5.9. Hard decays¶
The handler for decays of particles produced in the hard scattering
process (e.g. W, Z, top, Higgs) can be enabled and configured using
the HARD_DECAYS
collection of settings (and a small number
of other other top-level settings). Which (anti)particles IDs should
be treated as unstable is determined by the
PARTICLE_DATA:<id>:Stable
switch described in Models.
The syntax to configure HARD_DECAYS
sub-settings is:
HARD_DECAYS:
<sub-setting>: <value>
# more sub-settings ...
Channels:
"<channel id>":
<channel sub-setting>: <value>
# more sub-settings for <channel>
# more channels ...
The channel ID codes are of the form a -> b c ...
. The particle
IDs for the decay channels can be found in the decay table printed to
screen during the run.
This decay module can also be used on top of NLO matrix elements, but it does not include any NLO corrections in the decay matrix elements themselves.
Note that the decay handler is an afterburner at the event generation
level. It does not affect the calculation and integration of the hard
scattering matrix elements. The cross section is thus unaffected
during integration, and the branching ratios (if any decay channels
have been disabled) are only taken into account for the event weights
and cross section output at the end of event generation (if not
disabled with the HARD_DECAYS:Apply_Branching_Ratios
option,
cf. below). Furthermore any cuts or scale definitions are not
affected by decays and operate only on the inclusively produced
particles before decays.
5.9.1. Status¶
This sub-setting to each channel defined in HARD_DECAYS:Channels
allows to explicitly force or disable a decay channel. The status can take the
following values:
Status: -1
Decay channel is disabled and does not contribute to total width.
Status: 0
Decay channel is disabled but contributes to total width.
Status: 1 (default)
Decay channel is enabled.
Status: 2
Decay channel is forced.
For example, to disable the hadronic decay channels of the W boson one would use:
HARD_DECAYS:
Channels:
"24 -> 2 -1": { Status: 0 }
"24 -> 4 -3": { Status: 0 }
"-24 -> -2 1": { Status: 0 }
"-24 -> -4 3": { Status: 0 }
In the same way, the bottom decay mode of the Higgs could be forced using:
"25 -> 5 -5": { Status: 2 }
Note that the ordering of the decay products in <channel id>
is
important and has to be identical to the ordering in the decay table
printed to screen. It is also possible to request multiple forced
decay channels (Status: 2
) for the same particle, all other
channels will then automatically be disabled.
5.9.2. Width¶
This option allows to overwrite the calculated partial width (in GeV) of a given decay channel, and even to add new inactive channels which contribute to the total width. This is useful to adjust the branching ratios, which are used for the relative contributions of different channels and also influence the cross section during event generation, as well as the total width which is used for the lineshape of the resonance.
An example to set (/add) the partial widths of the H->ff
,
H->gg
and H->yy
channels can be seen in the following. The
values have been taken from LHCHXSWG):
PARTICLE_DATA:
25:
Mass: 125
Width: 0.00407
HARD_DECAYS:
Enabled: true
Channels:
"25 -> 5 -5": { Width: 2.35e-3 }
"25 -> 15 -15": { Width: 2.57e-4 }
"25 -> 13 -13": { Width: 8.91e-7 }
"25 -> 4 -4": { Width: 1.18e-4 }
"25 -> 3 -3": { Width: 1.00e-6 }
"25 -> 21 21": { Width: 3.49e-4 }
"25 -> 22 22": { Width: 9.28e-6 }
Another example, setting the leptonic and hadronic decay channels of W and Z bosons to the PDG values, would be specified as follows:
HARD_DECAYS:
Enabled: true
Channels:
"24 -> 2 -1": { Width: 0.7041 }
"24 -> 4 -3": { Width: 0.7041 }
"24 -> 12 -11": { Width: 0.2256 }
"24 -> 14 -13": { Width: 0.2256 }
"24 -> 16 -15": { Width: 0.2256 }
"-24 -> -2 1": { Width: 0.7041 }
"-24 -> -4 3": { Width: 0.7041 }
"-24 -> -12 11": { Width: 0.2256 }
"-24 -> -14 13": { Width: 0.2256 }
"-24 -> -16 15": { Width: 0.2256 }
"23 -> 1 -1": { Width: 0.3828 }
"23 -> 2 -2": { Width: 0.2980 }
"23 -> 3 -3": { Width: 0.3828 }
"23 -> 4 -4": { Width: 0.2980 }
"23 -> 5 -5": { Width: 0.3828 }
"23 -> 11 -11": { Width: 0.0840 }
"23 -> 12 -12": { Width: 0.1663 }
"23 -> 13 -13": { Width: 0.0840 }
"23 -> 14 -14": { Width: 0.1663 }
"23 -> 15 -15": { Width: 0.0840 }
"23 -> 16 -16": { Width: 0.1663 }
5.9.3. HARD_SPIN_CORRELATIONS¶
Spin correlations between the hard scattering process and the
following decay processes are enabled by default. If you want to
disable them, e.g. for spin correlation studies, you can specify the
option HARD_SPIN_CORRELATIONS: 0
. This is a top-level
setting as opposed to the other HARD_DECAYS
-related settings.
