A Strange Perspective – Preliminary Results from the STAR Detector at RHIC
description
Transcript of A Strange Perspective – Preliminary Results from the STAR Detector at RHIC
Helen CainesThe Ohio State
University
MSU – Nucl. Seminar
June 2001
A Strange Perspective –
Preliminary Results from the STAR
Detector at RHIC
Science is a wonderful thing if one does not have to earn one's living at it
– Einstein (1879—1955)
Helen Caines
MSU - 2001
The STAR Collaboration
Russia: MEPHI – Moscow, LPP/LHE JINR–Dubna, IHEP-Protvino
U.S. Labs: Argonne, Berkeley, Brookhaven National Labs
U.S. Universities: Arkansas, UC Berkeley, UC Davis, UCLA, Carnegie Mellon, Creighton, Indiana, Kent State, MSU, CCNY, Ohio State, Penn State, Purdue,Rice, Texas A&M, UT Austin, Washington, Wayne State, Yale
Brazil: Universidade de Sao Paolo
China: IHEP - Beijing, IPP - Wuhan
England: University of Birmingham
France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes
Germany: Max Planck Institute – Munich University of Frankfurt
Poland: Warsaw University, Warsaw University of Technology
Institutions: 36Collaborators: 415 Students: ~50
Spokesperson: John Harris
Helen Caines
MSU - 2001
QCD
Quarks confined within hadrons via strong force
v(r) = /r + *r
At large r -second term dominates
At small r -Coulomb-like part dominates
However function of q( mtm transfer) and -> 0 faster
than q (or 1/r) -> infinity (called asymptotic freedom)
This concept of asymptotic freedom among closely packed
coloured objects (q and g) has led to one of the most exciting
predictions of QCD !!
The formation of a new phase of matter where the colour degrees of freedom are liberated. Quarks and gluons are no longer confined within colour singlets.
The Quark-Gluon Plasma!
Helen Caines
MSU - 2001
most dangerous event in
human history: - ABC
News –Sept ‘99
Don’t Panic!!!
"Big Bang machine could
destroy Earth" -The
Sunday Times – July ‘99
the risk of such a catastrophe is essentially zero. – B.N.L. – Oct ‘99
- New Scientist
Will Brookhaven
Destroy the Universe? –
NY Times – Aug ‘99
No… the experiment will not tear our region of space to subatomic shreds.
- Washington Post – Sept ‘99
Apocalypse2 – ABC News – S
ept
‘99
Helen Caines
MSU - 2001
The Phase Space Diagram
TWO different phase transitions at work!
– Particles roam freely over a large volume
– Masses change
Calculations show that these occur at approximately the same point
Two sets of conditions:
High Temperature
High Baryon Density
Lattice QCD calc. Predict:
Tc ~ 150-170 MeV
c ~ 0.5-0.7 GeV/fm
Deconfinement transition
Chiral transition
Helen Caines
MSU - 2001
Tfireball < Tc(170MeV) Hadron gasHard to make S 0 particles
+ N + K (Ethresh 530MeV) + K + (Ethresh 1420MeV)
Mtm phase space suppressedNeed to create 3 qq pairs
(initially there are no q) with similar momenta in a region already containing many quarks.
Tfireball >Tc(170MeV) QGP
Easy to make s quarks E=2ms ( 300MeV)Free gluons
g-g fusion - dominate ss creation faster reaction time than qq
Pauli blocking may aid creation of ss quarks
( probably not true at high T, too many states).
_
_
__
_
_
_
Why are we interested in Strangeness?
Helen Caines
MSU - 2001
Introduction
When is Strangeness Produced – Resonances
Chemical content
– Yields
Thermal Freeze-out – Radii and Inverse slopes
Flow – How much and when does it start?
Chemical Freeze-out - Ratios
Helen Caines
MSU - 2001
Previous Strangeness Highlights
WA97
Evidence of strangeness enhancement between pA and AA collisions at the SPS – Not reproducible by
models
SPS s=17GeVEnhancement > > > h
|s|
Helen Caines
MSU - 2001
Strangeness Highlights (2)
AGS and SPS > 1
Need to consider p absorption
Multi-Strange Particles appear to
freeze out at a cooler temperature/ earlier or have
less flow
SPSAGS
_
Helen Caines
MSU - 2001
The CERN announcement
Strangeness was one of the corner stones of the CERN announcement.
