Helen CainesYaleUniversity

March 2003

Strangeness Experimental Techniques

An expert is a man who has made all the mistakes which can be made, in a narrow field.-Niels Bohr

Helen Caines

SQM - March 2003

The Goal

To design the Ultimate Strangeness Experiment

What we need:

To be able to measure charged and neutral decaysLots strangeness created.Measurements at low and high pt.Measurements at mid and high rapidity.

Only 5 charged particles are sufficiently stable to reach most detector:

Pions, Kaons, Protons, Electrons and Muons

(+1) Photon: the only neutral particle which can be efficiently detected.

Helen Caines

SQM - March 2003

V0s Reconstruction?

Strange Hadrons K0

S →+- (494 MeV/c2, 2.7cm, 68.6%), →p- (1.12 GeV/c2, 7.9cm, 63.9%), Ξ-→- (1.32 GeV/c2, 4.9cm, 99.9%), -→K- (1.67 GeV/c2, 2.5cm, 67.8%),

Strangely enough moststrange particles are neutralor decay into something neutral

Helen Caines

SQM - March 2003

Finding V0s

proton

pion

Primary vertex

Helen Caines

SQM - March 2003

• Extracting the particle yields– Consider each of the possible final states in turn

– Calculate the parent mass as a function of (y,pT)

– Count the number in the mass peak and correct for reconstruction losses

Invariant mass distributions

M m12 m2

2 2 E1E2 p 1

p 2

Helen Caines

SQM - March 2003

Unique identification of two-body decaying particles by studying the division, between the daughters, of the parent's mtm vector. - the fractional difference in momentum of the daughters pt - the mtm component of the +ve daughter transverse to the line of flight of the parent

All possible values are constrained to lie along a curved locus specific to the mass of the parent characterizes the decay asymmetry, <> - daughter mass difference

The Podolanski-Armenteros plot

pT p p

p||

p||

p|| p||

Helen Caines

SQM - March 2003

BeforeAfter

In case you thought it was easy…

Helen Caines

SQM - March 2003

Acceptance and Efficiency

Helen Caines

SQM - March 2003

Fine way to calibrate a detector!

• Large peaks at 2 o’clock and 8 o’clock

• TPC pad row “floating”

Helen Caines

SQM - March 2003

Event mixing method

• The measurement of hadronic resonances.

• These particles are short-lived compared to the reaction time.

• Resonances are reconstructed using a combinatorial technique. Consider all track combinations and calculate the invariant mass.

• The background is calculated using positive tracks from one event mixed with the negative tracks of another event.

Background subtracted K+K- invariant mass distribution

K+K- invariant mass distribution (11% central events)

Invariant mass [GeV/c2]

dN/d

M [M

eV-1]

dN/d

M [M

eV-1]

Same event distributionMixed event distribution

K+K-

STAR Preliminary

Helen Caines

SQM - March 2003

Kink reconstruction

Approx. 10% of a real event showing a reconstructed kaon decay K± ±(64%) or K± ±(21%). Lifetime c = 3.7 m

De

ca

y a

ng

le (

de

gre

es

)

Parent momentum (GeV/c)

Kaon limit

Pion limit

• In this method, one of the decay daughters is not observed.

• Main background is from -decay, which has a smaller Q-value

• A cut on the decay angle (momentum dependent) removes the contamination

• Remaining background from multiple scattering and split tracks.

• Find ~ few kink decays per event.

Helen Caines

SQM - March 2003

• So now we know how to reconstruct them.

•First question what kind of accelerator do we want?

