Post on 13-Aug-2019
What Does A Drift Chamber Do?
❂ Basic principle
� Charged particle in matter loses energy by ionization
� Matter in drift chamber is gas
� Ionization is our source of information
❂ Tracking
� Ionization along path : Timing➪ Reconstruct picture of track
� Track of charged particle bent in magnetic �eld
➪ Momentum
� Can also provide global timing information (t0)❂ Particle identi�cation (PID)
❂ Amount of ionization along path
➪ With momentum, gives PID
The BaBar Drift Chamber
IP1618
469236
324 681015 1749
551 973
17.1920235
❂ cylindrical, 40-layer design
� 7104 drift cells, hexagonal �eld wire pattern
� 80,120 �m Gold-plated Al �eld wires
� 20 �m gold-plated tungsten-rhenium sense wires
❂ Features dictated by multiple-scattering limit
� Low Z gas mixture (80:20 helium:isobutane)
� Al �eld wires, endplates
� Be inner core
❂ Features dictated by boost
� All electronics on rear endplate
� Forward endplate thinner in region of acceptance
Expected Performance
❂ Position Resolution
� 130 �m single-cell resolution
❂ Momentum Resolution
� Care about charged particles with pt between 100 MeV/C and 2.5 Gev/C
� Only just starting to get real idea of performance
❂ Particle ID
❂ dE/dX resolution of 7 percent
❂ `Good' �=K separation up to 700 MeV
✎ Electronics not to degrade intrinsic performance by more than 10 percent.
Tracking: Signal
❂ Creation of signal
❂ Charged particle collides with gas molecules
� Primary ionization: liberate electron
� Secondary ionization: electrons collide with gas molecules
❂ Drift, proportional ampli�cation
❂ Collection of signal
❂ electrons attracted to sense wires
❂ TDC readout
❂ Interpretation of signal : Tracks
Proportional Wire
❂ Basics of avalanche
� Electron drifts and collides with gas molecules
� Picks up energy from E �eld between collisions
� If enough, ionizes gas molecule, liberates electrons
➦ ...and photons
❂ Important quantities
� Mean free path between collisions
� Size of E �eld near wire
✎ thinner wire means larger �eld gradient
➪ want thinnest wire that won't snap
The Wire Continued
❂ Proportionality
� Signal proportional to number of primary electrons produced
� Condition: number of electrons in avalanche negligible w.r.t. linear charge
density of wire❂ Issues
❂ Space Charge
� Electrons collected, ions left behind
� Slowly move toward cathode (�eld wire)
� Meanwhile, reduces ampli�cation❂ Breakdown and Quenching
� ionization also produces photons
� photons can also spark new avalanches
� photons travelling on average further than avalanche length gives break-
down.
� quench gas absorbs photons, prevents breakdown.
The Cell
❂ Need position information from the signal
❂ Ideally, surround anode with cylindrical cathode
� circular cell
� circles of equal drift time
✎ too much material
� In practice, �eld wires approximate ideal cell shape
✎ surfaces of equal drift time called isochrones
The BaBar DCH Cell
❂ Typical cell size: 1.2 by 1.8 cm squared
❂ Typical shape:❂ Field wires in a hexagonal pattern
� Held at ground
� Sense wires at 1960V (1900V)
� 50n isochrones in 1.5 T magnetic �eld
HEX2 - Wire 168 Isocrones every 50 ns
BaBar DCH Cell Continued
❂ Position of hit within a cell
✎ Actually measure drift time, not distance
� conversion done by t-to-d calibration
� Iterative process
� right and left sides of cell �tted separately
-1
-0.5
0
0.5
1
0 100 200 300 400 500 600
Drift Time (ns)
Sig
ned
Dis
tanc
e to
wire
(cm
)
PEP-II / BaBar
d(t) layer 31
Resolution within the Cell
0
0.005
0.01
0.015
0.02
0.025
-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1
Mean 125 µm
Signed distance from wire (cm)R
esol
utio
n (c
m)
❂ x-axis: signed radial position within cell
❂ y-axis : residual
� Di�erence between �tted track doca and drift time
❂ Leaves us with questions
➪ What do we mean by a track?
➪ What are track parameters? (like doca)
➪ How is tracking done?
What is a Track?
