Regular Series


Vol. 39 (2008), No. 7, pp. 1521 – 1779


Higher Order Corrections to Heavy Flavour Production in Deep Inelastic Scattering

abstract

In the asymptotic limit \(Q^2 \gg m^2\), the non-power corrections to the heavy flavour Wilson coefficients in deep-inelastic scattering are given in terms of massless Wilson coefficients and massive operator matrix elements. We start extending the existing NLO calculation for these operator matrix elements by calculating the \({\cal O}(\varepsilon \)) terms of the two-loop expressions and performing first investigations of the three-loop diagrams.


Viscosity and Boost Invariance at RHIC and LHC

abstract

We consider the longitudinal hydrodynamic evolution of the fireball created in a relativistic heavy-ion collision. Nonzero shear viscosity reduces the cooling rate of the system and hinders the acceleration of the longitudinal flow. As a consequence, the initial energy density needed to reproduce the experimental data at RHIC energies is significantly reduced. At LHC energies, we expect that shear viscosity helps to conserve a Bjorken plateau in the rapidity distributions during the expansion.


Higgs Production in Association with Two Jets at the LHC

abstract

The Higgs boson is a keystone of the mechanism of ElectroWeak Symmetry Breaking (EWSB) in the Standard Model of particles. Besides the Higgs boson discovery, which will be widely pursued at the Large Hadron Collider (LHC) at CERN, in order to analyse EWSB it will be crucial to study the Higgs couplings. A key component of the programme to measure the couplings, and in particular the ones of the Higgs to the \(W\)- or \(Z\)-bosons, will be the vector-boson fusion (VBF) process, characterised by the production of a Higgs boson plus two jets. However, Higgs + 2-jet production occurs mostly via gluon fusion, which, while part of the Higgs signal, constitutes a background when trying to isolate the gauge couplings of the VBF process. We give an overview of Higgs production, and analyse some distributions, in particular the azimuthal correlation between the jets, which may distinguish the VBF process from gluon fusion.


Soft-Hadronic Observables for Relativistic Heavy-Ion Collisions at RHIC and LHC

abstract

The relativistic hydrodynamics together with the single-freeze-out statistical hadronization model is used to describe the mid-rapidity hadron production in ultra-relativistic heavy-ion collisions at RHIC and LHC. At the highest RHIC energy our approach gives a quite satisfactory global description of soft hadronic observables including the HBT radii. With the increased initial energy, from RHIC to LHC, we expect the saturation of the pion elliptic flow and the moderate increase of the correlation radii.


Exact Tree-Level QCD Spin-Amplitudes and their Gauge Invariant Sub-Structures

abstract

We discuss possible separations into gauge invariant parts of exact, massive, tree-level spin amplitudes of processes involving two quarks, two gluons and a color-neutral current. We search for forms compatible with parton shower languages, without applying approximations or restrictions on phase space regions. Special emphasis will be put on the isolation of parts corresponding to the running coupling constant and parts necessary for the construction of evolution kernels for individual splittings. Our representation is quite universal: any color-neutral current can be used, in particular our approach is not restricted to vector currents only.


CP Violation in the Chargino/Neutralino Sector of the MSSM

abstract

The CP-violating effects in the neutralino sector of the MSSM can be observed in event-counting type experiments, i.e. without the need of exploiting variables sensitive to beam or neutralino polarization. On the other hand, such CP-odd effects in the chargino sector can be generated only at the loop level. We contrast the two sectors and present results of full one-loop analysis of the CP-odd asymmetry in the non-diagonal chargino pair production in \(e^+e^-\) collisions.


Soft and Collinear Enhancements to Top Quark and Higgs Cross Sections

abstract

I present calculations of soft and collinear corrections to the cross-sections for single top quark production, and for Higgs production via \(b{\bar b} \rightarrow H\) at the Tevatron and the LHC. I show that the corrections provide significant enhancements to the cross-sections in both cases. For single top production the soft gluon corrections dominate the cross-section, particularly in the \(s\)-channel and in \(tW\) production. For Higgs production it is shown that purely collinear terms have to be included as well to provide an accurate calculation.