Helen Caines

SQM - March 2003

Collider vs Fixed Target

Collider:

Higher energyLab frame == CM frameLess focussed particles

Fixed Target:

Higher ratesKnown z vertexBoost gives longer c

Go with the Collider

Helen Caines

SQM - March 2003

Beam @ RHIC Complex

Helen Caines

SQM - March 2003

Beam at SPS Complex

West AreaWA

North Area NA

SPS

PS LINAC3 ECR

The experiments

Electron Cyclotron ResonanceLead Plasma is bombarded with an e beam

Pb+27 2.5A KeV

PSBooster

Heavy Ion LINAC1m carbon foilPb+53 4.2A MeV

PS BoosterPb+53 95.4A MeV

Proton Synchroton4.25A GeV1mm Al foil

Pb+82Super Proton Synchroton

Pb+82 160A GeV

Experimental AreasFix target experiments

natPb ~ 450 mg/cm2

107 PbPb collisions per burst

Fixed Target ExperimentsForward emission at mid-rapidity

Helen Caines

SQM - March 2003

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

SQM - March 2003

What’s the Main Tracking Device?

• Large highly segmented tracking volume at low cost– Permits over sampling, a big plus

• Simplifies tracking code

• Improves position/momentum resolution

• Improves dE/dx resolution

• Design simplicity, an empty volume of gas• Low mass – reduced multiple coulomb scattering

Advantages of a TPC (why there are 7 at RHIC)

Helen Caines

SQM - March 2003

Disadvantages

• Slow readout – can’t be used in level 0 trigger

• Two track resolution limited with classic design – (although improvements possible with radial magnification)

Helen Caines

SQM - March 2003

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

SQM - March 2003

Helen Caines

SQM - March 2003

Silicon Tracker?

Lots of material – Not so good – Lots of scattering

Helen Caines

SQM - March 2003

Charge Determination

Magnetic field Tracking detectors Trajectory

STAR, ALICE,CMS, CERES, NA49, NA57

PHENIX,NA50 & NA60,PHOBOS, BRAHMS,ALICE

In or Out of Magnetic Field

Helen Caines

SQM - March 2003

In a non-uniform magnetic field the drift velocity is not strictly perpendicular to the pad-row

Find the drift velocity by solving the Langevin equation

A(t) is a stochastic damping term, resulting from collisions in the gas is the mean time between collisions

v

1 2 2

E

E

B

B

E

B

B B

B

2 2

0 ,

vvtA

tmABvEqvm

Ev

m

qB ,

Charge transport correction (ExB)

The Solution:

where:

Horrible mess!!!!!!! Place in nice uniform Field

Helen Caines

SQM - March 2003

Now have Main detector

Want low momentum tracks , near primary vertexNeed fine pixelation

Helen Caines

SQM - March 2003

Fine Vertex Determination

6 Layers, three technologies (keep occupancy ~constant ~2%)

– Silicon Pixels (0.2 m2, 9.8 Mchannels)

– Silicon Drift (1.3 m2, 133 kchannels)

– Double-sided Strip Strip (4.9 m2, 2.6 Mchannels)

Rout=43.6 cm

Lout=97.6 cm

SPD

SSD

SDD

ALICE ITS

Helen Caines

SQM - March 2003

Position Sensitive Silicon Detector

Strip

Pad/Pixel

Drift

1eh/3.6eV, 300m 25000 e

Helen Caines

SQM - March 2003

Cluster reconstruction– Each pad-row crossing results in charge deposited in

several pad-time bins.– These are joined to form clusters which have certain

characteristics – The position is calculated as the weighted mean of

the cluster charge in the pad-time directions– The coordinates are determined by:

• The mean pad position (x)• The mean time bin x drift velocity (y)• The pad row position (z)

– Total charge can be used for particle identification (see later)

Charge cluster reconstruction

Helen Caines

SQM - March 2003

• Step-by-step– Find charge clusters in all TPCs– Apply charge transport correction – Track following in the main detector

• Start where the track density is lowest• First find high momentum • Form initial 3 point tracks seeds• Use (local) slope to extrapolate the track

– Tracks are propagated to inner detectors• No momentum measurement outside magnetic field• Assume all tracks are primary• Momentum assignment based on trajectory• Use trajectory to define a “road” in detector

– Search for non-primary vertex tracks • Do track following as a separate step• Momentum determined from curvature of tracks

