CMS ütközés
High Energy Experimental Particle and Heavy Ion Physics

Group Leader: Róbert Vértesi


The aim of our research group is to better understand the strong interaction through
collisions of nucleons and nuclei by performing basic and advanced measurements (cross
sections, particle spectra and correlations), and by testing various theoretical ideas (quarkgluon
plasma, gluon saturation, critical endpoint of the phase diagram). We participate in
several complementary experiments (mainly ALICE and CMS), both in data-taking and
physics analysis.

With the help of ultra-relativistic heavy-ion collisions, the properties of strongly-interacting
hadronic matter can be studied under extreme conditions of temperature and energy
density. Characteristics of this phase of matter are important for a better understanding of
the strong interaction as well as to address cosmological questions of the early Universe.
Recently the study of particle production in high-multiplicity events in small collision
systems at the LHC has revealed unexpected new collective-like phenomena. In particular,
for high-multiplicity pp and p-Pb collisions, radial flow signals, long-range angular
correlations, and strangeness enhancement have been reported. Our activities this year
focused on the above-listed topics.

Spectra of identified hadrons. — We have measured the transverse momentum spectra of
identified charged hadrons (pions, kaons, and protons) in proton-proton collisions at √s =
13 TeV. The pT spectra and integrated yields are compared to lower center-of-mass energy
pp results and to Monte Carlo simulations. The average pT increases with particle mass and
the charged-particle multiplicity of the event (Fig. 1, left). A comparison with lower energy
data shows only a moderate dependence of the average pT on the center-of-mass energy.
The PYTHIA8 CUETP8M1 event generator reproduces most features of the measured
distributions, but EPOS LHC also gives a satisfactory description of several aspects. Particle
production is strongly correlated with event multiplicity in all collision types, rather than
with the center-of-mass energy or collision system. The data supports the assumption that
the characteristics of particle production are constrained by the amount of initial parton
energy that is available in any given collision.

Sources of radial flow patterns. — We have proposed a tool to reveal the origin of the
collective-like phenomena observed in proton-proton collisions. We exploit the fundamental
difference between the underlying mechanisms, color reconnection, and hydrodynamics,
which produce radial flow patterns in PYTHIA8 and EPOS3 Monte Carlo event generators,
respectively. The strength of the coupling between the soft and hard components, by
construction, is larger in PYTHIA8 than in EPOS3. We studied the transverse momentum (pT)
distributions of charged pions, kaons and (anti) protons in inelastic pp collisions at √s =
7 TeV produced at mid-rapidity. Specific selections are made on an event-by-event basis as a
function of the charged particle multiplicity and the transverse momentum of the leading jet

reconstructed using the FastJet algorithm at mid-pseudorapidity. From our studies,
quantitative and qualitative differences between PYTHIA8 and EPOS3 are found in the pT
spectra when (for a given multiplicity class) the leading jet pT is increased. In addition, we
showed that for low-multiplicity events the presence of jets can produce radial flow-like
behavior, shown in Fig. 1 (right). The observed differences between the two event classes
(low and high multiplicities) similar to those seen in the hadrochemistry measured in the jet
and bulk regions in pp and Pb-Pb collisions by the ALICE collaboration. Motivated by our
findings, we proposed to perform a similar analysis using experimental data from RHIC and
the LHC.

Angular correlations. — Previous studies have shown that several mechanisms can play a
role in producing collective-like behavior. It has been demonstrated that multi-parton
interactions and color reconnection as implemented in PYTHIA MC event generator produce
radial flow patterns via boosted color strings. Also, azimuthal correlations have been studied
in A Multi-Phase Transport model (AMPT), where the ridge structure can be generated
assuming incoherent elastic scattering of partons and the string melting mechanism.
Besides, phenomenological studies (as described above) show that it is possible to find a
subclass of low-multiplicity events where radial flow patterns arise, despite the fact that at
very low multiplicity hydrodynamics cannot be applied and color reconnection effects are

Quantum correlations. — We have measured short-range two-particle correlation functions
of identified hadrons in pp, p-Pb, and peripheral Pb-Pb collisions. The extracted radii of the
particle emitting source (via Bose-Einstein correlations) are in the range 1-5 fm, reaching
highest values for very high multiplicity p-Pb and Pb-Pb collisions. The pp and p-Pb source is
elongated in the beam direction, while in the peripheral Pb-Pb case the source is symmetric.
The dependence of the radii on the multiplicity and kT factorizes and appears to be less
sensitive to the type of the collision system and center-of-mass energy (Fig. 2, right). The
observed similarities may point to a common critical hadron density reached in the

Heavy flavour production. — Heavy-flavour (beauty and charm) quarks are produced
almost exclusively in initial hard processes, and their yields remain largely unchanged
throughout a heavy-ion reaction. Nevertheless, they interact with the nuclear matter in all
the stages of its evolution. Thus, heavy quarks serve as ideal self-generated penetrating
probes of the strongly interacting QGP. Jets containing heavy flavour hadrons are also
sensitive to flavour-dependent fragmentation and gluon splitting. Recent heavy-flavour jet
measurements by the ALICE experiment, with contributions from our group, provide strong
constraints on theoretical models of heavy-flavour production and fragmentation.
Jet structure. — Non-trivial behavior of high multiplicity events in small systems have also
been observed in the heavy-flavour sector. Recent analyses of pp and p-Pb collisions show a
universal enhancement of heavy-flavour particles that is usually attributed to multiple
parton interactions and higher gluon radiation associated with short distance production
processes. We have carried out extensive studies using MC event generators. We have given
predictions for multiplicity-dependent jet structures, and proposed a way to validate the
presence and extent of effects such as multiple-parton interactions or color reconnection,
based on the detection of non-trivial jet shape modification in high multiplicity events. We
proposed a way to use the multiplicity-dependent jet structures to experimentally
differentiate between equally well-performing simulation tunes. We have also introduced a
definition of a characteristic jet size measure that is independent of multiplicity. These
studies can serve as a baseline for jet structure analyses in heavy-ion collisions as well as
flavour-dependent studies.
New method for tracking of charged particles at high multiplicities. — We have developed
a novel combination of established data analysis techniques for reconstructing charged
particles in very high multiplicity collisions. It uses all information available while keeping
competing choices open as long as possible. Suitable track candidates are selected by
transforming measured hits to a track parameter space with help of templates. The highly
connected network of track candidates and their corresponding hits is cut into very many
subgraphs by removing a few of its vulnerable components, edges, and nodes. Finally, the
hits distributed among the candidates by exploring a deterministic decision tree. A depthlimited
search is performed maximizing the number of hits on tracks, and also the sum of
track-fit quality measures.