Search for a Λnn bound state in Pb-Pb collisions with ALICE at the LHC

Annalisa Mastroserio,G.E Bruno,C Corianò,P Colangelo,F De Fazio,E Nappi,G Chiodini,D.M Creanza

doi:10.1051/epjconf/201819200045

Annalisa Mastroserio, G.E Bruno + Show 6 more

Open Access

https://doi.org/10.1051/epjconf/201819200045

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Abstract

The extreme energy densities reached at the LHC lead to the production of a significant amount of baryons and strangeness. Such a regime allows for an increased production of potentially existing exotic QCD bound states containing nuclei and strange hadrons. An interesting measurement for the phenomenology of the nuclear interaction is the presence of a neutral bound state constituted by one Λ and two neutrons. The excellent particle identification, tracking and vertexing performance of the ALICE experiment allow for the search of this exotic bound state in the decay channel Λnn →3H+π- in Pb-Pb collisions. In order to improve detection of this state despite the presence of a huge combinatorial background, the extraction of the signal is performed by means of a multivariate approach with the TMVA (Toolkit for Multivariate Data Analysis with ROOT). So far the indication for the existence of the Λnn state was reported only by the HypHI Collaboration, so the observation by ALICE would crucially contribute to the study of such an exotic state.

Highlights

The nuclear state composed by the Λ hyperon and two neutrons, namely the Λnn state, can be considered as an isospin partner state of the 3 ΛH which has a separation energy ≈keV and this is already a hint that the neutral state can hardly be formed
The Λnn state, if existing, can be eventually detected with ALICE via the decay channel Λnn →3H+π− and its observation would crucially contribute to the understanding of exotic nuclear bound states
The triton identification is the main issue here, because above 2 GeV/c the sample of candidates selected by the Time Projection Chamber (TPC) contain several low mass particles (e.g.: pions and electrons) which enlarge dramatically the combinatorial background

Summary

Introduction

The nuclear state composed by the Λ hyperon and two neutrons, namely the Λnn state, can be considered as an isospin partner state of the. KeV and this is already a hint that the neutral state can hardly be formed. The production of a significant amount of baryons and strangeness at LHC energies allows for an increased production of potentially existing exotic QCD bound states. The Λnn state, if existing, can be eventually detected with ALICE via the decay channel Λnn →3H+π− and its observation would crucially contribute to the understanding of exotic nuclear bound states

Analysis strategy for Λnn detection

Λnn signal extraction with a Machine Learning approach