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Register a new userDetermination of the strong coupling constant at LEP
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T. Wengler
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Multi-hadronic events produced in e+e- collisions provide an excellent laboratory to study QCD, the theory of strong interactions, and in particular to determine the strong coupling parameter alpha_s and demonstrate its predicted behavior as a function of the energy scale. Determinations of alpha_s at LEP will be reviewed with emphasis on event shape variables and jet rates in 3-jet and 4-jet events.
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The strong coupling constant is one of the fundamental parameters of the standard model of particle physics. In this review I will briefly summarise the theoretical framework, within which the strong coupling constant is defined and how it is connected to measurable observables. Then I will give an historical overview of its experimental determinations and discuss the current status and world average value. Among the many different techniques used to determine this coupling constant in the context of quantum chromodynamics, I will focus in particular on a number of measurements carried out at the Large Electron Positron Collider (LEP) and the Large Hadron Collider (LHC) at CERN.
The current status of measurements of the strong coupling constant from different reactions is reviewed. Including new results presented at the 1996 ICHEP conference, a global average alpha_s(Mz) = 0.118 +- 0.003 is obtained.
We calculate the vacuum polarization functions on the lattice using the overlap fermion formulation.By matching the lattice data at large momentum scales with the perturbative expansion supplemented by Operator Product Expansion (OPE), we extract the strong coupling constant $alpha_s(mu)$ in two-flavor QCD as $Lambda^{(2)}_{overline{MS}}$ = $0.234(9)(^{+16}_{- 0})$ GeV, where the errors are statistical and systematic, respectively. In addition, from the analysis of the difference between the vector and axial-vector channels, we obtain some of the four-quark condensates.
The strong coupling constant $alpha_s(M_Z)$ is determined from inclusive jet and dijet cross sections in neutral-current deep-inelastic $ep$ scattering (DIS) measured at HERA by the H1 collaboration using next-to-next-to-leading order (NNLO) QCD predictions. The dependence of the NNLO predictions and of the resulting value of $alpha_s(M_Z)$ at the $Z$-boson mass $m_Z$ are studied as a function of the choice of the renormalisation and factorisation scales. Using inclusive jet and dijet data together, the strong coupling constant is determined to be $alpha_s(M_Z)=0.1166,(19)_{rm exp},(24)_{rm th}$. Complementary, $alpha_s(M_Z)$ is determined together with parton distribution functions of the proton (PDFs) from jet and inclusive DIS data measured by the H1 experiment. The value $alpha_s(M_Z)=0.1147,(25)_{rm tot}$ obtained is consistent with the determination from jet data alone. The impact of the jet data on the PDFs is studied. The running of the strong coupling is tested at different values of the renormalisation scale and the results are found to be in agreement with expectations.
It is shown that $alpha_s(E)$, the strong coupling constant, can be determined in the non-perturbative regime from Bose-Einstein correlations (BEC). The obtained $alpha_s(E)$ is in agreement with the prescriptions dealt with in the Analytic Perturbative Theory approach. It also extrapolates smoothly to the standard perturbative $alpha_s(E)$ at higher energies. Our results indicate that BEC dimension can be considered as an alternative approach to the short range measure between hadrons.
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