Littérature scientifique sur le sujet « Hypertriton »

The lightest hypernucleus, the hypertriton, has been a benchmark in the field of hypernuclear physics. However, some of recent experiments employing energetic heavy-ion beams have revealed that the hypertriton lifetime is significantly shorter than 263 ps which is expected by considering the known weakly binding nature of the hypertriton. The STAR collaboration has also measured the hypertriton binding energy, and the deduced value is contradicting to its formerly known small binding energy. These measurements have indicated that the fundamental physics quantities of the hypertriton such as its lifetime and binding energy have not been understood, therefore, they have to be measured very precisely. Furthermore, an unprecedented Λnn bound state observed by the HypHI collaboration has to be studied in order to draw a conclusion whether or not such a bound state exists. These three-body hypernuclear states are studied by the heavy-ion beam data in theWASA-FRS experiment and by analysing J-PARC E07 nuclear emulsion data with machine learning.

Conflicting values of the hypertriton lifetime τ(3ΛH) were derived in relativistic heavy ion (RHI) collision experiments over the last decade. A very recent ALICE Collaboration measurement is the only experiment where the reported τ(3ΛH) comes sufficiently close to the free-Λ lifetime τΛ, as expected naively for a very weakly bound Λ in 3ΛH. We revisited theoretically this 3ΛH lifetime puzzle [1], using 3ΛH and 3He wave functions computed within the ab initio no-core shell model employing interactions derived from chiral effective field theory to calculate the two-body decay rate Γ(3ΛH → 3He + π−). We found significant but opposing contributions arising from ΣNN admixtures in 3ΛH and from π− − 3He final-state interaction. To derive τ(3ΛH), we evaluated the inclusive π− decay rate Γπ− (3ΛH) by using the measured branching ratio Γ(3ΛH → 3He + π−)/Γπ− (3ΛH) and added the π0 contributions through the ΔI = 1/2rule. The resulting τ(3ΛH) varies strongly with the rather poorly known Λ separation energy Esep(3ΛH) and it is thus possible to associate each one of the distinct RHI τ(3ΛH) measurements with its own underlying value of Esep(3ΛH).

We calculated the hypertriton production at RHIC-STAR and HIRFL-CSR acceptance, with a multi-phase transport model (AMPT) and a relativistic transport model (ART), respectively. In specific, we calculated the Strangeness Population Factor [Formula: see text] at different beam energy. Our results from AGS to RHIC energy indicated that the collision system may change from hadronic phase at AGS energies to partonic phase at RHIC energies. Our calculation at HIRFL-CSR energy supports the proposal to measure hypertriton at HIRFL-CSR.

We present the effects of Fermi motion and different off-shell assumptions on the differential cross sections of photo- and electroproduction of the hypertriton. We found that Fermi motion plays an important role in these processes. Different off-shell assumptions yield different cross section magnitudes. The few available electroproduction data favor the assumption that the initial nucleon is off-shell and the final hyperon is on-shell. Measurement of the hypertriton photoproduction is urgently required to further support our findings.

Recent heavy-ion collision experiments reported a surprisingly short lifetime for the hypertriton, which has been recognized as the hypertriton lifetime puzzle. Our J-PARC E73 experiment contributes to solve this puzzle with an independent experimental method by employing 3He(K−, π0) 3ΛH reaction. In this contribution, we will demonstrate our capability to provide 3ΛH binding energy information by deriving the production cross section ratio, σ3ΛH/σ4ΛH. The production cross section data for 3ΛH and 4ΛH are already available as the pilot run of E73 experiment and data analysis is in progress.

