Instrumentation for the Energy Frontier Ronald Lipton, Fermilab High Energy Physics has had remarkable success at the Frontier culminating with the discovery of.

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Transcript Instrumentation for the Energy Frontier Ronald Lipton, Fermilab High Energy Physics has had remarkable success at the Frontier culminating with the discovery of.

Instrumentation for the Energy Frontier
Ronald Lipton, Fermilab
High Energy Physics has had remarkable success at the Frontier
culminating with the discovery of the Higgs.
This success was enabled by equally remarkable progress in
technology and instrumentation. These lectures will look at past and
current work and perhaps offer a glimpse of the future
Tracking
Let’s think about designing a
tracker for a collider detector
• They all look pretty generic
• The solenoidal field defines the
overall geometry
• Transitions from a barrel to disk
geometry tend to be awkward
– Disks provide lower mass at
high eta, more normal
incidence
– The number of hits/area is
maximized with disks
combined with barrels
R. Lipton
D0
2
Designing a Tracker - 1
1
b
g ct
g
b = g ct ´ 1 » ct
g
» 30 - 300 microns
æ
Rinner 2 ö
) ÷
ç 1+ (
Router ÷
s ip = s meas ç
ç
Rinner ÷
(1) ÷
ç
Router ø
è
First let’s decide on the vertex detector
• Scale set by HQ lifetimes
• Minimize Rinner/Router
• Rinner set by occupancy, beam pipe diameter
• Router set by cost of pixelated detectors
• smeas set by technology, mass of sensors
• ILC ~ 5 microns at 1.5 cm (slow, rad soft, monolithic)
• LHC ~ 20 microns at 5 cm (fast, rad hard, hybrid)
Length set by luminous region, angular coverage
R. •Lipton
D0SMT
Tevatron luminous region
~25 cm long
3
Designing a Tracker - 2
2E-11
1.8E-11
1.6E-11
Layer Impact on momentum resolution
(Seiden SDC-91-00021)
1.4E-11
ALPHA
1.2E-11
1E-11
8E-12
6E-12
4E-12
2E-12
0
0.00
k=
Momentum Resolution
• Resolution proportional to s/BL2
• For a high momentum track f=f0+kr k=1/pt
• We effectively want to measure Df (circumferential
distance di)
• Most important information is at the outer radius and
near the origin
• Intermediate layers primarily provide pattern recognition
R. Lipton
20.00
40.00
60.00 80.00 100.00 120.00 140.00
Layer Radius (cm)
æ
rd ö
ç å i 2i ÷
æ f ö æ
öç
si ÷
ç 0 ÷ = ç A -B ÷ç
÷
2
ç k ÷
è
ø è -B C øç å ri di ÷
ç
s i2 ÷ø
è
ai = (C
ri2
si
-B
ri
s i2
)
k(curvature) = åai di
Alpha estimates the effect
of the layer on momentum
resolution
4
Designing a Tracker - 3
CMS FPIX Plaquette
LHCB VELO R and F sensors
The Forward Direction
• As we move forward the we begin to lose ∫Bdl and
momentum resolution
• Disks become more cost effective/hit than barrels
• We can recover some momentum resolution with
precision disks
• We want to measure phi well, r not as well, but this is
difficult in a disk geometry
Intermediate disks have little effect on resolution
R. •Lipton
Tiled 3D pixel structure5
Doublet strip modules
For track trigger
Doublet pixel/strip
modules for track trigger
Forward pixel diska
For extended h coverage
Possible design for CMS Phase 2 tracker with extension to improve
acceptance for forward physics (H➛tt, Higgs self coupling, WW scattering)
R. Lipton
6
Silicon Tracking
This has become the “baseline” technology
for the energy frontier. It is:
• Precise ~ micron-level resolution
• Moderate to low mass (depends on density,
cooling, electronics)
• Fast ~ can achieve sub-nanosecond resolution
• Radiation hard – can be designed to operate to 1016/cm2 fluence
• Costly? $10/cm2 for CMS sensors $3/cm2 for CMOS electronics
We can profit from the huge technical advances and infrastructure in
the semiconductor industry
R. Lipton
7
Signal and Noise
• What is the thinnest “practical” silicon
tracker?
