Dihadron correlations from AMPT

Transcription

1 Dihadron correlations from AMPT Che-Ming Ko Texas A&M University AMPT Anisotropic flows Dihadron azimuthal correlations 2D dihadron correlations Based on work with Jun Xu, PRC 83, (R) (2011); (2011); 84, (2011); 84, (2011) Supported by US National Science Foundation and Department of Energy, and the Welch Foundation 1

2 Triangular flow Alver & Roland, PRC 81, (2010) Contrary to elliptic flow, triangular flow is not sensitive to centrality. Similar results from hydrodynamic model (Petersen et al., PRC 82, (2010), Alver et al., PRC 82, (2010)). 2

3 A multiphase transport (AMPT) model Default: Lin, Pal, Zhang, Li & Ko, PRC 61, (00); 64, (01); 72, (05); Initial conditions: HIJING (soft strings and hard minijets) Parton evolution: ZPC Hadronization: Lund string model Hadronic scattering: ART String melting: PRC 65, (02); PRL 89, (02) Convert hadrons from string fragmentation into quarks and antiquarks Evolve quarks and antiquarks in ZPC When partons stop interacting, combine nearest quark and antiquark to meson, and nearest three quarks to baryon (coordinate-space coalescence) Hadron flavors are determined by the invariant mass of quarks 3

4 Lund string fragmentation model Fragmentation function (Schwinger mechanism) z : light-cone momentum fraction string tension κ ~ [b(2+a)] -1 HIJING: a = 0.5 and b = 0.9 GeV -2 (B) Suppression factor: 0.3 for strangeness AMPT: T. Sjostrand, CPC 82, 74 (1994) f(z) " z #1 1 # z 2 ( ) ( ) a exp # b m2 + p & t a = 2.2 and b = 0.5 GeV -2 (A) 7% larger κ Popcorn mechanism: BB, BMB Formation time: t f = τ 0 coshy with τ 0 = 0.7 fm/c $ %& z ' ) () 4

5 ZPC: Zhang s Parton Cascade B. Zhang, CPC 109, 193 (1998) Includes only elastic parton-parton scattering with cross section regulated by screening mass and taken to be energy independent dσ dt 9πα 2 1+ µ2 1 2, σ 9πα 2 2s 2 s t µ 2 2µ 2 Instead of determining the screening mass from the parton phase-space distribution, it is taken as a constant to fix the total cross section: µ = 3.2 fm -1, α s = 0.47 σ = 3 mb µ = 1.8 fm -1, α s = 0.47 σ = 10 mb (A) µ = 3.2 fm -1, α s = 0.33 σ = 1.5 mb but more isotropic (B) 5

6 Parton cascade vs 2+1D viscous hydro Molnar and Huovinen, JPG 38, (2011) Average transverse and Longitudinal pressure η=6t/5σ 4 Similar results are obtained from parton cascade and viscous hydro for same transport coefficient. 6

7 Charged particle Pseudorapidity distribution at LHC Results obtained with parameter set B are consistent with data. Final-state interactions reduce the charged particle yield at midrapidity. 7

8 Centrality dependence of charged particle rapidity density and transverse momentum spectra at LHC P T (GeV/c) Experimental data are reasonably described by parameter set B. 8

9 Elliptic flow at LHC Slightly larger and smaller v 2 in central and peripheral collisions, respectively. Larger v 2 at high p T due to overestimated non-flow effect. 9

10 Centrality dependence of anisotropic flow at LHC Experimental data on v 2, v 3, and v 4 are reasonably described. 10

11 Elliptic flow at RHIC Parameter set B, corresponding to smaller but more isotropic cross section, describes better the elliptic flow wrt the event plane than parameter set A. 11

12 Triangular flow at RHIC Parameter set B predicts smaller triangular flow than parameter set A. 12

13 Dihadron azimuthal correlations triggered by jets Adare et al. (PHENIX Collaboration), PRC 77, (R) (2008) Peaks at ~2π/3 and ~4π/3 in central and mid-central collisions are consistent with triangular flow 13

