Event Shape Engineering Research Articles

Event-by-Event Hydrodynamics+Jet Energy Loss: A Solution to the R_{AA}⊗v_{2} Puzzle.

High p_{T}>10 GeV elliptic flow, which is experimentally measured via the correlation between soft and hard hadrons, receives competing contributions from event-by-event fluctuations of the low-p_{T} elliptic flow and event-plane angle fluctuations in the soft sector. In this Letter, a proper account of these event-by-event fluctuations in the soft sector, modeled via viscous hydrodynamics, is combined with a jet-energy-loss model to reveal that the positive contribution from low-p_{T} v_{2} fluctuations overwhelms the negative contributions from event-plane fluctuations. This leads to an enhancement of high-p_{T}>10 GeV elliptic flow in comparison to previous calculations and provides a natural solution to the decade-long high-p_{T} R_{AA}⊗v_{2} puzzle. We also present the first theoretical calculation of high-p_{T} v_{3}, which is shown to be compatible with current LHC data. Furthermore, we discuss how short-wavelength jet-medium physics can be deconvoluted from the physics of soft, bulk event-by-event flow observables using event-shape engineering techniques.

Event-shape engineering for inclusive spectra and elliptic flow in Pb-Pb collisions atsNN=2.76TeV

We report on results obtained with the Event Shape Engineering technique applied to Pb-Pb collisions at $\sqrt{s_\rm{NN}}=2.76$ TeV. By selecting events in the same centrality interval, but with very different average flow, different initial state conditions can be studied. We find the effect of the event-shape selection on the elliptic flow coefficient $v_2$ to be almost independent of transverse momentum $p_\rm{T}$, as expected if this effect is due to fluctuations in the initial geometry of the system. Charged hadron, pion, kaon, and proton transverse momentum distributions are found to be harder in events with higher-than-average elliptic flow, indicating an interplay between radial and elliptic flow.

Collision geometry and flow in uranium + uranium collisions

Using event-by-event viscous fluid dynamics to evolve fluctuating initial density profiles from the Monte-Carlo Glauber model for U+U collisions, we report a "knee"-like structure in the elliptic flow as a function of collision centrality, located around the 0.5% most central collisions as measured by the final charged multiplicity. This knee is due to the preferential selection of tip-on-tip collision geometries by a high-multiplicity trigger. Such a knee structure is not seen in the STAR data. This rules out the two-component MC-Glauber model for initial energy and entropy production. Hence an enrichment of tip-tip configurations by triggering solely on high-multiplicity in the U+U collisions does not work. On the other hand, by using the Zero Degree Calorimeters (ZDCs) coupled with event-shape engineering such a selection is possible. We identify the selection purity of body-body and tip-tip events in full-overlap U+U collisions. By additionally constraining the asymmetry of the ZDC signals we can further increase the probability of selecting tip-tip events in U+U collisions.

Identified charged hadron production in Pb-Pb collisions with event shape engineering

The Event Shape Engineering technique allows the selection of different event shapes for a definite centrality and colliding system. The event selection is based on the azimuthal distribution of produced particles, using the so-called flow vector. For such shape selected events, the elliptic flow coefficient (v2) is significantly different with respect to the unbiased events. Moreover, recent Monte-Carlo studies show a strong correlation between the (final state) event shape selection and the (initial state) eccentricity of the collision. This opens the opportunity to characterize events according to the initial geometry. An approach to select the eccentricity of the event with the Event Shape Engineering is presented. Then the effect of this selection on identified particle spectra, mean transverse momentum and v2 of charged particles in heavy-ion collisions at = 2.76 TeV center-of-mass energy is discussed.

Final source eccentricity measured by HBT interferometry with the event shape selection

Azimuthal angle dependence of the pion source radii has been measured applying the event shape engineering technique at the PHENIX experiment. When events with higher magnitude of second-order flow vector are selected, the oscillation of the source radii is enhanced as well as v2 which leads to the enhancement of the measured final source eccentricity. The event twist effect in the spatial source distribution in the final state has been also explored with AMPT model. Results indicate a possible twisted source due to the initial longitudinal fluctuations.

Emission angle and particle mass dependence of HBT radii in Au+Au collisions at sNN=200 GeV

Abstract Hydrodynamic features of quark–gluon plasma (QGP) are being studied via various observables in high-energy heavy-ion collisions. Measurement of space–time structure via two-particle interferometry is one of those and is known as Hanbury Brown and Twiss (HBT) method. This gives geometrical and temporal information as well as dynamical information such as collective expansion during the freeze-out stage. The azimuthal-angle dependence of the HBT radii for various particle species ( ππ and KK ) and for various event plane orders (2nd and 3rd) are measured in s NN = 200 GeV Au+Au collisions at RHIC-PHENIX experiment. The extracted parameters for KK pairs are slightly different from those for ππ pairs for a given m T , which may indicate earlier freeze-out and longer emission duration for K than for π . The azimuthal dependences relative to the 2nd and 3rd order event planes are observed to be consistent with a strong and rapid expansion, which has also been known as sudden freeze-out with a short duration time. Large v 2 events are selected by the 2nd order flow vector magnitude Q 2 in order to test the Event Shape Engineering (ESE) via final eccentricity ϵ final measurement. The results confirm a significant correlation between elliptic flow v 2 and final eccentricity ϵ final , which is supposed to originate from the initial geometrical eccentricity ϵ initial .

Event shape engineering with ALICE

The strong fluctuations in the initial energy density of heavy-ion collisions allow an efficient selection of events corresponding to a specific initial geometry. For such "shape engineered events", the elliptic flow coefficient, $v_2$, of unidentified charged particles, pions and (anti-)protons in Pb-Pb collisions at $\snn = 2.76$ TeV is measured by the ALICE collaboration. $v_2$ obtained with the event plane method at mid-rapidity, $|\eta|<0.8$, is reported for different collision centralities as a function of transverse momentum, $\pt$, out to $\pt=20$ GeV/$c$. The measured $v_2$ for the shape engineered events is significantly larger or smaller than the average which demonstrates the ability to experimentally select events with the desired shape of the initial spatial asymmetry.

Results on flow from the ALICE Collaboration

This short overview includes recent results from the ALICE Collaboration on anisotropic flow of charged and identified particles in sqrt(sNN) = 2.76 TeV Pb-Pb collisions. We also discuss charge dependent and event plane dependent azimuthal correlations that are important in tests of the chiral magnetic effect, as well as understanding the dynamics of the system evolution and hadronization process. Lastly, we present ALICE results obtained with a new technique, the event shape engineering, which allows to perform a physical analysis on events with very large or small flow.

Ultra-relativistic nuclear collisions: Event shape engineering

The evolution of the system created in a high energy nuclear collision is very sensitive to the fluctuations in the initial geometry of the system. In this Letter we show how one can utilize these large fluctuations to select events corresponding to a specific initial shape. Such an “event shape engineering” opens many new possibilities in quantitative test of the theory of high energy nuclear collisions and understanding the properties of high density hot QCD matter.