∎

e1e-mail<EMAIL_ADDRESS>(corresponding author)

11institutetext: Université Paris-Saclay, CNRS, IJCLab, 91405 Orsay, France

#  NLOAccess: automated online computations for collider physics

Carlo Flore e1,addr1  0000-0002-1071-204X

(Received: date / Accepted: date)

###### Abstract

We present NLOAccess, an online platform acting as a virtual access to
automated perturbative computations of physical observables related to
collider physics within collinear factorisation. We discuss the design of the
project and illustrate how, with few steps and without any need of pre-code or
code compilation, a user can generate events and calculate cross sections
related for instance to quarkonium production and/or heavy-ion collisions at
leading and next-to-leading order in $\alpha_{s}$ using MadGraph5_aMC@NLO or
HELAC-Onia. Finally, we report about the obtained results in terms of number
of users and runs, and we illustrate the future plans of the project.

###### Keywords:

Heavy ions quarkonia hadronic physics event generators online computation

††journal: Eur. Phys. J. A

##  1 Introduction

Perturbative numerical computations are key ingredients in different research
areas in high-energy physics for searches of new particles, for studies of the
quark-gluon-plasma properties or of heavy-quark (onium) production. On the one
hand, there are many available codes allowing for the computation of cross
sections for different reactions. They enable their users to calculate such
physical observables for several processes at higher orders in Quantum
Electro- and Chromo-dynamics (QED and QCD) or within the full Standard Model
(SM), that is when one considers all the couplings and vertices in the
QCD+Electroweak theory. Some examples of such a kind of these codes can be
found in Refs. Mangano:2002ea ; Wang:2004du ; Gavin:2010az ; Alioli:2010xd ;
Bevilacqua:2011xh ; Hamilton:2012np ; Campbell:2019dru . On the other hand,
not all of them behave in a fully automated fashion, i.e. the generation of
dedicated codes starts from a generic request, (for instance, a process in a
certain model – within or beyond the SM). Some examples of automated codes and
event generators can be found in Refs. Corcella:2000bw ; Kilian:2007gr ;
Belyaev:2012qa ; Sherpa:2019gpd . Noticeable exceptions are represented by two
programs, MadGraph5_aMC@NLO Alwall:2014hca  (MG5 in what follows) and HELAC-
Onia Shao:2012iz ; Shao:2015vga . They are fully automated codes, respectively
at leading and next-to-leading order (LO and NLO) for SM and Beyond SM (BSM)
calculation, and at LO and approximated higher orders for single and multiple
quarkonium production within the SM.

Generally speaking, the full automation offers a vast flexibility in terms of
usage of different BSM models (see for instance the large quantities of the
so-called Universal FeynRules Output (UFO) models Degrande:2011ua  for
MG5_aMC@NLO111A non exhaustive list of UFO models can be found at
https://madgraph.mi.infn.it/models_db.dat) and of generation of various
processes within a model. However, at the same time, it requires these codes
to be as much user-friendly as possible. Another potential complication is
represented by their compilation, linking and usage with external
libraries/codes like for instance LHAPDF Buckley:2014ana  or PYTHIA
Bierlich:2022pfr .

To avoid these potential drawbacks, online tools surely represent a valuable
asset. They allow any non-expert to produce scientific output without needing
any deep technical knowledge of the codes that are used and, in turn, they can
be used to learn how to use such codes, to train non-experts and, potentially,
to help in stimulating new ideas and produce new scientific results. One
example of an online tool is for instance APFEL Web Carrazza:2014gfa , that
allows one to visualise the parton distribution functions (PDFs) and plot
parton luminosities at the same time for different users. The project we
present here, NLOAccess, goes beyond the relatively simple visualisation of
PDFs and computation of parton luminosities.

