從未知世界來的光:
CDF實驗的光子物理
余欣珊
費米實驗室
清華大學高能物理演講
西元2009年2月26日
Shin-Shan Yu
Physics with Photons at CDF
1
Invisible Light from the Unknowns:
Physics with Photons at CDF
Shin-Shan Yu
Fermi National Accelerator Laboratory
High Energy Physics Seminar
National Tsing Hua University
February 26th, 2009
Shin-Shan Yu
Physics with Photons at CDF
2
Standard Model: Success
• Predicted existence of W, Z,
and gluon
• Unification of weak and
electromagnetic interactions
• Theory and experiments
agree over a wide range of
measurements
 Observed all SM particles
except Higgs
 e.g. W, Z mass with great
precision (better than 0.01%)
Shin-Shan Yu
Physics with Photons at CDF
3
Standard Model: Puzzles
1. Why are some particles so much heavier than the others?
What is the origin of mass?
 Higgs not found yet
1 top quark =
336000 electrons
2. Why are there so many kinds of elementary particles?
3. Do all the forces become one?
 Gravity not included yet
4. How to explain the anti-matter and matter asymmetry of
today’s world?
5. How do neutrinos mix?
To be continued …
Shin-Shan Yu
Physics with Photons at CDF
4
Physics Beyond Standard Model
1. Why are some particles so much heavier than the others?
What is the origin of mass?
 Technicolor
2. Why are there so many kinds of elementary particles?
 4th Generation, Compositeness, Leptoquark
3. Do all the forces become one?
 Supersymmetry, Extra Dimension
To be continued …
Shin-Shan Yu
Physics with Photons at CDF
5
Circle of Particle Physics
Puzzles
SM Parameters
New Particles
New Theory Parameters
Models That
Describe
New Results
One Theory?
Measurement
s of SM
Parameters
(Test of SM)
Studies
of
New Phenomena
Search for
New Phenomena
Theory
A Miracle!
Experiment
Shin-Shan Yu
Physics with Photons at CDF
6
Outline
 Overview of Particle Physics
• Tevatron and CDF
 Signatures in the Detector
• Why Photons
• Measurement of Differential Photon
Production Cross Section
• Signature-based Search for New Physics
with Photons
• Outlook
Shin-Shan Yu
Physics with Photons at CDF
7
Outline
 Overview of Particle Physics
•
•
•
•
•
Tevatron and CDF
Why Photons
Test of Standard Model Parameters
Search for New Phenomena
Outlook
Shin-Shan Yu
Physics with Photons at CDF
8
Fermilab Tevatron
and
Collider Detector at Fermilab (CDF)
Shin-Shan Yu
Physics with Photons at CDF
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Fermilab Tevatron
~70 km west of Chicago
99.99995%
of the speed of light
TeV=1012 eV
p
Shin-Shan Yu
s=1.96 TeV
0.98 TeV
0.98 TeV
?
Physics with Photons at CDF
p
10
Facts about Fermilab Tevatron
• Run I (1990-1995)
 s = 1.8 TeV
 110/pb per experiment
• Run II (2001- ?)
 s = 1.96 TeV
 2.0-2.5/fb in this talk
 ~ 4.8/fb per exp. so far
 ≥ 8.0/fb per exp. by the end of Run II
 Run until 2009-2010 (proposed to run until 2012,
10-15/fb)
Shin-Shan Yu
Physics with Photons at CDF
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Collider Detector at Fermilab (CDF)
• 600+ physicists
 61 institutions
 15 countries
(including Taiwan)
• 435 publications
 180 in Run II
• I am on CDF
2800
Weight
Ford
~ 4500
Mustangs
tons
5Size
Eiko~ ×
7.65 m
Eiko
cube
× 5 Eiko
Shin-Shan Yu
Physics with Photons at CDF
12
CDF Sub-Components
Muon chamber
Calorimeter
Hadron Cal.
EM Cal.
Tracking
System
=
Silicon Vertex Detector
+
Shin-Shan Yu
Drift Chamber
Solenoid
+
Physics with Photons at CDF
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The Magnificent Seven Signatures in the Detector
Momentum imbalance
Muon Chamber n
MET
Hadronic
Layers
Calorimeter
m EM Layers
Tracking system
jet
g
e
Shin-Shan Yu
t
jet
b-jet
m n
Physics with Photons at CDF
Beam Axis
px, y (m )  px, y ( j )  0
14
Why Photons?
