# Collective effects in small collisions systems

Europe/Zurich
4/3-006 - TH Conference Room (CERN)

### 4/3-006 - TH Conference Room

#### CERN

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Description

Information: the agenda is full and the timetable is being finalized, so there is unfortunately not room for any more contributions.

Results from LHC have shown that the underlying event produced by multi-parton interactions in pp and p-Pb collisions have a much richer structure than previously expected. It exhibits flow like phenomena,such as ridges, and strangeness production increases with the charged particle multiplicity. Many effects are known from large collisional systems where they are understood in terms of Quark-Gluon Plasma properties, but there are many alternative ways to model these effects in small collision system.

How can we pin down the origin of these effects?

The goal of this workshop is:

• to discuss the latest results and calculations
• understand what we need to focus on from the sides of theory and experiment next
• to be inspired by new directions and ideas

To achieve this, the one day workshop will focus on experimental and theoretical summaries in the morning, complemented with individual contributions on new directions and ideas in the afternoon.

Confirmed speakers include:

Leif Lönnblad (Lund University, Sweden)
Peter Skands (Monash University, Australia)
Torbjörn Sjöstrand (Lund University, Sweden)
Antonio Ortiz (ICN-UNAM, Mexico)
Klaus Werner (Nantes University, France)

​VIDYO connection will be available.

For information relative to housing, access to CERN for those not holding a CERN card and laptop registration, please check
http://lpcc.web.cern.ch/LPCC/index.php?page=visit

Registration
Registration Form
Participants
• Alba Soto Ontoso
• Alexander Borissov
• Alexander Philipp Kalweit
• Alice Ohlson
• Andrzej Siodmok
• Antonio Ortiz
• Aobo Zhang
• Benedetto Di Ruzza
• Boris Hippolyte
• Carlos Pajares
• Chiara Oppedisano
• Christian Bourjau
• Constantinos Loizides
• Cristian Andrei
• David Krofcheck
• Dener De Souza Lemos
• Francesca Bellini
• Gennaro Corcella
• Grigorios Chachamis
• Guilherme Teixeira De Almeida Milhano
• Harri Niemi
• Jan Fiete Grosse-Oetringhaus
• Jochen Klein
• Klaus Werner
• Livio Bianchi
• Lizardo Valencia Palomo
• Manoj Bhanudas Jadhav
• Mark Strikman
• Martin Hentschinski
• Mauro Rogerio Cosentino
• Maxime Guilbaud
• Michele Floris
• Mihai Petrovici
• Milan Stojanovic
• Paolo Bartalini
• Peter Christiansen
• Peter Skands
• Prabhakar Palni
• Prabhat Ranjan Pujahari
• Quan Wang
• Ranjit Nayak
• Renata Kopecna
• Roberto Preghenella
• Salvatore Aiola
• Sascha Stahl
• Shengquan Tuo
• Souvik Priyam Adhya
• Sunil Bansal
• Torbjörn Sjöstrand
• Vytautas Vislavicius
• Yaxian Mao
• Zhou Jing
• Μaria Vasileiou
Webcast
There is a live webcast for this event
• 1
Introduction
Speaker: Peter Christiansen (Lund University (SE))
• 2
Two-particle angular correlations studies in pPb and pp in LHCb

Two-particle angular correlations are studied in proton-proton collisions at $\sqrt{s}$ =13TeV, collected with the LHCb detector at the LHC. LHCb detecor provides measurement in the very forward region, 2 < η < 5. This region is complementary to other experiments and allows to explore low Bjorken-x region. A dedicated trigger to study the highest-activity events was used. A total of about 180 million minimum-bias events and 48 million high-acitvity events were used in this analysis.
The two-dimensional correlation function is studied as a function of dif- ference in pseudorapidity (∆η) and azimuthal angle (∆φ). The study was done for four different activity classes and four $p_{\rm T}$ classes. In high-activity events an enhancement is observed in the long range near side region, 2 < ∆η < 2.5,∆φ∼0.

