Relativistic nucleus-nucleus collisions Strangeness production
In nuclear matter, strange (and antistrange) quarks do not exist. The production of strange particles in heavy ion collisions is then of particular interest. Once produced, strange quarks have a low probability to annihilate with antistrange quarks, so they can live inside hadrons. Strange particles interact only weakly with the surrounding nuclear matter The simplest systems with strange quark content are kaons K+ = us K0 = ds K0 = ds K- = us
For instance, the mean free path of K+ in nuclear matter is about 5 fm. At energies around 1 GeV the cross section is due almost entirely to the elastic process. Lab momentum (GeV/c)
The interaction of pions, positive and negative kaons is different in nuclear matter. K+ cannot be absorbed on a nucleon, while K- are easily absorbed, producing a Λ
K+ have a small interaction cross section, with a long mean free path (about 6 fm), while K- have shorter mean free path. Positive kaons are then ideal probes to study the early stage of the collision.
At BEVALAC/SIS energies (few A GeV) kaons are produced near the kinematical threshold 1.58 GeV for NN -> K+ ΛN 2.5 GeV for NN -> K+K-N At subthreshold energies, kaons must be created through multiple interactions, as Δ+N -> K Y N and π+N -> K Y N where Y = Λ or Σ
The question of the modification of hadron properties inside the nuclear medium is of special interest. For instance, calculations predict a kaon-nucleon potential weakly repulsive for kaons and strongly attractive for antikaons. Such medium effects produce remarkable consequences on the kaon yields and their azimuthal distributions in heavy ion collisions. In particular, the yield of antikaons is expected to be significantly enhanced in dense nuclear matter, such as reached in heavy ion collisions at intermediate and relativistic energies. In the ultrarelativistic limit, strangeness production is considered as a signal for the formation of quark-gluon-plasma (see next lectures)
Kaon production probability First experiments on kaon production from heavy ion collisions were carried out in the ’80s at LBL and Dubna. Extensive sets of measurements were done at GSI around 1-2 AGeV
The production probability of kaons (per participant nucleon) near the threshold increases strongly with beam energy as E**(5.3) Note that for pp, the same quantity scales as E**18 near the threshold ! The K+ multiplicity per average number of participant nucleon vs system size. The increase with system mass shows the relevance of multiple collisions
Ni+Ni Another piece of evidence for multiple collisions comes from the kaon multiplicity per participant vs number of participants (collision centrality). While pions stay roughly constant vs #of participants, kaons show a large increase, which show the importance of multiple collisions in central events.
Data on antikaon production in heavy ion collisions are rather scarse
Kaon spectral distributions At low c.m. energies, particle emission is nearly isotropic. Kinetic energy spectra may be parametrized by Maxwell-Boltzmann distributions K+ inverse slope parameters agree with those of high energy pions
Energy distributions for negative kaons Inverse slopes: see previous tables
Inverse slope parameters as a function of the collision centrality Trend and absolute values are similar for pions and kaons
Au+Au @ 1 A GeV Angular distributions of kaons Kaon angular distributions are not consistent with isotropic emission. Data are fitted by with n=2 (dashed line) and n=4 (solid line) Transport calculations predict such forward/backward anisotropy as due to kaon-nucleon rescattering and alignment of pion-nucleon collisions along the beam axis
Azimuthal distributions Azimuthal angular distributions are affected by kaon interactions in the nuclear medium In case of nucleons and light fragments, the in-plane (sideward flow) and out-of-plane (squeeze-out) emission is explained by the hydrodynamical expansion In case of pions, anisotropy is explained by rescattering and reabsorption effects of pions in the nuclear (spectator) matter.
Sideward flow of strange particles Λ, K0s and K+ in-plane flow for Ni+Ni at 1.93 A GeV (FOPI Collaboration) No evidence of flow seen for kaons, in contrast to protons and Λ. This is interpreted as due to a weakly repulsive in-medium kaon-nucleon potential.
Data from KaoS for Au+Au at 1 A GeV show anisotropy effects for different centrality classes Peripheral collisions Semicentral collisions Central collisions
Kaon production is a promising tool even for the study of the Equation-Of-State (EOS) and to probe microscopic transport models. The sensitivity to the EOS is especially seen at low energies (factor 2 difference between soft and hard EOS at low energies) and for heavy systems. Ni+Ni 1 A GeV
The properties of hadrons inside nuclear matter under extreme conditions may change. Possible changes include modification of masses and lifetimes. K mesons are good candidates for the experimental study of in-medium modifications. Several theoretical approaches predict that the kaon mass increases with the nuclear density, whereas the antikaon mass decreases.
