Transcript Slide 1

PH599 Graduate Seminar presents:
Discovery of Top Quark
Karen Chen
Stony Brook University
November 1, 2010
Abstract
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Karen Chen
The third generation of quarks was predicted by Kobayashi and Maskawa to
explain CP violation. When the bottom quark was discovered, the search to
find its isospin partner began. The discovery of the top quark completes
the family of six quarks in the Standard Model. Measurements of the top
quark mass were conducted by the CDF and D0 experiments at the
Fermilab Tevatron. The top quark mass was measured from events
consistent with top pair production. The top pairs decay into a pair of
bottom quarks and a pair of W bosons with a nearly 100% branching ratio.
The experiments looked at events that result in either dilepton or lepton
plus jets final states. It is possible for the W bosons to both decay into
quarks but measurements based on events with all jets have low precision.
More precise mass measurements were conducted after the top quark’s
initial discovery. The experimental uncertainty associated with the top
quark and W boson mass puts constraints on the mass of the Higgs boson.
The high precision of the top quark mass may have important implications
on the validity of the Standard Model.
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Outline
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Discovery
 3rd generation quarks to explain CP violation
Direct Measurement of Top Quark Mass
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Experiments at the Tevatron, detector basics
Decay of top pairs
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Event Selection
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Signatures of signal and background processes
Likelihood fits for mt
Top Quark Mass Relevance Today
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Possible final states: Dilepton, l+jets
Constraints on Higgs mass
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Discovery of the top quark
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1964 - CP violation found in kaons
1973 – (Cabibbo), Kobayashi and Maskawa
 Need a third generation of quarks to explain CP
violation
1977 – Bottom quark discovered!
 And thus begins the search for its isospin partner,
the top quark
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Discovery of the top quark
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Tevatron – particle
accelerator at Fermilab
Two experiments:
 Collider Detector at
Fermilab
 D0 (or DZero)
Proton, antiproton
collisions
CDF and D0 measured
the top quark mass
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Measurement of Top Quark Mass
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Invariant Mass, m = m(E,p)
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To find top quark mass, we need the energy and
momentum of the decay products.
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Karen Chen
E2 = (pc)2 + (mc2)2
Or more conveniently*:
 m2 = E2 – p2 = P2, where P is four vector
momentum,
 P2 = E2 – p2 = E2 - px2 - py2 - pz2
 Example of a two body decay
 A  B+C
 mA2 = (PB+ PC)2
Note: It is convenient to measure everything in units of GeV, so c is set to 1.
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Karen Chen
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Decays of top pairs
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Top pair decay with branching ratio ~100%
p p  tt  bbW  W 
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W decay
Branching ratios
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We
~1/9
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Wm
~1/9
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Wt
~1/9
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Wqq
~2/3
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Decays of top pairs
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Top pair decay with branching ratio ~100%
p p  tt  bbW  W 
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Final decay products
 Both W’s decay into leptons
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“Dilepton final state”
One decays into a lepton, the other into quarks
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tt->bb + ll 
tt->bb + l + qq
“Lepton + jets”
Both W’s decay into quarks
 tt->bb + qqqq
“All Jets”, “fully hadronic”
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Decays of top pairs
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What are “jets?”
Why don’t you see a single quark?
Quark confinement
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Gravity, EM ~ 1/r2
Strong force increases with distance!
1.
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3.
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Collision
produces quarks
Energy grows
with distance
More quarks are
created
Can combine to
form hadrons
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Measurement of Top Quark Mass
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Dilepton (~4.5%)
 Muons or electrons
 Pure signal, low yield
l+jets (~30%)
 Moderate yield and bg
All jets (~44.5%)
 Large backgrounds
Tau channels (~21%)
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At least one W decays into a t
Hard to identify t decays
Short lifetime, hadronize quickly
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Tevatron Average:
mt = 173.1 ± 0.6 (stat.) ± 1.1(syst.) GeV/c2
Hobbs et al.
