Strangeness Experimental Techniques

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Strangeness Experimental Techniques
An expert is a man who has made all the mistakes
which can be made, in a narrow field.-Niels Bohr
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The Goal
To design the Ultimate Strangeness Experiment
What we need To be able to measure charged and
neutral decays Lots 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.
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V0s Reconstruction?
Strangely enough most strange particles are
neutral or decay into something neutral
Strange Hadrons K0S ?pp- (494 MeV/c2, 2.7cm,
68.6), L?pp- (1.12 GeV/c2, 7.9cm, 63.9),
?-?Lp- (1.32 GeV/c2, 4.9cm, 99.9),
W-?LK- (1.67 GeV/c2, 2.5cm, 67.8),
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Finding V0s
proton
Primary vertex
pion
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Invariant mass distributions

• 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

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The Podolanski-Armenteros plot
Unique identification of two-body decaying
particles by studying the division, between the
daughters, of the parent's mtm vector. a - 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 a characterizes the decay asymmetry,
ltagt - daughter mass difference
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In case you thought it was easy
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Acceptance and Efficiency
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Fine way to calibrate a detector!

• Large peaks at 2 oclock and 8 oclock
• TPC pad row floating

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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.

STAR Preliminary
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Kink reconstruction

• In this method, one of the decay daughters is not
  observed.
• Main background is from p-decay, which has a
  smaller Q-value
• A cut on the decay angle (momentum dependent)
  removes the p contamination
• Remaining background from multiple scattering and
  split tracks.
• Find few kink decays per event.

Approx. 10 of a real event showing a
reconstructed kaon decay K ? m n (64) or K ?
p p0 (21). Lifetime ct 3.7 m
Decay angle (degrees)
Kaon limit
Pion limit
Parent momentum (GeV/c)
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• So now we know how to reconstruct them.

• First question what kind of accelerator do we
  want?

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Collider vs Fixed Target

• Collider
• Higher energy
• Lab frame CM frame
• Less focussed particles

Fixed Target Higher rates Known z vertex Boost
gives longer ct
Go with the Collider
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Beam _at_ RHIC Complex
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Beam at SPS Complex
The experiments
PS Booster
North Area NA
LINAC3
ECR
PS
SPS
West Area WA
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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
high resolution
Clearly identify individual tracks
Good tracking efficiency
Pt (GeV/c)
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Whats the Main Tracking Device?
Advantages of a TPC (why there are 7 at RHIC)

• 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

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Disadvantages

• Slow readout cant be used in level 0 trigger
• Two track resolution limited with classic design
  (although improvements possible with radial
  magnification)

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How a TPC works

• 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

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Silicon Tracker?
Lots of material Not so good Lots of
scattering
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Charge Determination
Magnetic field
Tracking detectors
Trajectory
PHENIX,
NA50 NA60, PHOBOS, BRAHMS, ALICE
STAR, ALICE, CMS, CERES, NA49, NA57
In or Out of Magnetic Field
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Charge transport correction (ExB)

• 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
• t is the mean time between collisions

The Solution
where
Horrible mess!!!!!!! Place in nice uniform Field
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Now have Main detector
Want low momentum tracks , near primary
vertex Need fine pixelation
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Fine Vertex Determination
ALICE ITS
SSD
SDD
SPD
Lout97.6 cm
Rout43.6 cm

• 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)

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Position Sensitive Silicon Detector
Strip
Pad/Pixel
Drift
1eh/3.6eV, 300mm 25000 e
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Charge cluster reconstruction

• 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)

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Track reconstruction
Row n-5 n-4 n-3 n-2 n-1 n

• 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

seed
Track following method
Row n-5 n-4 n-3 n-2 n-1 n
Track road method
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Calibration - Lasers
Using a system of lasers and mirrors illuminate
the TPC Produces a series of gt500 straight lines
criss-crossing the TPC volume

• Determines
• Drift velocity
• Timing offsets
• Alignment

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Momentum measurement
Measurement
X
P3
L3m, s10cm r11 m, B0.5T p1.7 GeV/c
Uncertainty
s
L
P2
P1
R
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Calibration Cosmic Rays
Determine momentum resolution
dp/p lt 2 for most tracks
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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

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The Bethe-Bloch equation

• Energy loss
• Bethe showed that energy loss is strictly a
  function of b v/c
• and the properties of the medium
• Including relativistic effects, the Bethe-Bloch
  equation is
• where,

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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)

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Comment on dE/dx measurements

• Practical considerations
• All energy loss distributions have inherently
  large spread
• Primary ionisation
• Number of primary electrons DE /W
• W energy to produce e--ion pair
• Follows Poisson statistics
• Secondary ionisation
• Energy distribution of primary electrons E-2
• If E gt 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
  DE increases)
• Momentum resolution worsens (multiple scattering)

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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
• Doesnt 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

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PID Techniques(1) - dE/dx
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Still need high momentum PID
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Time-of-Flight method

• Requirements
• Time measurement between two scintillation
  counters (or similar)
• For p gt 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 st 60 ps
• At 6 GeV/c p-K separation 2 st
• K-p separation 6 st

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Now Have the Ideal Strangeness Detector!
A magnetic field for charge and momentum
determination A TPC for main tracking An Innner
silicon detector for high precision vertexing and
low momentum tracking A TOF for high momentum
PID Tracking at high and mid-rapidity with large
acceptance
Sound Familiar?
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The STAR Detector
Magnet
Coils
TPC Endcap MWPC
ZCal
ZCal
Central Trigger Barrel
RICH
yr.1 SVT ladder

• Year 2000,

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Other Stuff
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Energy Loss Bethe-Bloch
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Calorimeters

• Electromagnetic Calorimeters
• e- and g 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)
• LeadScintillator
• Homogenous Calorimeters
• Resolution,
• LeadGlass, PbWO4

PHOS in ALICE ECAL in CMS
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Triggering/Centrality
Spectators Definitely going down the beam
line Participants Definitely created moving
away from beamline
Several meters
Spectators
Zero-Degree Calorimeter
Participants
Impact Parameter
Spectators
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RHIC ZDC
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V0 Efficiency
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Kinematics
Invariant cross-section
Lorentz Transformations
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Why Rapidity?

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

y y y0

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

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Pseudo-Rapidity
gSPS 9, qgtgt6o // gRHIC 100, qgtgt1.6o // gLHC
2750, qgtgt0.02o
Maximun Rapidity