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Mrinal Dasgupta

Publications and source records attributed to Mrinal Dasgupta.

At least 37 records · Page 2Linked to original sources
arXiv2015

On jet substructure methods for signal jets

We carry out simple analytical calculations and Monte Carlo studies to better understand the impact of QCD radiation on some well-known jet substructure methods for jets arising from the decay of boosted Higgs bosons. Understanding differences between taggers for these signal jets assumes particular significance in situations where they perform similarly on QCD background jets. As an explicit example of this we compare the Y-splitter method to the more recently proposed Y-pruning technique. We demonstrate how the insight we gain can be used to significantly improve the performance of Y-splitter by combining it with trimming and show that this combination outperforms the other taggers studied here, at high $p_T$. We also make analytical estimates for optimal parameter values, for a range of methods and compare to results from Monte Carlo studies.

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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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arXiv2013

QCD calculations for jet substructure

We present results on novel analytic calculations to describe invariant mass distributions of QCD jets with three substructure algorithms: trimming, pruning and the mass-drop taggers. These results not only lead to considerable insight into the behaviour of these tools, but also show how they can be improved. As an example, we discuss the remarkable properties of the modified mass-drop tagger.

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arXiv2013

Jet substructure with analytical methods

We consider the mass distribution of QCD jets after the application of jet substructure methods, specifically the mass-drop tagger, pruning, trimming and their variants. In contrast to most current studies employing Monte Carlo methods, we carry out analytical calculations at the next-to-leading order level, which are sufficient to extract the dominant logarithmic behaviour for each technique, and compare our findings to exact fixed-order results. Our results should ultimately lead to a better understanding of these jet substructure methods which in turn will influence the development of future substructure tools for LHC phenomenology.

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arXiv2013

Towards an understanding of jet substructure

We present first analytic, resummed calculations of the rates at which widespread jet substructure tools tag QCD jets. As well as considering trimming, pruning and the mass-drop tagger, we introduce modified tools with improved analytical and phenomenological behaviours. Most taggers have double logarithmic resummed structures. The modified mass-drop tagger is special in that it involves only single logarithms, and is free from a complex class of terms known as non-global logarithms. The modification of pruning brings an improved ability to discriminate between the different colour structures that characterise signal and background. As we outline in an extensive phenomenological discussion, these results provide valuable insight into the performance of existing tools and help lay robust foundations for future substructure studies.

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arXiv2012

On jet mass distributions in Z+jet and dijet processes at the LHC

The mass distribution of jets produced in hard processes at the LHC plays an important role in several jet substructure related studies involving both Standard Model and BSM physics, especially in the context of boosted heavy particle searches. We compute analytically the jet-mass distribution for both Z+jet and dijet processes, for QCD jets defined in the anti-k_t algorithm with an arbitrary radius R, to next-to-leading logarithmic accuracy and match our resummed calculation to full leading-order results. We note the important role played by initial state radiation (ISR) and non-global logarithms explicitly computed here for the first time for hadron collider observables, as well as the jet radius dependence of these effects. We also compare our results to standard Monte Carlo event generators and discuss directions for further studies and phenomenology.

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arXiv2012

Predictions for Drell-Yan ϕ* and Q_T observables at the LHC

We make theoretical predictions for the recently introduced variable ϕ* corresponding to the azimuthal angle between leptons produced in the Drell-Yan process at the LHC. As a consequence of this work we are also able to generate results for the more commonly studied transverse momentum Q_T of the lepton pair. Comparisons of these purely perturbative estimates for the Q_T case yield good agreement with ATLAS and CMS data, as we demonstrate. We anticipate that this work will help stimulate measurements of ϕ* at the LHC.

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arXiv2012

Probing the low transverse momentum domain of Z production with novel variables

The measurement of the low transverse momentum region of vector boson production in Drell-Yan processes has long been invaluable to testing our knowledge of QCD dynamics both beyond fixed-order in perturbation theory as well as in the non-perturbative region. Recently the DØ collaboration have introduced novel variables which lead to improved measurements compared to the case of the standard QT variable. To complement this improvement on the experimental side, we develop here a complete phenomenological study dedicated in particular to the new ϕ* variable. We compare our study, which contains the state-of-the-art next-to-next-to-leading resummation of large logarithms and a smooth matching to the full next-to-leading order result, to the experimental data and find excellent agreement over essentially the entire range of ϕ*, even without direct inclusion of non-perturbative effects. We comment on our findings and on the potential for future studies to constrain non-perturbative behaviour.

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arXiv2011

Accurate QCD predictions for new variables to study dilepton transverse momentum

We report on the computation of the angle-between-leptons distribution in Drell-Yan processes. More precisely we study the recently introduced variable phi*, which provides us with a more accurate probe of the low Q_T domain of Z boson production at hadron colliders. Our theoretical prediction is obtained by matching a next-to-next-to leading logarithmic (NNLL) resummation to a fixed order calculation at next-to-leading order (NLO). We find that the result significantly differs from a pure fixed order calculation in a wide range of the observable we are interested in, clearly indicating the need for resummation. We also perform a first comparison to the measurement by the D0 collaboration, finding good agreement between theory and experiment.

