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Towards a generic implementation of matrix-element maximisation as a classifier in particle physics

Fri, 16/08/2019 - 04:14

by: von Buddenbrock, Stefan
The so-called matrix-element method (MEM) has long been used successfully as a classification tool in particle physics searches. In the presence of invisible final state particles, the traditional MEM typically assigns probabilities to an event -- based on whether it is more signal or background-like -- through a phase space integration over all degrees of freedom of the invisible particles in the process(es). One inherent shortcoming of the traditional MEM is that the phase space integration can be slow, and therefore impractical for high multiplicity final states and/or large data sets. The recent alternative of matrix-element maximisation has recently been introduced to circumvent this problem, since maximising a highly-dimensional function can be a far more CPU-efficient task than that of integration. In this work, matrix-element maximisation is applied to the process of fully-leptonic top associated Higgs production, where the Higgs boson decays to two $b$-quarks. A variety of optimisation algorithms are tested in terms of their performance and speed, and it is explicitly found that the maximisation technique is far more CPU-efficient than the traditional MEM at the cost of a slight reduction in performance. An interesting consequence of using matrix-element maximisation is that the result of the procedure gives an estimate of the four-momenta for the invisible particles in the event. As a result, the idea of using these estimates as input information for more complicated tools is discussed with potential prospects for future developments of the method.

Computational challenges for MC event generation

Fri, 02/08/2019 - 03:51

by: Buckley, Andy
The sophistication of fully exclusive MC event generation has grown at an extraordinary rate since the start of the LHC era, but has been mirrored by a similarly extraordinary rise in the CPU cost of state-of-the-art MC calculations. The reliance of experimental analyses on these calculations raises the disturbing spectre of MC computations being a leading limitation on the physics impact of the HL-LHC, with MC trends showing more signs of further cost-increases rather than the desired speed-ups. I review the methods and bottlenecks in MC computation, and areas where new computing architectures, machine-learning methods, and social structures may help to avert calamity.

Dipole evolution: perspectives for collectivity and $\gamma^*$A collisions

Wed, 31/07/2019 - 04:09
LU TP 19-32

by: Bierlich, Christian
The transverse, spatial structure of protons is an area revealing fundamental properties of matter, and provides key input for deeper understanding of emerging collective phenomena in high energy collisions of protons, as well as collisions of heavy ions. In this paper eccentricities and eccentricity fluctuations are predicted using the dipole formulation of BFKL evolution. Furthermore, first steps are taken towards generation of fully exclusive final states of $\gamma^*$A collisions, by assessing the importance of colour fluctuations in the initial state. Such steps are crucial for the preparation of event generators for a future electron-ion collider. Due to the connection between an impact parameter picture of the proton structure, and cross sections of ep and pp collisions, the model parameters can be fully determined by fits to such quantities, leaving results as real predictions of the model.

The PYTHIA Event Generator: Past, Present and Future

Wed, 24/07/2019 - 04:14
LU TP 19-31

by: Sjöstrand, Torbjörn
The evolution of the widely-used PYTHIA particle physics event generator is outlined, from the early days to the current status and plans. The key decisions and the development of the major physics components are put in context.

Coherent Showers in Decays of Coloured Resonances

Tue, 23/07/2019 - 04:03

by: Brooks, Helen
We present a new approach to coherent parton showers in the decays of coloured resonances, based on the notion of ``resonance-final'' (RF) QCD antennae. A full set of mass- and helicity-dependent $2\to 3$ antenna functions are defined, with the additional requirement of positivity over the respective branching phase spaces. Their singularity structure is identical to that of initial-final (IF) antennae in $2\to N$ hard processes (once mass terms associated with the incoming legs are allowed for), but the phase-space factorisations are different. The consequent radiation patterns respect QCD coherence (at leading colour) and reduce to DGLAP and eikonal kernels in the respective collinear and soft limits. The main novelty in the phase-space factorisation is that branchings in RF antennae impart a collective recoil to the other partons within the same decay system. An explicit implementation of these ideas, based on the Sudakov veto algorithm, is provided in the VINCIA antenna-shower plug-in to the PYTHIA 8 Monte Carlo event generator. We apply our formalism, matched to next-to-leading order accuracy using POWHEG, to top quark production at the LHC, and investigate implications for direct measurement of the top quark mass. Finally, we make recommendations for assessing theoretical uncertainties arising from parton showers in this context.

