1. Yao-Bei Liu（刘要北） 河南科技学院
第二届中国 LHC 物理工作会议（CLHCP-2016）
Searches for the top-Higgs FCNC couplings from tH associated production signal with diphton channel at the LHC
Yao-Bei Liu（刘要北）
河南科技学院
Based on Yao-Bei Liu and Zhen-Jun Xiao, Phys Lett B 763 (2016) 458.
Dec. 17, PKU

2. Outline Motivation Calculation framework
Numerical results and discussions
Conclusion

3. 1.Motivation H,g,Z,γ J. A. Aguilar-Saavedra, hep-ph/0409342
Br(t-> qX) ~ – 10-12,
J. A. Aguilar-Saavedra, hep-ph/
In the SM, the FCNC couplings in the top sector are strongly suppressed due to the GIM mechanism. However, in many NP models, some FCNC processes can be greatly enhanced by the new particles and extending the flavor structures, such as SUSY, THDM, LHT,etc.
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J. M. Yang, a mini review, hep-ph/ ; arXiv: ; J. J. Cao et al., PRD 75 (2007) ; PRD76(2007) ; PRD79(2009)054003;EPJC 74 (2014) 3058;…
X.-F. Han, L. Wang, J. M. Yang, Phys. Rev. D80(2009)015018;
C.-X. Yue, J. Wang, Y. Yu, T.-T. Zhang, Phys. Lett. B705 (2011) 222; T. Han and R. Ruiz, Phys. Rev. D89 (2014) ; HDM
…… Sorry for limited space!
SUSY models
LHT
Any signal for top quark FCNC process at a measurable rate would be a robust evidence for NP!

4. In the specific NP models, the top FCNC couplings are constrained by the model parameters or new particles. An effective field theory is universal.
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[4] L. Wu, Enhancing thj Production from Top-Higgs FCNC Couplings, JHEP 1502 (2015) 061.
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…… Sorry for limited space!
Due to the the advantages of good resolution on the Higgs mass and small QCD backgrounds, we will study the observability of the top-Higgs FCNC tqh coupling through the process

5. 2.Calculation framework
In general, the effective Lagrangian describing the FCNC Yukawa interactions of a light
up-type quark with the top quark and a Higgs boson can be written as [1-3]
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After neglecting all the light quark masses, the branching ratio of t → qh can be approximately given by [4]
Here the factor κQCD is the NLO QCD correction to Br(t → qh) and equals about 1.1[5].
[4] A. Greljo, J. F. Kamenik and J. Kopp, JHEP 1407 (2014) 046.
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6. Current constraints: A. Direct limits:
Recently, the ATLAS[1] and CMS[2] collaborations have set the upper limits of Br(t → qH) < 0.79% and Br(t → cH) < 0.56% at 95% C.L.
[1] ATLAS Collaboration, Search for top quark decays t → qH with H → γγ using the ATLAS detector,
JHEP 1406 (2014) 008.
[2] CMS Collaboration, Combined multilepton and diphoton limit on t to cH, CMS-PAS-HIG
※ Same-sign top pair production process via thq couplings in t-channel.
Search for top quark FCNC in same-sign top quark production. PRD 91(2015)
Br(t → uH) < 4.21% at LHC 8 TeV
more weaker!
※ At 13 TeV, ATLAS Collaboration has searched for SUSY in final states with jets and two same-sign leptons. No significant excess over the SM expectation is observed for a total integrated luminosity of 13.2 fb−1.
ATLAS-CONF
two same-sign lepton + two b jet + missing energy

7. B. Indirect limits: from neutral meson oscillations;
the electroweak precision observables;
Top quark decay;
Higgs decay……
B. Indirect limits:
※ At loop level, the [3] mixing and [4] can be used to constrain the top quark violation in the tqh vertex.
[3] J. I. Aranda, A. Cordero-Cid, F. Ramirez-Zavaleta et al., Phys. Rev. D 81 (2010)
[4] H. Hesari, H. Khanpour and M. M. Najafabadi, Phys. Rev. D 92 (2015)
※ top quark decay
PDG-2014 data:
This bound is more weaker
However, these indirect limits can also be come from new particles and other model parameters in the specific NP models. Thus, we here only consider the direct limits.

8. At the LHC, the parton level signal process at the tree-level via the FCNC thq couplings can be expressed as qg → th
The LO cross section are computed using with CTEQ6L PDF, setting the renormalization and factorization scales to be µR = µF = µ0/2 = (mt + mh)/2. In this work, we assume λtqh ≤ 0.1 to satisfy the direct constraints from the ATLAS and CMS. The SM input parameters are taken as follows:
PDG-2014

9. For the same values, one can see that the sensitivity to the coupling λtuh will be
better than λtch.

10. 3. Numerical results and discussions
3.1 We perform the Monte Carlo simulation and explore the sensitivity of 14 TeV LHC to the top-Higgs FCNC couplings through the channel
Signal: two photons + one b jet + one lepton + missing energy,
Backgrounds:
too small

11. PYTHIA + Delphes + MadAnalysis5
All of these signal and backgrounds events are generated at LO. In our simulation, we generate 106 events for the signals and backgrounds respectively.
PYTHIA + Delphes + MadAnalysis5
The high order corrections for the dominant backgrounds are considered by including a K-factor, which is 1.12 for Whj[1] and 1.3 for Wjγγ[2], respectively. The K-factor for the LO cross section of signal is chosen as 1.5 at the 14 TeV LHC [3].
It should be mentioned that the process could also be considered as a source of top plus a Higgs boson if the light quark is missed by the detector.
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12. Basic cuts:
Cut 1:

13. Since there are only one b jet and one lepton in the final states, it is easily to reconstruct the top quark transverse cluster mass, which is defined as
Cut 2:
Cut 3:

15. G. Cowan, K. Cranmer, E. Gross, O. Vitells, arXiv:1007
G. Cowan, K. Cranmer, E. Gross, O. Vitells, arXiv: , EPJC 71 (2011) 1554.
The upper limits on the FCNC couplings λtuh(λtch) can be probed to (0.047) with L = 3000 fb−1. These limits can be converted to the 3σ C.L. upper limits on the branching ratio Br(t → uh) = 0.036% and Br(t → ch) = 0.13%, respectively.

16. Lei Wu explore the observability of top-Higgs FCNC couplings through
[71] G. Aad et al. [ATLAS Collaboration], JHEP 1406, 008 (2014), arXiv: [hep-ex]. [72] The CMS Collaboration, CMS-PAS-HIG
[73] K. Agashe et al. [Top Quark Working Group Collaboration], arXiv: [hep-ph].
Lei Wu explore the observability of top-Higgs FCNC couplings through
The results show that Br(t → uh) and Br(t → ch) can be respectively probed to 0.23% and 0.26% at 3σ sensitivity at 14 TeV LHC with an integrated luminosity of 3000 fb−1 .
Lei Wu, Enhancing thj Production from Top-Higgs FCNC Couplings, JHEP 1502 (2015) 061.

17. 3.2 Charge ratio R W+ W-

18. 4. Conclusion
In some parameter regions, the LHC may observe the above signals at the
5σ confidence level through the process
Otherwise, the branching ratios Br(t→uh) and Br(t→ch) can be respectively
probed to 0.036% and 0.13% at 3σ level at 14 TeV LHC with the integrated
luminosity of 3000 fb−1.
The charge ratio for the lepton in the signal can be not only used to
discriminate between signal and backgrounds, but also used to determine
that the signal comes from the up initiated production channel and charm
initiated production channel.

19. Thank you!
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