Search for New Phenomena in the CDF Top Quark Sample
Kevin LannonThe Ohio State UniversityFor the CDF Collaboration
SMU Seminar 2-5-07 K. Lannon 2
Why Look in Top Sample?
Top only recently discovered Top turned 10 in 2005 Samples still relatively small Still plenty of “room” for
unexpected phenomena
Top is really massive Comparable to gold nucleus! Yukawa coupling near unity Special role in EWSB?
Many models include new physics coupling to top
0.001
0.01
0.1
1
10
100
1000
u d
sc
b
t
Quark Masses
GeV/c2
5 orders of magnitude between quark masses!
SMU Seminar 2-5-07 K. Lannon 3
What Might We Find?
It’s not Standard Model top at all! Charge not 2/3? [Phys.Rev.D59:091503,1999; Phys.Rev.D61:037301,2000]
Spin not 1/2?
It’s not only Standard Model top Additional heavy particles decaying to high pt leptons, jets and
missing energy (t ’) [Phys.Rev.D64:053004,2001; Phys.Rev.D65:053002,2002]
Heavy resonance decaying to tt [Phys.Lett.B266:419,1991]
tH+b ttH production [Phys.Rev.D68:034022,2003]
Nothing but the Standard Model . . . . Not as bad as it sounds Test our abilities to calculate signal and background properties
Important at the LHC top becomes background to other searches Constrains models that put new physics in the top sample
[hep-ph/0504221]
SMU Seminar 2-5-07 K. Lannon 4
The Tevatron and CDF Tevatron accelerator
Highest energy accelerator in the world (Ecm = 1.96 TeV)
World record for hadron collider luminosity (Linst = 2.72E32 cm-2s-1)
Only accelerator currently making top quarks
Central Cal
Plug Cal
CentralTracker
Silicon Tracker
Muon Detectors
CDF Detector Trigger on high pT leptons,
jets and missing ET
Silicon tracking chamber to reconstruct displaced vertices from b decays
SMU Seminar 2-5-07 K. Lannon 5
Tevatron Performance
Integrated luminosity at CDF Total delivered: ~2.3 fb-1
Total recorded: ~1.9 fb-1 (~ 17 Run I!) So far for top analyses, used up to 1 fb-1
More analyses with 1.0-1.2 fb-1 in progress for winter and spring Doubling time: ~1 year Future: ~4 fb-1 by 2007, ~8 fb-1 by 2009
Peak LuminosityToday’s Presentation:
200 pb-1 ~ 1 fb-1
Integrated Luminosity
Analyzed by Summer
SMU Seminar 2-5-07 K. Lannon 6
Triggering on Top Need high efficiency, low fake
rate trigger for high pT leptons Relies on track trigger (XFT)
Fake rate increases with occupancy Occupancy increases with
luminosity 3x higher than original design
because Tevatron didn’t reduce bunch spacing (392 ns 132 ns)
Z ee at low lum.9 add. Int./crossing
fakeFake tracks can be made from segments of different real physical tracks.
Trigger for muons without upgrade
Missing segments
Instrumenting additional layers reduces fake rate. Efficiency stays high.
Upgrade put into operation in October Efficiency = 96% for high pT tracks Fake track rejection factor = 5-7
Reduction factor ~ 4
SMU Seminar 2-5-07 K. Lannon 7
Top Quark Production at Tevatron
~85% ~15% QCD pair production
NLO = 6.7 pb First observed at
Tevatron in 1995
EWK single-top production s-channel: NLO = 0.9 pb
t-channel: NLO = 2.0 pb Not observed yet
s-channel t-channel
Other?: ???,0 ttX Htt
(and LHC)
833 pb
~13% ~87%
10.6 pb
247 pb
Associated tW
62 pb
SMU Seminar 2-5-07 K. Lannon 8
Top Production Rates
Like finding a needle in a haystack . . . .
pb7.6)175@( GeVMttpp top
One top pair each 1010 inelastic collisions at s = 1.96 TeV
Efficient Trigger ~90% for high pT leptons
Targeted event selection Distinctive final state Heavy top mass
Advanced analysis techniques Artificial Neural Networks
Needle in haystack (approx.)
