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

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1 Journal of Physics: Conference Series PAPER OPEN ACCESS Final source eccentricity measured by HB interferometry with the event shape o cite this article: akafumi Niida and PHENIX Collaboration J. Phys.: Conf. Ser. Related content - Rescattering effects on HB interferometry Joseph Kapusta and Yang Li - Does HB measure the freeze-out source distribution? Cheuk-Yin Wong - Pion HB radii from RHIC to LHC Evan Frodermann, Rupa Chatterjee and Ulrich Heinz View the article online for updates and enhancements. his content was downloaded from IP address on //9 at :

2 Hot Quarks Journal of Physics: Conference Series () doi:.88/-9/// Final source eccentricity measured by HB interferometry with the event shape akafumi Niida for the PHENIX Collaboration University of sukuba, -- ennoudai, sukuba, Ibaraki -8, Japan Abstract. 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 v which leads to the enhancement of the measured final source eccentricity. he event twist effect in the spatial source distribution in the final state has been also explored with AMP model. Results indicate a possible twisted source due to the initial longitudinal fluctuations.. Introduction Higher-order flow coefficients v n are useful observables to constrain the properties of the quarkgluon plasma, such as a shear viscosity over entropy density ratio, in a heavy ion collision [,. he v is mainly caused by the almond shape of the nuclear overlap region, but higher-order flow, v, v,, especially their odd components are originating from the initial spatial fluctuations of participant nucleons. Recently ALAS experiment has presented results of event-by-event v n [, where v n values show large variations even in the fixed centrality bin due to initial fluctuations. o control such an initial fluctuation, the event shape engineering was suggested [. his could be a useful tool to study the response of initial state to the system evolution and connect the initial and final states. In these proceedings, we present results on HB measurements using charged pions and applying the event shape engineering technique for Au+Au collisions at s NN = GeV recorded with the PHENIX experiment. he event shape engineering focuses on the fluctuations in the transverse plane, but the presence of fluctuations in longitudinal direction could cause a twisted source along that direction [. he number of participants going to the forward and backward directions is not necessarily the same, and also participant eccentricities and participant planes might be different at both angles, which leads to different event plane angles between forward and backward rapidities. hus the initial twist may survive as a twisted flow in the final state [8, 9. In these proceedings, we examine the possibility of a spatially twisted source in the final state using HB interferometry in AMP model.. HB measurements with event shape at PHENIX he event shape was performed by selecting the magnitude of nd -order flow vectors, Q, which were measured by the Reaction Plane Detector (RXN, < <.8). he Q is Content from this work may be used under the terms of the Creative Commons Attribution. licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd

3 Hot Quarks Journal of Physics: Conference Series () doi:.88/-9/// Au+Au GeV RXN.S+N lower % higher % -% 8 Q Bessel-Gaussian -% 8 Q v.. Q -% no Q lower Q % higher Q % Au+Au GeV -% p (GeV/c) p (GeV/c) Figure. (Left) Q distributions measured with the RXN in Au+Au GeV collisions, indicating %(%) higher(lower) Q events. (Right) Charged hadron v as a function of p with and without event shape. defined as Q = Q,x + Q,y / Σw i, where Q,x = Σw i cos(ϕ), Q,y = Σw i sin(ϕ), and w i reflects the multiplicity within the element i of the RXN. Figure (left) shows Q distributions for two centrality bins, where the higher % and the lower % Q events are shown filled. We have tested the effect of the Q on charged hadron v as shown in Fig. (right), where the v is measured at mid-rapidity ( <.) and the systematic uncertainty from different event planes of south, north, and both combined RXN is included. It is confirmed that the Q enhances or decreases the strength of v. hen we have applied this technique to the HB measurement using charged pion pairs. Charged pions were identified by the electromagnetic calorimeter (EMCal, <.) and the pair cuts at the drift chamber and EMCal were applied to remove the effects of mis-reconstructed tracks and detector inefficiency. For the HB analysis, pion pairs were analyzed with the out-side-long parameterization [, in the longitudinally co-moving system. he effect of the event plane resolution was also corrected for both cases with and without Q. Figure (left) shows the extracted pion HB radii, R s and R o, as a function of azimuthal pair angle ϕ relative to the second-order event plane Ψ. Results show that the higher Q increases the oscillation strength compared to the case without Q. hese oscillations of HB radii are supposed to be sensitive to the final source eccentricity at freeze-out, ε final. Blast-wave studies suggest that the quantity of Rs, /R s, would be a good probe to ε final in the limit of k =, where k denotes a mean pair transverse momentum. he oscillation amplitudes of Rs and Ro in a form of the final eccentricity are plotted as a function of the number of participants calculated by Glauber model in Fig. (right). he higher Q enhances the measured ε final as well as v. It could be originating from a larger initial eccentricity, although there should be a contribution from the selected flow itself because the radii modulations also depend on the anisotropy in the momentum space [,.. HB measurements with event twist in AMP model o study the twisted source in the final state, we used the data of Pb+Pb. ev collisions simulated using AMP model (v. with string melting), where the impact parameter was fixed to 8 fm. For the HB study, the interference effect between two identical particles, + cos( r p), was calculated and weighted to the relative pair momentum distributions. hen the correlation functions were reconstructed by taking a ratio of the distributions with and without the weight. Also, all charged pions were allowed to make a pair with each other including π + π to increase the statistics (the consistency between results for positive and negative pairs was checked). he event plane was determined using particles in < <, where particles

