arXiv subjects

Gregory Soyez

Publications and source records attributed to Gregory Soyez.

At least 55 records · Page 3Linked to original sources

arXiv2018

Top tagging : an analytical perspective

In this paper we study aspects of top tagging from first principles of QCD. We find that the method known as the CMS top tagger becomes collinear unsafe at high $p_t$ and propose variants thereof which are IRC safe, and hence suitable for analytical studies, while giving a comparable performance to the CMS tagger. We also develop new techniques to identify three-pronged jet substructure, based on adaptations of the Y-splitter method and its combination with grooming. A novel feature of our studies, relative to previous calculations of two-pronged substructure, is our use of triple-collinear splitting functions, which owes to the presence of two mass scales of the same order, $m_t$ and $m_W$, in the signal jet. We carry out leading logarithmic resummed calculations for the various top-taggers, for both background and signal jets, and compare the results to those from parton showers. We also identify and comment on the main features driving tagger performance at high $p_t$ and discuss the role of non-perturbative effects.

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arXiv2018

Recursive Soft Drop

We introduce a new jet substructure technique called Recursive Soft Drop, which generalizes the Soft Drop algorithm to have multiple grooming layers. Like the original Soft Drop method, this new recursive variant traverses a jet clustering tree to remove soft wide-angle contamination. By enforcing the Soft Drop condition N times, Recursive Soft Drop improves the jet mass resolution for boosted hadronic objects like W bosons, top quarks, and Higgs bosons. We further show that this improvement in mass resolution persists when including the effects of pileup, up to large pileup multiplicities. In the limit that N goes to infinity, the resulting groomed jets formally have zero catchment area. As an alternative approach, we present a bottom-up version of Recursive Soft Drop which, in its local form, is similar to Recursive Soft Drop and which, in its global form, can be used to perform event-wide grooming.

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arXiv2017

The jet mass distribution after Soft Drop

We present a first-principle computation of the mass distribution of jets which have undergone the grooming procedure known as Soft Drop. This calculation includes the resummation of the large logarithms of the jet mass over its transverse momentum, up to next-to-logarithmic accuracy, matched to exact fixed-order results at next-to-leading order. We also include non-perturbative corrections obtained from Monte-Carlo simulations and discuss analytic expressions for hadronisation and Underlying Event effects.

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arXiv2017

Systematics of quark/gluon tagging

By measuring the substructure of a jet, one can assign it a "quark" or "gluon" tag. In the eikonal (double-logarithmic) limit, quark/gluon discrimination is determined solely by the color factor of the initiating parton (C_F versus C_A). In this paper, we confront the challenges faced when going beyond this leading-order understanding, using both parton-shower generators and first-principles calculations to assess the impact of higher-order perturbative and nonperturbative physics. Working in the idealized context of electron-positron collisions, where one can define a proxy for quark and gluon jets based on the Lorentz structure of the production vertex, we find a fascinating interplay between perturbative shower effects and nonperturbative hadronization effects. Turning to proton-proton collisions, we highlight a core set of measurements that would constrain current uncertainties in quark/gluon tagging and improve the overall modeling of jets at the Large Hadron Collider.

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arXiv2017

A study of jet mass distributions with grooming

We perform a phenomenological study of the invariant mass distribution of hadronic jets produced in proton-proton collisions, in conjunction a grooming algorithm. In particular, we consider the modified MassDrop Tagger (mMDT), which corresponds to Soft Drop with angular exponent $β=0$. Our calculation, which is differential in both jet mass and jet transverse momentum, resums large logarithms of the jet mass, including the full dependence on the groomer's energy threshold $z_\text{cut}$, and it is matched to fixed-order QCD matrix elements at next-to-leading order. In order to account for non-perturbative contributions, originating from the hadronisation process and from the underlying event, we also include a phenomenological correction factor derived from Monte Carlo parton-shower simulations. Furthermore, we consider two different possibilities for the jet transverse momentum: before or after grooming. We show that the former should be preferred for comparisons with upcoming experimental data essentially because the mMDT transverse momentum spectrum is not collinear safe.

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arXiv2016

Dichroic subjettiness ratios to distinguish colour flows in boosted boson tagging

$N$-subjettiness ratios are in wide use for tagging heavy boosted objects, in particular the ratio of 2-subjettiness to 1-subjettiness for tagging boosted electroweak bosons. In this article we introduce a new, \emph{dichroic} ratio, which uses different regions of a jet to determine the two subjettiness measures, emphasising the hard substructure for the 1-subjettiness and the full colour radiation pattern for the 2-subjettiness. Relative to existing $N$-subjettiness ratios, the dichroic extension, combined with SoftDrop (pre-)grooming, makes it possible to increase the ultimate signal significance by about $25\%$ (for $2\,\text{TeV}$ jets), or to reduce non-perturbative effects by a factor of $2{-}3$ at $50\%$ signal efficiency while maintaining comparable background rejection. We motivate the dichroic approach through the study of Lund diagrams, supplemented with resummed analytical calculations.