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arXiv2011

QCD predictions for new variables to study dilepton transverse momenta at hadron colliders

The D0 collaboration has recently introduced new variables, a_T and phi* to more accurately probe the low Q_T domain of Z boson production at hadron colliders than had been previously possible through a direct study of the Q_T distribution. The comparison of such accurate data to precise theoretical predictions from QCD perturbation theory will yield important information on the ability of resummed QCD predictions as well as parton shower models to describe the low Q_T domain and should enable more stringent constraints on non-perturbative effects. In the present paper we provide analytical predictions for the above mentioned variables, that contain resummation of large logarithms, including next-to-next-to leading logarithmic (NNLL) terms, supplemented by exact next-to-leading order calculations from MCFM.

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arXiv2010

Non-global logarithms and jet algorithms in high-pT jet shapes

We consider jet-shape observables of the type proposed recently, where the shapes of one or more high-pT jets, produced in a multi-jet event with definite jet multiplicity, may be measured leaving other jets in the event unmeasured. We point out the structure of the full next-to-leading logarithmic resummation specifically including resummation of non-global logarithms in the leading-Nc limit and emphasising their properties. We also point out differences between jet algorithms in the context of soft gluon resummation for such observables.

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arXiv2010

Jet masses for high p_T QCD jets at all orders

The study of the shape and sub-structure of high p_T jets produced in hadron collisions is becoming an increasingly important component of LHC phenomenology in the context of new particle discoveries. We study here the state of the art for masses of QCD jets in perturbation theory to all orders, up to next-to--leading logarithmic accuracy, with inclusion of non-global logarithms. We also study the precise role of the jet algorithm used to construct the jets and its impact on resummed predictions. Such analytical predictions, where available, offer an alternative to leading logarithmic predictions from Monte Carlo event generators and could, after fixed-order matching and accounting for non-perturbative effects, be used for direct comparison to early LHC data.

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arXiv2010

The aT distribution of the Z boson at hadron colliders

We provide the first theoretical study of a novel variable, $a_T$, proposed in Ref.[1] as a more accurate probe of the region of low transverse momentum $p_T$, for the $Z$ boson $p_T$ distribution at hadron colliders. The $a_T$ is the component of $p_T$ transverse to a suitably defined axis. Our study involves resummation of large logarithms in $a_T$ up to the next-to--leading logarithmic accuracy and we compare the results to those for the well-known $p_T$ distribution, identifying important physical differences between the two cases. We also test our resummed result at the two-loop level by comparing its expansion to order $α_s^2$ with the corresponding fixed-order results and find agreement with our expectations.

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arXiv2009

Hadronisation corrections for jets in the k_t algorithm

It has recently been established that hadronisation corrections to QCD jets vary as $1/R$, at small $R$, for jets of radius $R$. Here we demonstrate, using jets in the $k_t$ algorithm, that the magnitude of these $1/R$ corrections are unambiguously linked to the magnitude of $1/Q$ corrections to commonly studied event shapes in $e^{+}e^{-}$ annihilation.

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arXiv2009

The Z boson a_T distribution at hadron colliders

We provide a theoretical study of a novel variable introduced in Ref. \cite{WV} to study the transverse momentum of the Z boson at hadron colliders. The variable we consider has experimental advantages over the standard $p_T$ distribution enabling more accurate measurement at low $p_T$. We provide an all orders perturbative estimate for this variable at the next-to--leading logarithmic accuracy and compare the results to those for the standard $p_T$ distribution. We test our resummation, at the two-loop level, by comparing its expansion to fixed-order perturbative estimates and find agreement with our expectations.

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arXiv2009

On the universality of hadronisation corrections to QCD jets

We improve previously derived analytical estimates of hadronisation corrections to QCD jets at hadron colliders, firmly establishing at the two-loop level the link to the well-known power corrections to LEP event-shape variables. The results of this paper apply to jets defined in the kt and anti-kt algorithms but the general framework presented here holds also for other algorithms for which calculations are in progress.

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arXiv2008

Analytical studies for non-perturbative QCD of jets at hadron colliders

Inspired by the success of analytical models for non-perturbative effects, used to investigate event shape variables at LEP and HERA, we apply them to a study of jets at hadron colliders such as the Tevatron and the LHC. We find that simple analytical estimates are able to shed considerable light on issues that could previously be tackled only through Monte-Carlo simulations, for example the role of different non-perturbative effects in various jet algorithms. In this context, we also provide testable numerical results for the commonly studied inclusive-jet $p_t$ distribution, and we introduce new observables that could be employed to verify our calculations.

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