Abstract
The observation of the associated production of the Higgs boson with two top quarks in proton-proton collisions is one of the highlights of the LHC Run 2. Driven by the theoretical description of the physics processes, the Matrix Element Method (MEM) consists in computing a probability that an event is compatible with the signal hypothesis (ttH) or with one of the background hypotheses. It is a powerful classifying tool requiring high dimensional integral computations. The deployment of our MEM production code on GPU’s platform will be described. What follows will focus on the adaptation of the main components of the computations in OpenCL kernels, namely the Magraph matrix element code generator, VEGAS, and LHAPDF. Finally, the gain obtained on GPU’s platforms compared with classical CPU’s platforms will be assessed.
Highlights
The recent observation of the associated production of the Higgs boson with two top quarks [1] is one of the highlights of the Run 2 at the LHC
The δ4(xaPa + xbPb − ΣPk) term corresponds to the kinematic constraints between the p-p incoming partons and the outgoing particles considered in the Matrix Element
To analyze a data set of around 2500 events, it requires typically 14 hours of computation deployed on 96 cores (6 nodes with 2 Intel Xeon E5-2640 processors, 16 physical cores rates at 2.6 Ghz), thanks to MPI. This means that the analysis would take 55 days on one core. Once this reference implementation was achieved, rather than optimizing the MPI-Matrix Element Method (MEM) code we focused our efforts on GPU’s platform, with the idea to gather as many GPU devices as possible to have at our disposal a huge computing power to run the MEM analysis[9]
Summary
The recent observation of the associated production of the Higgs boson with two top quarks [1] is one of the highlights of the Run 2 at the LHC. The coupling of the Higgs boson to the b and τ light fermions have been inferred from the decay rate of the Higgs boson in bb[4] and ττ[5] respectively. This strategy does not apply to the coupling with the top quark, yt, as the decay of the Higgs boson into a top quark pair is kinematically forbidden. The combination of Run 1 and 2016 Run 2 data from LHC collisions allowed the first observation[1] of the simultaneous production of a Higgs boson with a ttpair in April 2018
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