CERN Accelerating science

Anti-nuclei in the cosmos and at the LHC

by A, Kalweit (CERN)

The ALICE collaboration has recently released several measurements on the production of anti-deuterons and anti-3He in proton-proton and Pb-Pb collisions at LHC energies. These results are not only  interesting for the study of the hot QCD medium produced in heavy- ion collisions, but they also provide crucial input for  astrophysicists to understand the nature and origin of the anti-
nuclei candidates which have been recorded by the AMS collaboration. Given the enormous challenges of identifying rare  particles with a space-borne detector, the AMS collaboration is  still clarifying if their observed events are of true nature or  the result of misidentification. Nevertheless, the potential  interpretation of the observed results is already ongoing.  

The observation of anti-nuclei heavier than anti-protons in cosmic  rays would have far reaching consequences as they might originate from either segregated primordial anti-matter or from the  
annihilation of dark matter particles. Thus, the search for these  particles addresses two of the most fundamental questions in modern physics: the baryon to anti-baryon asymmetry in the universe as well as the existence and nature of dark matter, which  is assumed to account for about 85% of the matter in the universe and to about a quarter of its total energy density.  

Integrated yields (dN/dy) of anti-protons, anti-deuterons and 3He nuclei as a function of the number of anti-nucleons in inelastic pp collisions at √ s = 7 TeV. 

Integrated deuteron-to-proton (d/p) and anti-deuteron-to-anti-proton (d/p) ratios in inelastic pp collisions as a function of the average charged particle multiplicity for different center-of-mass energies. The average d/p ratio in AA collisions lies two times above the highest value in pp collisions (not shown). Dashed and solid lines represent the expected values from EPOS (LHC) with afterburner and the bands their uncertainties. 

However, also high energetic collisions of cosmic rays can produce  anti-nuclei. These secondary anti-nuclei correspond to the background in the searches for anti-nuclei from primordial or dark  matter origin. It was long believed that the production rates of these secondary anti-nuclei are so low, that already the  observation of a single anti-3He nucleus (a bound state of two  
anti-protons and one anti-neutron) would be indicative of new  physics. For the estimation of these background rates, the LHC  data becomes crucial as it allows to study the production of light  anti-nuclei in the laboratory. As a matter of fact, while the anti-3He nucleus was discovered in the 1970s in proton-Ion  collisions, its production in proton-proton collisions was first  measured by the ALICE collaboration. Several theory groups are now  addressing quantitatively the question how many anti-nuclei from secondary anti-nuclei are expected in space-borne experiments  based on the recent LHC data. This turns the previously very qualitative estimates into quantitative studies.  

One of the most striking observations in the ALICE data is the fact that the yield of anti-nuclei drops by a factor between 1000  (proton-proton collisions) and 330 (Pb-Pb collisions) for each
additional anti-nucleon. In Pb-Pb collisions, this so-called penalty factor was measured up to the anti-alpha (anti-4He) and in  proton-proton collisions up to anti-3He. It is therefore very striking that the AMS collaboration lately reported that in the  same data sample two potential candidates of anti-4He were found and six potential candidates of anti-3He. Such a ratio would correspond to a much smaller penalty factor. This lead theorists  to speculations which even consider the existence of anti-stars containing anti-4He.  

The study of anti- and hyper-nuclei, including the measurement of  anti-4He in pp collisions, will also be a major part of the ALICE physics program in LHC Run 3 & 4. The expected large integrated luminosities and the upgraded ALICE detector in this data taking  campaign are ideal for the study of these rare objects.

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