V' Friese SQM04, Cape Town, September 2004 1

1
Energy Dependence of Strangeness
ProductionResults from NA49
V. Friese Gesellschaft für Schwerionenforschung Da
rmstadt, Germany v.friese_at_gsi.de
for the NA49 collaboration
Strangeness in Quark Matter 2004, Cape Town,
September 2004
2
Motivation

• Main questions
• Do we understand the strangeness production
  mechanism in HI collisions?
• Is it the same at all beam energies?
• Is strangeness an indicator for a phase
  transition?
• Do statistical hadron gas models work (and if
  yes, why) ?
• Do we need ?s in such models (and if yes, what
  does it mean) ?

Strangeness enhancement ? canonical suppression
Need for measurement of many strange particles at
many different beam energies (and system sizes)
Data base is growing fast RHIC, SPS, AGS
3
Analysis Status Central PbPb
? published
? submitted
? preliminary
? to come
4
The Experiment

• Large acceptance hadron spectrometer
• Tracking in four TPCs
• PID by dE/dx and TOF
• Centrality by zero-degree calorimeter

5
Kaon Identification
2.5 GeV lt p lt 10 GeV TOF dE/dx
p gt 4 GeV dE/dx
6
Mirapidity Kaon mt spectra
Well described by eponentials, no energy
dependence of inverse slopes
7
Kaon Rapidity Distributions
K
K-
Approximately Gaussian shape, width increasing
with beam energy
8
K / p ratio
Sharp peak at 30 AGeV
Not reproduced by HGM
Better description by SHM
UrQMD misses trend and magnitude
ltpgt 1.5 ( ltpgt ltp-gt)
9
A Word on the Data
? 4p yields depend strongly on midrapidity (TOF)
point

• ! Yes, but
• there PID is most reliable
• Good agreement between TOF and dE/dx where
  overlapping

? Are pion yields corrected for feeddown from
weak decays
! NA49 yes AGS yes BRAHMS No, will be
done soon, 4 correction (see talk by D. Röhrich)
10
4p versus midrapidity

• Mid-rapidity
• Stay away from fragmentation region
• Much more data available

• 4p
• Gaussian distributions of produced particles
• No handle for separation central/fragmentation
• Indepenence on kinematics (flow, )
• Statistical models describe average properties
  (EGC)
• 4p data increasing (NA49, BRAHMS)

Anyway Qualitative picture in K/p does not
change when going to midrapidity
11
K- / p ratio
Smooth evolution with beam energy (small
irregularity at 30 AGeV?)
HGM overpredicts ratio at higher SPS energies
SHM also systematically too high
UrQMD systematically low
ltpgt 1.5 ( ltpgt ltp-gt)
12
? Identification
20 AGeV
30 AGeV
?
Identification by decay topology and invariant
mass
?
13
? transverse mass spectra
158 AGeV
80 AGeV
40 AGeV
158 AGeV
30 AGeV
80 AGeV
20 AGeV
40 AGeV
30 AGeV
Spectra not exponential, fit in 0.4 GeV lt mt-m? lt
1.4 GeV
14
? Rapidity Distributions
? Gaussian at lower energies, flattens at top
SPS energy
? Gaussian at all energies
15
? / p ratio
Peak at 30 AGeV AGS data not conclusive
HGM predicts maximum at lower energies
SHM describes peak, but too high at top SPS
UrQMD too low at lower SPS
ltpgt 1.5 ( ltpgt ltp-gt)
16
Cascade Measurement
p -
(K- )
?-
(O-)
p
?
p -
?-
Identification by decay topology and invariant
mass
17
Cascade Transverse Mass Spectrum, 40 AGeV
Flow effect easily visible Strong deviation
from exponential Exponential fit in 0.2 GeV lt
mt-m? lt 1.4 GeV T (210 11) MeV (for 158
GeV T (261 6) MeV )
18
Cascade Rapidity Distributions
Gaussian shape at 40 and 158 AGeV
to come soon
Analysis of 20, 30 and 80 AGeV data started
19
?- / p ratio
Slight increase from 158 AGeV to 40 AGeV
HGM overpredicts both data points, expects
maximum above 40 AGeV
Fair description by SHM, predicts maximum at 30
AGeV
Underprediction by UrQMD, larger than for s1
ltpgt 1.5 ( ltpgt ltp-gt)
20
O Signals at 40 and 158 AGeV
21
O Transverse Mass Spectra
Exponential fit not too bad Flow fits (blast
wave) depend strongly on velocity profile
22
O Rapidity Distributions
Again Reasonable description by Gaussian
23
O / p ratio
Increase (?) from 40 to 158 AGeV
HGM reproduces data reasonably
SHM does so, too (why do not they differ so much?)
Strong underprediction by UrQMD
ltpgt 1.5 ( ltpgt ltp-gt)
24
F measurement
F observed in KK- decay channel Enrichement of
kaons by dE/dx cut Background reconstruction by
event mix method
25
F Transverse Mass Spectra
Approximate exponential behaviour in mt
Slopes rising towards RHIC, different trend
compared to kaons

26
F Rapidity Distributions
Approximate Gaussian shape width increases
faster than beam rapidity
27
F / p Ratio
General trend similar to K-
HGM overpredicts data
SHM predicts peak at 30 AGeV, not seen in data
UrQMD too low by factor 2 at 158 AGeV
ltpgt 1.5 ( ltpgt ltp-gt)
28
Once More The f Puzzle
Discrepancy NA50/NA49 still unsolved
Signal lost due to kaon rescattering?

• Indications
• harder mt spectrum
• broadening of rapidity distribution

29
The f Puzzle (contd.)

• But
• Should be concentrated at low pt
• UrQMD/RQMD gives small effect (10)

• Alternative explanations?
• Modified mass (K/f), width, branching ratio?
• But most of the f decay outside the dense medium
  (t45fm/c)

Are lifetime and size of the fireball drastically
different as those deduced from HBT?
30
Summary

• A multitude of strange particles has been
  measured by NA49 at five beam energies, analysis
  of energy scan almost completed, some more data
  (?) to come
• Total strangeness to pion ratio exhibits peak at
  30 AGeV, visible in K and ?, not reproduced by
  HGM nor UrQMD
• HGM overpredicts yields at higher SPS energies
  ?s seems to be needed
• SHM catches peak (due to ?s), but does not very
  well for K- and f
• UrQMD is generally too low, especially so for
  multistrange particles
• Cascade measurement at 20 and 30 AGeV could have
  much restrictive power
• SMES gives still the best description of the
  total strangeness / pion ratio

The sharp maximum in the strangeness to pion
ratio indicates a sudden change in the
strangeness production mechanism. It will be hard
to reproduce it with any continuous variation of
parameters.