SMU Seminar 2-5-07 K. Lannon 9
SM Top Quark Decays
Particular analysis usually focuses on one or two channels New physics can impact different channels in different ways Comparisons between channels important in search for new physics
BR(tWb) ~ 100%
SMU Seminar 2-5-07 K. Lannon 10
Top SignaturesElectron or
muon: pT > 20 GeV
Neutrino: Missing ET > 20-25 GeV
Jet: ET > 15-20 GeVcone = 0.4
b-jet: identified with secondary vertex tag
Dilepton Lepton + Jets All Hadronic
,e
bbtt
,e
bqqbtt
bqqqqbtt
SMU Seminar 2-5-07 K. Lannon 11
Top Event Yields
To give an idea of CDF sample sizes . . . . Based on top cross section of 6.7 pb Background and signal numbers based on event yields from
current analyses, scaled by luminosity Assume no changes in event selection, efficiency, etc.
Luminosity 1 fb-1 4 fb-1
Total Top Events 6700 26,800
Decay Mode Dil. L + J L + J (b-tag) Dil. L + J L + J (b-tag)
Before Event Selection
330 1985 1325
7940
Selected Signal Events
50 480 290 190 1910 1140
Expected Background
40 2290 160 150 9150 670 L+J: ~2k signal events with 4 fb-1 (signal:background ~ 1 : 5) L+J (b-tag): ~1k signal events with 4 fb-1 (signal:background ~ 2:1)
SMU Seminar 2-5-07 K. Lannon 12
Searching for New Physics Precision study of
top properties Non-SM behavior from
top quark Evidence of
something other than top in sample
Direct search for new phenomena in top sample Resonant production Non-SM decays New particles with
“top-like” signature New particles
produced in association with top
Vtb
SMU Seminar 2-5-07 K. Lannon 13
Top Properties Working Group
Studying all properties of top quark (except mass) ~ 150 faculty,
postdocs, students ~15 papers (so far) ~50 active analyses
Vtb
SMU Seminar 2-5-07 K. Lannon 14
Precision Study: Cross Section Cross section
Measured in different final states New physics can affect different
final states differently Different techniques used in
same final state Results combined at end for
most precise answer tt production calculated to NLO
Central value: 6.7 pb — 6.8 pb Uncertainties: 5.8pb — 7.4 pb For mtop = 175 GeV/c2
Combined result: 7.3 0.9 pb
A
NTop NTop = Nobs- Nbackground, or from fit
SMU Seminar 2-5-07 K. Lannon 15
Two Best Measurements
Both in Lepton + Jets Channel Vertex Tag (weight = 0.50, pull = + 0.88)
Uses b-tagging to increase ratio of signal to backgroundCounting experiment
Count W+jets events with a b-tagSubtract expected backgroundExcess attributed to top
Kinematic Artificial Neural Net (weight = 0.32, pull = -1.14)Uses kinematic variables to separate signal from backgroundCombines several variables in a neural network to increase
sensitivityFit for the number of top eventsDoes not use b-tagging (lower signal to background ratio)
SMU Seminar 2-5-07 K. Lannon 16
B-Tagging
b-tagging: Identifying jets containing a b quark Take advantage of
long b lifetime Look at precision
tracking information for tracks within jet
Reconstruct secondary vertices displaced from primary
Efficiency Per jet
40% for b jet 9% for c jet 0.5% for light jet
Per event (tt ) 60% for single tag 15% for double tag
SMU Seminar 2-5-07 K. Lannon 17
Sample Composition
Number of events with an identified W + 1 jets
Signal regionControl region695 pb-1
Backgrounds that produce W + jets signature
Excess of data over background attributed to top production
Agreement between data and background checks accuracy of background estimate
SMU Seminar 2-5-07 K. Lannon 18
Lepton + Jets Vertex Tag Result
One Tag + HT Cut 8.2 ± 0.6 (stat.) ± 1.0 (sys.)
pb
Two tags, no HT Cut Cross check 8.8 +1.2 -1.1 (stat.) +2.0 -1.3 (sys.)
