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34th International Conference on High Energy Physics. Single Top Quark Studies. Liang Li University of California, Riverside for the CDF, DØ and H1 Collaborations. Single Top Quark Production at Tevatron. tW production (not yet seen). ~ 0.25 pb. Tev : ~1 pb. t-channel (“ tqb ”). - PowerPoint PPT Presentation
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1Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Liang Li University of California, Riverside
for the CDF, DØ and H1 Collaborations
Single Top Quark Studies
34th International Conference on High Energy Physics
34th International Conference on High Energy Physics
2Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Single Top Quark Production at Single Top Quark Production at TevatronTevatron
Single Top Quark Production at Single Top Quark Production at TevatronTevatron
Single Top: Electroweak Interaction• Study Wtb coupling
• Direct measurement of the |Vtb| CKM matrix element• Test of CKM unitarity• Anomalous Wtb couplings
• New physics, example:• s-channel is sensitive to W’, H+
• t(s)-channel is sensitive to FCNC• 4th quark generation?
• Study top properties:• Polarization, decay width, lifetime, CP violation …
• Background study helps other physics searches, e.g. SM Higgs
Tev : ~1 pb ~ 2pb ~ 0.25 pb s-channel(“tb”)
t-channel(“tqb”)
tW production(not yet seen)
3Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
CKM Matrix Element |VCKM Matrix Element |Vtbtb||CKM Matrix Element |VCKM Matrix Element |Vtbtb||Direct measurement of |Vtb| from cross section measurement:
• Calculate posterior in |Vtb|2 : |Vtb|2
Assuming standard model production:• Pure V-A and CP conserving Wtb interaction• |Vtd|2 + |Vts|2 << |Vtb|2
• Additional theoretical errors needed (top mass, scale, PDF etc…)No need to assume three quark generations or CKM unitarity
|Vtb| = 0.88 ± 0.14 (exp.) ± 0.07 (theory)|Vtb| = 1.00 + 0.00 (exp.+ theory)- 0.12
4Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Anomalous Wtb CouplingsAnomalous Wtb CouplingsAnomalous Wtb CouplingsAnomalous Wtb Couplings
Top couplings to the W boson is a good place to look for deviations from SM
• Effective single top production cross section: = A (f1L)2 + B (f1
R)2 + C (f1
L + f2R)2 + D (f2
L + f1R)2
• f1: “Vector”-like, f2: “Tensor”-like• SM (left-handed vector coupling, LV ): f1
L=1, rest=0• Contributions also from left-handed left-handed tensor (LT), right-
handed vector (RV), right-handed tensor (RT) couplings
• Single top production directly sensitive to the Wtb interaction: rate, kinematics and angular distributions
Standard Model: ~ 2.9 pb
f2L(R)=1, rest=0 : ~ 10.4 pb
5Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Anomalous Wtb CouplingsAnomalous Wtb CouplingsAnomalous Wtb CouplingsAnomalous Wtb Couplings• Use same multivariate analysis
technique and event selection as for DØ “single top evidence” analysis (Phys. Rev. Lett. 98,181802(2007))
• Simultaneous limit setting for two signals by calculating 2 dimensional posterior probability density
arXiv:0807.1692 [hep-ex]
|Vtbf1L| = 1.31 + 0.25
- 0.21
First direct constraints on Wtb tensor couplings• Data favor the standard model
left-handed vector hypothesis over the alternative hypotheses
• Compare with SM value
6Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Heavy W’ ResonanceHeavy W’ ResonanceHeavy W’ ResonanceHeavy W’ ResonanceSingle top quark decay channel is a promising
searching background for heavy W’ with SM-like couplings• W’ decays leptonically into a single top
quark
• Same final states as single top production
• Concentrate on s-channel production• Enhancement from W’ resonance
• Consider purely left- and right-handed states• Left-handed couplings W’
• W-W’ interference
• DØ includes this term in signal model
• Right-handed W’R with M(W’R) > M(R)
• Decays to (l, ) and (q,q)
• Right-handed W’R with M(W’R) < M(R)
• Decays to (q,q) only
7Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Heavy W’ ResonanceHeavy W’ ResonanceHeavy W’ ResonanceHeavy W’ Resonance• Four-vectors of all final state objects are added to obtain the invariant
mass of the W’ boson• Use the mass distribution to separate W’ from background and set limit
8Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Charged Higgs ResonanceCharged Higgs ResonanceCharged Higgs ResonanceCharged Higgs ResonanceDiscovery of charged Higgs boson is an
unambiguous evidence for new physics: no “charged” Higgs boson in SM• Identical final state as s-channel single top
quark electroweak production• Three Two-Higgs Doublet Model (2HDM)
studied:• Type-I, II and III have different strategies to
give mass to fermions• tb decay channel dominates for large region
of parameter space
First direct search for heavy (mH+ > mtop ) H+ decaying to top• Reconstructed invariant mass M(jet1, jet2,
W) showed to be the single most sensitive variable, used to construct binned likelihood function
• Upper limits set close to predicted cross sections• Excluded parameter region for 2HDM Type-I:
180< mH+<184 GeV with 23 < tan< 7095%CL upper limit
on 2HDM type-I,III H+
9Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Flavor-Changing Neutral CurrentsFlavor-Changing Neutral CurrentsFlavor-Changing Neutral CurrentsFlavor-Changing Neutral CurrentsAny evidence for FCNC will be a signal
of new physics beyond the SM • FCNC cross sections scale with
(gtu/)2 and (gtc/)2
• Use same event selection as in CDF standard model single top production measurement (CDF note 9251) except:• Require exactly one hadronic jet
(instead of two jets)
Assume no anomalous coupling
10Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
Anomalous Single Top Anomalous Single Top Production at HERAProduction at HERA
Anomalous Single Top Anomalous Single Top Production at HERAProduction at HERA
Looking for new physics at HERA: anomalous single top production via FCNC• Highest sensitivity to tu
• Coupling to Vtuz suppressed due to high Mz
tc suppressed due to low c-density
• Events with e + PTmiss in HERA I+II e+p data
• Form top/W discriminator (MVA method)
@ 95% CL
478 pb-1
11Single Top Quark Studies, L. Li (UC Riverside) ICHEP 08, July 30 2008
SummarySummarySummarySummary
Single top quark production provides a rich testing ground for SM new physics• Five studies from three collaborations so far
• |vtb|, Anomalous Wtb coupling, W’/H+ tb resonances, FCNC production
• Many improved limits
• More studies and data to come, stay tuned!