Introduction

• Peter Christiansen (Lund University (SE))

Room: 4/3-006 - TH Conference Room

Two-particle angular correlations studies in pPb and pp in LHCb

• Renata Kopecna (Ruprecht-Karls-Universitaet Heidelberg (DE))

Room: 4/3-006 - TH Conference Room 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.

QGP like effects in small collisions systems: CMS perspective

• Maxime Guilbaud (Rice University (US))

Room: 4/3-006 - TH Conference Room 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.

Recent ATLAS measurements of azimuthal anisotropies in pp and p+Pb collisions

• Soumya Mohapatra (Columbia University (US))

Room: 4/3-006 - TH Conference Room 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.

Soft physics in pp and p-Pb collisions at ALICE

• Alice Ohlson (Ruprecht-Karls-Universitaet Heidelberg (DE))

Room: 4/3-006 - TH Conference Room

Understanding collectivity collectively - activities within the LPCC group

• Jan Fiete Grosse-Oetringhaus (CERN)

Room: 4/3-006 - TH Conference Room

Hydro perspective

• Harri Niemi (J. W. Goethe Universität)

Room: 4/3-006 - TH Conference Room

EPOS perspective

• Klaus Werner

Room: 4/3-006 - TH Conference Room

PYTHIA and extensions

• Torbjorn Sjostrand (Lund University (SE))

Room: 4/3-006 - TH Conference Room

DIPSY, ropes and shoving

• Leif Lönnblad (Lund University (SE))

Room: 4/3-006 - TH Conference Room

Exploring the extremes of the underlying event

• Peter Skands (Monash University (AU))

Room: 4/3-006 - TH Conference Room

Isolating the new physics using spherocity

• Antonio Ortiz Velasquez (Universidad Nacional Autonoma (MX))

Room: 4/3-006 - TH Conference Room

Underlying event model SHRIMPS

• Korinna Christine Zapp (LIP Laboratorio de Instrumentacao e Fisica Experimental de Part)

Room: 4/3-006 - TH Conference Room

Correlated wounded hot spots in proton-proton interactions

• Alba Soto Ontoso (UGR/FIAS)

Room: 4/3-006 - TH Conference Room

Collectivity observables in high-Q2 probes

• Ben Nachman (Lawrence Berkeley National Lab. (US))

Room: 4/3-006 - TH Conference Room 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.

Discussion session Room: 4/3-006 - TH Conference Room