Shin-Shan Yu
Physics with Photons at CDF
15
Photons in This Talk
• Focus on photons with energy > 25 GeV (25×109 eV)
Photon in this talk
10-16 m
1024 Hz
Visible light: 380-750 nm, 400-790 THz
Shin-Shan Yu
Physics with Photons at CDF
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Why I Don’t Like Photons
• Hard to get pure sample of photons for calibration
 Jets may be misidentified as photons, jet/photon ~ 1000
• Do not know where photon originates from
(contamination of non-collision background)
 Cosmic ray muons: bremsstrahlung due to interaction with
calorimeter
p
p
Cosmic ray muon
Shin-Shan Yu
Physics with Photons at CDF
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Why I Like Photons
• Copiously produced in p-pbar collisions, only after jets
 Test cross sections predicted by theory
• Energy well-measured by the EM calorimeter
 Measure mass of exotic particles
EM energy resolution > 2 times better than HAD energy resolution
• Photons do not decay
 No reduction in rates due to decay branching fractions
(BR ~10.8% for Wln and ~ 3.3% for Zll )
• High identification efficiency
 ~85% (if it does not pair-produce)
 For comparison, b-jet identification efficiency is 30-50%
• Many theories beyond the standard model predict
signatures with photons
Shin-Shan Yu
Physics with Photons at CDF
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Production Rates of SM Particles
Rate: N/sec
4 x 106
Jets (including b-jets)
cc
g
4 x 103
2 x 100
5 x 10-1
5 x 10-4 (2/hour)
Shin-Shan Yu
Physics with Photons at CDF
19
Models Which Predict Photon Signatures
•
Supersymmetry c1
0
~
g G or c20  gc10
 ggll MET, ggMET, ggjMET, gbjMET , gjjMET , gl MET , gMET ,
displaced gMET
•
Compositeness X*  g X
 ggbb, gll, ggll
•
Technicolor wT, rT  g pT
 gbb
•
Extra dimension G  gg or qq  Gg
 gg, gMET
•
Higgs gg (qq) H (W,Z) gg (W,Z), qq  H±h  4gW
 gg, ggl MET , ggMET, ggll , ggjj, gggg
•
4th Generation b'  Hb
 ggbbX
Shin-Shan Yu
Physics with Photons at CDF
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Published CDF Photon Analyses
Inclusive Photon
Cross Section (QCD)
Angular Distribution (QCD)
PRL 73 (94), PRD 48(93),
PRL 68 (92), PRL 71 (93),
PRD 65 (02), PRD 70 (04)
Photon + Dijet
Dijet Properties (QCD)
PRD 57 (98)
Photon + Muon
Intrinsic Charm (QCD)
PRL 77 (96), PRD 60 (99)
Photon + Jet + X
Jet h Distribution (QCD)
PRD 57 (98)
Photon + Lepton+X
Signature-based (Search)
PRD 66 (02), PRL 89 (02),
PRL 97 (06), PRD 75 (07)
Photon + Z or W
Anomalous Couplings (EWK)
Cross Section (QCD)
PRL 74 (95)
PRD 73 (06), PRL 94 (05)
Photon + b-jet + X
Techni-Omega, Signaturebased (Search)
PRL 83 (99), PRD 65 (02)
Photon + Ds
W -> photon + Ds (EWK)
PRL 77 (96), PRD 58 (98)
Photon + Track
W -> Photon + pion (EWK)
PRL 76 (96), PRL 69 (92)
PRL 58 (98)
Delayed Photon
GMSB (Search)
PRL 99 (07), PRD 78 (08)
Shin-Shan Yu
Physics with Photons at CDF
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Published CDF Photon Analyses
Exclusive Photon + Met
Extra Dimension (Search)
PRL 89 (02), PRL 101 (08)
Photon + Dilepton
Excited lepton (Search)
PRL 97 (06), PRL 94 (05)
Exclusive Diphoton
Diffractive (QCD)
PRL 99 (07)
Diphoton + X
Signature-based (Search)
PRL 81 (98), PRD 59 (99)
Diphoton + Met
GMSB (Search)
PRD 71 (05)
Diphoton
Cross Section (QCD)
PRL 70 (93), PRL 95 (05)
High-mass Diphoton
RS Graviton (Search)
PRL 99 (07)
Diphoton + W/Z
Fermiophobic Higgs (Search) PRD 64 (01)
• 38 Papers published in Physical Review Letters and
Physical Review D
 Electroweak and Strong forces (QCD), Physics beyond
Standard Model
 A big output from a small group
Shin-Shan Yu
Physics with Photons at CDF
22
Circle of Particle Physics
Puzzles
SM Parameters
New Particles
New Theory Parameters
Models That
Describe
New Results
One Theory?
