Heavy Ion ExperimentsHeavy Ion Experimentsat at

Heinz Pernegger/CERN,MIT

Vienna Conference on Instrumentation 2001 22/2/2001

STAR

Relativistic Heavy Ion Relativistic Heavy Ion Collider Collider @BNL@BNL

• A dedicated facility for Heavy Ion Physics at BNL

STAR

RHIC SpecificationsRHIC Specifications• Two independent superconducting rings

– 3.83 km rings– Beam crossing=106ns

• Can collide Au-Au– top energy 100+100 AGeV/c– in 60 bunches with 109/bunch– store time = 10 hours– average Luminosity = 2 x 1026 /cm2s

• But also for p+p– top energy 250+250 GeV/c– average Luminosity = 2 x 1032 /cm2s– polarized (for spin measurements)

• And nearly any nucleus on any nucleus– including asymmetric collisions

Rf storage cavities

Blue and yellow rings

Aim of Aim of RHIC’sRHIC’s heavy ion experimentsheavy ion experiments

• Study nuclear matter at extreme energy density– phase transition into a deconfined QGP

• RHIC is dedicated to heavy ion physics– it is a collider to get to top CM energy– with more than 30 weeks of running per year– allows to vary initial conditions (energy, collision system pp,pA,AA)

• Experiments at RHIC– a comprehensive set of detectors to look at many different

signatures

AGSAGS

SPSSPSRHICRHIC

What to look for ?What to look for ?

• Study “bulk” properties• Look at many different “parameters” and signatures

– energy density– flavour dynamics– in-media effects– soft & hard process– particle correlation

• Vary basic conditions– centrality– energy– system size

• The is no real SM of heavy ion physics & no “gold plated” events– predictions vary therefore maintain flexibility – avoid single-signatures experiments

– multiplicity– pT spectra– strangeness

enhancement– mini-jets– J/Ψ suppression– HBT– …

Difference to HEP?Difference to HEP?

Multiplicity!Multiplicity!

– Fine granularity– good track separation– detector with low

ambiguties– particles are low

momentum (multiple scattering)

– maybe not ultimate spatial resolution

• (low momentum -> large sagita)

• But also “low multiplicity” high rate pp collisions

• Need to handle this (at <1kHz)

STAR TPC L3 displaySTAR TPC L3 display

Difference to HEP?Difference to HEP?

Low and highLow and high ppTT matters matters • Most particles have low

momenta (few x 100 MeV/c)

• Flavour dynamics: want to study events for particle composition

• Jet quenching: Look at high momentum part of pTdistribution (2-20GeV/c)

• Need tracking with low pTacceptance

� π (>70MeV/c) ,K (>200MeV/c) , p (>300MeV/c)

• Low pT particle identification is crucial

Bild momentum distribution

• Good azimutal coverage at mid-rapidity– anisotropic particle production

(“elliptic flow”)– particle correlations (space-time

evolution of source)

Difference to HEP?Difference to HEP?

HermeticityHermeticity & & Rare signalsRare signals

• Acceptance up to extreme pseudo rapidities (|η| up to 5) – “exclusive” multiplicity

measurements– particle ratios in extreme

forward direction

• Acceptance to electrons– to be sensitive to heavy flavor

production (D0,B0), γ*->e+e-

• BUT they are rare!

• good electron identification by combing detectors (tracking+RICH+EMCal)

Al l tracks

E lec t ron enr i ched s amp l e

(us ing RICH)

E/p matching for

p>0.5 GeV/c t racks

Phenix

General requirements:General requirements:

DAQs and triggers for high rate + lowmultiplicity pp-collisions

Tracking with high granularity and lowamibuties to handle n x 1000 particles/collision

Layout with acceptance in low pT and wide rapidity range

Sensitivity to rare probes (e,µ,γ)combinedEMCals/Cherenkov

Experiments with dynamic rangeExperiments with dynamic range

TOF, dE/dx, RICH:Low & high pTparticleidenification

Be prepared for the unexpected:maintain flexibility

Detector technologies usedDetector technologies used

Brahms Phenix Phobos Star

Tracking TPC TEC, pad/drift chamber

Silicon pad detector

TPC

Particle ID TOF, RICH TOF (p,K,p) Threshold-RICH (e -)

dE/dx with sil icon, 1 TOF wall

dE/dx with TPC, RICH,

ET, P0 - Shachlik EMCal - Emcal

Multiplicity Scintilator Mult-detector

Pad chamber, Silicon multiplicity TPC (barrel+forward)