pbHT = scalar sum of lepton, jet, and missing ET
SMU Seminar 2-5-07 K. Lannon 19
Using Kinematics to Identify Top
Look for central, spherical events with large transverse energy Signal: PYTHIA tt monte carlo Background: ALPGEN + HERWIG W + 3p monte carlo
• Normalized to unit area
• HT scalar sum of lepton, jet, and missing ET
• Aplanarity uses lepton, jet and missing ET
• Max jet uses 3 highest ET jets; all others use 5 highest
SMU Seminar 2-5-07 K. Lannon 20
Statistical Sensitivity Evaluate expected fit
fractional error using MC-based pseudo experiments Single variable fits: fit signal
fraction using distributions of a single kinematic variable
Plotted Points: median fit fractional
error Error bars: 68% interval
SMU Seminar 2-5-07 K. Lannon 21
Multivariate Approach: Neural Nets
Structure Composed of nodes modeled
after neurons in nervous system Organized into layers
Input layer: initialized by input variables
Hidden layer: takes information from each input node and passes to output layer
Output node: new discriminating variable with range [0,1]
Training Neural net output determined by
exposure to training data Iteratively adjust parameters to
minimize error:
Training accomplished through JETNET program(Peterson et al. CERN-TH/7135-94)
7 kinematic variables 7 input nodes
Output node—range [0,1]—signal = 1
1 hidden layer, 7 hidden nodesInformation flow
SMU Seminar 2-5-07 K. Lannon 22
Statistical Sensitivity Evaluate expected fit
fractional error using MC-based pseudo experiments Single variable fits: fit signal
fraction using distributions of a single kinematic variable
NN: fit NN output of data to NN templates
Plotted Points: median fit fractional
error Error bars: 68% interval
NN Fit performs significantly better than single variable fits
SMU Seminar 2-5-07 K. Lannon 23
Using NN to Fit Data Basic Approach
Train NN to distinguish tt signal from backgrounds
PYTHIA tt MC as signal model ALPGEN + HERWIG W + 3p MC as
background model Use this NN to make templates
for fitting the data Use same signal model as above Also extract QCD multijet
template from data Supplement electroweak template
with contributions from other processes: WW,WZ, Z + jets, single top
Fit templates to NN distribution from data
Binned maximum likelihood fit Three component fit
Signal and electroweak float QCD constrained to value
estimated using isolation vs missing ET method
SMU Seminar 2-5-07 K. Lannon 24
Lepton + Jets Kinematic ANN Result
Sample Events Fitted tt (tt )
W + 3 Jets 2102 324.6 31.6 6.0 0.6 0.9 pb
W + 4-Jet 461 166.0 22.1 5.8 0.8 1.3 pb
SMU Seminar 2-5-07 K. Lannon 25
Kinematics of b-Tagged Events
Looks like top!
SMU Seminar 2-5-07 K. Lannon 26
Systematic Uncertainties
Main Systematic Uncertainties uncorrelated Lepton + Jets Vertex Tag
b-tagging efficiency: 6.5%Background estimation: 3.4%
Kinematic ANNBackground shape modeling: 10.2% Jet Energy Scale: 8.3%
For both results, uncertainty dominated by systematics
Both are working to reduce for 1.2 fb-1 publications
SMU Seminar 2-5-07 K. Lannon 27
Search for t H+b Compare top yield in four different channels
Measurements consistent with SM Consider correlated effect of tH+b decays on four channels Exclude when changes make expectation inconsistent with data Limits for 6 sets of MSSM parameters and less model-specific scenarios
Varying model parameters changes:BR(tH+b)
BR(H+)BR(H+cs)BR(H+t*b)BR(H+W+h0)BR(H+W+A0)
Shown here: Variations as a function of tan particular set of MSSM parameters
Phys.Rev.Lett. 96 (2006) 042003
SMU Seminar 2-5-07 K. Lannon 28
MSSM Limits
Calculate BR(tH+b) and H+ BR’s as a function of MH+ and tan()
Use 6 different MSSM “benchmarks” Results for “Benchmark #1” shown below
SMU Seminar 2-5-07 K. Lannon 29
Less Model Dependent Limit “Tauonic Higgs” Model
Assume BR(H+) = 1 i.e. MSSM with high tan()
“Worst” Limit Find arbitrary combination of
H+ BR’s that give least stringent limit
SMU Seminar 2-5-07 K. Lannon 30
t’ Production
Consider possible contribution to “top” sample from heavier particles with “top-like” signature (t’)
Examples 4th chiral generation consistent with precision EWK data
[Phys. Rev. D64, 053004 (2001)] “Beautiful Mirrors” Model: additional generation of quarks that
mix with 3rd generation [Phys. Rev. D65, 053002 (2002)]
Consider decay of t’Wq Happens when mt’ < mb’ + mW
Precision EWK data suggests mass splitting between b’ and t’ small
Search for by fitting HT vs Mreco
HT = sum of transverse momenta of all objects in event
Mreco = Wq invariant mass reconstructed with a 2 fitter (same technique used in top mass reconstruction)
SMU Seminar 2-5-07 K. Lannon 31
t’ Search Results
No evidence for t’ observed
Set 95% confidence level limits on t’BR(t’Wq)2
Exclude mt’ < 258 GeV for BR(t’Wq) = 100%
Interesting behavior in high mass tails
SMU Seminar 2-5-07 K. Lannon 32
Resonant top production:No evidence seenExclude Leptophobic Z’ with Mz’ < 725 GeV/c2
Top Quark Lifetime (~10-24s in SM)Result: c < 52.5 m at 95% confidence levelConsistent with detector resolution.
