Transcript Highlights from Top Physics

Highlights from Top
Quark Physics
Suyong Choi
Korea University
1
•
•
•
•
Introduction
Measurement of Production Cross Sections
Properties of the Top Quark
Summary and Outlook
Contents
2
CDF
CDF
DØ
DØ
Top Quark
Runfrom
2 results
Top Mass Distributions
1995 observation paper
3
Success of the SM
4
• Top quark is special
•
•
•
•
•
Most massive
Interaction only within 3rd generation
top-Higgs coupling ~ 1
2
𝑚𝑡 × 𝑚𝑍 ≈ 𝑚𝐻
Boundary between metastability
and stability
The Top Quark
5
5 fb-1 @ 7 TeV
20 fb-1 @ 8 TeV
LHC and Experiments
6
• Properties
•
•
•
•
Mass
Decay width
Spin
Coupling
• Cross section
measurements
• Production and decays
Physics with Top Quarks
7
• Higher cross section
and higher luminosity
at LHC
• Top quark factory
• Rare processes with top
quarks
• New physics with top
quarks
• Tevatron and LHC are
complementary
Cross Sections at
Tevatron and LHC
8
PRODUCTION
9
Pair production
diagams
• Strongly produced
• Contribution of 𝑞𝑞 and
𝑔𝑔 changes as 𝑠
𝑡𝑡 Pair Production
10
Dilepton:
ee, e, 
tau+X
Multijet
mu+jets
e+jets
Lepton+jets
•
•
•
•
𝐵𝑟 𝑡 → 𝑏ℓ𝜈 = 10.6% per lepton flavor
Multijet – Highest statistics, but large backgrounds and combinatorics
Lepton+jets – Highest statistics and usually yields best measurement
Dilepton – Smaller statistics but clean, less combinatoric, solving for 2
neutrino momenta not trivial
𝑡𝑡 channels
11
• Experimental error comparable to theory error
• QCD explains well the inclusive pair production
Pair Production Cross
Section
12
• Lepton asymmetry reflects
• Asymmetry in production
• Polarization of 𝑡𝑡: 𝑡𝑅 𝑡𝑅 vs 𝑡𝐿 𝑡𝐿
• SM predicts small asymmetry in production and no
polarization
Lepton ForwardBackward Asymmetry
13
𝐴ℓ
Channel
Luminosity
CDF Lepton+jets
9.4 fb-1
0.094+0.032
−0.029
D0 Lepton+jets
9.7 fb-1
0.047+0.025
−0.027
D0 Dilepton
9.7 fb-1
0.058 ± 0.053
SM prediction @ NLO: 𝐴𝑙 = 0.036 ± 0.002
Lepton AFB in 𝑡𝑡
14
s and t channel
• Electroweak production
• Cross section of the same
order as pair production
• Sensitive probe of |𝑉𝑡𝑏 | without
the assumption of 3 generation
of quarks
W associated
Single Top Production
15
Single Top Production
t-channel
16
Signal Region
𝜎 𝑝𝑝 → 𝑊𝑡 = 23.4+5.5
−5.4 𝑝𝑏
Control Region
6.0𝜎 significance
Observation of Wt Single
Top Production
17
• From single top quark production cross section, we can
measure |𝑉𝑡𝑏 | directly without assuming 3 generation of
quarks
• Current best direct measurement:
𝑉𝑡𝑏 = 1.020 ± 0.046(𝑒𝑥𝑝𝑡. ) ± 0.017(𝑡ℎ𝑒𝑜𝑟. )
Measurement of |𝑉𝑡𝑏 |
18
PROPERTIES
19
• Tevatron: 𝑚𝑡 = 173.20 ± 0.87 GeV – 0.5% accuracy
Mass of Top Quark
20
• CPT violated if 𝑚𝑡 ≠ 𝑚𝑡
• 𝑡 and 𝑡 distinguished by electric charged of lepton
Δ𝑚𝑡 = −272 ± 196 𝑠𝑡𝑎𝑡 ± 122(𝑠𝑦𝑠𝑡)
Mass Difference of 𝑡 and 𝑡
21
• In SM, top quark width at NLO is
• 1.29 GeV/c2
• Lifetime of 0.5 × 10−24 𝑠
• Decay width reflected
in reconstructed mass distribution
• CDF measures Γ𝑡 = 2.21+1.84
−1.11 GeV
• Γ𝑡 < 6.38 GeV @ 95% CL
Decay Width of Top Quark
22
• 𝑄𝑐𝑜𝑚𝑏 = 𝑄ℓ × 𝑄𝑏−𝑗𝑒𝑡
• B-jet charge calculated
from tracks associated
with b-jet
Electric Charge of Top
Quark
23
• W from top decays
are either left-handed
or longitudinal
W Polarization from Top
24
• 𝑞𝑞 → 𝑡𝑡 and 𝑔𝑔 → 𝑡𝑡 the spins
of top quarks are correlated
• Due to 𝜏𝑡 ≪ 1/Λ𝑄𝐶𝐷 , spin state
of top at production reflected in
decay products
• Lepton is the most sensitive
probe of top spin polarization
• Tevatron and LHC has different
contributions of 𝑞 𝑞 and 𝑔𝑔
• ATLAS observed spin
correlations at 5.1 s.d.
𝑓 𝑆𝑀 = 1.30 ± 0.14 𝑠𝑡𝑎𝑡
+0.27
−0.22 (𝑠𝑦𝑠𝑡)
Spin Correlation
25
Top Coupling with Vector
Bosons with 𝑡𝑡𝑊and 𝑡𝑡𝑍
26
• Major background to
𝑡𝑡 + 𝐻(→ 𝑏𝑏)
• Number of b-tagged
jets distribution
𝑡𝑡 + 𝑏𝑏 Production
27
• Approaching 20 years of rich physics program at hadron
colliders with top quark events
• Top quark production and properties consistent with SM
• Many measurements systematics limited.
What can you do with millions of top quark events?
Summary and Outlook
28
• When Υ was discovered in 1977, it was considered as a bound
state of 𝑏𝑏 quarks. Hence extra quark was thought to exist.
• It took a long time until top quark was discovered in 1995 by
CDF and D-Zero experiments using Fermilab Tevatron
accelerators
• Being the most massive quark, it may hold the key.
• With the luminosity and energy reach of the LHC at CERN,
top quarks can be studied with unprecedented precision.
• 1.96 TeV → 8 TeV
Introduction
29
• 𝜎𝑡𝑡 is a function of 𝑚𝑡 and 𝛼𝑠 𝑀𝑍
Strong Coupling Constant
30
SEARCHES WITH TOP QUARKS
31
Search for Resonances
Decaying into 𝑡𝑡
32
Anomalous Single Top
Search for 𝑡 → 𝑍𝑞
• 𝑔𝑞 → 𝑡
𝐵 𝑡 → 𝑍𝑞 < 0.0021 @ 95% 𝐶𝐿
Search for FCNC
33
• Top-Higgs coupling almost 1
• Consistent with backgrounds
• Cross section limits at
Search for 𝑡𝑡 + 𝐻
34