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DeSilva

VIEWS: 6 PAGES: 28

									The near-side long range correlation
  structure in two particle number
        correlations at RHIC


             L. C. De Silva
      for the STAR collaboration
         University of Houston



                 STAR

                                       1
                                           Outline
 Phys. Rev. C 80, 064912 (2009)                                   STAR preliminary

                         3<pt,trigger<4 GeV                         pT > 0.15
                                                     
                                                                    GeV/c
                         pt,assoc.>2 GeV              ref




       Δφ                         Δη                                                 Δη
                                                             Δφ

• Motivation of study
   – Triggered and un-triggered data in 200GeV Au+Au collisions
• Data and cuts
• Correlation measure
• Centrality dependent evolution
• Momentum dependent evolution
• Fit function and alterations
• Comparison to theory
• Summary                                                     2
                   Motivation – triggered “ridge”
                          Dihadron correlations       Phys. Rev. C 80, 064912 (2009)
                                                    Au+Au 0-10%: √sNN = 200GeV
                             associated
                                                                             3<pt,trigger<4 GeV
                         trigger
Structures:                                                                    pt,assoc.>2 GeV

• Near side “jet” peak comparable to p+p
• Near side “ridge” structure at intermediate pt




What do we know:(Phys. Rev. C 80, 064912 (2009))
• Ridge yield approximately independent of and pt,trig
• Ridge yield persists to highest trigger pt => correlated to jet production?
• Ridge only in A+A (not present in p+p or d+Au or peripheral A+A)
• Ridge pt-spectra are „bulk-like‟ and approximately independent on pt,trig
                                                                                       3
                Motivation – untriggered ridge
                                                    STAR preliminary
                                                               0 – 10%: Au+Au 200 GeV
                                                 ref
Structures:
• Near side “ridge like” structure
• Away side structure



What do we know: (M. Daugherity for STAR Collaboration, QM 2008)
• Strong centrality variations of the same side structure
• Multiple particle production mechanisms contribute to correlation function in
  soft sector
Why is it interesting:
• The same side structure is an elongated 2D Gaussian
• Significant soft particle contribution

                                                                                          4
              Investigations presented in this talk
• Centrality dependence: Investigate the evolution of the untriggered ridge in 200GeV
Cu+Cu collisions (pT > 0.15 GeV/c)
               40 – 50%                      40 – 30%   30 – 20%                   20 – 10%




    Δφ                   Δη

         STAR preliminary
         Cu+Cu 200GeV
                                   10 – 0%




• pt dependence: Investigate how the untriggered ridge evolves toward the triggered in
200GeV Cu+Cu and Au+Au collisions
                                                          Phys. Rev. C 80, 064912 (2009)
                                                                           3<pt,trigger<4 GeV
STAR preliminary 
Au+Au 0-10%       ref                                                     pt,assoc.>2 GeV




                              Δφ             Δη                                                 5
                                Data and cuts
•   Cu+Cu 200GeV minimum bias
•   Au+Au 200GeV central trigger
•   Event cuts: Primary vertex cut (+/- 25cm)
•   Track cuts:
     – pT ≥ 0.15 GeV/c for the untriggered analysis
     – pT ≥ x GeV/c lower limit x is varied for both particles for the pT evolution (
       0.15, 0.3, 0.5 GeV/c, ……)
     – -1 ≤ η ≤ 1
•   Centrality parameter  = 2<Nbin>/<Npart>


                 




                                                                                   6
                                                  Tracks produced in an event
                              Correlation Measure
• ρ = Two particle density
                                                                                       Sibling Pairs
• Sibling Pairs                           Event A                                 x2

                            na nb
     sib ( pt , ,  ) 
                            Area                                                                     x1
• Mixed Pairs
                                                                                   Mixed Pairs
                                          Event B                            x2
                          n a nb
     ref ( pt , ,  ) 
                          Area
• Final Measure:
                                                                                                x1
      sib   ref                               
                                      ref
                                               '

                                                    ref
                                                           Normalized Ref Pairs: Total number of sibling to mixed pairs
            ref              ref
                                                           dN ch
                                                           d d 
                                                                                             Slide from E. Oldag
• Number of correlated pairs per final state particle                                        UT Austin

                                                                                                          7
        Untriggered correlation plots – centrality evolution
                   pT > 0.15
                   GeV/c                80 – 70%        70 – 60%   60 – 50%
         proton-proton              STAR preliminary
                                  Cu+Cu 200GeV
 ref




                                   Δφ
                                                   Δη
                                        50 – 40%        40 – 30%   30 – 20%

                          
                            ref




                                        20 – 10%        10 – 0%

                          
                            ref




                                                                              8
     How we increase the momentum threshold
                                                              Phys. Rev. C 80, 064912 (2009)
                   STAR preliminary
                   Au+Au 0-10%                               Au+Au 0-10%: √sNN = 200GeV
     
      ref   pT > 0.15                                                              3<pt,trigger<4 GeV
             GeV/c                                                                  pt,assoc.>2 GeV




