arXiv subjects
Publications and source records attributed to Gregory Soyez.
We compute the ratio of the inclusive jet cross-sections obtained with the same jet algorithm at two different values of the jet radius. We perform a computation of that observable at NLO (O(alphas^2)) in perturbative QCD and compute non-perturbative corrections from soft-gluon emission. We discuss predictions for RHIC and the LHC.
We examine the problem of jet reconstruction at heavy-ion colliders using jet-area-based background subtraction tools as provided by FastJet. We use Monte Carlo simulations with and without quenching to study the performance of several jet algorithms, including the option of filtering, under conditions corresponding to RHIC and LHC collisions. We find that most standard algorithms perform well, though the anti-kt and filtered Cambridge/Aachen algorithms have clear advantages in terms of the reconstructed transverse-momentum offset and dispersion.
Obtaining a good momentum reconstruction of a jet is a compromise between taking it large enough to catch the perturbative final-state radiation and small enough to avoid too much contamination from the underlying event and initial-state radiation. In this paper, we compute analytically the optimal jet radius for dijet reconstructions and study its scale dependence. We also compare our results with previous Monte-Carlo studies.
In all modern hadronic colliders, jets recieve a large contribution from a soft background: pileup in the case of proton-proton collisions at the LHC, or the underlying event for heavy-ion collisions at RHIC or the LHC. In these proceedings, we present a generic and simple method, based on jet areas, to subtract the contribution of the soft background from the jets. This allows for more precise kinematic reconstruction of jets in dense environments.
From dedicated QCD studies to new physics background estimation, jets will be everywhere at the LHC. In these proceedings, we discuss two important recent series of improvements. In the first one, we introduce new algorithms and new implementations of previously existing algorithms, in order to cure limitations of their predecessors and to satisfy fundamental requirements. In the second part, we show that it is of prime importance to carefully choose the jet definition (algorithm and parameters) to optimise kinematic reconstructions at the LHC. Noticeably, we show that while at scales around 100 GeV, R~0.5 is an appropriate choice, clustering at the TeV scale requires R~1 for optimal efficiency. We finally show that our results are valid in the presence of pileup, provided that a subtraction procedure is applied.
We present a strategy to quantify the performance of jet definitions in kinematic reconstruction tasks. It is designed to make use exclusively of physical observables, in contrast to previous techniques which often used unphysical Monte Carlo partons as a reference. It is furthermore independent of the detailed shape of the kinematic distributions. We analyse the performance of 5 jet algorithms over a broad range of jet-radii, for sources of quark jets and gluon jets, spanning the energy scales of interest at the LHC, both with and without pileup. The results allow one to identify optimal jet definitions for the various scenarios. They confirm that the use of a small jet radius (R\simeq 0.5) for quark-induced jets at moderate energy scales, O(100 GeV), is a good choice. However, for gluon jets and in general for TeV scales, there are significant benefits to be had from using larger radii, up to R\gtrsim 1. This has implications for the span of jet-definitions that the LHC experiments should provide as defaults for searches and other physics analyses.
We discuss the description of the proton structure function within the dipole factorisation framework. We parametrise the forward dipole amplitude to account for saturation as predicted by the small-x QCD evolution equations. Contrarily to previous models, the saturation scale does not decrease when taking heavy quarks into account. We show that the same dipole amplitude also allows to reproduce diffractive data and exclusive vector meson production.
We illustrate how the midpoint and iterative cone (with progressive removal) algorithms fail to satisfy the fundamental requirements of infrared and collinear safety, causing divergences in the perturbative expansion. We introduce SISCone and the anti-kt algorithms as respective replacements that do not have those failures without any cost at the experimental level.
The k_t and Cambridge/Aachen inclusive jet finding algorithms for hadron-hadron collisions can be seen as belonging to a broader class of sequential recombination jet algorithms, parametrised by the power of the energy scale in the distance measure. We examine some properties of a new member of this class, for which the power is negative. This ``anti-k_t'' algorithm essentially behaves like an idealised cone algorithm, in that jets with only soft fragmentation are conical, active and passive areas are equal, the area anomalous dimensions are zero, the non-global logarithms are those of a rigid boundary and the Milan factor is universal. None of these properties hold for existing sequential recombination algorithms, nor for cone algorithms with split--merge steps, such as SISCone. They are however the identifying characteristics of the collinear unsafe plain ``iterative cone'' algorithm, for which the anti-k_t algorithm provides a natural, fast, infrared and collinear safe replacement.
