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\title{The LHCf experiment}

\author{A Tiberio$^{2,1}$, O Adriani$^{2,1}$, E Berti $^{2,1}$, L Bonechi$^{1}$, M Bongi$^{2,1}$, R D'Alessandro$^{2,1}$, S Ricciarini$^{1,3}$, and A Tricomi$^{4,5}$ for the LHCf Collaboration}
\address{$^1$ INFN, Section of Florence, I-50019 Sesto Fiorentino, Florence, Italy}
\address{$^2$ Department of Physics, University of Florence, I-50019 Sesto Fiorentino, Florence, Italy}
\address{$^3$ IFAC-CNR, I-50019 Sesto Fiorentino, Florence, Italy}
\address{$^4$ INFN, Section of Catania, I-95131 Catania, Italy}
\address{$^5$ Department of Physics, University of Catania, I-95131 Catania, Italy}

\ead{alessio.tiberio@fi.infn.it}

\begin{abstract}
The LHCf experiment is dedicated to the measurement of very forward particle production in the high energy hadron-hadron collisions at LHC, with the aim of improving the cosmic-ray air shower developments models. Most of the simulations of particle collisions and detector response are produced exploiting the resources available at CNAF. The role of CNAF and the main recent results of the experiment are discussed in the following.
\end{abstract}

\section{Introduction}
The LHCf experiment is dedicated to the measurement of very forward particle production in the high energy hadron-hadron collisions at LHC. The main purpose of LHCf is improving the performance of the hadronic interaction models, that are one of the important ingredients of the simulations of the Extensive Air Showers (EAS) produced by primary cosmic rays.
Since 2009 the LHCf detector has taken data in different configurations of the LHC: p-p collisions at center of mass energies of 900\,GeV, 2.76\,TeV, 7\,TeV and 13\,TeV, and p-Pb collisions at $\sqrt{s_{NN}}\,=\,5.02$\,TeV and 8.16\,TeV. The main results obtained in 2018 is shortly presented in the next paragraphs.

\section{The LHCf detector}
The LHCf detector is made of two independent electromagnetic calorimeters placed along the beam line at 140\,m on both sides of the ATLAS Interaction Point, IP1 \cite{LHCf_experiment, LHCf_detector}. Each of the two detectors, called Arm1 and Arm2, contains two separate calorimeter towers allowing to optimize the reconstruction of neutral pion events decaying into couples of gamma rays. During data taking the LHCf detectors are installed in the so called \"recombination chambers\", a place where the beam pipe of IP1 splits into two separate pipes, thus allowing small detectors to be inserted just on the interaction line (this position is shared with the ATLAS ZDC e.m. modules). For this reason the size of the calorimeter towers is very limited (few centimeters). Because of the performance needed to study very high energy particles with the requested precision to allow discriminating between different hadronic interaction models, careful simulations of particle collisions and detector’s response are mandatory. In particular, due to the tiny transversal size of the detectors, large effects are observed due to e.m. shower leackage in and out of the calorimeter towers. Most of the simulations produced by the LHCf Collaboration for the study and calibration of the Arm2 detector have been run exploiting the resources made available at CNAF.

\section{Results obtained in 2018}

During 2018 no experimental operations were performed in LHC tunnel or SPS experimental area, so all the work was concentrated to the analysis of data collected during the 2015 operation in p-p collisions at 13 TeV and during 2016 operation in p-Pb collisions at 8.16 TeV.
The final results of photon and neutron production spectra in proton-proton collisions at $\sqrt{s} =$ 13 TeV in the very forward region ($8.81 < \eta < 8.99$ and $\eta > 10.94$ for photons, $8.81 < \eta < 9.22$ and $\eta > 10.76$ for neutrons) were published on Physics Letters B and Journal of High Energy Physics, respectively \cite{LHCf_photons, LHCf_neutrons}.
These are the first published results of the collaboration at the highest available collision energy of 13 TeV at the LHC.
In addition to proton-proton results, preliminary results for photon spectrum in proton-lead collisions at $\sqrt{s_{NN}} = 8.16$ TeV were obtained and presented in several international conferences.

\section{LHCf simulations and data processing}

A full LHCf event involves two kinds of simulations: the first one was produced making use of the COSMOS and EPICS libraries, the second one making use of the CRMC toolkit. In both cases we used the most common generators employed in cosmic ray physics. For the second group only secondary particles produced by collisions were considered, whereas for the first group transport through the beam pipe and detector interaction were simulated as well. For this purpose, all this software was at first installed on the CNAF dedicated machine, then we performed some debug and finally we interactively run some test simulations.
In order to optimize the usage of resources, simulations production was shared between Italian and Japanese side of the collaboration. For this reason, the machine was used as well to transfer data from/to Japanese server.
In addition to simulations activity, CNAF resources were important for data analysis, both for experimental and simulation files. This work required to apply all reconstruction processes, from raw data up to a ROOT file containing all relevant physics quantities reconstructed from detector information. For this purpose, LHCf analysis software was installed, debugged and continuously updated on the system. Because the reconstruction of a single file can take several hours and the number of files to be reconstructed is large, the usage of the queue dedicated to LHCf was necessary to accomplish this task. ROOT files were then transferred to local PCs in Firenze, in order to have more flexibility on the final analysis steps, that does not require long computing time.

In 2018, the CNAF resources were mainly used by LHCf for mass production of MC simulations needed for the $\pi^0$ analysis of LHC data relative to proton-proton collisions at $\sqrt{s} = 13\,$TeV.
In order to extend the rapidity coverage, in $\pi^0$ analysis also the data acquired with the detector shifted 5 mm upward with respect to the nominal position are analysed.
As a consequence all the MC simulations involving the detector have to be generated again with that modified geometry.
The full sample of $10^8$ collisions was generated for QGSJET model, while about 50\% of the EPOS sample was completed.

\section*{References}

\begin{thebibliography}{9}
\bibitem{LHCf_experiment} O. Adriani {\it et~al.}, JINST \textbf{3}, S08006 (2008)
\bibitem{LHCf_detector} O. Adriani {\it et~al.}, JINST \textbf{5}, P01012 (2010)
\bibitem{LHCf_photons} O. Adriani {\it et~al.}, Physics Letters B \textbf{780} (2018) 233–239
\bibitem{LHCf_neutrons} O. Adriani {\it et~al.}, J. High Energ. Phys. (2018) \textbf{2018}: 73.
\end{thebibliography}

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