Have numerous pointers that there is evidence of a new state of matter even at SPS energies so why RHIC?
•Still a large baryon number so need models to understand what’s going on.
•Those models can probably be tuned to reproduce the experimental data but would require more “knobs”
•So want to go to “cleaner” system
•Less baryon number (only look at produced particles)
•Closer to the region where QCD predictions work – Definite theory not models
•Higher energies – further across phase transition boundary – In new regime for longer and more frequently
Helen Caines
MSU - 2001
Welcome to BNL- RHIC!
Helen Caines
MSU - 2001installation in 2003
Endcap Calorimeter
Year 2000,
The STAR Detector (Year-by-Year)
ZCal
Time Projection Chamber
Magnet
Coils
RICH * yr.1 SVT ladder
TPC Endcap & MWPC
ZCal
Central Trigger Barrel
FTPCs (1 + 1)
Silicon Vertex Tracker *
Vertex Position Detectors
year 2001,
+ TOF patch
Barrel EM Calorimeter
year-by-year until 2003,
Helen Caines
MSU - 2001
STAR Pertinent Facts
Field:
0.25 T (Half Nominal value)
(slightly worse resolution at higher p, lower pt acceptance)
TPC:
Inner Radius – 50cm
(pt>75 MeV/c)
Length – ± 200cm
( -1.5 1.5)
Events:
~300,000 “Central” Events –top 8% multiplicity
~160,000 “Min-bias” EventsL3-Real time display
Helen Caines
MSU - 2001
Needle in the Hay-Stack!
How do you do tracking in this regime?
Solution: Build a detector so you can zoom in close and “see” individual tracks
Good tracking efficiency
Clearly identify individual tracks
high resolution
Pt (GeV/c)
Helen Caines
MSU - 2001
Particle ID Techniques - dE/dx
dE/dx PID range: ~ 0.7 GeV/c for K/ ~ 1.0 GeV/c for K/p
12
Kp
d
edE
/dx
(keV
/cm
)
0
8
4
12
Kp
d
edE
/dx
(keV
/cm
)
0
8
4
Kp
d
edE
/dx
(keV
/cm
)
0
8
4
dE/dx
6.7%Design
7.5%With calibration
9 %No calibration
Resolution:
Even identified anti-3He !
Helen Caines
MSU - 2001
Particle ID Techniques - Topology
Decay vertices
Ks + + -
p + -
p + +
- + -
+ + +
+ K -
“kinks”:
K +
Vo
Helen Caines
MSU - 2001
Finding V0s
proton
pion
Primary vertex
Helen Caines
MSU - 2001
High Pt K+ & K- Identification Via “Kinks”
+/-
K+/-
Helen Caines
MSU - 2001
Particle ID Techniques Combinatorics
Ks + + - K+ + K-
p + - p + +
Combinatorics
from K+ K- pairs
K+ K- pairs
m inv
m inv
same event dist.mixed event dist.
background subtracted
dn/dm
dn/dm Breit-Wigner fit
Mass & width
consistent w. PDG
K* combine all K+ and -
pairs (x 10-5)
m inv (GeV)
Helen Caines
MSU - 2001
STAR STRANGENESS!
K0s
K+
(Preliminary)
Helen Caines
MSU - 2001
Triggering/Centrality
• “Minimum Bias”ZDC East and West thresholds set to lower edge of single neutron peak.