2
2 a1g 2kT
ENC
=
(C
+
C
)
• Noise –
det
gate
gm t s
•
•
Signal/Noise Ratio
•
•
Increasing gm costs power (gm~Id),
minimize Cdet->pixels ~ 10 ff possible
minimal coupling to other electrodes
Power – assume id=500 na, pitch 25 microns
10000
Signal – shoot for 25:1 s/n
–80 e/h pairs/micron
1000
Speed – let’s say 5 ns
Mechanical –
100
–Can thin to ~10 microns
10
0
R. Lipton
50
100
150
200
Detector Thickness (microns)
250
300
8
Silicon Detector
How we connect the
SiO2
detectors to the electronics,
cool them, and mount them
is the name of the game…
Al
p+
n+
Hybrid Pixel Interconnect using bump bonds
Analog
Analog
cable
cable
SVX4
SVX4
R. Lipton
hybrid
hybrid
9
Detector/Electronics
Integration Technologies
MAPS
source
p+
top gate
drain
clear
bulk
n+
p+
n+
n+
p
n
symmetry axis
~1µm
CCD
internal gate
- - -- - -
+
-
+
DEPFET
n-
- +
+
50 µm
• Monolithic active pixels –
collect charge in a few mm
epitaxial layer (STAR, ALICE)
• Charge coupled device (SLD)
• DEPFET (Belle II)
• Silicon on Insulator…
• 3D Integration…
-
p+
rear contact
3D
R. Lipton
SOI
10
Solving Problems - MAPS
MAPs – technology used in cameras using
charge collection by diffusion in a thin(~5-15
mm) epitaxial layer
Slow-charge
collection by
diffusion
Fully depleted
substrates
Low S/N
Thick, high
resistivity
epitaxial layers
Charge lost to
parasitic PMOS
4 Well process
3D assemblies
(RAL)
Thinning and
backside
processing
(IPHC-DRS)
(IPHC-DRS)
Technologies Device-scaling
Rapid initial decrease in cost
• Slower leveling
Voltage no longer scaling (P~CV2 f)
Analog becomes harder at feature sizes
below 65 nm
Designs become very costly
8” 130 nm - $500k
12” 65 nm - $1.9M
R. Lipton
(Deptuch, IF ASIC meeting)
12
TechnologiesBonding Costs and Yields
Current and projected costs and yields for sensor/readout integration technologies
TechnologiesThree Dimensional Electronics
Back-Face
A 3D-IC technology is composed of two or
more layers of active electronics or sensors
Face-Face
connected with through silicon vias
• It enables intimate interconnection
MIT-LL
between sensors and readout circuits
3D-IC process
• It enables unique functionality
FDSOI oxideoxide bonding
– Digital/analog/ and data
communication tiers
Ziptronix / licensed to Novati
– Micro/macro pixel designs
– Correlate information
• Wafer thinning enables low mass, high
resolution sensors
• Etching of vias (3D) through silicon bulk
• Bonding technologies enable very fine
pitch, high resolution pixelated devices
• Commercialization of 3D wafer bonding
can reduce costs for large areas
• Unique circuit/sensor
Xilinx 3D-based FPGA
Pixelization3D Interconnect
Technology based on:
• Bonding between layers
– Copper/copper
– Oxide to oxide fusion
– Copper/tin bonding
– Polymer/adhesive bonding
– Cu stud
• Through wafer via formation and
metalization
PCB Interconnect
Copper bonded two-tier IC
(Tezzaron)
micron pitch
pitch,DBI
50 micron
thick oxide
bonded
88 micron
(oxide-metal)
bonded
imager
(Lincoln
Labs)
PIN
imager
(Ziptronix)
IBM 32 nm 3D technology
15
Opportunities in 3 Dimensions
readout
IC and pads
200 micron
Buried
oxide
sensor
trenches
Handle wafer
CMS Level 1 track trigger
- Correlate hits in adjacent layers
to filter out low momentum tracks
Combine active edge and 3D
electronics to produce tiled sensors
combined with ROICs for large area arrays
CMS – Use stack of
3D tiers to emulate
tracker layers for
CAM –based track
recognition
Use TSVs to connect each SiPM subpixel to quenching,
timing, and control electronics
Example - Track Trigger
In CMS the L1 trigger will be saturated
with multiple interaction background
• Use tracking information in the L1
trigger
– Send hits from tracks with Pt>2
off detector for L1
– Correlate hits from sensors
separated by ~ 1 mm
– Correlation done on-module
• To do this we need novel