NLOAccess is one of the two Virtual Accesses among the different Working
Packages of the EU Horizon 2020 integrated initiative called “STRONG-2020”. As
mentioned above, it gives access to automated tools allowing anyone to compute
observables related to different scattering processes involving hadrons,
without any need of writing down any part of code before a run. More
information on NLOAccess can be found at https://nloaccess.in2p3.fr. In the
following, we will also refer to the codes available on NLOAccess as tools. At
present, the two available tools on NLOAccess are MG5 and HELAC-Onia.
NLOAccess and its tools are accessible at this address:

https://nloaccess.in2p3.fr/tools/

The paper is organised as follows: Section 2 illustrates the design of
NLOAccess in terms of its server scheme, while in Section 3 we explain how to
obtain results from the online platform. Finally, in Section 4 we gather the
conclusions and ideas for the future of such a project.

##  2 Design

Let us first present how the NLOAccess platform is designed and illustrate the
different sides needed to handle multiple computations for several users at
the same time.

NLOAccess relies on Flask222https://flask.palletsprojects.com, a Python micro-
framework, that allows one to design a web application in a relatively easy
way. By virtue of a Python application, i.e. a Python code, one is able to
serve at the same time the front- and back-end, with a built-in internal web
server and an external one, thanks to the Werkzeug
toolkit333https://werkzeug.palletsprojects.com. The rendering of the website
is done through different HTML templates, and it is handled by
Jinja444https://jinja.palletsprojects.com, a web template engine. For each
page one has to define the desired functions, that dictate the web application
actions, into the Python application, and assign the corresponding HTML
template.

To describe the NLOAccess design, we can divide the structure into three
pieces: (a) the server side, (b) the cluster side and (c) the cloud side. A
diagram illustrating this design and the interactions between these three
parts is depicted on Figure 1.

Figure 1: Deployment layout of NLOAccess. The three sides, together with the
allowed interactions between an external user and the NLOAccess platform are
illustrated.

The three sides serve the following functions:

  1. (a)

the server side consists in the Flask server and the internal NLOAccess
servers, where the code and the results are stored;

  2. (b)

the cluster side refers to the internal batch system, managed through
HTCondor555https://htcondor.org. The codes present on NLOAccess are able to
communicate with the batch system, submitting runs on the cluster, retrieving
and storing them in the NLOAccess servers;

  3. (c)

the cloud side is represented by an OwnCloud666https://owncloud.com server,
where the user personal folder, which contains the results for each of the
requested runs, is securely stored and synchronised.

As one can see from Figure 1, an external user is allowed to interact only
with the server and the cloud sides through a web browser. On the server side,
the user sees and interacts only with the front-end, served by the Flask
application through the NLOAccess website, on which the user can send run
requests and wait for the results. The users are hence notified via e-mail
when the run is finished. The results for each run are then stored in the
personal user folder on the OwnCloud server. Such a folder is protected via
the same password the user has chosen during the registration procedure needed
to use the codes present on NLOAccess.

Internally, on the cluster side, we notice that both HELAC-Onia and MG5 are
able to interact with the batch system. For each requested run, they create
their HTCondor job, submit it to the batch system, wait for the runs to be
finished and retrieve the corresponding results. NLOAccess is then able to
retrieve the output and storing it in the corresponding cloud user folder. As
already mentioned, the user is then also notified via e-mail for the run
completion.

Since NLOAccess allows every user to ask for a run request at any time, there
is the need of keeping track of the different runs, associate them with the
corresponding user, and put the results in their corresponding user folder. It
is then natural to store some data about the user and the runs. To do so, we
employ a PostgreSQL777https://www.postgresql.org/ database, that is ideal to
be integrated in a Python environment. The database securely stores data about
the users888Data security is warranted. Password are securely encrypted and
the user can ask for the cancellation of their data as the EU GDPR requires.,
the runs and the different processes that are generated (the latter
information is kept for practical and statistical purposes).

Let us finally notice that this design allows any code that can be compiled
and launched on a Unix shell to be included on the web platform.

