Single Top Quark Production

Single Top Quark Production

Mark Palenik

Physics 564, Fall 2007

•Two generations of matter were known until 1976, when the tau lepton () 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).

•Single Top quarks first produced in 2006 at D0

History of Top Quark

Image courtesy of

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– 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

• 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

tbtstd

cbcscd

ubusud

VVV

VVV

VVV

V

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.

TevatronAØ: The High Rise

BØ: The Competition

CØ: Future BTeV

FØ: The RF

EØ: This Space For Rent

DØ: Fermilab’s Best DetectorImage courtesy of [4]

Protons and anti protons are collided with COM energy of 1.96 TeV in two regions, CD and D0

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 non-interacting 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• Three levels of triggering are used to reduce data to a

recordable number of events– 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– 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

• [2] John Womersly, The Top Quark and Beyond, arXiv:hep-ex/0604008, April 4 2006

• [3] John Wormersly, Tevatron Physics, arXiv:hep-ex/0301007, January 1 2003

• [4] D0 Presentations: The D0 Experiment, http://www-d0.fnal.gov/Run2Physics/displays/presentations/lincoln_public_D0_mom_feb2001/lincoln_public_D0_Mom_Talk.ppt

• [5] D0 Presentations: The Top Quark, http://www-d0.fnal.gov/Run2Physics/displays/presentations/gerber_colloq_UICtop_feb2002/gerber_colloq_UICtop_feb2002.pdf

Backup slide

• Events are selected as top quark candidates if:– 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