Measurement
s of SM
Parameters
(Test of SM)
Studies
of
New Phenomena
Search for
New Phenomena
A Miracle!
Shin-Shan Yu
Physics with Photons at CDF
23
Measurement of Differential
Photon Production Cross Section
Work with R. Culbertson (Fermilab),
C. Deluca, S. Grinstein, M. Martinez (IFAE, Barcelona)
Shin-Shan Yu
Physics with Photons at CDF
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Test of Strong Interaction
X-section
Non-perturbative
Proton Structure
=
Universal

Proton
Process-dependent
structure
(PDF)
perturbative
SM
g
BSM
X
Parton Distribution Function (PDF):
fraction of proton momentum carried by parton
• Test perturbative QCD
 Use PDF obtained from DIS, Drell-Yan, jet cross-sections
• Potentially provide information on the non-perturbative
part (PDF)
 PDF used to predict X-section of new physics
Shin-Shan Yu
Physics with Photons at CDF
25
Why Is Gluon PDF Important
• Photon x-section advertised for being useful to gluon PDF
 Previous photon x-section results are not used in current PDF fits.
More discussions later.
• Dominant Higgs production: gluon gluon fusion
Shin-Shan Yu
Physics with Photons at CDF
26
Experimental Motivation
• Advantages over jets




Ease of trigger requirements allows access to lower pT
Do not hadronize, no ambiguity due to jet definitions
Better energy resolution
Less uncertainty on energy scale (1-2% vs. 2-3%)
• Probe photon techniques over a wide energy range
• First CDF Run II measurement
 Focus on central photons (|h| < 1.0)
 A new method to estimate backgrounds from jets
N total  f sig
d 

dpT dh pT hLU
2
Shin-Shan Yu
fsig: fraction of signal photons
L: number of p pbar per unit x-section
U: unfolding factor for efficiency, energy
scale, energy smearing
Physics with Photons at CDF
27
Major Backgrounds
Dominant at low pT
Dominant at high pT
Mesons in Jets Decaying to Multiple
Photons:p0, h0,Ks0
g
g
p0
p
BREM of Cosmic ray muon
g
p
Signal MC
Data
N total  f sig
d 

dpT dh pT hLU
2
Shin-Shan Yu
Physics with Photons at CDF
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Amount of Cosmic Background
MET/ET (g)< 0.8 reduces the
cosmic background to the
level of < 1%
Shin-Shan Yu
Physics with Photons at CDF
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Cosmic Events
• Use “photon arrival time at the EM calorimeter – collision
time” (EM Timing)
System intrinsic resolution ~ 0.6 ns
Shin-Shan Yu
Physics with Photons at CDF
30
Central Electromagnetic Calorimeter
• EM Calorimeter
Hadron Calorimeter
 (E)/E =
0.135/ET+0.02
 500-cm long
 f×h=0.26 × 0.11
• CMS 0.017 × 0.017
Electromagnetic Calorimeter
 18 X0
EM Cal. unit: 45.5 cm (width)
× 22.7 cm (length) × 34.5 cm (depth)
Minimum separation of two photons from p0:
50/ET(p0) [cm GeV], e.g. 5 cm for 10 GeV p0
Single EM shower size ~ 3.5 cm
Shin-Shan Yu
Physics with Photons at CDF
31
Photon Isolation
More isolation energy
Photons less isolated
Photons from jets have more surrounding
tracks and energy (more isolation)
g
Isolation
energy
Energy of the
jet
photon
Calorimeter isolation
g
DATA
signal
background
Isolation
Shin-Shan Yu
Physics with Photons at CDF
32
Fitting Isolation
E iso  Econe  Eg  E leakage  E MI
pT = 200-230 GeV
Data
Fit result
Signal
Background
Shin-Shan Yu
Physics with Photons at CDF
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Systematics Study (Signal Fraction)
• Use Z electron data for signal template
Electrons look like photons in the calorimeter
• Decrease the number of histogram bins to 2
Removing details on the shape
2-bin method
2 unknowns
2 equations
Shin-Shan Yu
Physics with Photons at CDF
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Fraction of Signal Photons
Signal fraction 70-100%
Systematic uncertainty 13% first bin, 5% for the rest
Shin-Shan Yu
Physics with Photons at CDF
35
Unfolding Factor
• Correct detector level
back to parton level
 energy scale, energy
resolution, acceptance,
efficiency
• Systematic uncertainties
 Photon energy scale (613%)
 ID efficiencies (3-5%)
N total  f sig
d 2

dpT dh pT hLU
Shin-Shan Yu
Physics with Photons at CDF
36
Data
rec
ET
true
ET
MC 
? data
Differential X-section
How Well Do We Know the Energy?