14 Possible mechanisms for the double-peak structure Mach cone shock waves J. Casalderrey-Solanaa, E.V. Shuryaka, and D. Teaney, Nucl. Phys. A774, 577 (2006). T. Renk and J. Ruppert, Phys. Rev. C 73, (2006). Jet deflections B. Betz, J. Noronha, G. Torrieri, M. Gyulassy, and D. Rischke, Phys. Rev. Lett. 105, (2010). Cherenkov radiation V. Koch, A. Majumder, and Xin-Nian Wang, Phys. Rev. Lett. 96, (2006). Medium-induced gluon radiation I. Vitev, Phys. Lett. B630, 78 (2005). A.D. Polosa and C.A. Salgado, Phys. Rev. C 75, (R) (2007). Path-length-dependent jet energy loss C.B. Chiu and R.C. Hwa, Phys. Rev. C 74, (2006). W. Li, S. Zhang, Y.G. Ma, X.Z. Cai, J.H. Chen, H.Z. Huang, G.L. Ma, and C. Zhong, Phys. Rev. C 80, (2009). 14

15 Hadron transverse momentum spectrum and Back-to-back jets in AMPT Thermal spectrum below 2 GeV/c and power law spectrum above 2 GeV/c Strong back-to-back correlations for hadrons above 2 GeV/c 15

16 Hadron azimuthal angular distribution ( ) = N ( 2# 1+ 2v n f " & ' $ $ ) % cos cos(n") + % 2v sin n sin(n") + * n =1 n =1 = N & $ 2# 1+ 2v ) ( % n cos(n(", - n )) + ' * n =1 Event plane Anisotroic flow v n = cos[n(" # $ n )] < >: average over particles in an event Including p T dependence 16

17 Dihadron azimuthal correlations in AMPT (I) Exact dihadron azimuthal correlations can be very well approximated by that obtained from the approximate hadron azimuthal angular distribution. 17

18 Dihadron azimuthal correlations Total correlation: < > e : average over all events dn pair dδφ = = 1 2π f trig ( φ) f asso ( φ + Δφ)dφ N trig N asso e + 2 n =2 e N trig v n trig N asso v n asso cos(δφ) e has contributions from both jet-induced correlation and anisotropic flow. Background correlations due to anisotropic flow dn pair dδφ back = 1 2π = N trig N trig e N asso e 2π e N asso e + 2 N trig v n trig n = v trig n v asso n cos( nδφ) n =2 e N asso v n asso e cos ( nδφ ) 18

19 Dihadron azimuthal correlations in AMPT (II) 0.15GeV c < p T asso < 2.5GeV c 2.5GeV c < p T trig < 6GeV c Away-side double peaks appear after subtraction of elliptic flow 19

20 Effects of initial triangularity on dihadron azimuthal correlations Peaks are enhanced (suppressed) in events of larger (smaller) triangularity. 20

21 Effects of high-order flows on dihadron azimuthal correlations Single away-side peak with shoulders after subtraction of triangular flow contribution. Subtraction of higher-order flow effects further weakens the shoulder punch through jet 21

22 Dihadron azimuthal correlations in central HIC Ma & Wang, PRL 106, (1911) Away-side double peaks remain after subtraction of flow effect importance of jet-induced medium excitation by radial flow in central collisions 22

23 Dihadron azimuthal correlations at LHC Background correlations due to flow contribution are evaluated from the long-range correlations. Away-side double peaks disappear after BG subtraction. 23

24 Two-dimensional dihadron correlations at LHC with final-state interactions without final-state interactions Near side (ΔΦ=0) peak (Δη=0) and ridge (up to Δη =4) as well as away-side broad structures are similar to experimental data. These structures are due to fluctuating initial conditions and final-state interactions. 24

25 Near-side associated particle yield Associated particle yield in the jet region increases while in the ridge region decreases with trigger particle momentum. 25

26 Summary Using refitted parameter set B, AMPT can reasonably describe LHC data on - charged particle psuedorapidity distribution; - transverse momentum spectrum; - anisotropic flows including the triangular flow; - near-side ridge and away-side broad structure in twodimensional dihadron correlations. Triangular flow is appreciable and less dependent on centrality of collisions. Triangular flow affects significantly away-side double peaks in dihadron azimuthal correlations. Initial fluctuations and final-state interactions NS peak and ridge as well as AS structures 26