~30K Events |Zvtx| < 200 cm
• “Central”
CTB threshold set to upper 15%REQUIRE:Coincidence ZDC East and West
REQUIRE:
Min. Bias + CTB over threshold
Helen Caines
MSU - 2001
The Collisions
The End Product
Helen Caines
MSU - 2001
Baryon Stopping/Transport
Anti-baryons - all from pair production
Baryons - pair production + transported
B/B ratio =1 - Transparent collision
B/B ratio ~ 0 - Full stopping, little pair production
Measure p/p, / , K-/K+
(uud/uud) (uds/uds) (us/us)
_
_
_ _
- - - - - - - -
Helen Caines
MSU - 2001
p/p Ratio_
Phys. Rev. Lett March 2001
Still finite baryon number
Ratio is flat as function of pt and y
Slight fall with centrality
Ratio = 0.65 ±0.03(stat) ±0.03(sys)
Helen Caines
MSU - 2001
Strange Baryon Ratios
Ratio = 0.73 ± 0.03 (stat)
~0.84 /ev, ~ 0.61/ev
Reconstruct: Reconstruct:_
STAR Preliminary
Ratio = 0.82 ± 0.08 (stat)
_ ~0.006 /ev, ~0.005/ev
Helen Caines
MSU - 2001
Preliminary /Ratio
= 0.73 0.03 (stat)_
Ratio is flat as a function of pt and y
Central events
|y|<0.5
Helen Caines
MSU - 2001
and from mixed event Studies
Good cross-check with
standard V0 analysis.
Low pt measurement
where there is no V0 analysis
High efficiency (yields are ~10X V0 analysis yields)
Background determined by mixed event
STAR preliminary
The ratio is in agreement with “standard” analysis
= 0.77
0.07 (stat)
_
Helen Caines
MSU - 2001
¯______
_
Anti-baryon/Baryon Ratios versus s
STAR preliminary
Baryon-pair production
increases dramatically with
s – still not baryon free
65.0
Trpair
pair
p
pbar
YY
Y
Y
Y
2Tr
pair
Y
Y
2/3 of protons from pair production , yet pt dist. the same
– Another indication of thermalization
Pair production is larger than baryon transport
Helen Caines
MSU - 2001
Particle Freeze-out Conditionsti
me
3. freeze-out
1. formation
Chemical Freeze out: inelastic scattering stops
2. hot / denseKinetic Freeze out: elastic scattering stops
Helen Caines
MSU - 2001
K+/K- Ratio - Nch
dE/dx Kinks
•K+/K-= 1.08±0.01(stat.)± 0.06(sys.) (dE/dx). (The kink method is systematically higher.)
STAR preliminary
STAR preliminary
K+/K- constant over measured centrality
Helen Caines
MSU - 2001
K-/- Ratios
K-/ ratio is enhanced by almost a factor of 2 in central collisions when compared to peripheral collisions
STAR preliminary
SPS
Helen Caines
MSU - 2001
K0* and K0* Identification
Short lifetime (c =4fm) – sensitive to the evolution of the system?
_
First measurement in heavy ion collisions
Helen Caines
MSU - 2001
K0*/h-
Strangeness Enhancement?
Represents a 50% increase
compared to K0*/ measured
in pp at the ISR.
Also look at K*/K
From spin counting
K*/K = vector meson/meson
= V/(V+P)
=0.75
e+e-(LEP)K*/K = 0.32 ±0.02
pp (ISR)K*/K = 0.6 ± .09 ± .03Au-Au (STAR)= 0.42
Helen Caines
MSU - 2001
Comparing to SPS
K+/K-(kink) = 1.2 ±
K+/K-(dE/dx) = 1.08 ±0.01 (stat.)± 0.06 (sys.)
K-/= 0.15 ± 0.02 (stat.)
K*/h- = 0.06 ± 0.006
(stat.)± 0.01 (sys.)
K*/h- = 0.058 ± 0.006
(stat.)± 0.01 (sys.)
p/p = 0.6 0.02 (stat.) 0.06 (sys.)
¯/ = 0.73 ± 0.03 (stat.)
± 0.08 (stat.)