##  3 Features, usage and results

We turn now to the description of the usage of NLOAccess. Generally speaking,
the run requests are done via some dedicated forms or, for more articulated
runs, via file input. The input file can either be built locally by the user
and uploaded through the web pages, or it can be directly written down and
uploaded via dedicated forms on the web platform. We stress that the building
of the input file is the sole effort requested to the users, to which is
offered all the computational power needed to perform their runs. The running
time is a priori not limited, but a time limit can be set by the server.

As mentioned in Section 1, at the moment, NLOAccess offers the possibility to
use two tools: MadGraph5_aMC@NLO and HELAC-Onia. Although for each tool the
commands needed to trigger a run are similar, we prefer to separately describe
the usage of both codes on NLOAccess. Indeed, these two codes, although
sharing some aspects (e.g. they both generate events, they involve Monte Carlo
–MC– integration, they are interfaced with external libraries etc.), are of
different nature. While HELAC-Onia is really a code, MG5 is a meta-code, i.e.
a code generating other codes. As such, there are some small but substantial
differences in the way the two codes are run, that reflect in some aspects,
for instance the code re-usage. Since HELAC-Onia is pre-compiled, it has a
single executable associated to itself. Therefore, one has to run again the
same process if some parameters such as PDFs, scale choices, or kinematical
cuts, have to be changed. On the other hand, MG5 allows one to generate a
specific code for the requested process and compiles it. This allows the user
to run the same code multiple times. Hence, while for HELAC-Onia there can be
a 1-to-$n$ correspondence between a process and the number of output folders,
for MG5 there is a 1-to-1 correspondence. Thus, for each run, the results will
be stored in a sub-folder into the main process folder. These infrastructure
differences, summarised in Table 1, reflect in the different usage of the two
tools on NLOAccess, as explained below in the following subsections.

Table 1: Summary of infrastructure differences between HELAC-Onia and MG5_aMC@NLO. | HELAC-Onia | MG5
---|---|---
code compilation | once |  once for every generate/output or launch commands
running |  run single executable for each run |  re-run the generated code for the requested process
code re-usage | ✗ | ✓

###  3.1 HELAC-Onia

Let us first start by illustrating how HELAC-Onia can be used on NLOAccess.
HELAC-Onia offers the generation of matrix elements and events for heavy
quarkonium physics within the SM at Born order. It is based on Non-
Relativistic QCD (NRQCD) Bodwin:1994jh  and uses the off-shell recursion
relations Berends:1987me  to speed up the the matrix-element computation. Such
a code is available at https://nloaccess.in2p3.fr/HO/.

Generally speaking, in the NRQCD framework, the cross section for a process
(for instance, inclusive quarkonium production in $pp$ collisions) is written
in a factorised form as follows:

$\displaystyle\sigma(pp\to{\pazocal{Q}}+X)$ $\displaystyle=\sum_{i,j,n}\int
dx_{1}dx_{2}f_{i/p}(x_{1})f_{j/p}(x_{2})$ (1)
$\displaystyle\times\hat{\sigma}(ij\to
Q\bar{Q}[n]+X)\langle\mathcal{O}_{n}^{\pazocal{Q}}\rangle$

where $f_{i/p}(x_{1})$, $f_{j/p}(x_{2})$ are the PDFs for the partons $i$, $j$
carrying a longitudinal momentum fraction $x_{1}$, $x_{2}$ from the two
protons in the inital state, $\hat{\sigma}(ij\to Q\bar{Q}[n]+X)$ is the
partonic cross section for producing a heavy-quark pair in the Fock state $n$,
with $n=^{2S+1}\\!L_{J}^{c}$ ($S$ being the spin, $L$ the orbital and $J$ the
total angular momentum of the heavy-quark pair, and $c=1,8$ indicating if the
pair is in a colour singlet – CS – or colour octet – CO – state), and
$\langle\mathcal{O}_{n}^{\pazocal{Q}}\rangle$ is the long distance matrix
element (LDME) for the specific Fock state $n$, describing the transition
between the heavy-quark pair and the observed quarkonium in the final state.

