Single Top Quark Production

1. Single Top Quark Production Mark Palenik Physics 564, Fall 2007

2. 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). • Single Top quarks first produced in 2006 at D0 Image courtesy of

3. 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]

4. Single Top Production • Top quark production split into s-channel and t-channel events • Other Processes occur, but with much lower frequency

5. 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

6. 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.

7. AØ: The High Rise FØ: The RF BØ: The Competition EØ: This Space For Rent CØ: Future BTeV DØ: Fermilab’s Best Detector Protons and anti protons are collided with COM energy of 1.96 TeV in two regions, CD and D0 Tevatron Image courtesy of [4]

8. 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]

9. Dense Stuff Undense Stuff 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. Image courtesy of [4]

10. 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]

11. Processes • D0 and CDF do not use fundamentally different physics • D0 uses calorimetry more heavily, while CDF relies more heavily on tracking

12. 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

13. 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]

14. 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

15. 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

16. 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

17. 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