Low-Mass Vector Mesons and Continuum at RHIC-PHENIX

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Low-Mass Vector Mesons and ContinuumatRHIC-PHENI
X

• Kenta Shigaki (Hiroshima University
  )
• for the X Collaboration
• Winter Workshop on Nuclear Dynamics 2009
• February 6, 2009
• Big Sky, Montana, U.S.A.

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Presentation Outline

• physics goals in low-mass region
• strategy of RHIC-PHENIX
• 1 chiral symmetry restoration
• low-mass vector mesons at low pT
• 2 quark tomography
• low-mass vector mesons at high pT
• 3 continuum enhancement
• low-mass continuum at low pT
• summary, remarks and outlook

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Four(-or-More)-Fold Low-Mass Physics

• probe of (partial) chiral symmetry restoration
• mass modification of low-mass vector mesons
• mass spectrum shapes
• branching ratios
• quark tomography
• flavor/spin dependence of quark energy loss
• (yet-to-be-understood) continuum enhancement
• search for thermal radiation
• thermal virtual photons

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RHIC-PHENIX, Relevant Data Sets

• fundamental strategy of PHENIX
• multiple probes with emphasis on transparent ones
• systematic studies of rare probes
• e.g. di-electron
• data sets most relevant to this presentation
• pp run 5 100 M ( 55 G min. bias equiv.)
• dAu run 3 50 M (3.8 G min. bias equiv.)
• AuAu run 4 800 M
• all data sets at ?s(NN) 200 GeV
• more statistics available in later runs

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A Key Measurement Di-Electron

• precise tracking and electron identification
• challenging background primarily from Dalitz
  decays
• statistical subtraction
• g conversion rejection
• correlated background also evaluated
• cross pairs
• jet components
• ref. Wed/PM3, S.Bathe

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Topic 1a f/w Mass Spectra

• naïve method to look at mass modification
• changes expected in low pT region
• lifetime of LVM in same order as that of partonic
  phase

R. Rapp, J. Phys G31 (2005) S217
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Topic 1b Decay Modes of f

• possible suppression of f ? KK- decay mode
• yield comparison between ee- and KK-
• ref. Mon/PM2, C.Markert, Mon/PM4 U.Mosel
• changes, again, expected in low pT region

T.Hatsuda and S.Lee, Phys.Rev.C46-1 (1992)
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Importance of Systematic Studies

• pT dependent information, e.g. spectra
• tf 46 fm/c, tw 23 fm/c
• low pT also essential for yield extraction
• multiple decay modes of multiple species
• f ? KK, ee (110-4)
• w ? ppp, pg, ee (710-5)
• r ? pp, ee (510-5)

high pT LVM
low pT LVM
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Baselines in pp

• clear peaks in all decay modes
• good mass resolution except in radiative mode

pp f ? KK- (single K ID)
pp w ? p0g
pp w, f ? ee-
pp w ? p0pp-
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and in dAu

• nearly as good peaks as in pp

dAu f ? KK- (double K ID)
dAu w, f ? ee-
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Solid Baselines Established

• agreement among all measured decay modes

pp, dAu w ? ee-, p0pp-, p0g
PHENIX preliminary
pp, dAu f ? ee-, KK-
PHENIX preliminary
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gt 1a f/w Mass Spectra in AuAu

• marginal statistics in featured ee- mode
• possible mass line shape analysis in later runs

AuAu w, f ? ee-
AuAu f ? KK- (double K ID)
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gt 1b Decay Modes of f in AuAu

• long awaited f ? ee- result (close to final)
• no signature of modification beyond errors
• centrality dependence of integrated yield, slope,
  spectra
• difference, if any, still to be quantified

MB
40-92
20-40
AuAu f ? ee-, KK-
PHENIX preliminary
AuAu f ? ee-, KK-
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Topic 2 Quark Tomography

• energy loss implied in partonic level
• same nuclear modifications for p/h (but not for
  direct g)
• cleaner signal in higher pT region
• mesons decaying into multi-photons advantageous
• systematic study for real tomography
• via multiple mesons/baryons/photon
• quark flavor dependence?
• pseudo-scalar mesons versus vector ones?
• photon decay modes of (vector) mesons motivated

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Extended f Baseline in pp/dAu

• double/single/no K identification methods
• extended pT reach to both low and high sides
• different efficiencies and signal/background
  ratios
• consistent results

pp, dAu f ? (ee-,) KK-
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Long Solid Baselines

• no suppression in dAu consistent with p and h

dAu 0-20 f ? KK-

• dAu 0-20 w ? p0pp-, p0g

dAu min.bias f ? KK-

• dAu min.bias w ? p0pp-, p0g

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Little pT Dependence in pp/dAu

• f, w/p (K) ratios flat above a few GeV/c
• another view of baseline

dAu w/p
dAu f/p
pp f/p (K)
pp w/p
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Highlight on High pT Side of AuAu f

• double/single/no K ID methods as in pp/dAu

double K ID
AuAu f ? KK-
no K ID
pp, AuAu f ? (ee-,) KK-
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Recent Highlights in AuAu w Analysis

• w ? p0g on high pT side up to 10 GeV/c
• long awaited w ? ee-, along with f ? ee-

0-20
MB
60-92
20-60
PHENIX preliminary
AuAu w ? p0g
AuAu w ? ee-, p0g
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f (, w) Meson As Heavy As Proton

• different behavior between f (, w) and protons
• protons not suppressed regardless of centrality
• not an mass effect partonic energy loss implied

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gt 2 Quark Tomography in AuAu

• quark flavor/spin dependent suppression?
• interesting behavior of nuclear modification for
  f, w
• Ks0 at high pT (? p0p0) also under study

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Topic 3 Low-Mass Continuum

• low-mass continuum enhancement at SPS
• melting r? broadening? thermal virtual photon?

CERES/NA45
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Di-Electron Continuum in pp

• signal/background 1
• peaks well separated due to high resolution
• agreement with hadron decay contributions
• charm consistent with PHENIX single electrons
• ref. Fri/AM6 I.Garishvili

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Continuum Extraction in AuAu

• signal/background 1/200
• large systematic error from tiny background
  uncertainty
• agreement with extra g converter runs

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Di-Electron Continuum Enhancement

• observed in low mass region below w/r
• 3.4 ? 0.2 (stat) ? 1.3 (sys) ? 0.7 (model)
  (150750 MeV)
• not in intermediate mass between f and J/Y
• agreement with PYTHIA with modified charm

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Continuum Enhancement Features

• concentrated in low pT
• faster increase than number of participants

0 lt pT lt 8.0 GeV/c
0 lt pT lt 0.7 GeV/c
0.7 lt pT lt 1.5 GeV/c
1.5 lt pT lt 8.0 GeV/c
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gt 3 Low-Mass Continuum in AuAu

• not described by any theoretical model
• SPS data (CERES, NA60) described by broadening

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cf. g at Low Mass Higher pT

• first measurement of thermal radiation at RHIC
• ref. Wed/PM3, S.Bathe
• pp small excess consistent with pQCD
• AuAu large excess at all pT

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Summary, Remarks and Outlook

• multi-fold physics in low-mass region addressed
• solid systematic baselines established in
  pp/dAu
• AuAu challenges identified, attacked, overcome
• electron/photon measurements, as well as hadrons
• continuum enhanced in AuAu still mysterious
• paper(s) submitted to PRL
• vector mesons in AuAu elaborated and being
  finalized
• papers in preparation
• further systematic studies laid out/underway
• HBD to make use of next AuAu run
• ref. Sat/AM1 R.Seto