2011/2012
The subject of the present PhD thesis is the study of the production of light hypernuclei in ultra-relativistic Pb-Pb collisions with ALICE (A Large Ion Collider Experiment), one of the four major experiments at the LHC (Large Hadron Collider). The main physics goal of the ALICE experiment is the investigation of the properties of the strongly interacting matter at high energy density ($>$ 10 GeV/fm$^3$) and high temperature ($\approx$ 0.2 GeV) conditions. According to the lattice Quantum Chromo Dynamics (QCD) calculations, under these conditions (i.e. high temperature and large energy density) hadronic matter undergoes a phase transition to a ``plasma'' of deconfined quarks and gluons (Quark Gluon Plasma, QGP). In the first chapter of the thesis a general introduction to the heavy-ion physics will be given. Then the main quantities related to QGP formation (i.e. \textit{probes}) will be described. Finally the most important results obtained at SPS, RHIC and LHC experiments will be shown and discussed. In the second chapter a short description of the LHC and its experimental conditions will be reported and an overview of the ALICE experiment will be given. A description of the different detectors and their performances during data taking will be described; in addition a description of the computing framework will be given. The third chapter will be devoted to an introduction of the (anti)(hyper)nuclei production in heavy-ion collisions. The two main approaches which are believed to govern nuclei production (i.e. coalescence and thermal models) will be described, and an overview on the results at different energies will be shown. A comparison of the theoretical results will be also shown, with particular regards to the energies at the LHC. The fourth chapter is devoted to the description of the analysis method used to get (anti)hypertriton production yield in \PbPb~collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76~TeV with the ALICE experiment via its mesonic decay \hyp~$\rightarrow$ \he + \pim (\antihyp $\rightarrow$ \antihe + \pip). In the beginning of the chapter the analysis technique used for particle identification and for the determination of secondary vertices will be described. The analysis will be divided into two distinct parts: the first one based on the data sample collected by the ALICE experiment during the first LHC heavy-ion run held at the end of 2010, while the second one based on data collected at the end of 2011. A detailed description of the study on efficiency evaluation and signal extraction will be shown for both analysis, together with a study of the systematic uncertainties. The results on the production yield of (anti)hypertriton will also be shown. The estimation of the hypertriton lifetime will be provided in the final section of the chapter.\\ In the fifth chapter the method used to obtain the \pt~spectrum of \he~will be presented. The raw spectra, the efficiency evaluation, systematic errors and feed-down from \hyp~will be presented. The final spectrum will be used to evaluate the production yield of \he(\antihe) in the whole \pt~region, from 0 to $\infty$. \\ Finally, in the last chapter, the present experimental results will be compared with published relevant results and with the most recent theoretical findings. Moreover, the measurement of the ``Strangeness Population Factor'' [S$_{3}$= \hyp/\he/($\Lambda$/p)] at the LHC energies will be provided. This quantity is a valuable tool to probe the nature of dense matter created in high-energy heavy-ion collisions and to validate theoretical models.
-------------------------------------------------------------------------------------------------- Questa tesi è dedicata allo studio della produzione di ipernuclei leggeri in collisioni ultra-relativistiche di ioni piombo (Pb) con l'esperimento ALICE (A Large Ion Collider Experiment), uno dei quattro grandi esperimenti del Large Hadron Collider (LHC) del CERN. Il principale obiettivo scientifico dell'esperimenento ALICE è lo studio delle proprietà della materia in condizioni estreme di energia (> 10 GeV/fm^3) e di temperatura (~ 0.2 GeV) mediante lo studio di collisioni di ioni piombo. Calcoli di Cromo Dinamica Quantistica (QCD) su reticolo prevedono, infatti, che in condizioni di alta temparatura e grande energia la materia adronica subisca un transizione di fase verso un ``plasma'' di quark e gluoni deconfinati (Quark Gluon Plasma, QGP). Nel primo capitolo della tesi verranno descritte in maniera generale la fisica degli ioni pesanti e le grandezze caratterische usate per provare la formazione del QGP (probes). Verranno quindi mostrati e discussi i risultati sperimentali che possono provare l'esistenza di uno stato deconfinato della materia nucleare ottenuti agli esperimenti a SPS, RHIC e LHC. Nel secondo capitolo saranno brevemente presentati il Large Hadron Collider (LHC) e le condizioni sperimentali di lavoro durante i primi tre anni di presa dati; in seguito verrà data un'ampia panoramica dell'esperimento ALICE. Saranno descritti i differenti sotto-rivelatori che formano l'esperimento e verranno inoltre mostrate le loro performance durante l'acquisione dati; inoltre verrà fornita una descrizione del framework di calcolo utilizzato nell'analisi dei dati. Il terzo capitolo sarà dedicato alla descrizione dei maccanismi di produzione di (anti)(iper)nuclei in collisioni di ioni pesanti: verranno descritti i due meccanismi di produzione che si ritiene governino la loro produzione (coalescenza e modello termico) e verrà mostrata una panoramica sui risultati ottenuti a diverse energie. Inotre saranno presentati diversi calcoli teorici, ponendo particolare attenzione ai risultati aspettati all'energia di LHC. Il quarto capitolo contiene la descrizione del metodo di analisi utilizzato per valutare lo yield di pruduzione dell'(anti)ipertritone attraverso il suo canale di decadimento mesonico \hyp~$\rightarrow$ \he + \pim (\antihyp $\rightarrow$ \antihe + \pip) in collisioni \PbPb~con energia nel centro di massa $\sqrt{s_{\rm{NN}}}$ = 2.76~TeV. Inizialmente verrà descritta la tecnica di analisi utilizzata per l'identificazione di particelle e dei vertici secondari, quindi sarà fornita la descrizione dettagliata della tecnica di analisi. L'analisi dei dati è stata siddivisa in due distinte parti: la prima è dedicata alla descrizione della procedura utilizzata per l'analisi dei dati raccolti da ALICE durante la prima acquisizione di collisioni Pb--Pb alla fine del 2010; nella seconda parte, invece, verrà descritta la procedura di analisi dei dati raccolti durante la seconda presa dati nel Dicembre 2011. Verranno quindi descritte in modo dettagliato l'estrazione del segnale, lo studio del fondo combinatoriale e gli errori sistematici. Infine, nella parte finale del capitolo, varrà fornita una stima della vita media dell'ipertritone.\\ Nel quinto capitolo sarà presentato il metodo usato per ottenere lo spettro in pT di (anti-3He)3He. Verranno descritti: la procedura di estrazione del segnale, la stima dell'efficienza in funzione del momento trasverso, la valutazione degli errori sistematici e la procedure usata per sottrarre il feed-down dovuto al decadimento dell'ipertitone. Lo spettro verrà quindi utilizzato per valutare lo yield di produzione di (anti-3He) 3He. Infine, nel sesto e ultimo capitolo, i risultati sperimentali ottenuti verranno confrontati con i risultati teorici discussi nel Capitolo 3.
XXV Ciclo
1985

