Suche nach einer vierten Fermion-Generation mit dem CMS Detektor am "Large Hadron Collider"

• Applicant: Professor Dr. Alexander Schmidt
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2011 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 200475464

• Final Report Year: 2018

Final Report Abstract

The discovery of the Higgs boson in the year 2012 by the experiments at the Large Hadron Collider (LHC) at CERN was a historic achievement for experimental high energy physics. Even though the Higgs boson has now become an integral part of the standard model of particle physics it leaves many fundamental questions unanswered. In many of the popular explanations of the open questions additional quarks beyond the known three generations are predicted. Such additional quarks are usually divided into two categories: chiral and vector-like heavy quarks. Heavy quarks with chiral couplings are disfavoured by indirect constraints from the measurements of the properties of the Higgs boson, but heavy quarks with vector-like couplings are not constrained in a similar way. The efforts of our Emmy Noether group have focused on two main topics: the development of new analysis methods for the identification of the origin of jets (b-tagging and jet substructure analysis), and the application of these new analysis methods in the search for physics beyond the standard model. Vector-like quarks are searched for in very high mass regions around 1 TeV and above. Therefore, the decay products such as top quarks and Higgs bosons will receive very high Lorentz boosts. A highly boosted hadronic top quark decay t → bqq will not be reconstructed as a three-jet signature, but as a single so-called “fat” jet. We have therefore commissioned algorithms for the identification of boosted top quarks and boosted Higgs bosons and we have measured their performance in data. A special highlight is the development of algorithms for b-jet identification in the substructure of jets which turned out to give substantial improvements to the whole concept of boosted object identification. We applied these new algorithms in searches for vector-like quarks in a large variety of channels and topologies. We were the first group to ever investigate fully hadronic final states in the search for exotic heavy quarks. The new analysis methods are able to reduce backgrounds from QCD multijet production by many orders of magnitudes. We were able to demonstrate that the hadronic channel is a realistic discovery candidate in addition to the semileptonic channels. At the end of LHC Run-I we performed a global interpretation of all T' quark searches that have been made with 8 TeV data. This interpretation is done by combining all available results to derive limits on model parameters. This effort led to the final legacy paper for vector-like quark searches of the CMS collaboration with 8 TeV data. With the first 13 TeV data of LHC Run-II we have continued the searches for VLQ in a broader range of topologies. We have done the first search for pair-produced VLQ in semi-leptonic final states, as well as singly produced VLQ. The search for singly produced VLQ was the first publication of the CMS collaboration on this topic. We also extended the search strategy to cover models with multiple new particles. For instance, we have conducted the first search for resonances decaying to VLQ (Z' → tT'). In all cases, the newly developed methods of b-jet identification and jet substructure analysis have been applied. We also introduced our new jet substructure analysis methods into related searches for BSM physics, in particular, the search for heavy Z bosons decaying into tt final states. Also in this case the top quarks receive very high Lorentz boosts. With the new methods the backgrounds in this analysis could be reduced by an order of magnitude compared to the previous results which have been published by the CMS collaboration. We have left almost no stone unturned. VLQ and BSM physics in general may still be hiding in difficult corners of phase space that may be probed with the High-Luminosity LHC. We have presented our results at numerous international conferences and workshops. I have been elected into the international advisory committee of the BOOST workshop. We have also organised and hosted workshops on fourth generation and vector-like quarks which attracted the international community of the ATLAS and CMS experiments as well as theoretical physicists. Furthermore, we have taken several responsibilities within the CMS collaboration. Most notably I have been charged to coordinate the b-tagging group as well as the “Beyond 2 Generations” analysis group of the CMS collaboration. The postdoctoral research associate has been appointed to coordinate the b-tagging group in 2016. These achievements show that the group was recognised as an internationally leading group in the field of exotic heavy quark searches and the development and application of new analysis methods.

Publications

• Measurement of the cross section and angular correlations for associated production of a Z boson with b hadrons in pp collisions at √s = 7 TeV,” JHEP 12 (2013) 039
  CMS Collaboration, S. Chatrchyan et al.
  
• “Identification of b-quark jets with the CMS experiment,” JINST 8 (2013) P04013
  CMS Collaboration
  
• “Long-lived heavy quarks : a review,” Adv.High Energy Phys. 2013 (2013) 690254
  M. Buchkremer and A. Schmidt
  
• “Performance of b √s = 8 TeV in multijet, tt¯ and boosted topology events,” CMS Physics Analysis Summary CMS-PAS-BTV-13-001, 2013
  CMS Collaboration
  
• “Search for vector-like T quarks decaying to top quarks and Higgs bosons in the all-hadronic channel using jet substructure,” JHEP 06 (2015) 080
  CMS Collaboration, V. Khachatryan et al.
  
• Jet Substructure at the Large Hadron Collider : Experimental Review
  L. Asquith et al.
  
• “Search for resonant tt¯ production in proton-proton collisions at √s = 8 TeV,” Phys. Rev. D 93 (2016) 012001
  CMS Collaboration, V. Khachatryan et al.
  
• “Search for vector-like charge 2/3 T quarks in proton-proton collisions at sqrt(s) = 8 TeV,” Phys. Rev. D 93 (2016) 01200
  CMS Collaboration, V. Khachatryan et al.
  
• “Search for a heavy resonance decaying to a top quark and a vector-like top quark at √s = 13 TeV,” JHEP 09 (2017) 053
  CMS Collaboration, A. M. Sirunyan et al.
  
• “Search for pair production of vector-like T and B quarks in single-lepton final states using boosted jet substructure in proton-proton collisions at √s = 13 TeV,” JHEP 11 (2017) 085
  CMS Collaboration, A. M. Sirunyan et al.
  
• “Search for single production of a heavy vector-like T quark decaying to a Higgs boson and a top quark with a lepton and jets in the final state,” Phys. Lett. B 771 (2017) 80
  CMS Collaboration, V. Khachatryan et al.
  
• “Identification of heavy-flavour jets with the CMS detector in pp collisions at 13 TeV,” JINST 13 (2018) P05011
  CMS Collaboration, A. M. Sirunyan et al.