Hidden Valley MC Generation with Pythia8.

Since version 8.150, Hidden Valley radiation has been available in Pythia.


Howto set up Pythia

Download latest (8.157) Pythia
wget http://home.thep.lu.se/~torbjorn/pythia8/pythia8157.tgz
tar -zxvf pythia8157.tgz
cd pythia8157

Our physics model

We are interested in generating lepton jets files as in this paper, C. Cheung, J. T. Ruderman, L-T Wang, I. Yavin A typical event looks like


Where we can identify three stages:

  1. quark-quark interaction with a mediating gluino to produce 2 squarks. Each squark decays into a quark (jet) and a neutralino. The Neutralino is our Portal into the dark sector.
  2. Neutralino decays to a dark boson and dark fermion. Dark particles are charged under the dark gauge group and can radiate (massive) dark photons (FSR). Depending on the spectrum of the dark sector, dark cascades can occur. Typically, the dark fermion is stable and carries away missing energy. The dark boson might be the dark gauge boson, or decay to the dark gauge boson.
  3. Decay of the dark gauge boson back into the SM due to kinetic mixing of the dark gauge group with the SM EM tensor.

The particle spectrum considered in the above mentioned paper is as following:

Dark Particle Mass (GeV) PDG ID Decays to
------------------------------------- -------- ----------------------- -----------------
Dark Pseudo-Scalar $a_{d}$ 1.5 4900001 $b_{\mu}h_{d}$
Dark Higgs (light) $h_{d}$ 0.1 4900002 $\ensuremath{{\not\mathrel{E}}_T}$
Dark Higgs (heavy) $H_{d}$ 1.2 4900003 $b_{\mu}b_{\mu}$
Dark Fermion $f_{d}$ 1. 4900004 $\ensuremath{{\not\mathrel{E}}_T}$
Dark Photon $b_{\mu}$ .15 4900022 $e^{+}e^{-}$

split1.png split2.png split3.png split4.png

Depending on the dark photon's mass it can decay back into e+e- and mu+mu- pairs and pion pairs. A heavier dark photon could decay into protons, but unlike for positrons, no cosmic proton excess has been observed at high energies by PAMELA and ATIC. The observed positron excess initiated the development of hidden-valley dark-matter theories, predicting lepton jet signatures in collider experiments link.

Setup our physics model in Pythia

Pythia8 allows showering in a hidden sector with a gauge structure of choice. To validate the showering we set up a model identical to the model provided by C. Cheung, J. T. Ruderman, L-T Wang, I. Yavin as described above. HV particle IDs are hard coded in Pythia8, therefore if we'd like to refrain from altering the Pythia8 source code, we'd better assign our dark particles these particle IDs. Table 1 in this paper shows all available HV particles. We need 4 particles (490001 - 4900004) together with the dark gauge boson for U(1) (4900022). The 4 particles' (490001 - 4900004) mass and quantum numbers need to be changed in order to reflect the model. In our LeptonJets.cmd file (similar to QNUMBERS in SLHA):

Change directory to examples and create LeptonJets.cmd:

4900001:m0 = 1.5
4900002:m0 = 0.1
4900003:m0 = 1.2
4900004:m0 = 1

4900001:spinType = 1
4900002:spinType = 1
4900003:spinType = 1
4900004:spinType = 2

4900001:chargeType = 0
4900002:chargeType = 0
4900003:chargeType = 0
4900004:chargeType = 0

4900001:colType = 0
4900002:colType = 0
4900003:colType = 0
4900004:colType = 0

4900001:name = darkPseudoScalar
4900002:name = darkLightHiggs
4900003:name = darkHeavyHiggs
4900004:name = darkFermion

4900001:antiname =
4900002:antiname =
4900003:antiname =
4900004:antiname = darkFermionBar

The following is probably unnecessary, but to make sure the other HV particles don't do anything we make them very massive:

4900005:m0 = 1000000
4900006:m0 = 1000000
4900011:m0 = 1000000
4900012:m0 = 1000000
4900013:m0 = 1000000
4900014:m0 = 1000000
4900015:m0 = 1000000
4900016:m0 = 1000000
4900021:m0 = 1000000
4900023:m0 = 1000000
4900101:m0 = 1000000
4900111:m0 = 1000000
4900113:m0 = 1000000
4900211:m0 = 1000000
4900213:m0 = 1000000
4900991:m0 = 1000000

The decay modes in the dark sector are set using a SLHA file. Pythia8 comes standard with a sps1a SLHA file, which are the same mass points used in the paper whose model we like to mimic for validation purposes. Open the sps1aWithDecays.spc file and add to the bottom the decays. First the Neutralino (100022) decay into the dark sector, then the splittings in the dark sector, and the decay of the dark gauge boson to two electrons.