Trigger Forward Scintilator+ZDC

Cherenkov beam-beam counter+ZDC

Forward Scinilator+ZDC

Scinilator Barrel+ZDC

RHIC Performance during RUN2000RHIC Performance during RUN2000

• Performance during the first physics run at RHIC (June-September 2000) :– 60 bunches per ring ü– 5×108 Au/bunch ü– Initial storage energy: 2 runs at different energy

• short run at γ = 30 [28 GeV/nucl.] • long run at γ = 70 [66 GeV/nucl.] ü• This energy is below the lowest quench of any DX magnet. • Full operating current for 100 GeV/nucl. reached at end of run)

– Luminosity: 2 × 1025 cm-2 s-1 ü

– Integrated luminosity: a few (µb) -1 ü

Accelerating a gold bunch in RHICAccelerating a gold bunch in RHIC

Injection Transition energy Storage energy

Bun

ch le

ngth

[ns

]

Transition energy crossingTransition energy crossing

Transition energy ∆Ε = 200 MeV

RHIC is first superconducting, slow ramping accelerator to crosstransition energy:

Cross unstable transition energy with radial energy jump (2000):

Beam energy

Slow and fast particles remain in step.ð increased particle interaction (space charge)ð short, unstable bunches

Cross unstable transition energy by rapidly changing transition energy (2001):

Transition energy

Beam energy

Avoids beam loss and longitudinal emittance blow-up

Year 2000 condition Year 2001 condition

Collision rate at experimentsCollision rate at experimentsC

olli

sion

rat

e [H

z]

BRAHMS: Lpeak = 3.3 × 1025 cm-2 s-1

Lave = 1.7 × 1025 cm-2 s-1

[ σ(Au+Au → ≥1n + ≥1n) = 10.7 b (theor.)]

Narrow bunches @ Brahms, Phenix

Wide bunches @ Phobos, Star *

* will be reduced during next run

RUN2000 integrated AuRUN2000 integrated Au--Au luminosityAu luminosity

Lave = 0.8 × 1025 cm-2 s-1

Availability: 47 %(last 6 days @ BRAHMS)

6.5 6.5 µµbb--11

STARSTAR

• Emphasis:– Track ~ 1000 charged particles in |η| < 1

ZCal

Silicon Vertex Tracker

Central Trigger Barrel or TOF

FTPCs

Time Projection Chamber

Barrel EM Calorimeter

Vertex Position Detectors

EndcapCalorimeter

Magnet

Coils

TPCEndcap & MWPC

ZCal

RICH

Events at StarEvents at Star

Data Taken June 25, 2000.

Pictures from Level 3 online display.

Central AuCentral Au--Au collision @ STARAu collision @ STAR

STAR detectorSTAR detector

• 0.5 Tesla magnet– 0.25 for year 1

• Trigger – CTB– ZDC– Level 3

• Year 1 detectors– TPC– RICH– 1 SVT ladder

Star TPCStar TPC

• Gas : P10 (Ar-CH4 90%-10%) @ 1 atm• Drift voltage : -31 kV

60 cm

127 cm

190 cm

Inner sector2.85 × 11.5 mm2 pad1750 pads

Outer sector6.2 × 19.5 mm2 pad3940 pads

TPC first preliminary resultsTPC first preliminary results

Track length (cm)

σ dE

/dx

/(

dE

/dx

) (%)

• Drift velocity– laser (coarse)– track matching between halfs

(fine)

• Tracking– Position resolution

• 500 µm

– 2-Track resolution• 2.5 cm

– Momentum resolution• 2%

• dE/dx resolution– gain monitored by pulser + offline– Good particle separation using dE/dx

• 7.5%

STAR RICH detectorSTAR RICH detector

• Extends STAR’s PID capabilities into high pTrange

• can study flavourdependence of hard processes– low rate, inclusive

measurement– can do with “1-arm”

• Developed by CERN RD-26 in ALICE framework headed

• ALICE RICH Prototype Module (1 m2)

• Radiator = C6F14 Liquid• Photo Converter

– CsI (λ < 210 nm)

