by Ramona Lea. Published: 10 November 2017

Ramona Lea, of the University and INFN of Trieste (Italy), has recently given a seminat at CERN on the study of the production of light (anti-) nuclei and (anti-) hypernuclei with ALICE at the LHC. Here she provides a summary of the topic.

Fig. 1 : B2 evaluated at pT/A = 0.75 GeV/c as a function of the average multiplicity of charged particles. In green are shown results from pp collisions at √s = 7 TeV, in blue from p-Pb collisions at  √sNN = 5.02 TeV, in red from  Pb-Pb collisions at  √sNN = 2.76 TeV and in black results from  Pb-Pb collisions at  √sNN = 5.02 TeV.

 

The study of the production of light (A<=4) (anti-)(hyper-)nuclei in high energy collisions is very important for different reasons. Their production mechanism inside the fireball is not well understood, in fact it is not clear if they are formed in the early stages of the collisions (i.e. at chemical freeze-out) or later by the coalescence of protons and neutrons. Moreover, due to low binding energy (few MeV) their formation can give us hint of the freeze-out conditions and to the dynamics of the emitting source. Finally, the study of their production can be used as baseline for exotic bound searches and the measurements of the production yield of anti-nuclei can also be used to estimate the background for secondary anti-nuclei in dark matter search.

At the LHC a large number of particles are produced (dNch/dη ~ 2000 in central collisions); among all the produced particles, almost 80% are pions and only ~5% are protons. This means that also in central Pb-Pb collisions the production of light nuclei is a rare process, because the penalty factor for each additional (anti)baryon is larger than 300. At the LHC  the fireball carries no net baryon number so the yields of the produced antiparticles closely coincide with the corresponding particle yields.

With ALICE, the identification of all the produced particles and, in particular, the measurement of light nuclei and Λ-hypernuclei, is possible because of the experiment’s excellent tracking and particle-identification capabilities via dE/dx and time-of-flight measurements.

Usually two different models are used to describe the production of (anti-)(hyper-)nuclei in high-energy collisions: the coalescence and the thermal model.

The coalescence approach assumes that (anti-)baryons which are close enough in the phase-space at kinetic freeze-out and match the spin state, can form a baryon state.  One of the observable related with this model is the the “coalescence parameter” BA that can be defined as the ratio between the measured pt-spectrum  of  the nucleus and the measured pt-spectrum of protons, elevated to the A of the nucleus. For deuterons, the BA is called B2 . The B2  at a fixed pT/A = 0./7 GeV/c is shown as a function of the average multiplicity in Fig.1. The different colors represent a  different colliding systems. A clear dependence with the average multiplicity is visible, going from the lower values in the most central Pb-Pb collisions (on the right side of Fig.1) to the higher values in pp collisions (on the left side of Fig.1).  This behavior is not predicted by the simple coalescence model, but can be qualitatively explained in coalescence models that take into account the volume of the expanding source. 

An other observable that can be used to test the coalescence model for light nuclei is the measure of their elliptic flow. In the left part of Fig.2, the measured v2(pT) is shown for three centrality intervals. The bands represent the predictions from a simple coalescence model: the simple coalescence model is not able to describe the data, while as can be observed on the right part of Fig.2, predictions from a model based on simplified hydrodynamics (Blast-Wave model) are able to describe the observed measurements of deuteron.

Fig.2: On the top , the  measured v2 of deuterons plus anti-deuterons is  compared with the expectations from simple coalescence  for different centrality intervals as indicated in the legend. On the bottom, the  v2(pT) of deuterons is compared with v2 of lighter particles. For  π± , K± and  p+p the long dashed curves represent the combined pT and v2 Blast-Wave fit. Deuteron curves (red dash dotted lines) are predictions from lighter particles Blast-Wave combined fit.

 

The thermal model allows to calculate the production yields of hadrons (dN/dy) created in the fireball (assumed to be in thermal equilibrium) when it reaches the chemical freeze-out temperature (Tchem),  where inelastic collisions cease.  The production yield of light particle can be described by this model, so it is interesting to understand if the yields of composite objects (like the light nuclei) can be understood in the same grand-canonical scheme as lighter particles.  Surprisingly, the yields of light nuclei and hypetriton (which is the lightest known hyeprnucleus, formed by a proton, a neutron and a lambda) can be well described by the thermal model, as can be seen in Figure 3.

Fig. 3: Thermal model fits, with three different implementations, to the light flavor hadron yields in central (0- 10%) Pb-Pb collisions at √sNN = 2.76 TeV. All the details can be found in arXiv:1710.07531 [nucl-ex].

 

This observation can be used to search for even more exotic states. ALICE has performed a search for two hypothetical strange dibaryon states. The first one is the H-dibaryon, which is a six-quark bound state of uuddss, first predicted by R. Jaffe in 1977. The second hypothetical bound state investigated by ALICE is a possible Λn bound state.

The two searches are performed in central (0–10%) Pb–Pb collisions at √sNN = 2.76TeV in the decay modes H-dibaryon→Λpπand Λn→dπ+. No signals are observed in either of the measured invariant-mass distributions, therefore setting upper limits for the production yields. These limits are well below the yields predicted using the thermal model. The differences between the upper limits at 99% CL obtained for the two states are a factor of the order of 25 below the predictions. Given the success of the model in predicting light nuclei and hypertriton yields, it appears that the existence of such bound states is very unlikely.

In summary, excellent ALICE performance allows for the detection of light (anti-)nuclei,(anti-)hypernuclei and to set upper limits for the production of exotic bound states. All the measurements done up to now lead to a kind of light nuclei “puzzle”: light nuclei  yield and v2 (pT ) in Pb-Pb collisions suggest an early “freeze out”, while a large effects of re-interactions (favoring late stage coalescence) should be expected. New and more precise results are expected in the near future, and these will provide stricter constraints to the theoretical models.

 

Further details on the topic can be found in:

1. Production of light nuclei and anti-nuclei in pp and Pb-Pb collisions at energies available at the CERN Large Hadron Collider, Phys. Rev. C 93, 024917 (2016)

2. Preliminary Physics Summary: Deuteron and anti-deuteron production in pp collisions at √s = 13 TeV and in Pb–Pb collisions at √sNN = 5.02 TeV, ALICE-PUBLIC-2017-006

3. Production of deuterons, tritons, 3He nuclei and their anti-nuclei in pp collisions at√s= 0.9, 2.76 and 7 TeV, arXiv:1709.08522

4. Production of 4He and 4He in Pb-Pb collisions at √sNN = 2.76 TeV at the LHC, arXiv:1710.07531 [nucl-ex]

5. Measurement of deuteron spectra and elliptic flow in Pb-Pb collisions at √sNN= 2.76 TeV at the LHC, arXiv:1707.07304

6. 3ΛH and 3Λ-H- production in Pb–Pb collisions at √sNN = 2.76 TeV, Phys. Lett. B 754, 360 (2016)