interconnect technology which
allows the chip to “see” signals from
top and bottom sensors
– Through-silicon-vias allows single
layer of electronics to see both
R. Lipton
Via-Last Module
(FNAL design)
•
•
•
•
250m
Analog
signals
Flex Jumper
50 x 250 micron through silicon vias
Bump bonded short strip sensors
Analog signals through flex jumper
2.5 cm long strips (set by chip size)
TSVs
ROIC
Short (0.125 cm) strips
Short (1.25 mm) strips
Carbon Foam Spacer
Long (2.5 cm) strips
17
High Speed silicon
Two techniques to attain ~10 ps resolution
• Fast parallel plate structure using 3D
detector technology
• Use amplification to produce a large signal
from initial electron arriving at gap
structure
Fast parallel plate structure
(Da Via)
TSV
Coarse SiPM Tier
Fine SiPM Tier
Coincidence and active quench
READOUT FLEX
Gain-based structure
(Sadrozinski)
Use two layers of 3D SiPMs to
produce fast, low power, low noise
trackers (Lipton)
R. Lipton Higgs Factory
Workshop[ 11/16/2012
18
Radiation Damage in Silicon
• Radiation
– Electromagnetic (g, b, x-ray).
• Ionization, e-hole pair creation.
– Hadronic (n, p, p). Damage to the
bulk material caused by
displacement of atoms from
lattice sites in addition to ionization
• Electronics are affected primarily by
ionization
– Charge buildup in insulating layers
– Charge injection into sensitive
nodes
• Sensors are affected by bulk damage
and ionization
– Crystal structure damage
– Introduction of traps
– Introduction of mid-band states
R. Lipton
A. Vasilescu (INPE Bucharest) and G. Lindstroem (University of Hamburg),
Displacement damage in silicon, on-line compilation
19
10-1
Radiation effects on Detectors
•
•
•
•
HEP silicon detectors used at the Tevatron
and LHC are primarily affected by bulk
damage. Associated electronics are
affected by primarily by ionization damage.
Detectors are unique
– Lightly doped silicon
– Thick structures
– Regular array of electrodes
Several different bulk effects:
– Increase in leakage current
– Changes in doping concentration
– Increased charge trapping
All of these depend on time and
temperature, sometimes in complex ways
3]
n-type FZ - 7 to 25 KWcm
n-type FZ - 7 KWcm
n-type FZ - 4 KWcm
n-type FZ - 3 KWcm
p-type EPI - 2 and 4 KWcm
10-2
m
c/ 10-3
A
[
n-type FZ - 780 Wcm
n-type FZ - 410 Wcm
n-type FZ - 130 Wcm
n-type FZ - 110 Wcm
n-type CZ - 140 Wcm
p-type EPI - 380 Wcm
-4
10
V
/
I -5
D10
10-6 11
10
1012
1013
Feq [cm-2]
1014
1015
[M.Moll PhD Thesis]
I det = I 0 + a F ´Volume
a = 2 - 3´10-17 A / cm
Depends on temperature
extrapolated values
electrons
holes
R. Lipton
Ref 4.
20
Designing Radiation Hard
Electronics
• Radiation generates e-hole
pairs in insulating oxides
– Electrons are mobile and are
removed by the gatesubstrate field
– Holes are trapped – either in
the bulk or by deeper traps
near the silicon-oxide
junction
– Holes can recombine with
tunneling electrons from the
silicon-> thin gate oxides in Tranisistor
DV/Rad
modern deep submicron
electronics are intrinsically
radiation hard
R. Lipton
Gate thickness (nm)
21
Designing Radiation Hard
Detectors
• Leakage current is universal
• Generates shot noise, thermal
effects
– Reduce thickness
– Run cold to reduce current, avoid
thermal runaway
• Trapping reduces signal mean free path
– Thin detectors
– Increase internal fields
• Run Cold (~-20 deg C)
– Freeze-in p-type impurities
• Use 3D detectors
– Etch electrodes deep into silicon
– Full thickness for charge collection,
short drift distance
• R. Lipton
Use Diamond sensors
(Parker, Kenney)
22
Mechanics
These are complex
engineered
systems
• Mechanics has
central effects
on physics
performance
• We sometimes
focus too much
on “physicsey”
things like
radiation
damage and
give short shrift
to mechanics
R. Lipton
CMS
23
Material
• Controlling material is critical to physics performance.