#         PDG            Width
DECAY   1000022     1.00000000E-00  # ~chi_10 decays
#          BR         NDA      ID1       ID2
     2.50000000E-01    2     4900022        4900004   # BR( ~chi_10 -> gammav hvfermion )
     2.50000000E-01    2     4900001        4900004   # BR( ~chi_10 -> hvpseudoscalar hvfermion )
     2.50000000E-01    2     4900002        4900004   # BR( ~chi_10 -> hvlighthiggs hvfermion )
     2.50000000E-01    2     4900003        4900004   # BR( ~chi_10 -> hvheavyhiggs hvfermion )
#            PDG       Width
DECAY   4900001   2.21729879e-15   #
#          BR         NDA      ID1       ID2
     1.00000000E+00    2     4900022     4900002       # BR( hvpseudoscalar -> gammav  hvlighthiggs )
#            PDG       Width
DECAY   4900003   2.00000000E-15   # width value ...
#          BR         NDA      ID1       ID2
     1.00000000E+00    2     4900022     4900022      # BR( hvheavyhiggs -> gammav  gammav )
#            PDG       Width
DECAY   4900022   0.80000000E-15   # width value set to arbitrary number
#          BR         NDA      ID1       ID2
     1.00000000E+00    2       11         -11      # BR( gammav -> 11  -11 )

Add the SLHA file to the beginning of LeptonJets.cmd and update the minDecayDeltaM in order to let the dark boson decay.
Turn ISR and FSR on and multiple interactions and hadronization off:

SLHA:file = sps1aWithDecays.spc
SLHA:minDecayDeltaM = 0.001

PartonLevel:MI = off               ! no multiple interactions
PartonLevel:ISR = on              ! no initial-state radiation
PartonLevel:FSR = on              ! no final-state radiation
HadronLevel:Hadronize = off        ! no hadronization

Also allow the dark gauge boson to decay and set its mass

4900022:mayDecay = true
4900022:m0 = 0.150        !mass of dark photon is now set to 200 MeV
4900022:0:meMode = 0 !pfff

And finally turn on HV radiation and set the dark sector to be Abelian, U(1). Here we also set the coupling of the hidden sector;

HiddenValley:Ngauge = 1
HiddenValley:doKinMix = on
HiddenValley:FSR = on
HiddenValley:alphaFSR = 0.1


The authors of ... simulated showering in the dark sector using a private Mathemica package. To compare dark sector radiation by Pythia8 against their simulation, we start with the same generated events up until the Neutralino production. Here are the LHE files with these events. Decays of the Neutralinos into the dark sector and the ensuing dark decays and radiation are separately simulated by Mathematica and Pythia8. In order to factorize the radiation from the decays in the dark sector, we first compare the dark gauge boson's $P_{T}$ spectrum for $\alpha_{d} = 0.0$ (no radiation).
The good agreement for the number of dark gauge bosons above 5 GeV and the the dark gauge boson's $P_{T}$ spectrum implies that our Pythia8 model is set up correctly.

nDP_hist_Pythia8_0.0_Mathematica_0.0.png pt_hist_Pythia8_0.0_Mathematica_0.0.png

With Radiation

pT_hist_50_Pythia8_0.1_Mathematica_0.1.png pT_hist_50_Pythia8_0.628_Mathematica_0.1.png
pT_hist_50_Pythia8_0.3_Mathematica_0.3.png pT_hist_50_Pythia8_1.885_Mathematica_0.3.png
-- LoekHooftVanHuysduynen - 19-Dec-2011
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Topic revision: r8 - 2012-04-02 - unknown
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