wiki:Octet_tcgg
Last modified 23 months ago Last modified on 11/03/15 23:22:06

A Coloron Model with a Decay to a Single Top Quark

This model introduces a heavy scalar color octet, the coloron. It is produced through gluon fusion and decays to a top quark and a charm quark.

Corresponding Authors

  • Elizabeth Drueke (Michigan State University)
  • Reinhard Schwienhorst (Michigan State University)
  • Natascia Vignaroli (Michigan State University)
  • Devin G. E. Walker (SLAC National Accelerator Laboratory)
  • Jiang-Hao Yu (The University of Texas at Austin)

Other Contributors

  • Joseph Nutter (Michigan State University)
  • R. Sekhar Chivukula (Michigan State University)
  • Elizabeth H. Simmons (Michigan State University)

Description of the Model

The color octet (GH) is a neutral heavy hadronic resonance. The Feynman diagram for the production of GH and its decay to a single top quark and a charm quark is shown below. Feynman Diagram 1 The single production of GH can be described by the effective coupling

$-\frac{1}{4} C_{ggG} d_{abc} G^a_{\mu\nu} G^{\mu\nu b} G^c_H$

with

$C_{ggG}=\sqrt{\frac{1}{6}}\frac{\alpha_s}{\pi }\frac{\mu}{M^2_{G_H}}\left(\frac{\pi^2}{9}-1\right) \ .$

Note that single production is suppressed by a factor

$(\pi^2/9 -1)^2$,

which is an accidental suppression factor coming from the loop. Above the threshold for decays into a single top quark, GH has two main decay modes: the decay into gluons, which occurs at loop-level similar to single coloron production, and the flavor-violating decay into tc. The corresponding rates are:

$\Gamma \left[G_H \to (\bar{c}_L t_R +\bar{t}_R c_L )\right] =\left(V_{cb}\right)^2 \frac{M_{G_H}}{16 \pi} \frac{m^2_t}{u^2}\left(1-\frac{m^2_t}{M^2_{G_H}}\right)^2 \,, $ \newline
$\Gamma \left[G_H \to gg \right]=\frac{5 \alpha^2_s}{1536 \pi^3}\frac{\mu^2}{M_{G_H}}\left(\frac{\pi^2}{9}-1\right)^2 \,.$

We set u=mu (the stability of the potential forbids mu>u); and consider for simplicity the set of

$(M_{G_H}, \mu)$

values that give a 50% GH decay into tc and 50% into gg. GH is a very narrow resonance, with a width of the order of 10-4 GeV.

See more details in

Model Files

Generation specifics

In 1409.7607v2, the samples were generated with the coloron mass as the scale, dsqrt_q2fact1, and dsqrt_q2fact2 in the run_card.dat file. These samples were also generated without MadGraph cuts as demonstrated in the run_card.dat for 500 GeV mass included above. The specific generations run were

p p > GH, GH > b c~ l+ vl @1 GHT=1 QED=2
p p > GH, GH > b~ c l- vl~ @2 GHT=1 QED=2

To generate the settings for a specific coloron mass, use the appropriate model directory contained in the Octet-tcgg zip file. Parameter card files are included for the combination of settings that gives a branching ratio of 0.5 to tc. For other BR values, please see 1409.7607v2.

Related Models

Attachments