UCT Seminar I: PHOBOS Experiment RHIC

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UCT Seminar IPHOBOS Experiment _at_ RHIC

• Peter Steinberg
• Brookhaven National Laboratory
• Fulbright Scholar Program

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PHOBOS Apparatus
Ring Multiplicity Detectors (Silicon)
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Collaboration
ARGONNE NATIONAL LABORATORY Birger Back, Alan
Wuosmaa BROOKHAVEN NATIONAL LABORATORY Mark
Baker, Donald Barton, Alan Carroll, Nigel George,
Stephen Gushue, George Heintzelman, Burt
Holzman, Robert Pak, Louis Remsberg, Peter
Steinberg, Andrei Sukhanov INSTITUTE OF NUCLEAR
PHYSICS, KRAKOW Andrzej Budzanowski, Roman
Holynski, Jerzy Michalowski, Andrzej Olszewski,
Pawel Sawicki, Marek Stodulski, Adam Trzupek,
Barbara Wosiek, Krzysztof Wozniak MASSACHUSETTS
INSTITUTE OF TECHNOLOGY Maartin Ballintijn, Wit
Busza (Spokesperson), Patrick Decowski, Kristjan
Gulbrandsen, Conor Henderson, Jay Kane, Judith
Katzy, Piotr Kulinich, Jang Woo Lee, Heinz
Pernegger, Corey Reed, Christof Roland, Gunther
Roland, Leslie Rosenberg, Pradeep Sarin,
Stephen Steadman, George Stephans, Carla Vale,
Gerrit van Nieuwenhuizen, Gábor Veres, Robin
Verdier, Bernard Wadsworth, Bolek
Wyslouch NATIONAL CENTRAL UNIVERSITY,
TAIWAN Chia Ming Kuo, Willis Lin, Jaw-Luen
Tang UNIVERSITY OF ILLINOIS AT CHICAGO Russell
Betts, Edmundo Garcia, Clive Halliwell, David
Hofman, Richard Hollis, Aneta Iordanova, Wojtek
Kucewicz, Don McLeod, Rachid Nouicer, Michael
Reuter, Joe Sagerer UNIVERSITY OF
MARYLAND Abigail Bickley, Richard Bindel, Alice
Mignerey, Marguerite Belt Tonjes UNIVERSITY
OF ROCHESTER Joshua Hamblen, Erik Johnson, Nazim
Khan, Steven Manly, Inkyu Park, Wojtek
Skulski, Ray Teng, Frank Wolfs
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Outline

• PHOBOS show-and-tell
• Hardware
• Event Selection
• Multiplicity Measurements
• PHOBOS Spectrometer
• Particle Identification
• Elliptic Flow (v2)
• HBT Interferometry
• Future ideas(?!)
• More generic physics discussion tomorrow!

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Silicon Detectors
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Silicon Pad Technology
Double Metal, single-sided, AC coupled,
polysilicon biased detectors produced by ERSO in
Taiwan
AC coupled pad (p-implant metal 1
pad) polysilicon bias resistor metal 2 readout
line contact hole metal 1- metal 2
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Silicon Pad Sensors
Typical pad layout
IDE VA-HDR1 Readout Chips

• High dynamic range ( gt 100 MIPs), peaking time
  1.1 ms
• Si latch-up system interlocks during single-event
  upset to protect against chip damage

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Silicon Everywhere!
137,000 channels in total
Octagon/Vertex
Spectrometer Arm
Ring
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Si Signal Simulation
Vertex signal response
angle corrected _at_ 300 mm of Si
dE dx
normalized hit energy x0
keV

• Full understanding of detector signal at the
  most basic level

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Sensor Uniformity
Octagon
counts
/- 3
Smp 93 keV
signal (keV)
/- 1
counts
Rings
Smp 85 keV
signal (keV)
No substantial signal variation due to different
layout (double metal line routing and/or varying
pad size)
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Signal/Noise Ratios

• S/N ratios better than 101 design specification
• Larger pads longer readouts lower S/N
  ratio
• Ave. noise in entire detector setup stable over
  time

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Event Selection
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Event Selection

• Trigger hardware
• Timing and topological selections (e.g.
  coincidences) while taking data
• Trigger software
• Detailed cuts on timing, topology,
  multiplicities, etc.
• Offline reconstruction
• Event vertex is this a useful event?
• Centrality is it a violent enough collision?

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Event Selection
Paddle Counters
Coincidence (38 ns) between paddle counters
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Centrality with Paddles
e.g., top 6 most central collisions
Entries
3lthlt4.5
Events/Bin
h
Energy deposited in paddle counters
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Centrality with ZDCs
ZDC sum vs. Paddle sum Independent methods to
determine centrality that correlate well
ZDC
ZDC Sum (au)

• See J.M. Katzy in Parallel Session II for details

central
peripheral
Paddle Sum (au)
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Vertex Determination
counts
For this event vertex _at_ Z -0.054 cm
cm

• Vertex Resolution
• sx 450 mm
• sy sz 200 mm

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Charged Multiplicity
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Why Count Everything?

