5.13. Colour_Reconnections

The colour reconnections setup covers the non-perturbative reshuffling of parton colours before the partons fragment into primordial hadrons. In the current implementation Sherpa collects all \(N\) colourful partons entering colour reconnections and probes \(N^2\) pairs of colour connections between two partons \(i\) and \(j\). Their Lund-inspired distance in phase space is given by \(d_{ij} = p_ip_j-m_im_j\), where the gluon momenta are divided equally between both colours they carry.

The reshuffling of colour from old pairings \(\langle ij\rangle\) and \(\langle kl\rangle\) to new pairings \(\langle il\rangle\) and \(\langle kj\rangle\) is decided probabilistically, with

\(P(\langle ij\rangle\langle kl\rangle\to\langle il\rangle\langle kj\rangle) = R\cdot \left\{1-\exp[-\eta_Q(D_{ij}+D_{kl}-D_{il}-D_{kj})]\right\}\).

For a power-law in the definition of the \(D_{ij}\) we also calculate the average length of all colour connections \(ij\) as \(\langle D_{ij}\rangle = \frac{1}{N}\sum_{ij}d_{ij}^\kappa\).

COLOUR_RECONNECTIONS:
  MODE:      On
  PMODE:     Log
  Q_0:       1.
  RESHUFFLE: 0.11

MODE (default: )

Switches the colour reconnections on or off.

PMODE (default: log)

This switch defines how the distances of two partons in colour space are being calculated. Available options define the distances \(D_{ij}\) used in the decision whether colours are reshuffled as follows:

• Log: \(D_{ij} = \log(1+d_{ij}/Q_0^2)\)

• Power: \(D_{ij} = \frac{d_{ij}^\kappa}{\langle D_{ij}\rangle}\)

Q_0 (default: 1.)

\(Q_0\) in the logarithmic version of the momentum-space distance.

ETA_Q (default: 0.1)

\(\eta_Q\) in the probability above.

RESHUFFLE (default: 1/9)

The colour suppression factor \(R\) in the probability above.

KAPPA (default: 1.)

The exponent \(\kappa\) in the equations above.