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G. P. Salam

Publications and source records attributed to G. P. Salam.

At least 55 records · Page 3Linked to original sources
arXiv2000

Revisiting non-perturbative effects in the jet broadenings

We show that taking into account the interplay between perturbative and non-perturbative effects, the power-suppressed shift to the broadening distributions becomes B dependent, and the non-perturbative contribution to the mean values becomes proportional to 1/(Q\sqrt{\as(Q)}). The new theoretical treatment greatly improves the consistency of the phenomenology with the notion of the universality of confinement effects in jet shapes.

hep-ph↗
arXiv2000

Universality of 1/Q corrections to jet-shape observables rescued

We address the problem of potential non-universality of the leading 1/Q power corrections to jet shapes emerging from the non-inclusive character of these observables. We consider the thrust distribution as an example and analyse the non-inclusive contributions which emerge at the two-loop level. Although formally subleading in $\as$, they modify the existing na{\"ı}ve one-loop result for the expected magnitude of the power term by a factor of order unity. Such a promotion of a subleading correction into a numerical factor is natural since the non-perturbative power terms are explicitly proportional to powers of the QCD scale $Λ$ which can be fixed precisely only at the two-loop level. The ``jet-shape scaling factor'' depends on the observable but remains perturbatively calculable. Therefore it does not undermine the universal nature of 1/Q power corrections, which remain expressible in terms of the universal running coupling and universal soft-gluon emission.

hep-ph↗
arXiv2000

On the universality of the Milan factor for 1/Q power corrections to jet shapes

We perform the two-loop analysis of the 1/Q power corrections to jet-shape variables. This step is necessary for producing reliable theoretical predictions for the relative magnitudes of genuine confinement effects. We show that the rescaling factor recently derived for the thrust case (the Milan factor) remains the same for the class of observables which includes the C-parameter, invariant jet masses, jet broadening and the energy-energy correlation measure. We list the expressions which should be used for extracting 1/Q power effects in jet shapes. We also envisage large non-perturbative effects, characterised by fractional powers of Q, in certain semi-inclusive observables such as the height of the spectrum of Drell-Yan lepton pairs with invariant mass Q and transverse momentum Q_t=0, and back-to-back energy flows in e^+e^- and DIS.

hep-ph↗
arXiv2000

The status of NLL BFKL

This talk summarises the current status of the NLL corrections to BFKL physics and discusses the question of small-x factorisation.

hep-ph↗
arXiv2000

QCD

We discuss issues of QCD at the LHC including parton distributions, Monte Carlo event generators, the available next-to-leading order calculations, resummation, photon production, small x physics, double parton scattering, and backgrounds to Higgs production.

hep-ph↗
arXiv1999

The resummed thrust distribution in DIS

We present preliminary results on the resummation of leading and next-to-leading logarithms for the thrust distribution in deep inelastic scattering. Our predictions, expanded to O(alpha_s^2), are compared to corresponding results from the Monte Carlo programs DISASTER++ and DISENT.

hep-ph↗
arXiv1999

A collinear model for small-x physics

We propose a simple model for studying small-x physics in which we take only the collinearly enhanced part of leading and subleading kernels, for all possible transverse momentum orderings. The small-x equation reduces to a second order differential equation in t=log k^2/Lambda^2 space, whose perturbative and strong-coupling features are investigated both analytically and numerically. For two-scale processes, we clarify the transition mechanism between the perturbative, non Regge regime and the strong-coupling Pomeron behavior.

hep-ph↗
arXiv1999

Renormalization Group Improved Small-x Equation

We propose and analyze an improved small-x equation which incorporates exact leading and next-to-leading BFKL kernels on one hand and renormalization group constraints in the relevant collinear limits on the other. We work out in detail the recently proposed omega-expansion of the solution, derive the Green's function factorization properties and discuss both the gluon anomalous dimension and the hard pomeron. The resummed results are stable, nearly renormalization-scheme independent, and join smoothly with the fixed order perturbative regime. Two critical hard pomeron exponents are provided, which - for reasonable strong-coupling extrapolations - are argued to provide bounds on the pomeron intercept.

hep-ph↗
arXiv1999

BFKL and CCFM final states

I give a brief presentation of recent results on the equivalence of BFKL and CCFM small-x final states, and discuss their implications for phenomenology.

hep-ph↗
arXiv1999

Soft emissions and the equivalence of BFKL and CCFM final states

This article demonstrates that the BFKL and CCFM equations, despite their different physical content, lead to equivalent results for any final-state observable at leading single-logarithmic order. A novel and fundamental element is the treatment also of the soft (z->1 divergent) part of the splitting function in the CCFM equation.

hep-ph↗
arXiv1999

Small-x one-particle-inclusive quantities in the CCFM approach

This article presents the results of a quantitative study of the small-x data at HERA, using the CCFM equation. The first step consists of choosing the version of the CCFM equation to be used, corresponding to selecting a particular subset of next-to-leading-logarithmic corrections --- the choice is constrained by requiring a phenomenologically reasonable small-x growth. For the time being, the parts of the splitting functions that are finite at z=0 have been left out. We then examine results for F_2^c, R, the transverse energy flow, the charged-particle transverse-momentum spectrum and the forward-jet cross section and compare to data. While some of the data is reproduced better than with DGLAP-based calculations, the agreement is not entirely satisfactory, suggesting that the approach developed here is not yet suitable for detailed phenomenology. We discuss why, and suggest directions for future work.

hep-ph↗
arXiv1998

BFKL at next-to-leading order

This is a summary of the contributions on the next-to-leading order corrections to the BFKL equation which were presented to the `Small-x and Diffraction' working group at the 1998 Durham Workshop on HERA Physics.

hep-ph↗
arXiv1998

A resummation of large sub-leading corrections at small x

The NLL corrections to the BFKL kernel are known to be very large, to the extent that even for small values of alpha_s, they lead to physical cross sections which are not positive definite. It is shown in the context of a toy model, that such pathological behaviour is an artifact of the truncation at NLL order, and is associated in particular with double transverse logarithms. These are resummed in a manner consistent with the full NLL kernel, and are shown to change its properties quite considerably.

hep-ph↗
arXiv1998

Associated quantities from the CCFM approach

Results are presented on structure functions and final state properties within the CCFM approach. Traditionally used forms of the CCFM equation have difficulty fitting the F_2 data, predicting too fast a growth at small x. A solution can be found in a particular treatment of formally subleading (1-z) terms, which dampens very considerably the small-x growth. Preliminary results are shown for the transverse energy flow, and future prospects and plans are discussed.

hep-ph↗
arXiv1998

On the QCD analysis of Jet Broadening

The perturbative all-order analysis of the jet-broadening B-distribution in the small-B region is carried out with single-logarithmic accuracy, which requires the control of both the sum of the moduli and the modulus of the sum of the transverse momenta of soft gluons. We confirm the master equation for the B-distribution derived by Catani, Turnock and Webber (CTW). Proper treatment of quark recoil is necessary at this accuracy. This effect was neglected in the CTW solution. We show that the answer can be expressed in terms of the CTW result but evaluated at a properly rescaled B value.

hep-ph↗