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arXiv2016

SoftKiller, a particle-level pileup removal method

Existing widely-used pileup removal approaches correct the momenta of individual jets. In this article we introduce an event-level, particle-based pileup correction procedure, SoftKiller. It removes the softest particles in an event, up to a transverse momentum threshold that is determined dynamically on an event-by-event basis. In simulations, this simple procedure appears to be reasonably robust and brings superior jet resolution performance compared to existing jet-based approaches. It is also nearly two orders of magnitude faster than methods based on jet areas.

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arXiv2016

Jet shapes for boosted jet two-prong decays from first-principles

Several boosted jet techniques use jet shape variables to discriminate the multi-pronged signal from Quantum Chromodynamics backgrounds. In this paper, we provide a first-principles study of an important class of jet shapes all of which put a constraint on the subjet mass: the mass-drop parameter ($μ^2$), the $N$-subjettiness ratio ($τ_{21}^{(β=2)}$) and energy correlation functions ($C_2^{(β=2)}$ or $D_2^{(β=2)}$). We provide analytic results both for QCD background jets as well as for signal processes. We further study the situation where cuts on these variables are applied recursively with Cambridge-Aachen de-clustering of the original jet. We also explore the effect of the choice of axis for $N$-subjettiness and jet de-clustering. Our results bring substantial new insight into the nature, gain and relative performance of each of these methods, which we expect will influence their future application for boosted object searches.

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arXiv2016

Improved jet substructure methods: Y-splitter and variants with grooming

It has recently been demonstrated with Monte Carlo studies that combining the well-known Y-splitter and trimming techniques gives rise to important gains in the signal significance achievable for boosted electroweak boson tagging at high $p_t$. Here we carry out analytical calculations that explain these findings from first principles of QCD both for grooming via trimming and via the modified mass-drop tagger (mMDT). We also suggest modifications to Y-splitter itself, which result in great simplifications to the analytical results both for pure Y-splitter as well as its combination with general grooming methods. The modifications also lead to further performance gains, while making the results largely independent of choice of groomer. We discuss the implications of these findings in the broader context of optimal methods for boosted object studies at hadron colliders.

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arXiv2016

Inclusive jet spectrum for small-radius jets

Following on our earlier work on leading-logarithmic (LLR) resummations for the properties of jets with a small radius, R, we here examine the phenomenological considerations for the inclusive jet spectrum. We discuss how to match the NLO predictions with small-R resummation. As part of the study we propose a new, physically-inspired prescription for fixed-order predictions and their uncertainties. We investigate the R-dependent part of the next-to-next-to-leading order (NNLO) corrections, which is found to be substantial, and comment on the implications for scale choices in inclusive jet calculations. We also examine hadronisation corrections, identifying potential limitations of earlier analytical work with regards to their $p_t$-dependence. Finally we assemble these different elements in order to compare matched (N)NLO+LLR predictions to data from ALICE and ATLAS, finding improved consistency for the R-dependence of the results relative to NLO predictions.

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arXiv2015

Small-radius jets to all orders in QCD

As hadron collider physics continues to push the boundaries of precision, it becomes increasingly important to have methods for predicting properties of jets across a broad range of jet radius values R, and in particular for small R. In this paper we resum all leading logarithmic terms, $α_s^n \ln^n R$, in the limit of small R, for a wide variety of observables. These include the inclusive jet spectrum, jet vetoes for Higgs physics and jet substructure tools. Some of the quantities that we consider are relevant also for heavy-ion collisions. Furthermore, we examine and comment on the underlying order-by-order convergence of the perturbative series for different R values. Our results indicate that small-R effects can be substantial. Phenomenological studies will appear in a forthcoming companion paper.