Assuming such mass modification of K mesons a better fit to data is sometimes obtained
At relativistic and ultrarelativistic beam energies, other particles may carry strangeness content. However, most of the information concerning strange particles is still contained in K mesons. Only the strange quarks carried out by hyperon-antihyperon pairs are not accounted for. What are the main features of kaon production at higher energies?
Rapidity distributions at AGS (10 A GeV) and SPS (160 A GeV) are wider than expected from a thermal source
mt - distributions are described by exponential shapes with inverse slope parameters in the order of 170-200 MeV, except at very low mt values.
At SPS energies, transverse mass spectra exhibit large inverse slope parameters
One important aspect of the production of strange particles is the strangeness enhancement. The simplest way to look at this effect is to study the ratio K/π This ratio increases from about 0.1 in peripheral collisions to 0.16 in central collisions
At SPS, strange particle yields are enhanced by a factor 2 when compared to nucleon-nucleon collisions However, such enhancement is observed also at lower energies (SIS, AGS) and it was explained as due to multiple interactions of participants.
The kaon/pion ratio may be ambiguous as a signal of strangeness enhancement (particles with different masses such as the pion and the kaon populate the phase space differently). Anyway, these ratios seem to point out something interesting. Why the maximum is not observed for negative kaons? Why it is observed for Lambdas?
It has been suggested to use other indicators of the strangeness enhancement: Kaon/eta Kaon/(high energy pions) AntiLambda/Antiproton Strangeness enhancement will be one of the striking signals investigated as a signature of the quark-gluon-plasma formation
Another interesting aspect is: why the ratio between positive and negative kaons is nearly constant for different centralities? One should expect that at low energies there should be an influence of the energy threshold and of the medium density.
In ultrarelativistic AA collisions, the study of antibaryon production (p, Λ) may help to understand the mechanism of antiquark production. In particular the ratio Λ/p (about 0.25 in pp collisions) is a measure of the s/u ratio. This is expected to be 1 in the deconfined phase. The search for such enhancement has motivated the study of such particles at AGS and SPS Transverse mass spectra of antiprotons and antilambdas at 200 A GeV
Ratio Λ/p System 0.25 p+p 0.5 p+S 0.3 p+Au 1.9 S+S 1.1 S+Au Large errors on such ratios Such measurements are difficult however to carry out and to interpret: Lambdas may be produced also from the decay of heavier hyperons The detection acceptance is usually very small
So far, there is no conclusive answer to the question of whether the simple strangeness enhancement (kaon/pion or antilambda/antiproton) can be understood in a purely hadronic scenario or require something new. This led to the study in recent years of other particles with strangeness content Λ0= uds Σ+ = uus Σ- = dds Σ0 = uds Ξ0 = uss Ξ- = dss Ω- = sss
In particular, the study of multistrange hyperons seems more promising, since their production requires the creation of 2 or 3 strange quarks and is highy suppressed in a baryon environment. On the other side, in a quark gluon plasma environment, multistrange hyperons could be formed more easily. The study of the ratio antihyperon/hyperon and between different hyperons is relatively new in heavy ion collisions (from SPS on, 1994).
The detection of all these particles require the reconstruction of their decay topology. For lambdas and antilambdas, this involves the detection of protons and pions (BR=63.9%) from a characteristic V0 topology: p Λ0 Π- cτ = 7.9 cm
For multistrange hyperons, one has to follow a more complicated topology, since they decay into lambda and pion (kaon) p Π- Λ0 Π- Ξ- cτ = 2.5 cm
The Ω decay topology p Π- Λ0 K- Ω- cτ = 7.9 cm
More recent results (WA97/NA57) on the strangeness enhancement for hyperons will be discussed in connection with the signatures of quark-gluon-plasma formation. ALso the production of the φ(1020) meson, with hidden strangeness content, which decays into K+K- (B.R.= 49.2%), has been studied, as well as other kaon resonances with short lifetime.
Conclusions: Hadronic particle production is the dominant process in nucleus-nucleus collisions at high energies. The study of particle production and their properties is essential for the understanding the physics of highly compressed and hot nuclear matter. Due to the difficulty to use full QCD calculations, phenomenological models are often used to investigate the differences between NN and AA collisions. Several aspects of particle production have been studied in connection with fundamental questions: the nuclear EOS, in-medium modifications of hadron properties, deconfinement phase transition, thermal and chemical equilibrium. Strangeness production in particular exhibits still many intriguing data, and has many aspects of interest.