Radiative Corrections to DIS

abstract

Early deep inelastic scattering (DIS) experiments at SLAC discovered partons, identified them as quarks and gluons, and restricted the set of the candidate theories for strong interactions to those exhibiting the asymptotic freedom property. The next generation DIS experiments at FNAL and CERN confirmed the predictions of QCD for the size of the scaling violation effects in the nucleon structure functions. The QCD fits to their data resulted in determining the momentum distributions of the point-like constituents of nucleons. Interpretation of data coming from all these experiments and, in the case of the SLAC experiments, even an elaboration of the running strategies, would not have been possible without a precise understanding of the electromagnetic radiative corrections. In this note I present my personal recollection of the important milestones, achieved in the period preceding the HERA era, in the high precision calculations of the radiative corrections to DIS, and in the development of the methods of their experimental control. I present subsequently the measurement strategies and discuss the advanced radiative correction tools used in the initial phase of the HERA experimental program, with an emphasis on their role in the first, model independent, measurement of the partonic densities in the small-\(x_{\rm Bj}\) region.


A Gauge Model of Data Selection, Acquisition and Analysis for LHC

abstract

A novel model of the data selection, acquisition and analysis for a multi-purpose and multi-component high-energy-physics experiment is presented. Its departure point is the freedom and the responsibility given to the different physics groups of the experiment to impose, on the event-by-event basis, their physics-goal-optimal configurations of (i) the sub-detectors, (ii) the trigger and data acquisition system, and (iii) the reconstruction and analysis framework. Its target is to develop, in a close analogy to the construction of the gauge models in particle physics, the overall data handling scheme, in which a multi-purpose experiment becomes an association of coexistent, yet largely independent, physics-group-based sub-experiments sharing common hardware maintenance, data-acquisition, and data reconstruction resources.


Hard Scattering and Electroweak Corrections at High Energies

abstract

After a brief recollection of joint scientific work with Staszek Jadach, recent results on electroweak radiative corrections for scattering processes in the TeV region are presented. The status of the four-fermion scattering amplitudes is discussed, with emphasis on logarithmically enhanced contributions in two-loop approximation. Predictions for the production of \(\gamma \), \(Z\) and \(W\) with large transverse momenta together with a jet are presented. For \(p_{\rm T}\) above 1 TeV the electroweak corrections may well reach several tens of percent. A similar situation is observed for top-antitop quark production at large invariant mass of the \(t\bar {t}\) system.


NLO–QCD Event Generators in GRACE

abstract

Automatic Feynman-amplitude calculation system, GRACE, has been extended to treat next-to-leading order (NLO) QCD calculations. Matrix elements of loop diagrams as well as those of tree level ones can be generated using the GRACE system. A soft/collinear singularity is treated using a leading-log subtraction method. Higher order re-summation of the soft/collinear correction by the parton shower method is combined with the NLO matrix-element without any double-counting in this method. An example of the event generator for \(W+\) jet and di-photon processes are given for demonstrating a validity of this method.


Unstable Quark–Gluon Plasma at LHC

abstract

Coupling of the quark–gluon plasma from the early stage of heavy-ion collisions is argued to be significantly weaker at LHC than at RHIC. For this reason, the role of instabilities — the pre-equilibrium plasma is unstable with respect to chromomagnetic modes — will be enhanced. The instabilities isotropize the system and speed up the process of equilibration. A possibility to observe direct signals of the instabilities is considered.


all authors

G. Balossini, G. Montagna, C.M. Carloni Calame, M. Moretti, M. Treccani, O. Nicrosini, F. Piccinini, A. Vicini

Electroweak and QCD Corrections to Drell–Yan Processes

abstract

The relevance of single-\(W\) and single-\(Z\) production processes at hadron colliders is well known: in the present paper the status of theoretical calculations of Drell–Yan processes is summarized and some results on the combination of electroweak and QCD corrections to a sample of observables of the process \(p p \to W^\pm \to \mu ^\pm + X\) at the LHC are discussed. The phenomenological analysis shows that a high-precision knowledge of QCD and a careful combination of electroweak and strong contributions is mandatory in view of the anticipated LHC experimental accuracy.