HELAC-Onia allows to calculate the convolution of these different terms for
all such a kind of processes as in Eq. (1), not only for single but also for
multiple quarkonia in the final state. It operates for different kinds of
reactions (hadro-production, electro-production, photo-production etc.). This
code is interfaced with LHAPDF, allowing also for the computation of cross
sections for collisions involving heavy ions by using nuclear PDFs (nPDFs).
The code is also interfaced with parton shower generators like PYTHIA. It also
allows one to generate Les Houches Events (LHE, with the corresponding
extension .lhe) Alwall:2006yp .

In general, to compute a cross section with HELAC-Onia on NLOAccess , it is
sufficient to write and submit an input file in the following general form:

generate $\\{$ process $\\}$

set $\\{$ parameter $\\}$ = $\\{$ value $\\}$

$\vdots$

decay $\\{$ particle $\\}$ @ $\\{$ BR $\\}$

$\vdots$

launch

One can highlight four different kinds of commands:

  1. (i)

the generate command is needed to indicate the desired process to be
generated. For instance, if one wishes to generate our example in Eq. (1) for
a $J/\psi$ in CS, it is sufficient to write “generate p p > cc$\sim$(3S11) j”,
where the corresponding syntax for the $J/\psi$ in the CS state and for a jet
(representing the set of partons created in the hard scattering which is
denoted as $X$ in Eq. (1)) is used. The syntax for the particles and the
quarkonium states can be found on NLOAccess999See
https://nloaccess.in2p3.fr/tools/sm-particles and
https://nloaccess.in2p3.fr/tools/quarkonia for SM particles and quarkonia
respectively.;

  2. (ii)

the set command allows one to change every parameter defined within HELAC-
Onia. Through this command, one has control on several classes of parameters:

     * •

collision parameters, i.e. which particles are in the initial state;

     * •

theory parameters such as the factorisation and renormalisation scales or the
value of the coupling constants;

     * •

PDF related parameters (e.g. whether to use LHAPDF and which (n)PDF set to
use);

     * •

MC setup parameters;

     * •

kinematical cuts;

     * •

quarkonium specific parameters (e.g. the value of the LDMEs);

     * •

physical constants (like the quark, bosons and lepton masses);

     * •

parameters specifying the desired kind of output (ROOT, Gnuplot, TopDrawer
and/or .lhe files).

A list of set commands is then required if the user wishes to use different
parameters for a run101010A dedicated user guide for the HELAC-Onia syntax is
available at
https://nloaccess.in2p3.fr/tools/downloads/HO_online_guide_v01.pdf.;

  3. (iii)

the decay command allows one to define a specific BR for the decay of a
particle. For instance, if one wants a $J/\psi$ to decay in di-muons with a BR
of 6%, is it sufficient to use decay cc$\sim$(3S11) > m+ m- @ 0.06; the
corresponding computed cross section will then be multiplied by the desired
BR;

  4. (iv)

the launch command triggers the computation.

The syntax for the particles and the naming of the most important variables
are illustrated on the online guide for HELAC-Onia which we have developed and
made available on NLOAccess.

An input file can be directly uploaded or built on the web platform using the
dedicated forms. This will trigger the calculation for the desired process and
parameters and, if the calculation will be successful, the user will receive
an e-mail containing the link of their personal OwnCloud folder, into which
they will find the plots done for them on the fly. This relatively simple and
automated procedure allows the NLOAccess users to obtain results with HELAC-
Onia in a straightforward way.

A last comment about the usage of HELAC-Onia is in order. As mentioned at the
beginning of Section 3 and also summarised in Table 1, this tool provides a
single executable that is used for every run. Linked to this executable, there
is an analysis code, that is used to create and fill the histograms given as
output of a calculation. At the moment, we are then limited to offer to the
users the generation of the plots for the total cross section, and the single
differential cross sections in terms of rapidity $y$ and transverse momentum
$P_{T}$ of each particle in the final state. If a user is interested to get
more complex kinematical distributions, they can be obtained through the
analysis of the LHE event file.