• Investigate how the untriggered ridge evolves toward the triggered as a function of pt


                           pt2




                                                               pt1                          9
                     The momentum dependent evolution of the ridge
        STAR preliminary
        Cu+Cu 200GeV : 0–10%
                  pT >150 MeV/c       pT >300 MeV/c                pT >500 MeV/c

 ref




             Δφ                Δη

                  pT >700 MeV/c       pT >900 MeV/c                pT >1100 MeV/c




                                                        Phys. Rev. C 80, 064912 (2009)
                                                        Au+Au 0-10% √sNN = 200GeV
                  pT >1300 MeV/c       pT >1500 MeV/c                     3<pt,trigger<4 GeV
                                                                             pt,assoc.>2 GeV




                                                                                         10
STAR preliminary
Au+Au 200GeV : 0–10%
   pT > 0.15           pT > 0.30           pT > 0.50          pT > 0.70
   GeV/c               GeV/c               GeV/c              GeV/c
   
    ref




   pT > 0.90           pT > 1.10           pT > 1.30          pT > 1.50
   GeV/c               GeV/c               GeV/c              GeV/c




   pT > 1.70            pT > 1.90          pT > 2.10          pT > 2.30
   GeV/c                GeV/c              GeV/c              GeV/c




   pT > 2.50            pT > 2.70          pT > 2.90          pT > 3.10
   GeV/c                GeV/c              GeV/c              GeV/c




               • Disappearance of long range correlation (“ridge”) at high pT
               • Emergence of “Jet” like (unmodified jet?) at high pT           11
           Three possible physics explanations

• The initial energy density fluctuations and third harmonic component in
azimuth
     P. Mishra et.al: Phys.Rev.C77:064902,2008
     B. Alver and G. Roland - arXiv:1003.0194

• CGC flux tubes + radial flow model
     Gavin, McLerran and Moschelli: Phys.Rev.C79:051902,2009; Bulk-Bulk
     Moschelli and Gavin: Nucl.Phys.A836:43-58,2010; Jet-Bulk

• Medium modified jet model (pQCD related explanation)
     T.A. Trainor and D.T. Kettler – arXiv:1008.4759
     T.A. Trainor – Phys.Rev.C80:044901,2009


 In this talk we will address all three models via our fit function
     Uses a cos(3Δϕ) term to discuss about energy density fluctuations
     Uses an asymmetric 2D Gaussian term to discuss pQCD and CGC
    models

                                                                            12
                                 Fit function
                 f  f1  f 2  f 3  f 4  f 5  f 6  f 7
                 f1  c 0
                 f 2  c1  cos( )
                 f 3  c 2  cos(2 )
                 f 4  c 3  cos(3 )                                • f4 and f5 focuses on
                 f 5  c 4  exp(0.5 * (( /c 5 )  ( /c 6 ) ))
                                                         2      2     the same side long
                                                                      range correlation
                 f 6  c 7 * exp(0.5 * ( /c 8 ) 2 )
                                                                      • What do we require;
                 f 7  c 9 * exp(1* ( /c10 )  ( /c11 ) )
                                                     2          2
                                                                      f4 or f5 or f4 + f5

f1  offset
f 2  cos( )                        "away side momentum conservation"
 cos(2 )
f3                                   "v 2 like correlation"
f 4  cos(3 )                       "v 3 like correlation"
f 5  2D asymmetric Gaussian  "near side longrange correlation in "
f 6  1D Gaussian in                "string fragmentationcorrelation"
f 7  2D Exponential                 "HBT&e e  "
                                                                                     13
              Residual and fit decomposition example : 30 – 40%
                         cos(Δφ)                 cos(2 Δφ)     cos(3 Δφ)          2d Gaussian
Decomposition

STAR preliminary

Please note that the
scales are different
                       Offset      1d Gaussian      Exponent      Which describes the same side
                                                                  better




                        Data                        Fit              Residual
   STAR preliminary
   Cu+Cu 200GeV
                                                                                        χ2/#dof ≈ 1.6




                                                                                                  14
  Parameter evolution – centrality (CuCu 200GeV)
       STAR preliminary
       Cu+Cu 200GeV




• We observe a non zero v2(2D) value at the central bin
• v3 increases with increasing centrality
• The evolution of long range correlation data as reflected via the 2D Gaussian
parameters




                                                                            15
     Do we need v3 to describe the same side structure
                     Pr ojections made on   2

              - Data
              - Fit – cos(3Δϕ)
                                                   - Data
                                                   - Fit – 2D Gaussian   Example at 10 – 20%
         
                                                                         pT ≥ 0.15GeV/c
                                                                         STAR preliminary
                                                                         Cu+Cu 200GeV




• The projections indicate that the v3 contribution to the long range correlation on the
same side is relatively small compared to a 2D Gaussian
• Furthermore the centrality dependence indicate that the v3 contribution is less
relevant in more peripheral bins
• The sharp amplitude and Δη width evolution follow a smoother evolution when v3
taken in to account