• Ionization Detector– MWPC pad chamber & CH4 Gas

STAR RICH acceptanceSTAR RICH acceptance

• Extend PID beyond TPC TOF: 1 < p < 3 GeV/c π K2 < p < 5 GeV/c p

• 160 x 85 cm2 ⇒ 1 m2

• Radial Distance of 2.4 m• |y| < 0.2

Star PID withStar PID with dEdE//dxdx and RICHand RICH

Star PID through track topologyStar PID through track topology

25

STAR SVT (silicon drift vertex detector)STAR SVT (silicon drift vertex detector)

Silicon Drift DetectorsSilicon Drift Detectors

• design resolution <20µm• 1st year commissioning run

PHOBOSPHOBOS– Emphasis

è very large |η|<5.4 for multiplicity & flow measurements

è very low pT acceptance (π:>50MeV/c)

– Multiplicity array + 2-arm spectrometer

• with full PID, momentum measurement

– Minimize the number of technologies:

• All Si-strip tracking• Si multiplicity detection• PMT-based TOF

– Unbiased global look at very large number of collisions (~109)

• through fast DAQ (n x 100Hz)• small detector

Octagon/Vertex

Spectrometer Arm Ring

Silicon everywhereSilicon everywhere• Multiplicity array

– 1 layer barrel + 2x 3 rings

• Spectrometer– 16 layer silicon pad

detectors

1 of 10 layouts:

14 cm• Thin detectors

– low multiple scattering– less background

• Compact detector close to IP

• Pad detectors: same technology for– multiplicity measurement by signal

integration in larger pads– PR+tracking+dE/dx PID with

smaller pads

PHOBOS: Why silicon pads everywhere?PHOBOS: Why silicon pads everywhere?

PHOBOS: Full coverage multiplicity measurementPHOBOS: Full coverage multiplicity measurement

ηη

Octagon 3 Rings3 Rings

Run 5374Event 79495

dN/d

η

• Charged multiplicity for forward+mid rapidity (on

event-by-event basis)• full phi coverage

– anisotropy of particle production

• can deal with occupancies >80%

0 +3-3 +5.45.4

φφ

ηη

Rings RingsOctagon

ππ

p

K

protons

Kaons

pions

PHOBOS SpectrometerPHOBOS Spectrometer• Tracking and vertex determination

– momentum resolution 2%– vertex resolution 300-400µm

• PID with dE/dx in silicon– dE/dx resolution = 7.5% – identical to STAR TPC dE/dx resolution– high dynamic range for stopping particle

Multiplicity’s trivial dependence:Multiplicity’s trivial dependence:

Centrality measurement at RHICCentrality measurement at RHIC

“Spectators”

Zero-degreeCalorimeter

“Spectators”

Many things scale with Npart:• Transverse Energy• Particle Multiplicity• Particle Spectra

“Participants”

Only ZDCs measure Npart

specpart NAN −=

Detectors at 90o

The collision geometry (i.e. the impact parameter) determines the number of nucleons that participate in the collision

RHIC’s RHIC’s ZDCZDC

• Based on Tungsten/fiber sampling cal• each experiment uses 3 segments

forward/backward• The ZDC provides

– measurement of spectator neutrons (protons are bend away), i.e. Event selection

– timing information, i.e. Trigger– Luminosity monitor for RHIC (σtot =

10.7b)

Provides normalization between experiments, i.e. makes their results comparable

Event Selection & Event Selection & NNparticipantparticipant (e.g. PHOBOS)(e.g. PHOBOS)

• Combine– ZDC with– forward scintilator array

(“paddle counters”)

Paddle signal

ZDC

sign

al

Paddle signal (a.u.)

Npart

• Define centrality classes (fraction of cross section)

MC

Data

• Use model calculation to extract Npart (Hijing + Geant)

Centrality selection + estimate for Npart

35

PHENIX LayoutPHENIX Layout

– 2 central spectrometers

– 2 forward spectrometers

– 3 global detectors

West

EastSouth

North

W.A. Zajc 36

PHENIX during installationPHENIX during installation

January, 1999

• Event Characterization– Si strips and pads (MVD)– Cerenkov (Beam-Beam)

• Tracking– Central Arms

• Drift Chambers• Pad Chambers• Time Expansion Chamber (TEC)

– Muon Arms• Cathode Strip Chambers (muTr)• Iarocci Tubes (muID)

