arXiv subjects

Gavin P. Salam

Publications and source records attributed to Gavin P. Salam.

At least 73 records · Page 4Linked to original sources

arXiv2010

Jet Reconstruction in Heavy Ion Collisions

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.

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arXiv2010

Giant QCD K-factors beyond NLO

Hadronic observables in Z+jet events can be subject to large NLO corrections at TeV scales, with K-factors that even reach values of order 50 in some cases. We develop a method, LoopSim, by which approximate NNLO predictions can be obtained for such observables, supplementing NLO Z+jet and NLO Z+2-jet results with a unitarity-based approximation for missing higher loop terms. We first test the method against known NNLO results for Drell-Yan lepton pt spectra. We then show our approximate NNLO results for the Z+jet observables. Finally we examine whether the LoopSim method can provide useful information even in cases without giant K-factors, with results for observables in dijet events that can be compared to early LHC data.

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arXiv2010

On the characterisation of the underlying event

The measurement of the underlying event (UE) and its separation from hard interactions in hadron-collider events is a conceptually and practically challenging task. We develop a simple, mostly analytical toy model for the UE in order to understand how different UE measurement approaches fare on the practical aspects of this problem, comparing the traditional approach used so far at Tevatron with a recently proposed "jet-area/median" approach. Both are found to perform comparably well in measuring average properties of the UE, such as the mean transverse momentum flow, but the jet-area/median approach has distinct advantages in determining its fluctuations. We then use the jet-area/median method to investigate a range of UE properties in existing Monte Carlo event-generator tunes, validating the main results of the toy-model and highlighting so-far unmeasured characteristics of the UE such as its rapidity dependence, as well as its intra- and inter-event fluctuations and correlations.

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arXiv2010

Towards Jetography

As the LHC prepares to start taking data, this review is intended to provide a QCD theorist's understanding and views on jet finding at hadron colliders, including recent developments. My hope is that it will serve both as a primer for the newcomer to jets and as a quick reference for those with some experience of the subject. It is devoted to the questions of how one defines jets, how jets relate to partons, and to the emerging subject of how best to use jets at the LHC.

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arXiv2010

A note on the CDF high-p_t charged particle excess

It has recently been pointed out that CDF data for the cross section of high-p_t charged particles show an excess of up to three orders of magnitude over QCD predictions, a feature tentatively ascribed to possible violations of factorisation. We observe that for p_t > 80 GeV the measured charged-particle cross sections become of the same order as jet cross sections. Combining this information with data on charged particle distributions within jets allows us to rule out the hypothesis that the CDF data could be interpreted in terms of QCD factorisation violation. We also comment on the difficulty of interpreting the excess in terms of new physics scenarios.

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arXiv2010

Phenomenology of event shapes at hadron colliders

We present results for matched distributions of a range of dijet event shapes at hadron colliders, combining next-to-leading logarithmic (NLL) accuracy in the resummation exponent, next-to-next-to leading logarithmic (NNLL) accuracy in its expansion and next-to-leading order (NLO) accuracy in a pure alpha_s expansion. This is the first time that such a matching has been carried out for hadronic final-state observables at hadron colliders. We compare our results to Monte Carlo predictions, with and without matching to multi-parton tree-level fixed-order calculations. These studies suggest that hadron-collider event shapes have significant scope for constraining both perturbative and non-perturbative aspects of hadron-collider QCD. The differences between various calculational methods also highlight the limits of relying on simultaneous variations of renormalisation and factorisation scale in making reliable estimates of uncertainties in QCD predictions. We also discuss the sensitivity of event shapes to the topology of multi-jet events, which are expected to appear in many New Physics scenarios.

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arXiv2010

Discovering baryon-number violating neutralino decays at the LHC

Recently there has been much interest in the use of single-jet mass and jet substructure to identify boosted particles decaying hadronically at the LHC. We develop these ideas to address the challenging case of a neutralino decaying to three quarks in models with baryonic violation of R-parity. These decays have previously been found to be swamped by QCD backgrounds. We demonstrate for the first time that such a decay might be observed directly at the LHC with high significance, by exploiting characteristics of the scales at which its composite jet breaks up into subjets.

