Cross sections with cut lines vs. inclusive do not agree

[Frank Siegert]

Bill Murray looks at three runs:

1. "inclusive" process pp -> gamma b bbar
2. "background" process pp -> gamma b bbar (with Z disabled as particle)
3. "signal" process pp -> gamma Z[b bbar].

Looking at the b bbar mass around the Z mass he sees a discrepancy between run 1 and the sum of run 2 + run 3 (the former shows a higher peak above the background by about a factor of 2).

[anonymous]

The factor 3 reported above was not found from putting the higgs mass to 400 GeV (although I did try that, I saw H to ZZ to mumubb which confused the issue)

I simply disabled the higgs in the control cards - that gave the

discrepancy. So I think this is a different probblem.

Bill

[Frank Siegert]

Thanks for sending me your setup files, I have tried to reformulate the problem description in light of these, hopefully correct this time?

Some remarks:

• Since your final state is not one to apply CKKW merging on, you should switch it off. To do that, set "SUDAKOV_WEIGHT=0" and "SCALE_SCHEME=G_MEAN_PT2" (or whatever you think appropriate) in the (me) section.

• In your "no Z" run 2 you are missing lots of diagrams from the "inclusive" run 1, on top of the ones that you are using in the "forced Z" run 3. Some of these are s-channel types, which one can try to include in run 3 (or observe that their cross section is low), but there are also t-channel ones, which might have an impact. That makes it unclear to me whether the sum of 2 and 3 should really coincide with 1.

• I can't judge which interference effects might appear here between the diagrams in run 1 that you are separating in run 2 and 3?

• (PS: If you set EVENT_GENERATION_MODE=Weighted you get cross-section normalised histograms automatically.)

[Bill Murray]

First comment: The comment I inserted yesterday has not appeared. Bother. Basically I was saying that the same problem appears in ZH with/without a Higgs as in gamma/Z with/without a Z, and as the higgs is narrow interference did not feel like an issue.

Anyway, I changed the scale_scheme and Sudakov weight as you suggest, with interesting results:

• The gamma-Z cross section is unchanged
• The gamma bb cross sections roughly halve

==> The exclusive rate is now 40% of the inclusive, instead of 20%, basically

because the inclusive rates are all halved.

So things get better looking. But now I wonder why one process depends upon the scale and not the other.

Bill

[Frank Siegert]

Replying to Bill Murray:

Basically I was saying that the same problem appears in ZH with/without a Higgs as in gamma/Z with/without a Z, and as the higgs is narrow interference did not feel like an issue.

But what about the missing diagrams? I haven't looked at the diagrams in ZH yet, but for gamma/Z there were some (see my second point above).

Anyway, I changed the scale_scheme and Sudakov weight as you suggest, with interesting results:

• The gamma-Z cross section is unchanged
• The gamma bb cross sections roughly halve

==> The exclusive rate is now 40% of the inclusive, instead of 20%, basically

because the inclusive rates are all halved.

So things get better looking. But now I wonder why one process depends upon the scale and not the other.

That's because the gamma Z cross section does not contain diagrams with alpha_s, while the gamma bb does.

[anonymous]

missing lots of diagrams ​from the "inclusive" run 1, on ​top of the ones that you are using in the "forced Z" run 3. Some of these are s-channel types, which one can try to include in run 3 (or observe that their cross section is low), but there are also t-channel ones,

[Frank Siegert]

Sorry for the spam that has been entered in the last comment. As a short update to people reading this bug report: Bill has simplified the problem further, to avoid some other issues we came across when discussing the problem. I'll copy his mail and add two Run.dat's which demonstrate the simpler problem. His main (and, as far as I can see, valid) point is that any missing diagrams shouldn't affect the cross section in the Z peak region that much.

PS: Can we remain in this bug report when we discuss this issue, please? I don't want to keep copying back and forth. It might be a bit inconvenient, that the bug tracker does not (yet?) have a mail interface to post to. But it still ensures that nothing gets lost or forgotten, and the discussion that might be interesting to other users as well stays public. Thanks!

[Frank Siegert]

Bill Murray:

Here are cross-sections, in pb, for "Gamma b bbar" with 20GeV pT on the bs and 200GeV on the gamma, plus 0.6dR isolation of the gamma from both b quarks.

79-83 89-93 99-103

Pbb 0.0026 0.0157 0.0024 PZ,Z->bb 0.0001 0.0056 0.0002

I assume the background is pretty flat, it looks it, and deduce a Z cross-section, in the range 89-93, of:

Pbb 0.0132 PZ,Z->bb 0.0055

[Frank Siegert]

This is the same issue as the one described in bug #69

[Stefan Hoeche]

Milestone old deleted