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Another example of pheno-exp interaction at CP3: search for single top + Higgs in the H->bb decay channel

One of the most praised aspects of working at CP3 is the strong interaction between the local theorists and experimentalists. A very recent example of that is the search for single top + Higgs in the H->bb decay channel, which was studied in depth at CP3 from the phenomenological point of view in [1], and searched in 20/fb of CMS data collected in 2012 at a center-of-mass energy of 8 TeV as reported in [2].
By Andrea Giammanco on October 9, 2014
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Quantum interference makes single top + Higgs a powerful handle on the relative phase between the top-Higgs and the W-Higgs couplings. Unitarity predicts that a cancellation should occur at tree level in the Standard Model between diagrams where a Higgs boson is radiated from the single top quark and from the intermediate W boson, causing a suppression of this process. If the relative sign between the two couplings is opposite to what the Standard Model assumes, the production rate gets enhanced by one order of magnitude.
The H->bb channel is chosen because it is the dominant decay mode for the known Higgs mass, but this choice also brings formidable challenges because of the large background from ttbar production accompanied by additional heavy quarks (b,c), and because of the many ways to associate the observed hadronic jets to partons coming for the decay of top quarks or of the Higgs boson.
Members of the CMS experiment inside CP3 had the privilege of frequent face-to-face discussions with the authors of [1] and initiated the effort in CMS. The initial work, done in relative solitude, was presented in internal CMS meetings at CERN with the effect of exciting and attracting additional collaborators from other countries. Collaborators from the Karlsruhe Institute of Technology (Germany), the University of Nebraska (USA), the University of Malaya (Malaysia) were crucial towards the achievement of a timely completion of the analysis [2], which complements a parallel effort in the H->γγ channel (which has the bonus of a very clean selection and of being sensitive to interference also through the decay vertex, but suffers from a very low rate) [3]. Our analysis was presented at the topical top-quark workshop TOP2014 in Cannes last week in the form of a talk and two posters, detailing different aspects of this complex analysis.
The upper limit obtained in the bb channel with the 8 TeV dataset is 1.78 pb (1.22 pb were expected from simulation), still far from being sensitive to the cross section expected even in the most favorable case (0.23 pb if the top-Higgs and W-Higgs couplings have opposite sign). A combination with the γγ channel and possibly with other less sensitive channels is envisaged, exploiting the very small correlation between all these measurements.
However, this result can be considered as an excellent warm-up in view of the LHC Run II: according to [1], the signal cross section is expected to increase by a factor four from 8 to 13 TeV, and some of the most discriminating observables depend on energy in such a way that they separate the signal and the background better at the larger energy. The residual degeneracies in Higgs couplings could therefore be lifted very soon for good!

[1] Marco Farina, Christophe Grojean, Fabio Maltoni, Ennio Salvioni, Andrea Thamm , "Lifting degeneracies in Higgs couplings using single top production in association with a Higgs boson", JHEP 1305 (2013) 022

[2] CMS collaboration, "Search for H->bb in association with single top quarks as a test of Higgs boson couplings", CMS-HIG-14-015

[3] CMS collaboration, "Search for a Higgs boson produced in association with a single top quark and decaying to photons", CMS-HIG-14-001

Reference url: http://cds.cern.ch/record/1952829

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