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arXiv2009

Fat Jets for a Light Higgs

At the LHC associated top quark and Higgs boson production with a Higgs decay to bottom quarks has long been a heavily disputed search channel. Recently, it has been found to not be viable. We show how it can be observed by tagging massive Higgs and top jets. For this purpose we construct boosted top and Higgs taggers for Standard Model processes in a complex QCD environment.

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arXiv2009

A posteriori inclusion of parton density functions in NLO QCD final-state calculations at hadron colliders: The APPLGRID Project

A method to facilitate the consistent inclusion of cross-section measurements based on complex final-states from HERA, TEVATRON and the LHC in proton parton density function (PDF) fits has been developed. This can be used to increase the sensitivity of LHC data to deviations from Standard Model predictions. The method stores perturbative coefficients of NLO QCD calculations of final-state observables measured in hadron colliders in look-up tables. This allows the posteriori inclusion of parton density functions (PDFs), and of the strong coupling, as well as the a posteriori variation of the renormalisation and factorisation scales in cross-section calculations. The main novelties in comparison to original work on the subject are the use of higher-order interpolation, which substantially improves the trade-off between accuracy and memory use, and a CPU and computer memory optimised way to construct and store the look-up table using modern software tools. It is demonstrated that a sufficient accuracy on the cross-section calculation can be achieved with reasonably small look-up table size by using the examples of jet production and electro-weak boson (Z, W) production in proton-proton collisions at a center-of-mass energy of 14 TeV at the LHC. The use of this technique in PDF fitting is demonstrated in a PDF-fit to HERA data and simulated LHC jet cross-sections as well as in a study of the jet cross-section uncertainties at various centre-of-mass energies.

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arXiv2009

Moriond 2009, QCD and High Energy Interactions: Theory Summary

These proceedings provide a brief summary of the theoretical topics that were covered at Moriond QCD 2009, including non-perturbative QCD, perturbative QCD at colliders, a small component of physics beyond the standard model and heavy-ion collisions.

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arXiv2008

Quantifying the performance of jet definitions for kinematic reconstruction at the LHC

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.

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arXiv2008

Jet substructure as a new Higgs search channel at the Large Hadron Collider

We show that W H and Z H production where the Higgs boson decays to bbbar can be recovered as good search channels for the Standard Model Higgs at the Large Hadron Collider. This is done by requiring the Higgs to have high transverse momentum, and employing state-of-the-art jet reconstruction and decomposition techniques.

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arXiv2008

Jet substructure as a new Higgs search channel at the LHC

These proceedings discuss a possible new search strategy for a light Higgs boson at the LHC, in high-pt WH and ZH production where the Higgs boson decays to a single collimated b-bbar jet. Material is included that is complementary to what was shown in the original article, arXiv:0802.2470.

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arXiv2008

What HERA may provide ?

More than 100 people participated in a discussion session at the DIS08 workshop on the topic What HERA may provide. A summary of the discussion with a structured outlook and list of desirable measurements and theory calculations is given.

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arXiv2008

Jet substructure as a new Higgs search channel at the LHC

It is widely considered that, for Higgs boson searches at the Large Hadron Collider, WH and ZH production where the Higgs boson decays to b anti-b are poor search channels due to large backgrounds. We show that at high transverse momenta, employing state-of-the-art jet reconstruction and decomposition techniques, these processes can be recovered as promising search channels for the standard model Higgs boson around 120 GeV in mass.

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arXiv2008

A Higher Order Perturbative Parton Evolution Toolkit (HOPPET)

This document describes a Fortran 95 package for carrying out DGLAP evolution and other common manipulations of parton distribution functions (PDFs). The PDFs are represented on a grid in x-space so as to avoid limitations on the functional form of input distributions. Good speed and accuracy are obtained through the representation of splitting functions in terms of their convolution with a set of piecewise polynomial basis functions, and Runge-Kutta techniques are used for the evolution in Q. Unpolarised evolution is provided to NNLO, including heavy-quark thresholds in the MSbar scheme, and longitudinally polarised evolution to NLO. The code is structured so as to provide simple access to the objects representing splitting functions and PDFs, making it possible for a user to extend the facilities already provided. A streamlined interface is also available, facilitating use of the evolution part of the code from F77 and C/C++.

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arXiv2008

The anti-k_t jet clustering algorithm

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.

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arXiv2008

The Catchment Area of Jets

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.

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