Multi-differential studies to explore strangeness enhancement in pp with ALICE
at the LHC

Francesca Ercolessi1 on behalf of the ALICE Collaboration

1 University and INFN, Bologna, Italy.

<EMAIL_ADDRESS>
50th International Symposium on Multiparticle Dynamics
(ISMD2021)
12-16 July 2021 10.21468/SciPostPhysProc.?

## Abstract

The study of energy and multiplicity dependence of strange hadron production
in proton-proton collisions provides a powerful tool to understand
similarities and differences between small and large collision systems. In
order to better understand the origin of strangeness enhancement in pp new
multi-differential analyses have been performed. The first separates the
contribution of soft and hard processes, such as jets, to strange hadron
production through two-particle correlation techniques. The second exploits
the concept of the effective energy available for particle production in the
event, which is estimated by an anti-correlation with the energy deposited in
ALICE’s Zero Degree Calorimeters. The results indicate that strangeness
production emerges from the growth of the underlying event and confirm it is
related to the final state multiplicity.

##  1 Introduction

The primary goal of the ALICE experiment is to study the physics of strongly
interacting matter, including the properties of the quark-gluon plasma (QGP),
which is expected to be produced in ultra-relativistic heavy-ion collisions.
The relative production of strange hadrons with respect to non-strange hadrons
in heavy-ion collisions is an important observable to investigate the strongly
interacting medium created in the interaction. Strangeness enhancement was one
of the first proposed signatures of QGP formation [1], which is expected to
lead to an increased production of strange hadrons relative to pp
interactions, following a hierarchy related to the strangeness content of the
particle. This phenomenon was observed for the first time at SPS [2], then at
RHIC [3] and later at the LHC [4] at increasing collision energies. However,
the ALICE experiment first observed that small systems, such as p–Pb and pp,
show striking similarities with A–A collisions when multiplicity dependent
studies are performed. The ratio of strange particle yields to pion yields as
a function of the charged-particle multiplicity produced in the event is found
to increase across multiplicities, evolving smoothly within different
collision systems and energies [5]. The results can be compared to various
model predictions, from a statistical hadronization description using the
canonical suppression approach [6], to rope hadronization models including
colour reconnection effects [7], to two-component (core-corona) models [8].
However, the mechanisms responsible for this phenomenon in small systems are
still unclear and further experimental studies are needed.

##  2 Multi-differential studies

The ALICE experiment detects $K_{\textrm{S}}^{0}$, $\Lambda$, $\Xi$ and
$\Omega$ baryons by reconstructing their weak decay tracks in the central
pseudorapidity region. The main subdetectors deployed for particle
identification (PID) and tracking include the six-layer high resolution Inner
Tracking System (ITS) and the large volume Time Projection Chamber (TPC). The
Time-Of-Flight detector (TOF) is exploited to suppress the out-of-bunch
pileup. Two forward detectors placed on both sides of the ALICE interaction
point, the V0 and the Zero Degree Calorimeters (ZDC), are used to classify
events in multiplicity and energy event classes, respectively. A detailed
description of the ALICE apparatus and its performance can be found in [9].
In these proceedings a selection of the ALICE results that explore strangeness
production in proton-proton collisions at $\sqrt{s}$ $=$ $13$ TeV will be
discussed.

###  2.1 Strangeness in and out of jets

The processes involved in particle production in pp collisions can be
classified into hard and soft ones, depending on the momentum transfer. The
relative contribution of hard and soft processes to strangeness production in
pp is still not clear and can be studied through techniques involving full-jet
reconstruction or two-particle correlations.
In this analysis, ALICE exploits the angular correlation method to separate
$\textrm{K}_{\textrm{S}}^{0}$ and $\Xi$ hadrons produced in jets (hard
processes) from those produced out-of-jets (soft processes). This technique is
based on the fact that particles produced in the near-side jet region are
characterised by a small angular separation from the leading particle of the
jet, which is identified as the particle with the highest transverse momentum
in the collision and with $p_{\textrm{T}}$ $>$ $3$ GeV/$c$. The angular
correlation distribution of the jet trigger particle and the associated
strange hadron is studied in the $(\Delta\eta,\Delta\phi)$ plane, which is
divided into a near-side-jet region, an out-of-jet region, and a full
inclusive region. The near-side-jet contribution is obtained subtracting the
full and out-of-jet in the corresponding $(\Delta\eta,\Delta\phi)$ region. The
$\Xi$ yields per trigger particle and per unit of $\Delta\eta\Delta\phi$ are
displayed in Fig. 1 as a function of the charged-particle multiplicity
produced at midrapidity. The full and out-of-jet yields increase with
multiplicity, while the near-side-jet yields show a very mild to no dependence
on particle production at midrapidity. Similar results are obtained for
$\textrm{K}_{\textrm{S}}^{0}$ yields.

Figure 1: $\Xi$ yields per trigger particle and per unit of
$\Delta\eta\Delta\phi$ as a function of the charged-particle multiplicity
produced at midrapidity.

The ratio between $\Xi$ and $\textrm{K}_{\textrm{S}}^{0}$ yields as a function
of charged particle production at midrapidity is shown in Fig. 2. The out-of-
jet and the full yield ratios increase with multiplicity, confirming the
observation that strangeness enhancement is larger for particles with larger
strangeness content. The near-side-jet yield ratio shows a hint of increase
with multiplicity as well. These results suggest that out-of-jet (soft)
processes are the dominant contribution to strange particle production in pp
collisions.