• Particle Identification– Time-of-Flight scintillators– dE/dx (TEC)– threshold RICH – TOF in EmCal

• Calorimetry– Lead-scintillator (PbSc)– Pb-glass (PbGl)

PHENIX year 1 configurationPHENIX year 1 configuration

• Tracking– pad chamber– drift chamber

• PID– TOF– EMCal + RICH (e-)

• Global observables + event selection– Cherenkov BBC– ZDC– silicon MVD (engineering run)

PHENIX PHENIX EMCalEMCal

PHENIX PHENIX EMCal EMCal performanceperformance

• Rel iable measurement of total transverse energy E T = (1.17±0.05) E E M C a l

• good energy resolut ion

� π0 identification

• + TOF information (resolution 200ps)

PHENIX electron ID: all subPHENIX electron ID: all sub--systems in concertsystems in concert

High pT electrons in PHENIX:

PHENIXPHENIX HadronHadron Identification TOFIdentification TOFCombined

– Tracking– Beam-Beam Counter– Time-of-Flight array

provides excellent hadron identification over broad momentum band:

BRAHMSBRAHMS

• Two magnetic dipole spectrometers– forward & mid rapidity– rotating segments with

magnet+TPC+RICH

• Cover large(st) pT-y by scanning • Event & Vertex selection

– multiplicity tile, silicon pad

• Goal:– identified spectra over a broad range of rapidity and pT

BRAHMS AcceptanceBRAHMS Acceptance

• Tracking based– TPC (vertical drift) in small

azimuthal slice• PID based

– TOF hodoscopes– Cherenkov

BRAHMS TOF PID separation in year 1BRAHMS TOF PID separation in year 1

Example: Particle Identification achieved in Mid-Rapidity Spectrometer

p/K to 2.2 GeV/cK/ππ to 1.5 GeV/c

π

K p

time of flight resolution 120 ps

Vertex Determination in BRAHMS Vertex Determination in BRAHMS (as an example ...)(as an example ...)

• Large RMS of interaction diamond– next year σ=15-20 cms

• Select “useable” vertex range with beam-beam counters– fast cherenkov array on +z & - z– vertex by time difference σ(BB)~2.6cm

2 m

• ... And use tracking to get precision (BRAHMS TPC)

• Disadvantage– many recorded event are

rejected in analysis– with small acceptance ->

large corrections

• But also opportunity for small experiments– acceptance can be “artificially”

increased by using different vertex samples

1.25 atm of C4 F10 and C5 F12 mixture

pe detection

Measured refraction index: n = 1.00203

cm

cm

Average # of p.e : 20

Preview for next year: BRAHMS RICHPreview for next year: BRAHMS RICH

RICH radius vs FS momentum

Summary ISummary I

• With RHIC the heavy-ion physics community has entered a new era of better understanding nuclear matter and its phenomena

• RHIC accelerator is the first dedicated heavy-ion collider and provides unparalleled capabilities.

• The RHIC community has put together a very comprehensive set of experiments and met the challenges of

– segmentation– dynamic range– diversity & flexibility– data analysis

How to summarize the detector performance ?How to summarize the detector performance ?

• Phobos, Phenix & Star already published papers on charged particle multiplicity, anti-p/p ratio, elliptic flow

• … and presented a real firework display of first preliminary resultsat Quark Matter 2001:

… by physics results in just 5 … by physics results in just 5 months after data taking !months after data taking !

• STAR

- h- multiplicity- identified pTdistribution - particle ratios- elliptic flow (vs pT)- particle correlation- pT fluctuations

• PHOBOS

- elliptic flow- particle ratios- dNch/dη vs centrality- dNch/dη @η=0- full 4-π dNch/dη

• PHENIX

- dNch/dη @η=0 & Et- elliptic flow (vs pT)- particle correlations - identified pT spectra for charged particles- π0 pT spectra- first electron spectra

• BRAHMS

- Particle ratios at η ~0 and 3- particle ratios vs pT and centrality- dNch/dη @η=0

Thanks toThanks to

W. Busza, J. Harris, F. Videbaek, W. Zajc,

T. Roser, S. Ozaki,

P. Steinberg, R.Pak, S. White, A.Drees,

G. Roland, G. van Nieuwenhuizen,

F. Retiere, E. Schyns, B. Lasiuk, B. Nielsen