• Event Selection / Quality
• Broad view of the whole event
• Centrality selection
• Reaction plane (for flow)
• Physics
• Integral over all production processes
• Initial state, parton dynamics, hadron dynamics
• Fluctuations more detail

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Multiplicity Vertex Array
Vertex
Ring
Octagon
Multiplicity
Vertex

• Single layer of Si with large pads

• Two layers of Si with strip pads

Count single hits or sum of analog signals in a
detector area as a measure of particle
multiplicity
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Multiplicity Array Unrolled
f
Vertex
Spec
Spec
3
-3
5.4
-5.4
0
h
Rings
Rings
Octagon
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Background Suppression
D E vs. h in the Octagon

• Good agreement between data and simulation
• Powerful method to reject background

not from vertex
Si
from vertex
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PHOBOS Data on dN/dh
130 GeV
200 GeV
19.6 GeV
PHOBOS Preliminary
dN/dh
Most Central
Npart
h
h
h

• AuAu collisions at ?s19.6, 130, 200 GeV
• dN/dh for hlt5.4 over full azimuth
• Centrality from paddles (130/200) Nhits (19.6)
• Top 50 of total cross section (Npart65-360)

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Particle density near midrapidity
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PHOBOS Spectrometer
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The PHOBOS Spectrometer
10cm

• Outer layers situated in 2T magnetic field
• High segmentation in bending direction
• Tracking within 10 cm of interaction point
• Coverage near mid-rapidity
• Phi acceptance of 3 per Arm

70 cm
z
x
y
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PHOBOS Magnet

• B Field Map

By (T)
Reproducibility of absolute field strength better
than 1
cm
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Particle Tracking In Spectrometer

• Road-following algorithm finds straight tracks in
  field-free region
• Curved tracks in B-field found by clusters in
  (1/p, ?) space
• Match pieces by ?, consistency in dE/dx and fit
  in yz-plane
• Covariance Matrix Track Fit for momentum
  reconstruction and ghost rejection

z
By
Beam
2
1
x
10 cm
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Spectrometer Performance
Acceptance
Momentum Resolution

• Data Sample Production Run 2001(200 GeV)
• 7.8 M AuAu Events, Min. Bias Trigger
• 32 M reconstructed particles

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PHOBOS-Spectra _at_ 200GeV

• Corrections
• Acceptance/Efficiency
• Ghost Tracks
• Momentum resolution
• Variable bin width
• Secondaries

0.2ltyp lt1.4
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Particle Identification
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pT measurements with
Z
dN/dpT
near mid-rapidity
up to 5 GeV/c
0.03 0.2 1.0
pT, GeV/c
Charge
PID
Mass Charge
Mass
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Particle identification
Based on dE/dx measurements in Si
sensors (resolution ?7)
Positive charges
p
Negative charges
K
p
K
?
?
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Antiparticle to particle ratios measurements
Acceptance of the spectrometer
y pT (GeV/c)
B2T
??
0.35 1.3 0.10 0.6 0.25 0.8 0.10
0.5 0.20 0.7 0.15 0.9
K?
p,p
Momentum resolution 1 2
Reversible 2T magnetic field Two symmetric
spectrometer arms

• Independent measurements
• Acceptance efficiency cancels

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200 GeV Ratios
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Energy dependence of particle ratios
AA central collisions
??PHOBOS 130 GeV PRL 87,102301,2001
K/K
??PHOBOS 200 GeV Submitted to PRC
nucl-ex/0206012
p/p

• Rapidly decreasing net baryon density near
  midrapidity

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Centrality and pT dependence
AuAu ?sNN 200 GeV
10 most central
? p/p K/K ??/?
? p/p K/K ??/?
PHOBOS preliminary
PHOBOS preliminary

• Ratios, within the errors, are independent of pT
  and Npart

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Tracking _at_ low pT
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Particle measurements at very low pT
Search for particles ranging out in the 5th
spectrometer plane
Mass measurements (energy-range method)
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• Eloss?dE
• Mp ltdE/dxgtEloss ?m
• (?1/?2) ( ?m?2)

10
0

• Cuts on dE/dx per plane
• MASS HYPOTHESIS

0 10 20
Z cm
P Ek21 MeV

• Cuts on Eloss (Ekkinetic energy)
• MOMENTUM HYPOTHESIS

K Ek19 MeV
dE/dx
? Ek 8 MeV

• Corrections
• acceptance
• efficiency
• background

A B C D E silicon plane
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Particle mass measurements at very low pT
Results AuAu ?sNN200 GeV 15 central
Test of the method
Reconstruction of low momentum MC particles
(p,p)
(p,p)
Eloss ltdE/dxgt10-3GeV2/cm
Eloss ltdE/dxgt10-3GeV2/cm
(K,K)
(K,K)
(?,??)
(?,??)
MC
DATA
Eloss MeV
Eloss MeV
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Invariant yields of very low pT particles
Analysis details
AuAu ?sNN200 GeV 15 central -0.1lt y lt0.4