❂ For us, a reconstructed quantity
✎ Made of hits (cell signals)
➦ We'll leave the how for later
❂ A helix (�t assuming uniform B)
❂ Relevant parameters
❂ Space Point: d0, �, z
❂ Shape: curvature (!), dip angle (�)
� Relation to cell: doca, entrance angle, ambiguity
� Quality of reconstruction
� Global timing information (T0)
✎ Parameter of track �t
✎ Source of event time
➪ Leave last two for other talks
Track Parameters� Doca (distance of closest approach)
� Entrance Angle: �track � �hit
� Wire Ambiguity
� Relating drift time to position not always unique
� Resolve ambiguity with pattern-recognition methods
� Becomes a signed label
❂ This picture is 2d, but we have 3d information
✎ Our picture is incomplete
➪ It is time to complete it
The Larger Picture
❂ Stringing pattern of chamber
❂ layers organized into superlayers
� 40 layers, 10 superlayers
� Guard wires (340V)
� Axial superlayers|wires strung along axis
✎ 4 axial superlayers
� U and V stereo layers|wires are tilted✎ 6 stereo superlayers (3 of each)
✎ 40 mrad to 70 mrad (need larger radius for larger angle)
✎ U and V wires tilt in opposite directions
✎ Superlayers correspond to track segments
✎ Axial segments used in L1 trigger
The Larger Picture Continued
Using Stereo Information
❂ Axial layers do not provide z information
� Corresponds to projection of track onto rear endplate
➦ Technically can extract z information from charged separation
� Poor in practice
� Need electronics at both ends of chamber
� Axial track info forms a circle (!, �, d0)
➪ Stretch in Z, forms a cylinder
❂ Take stereo segment (i.e. U superlayer segment)
� Projection on endplate displaced
� Move in z until aligns with track
� \slice" cylinder
❂ One stereo segment isn't enough
� Need two z-related parameters (z, dip angle)➪ Need two independent lines
� Could use adjacent wires in superlayer
✎ Doesn't work well
� Use wire from other stereo avor (i.e. V for U)
A SampleEvent
From the Large to the Small
❂ Cell shape variations
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161162� Guard wires
� Field distortion across superlayer boundaries distort cell shape in outer layers
❂ Lorentz Angle
� Tracking accuracy best when electron drift at right angles to track
� B �eld a�ects drift: E�B
� Lorentz angle : angle between drift direction and E❂ Crosstalk
� Ions attracted to �eld wires
➪ Induce pulse on neighboring sense wire
Gas Choice
❂ 20 percent Isobutane, 80 percent Helium
❂ Why Helium?
� Multiple scattering (low momentum tracks in BaBar)
� He low Z
❂ Isobutane provides 80 percent of ionization
➪ Isobutane determines resolution✎ More Isobutane and multiple scattering worsens resolution
Gas Z A Density [g/l] dE/dx [kev/cm] W [ev]
Helium 2 4.003 0.178 0.32 41
Argon 18 39.95 1.78 2.44 26
Isobutane 34 58.12 2.67 4.50 23
Carbon Dioxide 22 44.00 1.98 3.01 33
Wire Choice
❂ Issues
❂ Mechanical Strength
� 20 micron sense wires: tradeo� with cell size, �eld gradient
� 80, 120 micron �eld wires
❂ Multiple Scattering
� Al low Z
❂ Resistivity
✎ Sense wire primary source of noise in readout circuit
� Tungsten, gold plating
❂ Surface Quality
✎ Sharp �eld gradients : sparking
� Rhenium in sense wire alloy
Electronics and Timing
❂ Sense wire dominant source of noise
❂ 4-channel ampli�er discriminator chip
❂ Di�erential output of amp. channel goes to discriminator
✎ Time over threshhold plus 40 nsec➪ goes to 6-bit TDC
� 1ns resolution
❂ FEX
❂ Keep up to 11 TDC hits for event
Particle ID and dE/dX
❂ Bethe-Bloch distribution
❂ Basic principle of PID with dE/dX❂ Mechanics of dE/dX
❂ BaBar Performance
The Bethe-Bloch Distribution
Muon momentum
1
10
100
Sto
ppin
g po
wer
[M
eV c
m2 /
g]
Lin
dhar
d-S
char
ff
Bethe-Bloch
Radiativeeffects
reach 1%
µ+ on Cu
Without δ
Radiativelosses
βγ0.001 0.01 0.1 1 10 100
1001010.1
1000 104 105 106
[MeV/c]100101
[GeV/c]100101
[TeV/c]
Anderson-Ziegler
Minimumionization
Eµc
Nuclearlosses
µ−
1
2
3
4
5 6
8
10
1.0 10 100 1000 10 0000.1
Pion momentum (GeV/c)
Proton momentum (GeV/c)
1.0 10 100 10000.1
1.0 10 100 10000.1
1.0 10 100 1000 10 0000.1
−d
E/
dx
(MeV
g−1
cm2 )
βγ = p/Mc
Muon momentum (GeV/c)
H2 liquid
He gas
CAl
FeSn
Pb
❂ Models energy loss in material of a relativistic particle
� dE/dx (energy lost per unit path length)
