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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.

Logarithmic Accuracy of Angular-Ordered Parton Showers

Mon, 29/04/2019 - 03:47

by: Bewick, Gavin
We study the logarithmic accuracy of angular-ordered parton showers by considering the singular limits of multiple emission matrix elements. This allows us to consider different choices for the evolution variable and propose a new choice which has both the correct logarithmic behaviour and improved performance away from the singular regions. In particular the description of $e^+e^-$ event shapes in the non-logarithmic region is significantly improved.

Collider Constraints on $Z^\prime$ Models for Neutral Current $B-$Anomalies

Thu, 25/04/2019 - 03:59

by: Allanach, B.C.
We examine current collider constraints on some simple $Z^\prime$ models that fit neutral current $B-$anomalies, including constraints coming from measurements of Standard Model (SM) signatures at the LHC. The `MDM' simplified model is not constrained by the SM measurements but is strongly constrained by a 139 fb$^{-1}$ 13 TeV ATLAS di-muon search. Constraints upon the `MUM' simplified model are much weaker. A combination of the current $B_s$ mixing constraint and ATLAS' $Z^\prime$ search implies $M_{Z^\prime}>1.9$ TeV in the Third Family Hypercharge Model example case. LHC SM measurements rule out a portion of the parameter space of the model at $M_{Z^\prime}<1.5$ TeV.

Multi-jet merging in a variable flavor number scheme

Tue, 23/04/2019 - 03:54

by: Höche, Stefan
We propose a novel technique for the combination of multi-jet merged simulations in the five-flavor scheme with calculations for the production of b-quark associated final states in the four-flavor scheme. We show the equivalence of our algorithm to the FONLL method at the fixed-order and logarithmic accuracy inherent to the matrix-element and parton-shower simulation employed in the multi-jet merging. As a first application we discuss Zbb production at the Large Hadron Collider.

Jet cross sections at the LHC and the quest for higher precision

Mon, 01/04/2019 - 03:45
SLAC-PUB-17411, UWTHPH-2019-9
ZU-TH 16/19
LU-TP-19-10, MCnet-19-06

by: Bellm, Johannes
We perform a phenomenological study of $Z$ plus jet, Higgs plus jet and di-jet production at the Large Hadron Collider. We investigate in particular the dependence of the leading jet cross section on the jet radius as a function of the jet transverse momentum. Theoretical predictions are obtained using perturbative QCD calculations at the next-to and next-to-next-to-leading order, using a range of renormalization and factorization scales. The fixed order predictions are compared to results obtained from matching next-to-leading order calculations to parton showers. A study of the scale dependence as a function of the jet radius is used to provide a better estimate of the scale uncertainty for small jet sizes. The non-perturbative corrections as a function of jet radius are estimated from different generators.

HEJ 2: High Energy Resummation for Hadron Colliders

Mon, 25/02/2019 - 03:13
DESY 19-028

by: Andersen, Jeppe R.
We present HEJ 2, a new implementation of the High Energy Jets formalism for high-energy resummation in hadron-collider processes as a flexible Monte Carlo event generator. In combination with a conventional fixed-order event generator, HEJ 2 can be used to obtain greatly improved predictions for a number of phenomenologically important processes by adding all-order logarithmic corrections in $\hat{s}/p_\perp^2$. A prime example for such a process is the gluon-fusion production of a Higgs boson in association with widely separated jets, which constitutes the dominant background to Higgs boson production in weak-boson fusion.