¯
¯
¯
Helen Caines
MSU - 2001
Simple Model
Assume fireball passes through a deconfined state can estimate particle ratios by simple quark-counting models
*Duds
sdu*
s
s
u
u
uss
ssu
p
p*D
uud
duu
p
p*
s
s
u
u
uds
sdu D=1.12
D=1.12
No free quarks so all quarks have to end up confined within a hadron
Predict
Predict
D=1.08± 0.08
su
su
K
K
s
s
u
uD
Measure
System consistent with having a de-confined phase
Helen Caines
MSU - 2001
s1
qs
2qs
2sq
2s
2s
4q
s2q
1s
2q
T/
uds
dss
uds
sdu
e
Particle Ratios and Chemical Content
j= Quark Chemical Potential
T = Temperature
Ej – Energy required to add quark
j– Saturation factor Use ratios of particles to determine Tch and
saturation factor
ij
i ejNT
j
Helen Caines
MSU - 2001
Chemical Fit Results
Not a 4-yields fit!
s 1
2 1.4
Thermal fit to preliminary data:
Tch (RHIC) = 0.19 GeV
Tch (SPS) = 0.17 GeV
q (RHIC) = 0.015 GeV
<< q (SPS) = 0.12-0.14 GeV
s (RHIC) < 0.004 GeV
s (SPS)
Helen Caines
MSU - 2001
P. Braun-Munzinger, nucl-ex/0007021
Chemical Freeze-out
Baryonic Potential B [MeV]
Chem
ical Tem
pera
ture
Tch
[M
eV
]
0
200
250
150
100
50
0 200 400 600 800 1000 1200
AGS
SIS
LEP
/ SppS
SPS
RHIC quark-gluon plasma
hadron gas
neutron stars
early universe
thermal freeze-out
deconfinementchiral restauration
Lattice QCD
atomic nuclei
Helen Caines
MSU - 2001
Kinetic Freeze-out and Radial Flow
If there is transverse flow
Look at mt = (pt2 + m2 )
distributionA thermal distribution gives a linear distribution
dN/dmt e-(mt/T)
mt
1/m
t d2N
/dyd
mt
Slope = 1/T
Slope = 1/Tmeas
~ 1/(Tfo+ 0.5mo<vt>2)
Want to look at how energy distributed in system.
Look in transverse direction so not confused by longitudinal expansion
Helen Caines
MSU - 2001
T = 190 MeV
T = 300 MeV
Tp = 565 MeV
mid-rapidity
mt slopes vs. Centrality
• Increase with collision centrality
consistent with radial flow.
Helen Caines
MSU - 2001
Radial Flow: mt - slopes versus mass
Naïve: T = Tfreeze-out + m r 2 where r = averaged flow velocity
Increased radial flow at RHICßr (RHIC) ßr (SPS/AGS) = 0.6c = 0.4 - 0.5cTfo (RHIC) Tfo (SPS/AGS) = 0.1-0.12 GeV = 0.12-0.14 GeV
Helen Caines
MSU - 2001
No evidence of mass modification
Identification
STAR Preliminary
Helen Caines
MSU - 2001
Inverse slope for Hyperons
T=352±6(stat) MeV
15% Most Central
As / ratio is flat as a function of pt can infer that the slope is the same – backed up by fitting to corrected spectrum
Some evidence that a single exponential fit is not the best fit to the data
e(-mt/
T)
Same slope as
Helen Caines
MSU - 2001
Radial Flow and Strange Particles
Do not follow “radial flow systematics” early kinetic freeze out?
STAR Preliminary
Neither the or the proton are corrected for feed-down.
Correction would drive the p slope up.
What about p absorption/annihilation?
Lower momentum more collisions more absorption/annihilation.
_
Helen Caines
MSU - 2001
C (Q
inv)
Qinv (GeV/c)
1
2
0.05 0.10
Width ~ 1/R
1D: overallrough “size”
K
RoutRside
Measuring the Source “Size” (HBT)
222111 xyipxyip ee~
~5 fm
x1
x2
y1
y2 ~1 m 122211 xyipxyip ee
)xpcos(1~)p,p(P *21
3D decomposition of relative momentum provides handle on shape and time as well as size
Helen Caines
MSU - 2001
K0s-K0
s Correlations
= 0.7 ±0.5
R = 6.5 ± 2.3
•No coulomb repulsion
•No 2 track resolution
•Few distortions from resonances
•K0s is not a strangeness eigenstate -
unique interference term that provides additional space-time information
K0s Correlation will
become statistically meaningful once we have ~10M events
Helen Caines
MSU - 2001
What have we “learnt” so far
• Mapping out “Soft Physics” Regime
Net-baryon 0 at mid-rapidity! ( y = y0-ybeam ~ 5 )
Chemical parameters
Chemical freeze-out appears to occur at same ~T as SPS
Strangeness saturation similar to SPS Kinetic parameters
Higher radial flow than at SPS
Thermal freeze out same as at SPS
Strange Particles The and do not seem to flow with the other particles.