W Helicity in Top Decay:SM: F0 = 0.7, F- = 0.3, F+ = 0.0Result: F0 = 0.610.13, F+ < 0.09
Summary
There are many more CDF results than I could show here.
Top Mass measured to 2.4 GeV/c2 (1.4%) uncertainty!
Even More results on the public webpagehttp://www-cdf.fnal.gov/physics/new/top/top.html No deviations from Standard Model so far
Many results statistically limited More results with 1-1.2 fb-1 coming soon Results for ~2fb-1 by this summer
Many new and updated analyses in progress Improved cross section measurements Single-top Top charge Flavor changing neutral currents Direct search for tH+b
http://www-cdf.fnal.gov/physics/new/top/top.html
SMU Seminar 2-5-07 K. Lannon 33
The Future: Top at LHC “Top physics will be easy at
the LHC” Top cross section increases
by factor of ~ 100 Background cross sections
increase by factor of ~10 Probe for new Physics
Mtt distribution Associated Higgs production:
ttH Even used for LHC detector
calibrations High precision results from
Tevatron important Discover new physics ~ 1-2 GeV/c2 precision on
mass Production and decay well
understood
Looks a little like B physics at the Tevatron
precision physics
SMU Seminar 2-5-07 K. Lannon 34
Extra Slides
SMU Seminar 2-5-07 K. Lannon 35
Top Cross Section vs Mass
SMU Seminar 2-5-07 K. Lannon 36
Search for Resonant Production Motivation
Some models predict particles decaying to top pairs
Should be visible as resonance in tt invariant mass spectrum
Example model: Topcolor assisted technicolor Extension to technicolor
that includes new strong dynamics
Couples primarily to 3rd generation
Includes new massive gauge bosons: topgluons and Z’
pp X 0 tt
SMU Seminar 2-5-07 K. Lannon 37
Search for Resonant Production Look for generic, spin
1 resonance (X0) decaying to top pairs Assume X0 =
1.2%MX0
Test masses between 450 GeV and 900 GeV in 50 GeV increments
Results No evidence for
resonance Set 95% confidence
level limit for X0 at each mass
Exclude leptophobic Z’ with Mz’ < 725 GeV
SMU Seminar 2-5-07 K. Lannon 38
W Helicity in Top DecayW Helicity in Top Decay
V-A Forbidden
W0 Longitudinal fraction
F0
W- Left-Handed fraction F-
tb
W
+1/2-1/2
+1
W+ Right-Handed fraction F+
tW
b
+1/2+1
-1/2
Helicity of W determined by V-A structure of EWK interaction 70% longitudinal 30% left-handed Right handed forbidden
tW+1/2
+1/2
0W
b
SMU Seminar 2-5-07 K. Lannon 39
W Helicity in Top Decay
Can be tested by measuring W helicity angle: *
* = angle of the lepton relative to negative the direction of the top in the W rest frame.
Can also use Mlb
2 0.5(mt2-mW
2)cos *
SMU Seminar 2-5-07 K. Lannon 40
W Helicity Results
Two CDF results with 955 pb-1
Use different kinematic fitters to reconstruct tt system: cos*
Very consistent measurements of F0 and limits on F+
F0 = 0.61 0.12(stat) 0.04 (syst) and F+ < 0.11 at 95% C.L.