###  3.2 MG5_aMC@NLO

We now turn to the MG5 usage on NLOAccess. First, we emphasise that the
availability of MG5 on NLOAccess represents for the first time the opportunity
to use such a code on a web interface in its full version, i.e. allowing also
for NLO computations. Indeed, in the past, an online platform for MG5 existed,
with a couple of mirrors around the world, but the allowed calculations were
limited to leading order. MG5 on NLOAccess is reachable at
https://nloaccess.in2p3.fr/MG5/.

Although such a code is very well known and widely used, especially for the
LHC physics, let us remind that MG5 is a framework for SM and BSM
phenomenology that provides all the necessary elements to compute cross
sections, generating pertubative short-distance events and matching them with
parton-shower programs. The processes can be simulated at LO accuracy for any
user-defined Lagrangian, and at NLO for a subset of models. We notice here
that the current version of MG5 already allows the computation of cross-
sections and the generation of events at NLO in collinear QCD for symmetric
nuclear beam ($AA$) collisions, as this requires only to indicate an LHAPDF ID
for nPDFs in the run card. The extension to asymmetric hadronic collisions
(e.g. $pA$ collisions), that will allow the automated computation of nuclear
modification factors at fixed-order in QCD, is now being finalised.
Preliminary results are presented in Ref. Safronov:2022uuy . We guide the
reader to Ref. Alwall:2014hca  and to their Launchpad
website111111https://launchpad.net/mg5amcnlo for all the information about
this framework.

Moving on, we first recall that MG5 is a meta-code. This allows one to
separate the usage of such a tool in two stages: the code generation and the
code running. This clearly reflects in the usage of MG5 on the web platform.
The first stage consists in the generation of the code via a simple form, that
requests three values: the name of the model, the process to be generated, the
name of the corresponding output folder. Such a first stage can be also
obtained uploading a simple input file in the following form:

import model $\\{$ model_name $\\}$

generate $\\{$ process $\\}$

output $\\{$ folder_name $\\}$

where again the curly brackets indicates specific values for the three
parameters. By default, the SM is used as a model. If an NLO computation is
requested using the corresponding MG5 syntax, the loop_sm model will be then
imported. Notice that, although both HELAC-Onia and MG5 works for $2\to n$
processes, in the latter tool no hadron can be generated with a fixed-order
(FO) computation, but only through parton shower generators. Moreover, we
underline that, at present, the sole way to compute quarkonium-production
cross sections at FO in MG5 is by applying the Colour-Evaporation Model
Fritzsch:1977ay ; Halzen:1977rs  (CEM). This approach relies on the principle
of quark-hadron duality and, to be applied, it requires dedicated cuts on the
invariant mass of the heavy-quark pair. These cuts can be implemented in an
analysis code for a specific process generated with MG5 involving an heavy-
quark pair in the final state121212As for the moment the upload of parts of
code is not allowed on NLOAccess, we invite the users to contact the NLOAccess
Team if different analysis files from the ones already available are needed..
An example of such a calculation can be found in Ref. Lansberg:2020rft , where
MG5 was used to apply the CEM at NLO for single and double quarkonium hadro-
production at Tevatron and the LHC.

Figure 2: Example of the form on NLOAccess to run the NLO MG5 code.

The second stage to use MG5 on NLOAccess is the code running. The users can
find their generated codes on their personal MG5 code database on NLOAccess.
Once they select the desired code to be run, the user finds a form with few
drop-down menus and a form where to upload their cards. The form changes
whether the code was generated using the LO or NLO syntax of MG5. An example
of a form for an NLO code is shown in Fig. 2. Both kind of codes can be run
either in the FO mode, or can be interfaced with a parton shower. At the
moment, within NLOAccess, two parton-shower generators are interfaced with
MG5: PYTHIA (both version 6 and 8) and HERWIG 6 Corcella:2000bw . Other tools
can be used for a run, for instance MadAnalysis5 Conte:2012fm  (for plotting
and analysis purposes), MadSpin Artoisenet:2012st  (for particle decays) or to
apply some reweighting procedures Mattelaer:2016gcx ; Kalogeropoulos:2018cke .
Again, once a run is performed and completed, the user is alerted via e-mail
and the results are uploaded and securely stored in the user personal OwnCloud
folder. Contrary to HELAC-Onia, for which the generation of .lhe files has to
be requested, MG5 automatically provides the events under the LHE format for
all the runs but for the FO NLO ones. In the latter case, plots for total and
differential cross-sections are automatically generated. Once again, the user
can obtain the desired results, with a couple of simple steps, with this
automated two-step procedure.