4 Hot Quarks Journal of Physics: Conference Series () doi:.88/-9/// R s Q no Q higher Q % R o Au+Au GeV, π + π + +π π - - /R ν, R µ,. µ=s, ν=s µ=o, ν=o Q no Q higher Q % /R ν, R µ,. µ=os, ν µ=o, ν= φ - Ψ [rad φ - Ψ [rad N part Figure. (Left and center) Azimuthal angle dependence of R s and R o relative to Ψ. (Right) nd -order azimuthal oscillation on R s and R o. In both panels, results with and without higher Q are shown. were divided into two sub-groups. A set of forward and backward event planes (Ψ F, ΨB ) were used for the event cut which requires finite difference between Ψ F and ΨB as a event twist, whereas the other set of events was used for a reference angle of azimuthal HB measurement. Figure (left) shows Rs, Ro, and Ros as a function of azimuthal pair angle relative to the Ψ B, ϕ, for four regions, where (ΨB ΨF ) >. was required. he oscillations of three HB radii measured in positive regions are shifted to negative direction, which is the direction of Ψ F in the current event cut. his phase shift can be understood to be a possible twist effect in the final source distribution. hese oscillations were fitted with the following functions: R µ( ϕ) = R µ, + R µ, cos( ϕ + α) (for µ = o, s), () R µ( ϕ) = R µ, + R µ, sin( ϕ + α) (for µ = os), () to extract the magnitude of the phase shift, which is taken into account with α. he phase R s 9 AMP Pb+Pb.eV, ππ.<k < GeV/c, EP: -<<- R o B F (Ψ -Ψ )>. R os 8 -.< <-..<<. φ [rad -.< <-..<<. φ [rad - φ [rad Figure. Azimuthal angle dependence of Rs, Ro, and Ros relative to the backward Ψ with the event cut of (Ψ B ΨF ) >., where the dashed lines show R os =. R l R ol R sl AMP Pb+Pb.eV ππ-hb.<k < GeV/c, EP: << - -.< <-..<<. -.< <-..<<. -.< <..<<.

5 Hot Quarks Journal of Physics: Conference Series () doi:.88/-9/// [rad α side... [rad α out... B F (Ψ -Ψ )>. [rad α os... AMP Pb+Pb.eV B w.r.t Ψ F w.r.t Ψ Figure. Phase shift parameters α obtained from the Rs, Ro, and R os as a function of. Results measured with respect to forward and backward event planes (Ψ F, ΨB ) are shown, with event cut of (Ψ B ΨF ) >.. shift parameter α obtained from the results with respect to Ψ F and ΨB is plotted as a function of in Fig.. he α increases with going from backward to forward angle in all cases. he variation of α in the dependence is comparable to the difference between results relative to Ψ F and Ψ B at the same. hese results indicate that the source in the final state is also twisted due to longitudinal fluctuations in the initial state, as well as the twisted event plane and flow as discussed in Ref. [8, 9.. Summary We presented the results of HB measurements using event shape engineering for collisions recorded with the PHENIX experiment. We found that the higher Q enhances the measured final source eccentricity as well as v. Although the model comparison is needed to disentangle both spatial and dynamical effects on the HB radii, this study clarifies the relation between initial and final eccentricity and constrains better the system dynamics. We have also studied the event twist effect using the AMP model. When selecting events with finite difference between forward and backward event plane angles, the oscillations of HB radii are shifted in the phase and the phase shift increases with. he results indicate a possible twisted source in the final state preserving the initial twist due to the longitudinal fluctuations. his effect could be measured in experiments at RHIC and the LHC. Both techniques could be useful to probe and control initial fluctuations in transverse plane and longitudinal directions, as well as to study the response of the system to the space-time evolution. References [ A. Adare et al. (PHENIX Collaboration), Phys. Rev. Lett., (). [ C. Gale, S. Jeon, B. Schenke, P. ribedy, and R. Venugopalan, Phys. Rev. Lett., (). [ G. Aad et al. (ALAS Collaboration), JHEP () 8. [ J. Schukraft, A. immins, and S. A. Voloshin, Phys. Lett. B 9, 9 (). [ P. Bożek, W. Broniowski, and J. Moreira, Phys. Rev. C 8, 9 (). [ F. Retière and M. A. Lisa, Phys. Rev. C, 9 (). [ C. J. Plumberg, C. Shen, and U. Heinz, Phys. Rev. C 88, 9 (). [8 J. Jia and P. Huo, Phys. Rev. C 9, 9 (). [9 J. Jia and P. Huo, Phys. Rev. C 9, 9 (). [ S. Pratt, Phys. Rev. D, (98). [ G. Bertsch, M. Gong, and M. ohyama, Phys. Rev. C, 89 (988).