Protection to stabilize the behavior of PFClusterAlgo #2490
Conversation
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A new Pull Request was created by @lgray (Lindsey Gray) for CMSSW_7_1_X. Protection to stabilize the behavior of PFClusterAlgo It involves the following packages: RecoParticleFlow/PFClusterProducer @nclopezo, @cmsbuild, @anton-a, @thspeer, @slava77, @Degano can you please review it and eventually sign? Thanks. |
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-1 runTheMatrix-results/50101.0_SingleMuPt10+SingleMuPt10FSIdINPUT+SingleMuPt10FS_ID/step2_SingleMuPt10+SingleMuPt10FSIdINPUT+SingleMuPt10FS_ID.log you can see the results of the tests here: |
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This was a DAS error, retest. |
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@slava77 if it is possible, could we get this in pre3? |
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FYI: I looked at the automatic comparisons and they show no issues. |
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checked the effect of the change using fwlite comparisons for single electrons (pt 35, 1000), photons (pt 10, 35), muons (pt 10, 1000), QCD (wf38.0), and ttbar (wf202.0). Some differences in the distributions for both low-level and high-level objects are observed. They are consistent with the expected small variations of energy and position of clusters and do not exhibit trends. Possible exceptions are the PS clusters where change in the spectrum is observed (a plot attached below for ttbar). Need to run some DQM comparisons before signing off as the one from jenkins are low stats. |
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@anton-a I think I've identified the problem. The energy-scaled distance weight needs to be normalized to the clustering threshold. The preshower has a threshold of 6e-5, and everything else has thresholds O(1). This could cause PS hits that are actually close by to not be considered, since the 1e-20 cut is effective ~1e-15 for the PS. If we normalize this it should fix this issue. I'll push a patch in an hour or so. |
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@anton-a Yes, this is definitely it since the shower sigma for the PS is 0.2. 1e-20 going to 1e-15 would definitely cause the effect you see. |
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@anton-a here is the proposed fix |
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@anton-a I need to fix a minor inconsistency. Just a moment. |
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@anton-a Everything is good to go now. Please retest! |
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@anton-a I have added a silent protection against this case as discussed. If you give a zero threshold, it will simply normalize to one. This reproduces the original behavior of the algorithm. |
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The changes did not fix the trend observed for the PS clusters in wf202. The new plot is virtually identical to the posted above. Will review the rest in the morning. |
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Ok, I'll will see if I can find out what's up on my side as well. Question: does the preshower spectrum change in the single electron or single photon relvals? If it does there are two clear possibilities: Since the shower sigma is so small for the PS and it has long, thin strips (6.3 cm x 0.2 cm) stacked in either the X or Y directions, one case could be that we're now dropping energy that was being improperly clustered in because of numerical issues. Another, more likely, scenario comes from physics-based arguments. Right now the shower sigma for the PS is 0.2. This means that one strip is one sigma away. The Moliere radius of lead is 1.6 cm, or 8 sigmas, which translates to a distance weight of 1e-14, which is safe in terms of numerics. However, the Moliere radius only defines a region of 90% containment, so there could be additional energy that actually belongs to the shower in the "unsafe" 10-11 sigma region that is cut away at present. I'll check with some preshower experts to make sure this reasoning is sound and check in ttbar. Update:
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Change in energy spectrum is expected.
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After talking to preshower experts, the change in the energy spectrum is actually expected. The shower size in the preshower is actually very narrow! Of the order of the size of a PS strip, 0.2cm. The numerical protection is doing its job here and restoring sane behavior, since with out the protection basically any PS hit in the topological cluster was contributing to a random PS cluster! I will try to confirm in a bit that things are ok for real EM showers with a single photon gun. |
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Here are the equivalent plots for photon gun 35 GeV using the original value for shower sigma but with and without the numerical protection. High statistics will help clear up the case, but there's really not a visible change for true EM objects.
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The effect on clusters extends mostly to PS. HCAL, and ECAL are less affected as shown on the plots for wf202 Higher level objects are affected, but no trends are observed and the effect is small. |
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@anton-a Could you post plots for electrons as well? |
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Single electron pt35 |
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+1 |
Reco updates -- Protection to stabilize the behavior of PFClusterAlgo
Require that the sum of rechit fractions be of reasonable size to be sure you're not adding some rechit that is far away any cluster position.
An example case of the instability is as follows:
some rechit is, at closest, ~7-8 crystals away from any cluster position. This means that the largest rechit fraction is going to be ~1e-23. The smaller fractions from clusters even farther away can be ~1e-50, which is outside of double precision when you add them together. The resulting fraction can then randomly be one or very very slightly less than one.
The means that when the fractions are normalized a rechit that is very far away from any cluster can be randomly added in its entirety to a cluster it simply doesn't belong to, and it can vanish for no reason when doing the position fit in PFClusterAlgo. This makes the algorithm convergence unstable.
Requiring the rechit fraction sum to be larger than 1e-20 fixes this issue, since it ensures that at least one cluster is within about 10 shower-sigmas of the rechit. The only exception to this case are the rechit seeds, which shouldn't move very from from the cluster position on average.
Note: this can change the final positions and energies of PFClusters (especially low-energy clusters) in all sub-detectors, but should not affect higher-level physics objects.