Fitting the Strong Coupling Constant with Soft-Drop Thrust

Wed, 26/06/2019 - 03:57

by: Marzani, Simone
Soft drop has been shown to reduce hadronisation effects at $e^+e^-$ colliders for the thrust event shape. In this context, we perform fits of the strong coupling constant for the soft-drop thrust distribution at NLO+NLL accuracy to pseudo data generated by the \textsf{Sherpa}~event generator. In particular, we focus on the impact of hadronisation corrections, which we estimate both with an analytical model and a Monte-Carlo based one, on the fitted value of $\alpha_s(m_Z)$. We find that grooming can reduce the size of the shift in the fitted value of $\alpha_s$ due to hadronisation. In addition, we also explore the possibility of extending the fitting range down to significantly lower values of (one minus) thrust. Here, soft drop is shown to play a crucial role, allowing us to maintain good fit qualities and stable values of the fitted strong coupling. The results of these studies show that soft-drop thrust is a promising candidate for fitting $\alpha_s$ at $e^+ e^-$ colliders with reduced impact of hadronisation effects.

A Monte-Carlo Simulation of Double Parton Scattering

Wed, 12/06/2019 - 04:04

by: Cabouat, Baptiste
In this work, a new Monte-Carlo simulation of double parton scattering (DPS) at parton level is presented. The simulation is based on the QCD framework developed recently by M. Diehl, J. R. Gaunt and K. Sch\"{o}nwald. With this framework, the dynamics of the $1\to2$ perturbative splittings is consistently included inside the simulation, with the impact-parameter dependence taken into account. The simulation evolves simultaneously two hard systems from a common hard scale down to the hadronic scale. The evolution is performed using an angular-ordered parton shower which is combined with a set of double parton distributions that depend explicitly on the inter-parton distance. An illustrative study is performed in the context of same-sign WW production at the LHC, with the quark content of the proton being limited to three flavours. In several distributions we see differences compared to DPS models in Herwig, Pythia, and the DPS "pocket formula".

Event Generation with Sherpa 2.2

Thu, 23/05/2019 - 09:15

by: Bothmann, Enrico
Sherpa is a general-purpose Monte Carlo event generator for the simulation of particle collisions in high-energy collider experiments. We summarize essential features and improvements of the Sherpa 2.2 release series, which is heavily used for event generation in the analysis and interpretation of LHC Run 1 and Run 2 data. We highlight a decade of developments towards ever higher precision in the simulation of particle-collision events.

Parton branching at amplitude level

Wed, 22/05/2019 - 03:58

by: Forshaw, Jeffrey R.
We present an algorithm that evolves hard processes at the amplitude level by dressing them iteratively with (massless) quarks and gluons. The algorithm interleaves collinear emissions with soft emissions and includes Coulomb/Glauber exchanges. It includes all orders in $N_{\mathrm{c}}$, is spin dependent and is able to accommodate kinematic recoils. Although it is specified at leading logarithmic accuracy, the framework should be sufficient to go beyond. Coulomb exchanges make the factorisation of collinear and soft emissions highly non-trivial. In the absence of Coulomb exchanges, we show how factorisation works out and how a partial factorisation is manifest in the presence of Coulomb exchanges. Finally, we illustrate the use of the algorithm by deriving DGLAP evolution and computing the resummed thrust, hemisphere jet mass and gaps-between-jets distributions in $e^+ e^-$.

Simulation of vector boson plus many jet final states at the high luminosity LHC

Tue, 14/05/2019 - 03:52
LU-TP 19-14

by: Hoeche, Stefan
We present a novel event generation framework for the efficient simulation of vector boson plus multi-jet backgrounds at the high-luminosity LHC and at possible future hadron colliders. MPI parallelization of parton-level and particle-level event generation and storage of parton-level event information using the HDF5 data format allow us to obtain leading-order merged Monte-Carlo predictions with up to nine jets in the final state. The parton-level event samples generated in this manner correspond to an integrated luminosity of 3ab-1 and are made publicly available for future phenomenological studies.