B or T production with coupling to 3rd generation SM quarks¶
Khadeejah Bepari, Jack Burton, Jon Butterworth, Joanna Huang, Ben Waugh, David Yallup
Preliminary/Work in Progress
Vector-Like quarks (VLQ) are hypothetical coloured, non-chiral, spin-1/2 particles. While fourth-generation quarks with chiral couplings are in general excluded by measurements at the LHC and elsewhere , vector-like quarks are not, and remain the simplest example of coloured fermions not yet excluded by data . VLQs occur in several non-supersymmetric theories beyond-the-standard model that propose solutions to the hierarchy problem, offering mechanisms by which the Higgs mass is stabilised . New sources of CP violation can also be introduced by these new particles .
VLQs are produced in SU(2) singlets, doublets or triplets of flavours X, T, B or Y, where T and B have the same charge as the SM top and bottom quarks (t and b), whilst Y and X have charges −4/3 and +5/3, respectively. VLQs can be produced in pairs via QCD or singly in association with SM quarks via electroweak interactions. They decay into to Z, W or Higgs bosons in association with SM quarks.
Direct experimental searches for VLQ contributions set lower limits on their mass with various values depending in detail upon the production mechanism, flavour or branching ratio assumptions considered, but generally between 700 GeV and nearly 2 TeV . The eventual sensitivity of the LHC and its upgrades is projected to approach 4 TeV .
Here we consider an effective model , where as a generic framework the four VLQ states X, T, B and Y are added as new coloured spin-1/2 objects to the SM, embedded in complete \(SU(2)_L\) representations. They have fixed electric charge and can only decay to SM quarks in association with the production of W, Z or Higgs bosons. We use the UFO file (without the chromomagnetic extension) provided at http://feynrules.irmp.ucl.ac.be/wiki/VLQ , read into Herwig 7.1.4 to study the single production of B VL quarks, under the following assumptions:
the VLQ only couples to the third generation quarks of the SM.
the X and Y masses, and either B or T, are so high (10 TeV) they are effectively decoupled
Following , we then scan the branching fractions to W, Z or Higgs, visualing the results in the lower left half of a plane in the W and H branching fractions. The top right half is unphysical, sign the branching fractions sum to mor than one. All relevant 7, 8 and 13 TeV from ATLAS and CMS available in rivet as of 30/3/2019 are used. The results can be compared to Fig.9 of , although rather than show the 95% c.l. for all masses in each plot, we show the exclusion sensitivity heatmap for a fixed mass, and indicate the 95% excluded region for the appropriate mass from Fig.9 of . The observed B production limits (Fig.9a and 14d of  and  respectively) are shown below.
Since the ATLAS results are based on the signature of looking for like-sign leptons plus b-jets, they are sensitive to the decay \(B \rightarrow Wt\), and thus are strongest in the bottom right of the plane.
The datasets used by Contur have final states sensitive to all three possible decays, but the \(Z \rightarrow ll\) measurements (for example ), especially those in association with b-jets (e.g. ). are the most sensitive, meaning Contur has most sensitivity to the \(B \rightarrow Zb\) decay, giving a complementary exclusion to  at high \(M_B\), and similar in shape to . At lower \(M_B\) values the whole physical plane is excluded by both sets of results.
In the upper left region, decays to a Higgs dominate and the most sensitive measurements are the \(H \rightarrow \gamma\gamma\) fiducial cross sections, especially .
and Contur so far…
The sensitivity is not as strong as the searches, but there is a region at high \(BR(T \rightarrow bW)\) and high \(M_T\) where there is some complementarity. This is the region where single \(T\) production dominates over pair production.
The model files are available in the VLQ_UFO directory here .