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Background sources
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Signal (Dilepton)
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t t  bb W W
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e e
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e e
Background (Diboson)
p p W Z
qq
 
ee
Both have final states of ee pair and two jets.
What’s the difference?
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Neutrinos appear in the signal process.
Problem: Neutrinos are weakly interacting, we can’t
really see them!
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Background discrimination: ET
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Total ET = 0, Missing ET must be from neutrinos.
Dilepton: ET > 35GeV
l+jets: ET > 15GeV
Abazov 2009
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Background sources
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Signal
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t t  bb W W
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p p W W
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Both have W boson pair, so ET may be the same.
What’s the difference?
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
Background
The signal has two bottom quarks. You expect more
jets in the signal than in the background.
Can you check if the jet is from a bottom quark or a
lighter quark?
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b-tagging
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C
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t~10-13s
t
b-tagging efficiency
W+
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~50% per jet
Misidentification
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P(b-tag|q) = 1%
P(b-tag|c) = 15%
t
Wb
b-tagged jet
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t~10-12s
Vertex is farther
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Background discrimination: # jets
Dilepton final state
l + jets final state
with one b-tagged jet
≥
Expect 2 jets
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Expect 4 jets
http://www-d0.fnal.gov/
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Background discrimination: # jets
η = -ln(tan(θ/2))
θ: azimuthal angle
from beam line
CDF detector crack at η = 1.1
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http://www-cdf.fnal.gov/physics/new/top/2004/jets/cdfpublic.html
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Measurement of Top Quark Mass
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At this point:
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Few unknowns
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Neutrino momentum
Jet combinatorics
What you can do:
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Have candidate tt events (using ET, b-tagging, and
other cuts) with lowered background
A mix of template method and weighing methods that
depend on the kinematic observables to determine a
best fit for mt.
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Measurement of Top Quark Mass
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Example: l+jets
Consider jet combinatorics under these constraints:
Hobbs et al.
Mt  Mt  Mlb  M qq'b
MW   MW   Ml  M qq'
Likelihood function as a
function of jet energy
scale and Mt.
Energy = [1+ DJES] f(s)
DJES = 0 -> perfect calibration
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Constraints on Higgs mass
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Higgs boson explains why
weak force carriers, W and Z,
are massive.
The mass of the top quark is
HUGE! compared to other
elementary particles.
Higgs mass is related to W
boson and top mass
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DMW ~ log(MH)
DMW ~ Mt2
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Constraints on Higgs mass
Results from 1995
P. Renton 1995
Karen Chen
Results from 2006
Heinemeyer et al., 2006
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Summary
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The main decay channel used for top quark mass
measurement:
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p p  tt  bbW W
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With appropriate cuts (ET, # of jets, b tagging), you
can increase the purity of the tt signal.
Mass measurement of top quark was done with
likelihood fits of mt using a combination of template
and weighting methods.
Precision of top quark and W boson mass puts
constraints on Higgs mass.
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References
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M. Kobayashi, T. Maskawa (1973). "CP-Violation in the Renormalizable
Theory of Weak Interaction". Progress of Theoretical Physics 49 (2): 652–
657.
P Renton, arXiv:hep-ph/0206231v2 1 Aug 2002
P. Renton, Review of Experimental Results on Precision Tests of
Electroweak Theories, Lepton-Photon 95, p35 (1995), published by World
Scientific.
P. Renton, arXiv:0809.4566v1 [hep-ph] 26 Sep 2008
S. Heinemeyer, W. Hollik, D. St ockinger, A.M. Weber, G. Weiglein,
arXiv:hep-ph/0604147v2 10 Oct 2006
V. Abazov et al. Measurement of the top quark mass in final states with two
leptons. Phys. Rev., D80:092006,2009.
http://www-d0.fnal.gov/Run2Physics/WWW/results/summary.htm
John D. Hobbs, Mark S. Neubauer, Scott Willenbrock, Tests of the Standard
Electroweak Model at the Energy Frontier, arXiv:1003.5733, 2010.
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