Figure 2: Ratio between $\Xi$ and $\textrm{K}_{\textrm{S}}^{0}$ yields as a
function of charged particle multiplicity at midrapidity.

###  2.2 Influence of initial and final state effects on strangeness
production

The multiplicity distribution of charged particles is characteristic of the
hadronic final state of any collision, but it also reflects its initial
dynamics being strongly correlated with the energy effectively available for
particle production in the initial stages of the interaction (effective
energy). In pp collisions, the effective energy is reduced with respect to the
full centre-of-mass energy due to the leading baryon effect, which consists of
a high probability to emit baryons at forward rapidity, namely with large
longitudinal momenta along the direction of the incident beams. ALICE’s Zero
Degree Calorimeters are able to detect the energy deposits of the forward
leading baryons in each event hemisphere as
$E_{\textrm{effective}}=\sqrt{s}-E_{\textrm{leading}}$. More details can be
found in [10] and references therein. In this analysis, the ZDC and V0
detectors are used to classify events in effective energy and multiplicity
percentile classes, respectively. The goal of this study is to disentangle
initial and final state effects on strange hadron production. Multiplicity and
effective energy are two correlated quantities, in particular recent ALICE
results showed that the forward energy deposit in the ZDC is anti-correlated
with particle production at midrapidity [11]. This correlation is studied in
this analysis using the PYTHIA 8 event generator and displayed in Fig. 3 (a),
which shows that events selected using a V0 or ZDC111 The ZDC based percentile
estimator is labelled as $\sqrt{s}-ZDC$ and it represents the percentile
classes of the quantity $\sqrt{s}-E_{\textrm{ZDC}}$. based estimator alone are
sensitive to both initial and final state. Fig. 3 (b) displays the ratio of
$\Xi$ yields to the charged-particle multiplicity in the event (self-
normalized to INEL>0222INEL¿0 is a conventional event class which contains
events with at least one charged particle in —$\eta$—$<$ 1.) selected using V0
or ZDC, as a function of particle production at midrapidity. The $\Xi$
production is found to increase with multiplicity following a similar trend,
independent of the estimator used.

Figure 3: (a) Multiplicity and effective energy correlation in PYTHIA 8 using
V0 and ZDC event classes. (b) Ratio of $\Xi$ yields to the charged-particle
multiplicity (self-normalized to INEL>0) as a function of particle production
at midrapidity, using V0 and ZDC standalone selections.

In order to separate initial and final state contributions to strange particle
production, V0-ZDC combined classes are exploited. Their discriminating power
in terms of effective energy and multiplicity is studied in Fig. 4 via the
PYTHIA 8 event generator. Events are selected using ZDC percentile selections,
by fixing the multiplicity through the V0 estimator in a high and a low range
(red points), and vice-versa using V0 percentile selections, by fixing the
effective energy through the ZDC estimator to a high and a low range (blue
points). The ratio of $\Xi$ yields to the charged-particle multiplicity
selected using V0-ZDC combined event classes is shown in Fig. 5 (a) and (b).
When fixing the multiplicity through the V0 estimator and selecting events in
ZDC percentile classes (Fig. 5 (a)) the points are distributed in a flat trend
showing no dependence on the energy percentile estimator. The different V0
selection is responsible for the enhancement observed between the two sets of
points. Fig. 5 (b) shows the ratio of $\Xi$ yields to the particle
multiplicity selected using V0 event classes, fixing the effective energy
through the ZDC estimator in a high and a low range. The two trends are
consistent within the experimental uncertainties and the selection in
effective energy does not influence the observed enhancement with
multiplicity. These preliminary results suggest that the effective energy does
not play a significant role in the strangeness enhancement observed in pp
collisions, confirming a strong role of the final state multiplicity. More
studies are ongoing in order to improve the discriminatory power of multi-
differential selections in terms of multiplicity and effective energy.

##  3 Conclusion

The ALICE Collaboration has provided a comprehensive set of results on the
production of strange hadrons in pp collisions. These results show striking
commonalities with heavy-ion collisions, although the mechanisms responsible
of the strangeness enhancement with multiplicity in small systems are still
not fully understood. The set of preliminary results presented in these
proceedings exploit multi-differential approaches to study strange hadron
production in pp collisions at $\sqrt{s}$ = 13 TeV. The first analysis
separates the contribution of soft and hard processes, such as jets, to
strangeness production through two-particle correlation techniques. Soft out-
of-jet processes are found to be the dominant contribution, with respect to
in-jet hard processes. The second analysis aims at disentangling initial and
final state effects on strange hadron production selecting the events in
multiplicity and effective energy classes. The results confirm that
strangeness enhancement is connected to the final particle multiplicity
produced in the collision and suggest no significant dependence on the initial
effective energy.

Figure 4: Multiplicity and effective energy correlation in PYTHIA 8 using
V0-ZDC combined classes.

Figure 5: Ratio of $\Xi$ yields to the charged-particle multiplicity extracted
using V0-ZDC combined selections. (a) Events are selected using ZDC event
classes, fixing the multiplicity through the V0 estimator. (b) Events are
selected using V0 event classes, fixing the effective energy through the ZDC
estimator.

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