From 6aa7ea32b106e245c01b61c8da14814e3796b073 Mon Sep 17 00:00:00 2001 From: Wren Vetens Date: Wed, 16 Oct 2024 12:33:41 -0400 Subject: [PATCH 1/3] Added warning for analysts to be careful with pre-existing decfiles and to verify for themselves the consistency and relevance of the decfiles they want to use --- second-analysis-steps/simulation-dkfiles.md | 2 ++ 1 file changed, 2 insertions(+) diff --git a/second-analysis-steps/simulation-dkfiles.md b/second-analysis-steps/simulation-dkfiles.md index 6c0b7326..76d3b543 100644 --- a/second-analysis-steps/simulation-dkfiles.md +++ b/second-analysis-steps/simulation-dkfiles.md @@ -60,3 +60,5 @@ The "Cuts" field specifies which one of a predetermined set of cut tools are use The bottom part of the decay file specifies the decay itself: This DecFile defines a signal `D*+` which decays 100% to `D0` `pi+`, and the D0 in turn decays 100% into `K+`, `K-`, `mu+` and `mu-`. Important is the definition of "MyD0". If the decay was to "D0" rather than "MyD0", the D0 would decay via all of the decay modes implemented in DECAY.DEC. The final part of each decay is the actual physics model used - in this case "VSS", the generic matrix element for vector to scaler-scaler transitions, and "PHSP", which is phase space only (matrix element = constant). Note that with PHSP, the decay products are completely unpolarized - for anything other than (spin0) to (spin0 spin0) this will get the angular momentum wrong! + +When using pre-existing decay files, it is important to check them closely for consistency first. Many Decay files are created for a specific physics use case, and some may even get physics wrong when it comes to e.g. the relative frequencies of intermediate states in a cocktail. From 2dfb0e741f634b80d132822f7c440f6fe71b4765 Mon Sep 17 00:00:00 2001 From: Wren Vetens Date: Wed, 16 Oct 2024 12:35:56 -0400 Subject: [PATCH 2/3] Added warning about cocktails of intermediate final states forced to one or more final states with description of a common mistake and how to avoid it. --- .../simulation-advanced-dkfiles.md | 54 +++++++++++++++++++ 1 file changed, 54 insertions(+) diff --git a/second-analysis-steps/simulation-advanced-dkfiles.md b/second-analysis-steps/simulation-advanced-dkfiles.md index 00d9176e..1e9df588 100644 --- a/second-analysis-steps/simulation-advanced-dkfiles.md +++ b/second-analysis-steps/simulation-advanced-dkfiles.md @@ -79,6 +79,60 @@ CDecay MyD_s- ``` Note that the fractions will always be renormalised to sum to 1 - you can directly use PDG branching fractions without having to rescale by hand. +## A Warning about intermediate state cocktails with forced final states + +*Be careful!* - if you are simulating a cocktail of intermediate states and forcing a decay to one or more final states, you may need to correct the branching fractions of all the intermediate decays as well. For example, if studying inclusive decays for some ``Xc0 -> D0X`` or ``Xc0 -> D+X`` with intermediate ``D*+`` and ``D*0`` states: + +If ``Xc0`` decays to ``D*0 pi0`` and ``D*+ pi-`` with equal probability and you are interested in the ``D0`` final state, to have a decfile such as: + +``` +Decay MyXc0 + 0.5000 MyD*0 pi0 PHOTOS ; + 0.5000 MyD*+ pi- PHOTOS ; +Enddecay +CDecay MyAntiXc0 +# +Decay MyD*+ + 0.677 D0 pi+ PHOTOS VSS; +Enddecay +CDecay MyD*- +# +Decay MyD*0 + 0.647 D0 pi0 PHOTOS VSS; + 0.353 D0 gamma PHOTOS VSP_PWAVE; + 0.00391 D0 e+ e- PHOTOS PHSP; +Enddecay +CDecay MyAntiD*0 +``` + +would result in a sample where an equal number of ``D0`` are coming from intermediate ``D*+`` as ``D*0``, even though the branching fraction for the former is only 67.7% - thus the ``D0 pi+ pi-`` final state would be overrepresented in the sample. + +This is to say that the fact that EvtGen normalizes all branching fractions for a decay to 1 matters for cocktails with multiple intermediate final states decaying to a forced final state! The proper handling would instead be: + +``` +Decay MyXc0 + 0.5000 MyD*0 pi0 PHOTOS ; + 0.3385 MyD*+ pi- PHOTOS ; +Enddecay +CDecay MyAntiXc0 +# +Decay MyD*+ + 0.677 D0 pi+ PHOTOS VSS; +Enddecay +CDecay MyD*- +# +Decay MyD*0 + 0.647 D0 pi0 PHOTOS VSS; + 0.353 D0 gamma PHOTOS VSP_PWAVE; + 0.00391 D0 e+ e- PHOTOS PHSP; +Enddecay +CDecay MyAntiD*0 +``` + +If you have multiple generations of intermediate cocktail states, the forced final state decay needs to be back-propagated through each generation in this manner. + +It is important to be careful about this even when using **pre-existing** decfiles and always check them for consistency, as even many decfiles within the database do not take this important factor into account. + ## Final state radiation After generating the decay, final state radiation is added using PHOTOS. Note that PHOTOS is enabled by default, even though many decfiles explicitly specify it. It needs to be explicitly removed via "noPhotos" From 97593c7c0521fc0f7a051f59f8f2e788ca6b36f4 Mon Sep 17 00:00:00 2001 From: Wren Vetens Date: Tue, 29 Oct 2024 16:20:23 -0400 Subject: [PATCH 3/3] Make the warning about decfiles stand out more clearly (& edits for language precision) --- second-analysis-steps/simulation-dkfiles.md | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/second-analysis-steps/simulation-dkfiles.md b/second-analysis-steps/simulation-dkfiles.md index 76d3b543..15b0d77f 100644 --- a/second-analysis-steps/simulation-dkfiles.md +++ b/second-analysis-steps/simulation-dkfiles.md @@ -61,4 +61,8 @@ The bottom part of the decay file specifies the decay itself: This DecFile defines a signal `D*+` which decays 100% to `D0` `pi+`, and the D0 in turn decays 100% into `K+`, `K-`, `mu+` and `mu-`. Important is the definition of "MyD0". If the decay was to "D0" rather than "MyD0", the D0 would decay via all of the decay modes implemented in DECAY.DEC. The final part of each decay is the actual physics model used - in this case "VSS", the generic matrix element for vector to scaler-scaler transitions, and "PHSP", which is phase space only (matrix element = constant). Note that with PHSP, the decay products are completely unpolarized - for anything other than (spin0) to (spin0 spin0) this will get the angular momentum wrong! -When using pre-existing decay files, it is important to check them closely for consistency first. Many Decay files are created for a specific physics use case, and some may even get physics wrong when it comes to e.g. the relative frequencies of intermediate states in a cocktail. +{% callout "Double Check Pre-Existing Decay Files!" %} + +When using pre-existing decay files, it is important to check them closely for consistency first. Many Decay files are created for a specific physics use case, and some may even get physics wrong when it comes to e.g. the relative branching fractions of intermediate states in a cocktail with a forced final state. + +{% endcallout %}