Results from (anti-)(hyper-)nuclei production and searches for exotic bound states with ALICE at the LHC

The momentum-over-charge ($p/z$) measurement from the TPC and velocity measurement using the TOF allow to obtain mass-over-charge distributions for (anti-)deuterons and (anti-)${}^{3}He$. Figure 1 shows the ALICE measurements of the mass-over-charge ratio difference for d-$\overline{d}$ and ${}^{3}He$-${}^3\overline{He}$. The results are compared with the CPT invariance expectation and with the existing mass measurements. These measurements show that the mass and binding energies of nuclei and anti-nuclei are compatible within uncertainties, confirming CPT invariance for light nuclei Adam:2015pna .

The deuteron transverse momentum ($p_T$) distributions are obtained for Pb–Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV Adam:2015vda , and for p–Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV for various centrality classes, and also for pp collisions at $\sqrt{s}$ = 7 TeV Adam:2015vda . A hardening of the spectrum with increasing centrality is observed in both Pb–Pb and p–Pb collisions. The $p_T$ distributions of ${}^{3}He$ are obtained for two centrality classes in Pb–Pb collisions at $\sqrt{s_{NN}}$= 2.76 TeV Adam:2015vda and for non-single diffractive (NSD) p–Pb collisions at $\sqrt{s_{NN}}$= 5.02 TeV. As an example, Fig. 2 shows the $p_T$ distribution of deuterons and ${}^{3}He$ in p–Pb collisions. The V0A multiplicity classes (Pb-side) corresponds to the measurement of the multiplicity in the Pb-going direction using the V0A detector. In order to extrapolate the yield into the unmeasured $p_T$ region, the spectra are fitted individually with a Blast-Wave function.

If nuclei production is described by the thermal model then the d/p ratio is expected to remain constant for all colliding systems and for all multiplicities (because ${\mu }_B$$\approx$0). Figure 3 shows the ratio of deuteron-to-proton yields as a function of event multiplicity for pp, p–Pb, and Pb–Pb collisions. Within the uncertainties no significant variation with multiplicity is observed in Pb–Pb, which is consistent with the thermal model expectations Adam:2015vda . In p–Pb, the d/p ratio increases with multiplicity, which is incompatible with the thermal model expectations for chemical freeze-out temperatures that do not depend on multiplicity. The ratio in pp collisions is a factor of 2.2 lower than in Pb-Pb.

The left panel of Fig. 4 shows the mass dependence of light nuclei yield for central Pb–Pb collisions and for NSD p–Pb collisions. The lines represent a fit with an exponential function. The figure also shows the anti-alpha yield measured in Pb–Pb collisions. Nuclei yields follow an exponential decrease with the mass. The penalty factor, namely the reduction of the yield by adding one nucleon, is $\sim$300 for Pb–Pb collisions and is $\sim$600 for NSD p–Pb collisions. The thermal model predicts the exponential dependence of the yield on the temperature given by d$N$/d$y$ $\propto$ $\exp (-m/T_{chem})$. The slope of the exponential fits can be used to extract $T_{chem}$. For Pb–Pb collisions, the obtained value is consistent with the thermal model expectation. However, the corresponding value is much lower for p–Pb collisions. Figure 4 (right) shows the thermal model fit to various particle yields including light (hyper-)nuclei for 0-10% central Pb–Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. Different models like Andronic:2010qu ; Wheaton:2004qb ; Torrieri:2006xi describe particle yields including deuterons, ${}^{3}He$ and ${}_{\Lambda }^{3}H$ yield well using $T_{chem}$$\sim$156 MeV. Exclusion of the nuclei from the fit does not cause any significant change in $T_{chem}$.

The thermal model describes the (hyper)nuclei yields well for Pb–Pb collisions and therefore can be used for predicting the expected yields of weakly decaying exotic bound states like H-dibaryon ($\Lambda$$\Lambda$) and $\overline{\Lambda n}$. The possible existence of these weakly bound states has been investigated via the decay channel $\Lambda$$\Lambda$$\to$ $\Lambda$+ p + ${\pi }^{-}$ and $\overline{\Lambda n}$ $\to$ $\overline{d}$ + ${\pi }^{+}$. No evidence has been seen in the invariant-mass distribution, neither for the $\Lambda$$\Lambda$ nor for the $\overline{\Lambda n}$ bound state. The upper limits on the yield of $\Lambda$$\Lambda$ and $\overline{\Lambda n}$ are about a factor 20 lower than the thermal model predictions when assuming reasonable lifetimes and branching ratios (BR). Figure 5 shows the experimentally determined upper limit for $\Lambda$$\Lambda$ and $\overline{\Lambda n}$ bound states compared with the model calculation as a function of BR, see Ref. Adam:2015nca for more details.