The area of a jet is a measure of its susceptibility to radiation, like pileup or underlying event (UE), that on average, in the jet's neighbourhood, is uniform in rapidity and azimuth. In this article we establish a theoretical grounding for the discussion of jet areas, introducing two main definitions, passive and active areas, which respectively characterise the sensitivity to pointlike or diffuse pileup and UE radiation. We investigate the properties of jet areas for three standard jet algorithms, k_t, Cambridge/Aachen and SISCone. Passive areas for single-particle jets are equal to the naive geometrical expectation πR^2, but acquire an anomalous dimension at higher orders in the coupling, calculated here at leading order. The more physically relevant active areas differ from πR^2 even for single-particle jets, substantially so in the case of the cone algorithms like SISCone with a Tevatron Run-II split--merge procedure. We compare our results with direct measures of areas in parton-shower Monte Carlo simulations and find good agreement with the main features of the analytical predictions. We furthermore justify the use of jet areas to subtract the contamination from pileup.
Considering the Balitsky-Kovchegov QCD evolution equation in full momentum space, we derive the travelling wave solutions expressing the nonlinear saturation constraints on the dipole scattering amplitude at non-zero momentum transfer. A phenomenological application to elastic vector meson production shows the compatibility of data with the QCD prediction: an enhanced saturation scale at intermediate momentum transfer.
Current cone jet algorithms, widely used at hadron colliders, take event particles as seeds in an iterative search for stable cones. A longstanding infrared (IR) unsafety issue in such algorithms is often assumed to be solvable by adding extra `midpoint' seeds, but actually is just postponed to one order higher in the coupling. A proper solution is to switch to an exact seedless cone algorithm, one that provably identifies all stable cones. The only existing approach takes N 2^N time to find jets among N particles, making it unusable at hadron level. This can be reduced to N^2 ln(N) time, leading to code (SISCone) whose speed is similar to that of public midpoint implementations. Monte Carlo tests provide a strong cross-check of an analytical proof of the IR safety of the new algorithm, and the absence of any 'R_{sep}' issue implies a good practical correspondence between parton and hadron levels. Relative to a midpoint cone, the use of an IR safe seedless algorithm leads to modest changes for inclusive jet spectra, mostly through reduced sensitivity to the underlying event, and significant changes for some multi-jet observables.
In these proceedings, I shall review the basic concepts of perturbative QCD in its high-energy limit. I shall concentrate on the approach to the unitarity limit, usually referred to as saturation, as well as on the gluon-number fluctuations the importance of which has recently been discovered. I shall explain the basic framework showing the need for those phenomena, first, from a simple picture of the high-energy behaviour, then, giving a short derivation of the equation driving this evolution. In the second part, I shall exhibit an analogy with statistical physics and show how this allows to derive geometric scaling in QCD with saturation. I shall finally consider the effects of gluon-number fluctuations on this picture and draw the physical consequences, i.e. a new scaling law, arising from those results.
In this paper I show that it is possible to use Regge theory to constrain the initial parton distribution functions of a global DGLAP fit. In this approach, both quarks and gluons have the same high-energy behaviour which may also be used to describe soft interactions. More precisely, I show that, if we parametrise the parton distributions with a triple-pole pomeron, {\em i.e.} like $\log^2(1/x)$ at small $x$, at $Q^2=Q_0^2$ and evolve these distribution with the DGLAP equation, we can reproduce $F_2^p$, $F_2^d$, $F_2^n/F_2^p$, $F_2^{νN}$ and $xF_3^{νN}$ for $W^2\ge 12.5$ GeV$^2$. In this case, we obtain a new leading-order global QCD fit with a Regge-compatible initial condition. I shall also show that it is possible to use Regge theory to extend the parton distribution functions to small $Q^2$. This leads to a description of the structure functions over the whole $Q^2$ range based on Regge theory at low $Q^2$ and on QCD at large $Q^2$. Finally, I shall argue that, at large $Q^2$, the parton distribution functions obtained from DGLAP evolution and containing an essential singularity at $j=1$ can be approximated by a triple-pole pomeron behaviour.
Working in light-cone coordinates, we study the zero-modes and the vacuum in a 2+1 dimensional SU(3) gauge model. Considering the fields as independent of the tranverse variables, we dimensionally reduce this model to 1+1 dimensions. After introducing an appropriate su(3) basis and gauge conditions, we extract an adjoint field from the model. Quantization of this adjoint field and field equations lead to two constrained and two dynamical zero-modes. We link the dynamical zero-modes to the vacuum by writing down a Schrodinger equation and prove the non-degeneracy of the SU(3) vacuum provided that we neglect the contribution of constrained zero-modes.