� vs. � or momentum
❂ Form� Minimal dependence on particle type; for practical purposes function of �
� Signi�cant material dependence
dEdx=4�Ne4
mc2
1�2Z2[ln(p
2mc2Emax�
I
)��2
2�
�(�)2
]
❂ Restricted energy loss formula
� Doesn't include high-energy transfers that do not contribute to a track
� Universal validity
Bethe-Bloch Continued
❂ Relativistic rise
� Corresponds to logarithmic term
� Density dependence
� Physical origins
� In time behavior of electromagnetic �eld of travelling particle
� Maximal energy transfer increases with �
❂ Fermi plateau
� �(�) curtails relativistic rise
� Complete cancellation of dependence as � ! 1
❂ Minimum
� At around � = 4
❂ Characterize curve
� Usually look at ratio of dE/dx on plateau to dE/dx at minimum
� R `amount of relativistic rise'
� � x-coordinate of plateau
� Depend on gas, density(T,P), sample length
Bethe-Block and Babar
❂ Distribution material and conditions dependent
❂ Using direct formula impossible
� Can't calculate all constants from �rst principles
➪ Parametrized �t to data instead
� BaBar DCH: 5-parameter �t
dE/dx and PID
❂ Remember energy loss results in ionization
� Will speak interchangeably of dE/dx and I
(measure of ionization strength along track)
❂ Theoretically
� Measured ionization ! particle mass
� Measured momentum
v =
pc2
pp2c2 +m2c4
➪ Particle ID
❂ To elaborate
� I � Z2Fg(v)
� Divide by minimal value at vmin ! normalized curve
❂ In practice
� Fg(v) not monotonic
� No closed form m = m(I; p)
➪ Plot Fg(v) curves for di�erent particles instead
Principles ContinueddE/dx vs momentum
10 3
10 4
10-1
1 10Track momentum (GeV/c)
80%
trun
cate
d m
ean
(arb
itrar
y un
its)
eµ
π
K
p dBABAR
❂ Above are curves for pions, kaons, electrons, muons, and protons
� Identical shape
� Displaced (on log scale) by logs of particle mass ratios
� Point on diagram: (Momentum, Ionization (dE/dx)
❂ PID not perfect or unique
� Curves overlap
� Ionization measurement has �nite accuracy
❂ Accuracy of ionization measurement (dE/dx resolution) obviously crucial
➪ Factors in accuracy? Measurement technique?
Statistics of Ionization
❂ Remember particle = track
❂ One track
� Approx. 40 hits➪ approx 40 ionization/charge measurements
✎ Average of all measurements bad estimator
� Mean energy loss dEdx�x has large uctuations
� Distribution skews towards high values (Landau tail)
❂ Solutions:
� Fit to shape of measured distribution
� Theoretically better
� Truncated mean method
� Take subsample, exclude high values
� Simple, similar performance in practice
dE/dx Resolution in BaBar
❂ Resolution rms width of dE/dx distribution
dedx/BB -1IDEntriesMeanRMSUDFLWOVFLWALLCHAN
130 2458 0.6094E-02 0.7721E-01
0. 0.
2458. 25.74 / 26
Constant 256.7Mean 0.5125E-02Sigma 0.7561E-01
99/08/31 18.07
Electron
IDEntriesMeanRMSUDFLWOVFLWALLCHAN
230 721
-0.4397E-02 0.7888E-01
0. 0.
721.0 13.74 / 22
Constant 73.13Mean -0.6906E-02Sigma 0.7753E-01
MuonIDEntriesMeanRMSUDFLWOVFLWALLCHAN
330 1802 0.3774E-02 0.7568E-01
0. 0.
1802. 49.53 / 18
Constant 185.1Mean 0.4962E-02Sigma 0.7642E-01
Pion
IDEntriesMeanRMSUDFLWOVFLWALLCHAN
430 177
-0.1678E-01 0.9047E-01
0. 0.
177.0 17.90 / 22
Constant 15.28Mean -0.8338E-02Sigma 0.8335E-01
KaonIDEntriesMeanRMSUDFLWOVFLWALLCHAN
530 19817-0.7998E-03 0.8709E-01
0. 0.
0.1982E+05 181.2 / 27
Constant 1837.Mean -0.1596E-02Sigma 0.8533E-01
Proton
IDEntriesMeanRMSUDFLWOVFLWALLCHAN
630 1792 0.4542E-02
0.1002 0. 0.
1792. 48.69 / 27
Constant 145.0Mean 0.8299E-02Sigma 0.9612E-01
Deuteron
0
100
200
300
-0.5 0 0.50
20406080
-0.5 0 0.5
050
100150200
-0.5 0 0.505
101520
-0.5 0 0.5
0500
100015002000
-0.5 0 0.50
50
100
150
-0.5 0 0.5
❂ 80 percent truncated mean
❂ Better than 2� �=K separation up to 700 MeV
Electronics and dE/dx
❂ Pulse
� Slow shaping
� Digitization: 15 Mhz FADC
� Interleaved with TDC hits
➪ Holes (Interpolation)
❂ Charge
� Integrated area of pulse
� Calibrated
❂ FEX output
� Charge
� Some shape information
Omissions
❂ Many important issues
� Not enough time to discuss them all
❂ DCH as Trigger❂ DCH and Event Time❂ Systematics and Corrections
❂ In tracking, dE/dx
❂ e.g. crosstalk, charge screening, entrance angle
❂ System Details
� Electronics
� Gas
� Mechanical
� Controls
❂ Aging
� Hydrocarbons can be a problem
� Current di�culties