Track reconstructionRow: n-5 n-4 n-3 n-2 n-1 n

seed

Track following method

Row: n-5 n-4 n-3 n-2 n-1 n

Track road method

Helen Caines

SQM - March 2003

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

SQM - March 2003

Momentum measurement

B~ constant

X+

Bvce

dtpd

GeV/c mT3.0 Bp

Measurement :

Uncertainty:

RP1

P2

P3s

L8

2

ss

LL

2

ss

LL

pp 2

BB

L s

Constant

Proportional to p23.082

BB

LBsp

LL

pp

pbpp a

L=3m, s=10cm=11 m, B=0.5T

p=1.7 GeV/c

Helen Caines

SQM - March 2003

Calibration – Cosmic Rays

Determine momentum resolution

p/p < 2% for most tracks

Helen Caines

SQM - March 2003

Tracking Efficiency

• Reconstruction losses can be divided into two types:– Geometrical Acceptance

• Consequence of limited detector coverage• Straightforward correction, calculated by Monte Carlo

– Reconstruction Efficiency• Particles in acceptance but not reconstructed• Possible reasons for loss:

– Hardware losses– Detector resolution– Merged/split tracks– Reconstruction algorithm

– Efficiency correction• Needs a detailed understanding of the detector response

– Embedding Method• Tune Monte-Carlo simulation to reproduce the data

– Cluster characteristics– Number of space-points on tracks

• Embed a few simulated tracks into real events

Helen Caines

SQM - March 2003

The Bethe-Bloch equation• Energy loss

– Bethe showed that energy loss is strictly a function of v/c

and the properties of the medium

– Including relativistic effects, the Bethe-Bloch equation is

where,

dE

dx4N0re

2mec2 Z

A

1

2 z2 ln2mec2

I22

2

2

v c , (1 2 ) 1 2

N0 Avagadro Number

re e2 me , the classical electron radius

, Z, A density, atomic and mass no.

I ionisation potential

parameterises saturation

Helen Caines

SQM - March 2003

Corrections• Experimental factors that affect the measured charge

– Temperature• Controlled to better than ± 0.1o C

– Pressure• TPCs are operated at “atmospheric” pressure• Ionisation varies by 0.6% mbar-1

– monitored and normalised to 970 mbar

– Correction for O2 and H2O

• Both highly electronegative• Results in linear charge loss with drift distance (few % in TPCs)

– Effective path length (dx) - depends on the track crossing angle• Two angles: one in pad direction, the other in drift direction

– Equalise the electronic response• Electronics and gas gain correction

– In practice an absolute gain calibration is difficult to obtain• Inter-sector calibration (relative gain correction)

Helen Caines

SQM - March 2003

Comment on dE/dx measurements

• Practical considerations– All energy loss distributions have inherently large spread

• Primary ionisation

Number of primary electrons = E /W

W = energy to produce e--ion pair

Follows Poisson statistics• Secondary ionisation

Energy distribution of primary electrons ~ E-2

If E > W they can produce further ionisation– Convolution produces Landau distribution– TPC samples dE/dx from this distribution– Use truncation to better estimate the mean

• Reduces sensitivity to fluctuations • Typically drop ~20% highest dE/dx samples• Truncation ratio must be optimised experimentally

– What happens in higher density media ? Fluctuations are reduced … but ... Height of rise decreases (probability for large E increases) Momentum resolution worsens (multiple scattering)

Helen Caines

SQM - March 2003

Electronics and Gas Gain Calibration

• Two methods– Pulse the sense wires above the padrow

• Induces charge on all pads simultaneously– Easy and quick to perform, but ...– Measures electronics response at maximum load– Doesn’t measure the gas gain

– Measure response to 83mKr added to the detector gas (ALEPH)• Simultaneous calibration of electronics and gas gain

variations – 9 keV peak used to calibrate to MIP peak in data– Provides linearity check up to several MIPs,

( depends on dynamic range of electronics)MIP = Minimum Ionising Particle

Helen Caines

SQM - March 2003

PID Techniques(1) - 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

SQM - March 2003

Still need high momentum PID

Helen Caines

SQM - March 2003

Time-of-Flight method

• Requirements

– Time measurement between two scintillation counters (or similar)