Data
ET(g)
• Photon energy affect the shape of differential x-section
• Compare reconstructed Zee mass peak in data and MC
 Apply time-dependent energy calibration
• Assign 1.5% systematic uncertainty
 Fitting model, energy dependence of scale
Shin-Shan Yu
Physics with Photons at CDF
37
Systematic Uncertainty (X-section)
• Major sources
 Signal fractions at
low energy
 Photon energy scale
at high energy
• Total systematic
uncertainty 12-15%
Shin-Shan Yu
Physics with Photons at CDF
38
Cross Section Result
• X-sections measured over 6 orders of magnitude
• Cover the energy range from 30 to 400 GeV
• Excess in data below 50 GeV?
Shin-Shan Yu
Physics with Photons at CDF
39
Why Are Photon X-sections Not Used in PDF?
• Data have steeper
dependence on xT
• Due to mis-modeling
of soft gluon emission
in the initial state
 Additional transverse
momentum smearing
of initial parton (kT)
resolved the
data/theory difference
xT=2pT/s
Shin-Shan Yu
Physics with Photons at CDF
40
Remarks
• Effects other than kT at low pT?
• kT is ad-hoc, better theoretical calculation needed
 Resummation of soft gluon emission
 Need to understand the difference between Run II CDF and D0
results
 10% difference in normalization
• May provide input to gluon PDF for photons pT > 50 GeV?
• QCD Compton process dominant for all pT at LHC
• Low pt region may be useful when the theoretical
calculation improves in the future
Shin-Shan Yu
Physics with Photons at CDF
41
Circle of Particle Physics
Puzzles
SM Parameters
New Particles
New Theory Parameters
Models That
Describe
New Results
One Theory?
Measurement
s of SM
Parameters
(Test of SM)
Studies
of
New Phenomena
Search for
New Phenomena
A Miracle!
Shin-Shan Yu
Physics with Photons at CDF
42
Search for New Physics with
Photons
Shin-Shan Yu
Physics with Photons at CDF
43
Two Approaches
Model-dependant search
Model  New Particles  Signatures
Supersymmetry  Bosonic Higgs  2 photons with a W or Z boson
 Optimize selections based on model
 Report limits on theory parameters (e.g. MH > 106 GeV)
Signature-based search
Form 2-object combinations out of the 7 magnificent objects,
Start from these 27 base signatures and look for additional objects
gg + X, g MET + X




Aim to maximize the chance of discovery
Apply a set of various requirements
Report event counts and kinematic distributions
Signature chosen based on past experience, detector strength, etc.
Shin-Shan Yu
Physics with Photons at CDF
44
The Magnificent Seven Signatures in the Detector
Muon Chamber
Hadronic Layers
Calorimeter
EM Layers
Tracking system
g
e
Shin-Shan Yu
t
jet
b-jet
m n
Physics with Photons at CDF
Beam Axis
45
CDF Photon Signature-based Search
•
•
•
•
•
HW
gg + X, X = e, m, t, MET
g + e or m + X, X = b-jet, MET, e, m, g
gMET + b-jet + jet
Displaced g + jet
gMET+ n jets
 ggen
Analyses I worked on
Shin-Shan Yu
Physics with Photons at CDF
46
Search for New Physics in gge/ggm
g
NP
g
m (e)
Data consistent with background
prediction. No evidence of new
physics, yet.
