10923413
Infrared renormalization-group flow for heavy-quark masses.
A short-distance heavy-quark mass depends on two parameters: the renormalization scale mu and a scale R controlling the absorption of infrared fluctuations. The radius for perturbative corrections that build up the mass beyond its pointlike definition in the pole scheme is approximately 1/R. Treating R as a variable gives a renormalization-group equation. R evolution improves the stability of conversion between short-distance mass schemes, allowing us to avoid large logs and the renormalon. R evolution can also be used to study IR renormalons without using bubble chains, yielding a convergent sum rule for the coefficient of the O(Lambda(QCD)) renormalon ambiguity of the pole mass.
12390720
A formalism for the systematic treatment of rapidity logarithms in Quantum Field Theory
A bstractMany observables in QCD rely upon the resummation of perturbation theory to retain predictive power. Resummation follows after one factorizes the cross section into the relevant modes. The class of observables which are sensitive to soft recoil effects are particularly challenging to factorize and resum since they involve rapidity logarithms. Such observables include: transverse momentum distributions at pT much less then the high energy scattering scale, jet broadening, exclusive hadroproduction and decay, as well as the Sudakov form factor. In this paper we will present a formalism which allows one to factorize and resum the perturbative series for such observables in a systematic fashion through the notion of a “rapidity renormalization group”. That is, a Collin-Soper like equation is realized as a renormalization group equation, but has a more universal applicability to observables beyond the traditional transverse momentum dependent parton distribution functions (TMDPDFs) and the Sudakov form factor. This formalism has the feature that it allows one to track the (non-standard) scheme dependence which is inherent in any sce- nario where one performs a resummation of rapidity divergences. We present a pedagogical introduction to the formalism by applying it to the well-known massive Sudakov form fac- tor. The formalism is then used to study observables of current interest. A factorization theorem for the transverse momentum distribution of Higgs production is presented along with the result for the resummed cross section at NLL. Our formalism allows one to define gauge invariant TMDPDFs which are independent of both the hard scattering amplitude and the soft function, i.e. they are universal. We present details of the factorization and re- summation of the jet broadening cross section including a renormalization in p⊥ space. We furthermore show how to regulate and renormalize exclusive processes which are plagued by endpoint singularities in such a way as to allow for a consistent resummation.
62817139
R evolution: Improving perturbative QCD
Perturbative QCD results in the MS scheme can be dramatically improved by switching to a scheme that accounts for the dominant power law dependence on the factorization scale in the operator product expansion. We introduce the ''MSR scheme'' which achieves this in a Lorentz and gauge invariant way and has a very simple relation to MS. Results in MSR depend on a cutoff parameter R, in addition to the {mu} of MS. R variations can be used to independently estimate (i.) the size of power corrections, and (ii.) higher-order perturbative corrections (much like {mu} in MS). We give two examples at three-loop order, the ratio of mass splittings in the B*-B and D*-D systems, and the Ellis-Jaffe sum rule as a function of momentum transfer Q in deep inelastic scattering. Comparing to data, the perturbative MSR results work well even for Q{approx}1 GeV, and power corrections are reduced compared to MS.
121727752
Phase space description of the production of quark gluon plasma in heavy-ion collisions
A new source term is proposed to describe the production of quark gloun plasma in heavy-ion collisions.
119262820
Parton fragmentation within an identified jet at NNLL
The fragmentation of a light parton i to a jet containing a light energetic hadron h, where the momentum fraction of this hadron as well as the invariant mass of the jet is measured, is described by “fragmenting jet functions”. We calculate the oneloop matching coefficients $ {\mathcal{J}_{ij}} $ that relate the fragmenting jet functions $ \mathcal{G}_i^h $ to the standard, unpolarized fragmentation functions Dhj for quark and gluon jets. We perform this calculation using various IR regulators and show explicitly how the IR divergences cancel in the matching. We derive the relationship between the coefficients $ {\mathcal{J}_{ij}} $ and the quark and gluon jet functions. This provides a cross-check of our results. As an application we study the process e+e- → Xπ+ on the Υ(4S) resonance where we measure the momentum fraction of the π+ and restrict to the dijet limit by imposing a cut on thrust T. In our analysis we sum the logarithms of τ = 1-T in the cross section to next-to-next-to-leading-logarithmic accuracy (NNLL). We find that including contributions up to NNLL (or NLO) can have a large impact on extracting fragmentation functions from e+e- → dijet + h.
119181326
Two-loop jet function and jet mass for top quarks
We compute the two-loop heavy-quark jet function in the heavy-quark limit. This is one of the key ingredients in next-to-next-to-leading order and next-to-next-to-leading-log order computations of the invariant mass distribution of top-jets at a future e{sup +}e{sup -} collider. The shape of the top-invariant mass distribution is affected by large logs which we compute at next-to-next-to-leading-log order. Exploiting the non-Abelian exponentiation theorem, a definition of the top jet mass is given which is transitive and whose renormalization group evolution is determined by the cusp-anomalous dimension to all orders in perturbation theory. Relations of the jet mass to the pole, MS, and 1S masses are presented at two-loop order.
119252368
Anisotropic distribution of nucleon participating in elliptical flow
Abstract Using the isospin dependent quantum molecular dynamics model, we study the effect of charge asymmetry and isospin dependent cross-section on d N d ( 〈 cos 2 ϕ 〉 ) and d N p t d p t . Simulations have been carried out for the reactions of X m 124 + X m 124 , where m = ( 47 , 50 and 59 ) and S 16 40 + S 16 40 . Our study shows that these parameters depend strongly on the isospin of cross-section and charge asymmetry. The distribution of free nucleons is symmetric but the distribution of nucleons constituting the light mass fragments, which are contributing in elliptical flow, is not symmetric around the beam axis.
117335698
Effect of isospin dependent cross-section on nuclear stopping
Nuclear stopping in heavy ion collisions (HIC) has been studied by means of rapidity distribution and asymmetry of nucleon momentum distribution. It is an important quantity in determining the outcome of a reaction. Fen Fu {\it et al.,} \cite{2} calculate both the radial flow and the degree of nuclear stopping in Pb + Pb and Ni + Ni at 0.4, 0.8 and 1.2 GeV/neucleon. They found that the expansion velocity as well as the degree of nuclear stopping are higher in the heavier system at all energies.
117283830
Isospin effects on fragment production
To understand the isospin effects in nearly symmetric nuclear matter we performed a complete systematical theoretical study within an Isospin dependent Quantum Molecular Dynamical Model (IQMD) and using Minimum Spanning Tree (MST) algorithm. Simulations are carried out for the reactions $_{124}Sn^{50}+_{124}Sn^{50}$ and $_{107}Sn^{50}+_{124}Sn^{50}$. The collision geometry is varied from central to peripheral. We find that neutron rich colliding nuclei are better candidate to study the isospin effects