– For p > 1 GeV/c, very good time resolution and long flight path

• For example

– The time difference between two particles, m1 and m2, over a flight path, L, is

which for p2 » m2c2 becomes

• NA49 Experiment

– The flight path is 13 m

– The time resolution t = 60 ps

– At 6 GeV/c: -K separation = 2 t

K-p separation = 6 t

t L

1c

L

2c

L

c1

m12c2

p2 1m2

2c2

p2

t ~m1

2 m22 Lc

2p2

Helen Caines

SQM - March 2003

Now Have the Ideal Strangeness Detector!

A magnetic field for charge and momentum determinationA TPC for main trackingAn Innner silicon detector for high precision vertexingand low momentum trackingA TOF for high momentum PIDTracking at high and mid-rapidity with large acceptance

Sound Familiar?

Helen Caines

SQM - March 2003installation in 2003

Endcap Calorimeter

Year 2000,

The STAR Detector

ZCal

Time Projection Chamber

Magnet

Coils

RICH * yr.1 SVT ladder

TPC Endcap & MWPC

ZCal

Central Trigger Barrel

FTPCs

Silicon Vertex Tracker *

Vertex Position Detectors

year 2001,

+ TOF patch

Barrel EM Calorimeter

year-by-year until 2003,

Helen Caines

SQM - March 2003

Other Stuff

Helen Caines

SQM - March 2003

Energy Loss: Bethe-Bloch

Helen Caines

SQM - March 2003

Calorimeters

• Electromagnetic Calorimeters • e- and deposit their total energy in the Calorimeter

• Hadronic calorimeter (may be in the future at mid-rapidity)• Zero Degree Calorimeters are largely used

• High Multiplicity : • Small RM ~ 2-5 cm• Distance 4-5 m from IP• Spectrometer

• Sampling Calorimeters:• cheap (acceptance)

• Lead+Scintillator• Homogenous Calorimeters :

• Resolution, • LeadGlass, PbWO4

PbWO4

X0 0.89RM 2 cmI 19.5 cmn 2.16Res 3% at >3GeV PHOS in ALICE & ECAL in CMS

Helen Caines

SQM - March 2003

Spectators – Definitely going down the beam line

Participants – Definitely created moving away from beamline

Triggering/Centrality

ImpactParameter

Spectators

Spectators

Zero-Degree Calorimeter

Participants

Several meters

• “Minimum Bias”ZDC East and West thresholds set to lower edge of single neutron peak.

REQUIRE:Coincidence ZDC East and West

• “Central”CTB threshold set to upper 15%

REQUIRE: Min. Bias + CTB over threshold

~30K Events |Zvtx| < 200 cm

Helen Caines

SQM - March 2003

RHIC ZDC

Helen Caines

SQM - March 2003

V0 Efficiency

Decay Length

Daughter DCA

DCA to PV Pos. Daughter DCA

Neg. Daughter DCA Mult.

Helen Caines

SQM - March 2003

Kinematics

N

ipmEvent

0,,,

pd

dE 3

3Invariant cross-section

ppE

ppE

TT

//*

*//

*

1000

0

N

iT ypmEvent

0,,,

Lorentz Transformations

dpdypd

TT

2

2

Helen Caines

SQM - March 2003

Why Rapidity?

pEpE

yz

zln5.0 y* = y + y0

Kinematical reason:•The shape of the rapidity distribution, dn/dy, is invariant

dynd

dppd

dpdypd

TT TT

22

2

Dynamical reason:• The invariant cross-section can be factorized

Helen Caines

SQM - March 2003

/1 , , mpy

SPS =9, >>6o // RHIC =100, >>1.6o // LHC =2750, >>0.02o

2/y

Maximun Rapidity

m

sy lnmax 3.5

maxyRHIC

8.2max

ySPS

6.8max

yLHC

2tanln

Pseudo-Rapidity