198
L. Kumar
and Pb-Pb collisions at
√ s N N = 2.76 TeV, in different multiplicity classes [13, 15,
36–38]. It is quite astonishing that the blast wave model works very well even for the
small systems. The extracted T kin and β T are similar for pp and p-Pb for similar
multiplicities. However, when compared with Pb-Pb, the β T is higher for smaller
systems at similar multiplicities.
It becomes interesting to observe the behavior of various observables as a function
of multiplicity with different systems plotted together. One such plot is shown in
Fig. 14.10 where various particle ratios, such as ( p + ¯
p)/φ, (K
+ + K
− )/(π
+ + π
− ),
( p + ¯
p)/(π
+ + π
− ), and /K
0
S , are plotted as a function of multiplicity for three
different p T regions, i.e., low- p T (0.50–0.55 GeV/c), mid- p T (2.50–3.00 GeV/c),
and high p T (7.00–10.00 GeV/c), for different collision systems pp, p-Pb, and PbPb [13, 15, 36–38]. It is observed that all the particle ratios evolve smoothly as a
function of charged particle multiplicity across different systems. This suggests that
the particle production depends only on the final-state multiplicity, irrespective of
difference in energy, and collision system.
The strangeness enhancement in heavy-ion collisions relative to pp collisions
was originally proposed as a signature of QGP formation. It is also attributed to the
canonical suppression in small systems. However, the microscopic understanding
of strangeness enhancement is not known completely. Figure 14.11 shows the multiplicity dependence of ratio of identified particle yields to the pion yield for pp
collisions at
√
s = 7 TeV, p-Pb collisions at
√ s N N = 5.02 TeV, and Pb–Pb collisions
at
√
s N N = 2.76 TeV [13, 15, 36–38]. The ratios shown are the K /π , K
∗
/π , p/π ,
φ/π π/π, /π and There is a smooth evolution of all particle ratios also
including strange particles as a function of charged particle multiplicity across different systems. This suggests that the strange particle production exhibits universal
final-state multiplicity dependence.
The particle ratios involving strange particles increase with increasing multiplicity
(K
∗ is an exception due to the rescattering effect). The strangeness enhancement is
observed for the first time in small systems. The enhancement is observed to increase
with increasing strangeness contents of the particles. As an example, the /π ratio
with total strangeness content of 3 units shows more enhancement compared to other
ratios. The increase due to mass of the particles is ruled out because the p/π ratio is
observed to be constant while φ/π shows enhancement. However, the enhancement
of φ/π is also surprising because the total strangeness of φ meson (s ¯
s) is zero units.
The canonical suppression does not explain the increase in φ/π ratio [15]. The
investigations based on models to understand this effect are ongoing. A recent study
based on assumption of total effective strangeness of φ meson as 2 units (s+¯ s) could
explain this behavior [25].
L. Kumar
and Pb-Pb collisions at
√ s N N = 2.76 TeV, in different multiplicity classes [13, 15,
36–38]. It is quite astonishing that the blast wave model works very well even for the
small systems. The extracted T kin and β T are similar for pp and p-Pb for similar
multiplicities. However, when compared with Pb-Pb, the β T is higher for smaller
systems at similar multiplicities.
It becomes interesting to observe the behavior of various observables as a function
of multiplicity with different systems plotted together. One such plot is shown in
Fig. 14.10 where various particle ratios, such as ( p + ¯
p)/φ, (K
+ + K
− )/(π
+ + π
− ),
( p + ¯
p)/(π
+ + π
− ), and /K
0
S , are plotted as a function of multiplicity for three
different p T regions, i.e., low- p T (0.50–0.55 GeV/c), mid- p T (2.50–3.00 GeV/c),
and high p T (7.00–10.00 GeV/c), for different collision systems pp, p-Pb, and PbPb [13, 15, 36–38]. It is observed that all the particle ratios evolve smoothly as a
function of charged particle multiplicity across different systems. This suggests that
the particle production depends only on the final-state multiplicity, irrespective of
difference in energy, and collision system.
The strangeness enhancement in heavy-ion collisions relative to pp collisions
was originally proposed as a signature of QGP formation. It is also attributed to the
canonical suppression in small systems. However, the microscopic understanding
of strangeness enhancement is not known completely. Figure 14.11 shows the multiplicity dependence of ratio of identified particle yields to the pion yield for pp
collisions at
√
s = 7 TeV, p-Pb collisions at
√ s N N = 5.02 TeV, and Pb–Pb collisions
at
√
s N N = 2.76 TeV [13, 15, 36–38]. The ratios shown are the K /π , K
∗
/π , p/π ,
φ/π π/π, /π and There is a smooth evolution of all particle ratios also
including strange particles as a function of charged particle multiplicity across different systems. This suggests that the strange particle production exhibits universal
final-state multiplicity dependence.
The particle ratios involving strange particles increase with increasing multiplicity
(K
∗ is an exception due to the rescattering effect). The strangeness enhancement is
observed for the first time in small systems. The enhancement is observed to increase
with increasing strangeness contents of the particles. As an example, the /π ratio
with total strangeness content of 3 units shows more enhancement compared to other
ratios. The increase due to mass of the particles is ruled out because the p/π ratio is
observed to be constant while φ/π shows enhancement. However, the enhancement
of φ/π is also surprising because the total strangeness of φ meson (s ¯
s) is zero units.
The canonical suppression does not explain the increase in φ/π ratio [15]. The
investigations based on models to understand this effect are ongoing. A recent study
based on assumption of total effective strangeness of φ meson as 2 units (s+¯ s) could
explain this behavior [25].