Strangeness at RHIC: what do we learn

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Strangeness at RHIC what do we learn?
Steffen A. Bass
Duke University RIKEN BNL Research Center

• Motivation
• Lessons from strangeness
• in the deconfined phase
• at hadronization
• in the hadronic phase

• in collaboration with
• Daphne Chang
• Adrian Dumitru
• Rainer Fries
• Berndt Mueller
• Chiho Nonaka
• Dinesh K. Srivastava

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• Deconfined Strangeness
• initial state vs. production
• enhancement vs. pp

- see also Daphne Changs talk this afternoon
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Basic Principles of the PCM

• degrees of freedom quarks and gluons
• classical trajectories in phase space (with
  relativistic kinematics)
• initial state constructed from experimentally
  measured nucleon structure functions and elastic
  form factors
• an interaction takes place if at the time of
  closest approach dmin of two partons
• system evolves through a sequence of binary
  (2?2) elastic and inelastic scatterings of
  partons and initial and final state radiations
  within a leading-logarithmic approximation (2?N)
• binary cross sections are calculated in leading
  order pQCD with either a momentum cut-off or
  Debye screening to regularize IR behaviour
• guiding scales initialization scale Q0, pT
  cut-off p0 / Debye-mass µD,
  intrinsic kT / saturation momentum QS, virtuality
  gt µ0

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Initial State Parton Momenta

• flavour and x are sampled from PDFs at an
  initial scale Q0 and low x cut-off xmin
• initial kt is sampled from a Gaussian of width
  Q0 in case of no initial state radiation

• virtualities are determined by

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Parton-Parton Scattering Cross-Sections

• a common factor of pas2(Q2)/s2 etc.
• further decomposition according to color flow

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Initial and final state radiation
Probability for a branching is given in terms of
the Sudakov form factors
space-like branchings
time-like branchings

• Altarelli-Parisi splitting functions included
  Pq?qg , Pg?gg , Pg?qqbar Pq?q?

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Strangeness in the PCM

• Intrinsic strangeness
• (anti-) strange quarks contained in the
• Dirac sea
• intrinsic strangeness is released through
• scattering

g
g
s
s

• Produced strangeness
• production mechanisms
• 1. binary collisions
• 2. radiation
• accounts for strangeness enhancement

s
q,g
s
q,g
s
s
s
s
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Strangeness Production How?

• intrinsic strangeness accounts for 39 of final
  ss yield
• momentum distribution of strangeness released
  differs from that
• produced in binary collision and branching
  processes
• PCM accounts for 55 of estimated measured yield
  at mid-rapidity
• total ss yield at mid-rapidity ? STAR
  estimation 271
• 
  PCM calculation 148

(
)
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Dynamics of Strangeness Production

• p p results scaled by Nelastic, AuAu/
  Nelastic, pp
• strangeness suppressed at high parton-parton
  sqrt(s)
• thermal re-scattering more important than
  initial hard
• processes for strangeness yield!

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Strange quark-antiquark rapidity correlations

• strange quarks and antiquarks are always produced
  pairwise
• initial state correlation extremely narrow in ?y
• release rescattering of strangeness as well as
  gluon splittings result in Gaussian distribution
  of correlated ?y
• contribution to strangeness Balance Function

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Probing Hadronization Time Balance Functions

• B(?y) narrower for late stage hadronization for
  two reasons

• lower temperature
• High initial dv/dz diffusion separates early
  produced pairs

• B(?y) provides clear signature of late stage
  hadronization

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• Strangeness at Hadronization
• Is strangeness in thermal/chemical equilibrium?
• What do we learn about hadronization mechanisms?

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The baryon puzzle _at_ RHIC

• where does the large proton over pion ratio at
  high pt come from?
• why do protons not exhibit the same suppression
  as pions?
• fragmentation yields Np/Npltlt1
• fragmentation starts with a single fast parton
  energy loss affects pions and protons in the same
  way!

ratio of KKP fragmentation functions for p and p
from u quarks
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Species dependent saturation of elliptic flow

• hyperon v2 saturates later and higher than kaon
  v2.
• same effect observed for protons and pions.
• at low pT the phenomenology seems better
  described in mT m0 than pT , indicating hydro
  scaling, yet scaling breaks down for high pT
• what drives the different pT scales for KS and ?
  v2?
• novel mechanism of baryon formation?

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RecombinationFragmentation Model

• basic assumptions
• at low pt, the quarks and antiquark spectrum is
  thermal and they recombine into hadrons locally
  at an instant
• features of the parton spectrum are shifted to
  higher pt in the hadron spectrum
• at high pt, the parton spectrum is given by a
  pQCD power law, partons suffer jet energy loss
  and hadrons are formed via fragmentation of
  quarks and gluons

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Recombination Pros Cons

• Pros
• for exponential parton spectrum, recombination
  is more effective than fragmentation
• baryons are shifted to higher pt than mesons,
  for same quark distribution
• understand behavior of protons!
• Cons

fragmenting parton ph z p, zlt1
recombining partons p1p2ph

• simple recombination violates entropy
  conservation
• gluons at hadronization need to be converted

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Recombination nonrelativistic formalism

• use thermal quark spectrum given by w(p)
  exp(-p/T)
• for a Gaussian meson wave function with momentum
  width ?M, the meson spectrum is obtained as

• similarly for baryons

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Recombination vs. Fragmentation
Fragmentation
never competes with recombination for a thermal
(exponential) spectrum
but it wins out at large pT, when the spectrum
is a power law (pT)-b
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Flavor Dependence of high-pt Suppression

• RF model describes different RAA behavior of
  kaons and hyperons
• phi measurement confirms constitutent quark
  number dependence
• in the fragmentation region all hadron flavors
  exhibit jet-quenching

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Elliptic Flow

• recombinationv2 of baryons saturates at a
  higher value than that of mesons
• at high PT, v2 is dominated by fragmentation
• identical v2 for baryons and mesons
• assume same v2 for s and u,d quarks
• v2 of F and K are identical
• Hydrodynamical model
• limits on hydrodynamic behavior
• saturation driven by recombination

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Parton Number Scaling of v2

• in leading order of v2, recombination predicts

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• Strangeness in the hadronic phase
• The appeal of multistrange baryons
• kaon HBT

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Flavor Dynamics Radial Flow

• Hydro linear mass-dependence of slope parameter,
  strong radial flow
• HydroMicro softening of slopes for multistrange
  baryons
• early decoupling due to low collision rates
• nearly direct emission from the phase boundary

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HBT with kaons

• less contamination from long-lived hadronic
  resonances (hadronic halo)
• lower phase-space occupancy than pions less
  contamination through multi-particle correlations
• probability of being directly emitted from the
  phase-boundary increases with pt
• increased sensitivity to EoS, particularly at
  high pt

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Connecting the dots
jet production
fragmentation
jet quenching
parton recombination
HBT
radial flow
reco/SM?
shattered color-glas
hydrodynamic evolution
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Summary and Outlook

• Deconfined phase
• PCM can account for 55 of the observed
  strangeness (1/3 initial state)
• strangeness production is sensitive to
  quasi-thermal rescattering
• distinctive rapidity correlation for associated
  particles
• Hadronization
• F and O confirm predictions of recombination
  model
• constituent quark scaling of strange hadron v2
  indicates thermalization
• Hadronic phase
• multi-strange baryons as undisturbed messengers
  of hadronization
• kaon HBT better control over FSI
• Outlook
• measurement/interpretation of strangeness
  balance functions
• transfer of strangeness techniques concepts to
  charm analysis

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The End