Reduced rescattering for the kaons from decay and/or
feel less flow
More than we ever hoped for after the first run !!!
Helen Caines
MSU - 2001
This Year – RICH,TOF Patch,SVT,FTPC
RICH and TOF:
Increase K identification in pt over a limited geometric acceptance
Centered at mid-rapidity they provide complimentary pt coverage
TOF patch 0.3< pt <1.5 GeV/c
RICH 1.1 < pt < 3.0 GeV/c
Overlaps with the TPC kink and dE/dx measurement
kink pt < 5 GeV, dE/dx pt < 0.8 GeV
SVT: Increased efficiency for all strange particles and resonances due to improved tracking
Should measure spectra for all particles this year.
HBT with strange particles
Exotica
FTPC: Strange particles at high y
Helen Caines
MSU - 2001
The Silicon Vertex Tracker
Radii – 6,10 15 cm
Length ±12.4 cm
± 18.6 cm
± 21.7cm
(-1 < < 1)
Helen Caines
MSU - 2001
SVT
STAR detector gets new silicon heart – CERN Courier
SVT installed and operational April 2001!!
91% live (out of 103,680 channels)
Helen Caines
MSU - 2001
SVT being Assembled
Radiation Length 1.5%/layer (including Electronics+Cooling)
216 wafers on 36 Ladders
0.7m2 Silicon
Half assembled.
Fully assembled
Helen Caines
MSU - 2001
Principle of Operation
SDD gives unique position in X-Y
* 6.3 cm x 6.3 cm area
* 280 m thick n-type Si wafer
* 20 m position resolution
Ionizing particle
X-position from drift time
Electron cloud
X
SDD
Y-position from readout anode number
Helen Caines
MSU - 2001
SVT Performance
1ch=2mV
Noise
Hits from Au-Au Event
Cosmic Ray Event–L3 TriggerThreshold at 4mV 6% live
Helen Caines
MSU - 2001
Conclusion
Lots doneLots still to be done
The future looks bright and exciting
Helen Caines
MSU - 2001
Comparison of AGS, SPS, RHIC
AGS SPS RHIC
Energy density 1GeV/fm3 5.3GeV/fm3 17GeV/fm3
Multiplicity 1,000 3,000 10,000
Baryon chemical potential b 520MeV 167MeV 47MeV
Freeze-out temperature (T) 120MeV 130 MeV 160MeV
Helen Caines
MSU - 2001
How a TPC works
420 CM
• Tracking volume is an empty volume of gas surrounded by a field cage
• Drift gas: Ar-CH4 (90%-10%)
• Pad electronics: 140000 amplifier channels with 512 time samples – Provides 70 mega pixel, 3D image
Helen Caines
MSU - 2001
Calibration - Lasers
Using a system of lasers and mirrors illuminate the TPC
Produces a series of
>500 straight lines criss-crossing the TPC volumeDetermines:
• Drift velocity
• Timing offsets
• Alignment
Helen Caines
MSU - 2001
Calibration – Cosmic Rays
Determine momentum resolution
p/p < 2% for most tracks
Helen Caines
MSU - 2001
K+/K- vs pt
Helen Caines
MSU - 2001
K- Inverse Slope Results
KinkdE/dx
h- mid rapidity dN/d
Incre
asin
g
cen
trality
Helen Caines
MSU - 2001
Helen Caines
MSU - 2001
Large h- multiplicityNearly Boost invariant
Helen Caines
MSU - 2001
The central rapidity region
ExcitedVacuum
hadrons
hadrons
• Almost net-baryon free dNnet-B/dy ~ 30 • Large particle multiplicity dN/d ~ 800
C. Bernard et al, PRD 55, 6861 (1997)
Helen Caines
MSU - 2001
Helen Caines
MSU - 2001
In case you thought it was easy…
BeforeAfter