###  3.3 Further general features

Having described the usage of both tools accessible via NLOAccess, we now
conclude by describing some further general features of the web platform.

To enhance the user-friendliness of the website, we allow the users for the
management of their runs. They are in fact able to check their live run status
on a dedicated page, and to remove a run - or multiple runs - if needed. For
each run, the number of sub-jobs running, in idle state or completed is also
shown. The information on every run is then retrieved in the personal run
history of the user. In this way, the user will retrieve information on the
tool that was used, the generated process, the date and time of the running
and the duration of the run itself. The total amount of time taken by all the
runs performed for the user is also given.

##  4 Summary and outlook

In this paper, we presented NLOAccess, an online platform devoted to automated
computation of physical observables for collider physics, part of the EU
Horizon 2020 integrated initiative “STRONG-2020”. At present, NLOAccess allows
any kind of user, from students and non-experts to people with experience, to
generate events and compute the cross section, in the collinear factorisation
approach, for several kinds of processes and for different initial and final
state particles involving also heavy-ion collisions and quarkonium production.
Noticeably, NLOAccess also allows for the first time to run on the web MG5 in
its full NLO version. We showed how, with few steps, any user is able to
generate and run codes, obtaining automatically plots done on the fly,
securely stored in a personal cloud folder. Such an online platform allowed so
far around 400 users from all over the world to perform over 4000 runs. Thanks
to its design, NLOAccess allows in principle any code that can be compiled and
launched on a Unix shell to be included, hence it represents an opportunity
for who is interested in promoting their own scientific codes.

Within the NLOAccess project, the team is also involved in code developments.
We mention here the extension of MG5 to the case of asymmetric hadronic
collisions, i.e. collisions between two hadronic beams of different nature
(e.g. proton-nucleus, nucleus $A$-nucleus $B$, pion-proton). Preliminary
results are presented in Ref. Safronov:2022uuy , and will be made available
soon on NLOAccess. These results will allow one to automatically calculate at
fixed-order the nuclear modification factors, that describes the nuclear
effects affecting the PDFs of quarks and gluons inside a bound nucleon. In
parallel to this development, there is also ongoing effort for extension and
validation of lepton-hadron collisions in MG5. Finally, we also mention the
creation of a Python3 compliant version of HELAC-Onia, that is available under
request.

Given the experience maturated since the beginning of the project, where
NLOAccess was used not only as a service for the community, but also to train
students and non-expert during several tutorials and hands-on session, we
believe that such a kind of project successfully shows all the benefits of
online access to such automated codes for everyone.

###### Acknowledgements.

We thank L.-A. Couturié, V. Lafage, J.-P. Lansberg, O. Mattelaer and H.-S.
Shao for their help and suggestions at different stages of the developement of
the NLOAccess platform. We also thank O. Mattelaer for a careful reading of
the manuscript. This project is supported by the European Union’s Horizon 2020
research and innovation programme under Grant agreement no. 824093. This
project has also received funding from the Agence Nationale de la Recherche
(ANR) via the grant ANR-20-CE31-0015 (“PrecisOnium”) and via the IDEX Paris-
Saclay ”Investissements d’Avenir” (ANR-11-IDEX-0003-01) through the
GLUODYNAMICS project funded by the ”P2IO LabEx (ANR-10-LABX-0038)”. This work
was also partly supported by the French CNRS via the IN2P3 project GLUE@NLO,
via the Franco-Polish agreement COPIN-IN2P3 and via the Franco-Polish EIA
(GlueGraph).

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