Shin-Shan Yu
Physics with Photons at CDF
47
Search for New Physics in
gbjMET
Work with R. Culbertson (Fermilab),
H. Frisch, D. Krop, C. Pilcher, S. Wilbur (U. Chicago)
b
NP
g
j
Shin-Shan Yu
Physics with Photons at CDF
48
Major Backgrounds
• Combination of real and fake objects
 CES/CPR method
for fake photons from jets
EM Calorimeter
CES
CPR
Solenoid
Central Shower Profile Detector (CES)
Copper Strips
on PC board
Central Preshower Detector (CPR)
2 mm resolution
Au-plated W Wires
Shin-Shan Yu
Physics with Photons at CDF
49
CES Method
• A method to estimate how likely a photon candidate is a jet
• Jet has wider profile
Compare measured
with expected and
form a c2
Jet
g
p0
Shin-Shan Yu
g
g
Physics with Photons at CDF
50
CPR Method
• A method to estimate how likely a photon candidate is a jet
• Jets have more photons to covert into electrons
g
g
CPR
electron CPR efficiency ~ 100%
Jet
g
e
e
p0
Shin-Shan Yu
g
g
  1 e
Physics with Photons at CDF
CPR

7 M  Ng
9
51
Statistical Technique
• CES: edata = 1 if c2 < 4; otherwise edata = 0
• CPR: edata = 1 if there is a CPR hit; otherwise
edata = 0
• esig and ebkg are calibrated
E(g) [GeV]
Shin-Shan Yu
Physics with Photons at CDF
52
Systematic Uncertainties (CES/CPR)
• Uncertainty 10-15%
• Dominant source
 CPR: hits contributed by
backscattered low-energy
particle (albedo)
EM Calorimeter
Preshower
Detector
Albedos
Shin-Shan Yu
Physics with Photons at CDF
53
Kinematic Distributions (gbjMET)
HT
Shin-Shan Yu
Physics with Photons at CDF
54
Kinematic Distributions (gbjMET)
Shin-Shan Yu
Physics with Photons at CDF
55
Event Counts
Data consistent with
background prediction. No
evidence of new physics, yet.
Observed 617 events in data
Apply tighter cuts. Then, compare data and background estimate.
Shin-Shan Yu
Physics with Photons at CDF
56
Model Phase
• After a large set of signatures are looked for, perform a
global constraint on models
Parameter 2
eejj
gbjMET
gge
ggm
ggt
gggg
eeee
Shin-Shan Yu
Physics with Photons at CDF
Parameter 1
57
Model-dependant vs. Signature-based
• Optimized for model A
 Precise limits
• Less sensitive to model A
 Maximize the phase space
probed
• Background estimate for
the optimized cuts
 Good training ground for
students
• Report limits on new
theory parameters
 Immediately useful for
theorists
 Easy for comparison
between experiments
• Background estimate for
several sets of cuts
 Advance techniques
• Report event counts and
kinematic distributions
 Results will not be obsolete
• May miss very peculiar
signatures or distributions
 Guidance from theorists
Shin-Shan Yu
Physics with Photons at CDF
58
Outlook (Tevatron)
• Searches with more data
 More sensitive to new physics
S
B L
• Apply isolation and CES/CPR methods to other crosssection measurements and searches
• Explore forward photons
 Increase di-photon data by at least a factor of 3
 Probe parton in a wider x range ? x = M/s exp(±y)
• Advantages over Large Hadron Collider (LHC)
experiments
 Less tracking material (2-5 times smaller), good for photons
 Access to b-jets (before LHC silicon vertex detectors are fully
commissioned)
 Access to peculiar signatures
Shin-Shan Yu
Physics with Photons at CDF
59
Outlook (LHC)
• Expect to have 1st collision
by the end of 2009 at 10
TeV
• Advantages over Tevatron
 5-7 times s
 10-100 times instantaneous
luminosity
 Better detector
• 5x better EM energy
resolution
• 10x better momentum
resolution
• More coverage
• More uniform
• Higher granularity
Shin-Shan Yu
Physics with Photons at CDF
60
Conclusions
• Presented physics results with photons
 Test of Standard Model with cross-section
measurement
 Systematic signature-based search for new physics
with photons
• Both Tevatron and Large Hadron Collider have
excellent prospects of discoveries. Will be lots
of fun!
Shin-Shan Yu
Physics with Photons at CDF
61
Circle of Particle Physics
Puzzles
SM Parameters
New Particles
New Theory Parameters
Models That
Describe
New Results
One Theory?
Measurement
s of SM
Parameters
(Test of SM)
Studies
of
New Phenomena
Search for
New Phenomena
A Miracle!
Shin-Shan Yu
Physics with Photons at CDF
Theory
Experiment
62