Reduction of One-Loop Amplitudes at the Integrand Level — NLO QCD Calculations

abstract

The recently proposed method (OPP) to extract the coefficients of the scalar one-loop integrals to any multi-particle (sub)-amplitude is described. Within this method no analytical information on the structure of the amplitude is needed, allowing for a purely numerical, but still algebraic, implementation of the algorithm. The algorithm can be used to automatically perform one-loop calculation both in QCD and in the EW Theory. As an application, we give QCD one-loop results for the process \(p p \to ZZZ\) at the LHC.


\(b\) Physics at LHC

abstract

Prospects for \(b\) physics study at LHC is described.


(N)LO Simulation of Chargino Production and Decay

abstract

We consider NLO chargino production and decays at the ILC. For this, we present an NLO extension of the Monte Carlo Event Generator WHIZARD including the NLO production. For photonic corrections, we use both a fixed order and a resummation approach. The latter method evades the problem of negative event weights and automatically includes leading higher order corrections. We present results for cross sections and event generation for both methods. As a first step towards a full NLO Monte Carlo, we consider a LO implementation of the chargino production and subsequent leptonic decay and investigate the precision of the sneutrino mass determination by means of lepton energy distributions in chargino decays. The SM and SUSY backgrounds are included in our study using full matrix elements as well as smearing effects from ISR and beamstrahlung. Without using energy distribution fits, the sneutrino mass can be determined with an error in the percent regime.


Fully Unintegrated Parton Correlation Functions

abstract

We summarize recent progress in the formulation of QCD factorization theorems in terms of parton correlation functions and discuss their relevance to LHC physics. We describe open problems and directions for future work.


Glauber Monte Carlo Predictions for LHC

abstract

In the framework of various Glauber-like models we compute several correlation observables in nuclear collisions at the SPS, RHIC, and LHC energies. We analyze fluctuations of the eccentricity of the fireball created in the collision, in particular the variable-axes harmonic moments \(\varepsilon ^\ast \), as well as the fluctuations of multiplicity of charged particles. We find moderate model dependence of the scaled standard deviation \(\sigma (\varepsilon ^\ast )\)/\(\varepsilon ^\ast \) on the choice of the particular Glauber model. For all considered models the values of \(\sigma (\varepsilon ^\ast )\)/\(\varepsilon ^\ast \) range from \(\sim \) 0.5 for central collisions to \(\sim \) 0.3–0.4 for peripheral collisions. The results are confronted to the recent measurement of the elliptic-flow fluctuations at RHIC. We also find that the dependence of multiplicity fluctuations on the centrality of the collision is too weak to explain the measurements at the SPS energies. The magnitude of the Glauber multiplicity fluctuations increases by about 20% from the RHIC to LHC energies.


Freeze-Out by Bulk Viscosity Driven Instabilities

abstract

We describe a new scenario (first introduced in G. Torrieri, B. Tomasik, I. Mishustin, arXiv:0707.4405[nucl-th]) for freezeout in heavy ion collisions that could solve the lingering problems associated with the so-called HBT puzzle. We argue that bulk viscosity increases as \(T\) approaches \(T_{\rm c}\). The fluid than becomes unstable against small perturbations, and fragments into clusters of a size much smaller than the total size of the system. These clusters maintain the pre-existing outward-going flow, as a spray of droplets, but develop no flow of their own, and hadronize by evaporation. We show that this scenario can explain HBT data and suggest how it can be experimentally tested.


Review of Applications of YFS-Style Resummation in Quantum Field Theory via Monte Carlo Methods

abstract

We review the application of exact, amplitude-based, YFS-style resummation in quantum field theory via Monte Carlo methods.


Historical and Mathematical Aspects of Iterative Solutions for Monte Carlo Simulations

abstract

Over the last 25 years Monte Carlo programs were being developed in Cracow in the group guided by Prof. Stanislaw Jadach. Many of those programs became standard in their application domains. In the following let us review some aspects of such projects which were probably at the foundation of their success. We will concentrate on mathematical aspects of their design and history of their construction. It is rather difficult to cover 25 years of the research in a single talk. That is why, I have organised my presentation around Monte Carlo PHOTOS but stressing its relation to other activities and projects often realized together with Prof. Jadach. Many of omitted aspects will find their way into other presentations collected in this volume. I will concentrate on issues related to phase-space parametrisation and spin amplitudes as used in our Monte Carlo programs such as MUSTRAAL, TAUOLA or KKMC and their similarities and differences with respect to solution used in PHOTOS.


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