Speaker: Renata Kopecna (Ruprecht-Karls-Universitaet Heidelberg (DE))
• 3
QGP like effects in small collisions systems: CMS perspective

The observation of a long-range, near-side, two-particle correlation (known as the ridge) has been over the past decade a key signature of the hydrodynamic evolution of the hot and strongly interacting matter produced in heavy-ion collisions. Indeed, the Quark-Gluon Plasma (QGP) appears to behave as a perfect fluid and latest results from LHC experiments in Pb-Pb collisions at 2.76 and 5.02 TeV show a nice agreement with hydrodynamic expectations, either for inclusive charged hadrons or identified particles. The observation of the ridge in high-multiplicity p-p and p-Pb collisions opened up new opportunities of exploring novel QCD dynamics in small colliding systems. While extensive studies of this long-range correlation phenomenon in p-Pb and p-p collisions have revealed its collective properties, the underlying mechanism that drives the collectivity in small colliding system remains unknown. A detailed investigation is needed to establish the cause of the observed collective behavior and to determine if, indeed, a QGP medium is being created or if another mechanism is responsible. Over the past year, the CMS experiment at the LHC has recorded a large amount of Pb-Pb, p-Pb and p-p collisions, opening new opportunities in the understanding of collective phenomena in high-multiplicity hadronic final state. Upon reviewing the experimental data from the CMS experiment and confronting them with theoretical models, a unified paradigm in describing the observed collectivity across all hadronic collision system is emerging. Potential future paths toward addressing key open questions, especially on collectivity in small colliding systems (p-A and p-p), will be discussed.

Speaker: Maxime Guilbaud (Rice University (US))
• 4
Recent ATLAS measurements of azimuthal anisotropies in pp and p+Pb collisions

The azimuthal anisotropies of particle yields observed in relativistic heavy-ion collisions have been traditionally considered as a strong evidence of the formation on a deconfined quark-gluon plasma produced in these collisions. However multiple recent measurements from the ATLAS Collaboration in $pp$ and $p$+Pb systems show similar features as those observed in A+A collisions, indicating the possibility of the production of such a deconfined medium in smaller collision systems. This talk presents a comprehensive summary of these ATLAS measurements in $pp$ collisions at 2.76, 5.02 and 13 TeV and in $p$+Pb collisions at 5.02 and 8.16 TeV. It includes measurements of two-particle hadron-hadron and muon-hadron correlations in $\Delta\phi$ and $\Delta\eta$, with a template fitting procedure used to subtract the dijet contributions. Measurements of multi-particle cumulants $c_n\{2$--$8\}$ are alsopresented. The cumulant measurements confirm presence of collectivephenomena in $p$+Pb collisions, but are biased by non-flow correlations and are not able to provide evidence for collectivity in $pp$ collisions. To address this, measurements from a new sub-event cumulant method that suppresses the contribution of non-flow effects are presented.

Speaker: Soumya Mohapatra (Columbia University (US))
• 5
Soft physics in pp and p-Pb collisions at ALICE
Speaker: Alice Ohlson (Ruprecht-Karls-Universitaet Heidelberg (DE))
• 6
Understanding collectivity collectively - activities within the LPCC group
Speaker: Jan Fiete Grosse-Oetringhaus (CERN)
• 10:30 AM
Coffee break
• 7
Hydro perspective
Speaker: Dr Harri Niemi (J. W. Goethe Universität)
• 8
EPOS perspective
Speaker: Klaus Werner
• 9
PYTHIA and extensions
Speaker: Torbjorn Sjostrand (Lund University (SE))
• 10
DIPSY, ropes and shoving
Speaker: Leif Lönnblad (Lund University (SE))
• 12:40 PM
Lunch
• 11
Exploring the extremes of the underlying event
Speaker: Peter Skands (Monash University (AU))
• 12
Isolating the new physics using spherocity
Speaker: Antonio Ortiz Velasquez (Universidad Nacional Autonoma (MX))
• 13
Underlying event model SHRIMPS
Speaker: Korinna Christine Zapp (LIP Laboratorio de Instrumentacao e Fisica Experimental de Part)
• 14
Correlated wounded hot spots in proton-proton interactions
Speaker: Alba Soto Ontoso (UGR/FIAS)
• 15
Collectivity observables in high-Q2 probes

Recent results from the ALICE experiment on strangeness enhancement in high multiplicity pp collisions give further evidence of collective behaviour similar to what arises from QGP formed in central heavy ion collisions. We consider here the behaviour at high multiplicity of some pp observables with high-Q2 probes, proposing tests that might help shedding light on the origin of such collective phenomena, and assess models proposed for their interpretation.

Speaker: Ben Nachman (Lawrence Berkeley National Lab. (US))
• 3:50 PM
Coffee break
• 16
Discussion session