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arXiv2015

On the use of charged-track information to subtract neutral pileup

The use of charged pileup tracks in a jet to predict the neutral pileup component in that same jet could potentially lead to improved pileup removal techniques, provided there is a strong local correlation between charged and neutral pileup. In Monte Carlo simulation we find that the correlation is however moderate, a feature that we attribute to characteristics of the underlying non-perturbative dynamics. Consequently, `neutral-proportional-to-charge' (NpC) pileup mitigation approaches do not outperform existing, area-based, pileup removal methods. This finding contrasts with the arguments made in favour of a new method, "jet cleansing", in part based on the NpC approach. We identify the critical differences between the performances of linear cleansing and trimmed NpC as being due to the former's rejection of subjets that have no charged tracks from the leading vertex, a procedure that we name "zeroing". Zeroing, an extreme version of the "charged-track trimming" proposed by ATLAS, can be combined with a range of pileup-mitigation methods, and appears to have both benefits and drawbacks. We show how the latter can be straightforwardly alleviated. We also discuss the limited potential for improvement that can be obtained by linear combinations of the NpC and area-subtraction methods.

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arXiv2014

Soft Drop

We introduce a new jet substructure technique called "soft drop declustering", which recursively removes soft wide-angle radiation from a jet. The soft drop algorithm depends on two parameters--a soft threshold $z_\text{cut}$ and an angular exponent $β$--with the $β= 0$ limit corresponding roughly to the (modified) mass drop procedure. To gain an analytic understanding of soft drop and highlight the $β$ dependence, we perform resummed calculations for three observables on soft-dropped jets: the energy correlation functions, the groomed jet radius, and the energy loss due to soft drop. The $β= 0$ limit of the energy loss is particularly interesting, since it is not only "Sudakov safe" but also largely insensitive to the value of the strong coupling constant. While our calculations are strictly accurate only to modified leading-logarithmic order, we also include a discussion of higher-order effects such as multiple emissions and (the absence of) non-global logarithms. We compare our analytic results to parton shower simulations and find good agreement, and we also estimate the impact of non-perturbative effects such as hadronization and the underlying event. Finally, we demonstrate how soft drop can be used for tagging boosted W bosons, and we speculate on the potential advantages of using soft drop for pileup mitigation.

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arXiv2012

Pileup subtraction for jet shapes

Jet shapes have the potential to play a role in many LHC analyses, for example in quark-gluon discrimination or jet substructure analyses for hadronic decays of boosted heavy objects. Most shapes, however, are significantly affected by pileup. We introduce a general method to correct for pileup effects in shapes, which acts event-by-event and jet-by-jet, and accounts also for hadron masses. It involves a numerical determination, for each jet, of a given shape's susceptibility to pileup. Together with existing techniques for determining the level of pileup, this then enables an extrapolation to zero pileup. The method can be used for a wide range of jet shapes and we show its successful application in the context of quark/gluon discrimination and top-tagging.

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arXiv2012

Jet Fragmentation Function Moments in Heavy Ion Collisions

The nature of a jet's fragmentation in heavy-ion collisions has the potential to cast light on the mechanism of jet quenching. However the presence of the huge underlying event complicates the reconstruction of the jet fragmentation function as a function of the momentum fraction z of hadrons in the jet. Here we propose the use of moments of the fragmentation function. These quantities appear to be as sensitive to quenching modifications as the fragmentation function directly in z. We show that they are amenable to background subtraction using the same jet-area based techniques proposed in the past for jet p_t's. Furthermore, complications due to correlations between background-fluctuation contributions to the jet's p_t and to its particle content are easily corrected for.

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arXiv2011

FastJet user manual

FastJet is a C++ package that provides a broad range of jet finding and analysis tools. It includes efficient native implementations of all widely used 2-to-1 sequential recombination jet algorithms for pp and e+e- collisions, as well as access to 3rd party jet algorithms through a plugin mechanism, including all currently used cone algorithms. FastJet also provides means to facilitate the manipulation of jet substructure, including some common boosted heavy-object taggers, as well as tools for estimation of pileup and underlying-event noise levels, determination of jet areas and subtraction or suppression of noise in jets.

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arXiv2011

Fluctuations and asymmetric jet events in PbPb collisions at the LHC

Recent LHC results concerning full jet-quenching in PbPb collisions have been presented in terms of a jet asymmetry parameter, measuring the imbalance between the transverse momenta of leading and subleading jets. We examine the potential sensitivity of this distribution to fluctuations from the heavy-ion background. Our results suggest that a detailed estimate of the true fluctuations would be of benefit in extracting quantitative information about jet quenching. We also find that the apparent impact of fluctuations on the jet asymmetry distribution can depend significantly on the choice of low-pt threshold used for the simulation of the hard pp events.

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arXiv2011

Seeing jets in heavy-ion collisions

In these proceedings, we briefly review how jets can be reconstructed in heavy-ion collisions. The main point we address is the subtraction of the large contamination from the underlying event background. We first present the main ingredients needed to define the jets and perform the background subtraction and then discuss the efficiency of the subtraction for different jet algorithms and background-estimation methods.

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