– That is apparent in vertex detectors and trackers, where
multiple scattering limits spatial and momentum
resolution.
– The production of additional particles increases
backgrounds and occupancies and complicates track
finding, track tracing, and event reconstruction.
– Stability, deflections, and distortions depend on the
weight to be supported, the geometry of structures,
environmental changes from fabrication to operation,
and material properties.
R. Lipton
24
New Materials
•
•
•
•
•
•
•
•
•
•
•
Carbon fiber composites
Carbon derivatives (C-C, Pyrolytic graphite, etc.)
Beryllium
Titanium alloys
Ceramics
Advanced compounds (SiC, BN, SiN, diamond, etc.)
Conducting polymers and carbon conductors
Foams
Adhesives
Electrical circuit components
Liquid / 2-phase cooling tubes
R. Lipton
25
Power
Current LHC detectors dissipate more than half their power in the
cables. Future, more ambitious detectors will utilize even more
power:
• High speed front end electronics
• GHz Waveform digitizers
• Pixelated sensors
• Higher readout bandwidth
To address these problems all future
experiments are examining power delivery
options
• Pulsed power (ILC, CLIC)
• DC-DC conversion (CMS, ILC, CLIC)
– High efficiency, rad hard high voltage ratio converters capable
of operating in a magnetic field.
• Serial powering (ATLAS, think Xmas tree lights)
R. Lipton
26
Cooling
An efficient, low mass cooling
scheme should have:
• Efficient heat transfer (2-phase)
– CO2 systems
• Low mass
• Good thermal contact to
electronics and sensor
• Well engineered
– Almost all hadron collider
experiments (except D0)
have had serious cooling
issues
R. Lipton
Super B, LHCb micromachined
channels
DEPFET air cooling
thermal tests
27
Power and Cooling
Readout
Amplifier
Data
Transmission
Pixelated Sensor
DC-DC
conversion
Support structure
Cooling pipes
• Data transmission – 10-200 pj/bit ~ 5-10 Gbit/sec
• Amplifier/readout ~100 mW/cm2
• Sensor IL ~ 1ma/100 cm2 x 500 V (high radiation) @ -25 deg C
• DC-DC converter supplies power at 60-80% efficiency
5x10 cm module – 7.5 Watts
If our tracker is 100 m2 -> 150 kW !!!
R. Lipton
28
What do we do?
• Data transmission
– Low power (less rad had?) transmission (10pj/bit)
– Lower bandwidth (process on detector) (2.5 Gb/sec)
• Amplifier/readout
– Low power design – limit functionality?
– Smaller feature size no longer too helpful (Vdd~1V)
– May be be able to achieve 75 mW/cm2
• Thin Sensor to 100 microns
– Vd~T2, lower volume 0.3 ma @ 50V
• High frequency DC-DC converter
– 90% efficiency
Can get to 85 kW – not so different than current CMS
R. Lipton
29
Data Transmission
Industry is driving low power,
high bandwidth data transmission
• Low power optical data transmission
– Modulators rather than laser diodes
– Mach-Zender – interferometer utilizing material with strong electrooptic effects
– Radiation hard transceivers
Elec. Tx
Current driver
Laser
(VCSEL)
Optical Tx
Elec. Rx
Receiver
PIN diodes
Optical Rx
Laser (CW)
Elec. Tx
Voltage driver
Modulator
Receiver
PIN diodes
Elec. Rx
Optical Tx
Optical Rx
R. Lipton
30
Monolithically integrated Silicon photonic device
Muon Collider - Accelerator
A muon collider would accelerate and cool a beam of muons and bring them into
collision for ~1000 turns in a circular collider
• It is the only lepton collider that can plausibly scale beyond 2-3 TeV with
acceptable cost and power
– Given the lack of new physics at 8 TeV LHC such a capability becomes
increasingly interesting
– Physics capabilities are similar to e+e- colliders, with additional ability to
explore s-channel h and H/A, but worse beam background, lower polarization
• It can provide a phased approach to implementation
– Move gracefully from n factory to Higgs factory to high energy collider –
complementing the rare decay and neutrino programs
– The phasing and small footprint makes the program affordable
• But the Muon beam decays:
– For 62.5-GeV muon beam of 2x1012, 5x106 dec/m per bunch crossing
– For 0.75-TeV muon beam of 2x1012, 4.28x105 dec/m per bunch crossing, or
1.28x1010 dec/m/s for 2 beams; 0.5 kW/m.
