Testing the running of the QCD coupling in particle energy spectra

Abstract

The spectra of charged particles in the energy variable $\xi = \log 1/x$ (``the hump backed plateau'') are analyzed in terms of moments in order to investigate the sensitivity to the running of $\alpha_s$ in particular towards small scales, and to further test thereby the local parton hadron duality picture. The modified leading log approximation (MLLA) of QCD provides a very satisfying description of the $e^+e^-$ data in the full range of c.m. energies from about 1 up to 91 GeV. The same calculation but with fixed coupling $\alpha_s$ yields moments which are inconsistent with the data already close to threshold. In particular the soft particles ($E\le 1$~GeV) reflect directly the running of $\alpha_s$.Comment: 14 pages including 3 figures, uuencoded gz-compressed PS-fil

Low and high energy limits of particle spectra in QCD jets

“…The perturbative calculations at low scales with running coupling seem to simulate the production and decay of resonances implemented in Monte Carlo programs like JETSET. This supports the idea that the parton cascade with running coupling down to small scales is dual to the cascade with a shorter perturbative phase but with hadronic resonances in the last stage [18].…”

“…(3) contains the same data as Fig. (2), but the theoretical predictions refer now to the MLLA with fixed α s [18]. The curves correspond to three different values of the coupling α s , i.e., of the anomalous dimension γ 0 .…”

“…Here the inclusive charged particle ξ-spectra with different mass assignments, namely Q 0 = 0, Q 0 = m π and Q 0 = 270 MeV, are shown. The latter value is suggested from the moment analysis [18], see below. The upper limits of ξ corresponding to the above effective masses Q 0 are also shown in the figures.…”

“…If this mass is taken equal to the cutoff Q 0 , then for partons and charged particles there is the same upper limit ξ E = Y . The hadron spectra in the jet from the primary parton A are then calculated from the parton distribution according to the LPHD hypothesis from [22,18] …”

Angular multiplicity fluctuations in hadronic Z decays and comparison to QCD models and analytical calculations

“…For our comparison of the data with the theoretical calculations quoted above, we use the following parameters: n f = 3 ; = 0 : 16 GeV: This value of n f is chosen since even at high energies the production of heavy avors will rarely happen in the jet and consequently its evolution is still dominated by the light a v ors [17]. The value of chosen is that found in tuning the jetset 7.4 matrix element program [18] on l3 data [19] and in our recent determination of s (m Z ) [20].…”