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Squark decays beyond MFV at LHC

People: Priscilla Pani, Giacomo Polesello, Amit Chakraborty, Björn Herrmann, Benjamin Fuks, Mihoko Nojiri, Abishek Iyer, Ramona Gröber



  • Investigate QFV signature: pp → t j ETmiss
  • Start by implementing a simplified model, then try to move on to more realistic case.


We start by investigating a simplified model setup as follows. We complement the SM by a right-handed scharm (scR) and stop (stR).

In the gauge eigenbasis, the two-squark system is parametrized with the three parameters:

  • Mst (stop mass parameter),
  • Msc (scharm mass parameter),
  • Mstc (mixing parameter).

After diagonalisation, in the mass eigenbasis, the three physical parameters are:

  • Msq1 (lighter squark),
  • Msq2 (heavier squark),
  • theta (mixing angle),

such that sq1 = ct*scR + st*St (and the corresponding relation for the second squark state sq2).

In addition, we add to the model a gluino (go), with the appropriate SUSY mixings (so that it will contribute to the various production process) and a bino (chi) that will allow both squarks to decay. The corresponding mass parameters are

  • Mchi (neutralino mass),
  • Mgo (gluino mass).

The FeynRules model file is available here, and the corresponding UFO there. Do not forget, in madgraph, to load the model as

 import model stop-scharm_UFO -modelname 

and that the neutralino is called chi (and not n1).

Working plan

1. Compute diagonal production cross-sections (at LO): function of mSq1 and mSq2, but independent of thSq.

2. Compute non-diagonal production cross-sections (at LO): function of mSq1, mSq2, and thSq. Evaluate, e.g., for thSq=(0, pi/4, pi/2) and mGl=3 TeV, and check importance w.r.t. diagonal production.

3. Compute branching ratios of Sq1 and Sq2 into t+N1 and into c+N1: function of mSq1, mSq2, thSq, mN1.

4. From the above, deduce cross-sections pp → t t ETmiss, pp → c c ETmiss, pp → t c ETmiss: functions of mSq1, mSq2, thSq, mN1. Make sure to seperate contributions from different subchannels, since this information is necessary to determine the experimental acceptance!

5. Compute the above signal cross-sections in the full model (i.e. with 4×4 mixing) and use the obtained acceptances to deduce the limits. For this, we will use analytical formulas tu compute the BR. NLO+NLL cross sections for the signal.

6. At the end, try a different bino mass (50 GeV as default, and let's increase it). Try also to include BR of the squarks into something else (that is not populating the signal regions). Finally, maybe get the gluino mass dependence.

Plots and documents

Rescaling of exclusion by BR

This set of macro was created to automatically rescale the scharm and stop analyses in the reference (8TeV 20ifb results) for any assumptions of mixing and U-masses. Download the full directory: nmfvstop.tgz

Execution Command (to be run in the working directory - run python –help for parameters explanation )

python –u1max 1000 –u1min 400 –step 100 –lumi 20 –alphalist 4,8 –Mchi 50

(NOTE before each parameter you need two dashes, I cannot make it appear in the twiki FIXME)


  • “Squark and gaugino hadroproduction and decays in non-minimal flavour violating supersymmetry” (arXiv)
  • “Impact of squark generation mixing on the search for squarks decaying into fermions at LHC”, (arXiv)
  • “General squark flavour mixing: constraints, phenomenology and benchmarks” (arXiv)
  • “Gluino Meets Flavored Naturalness” (arXiv)
  • “Light stop decays” (arXiv)
  • “Light stop decays into WbN1 near the kinematic threshold” (arXiv)
  • “Search for top squark pair production in final states with one isolated lepton, jets, and missing transverse momentum in s√= 8 TeV pp collisions with the ATLAS detector” (arXiv)
  • “Search for invisible particles produced in association with single-top-quarks in proton-proton collisions at s√s = 8 TeV with the ATLAS detector” (arXiv)
2017/groups/np/qfvlhc.txt · Last modified: 2017/09/11 15:29 by priscilla.pani