R. Lipton
31
Ionization Cooling
Muons produced by a high
intensity target are collected
and initially cooled by bunch
rotation.
• Ionization cooling is based
on the idea that energy
losss occurs in x,y,z but
momentum is restored by
RF in z only.
• Cooling is limited by the
heating effect of multiple
scattering
• Low Z absorber in RF cavity
with solenoid field
R. Lipton
Emittance
change
Energy loss
cooling term
Multiple scattering
Heating term
32
Accelerator Challenges
• Ionization Cooling
– Very high field (40T) high
temp superconducting
magnets
– 6 dimensional cooling
• RF breakdown in magnetic fields
– Seems to be solved
• Neutrino radiation ( < 10% x DOE
limit at site boundary?)
– Probably OK at 3 TeV, harder
at 6 TeV
– Must limit length of straight
sections (~ meters)
• Magnet shielding from beam decay
heat loads
Are any of these deadly to the Muon
Collider concept? – subject of MAP
Ronald Lipton 8/11/2011
33
Figure of merit:
Integrated Luminosity/Wall plug power
100.00
𝑭𝒐𝑴 ∝ 𝑳TOT / PWT
Muon Collider
TLEP
luminosity
X 1031 per
MW
PWFA
IHEP
PWFA
IHEPPWFA
PWFA
ILC
CLIC
ILC
10.00
Muon Collider
CLIC
Muon ColliderLEP3
ILC
CLIC
Super-Tristan
SLAC/LBL
FNAL
1.00
0.00
0.50
1.00
CLIC
2.00
2.50
3.00
C.M. colliding beam energy (TeV)
HIGGS
ILC
1.50
PWFA
LEP3
J.P.Delahaye @ MIT Workshp; April 10,2013
TLEP
Super-Tristan
34
FNAL
IHEP
SLAC/LBL
Muon Collider
Review of HIGGS Factory technology options
Preliminary
work in progress
(collab. project X)
Evolution of Muon Facilities
To F
San ar Dete
ford
c
(130 tor in
0km
)
LBNE
Buncher/
Accumulator
Rings & Target
N F D 5 Ge
ns t ecay V
o Sa Ring
nfor :
Front
d
End + 4
D +6 D
RLA to 63 GeV +
300m Higgs Factory
Linac + RLA
SC 325MHz
to ~5 GeV
nSTORM + Muon Beam
R&D Facility
Accum
1-3 MW Neutrino
factory
Compr
Front End
PX2 (3 GeV, 3 MW)
MAP Collaboration workshop (June 19, 2013)
Acceleration
0.2–0.8 GeV
26
m Storage Ring
0.8 – 2.8
GeV
Proton Driver
Targe
t
J.P.Delahaye
235m
Linac + 2RLA
4 MW Higgs factory
3-10 TeV Muon Collider
J.P.Delahaye
PX4
(8 GeV, 4 MW)
MAP Collaboration workshop (June 19,
2013)
35
Muon collider Higgs factory beam transport and
detector
R. Lipton
36
Muon Collider Background – 1.5 TeV
Detectors must be rad hard
Dominated by neutrons –
smaller radial dependence
Non-ionizing background ~ 0.1 x LHC
But crossing interval 10ms/25 ns 400 x
37
Muon Collider Detector
How do we design a detector for a
muon collider?
• Start with design for physics –
ILC, CLIC detectors
– SiD is the best match
• Background rejection is clearly
the dominant issue
– Design the machine-detector
interface and model bkd
– Understand the
compromises needed to
reject background
• Is it plausible, what are the
physics impacts?