The thesis is in two parts. Part I is covered in chapters 1 and 2 and concerns a simple model of the hypertriton developed by the author. The model is based on the fact that the lambda particle is loosely bound and so a lambda-inert core approach should be reasonable. The core is taken to be exactly like the free deuteron and a separable AN potential is used to construct the binding potential for the A particle. The model is tested in chapter 2 by calculating the ratio of two body to all pionic decay rates of the hypertriton and the result is found to agree well with experiment. Chapters 3 to 7 concern muon capture by 3He. Using the elementary particle model it is shown that the spin observables for quasi-elastic muon capture by 3He are much more sensitive to the nuclear pseudoscalar form factor ( and hence the nucleon pseudoscalar form factor ) than is the rate. Reliable and sophisticated wavefunctions for 3He and are then used to find the muon capture Hamiltonian in the impulse approximation. The result differs from that found in the elementary particle model in that the magnetic ( and dominant ) part of the Hamiltonian lacks strength. In chapter 5 new theory is developed for the muon wavefunction overlap reduction factor leading to the result C = 0.979. Chapter 6 details a calculation of muon capture by 3He leading to the deuteron neutron break-up final state in the plane wave impulse approximation. Finally, the processes leading to muonic atom formation are considered in chapter 7 with particular reference to final hyperfine population densities and their dependencies on target and beam polarization. It is shown that if only the intra-atomic processes are included, the results for the hyperfine population densities are unreliable.