F0 =0.59 0.12(stat) +0.07-0.06(syst)
and F+ < 0.10 at 95% C.L.
One measurement with 750 pb-
1
Uses Mlb and measures fraction of V+A
FV+A < 0.29 at 95% C.L.
Assuming F0 = 0.7 F+ < 0.09 at 95% C.L.
SMU Seminar 2-5-07 K. Lannon 41
Top Quark Lifetime Measure impact parameter of lepton
from Lepton + Jets top decay Evidence of displaced top suggests
Production via decay of long-lived particle New long-lived particle in top sample Anomalous top lifetime
Templates for SM processes
Result: c < 52.5 m at 95% confidence level
SMU Seminar 2-5-07 K. Lannon 42
Sample Composition
W+light flavor:From pretag using mistag matrixW+heavy flavor:From pretag using MC for HF fraction and b-tagging eff.
Single Top and Diboson: Estimated using theoretical cross section
Non-W QCD: Estimated from MET and lepton isolation side-bands
Difference between observed and predicted background attributed to top
Event count before applying b-taggingNumber of events with an identified W + 1 jets
SMU Seminar 2-5-07 K. Lannon 43
The Search for Single Top Standard Model
Rate |Vtb|2
Spin polarization probes V-A structure
Background for other searches (Higgs)
Beyond the Standard Model Sensitive to a 4th generation Flavor changing neutral
currents Additional heavy charged
bosonsW’ or H+
New physics can affect s-channel and t-channel differentlyTait, Yuan PRD63, 014018(2001)
SMU Seminar 2-5-07 K. Lannon 44
Signal and Backgrounds
W + Heavy Flavor
W + Light Flavor (Mistags)
Multi-jet QCD
tt
Other EWK
Total Background: 64696 events
Expected Single-Top: 28 3 events
Signal / Background ~ 1/20
e or : pT > 20 GeV: MET> 20 GeV
2 jets: ET > 15 GeV, 1 b-tag
Single-top Signature Backgrounds
Must use multivariate, kinematic techniques to separate signal from background
SMU Seminar 2-5-07 K. Lannon 45
Multivariate Discriminants
Improve signal discrimination by combining several variables into a multivariate discriminant Neural Network and multivariate likelihood function both used
Variables: ℓb and dijet invariant masses, HT, Q, angles, jet ET and , W-boson , kinematic fitter quantities, NN b-tag output
ZOOM
SMU Seminar 2-5-07 K. Lannon 46
Single Top Multivariate Likelihood Result
Best fit result for s- and t-channel separately s-channel: t-channel:
95% CL upper limit on combined s- + t-channel:
pb2.0 9.02.0
pb1.0 7.01.0
SMU Seminar 2-5-07 K. Lannon 47
Single Top Neural Network Result
Combined search: s-channel + t-channel
combined in SM ratio Best fit:
95% CL Limit:
pb(syst.).)(8.0 2.03.0
3.18.0
stat
pb4.3
Separate search s- and t-channel vary separately Best Fit:
t-channel: s-channel:
95% CL Limit: t-channel: s-channel:
pb(syst.)1.0(stat.)6.0 9.16.0
pb(syst.)(stat.)3.0 5.03.0
2.23.0
pb1.3pb2.3
SMU Seminar 2-5-07 K. Lannon 48
Single Top Matrix Element Result
Best fit result: pb7.2 5.13.1
SMU Seminar 2-5-07 K. Lannon 49
Summary This is an exciting time to be at the Tevatron
1.2 fb-1 sample currently in hand and being analyzed Top sample has grown from ~30 events in Run I to ~ several hundred
Larger samples coming soon (almost 2 fb-1) by summer Analysis techniques becoming increasingly mature and sophisticated Look forward to 1 fb-1 publications this winter
No evidence for new physics in top sample so far Have many more top measurements than covered in this talk (see CDF
public results webpage) Increasing precision continues to test consistency of measurements in
different channels Many new analyses on their way (as well as updates of current results)
Improved cross section measurements Single-top Top charge Flavor changing neutral currents Direct search for tH+b
Top Related