Stefano Forte

DOUBLE ASYMPTOTIC SCALING AT HERA

Perturbative QCD predicts that at sufficiently largeQ 2 and small x nucleon structure functions should exhibit scaling in the two variables p ln 1 lnln Q 2 and q ln 1 x � lnln Q 2 , provided only that the small-x behaviour of the input to the perturbative QCD evolution is sufficiently soft. We derive these asymptotic results by writing the gluonic Altarelli–Parisi equation at small x as a two–dimensional wave equation, which propagates the gluon distribution from its boundaries into the asymptotic region. We then show that the existing experimental data on F p

Next-to-leading order determination of the singlet axial charge and the polarized gluon content of the nucleon

We perform a full next-to-leading analysis of the the available experimental data on the polarized structure function g1 of the nucleon, and give a precise determination of its singlet axial charge together with a thorough assessment of the theoretical uncertainties. We find that the data are now sufficient to separately determine first moments of the polarized quark and gluon distributions, and show in particular that the gluon contribution is large and positive.

A direct test of perturbative QCD at small x

Abstract We show that recent data from HERA on the proton structure function F 2 at small x and large Q 2 provide a direct confirmation of the double asymptotic scaling prediction of perturbative QCD. A linear rise of ln F 2 with the scaling variable σ is observed throughout the kinematic region probed at HERA, and the measured slope is in excellent agreement with the QCD prediction. This provides a direct determination of the leading coefficient of the beta function. At large values of the scaling variable ϱ the data display a small but statistically significant scaling violation.

SUMMATION OF LEADING LOGARITHMS AT SMALL x

We show how perturbation theory may be reorganized to give splitting functions which include order by order convergent sums of all leading logarithms of x. This gives a leading twist evolution equation for parton distributions which sums all leading logarithms of x and Q 2 , allowing stable perturbative evolution down to arbitrarily small values of x. Perturbative evolution then generates the double scaling rise of F2 observed ∞

PHYSICS AT THE FRONT-END OF A NEUTRINO FACTORY: A QUANTITATIVE APPRAISAL

We present a quantitative appraisal of the physics potential for neutrino experiments at the front-end of a muon storage ring. We estimate the forseeable accuracy in the determination of several interesting observables, and explore the consequences of these measurements. We discuss the extraction of individual quark and antiquark densities from polarized and unpolarized deep-inelastic scattering. In particular we study the implications for the undertanding of the nucleon spin structure. We assess the determination of S from scaling violation of structure functions, and from sum rules, and the determination of sin 2 W from elastic e and deep-inelastic p scattering. We then consider the production of charmed hadrons, and the measurement of their absolute branching ratios. We study the polarization of baryons produced in the current and target fragmentation regions. Finally, we discuss the sensitivity to physics beyond the Standard Model.

Scale dependence and small-χ behaviour of polarized parton distributions

Abstract We discuss perturbative evolution of the polarized structure function g 1 in the ( χ , Q 2 ) plane, with special regard to the small- χ region. We determine g 1 in terms of polarized quark and gluon distributions using coefficient functions to order α s . At small χ g 1 then displays substantial scale dependence, which necessarily implies a corresponding scale dependence in the large- χ region. This scale dependence has significant consequences for the extraction of the first moment from the experimental data, reducing its value while increasing the error. Conversely, the scale dependence may be used to constrain the size of the polarized gluon distribution.

Determination of αs from F2p at HERA

We compute the proton structure function F_2^p at small x and large Q^2 at next-to-leading order in alpha_s(Q^2), including summations of all leading and subleading logarithms of Q^2 and 1/x in a way consistent with momentum conservation. We perform a detailed comparison to the 1993 HERA data, and show that they may be used to determine alpha_s(M_Z^2)=0.120 pm 0.005(exp) pm 0.009(th). The theoretical error is dominated by the renormalization and factorization scheme ambiguities.

Momentum conservation at small x

Abstract We discuss how momentum conservation is implemented in perturbative computations based on expansions of anomalous dimensions appropriate at small x . We show that for any given choice of F 2 coefficient functions there always exists a factorization scheme where the gluon is defined in such a way that momentum is conserved at next to leading order.

Anomalous evolution of nonsinglet nucleon structure functions

Abstract We calculate the scale dependence of nonsinglet nucleon structure functions. Due to anomalous axial symmetry breaking alarge flavour asymmetry of the quark-antiquark sea is generated nonperturbatively. This produces a strong scale dependenceof the nonsinglet structure function in an intermediate range of Q2. Evolving nonperturbatively a pure valence distributionfrom an infrared scale we can thus compute F2p−F2n as measured by the NMC, and give detailed predictions for its Q2dependence at fixed x. We also compare our results with Drell-Yan data.

ANOMALOUS EVOLUTION OF NONSINGLET STRUCTURE FUNCTIONS

We review a formalism that includes the effects of nonperturbative U(1) symmetry breaking on the QCD evolution of nonsinglet structure functions. We show that a strong scale dependence is generated in an intermediate energy range 0.5⪝Q⪝5 GeV2 GeV2 for all values of x. We show that this explains naturally the observed violation of the Gottfried sum, and allows a determination of the shape of the nonsinglet structure function, in excellent agreement with experiment. We argue that these effects may affect the determination of α, from deep-inelastic scattering.