S. Albino

Resummation of soft gluon logarithms in the DGLAP evolution of fragmentation functions

We define a general scheme for the evolution of fragmentation functions which resums both soft gluon logarithms and mass singularities in a consistent manner and to any order, and requires no additional theoretical assumptions. Using the double logarithmic approximation and the known perturbative results for the splitting functions, we present our scheme with the complete contribution from the double logarithms, being the largest soft gluon logarithms. We show that the resulting approximation is more complete than the modified leading logarithm approximation even with the fixed order contribution calculated to leading order only, and find, after using it to fit quark and gluon fragmentation functions to experimental data, that this approximation in our scheme gives a good description of the data from the largest x{sub p} values to the peak region in {xi}=ln(1/x{sub p}), in contrast to other approximations. In addition, we develop a treatment of hadron mass effects which gives additional improvements at large {xi}.

Strong coupling constant from the photon structure function

We extract the value of the strong coupling constant alpha(s) from a single-parameter pointlike fit to the photon structure function F(gamma)(2) at large x and Q(2) and from a first five-parameter full (pointlike and hadronic) fit to the complete F(gamma)(2)data set taken at PETRA, TRISTAN, and LEP. In next-to-leading order and the MS renormalization and factorization schemes, we obtain alpha(s)(m(Z))=0.1183+/-0.0050(expt)(+0.0029)(-0.0028)(theor) (pointlike) and alpha(s)(m(Z))=0.1198+/-0.0028(expt)(+0.0034)(-0.0046)(theor) (pointlike and hadronic). We demonstrate that the data taken at LEP have reduced the experimental error by about a factor of 2, so that a competitive determination of alpha(s) from F(gamma)(2) is now possible.

Generalizing the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi Evolution of Fragmentation Functions to the Smallest x Values

An approach which unifies the Double Logarithmic Approximation at small x and the leading order DGLAP evolution of fragmentation functions at large x is presented. This approach reproduces exactly the Modified Leading Logarithm Approximation, but is more complete due to the degrees of freedom given to the quark sector and the inclusion of the fixed order terms. We find that data from the largest x values to the peak region can be better fitted than with other approaches.

Timelike Single-logarithm-resummed Splitting Functions

We calculate the single logarithmic contributions to the quark singlet and gluon matrix of timelike splitting functions at all orders in the modified minimal-subtraction (MS¯) scheme. We fix two of the degrees of freedom of this matrix from the analogous results in the massive-gluon regularization scheme by using the relation between that scheme and the MS¯ scheme. We determine this scheme transformation from the double logarithmic contributions to the timelike splitting functions and the coefficient functions of inclusive particle production in e+e− annihilation now available in both schemes. This approach fixes two of the four degrees of freedom, and a third degree of freedom is fixed by reasonable physical assumptions. The results agree with the fixed-order results at next-to-next-to-leading order in the literature.

Confronting fragmentation function universality with single hadron inclusive production at HERA and e{sup +}e{sup -} colliders

Predictions for light charged hadron production data in the current fragmentation region of deeply inelastic scattering from the H1 and ZEUS experiments are calculated using perturbative quantum chromodynamics at next-to-leading order, and using fragmentation functions obtained by fitting to similar data from e{sup +}e{sup -} reactions. General good agreement is found when the magnitude Q{sup 2} of the hard photons virtuality is sufficiently large. The discrepancy at low Q and small scaled momentum x{sub p} is reduced by incorporating mass effects of the detected hadron. By performing quark tagging, the contributions to the overall fragmentation from the various quark flavours in the ep reactions are studied and compared to the contributions in e{sup +}e{sup -} reactions. The yields of the various hadron species are also calculated.

Fully double-logarithm-resummed cross sections

Abstract We calculate the complete double logarithmic contribution to cross sections for semi-inclusive hadron production in the modified minimal-subtraction ( MS ¯ ) scheme by applying dimensional regularization to the double logarithm approximation. The full double logarithmic contribution to the coefficient functions for inclusive hadron production in e + e − annihilation is obtained in this scheme for the first time. Our result agrees with all fixed order results in the literature, which extend to next-to-next-to-leading order.

Factorization Breaking in High-Transverse-Momentum Charged-Hadron Production at the Tevatron?

We compare the transverse-momentum (pT) distribution of inclusive light-charged-particle production measured by the CDF Collaboration at the Fermilab Tevatron with the theoretical prediction evaluated at next-to-leading order in quantum chromodynamics using fragmentation functions recently determined through a global data fit. While in the lower pT range the data agree with the prediction within the theoretical error or slightly undershoot it, they significantly exceed it in the upper pT range, by several orders of magnitude at the largest values of pT, potentially challenging the factorization theorem.

Medium-modified DGLAP evolution of fragmentation functions from large to small x

Abstract The unified description of fragmentation function evolution from large to small x which was developed for the vacuum in previous publications is now generalized to the medium, and is studied for the case in which the complete contribution from the largest class of soft gluon logarithms, the double logarithms, are accounted for and with the fixed order part calculated to leading order. In this approach it proves possible to choose the remaining degrees of freedom related to the medium such that the distribution of produced hadrons is suppressed at large momenta while the production of soft radiation-induced charged hadrons at small momenta is enhanced, in agreement with experiment. Just as for the vacuum, our approach does not require further assumptions concerning fragmentation and is more complete than previous computations of evolution in the medium.

Constraints on SUSY Seesaw Parameters from Leptogenesis and Lepton Flavor Violation

We study constraints on the fundamental parameters of super ymmetric type I seesaw models imposed by neutrino data, charged lepton flavor viola tion, thermal leptogenesis and perturbativity of the neutrino Yukawa couplings.We study constraints on the supersymmetric type I seesaw model imposed by neutrino data, charged lepton flavor violation and thermal leptogenesis. It is shown that the three heavy neutrino masses as well as the complex neutrino Yukawa coupling matrix can be constrained.

SOFT GLUON LOGARITHMIC RESUMMATION AND HADRON MASS EFFECTS IN SINGLE HADRON INCLUSIVE PRODUCTION

We define a general scheme for the evolution of fragmentation functions which resums soft gluon logarithms in a manner consistent with fixed order evolution. We present an explicit example of our approach in which double logarithms are resummed using the Double Logarithmic Approximation. We show that this scheme reproduces the Modified Leading Logarithm Approximation in certain limits, and find that after using it to fit quark and gluon fragmentation functions to experimental data,a good description of the data from the largest x values to the peak region in ln (1/x) is obtained. In addition, we develop a treatment of hadron mass effects which gives additional improvements at small x.