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REsummation-Improved moNtecarlo eVEnt geNeraTor

Periodic Reporting for period 2 - REINVENT (REsummation-Improved moNtecarlo eVEnt geNeraTor)

Reporting period: 2019-05-01 to 2020-10-31

The ultimate goal of physics is to understand in a quantitative manner the fundamental laws which govern the interactions among the most basics constituents of the universe. Our current understanding is based on the Standard Model (SM) of particle interactions, a quantum field theory that can be used to calculate the probability of a certain interaction to take place. The limits of this theory are tested at the Large Hadron Collider (LHC), where experiments perform very precise measurements of large statistical samples of particle collisions, probing the SM validity at the highest energies and tiniest distances ever accessed by humans. This is a formidable task, both experimentally and theoretically: on the experimental side, an extremely precise understanding of the statistical and systematic errors of the detectors is required; on the theoretical side, the most precise and accurate theory predictions are needed to match with the experimental precision, in a form which makes the comparison with data straightforward.

At hadronic colliders like the LHC, accurate modeling of the strong interactions in the framework of Quantum Chromodynamic (QCD) is crucial to interpret the experimental outcomes. The goal of this project is to push forward the frontier of precision QCD for event simulations. The key idea is to combine the three possible theoretical descriptions (fixed-order perturbative expansion, resummed calculations, and parton showers) into the same theoretical framework, in order to benefit from the advantages of each. The innovative approach proposed here improves over past efforts thanks to the inclusion of higher-logarithmic resummation, which bridges the gap between the perturbative description of hard radiation and the shower domain. This brings together three important advantages: the ability to use the best theoretical description in each region, the sizable reduction of the theoretical uncertainties gained by replacing the shower evolution with the higher-logarithmic resummation, and the ability to produce hadron-level events that can be directly interfaced to detector simulations.

By going beyond the state-of-the-art, REINVENT obtains the most precise theoretical predictions for the LHC in an event generator form that allows for direct comparison to data, producing results directly usable by experimental collaborations.
The work of REINVENT is organized into 3 different subprojects, each focusing on different directions of improvement of the state-of-the-art for event simulation.

The first Subproject A -“Color-neutral production processes for the LHC“ focuses on the immediate application of the novel approach matching higher-order resummation with fixed-order corrections and parton showering proposed by REINVENT to color-neutral production processes of high relevance for the LHC phenomenology. This serves also as a test the generality of the approach. For this subproject, we have completed the studies for the cases of W-boson production, HiggsStrahlung, and diphoton pair production. This HiggsStrahlung process was not originally part of the proposal, but it was added after it was realized it was well suited for our approach and that it could be used as a testing ground for a further extension of the method aiming at including the same level of higher-order corrections also to the Higgs boson decay to pair of bottom quarks, which is currently under development. The diphoton pair production process is instead an important milestone because it delivers the first program with this level of accuracy for this process and it is also proof of the capability of the method to deal with processes that require a nontrivial theoretical definition. e.g. phase space cuts or photon isolation already at the leading-order. In this subproject, we are also currently investigating Higgs production via gluon fusion and vector boson pair production.

Extending the Geneva approach to deal with more complicated processes, like vector boson plus one jet or Higgs boson plus one jet production, and merging these results with lower multiplicities is the second goal of REINVENT, pursued in Subproject B - “NNLO merging and N3LO exclusive generation“. For these processes where one jet is already present at the leading order, the principal resolution variable must distinguish between one or more jets, one-jettiness being the natural candidate.
Here the work is developing in two main directions, on one side we are rederiving some of the ingredients for the NNLL resummation of one-jettiness, the soft function in particular, on the other hand, we have devised a phase space map that preserves the value of zero and one jettiness while performing the necessary higher-order calculations. The existence of such a map is a requirement for the very definition of events used in our approach. We are now testing its implementation in a more local form of the one-jettiness subtraction procedure, with the goal of using it for the full implementation.


The last direction in the REINVENT research proposal is to investigate the usage of alternative resolution parameters and to perform the resummation of multiple observables at the same time. This has been explored in Subproject C - “Alternative and multiple resummations” We have studied the vector boson transverse momentum resummation, which is more suited to direct experimental measurements and for which the neglected nonsingular contributions are numerically more suppressed. This has been achieved after interfacing to RadISH, a program that delivers the state-of-the-art N3LL resummation for this quantity. This results proved two important points: one the one hand we showed how the REINVENT approach is independent of the resolution variable being used, while on the other hand we also proved how it can be interfaced to external tools, making it possible to increase its accuracy when more precise calculations become available. The interface with RadISH also opens up the possibility to perform the resummation
of two jet-resolution parameters at the same time, like the transverse momentum with a jet-veto, which has been studied in a separate publication.
With the work carried on until now, we have achieved results that were not available before for charged-current Drell-Yan production, HiggsShtrahlung and diphoton pair production. For diphoton-pair production, our implementation is the first-ever at this level of accuracy in a shower Monte Carlo program. At the same time, we have established the most precise simulation of the transverse momentum of a vector boson on a fully showered and hadronized form, that can be directly used form data comparison or Monte Carlo tuning. The work is proceeding as expected and we anticipate producing soon the best overall description of HiggsStrahlung production followed by Higgs decay to bottom quarks pairs. We are also working at similar state-of-the-art first Monte Carlo implementation for diboson pair production.

On the longer-term, the objectives of the project remain unchanged, we plan to deliver the most accurate predictions for the LHC phenomenology by extending the method to more processes, higher multiplicities, and performing more differential resummations.