• Particle momenta calibrated
• with reconstr.GEANT tracks
• (pT resolution 5)
• Data corrections
• ? acceptance efficiency
• (embedding single tracks)
• ? background corrections
• feeddown, secondaries,
• misID, ghosts
• (pp) 418
• (KK) 1611
• (???) 393
• based on DCA distributions
• rescaled HIJING
• Current systematic errors
• ?20(?) 40(K) 50(p)

(???)
(KK)
1/ (2?pT)d2N/dydpT
(pp)
PHOBOS preliminary
pT GeV/c
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?sNN 200 GeV Comparison to models
HIJING RQMD HYDROTHERMAL W.Broniowski,W.Florkow
ski (PRL87,2001,272302PRC65,2002,064905)
AuAu yields scaled by 1/ltNpart/2gt
(???)
(KK)
(pp)
10
10
10
1
1
1
1/ (2?mT) d2N/dydmT 1/ltNpart/2gt
10-1
10-1
10-1
PHOBOS preliminary
PHOBOS preliminary
PHOBOS preliminary
10-2 10-1 1
10-2 10-1 1
10-2 10-1 1
mT m0
PHENIX 130 GeV PRL88,2002,242301
(pp) corrected for feeddown

• very low pT
• (pp) models differ by a factor 2 to 6

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Elliptic Flow (v2)
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Elliptic flow
dN/d(f -YR ) N0 (1 2v1cos (f-YR) 2v2cos
(2(f-YR) ... )
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Flow basic method

• Subevent technique correlate
  qqqreaction plane in one part of
  qqqdetector to ? asymmetry in qqqtrack pattern in
  other part of qqqdetector
• Correct for imperfect reaction plane resolution

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Flow Hit-based method
nucl-ex/0205021 submitted to PRL
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Flow Hit-based method
nucl-ex/0205021 submitted to PRL
Select vertices offset in Z, symmetric coverage
in ?, ?
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Flow Hit-based method
nucl-ex/0205021 submitted to PRL
Select vertices offset in Z, symmetric coverage
in ?, ?
Reaction plane determined by subevents in -2lt?lt2
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Flow Track-based
New for QM2002!
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Flow Track-based
New for QM2002!
Reaction plane determined by hits in widely
separated subevent regions, symmetric in ?, ?
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Hit-based analysis
Track-based analysis

• pt dependence
• Tracks less background sensitive
• Minimal MC dependence
• Subevents and tracks wwwidely separated in h
• (Species dependence)

• Large h coverage
• Event-by-event
• Uniform acceptance in ?
• Separated subevents

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v2 vs. centrality and energy
hlt1
v2
200
130
PHOBOS Au-Au
Hit-based result v2200 v2130 similar
ltNpartgt
130 GeV result nucl-ex/0205021, submitted to PRL

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v2 vs. ?pT
0lthlt1.5
v2
PHOBOS preliminary h h- 200 GeV Au-Au
track-weighted centrality averaging
(top 55)
17 scale error
v2 appears to saturate for pTgt2 Similar to
published results at 130 GeV (STAR, PHENIX)
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v2 vs. ? and energy
ltNpartgt190
v2
PHOBOS Au-Au
200
130
Hit-based result v2200 v2130 similar
h
130 GeV result nucl-ex/0205021, submitted to PRL
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HBT Interferometry
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HBT
Another way to look at source dynamics ...
? phase space
Identified ?s
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Data sample
PHOBOS preliminary
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HBT results
0.2ltylt1.5
0.15ltkTlt0.35
Phobos preliminary 200 GeV Au-Au
15 most central events
?
Rout
Rside
Rlong
R2out-long
?- ?-
0.54?0.02
5.8?0.2
5.1?0.4
6.8?0.3
4.9?1.7
??
0.57?0.03
4.9?0.4
7.3?0.3
5.8?0.2
4.5?1.9
Systematic error on radii of 1 fm, on ? of 0.06
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Scaling with energy
?-?- data
Compilation of non-PHOBOS data from C. Adler et
al., Phys. Rev. Lett. 87, 082301 (2001)
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Future Plans
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PHOBOS Charm upgrade

• Open charm measurement is essential for
  understanding total charm production
• D e_ X is best suited for small acceptance
  near midrapidity

• Study single electrons at high pT from displaced
  vertices
• Suppress the hadronic background
• Upgrade PHOBOS with
• microvertex detector
• ALICE Transition Radiation Detector prototypes
• Electromagnetic calorimeter?

EMCal
TRD
mvertex
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Conclusions

• PHOBOS is small but versatile
• Competitive with big experiments in many aspects
  of heavy ion physics
• Full acceptance is unique niche for multiplicity
  and flow measurements (fluctuations?!)
• Capabilities have been limited by
• Rate fixed already for FY2003 (400 Hz)
• People lots of ideas, but few hands
• Interesting opportunities for new collaborators