                                                                                            16
    Residual and fit decomposition example without v3: 30 – 40%
                      Offset        cos(Δφ)         cos(2 Δφ)
Decomposition
STAR preliminary




                     2d Gaussian   1d Gaussian      Exponent




                       Data           Fit           Residual
  STAR preliminary
  Cu+Cu 200GeV
                                                                  χ2/#dof ≈ 2




                                                                  17
                                Disclaimer
STAR preliminary
Cu+Cu 200GeV
                   v3 included           v3 excluded




                    χ2/#dof ≈ 1.6            χ2/#dof ≈ 2


      • Residuals indicate that both fits have similar validity

      • Complex nature of the fits require further tests (global
      minima/ local minima, scanning the residual)

                                                                  18
            Comparison to theory (centrality dependence)
   •    A 2D asymmetric Gaussian alone fit the same side
   •    Compare to theory based on CGC flux tubes and radial (blast wave) flow (Gavin,
        McLerran, Moschelli, Phys.Rev.C79:051902,2009)

2D Gaussian amplitude                             2D Gaussian  width



                     Theory (shaded error)   
                 .   CuCu 200GeV




                                                                                               

              • Theory does not predict the Δη dependence
              • Ridge amplitude evolution agrees with data
                                                                                        
                    In the model the amplitude in AuAu has been scaled to match the central
                   data point
              • ΔΦ width prediction agrees with data                                           19
              • 2D asymmetric Gaussian alone on the same side is sufficient
                       Momentum dependence
pT > 0.15                      pT > 1.10                   pT > 1.70
GeV/c                          GeV/c                       GeV/c




                 STAR preliminary
pT > 2.30        Au+Au 200GeV       pT > 2.50              pT > 3.10
GeV/c                               GeV/c                  GeV/c




            • Long range correlation strength drops at high pT
            • A “Jet” like peak emerges on top of the long range correlation
            • Is it emerging from an unmodified “Jet”?
            • We incorporate a symmetric 2D Gaussian on the same side
            • Extracted parameters are compared to p+p data
                                                                               20
       Asymmetric 2D Gaussian momentum dependence




                                                 STAR preliminary
• Both systems follow similar trends             Au+Au 200GeV

• Au+Au 200GeV data show that the “ridge” yield approaches zero at high
pT
         Symmetric 2D Gaussian momentum dependence


                       • Extracted width indicates that the observed “jet” like
                       peak could be in fact an unmodified jet peak

                       • Further studies are required (amplitude, volume
                       and efficiency corrections) to draw any conclusions
                                                                           21
                 Comparison to theory
•   Compare to theory based on CGC flux tubes and blast wave
    (Gavin, McLerran and Moschelli: Phys.Rev.C79:051902,2009; Bulk-Bulk)
    (Moschelli and Gavin: Nucl.Phys.A836:43-58,2010; Jet-Bulk)




•   Initial comparison of CuCu 200GeV followed by AuAu 200GeV suggests that
    “ridge” amplitude does not favor only a pure medium effect (Bulk-Bulk) and the
    “ridge” pT spectrum seems harder (suggesting the importance of “jet” like
    contributions at high pT)
•   The pT evolution of the ridge Δϕ width is inconclusive (note that jet-jet
    correlations are not yet taken into account)
                                                                                 22
                   Summary and conclusions
•   Centrality dependence

     •   Determined a complex fit function to account for all the contribution to the two particle
         correlation structure (Triggered + untriggered, same side and away side)

     •   Tested the applicability of a v3 term to describe the Δη elongated correlation structure on
         the same side (“ridge”)

     •   v3 contribution on the long range structure is relatively small, an asymmetric 2D Gaussian
         alone can describe the same side structure within STAR acceptance

     •   Little evidence on away side for a v3 term (no double hump structure in residuals)

     •   CGC + radial flow model which yield an asymmetric 2D Gaussian describes the data in
         terms of amplitude and Δϕ width

     •   The inclusion of a v3 leads to a non zero v2 term in central collisions


•   Momentum dependence
     •   We observe the emergence of a symmetric jet like peak (unmodified?) on the same side

     •   The relative jet yield gets much larger compared to the ridge at high pT –
         long range correlation amplitude drops by an order of magnitude

     •   In the CGC + radial flow picture the jet-bulk correlation contribution seems to be small
         (mostly jet-jet and bulk-bulk correlations)                                              23
Backup slides




                24
       Projections at large Δη




                Pr ojections on   1.04



• cos(3Δϕ) term contribution is relatively higher
        

                                                    25
(v3(2D)/v2(2D))^2 plot




                         26
             Projections – cos(3Δϕ) only

     Projectionsmade on   2        Projections made on  1.04



                               




                                                                     27
         Projections – 2D Gaussian only

     Projectionsmade on   2        Projections made on  1.04



                               




                                                                     28

								
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