1. Discovering Z’ bosons in Minimal Walking Technicolor
Placement Partners
The Authors would like to acknowledge: Prof. A. Belyaev[1], Dr. C. Shepherd-Themistocleous[2], Dr. E. Olaiya[2], Mr. K. Mccarron[2], Dr. A. Pukhov[3]
Conclusion
By combining theoretical predictions and experimental data, we successfully produced
the first limits on the combined process of Z’ and Z’’ production and decay in the Walk-
ing Technicolor model (Figure 4).
This was the most comprehensive study of these WTC particles to date, and setting
the respective improved limits exclude a new region of the parameter space, and these
results will be published soon.
Introduction and Motivation
The Standard Model of Particle Physics (SM) successfully describes the strong, weak
and electromagnetic interactions of the elementary particles. However, we know the
theory fails at high energy and this demands theories Beyond the SM to solve.
One known issue with the SM is that it fails to explain why the mass of the Higgs
boson is so much smaller than the Planck scale, called the hierachy problem (Figure 1a).
A result of this means that the Higgs’ mass diverges quadratically with the energy
scale which leads to the fine tuning problem (Figure 1b).
To solve these and other SM issues, one well-motivated BSM theory is Technicolor,
where the mass scale of symmetry breaking (~TeV) provides a natural scale for the
Higgs mass.
Minimal Walking Technicolor
TC is an up-scaled version of Quantum Chromodynamics (QCD), which describes had-
rons as bound states of quarks. Similarly, TC describes new higher mass particles as
bound states of ‘techni-quarks’.
One viable version of Technicolor theory is Minimal Walking Technicolor, so called as
to be consistent with observation, the coupling becomes “walking” or slowly-changing
with energy scale (Figure 2a). MWTC contains 2 new, higher mass versions of the Z and
W± bosons, called Z’, Z’’ and W±’,W±’’. These bound states are called “techni-rho” and
“techni-eta” respectively.
We investigated the Z’ and Z’’ particles in the di-lepton decay (Drell-Yan) channel
(Figure 2b), in which the overall process is pp-> Z’/Z’’->l+l– .
Uniting theory and experiment
We conducted a comprehensive study of the production and decay of the Z’ and Z’’
particles initially using the CalcHEP Matrix Element Generator tool to investigate
how the properties of the particles changed with varying values of MA and g-tilde,
providing valuable insight into how the theory operates.
We wrote C code to scan through the parameters of the model in CalcHEP and
find the properties of the Z’ and Z’’, displayed as contour plots which were pro-
duced by Python code (Figure 3). To find the theoretical cross-sections, we ran the
same scans using the High Energy Physics Model Database (“HEPMDB”), which is
linked to the IRIDIS HPC supercluster.
Affiliations: [1] University of Southampton [2] PPD, Rutherford Appleton Laboratory [3] Moscow State University
Particle Physics Department
A. Coupe[1], J. Blandford[1]
Figure 4: a) Final exclusion in parameter space of MWTC, displaying the contributions from the
Z’ and Z’’ limits.
b) The purple region is our combined limit from Figure 4a, and the extra grey region is the im-
provement from the statistical two peak analysis.
Allowed region
Production and Decay of the Z’ and Z’’
The Drell-Yan decay channel was chosen specifically because it is the cleanest channel
to discover resonances; it allows us to precisely limit the parameters MA and g-tilde. MA
is the mass scale of WTC, which we found to be almost exactly equal to the mass of the
Z’ boson, and g-tilde defines the coupling of these new particles to SM particles, pro-
portional to 1/g-tilde.
Therefore we used the MA—g-tilde parameter space to display the dependence of Z’
and Z’’ properties and set up new limits on this theory.
Figure 2: a) Comparison of Running and Walking coupling.
b) The production and decay process with the Z’ or Z’’ decaying to a lepton-antilepton pair. This
would be observed as a new resonance (peak) in the invariant di-lepton mass distribution.
Combining Exclusions
The experimental cross sections, were obtained from the latest CMS limits. We then
found the difference between σtheory and σexp for the parameter space, first for Z’, then
Z’’. The areas where σtheory>σexp were excluded, as large cross sections would have
been discovered. The zero-level contour for Z’ and Z’’ was given on the same graph,
and the total area excluded by both gave the first combined exclusion limits for WTC.
In collaboration with RAL[3], a statistical analysis of the two resonant Z’ and Z’’
peaks was conducted in C++, which improved upon our original limits.
Figure 3: a) Contour plot of the Z’ mass in the MA-g-tilde parameter space.
b) Cross-section for the Z’ boson production and decay in the MA-g-tilde parameter space.
Figure 1: a) Scale of particle physics energies, the 16 orders of magnitude between the
SM scale and Planck scale causes the hierarchy problem.
b) One loop corrections cause the Higgs mass to diverge with the scale of the theory,
requiring huge fine tuning to cancel these out and give to the correct mass.
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