Single Top Quark Production
Mark Palenik
Physics 564, Fall 2007
History of Top Quark
•Two generations of matter were known until
1976, when the tau lepton (t) was discovered.
•Third generation quarks, top (t) and bottom
(b) were postulated to preserve symmetry.
•Top quark was finally discovered in 1995 at
Fermilab.
•Delayed discovery due to 175 GeV mass
(175x proton mass).
Image courtesy of
•Single Top quarks first produced in 2006 at
D0
Top Production
•
First top quarks were produced in
ttbar pairs.
Br(t->Wb)~1
A W boson can decay into two
quarks or a charged lepton and
neutrino.
We can get
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•
•
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–
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6 quarks
4 quarks, a charged lepton and a neutrino
2 quarks, 2 charged leptons, 2 neutrinos
We detect hadron jets produced
from free quarks
Image courtesy of [4]
Single Top Production
• Top quark production split into s-channel and t-channel
events
• Other Processes occur, but with much lower frequency
Single Top and CKM
• CKM Matrix is required to be unitary
Vud Vus Vub


V  Vcd Vcs Vcb 
Vtd Vts Vtb 
• For unitarity, |Vtd|2 + |Vts|2 + |Vtb|2 = 1
• Limits based on unitarity place Vtb = 0.999100+0.000034-0.000004 ,
direct measuremtns place Vtb > 0.78
Challenges in Detection
• Single top production has higher background that ttbar
production.
• Single top production is mimicked by many events,
including ttbar production (also Wc, and Wccbar)
• Sophisticated selection procedures must be put in place
• Variables are constructed from events that pass selection
and combined into likelihood functions.
Protons and anti protons are collided with COM energy of 1.96 TeV
in two regions, CD and D0
Tevatron
AØ: The High Rise
FØ: The RF
EØ: This Space
For Rent
DØ: Fermilab’s Best Detector
Image courtesy of [4]
BØ: The Competition
CØ: Future BTeV
Detection
• Different particles
detected in layers
– Innermost, silicon detects
charged particle
trajectories (precise)
– Next layer is calorimeter,
made of denser material
– Outermost layer is muon
detector
– Missing energy is in noninteracting particles
(neutrinos)
Image courtesy of [3]
Calorimetry
• Calorimeter detects photons and charged particles
• Cascades of particle showers are set off. Energy is
proportional to number of particles scattered at the end
• Electromagnetic particles are absorbed
• Hadrons usually pass through, muons do not shower.
Dense Stuff
Undense
Stuff
Image courtesy of [4]
Tracking
• b quarks form B-mesons, which travel~1mm then decay
• Silicon detectors search for particles with significant
impact parameter from primary vertex.
Image courtesy of [3]
Processes
• D0 and CDF do not use fundamentally different physics
• D0 uses calorimetry more heavily, while CDF relies more
heavily on tracking
Data Analysis
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Three levels of triggering are used to reduce data to a
recordable number of events
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First level selects 10-40kHz of collisions
Second level reduces this to a few hundred using
microprocessors
Third level uses a farm of computers to reduce to 50Hz
Topology and particle variables are tracked to match
single-top events (e.g. cosine of angle between lepton
and jet)
Data is analyzed with a Monte Carlo simulation
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–
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Monte carlo can tell you if the choice of variables is optimal
With optimal variable choice, signals move to last bin, and noise
to the first
Results
The original D0 single top quark
detection found a Vtb consistant with the
standard model, 0.68-1.0
Recent top quark data analysis found the
cross sections for s and t channel events
to be 0 and 0.3 pb, and thus Vtb outside
of the range of the standard model to 95%
confidence.
Further analysis has shown the error to lie
within that 5%
Image courtesy of [1]
Future: LHC
• Large Hadron Collider (LHC) scheduled for activation in
May 2008
• Will accelerate protons to 7 TeV, as opposed to the
Tevatron’s 980 GeV
• t-channel process cross section increases by a factor of
120, s-channel cross section increases by a factor of 10
TW Process
• LHC should be able to measure the tW process, which is
negligable at Tevatron
• Theoretical definition of this process is “a work in
progress”, new aspects are being explored
• The only single top process where W is directly observed
• Measure of top coupling to W and bottom-type quark
References
•
[1] Michael Wren, Search for Single-top production in 1 fb-1 with CDF, (unpublished thesis)
December 16 2006
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[2] John Womersly, The Top Quark and Beyond, arXiv:hep-ex/0604008, April 4 2006
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[3] John Wormersly, Tevatron Physics, arXiv:hep-ex/0301007, January 1 2003
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[4] D0 Presentations: The D0 Experiment, http://wwwd0.fnal.gov/Run2Physics/displays/presentations/lincoln_public_D0_mom_feb2001/lincoln_pu
blic_D0_Mom_Talk.ppt
•
[5] D0 Presentations: The Top Quark, http://wwwd0.fnal.gov/Run2Physics/displays/presentations/gerber_colloq_UICtop_feb2002/gerber_collo
q_UICtop_feb2002.pdf
Backup slide
• Events are selected as top quark candidates if:
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Have one lepton, ET>15 GeV
2 Jets, at least one b-tagged ET>15 GeV
Pseudorapidity < 2.8 (-ln tan(theta/2))
Events from QCD, containing Z bosons, dileptons, conversions, or
cosmic rays are removed