page_title: Strangeness and quark–gluon plasma
section_title: ALICE: Resolution of remaining questions about strangeness as signature of quark–gluon plasma
hierarchical_section_title: Strangeness and quark–gluon plasma / ALICE: Resolution of remaining questions about strangeness as signature of quark–gluon plasma
caption_attribution_description: English: Ratio to pion of integrated yields for p , K s 0 , &#x039B; , &#x03D5; , &#x039E; {\displaystyle p,K_{s}^{0},\Lambda ,\phi ,\Xi } and &#x03A9; {\displaystyle \Omega } . The evolution with multiplicity at mid-rapidity, d N c h / d &#x03B7; | &lt; 0.5 {\displaystyle \operatorname {d} \!N_{ch}/\operatorname {d} \!\eta {|}_{&lt;0.5}} , is reported for several systems and energies, including pp at s = 7 {\displaystyle {\sqrt {s}}=7} TeV, p-Pb at s N N = 5.02 {\displaystyle {\sqrt {s_{\operatorname {N} \!\operatorname {N} \!}}}=5.02} TeV, and also the ALICE preliminary results for pp at s = 13 {\displaystyle {\sqrt {s}}=13} TeV, Xe-Xe at s N N = 5.44 {\displaystyle {\sqrt {s_{\operatorname {N} \!\operatorname {N} \!}}}=5.44} TeV and Pb-Pb at s N N = 5.02 {\displaystyle {\sqrt {s_{\operatorname {N} \!\operatorname {N} \!}}}=5.02} TeV are included for comparison. Error bars show the statistical uncertainty, whereas the empty boxes show the total systematic uncertainty.
context_page_description: Strangeness production in relativistic heavy ion collisions is a signature and a diagnostic tool of quark–gluon plasma formation and properties. Unlike up and down quarks, from which everyday matter is made, heavier quark flavors such as strangeness and charm typically approach chemical equilibrium in a dynamic evolution process. QGP is an interacting localized assembly of quarks and gluons at thermal and not necessarily chemical equilibrium. The word plasma signals that color charged particles are able to move in the volume occupied by the plasma. The abundance of strange quarks is formed in pair-production processes in collisions between constituents of the plasma, creating the chemical abundance equilibrium. The dominant mechanism of production involves gluons only present when matter has become a quark–gluon plasma. When quark–gluon plasma disassembles into hadrons in a breakup process, the high availability of strange antiquarks helps to produce antimatter containing multiple strange quarks, which is otherwise rarely made.
context_section_description: The very high precision of (strange) particle spectra and large transverse momentum coverage reported by the ALICE Collaboration at the Large Hadron Collider (LHC) allows in-depth exploration of lingering challenges, which always accompany new physics, and here in particular the questions surrounding strangeness signature. Among the most discussed challenges has been the question if the abundance of particles produced is enhanced or if the comparison base line is suppressed. Suppression is expected when a otherwise absent quantum number, such as strangeness, is rarely produced. This situation was recognized by Hagedorn in his early analysis of particle production and solved by Rafelski and Danos. In that work it was shown that even if just a few new pairs of strange particles were produced the effect disappears. However, the matter was revived by Hamieh et al. who argued that is possible that small sub-volumes in QGP are of relevance. This argument can be resolved by exploring specific sensitive experimental signatures for example the ratio of double strange particles of different type, such yield of  () compared to (). The ALICE experiment obtained this ratio for several collision systems in a wide range of hadronization volumes as described by the total produced particle multiplicy. The results show that this ratio assumes the expected value for a large range volumes (two orders of magnitude). At small particle volume or multiplicity, the curve shows the expected reduction: The  () must be smaller compared to  () as the number of produced strange pairs decreases and thus it easier to make () compared to  () that requires two pairs minimum to be made.  However, we also see an increase at very high volume—this is an effect at the level of one-two standard deviations. Similar results were already recognized before by Petran et al. .
  Another highly praised ALICE result is the observation of same strangeness enhancement, not only on AA  (nucleus-nucleus) but also in pA (proton-nucleus) and pp (proton-proton) collisions when the particle production yields are presented as a function of the multiplicity, which, as noted, corresponds to the available hadronization volume. ALICE results display a smooth volume dependence of total yield of all studied particles as function of volume, there is no additional "canonical" suppression. This is so since the yield of strange pairs in QGP is sufficiently high and tracks well the expected abundance increase as the volume and lifespan of QGP increases. This increase is incompatible with the hypothesis that for all reaction volumes QGP is always in chemical (yield) equilibrium of strangeness. Instead, this confirms the theoretical kinetic model proposed by Rafelski and Müller. The production of QGP in pp collisions was not expected by all, but should not be a surprise. The onset of deconfinement is naturally a function of both energy and collision system size. The fact that at extreme LHC energies we cross this boundary also in experiments with the smallest elementary collision systems, such as pp, confirms the unexpected strength of the processes leading to QGP formation. Onset of deconfinement in pp and other "small" system collisions remains an active research topic.
  Beyond strangeness the great advantage offered by LHC energy range is the abundant production of charm and bottom flavor. When QGP is formed, these quarks are embedded in a high density of strangeness present. This should lead to copious production of exotic heavy particles, for example 
  D
  ₛ.  Other heavy flavor particles, some which have not even been discovered at this time, are also likely to appear.
caption: Ratio to pion of integrated yields for 
    
      
        
          p
          ,
          
            K
            
              s
            
            
              0
            
          
          ,
          Λ
          ,
          ϕ
          ,
          Ξ
        
      
      {\displaystyle p,K_{s}^{0},\Lambda ,\phi ,\Xi }
    
   and 
    
      
        
          Ω
        
      
      {\displaystyle \Omega }
    
  . The evolution with multiplicity at mid-rapidity, 
    
      
        
          d
          
          
            N
            
              c
              h
            
          
          
            /
          
          d
          
          η
          
            
              
                |
              
            
            
              <
              0.5
            
          
        
      
      {\displaystyle \operatorname {d} \!N_{ch}/\operatorname {d} \!\eta {|}_{<0.5}}
    
  , is reported for several systems and energies, including pp at 
    
      
        
          
            
              s
            
          
          =
          7
        
      
      {\displaystyle {\sqrt {s}}=7}
    
   TeV, p-Pb at 
    
      
        
          
            
              
                s
                
                  N
                  
                  N
                  
                
              
            
          
          =
          5.02
        
      
      {\displaystyle {\sqrt {s_{\operatorname {N} \!\operatorname {N} \!}}}=5.02}
    
   TeV, and also the ALICE preliminary results for pp at 
    
      
        
          
            
              s
            
          
          =
          13
        
      
      {\displaystyle {\sqrt {s}}=13}
    
   TeV, Xe-Xe at 
    
      
        
          
            
              
                s
                
                  N
                  
                  N
                  
                
              
            
          
          =
          5.44
        
      
      {\displaystyle {\sqrt {s_{\operatorname {N} \!\operatorname {N} \!}}}=5.44}
    
   TeV and Pb-Pb at 
    
      
        
          
            
              
                s
                
                  N
                  
                  N
                  
                
              
            
          
          =
          5.02
        
      
      {\displaystyle {\sqrt {s_{\operatorname {N} \!\operatorname {N} \!}}}=5.02}
    
   TeV are included for comparison. Error bars show the statistical uncertainty, whereas the empty boxes show the total systematic uncertainty.[36]
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