamma}{yields}{bar p}p and {bar p}p{yields}{gamma}{gamma}, can provide definitive information on the boundstate distributions of quarks in hadrons at the amplitude level. Recent theoretical work has shown that QCD predictions based on the factorization of long and short distance physics are already applicable at momentum transfers of order of a few GeV.

Exclusive twophoton processes: Tests of QCD at the amplitude level
Exclusive twophoton processes at large momentum transfer, particularly Compton scattering {gamma}p{yields}{gamma}p and its crossedchannel reactions {gamma}{gamma}{yields}{bar p}p and {bar p}p{yields}{gamma}{gamma}, can provide definitive information on the boundstate distributions of quarks in hadrons at the amplitude level. Recent theoretical work has shown that QCD predictions based on the factorization of long and short distance physics are already applicable at momentum transfers of order of a few GeV. 
TwoPhoton Exclusive Processes in QCD
Exclusive twophoton reactions such as Compton scattering at large angles, deeply virtual Compton scattering, and hadron production in photonphoton collisions provide important tests of QCD at the amplitude level, particularly as measures of hadron distribution amplitudes and skewed parton distributions. 
Experimental review of exclusive processes in two photon reactions
Recent experimental results on exclusive final stated produced in photonphoton interactions are reviewed. Comparisons between experiments and with perturbative QCD calculations are made for meson and baryon pair production. New results on vector meson pair (rho/sup 0/rho/sup 0/,rho/sup 0/omega,rho/sup 0/phi,...) production and production of multiparticle (KK..pi pi..,K*K..pi..,...) final states are summarized. 34 refs. 
Twophoton exclusive processes in quantum chromodynamics
QCD predictions for ..gamma gamma.. annihilation into single mesons, meson pairs, and baryon pairs are reviewed. Twophoton exclusive processes provide the most sensitive and practical measure of the distribution amplitudes, and thus a critical confrontation between QCD and experiment. Both the angular distribution and virtual photon mass dependence of these amplitudes are sensitive to the shapes of the phi (chi, Q). Novel effects involving the production of qq anti q anti q states at threshold are also discussed, and a new method is presented for systematically incorporating higherorder QCD corrections in ..gamma gamma.. reactions.