5.9.4. Store_Results¶
The decay table and partial widths are calculated on-the-fly during
the initialization phase of Sherpa from the given model and its
particles and interaction vertices. To store these results in the
Results/Decays
directory, one has to specify HARD_DECAYS:
{ Store_Results: 1 }
. In case existing decay tables are to be read
in the same configuration should be done. Please note, that Sherpa
will delete decay channels present in the read in results but not in
the present model with present parameters by default. To prevent
Sherpa from updating the decay table files accordingly specify
HARD_DECAYS: { Store_Results: 2 }
.
5.9.5. Result_Directory¶
Specifies the name of the directory where the decay results are to be stored. Defaults to the value of the top-level setting RESULT_DIRECTORY.
5.9.6. Set_Widths¶
The decay handler computes LO partial and total decay widths and
generates decays with corresponding branching fractions, independently
from the particle widths specified by
PARTICLE_DATA:<id>:Width
. The latter are relevant only for
the core process and should be set to zero for all unstable particles
appearing in the core-process final state. This guarantees
on-shellness and gauge invariance of the core process, and subsequent
decays can be handled by the afterburner. In constrast,
PARTICLE_DATA:<id>:Width
should be set to the physical width
when unstable particles appear (only) as intermediate states in the
core process, i.e. when production and decay are handled as a full
process or using Decay
/DecayOS
. In this case, the option
HARD_DECAYS: { Set_Widths: true }
permits to overwrite the
PARTICLE_DATA:<id>:Width
values of unstable particles by the
LO widths computed by the decay handler.
5.9.7. Apply_Branching_Ratios¶
By default (HARD_DECAYS: { Apply_Branching_Ratios: true }
),
weights for events which involve a hard decay are multiplied with the
corresponding branching ratios (if decay channels have been
disabled). This also means that the total cross section at the end of
the event generation run already includes the appropriate BR
factors. If you want to disable that, e.g. because you want to
multiply with your own modified BR, you can set the option
{HARD_DECAYS: { Apply_Branching_Ratios: false }
.
5.9.8. Mass_Smearing¶
With the default of HARD_DECAYS: { Mass_Smearing: 1 }
the
kinematic mass of the unstable propagator is distributed according to
a Breit-Wigner shape a posteriori. All matrix elements are still
calculated in the narrow-width approximation with onshell
particles. Only the kinematics are affected. To keep all intermediate
particles onshell {HARD_DECAYS: { Mass_Smearing: 0 }
.
5.9.9. Resolve_Decays¶
There are different options how to decide when a 1->2 process should be replaced by the respective 1->3 processes built from its decaying daughter particles.
Resolve_Decays: Threshold
(default) Only when the sum of decay product masses exceeds the decayer mass.
Resolve_Decays: ByWidth
As soon as the sum of 1->3 partial widths exceeds the 1->2 partial width.
Resolve_Decays: None
No 1->3 decays are taken into account.
In all cases, one can exclude the replacement of a particle below a
given width threshold using Min_Prop_Width:
(default 0.0). Both settings are sub-settings of
HARD_DECAYS
:
HARD_DECAYS:
Resolve_Decays: <mode>
Min_Prop_Width: <threshold>
5.9.10. Decay_Tau¶
By default, the tau lepton is decayed by the hadron decay module,
Hadron decays, which includes not only the leptonic decay
channels but also the hadronic modes. If Decay_Tau: true
is
specified, the tau lepton will be decayed in the hard decay handler,
which only takes leptonic and partonic decay modes into account. Note,
that in this case the tau needs to also be set massive:
PARTICLE_DATA:
15:
Massive: true
HARD_DECAYS:
Decay_Tau: true
5.9.11. Decay table integration settings¶
Three parameters can be used to steer the accuracy and time
consumption of the calculation of the partial widths in the decay
table: Int_Accuracy: 0.01
specifies a relative accuracy for
the integration. The corresponding target reference is either the
given total width of the decaying particle (Int_Target_Mode:
0
, default) or the calculated partial decay width
(Int_Target_Mode: 1
). The option Int_NIter: 2500
can be used to change the number of points per integration iteration,
and thus also the minimal number of points to be used in an
integration. All decay table integration settings are sub-settings of
HARD_DECAYS
.