R. Lipton
Neutrons/cm^2/bunch
38
g
Much of the
Background is
Soft
m-
g
And Out of
Time
Timing is clearly crucial to reduce
backgrounds
m(Striganov)
e+/-
m+
h0
h+-
e+/-
m+
h0
h+39
Background Inside a silicon detector:
Detector thickness
Angled tracks
dE/dX
MIP
Path in detector
• Background Path length in silicon detector vs de/dx
40
Time of energy deposit
with respect to TOF from
IP
Neutrons
electrons
positrons
Compton
High energy
conversions
soft
conversions
41
Tracker Implementation
• Tracker sufficiently pixelated so
background occupancy is
acceptable
– 20 micron vertex
– 100 micron x 1 mm tracker
• Multi-hit/waveform digitize hits
within ~20 ns window with ~0.5
ns resolution
– Plausible given signal/noise,
power requirements
– Track fit now includes time
of hit to accommodate
slower particles from IP
Problems are really power and
interconnect
R. Lipton
Simulation
of 6 ns peak,
100 ps jitter
100 x 1 mm
pixel 65 nm
Front end
(J. Kaplon,
CERN)
In time slow k or p
Threshold
Chan N
Pixel waveforms
Chan M
Out of time n, g …
In time pion
42
SiD(ILC)-Like Tracker
20 micron pixel
Vertex barrel
SiD-like tracker
with CMS-like
100m x 1 mm strips
50 micron pixel
Vertex disks
Tungsten absorber
cone
R. Lipton
43
Calorimeter Implementation
Fast timing will lose some
information from neutrons
Backgrounds form a pedestal in
each cell – fluctuations
determine resolution
• Segmented total absorbtion
calorimeter
– Merge PFA and Dual RO
concepts
– Design to control neutrons
– Utilize prompt arrival and
EM shower shape to
identify photons
andelectrons
R. Lipton
20 GeV pNo DR
correction
20 GeV pWith DR
correction
No slow neutron signal:
Before Dual Read out correction:
Mean: 15.5 GeV (reduced by 13.6 %)
s: 1.21+/-0.04 GeV
After DR correction: Mean: 20.5 GeV
s: 0.68+/-0.02 GeV
44
(Wenzel)
Event Yields
Based on counting experiment
stepping the beam across the
Higgs resonance
• I expect that detector
efficiencies and analysis cuts
will reduce yields by 10-20%
These results will have to be
confirmed with full simulation
including background
Summary and Conclusions
This was a glimpse of instrumentation at the energy frontier
I gave short shrift to or, neglected many things:
• Diamond detectors
• Triggering
• Data processing
Hopefully our Instrumentation Frontier report will provide a more
balanced overview.
There are many opportunities
for young people to get involved
• at Snowmass
• On LHC upgrades
• Generic detector R&D projects
R. Lipton
46
references
•
•
•
•
•
•
•
•
•
•
•
•
Particle Data Group web site
V. Radeka, Ann. Rev. Nucl. Particle Sci. 38 (1988) 217.
F. Sauli (GEM) Nucl. Instr. and Meth. A, 386 (1997), p. 531
Spieler - http://www-physics.lbl.gov/~spieler/USPAS-MSU_2012/index.html
Jaakko Härkönen, GSI/FAIR/NUSTAR/S-FRS seminar, Kumpula 6 October 2008
Systematic Errors and Alignment for Barrel Detectors, A. Seiden. Mar 1991. 8
pp. SDC-91-021
Velo –D.E. Hutchcroft, Initial results from the LHCb Vertex Locator, Nuclear
Instruments and Methods in Physics Research Section A: Accelerators,
Spectrometers, Detectors and Associated Equipment, Volume 648, Supplement
1, 21 August 2011, Pages S49-S50, ISSN 0168-9002,
http://dx.doi.org/10.1016/j.nima.2010.12.216.
Ren-yuan Zhu (CalTech) http://psec.uchicago.edu/workshops/fast_timing_conf_2011/
J.B. Birks, The Theory and Practice of Scintillation Counting, New York, 1964
G.F. Knoll, Radiation Detection and Measurement,New York, 1989
http://www.kip.uniheidelberg.de/~coulon/Lectures/Detectors/Free_PDFs/Lecture4.pdf
David Neuffer – Introduction to Muon Cooling
R. Lipton
47
Effect of dual read out correction: g ‘s from neutron
Capture discarded
20 GeV pNo DR
correction
20 GeV pWith DR
correction
May 8th 2013
Before Dual Read out correction:
Mean: 15.5 GeV
(reduced by 13.6 %)
s: 1.21+/-0.04 GeV
Hans Wenzel
After DR correction:
Mean: 20.5 GeV
s: 0.68+/-0.02 GeV