No events with Ecm below intermediate on-shell threshold when using parton distribution functions

Hi Wouter,

I've computed the cross-section for the following two syntax in the heft:
g g > h h, h > b b~, h > a a
and
g g > h , h > b b~ a a / z a b
They both results in the same (unique) diagram but with different on-shell particles.
The first one give a cross-section of 5e-6, while the second is at: 2e-12
So in order to expect one events with an invariant mass lower than 2*mh, you need to generate more than a million of events.

Now going to physical meaning, note that the approximation of the heft model doesn't work for a pair production:
Please look at:
http://cp3.irmp.ucl.ac.be/~alaureys/gg_hh/gg_hh.html

Cheers,

Olivier

On Aug 18, 2013, at 11:46 AM, Wouter van den Wollenberg <email address hidden> wrote:

> Question #234228 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/234228
>
> Description changed to:
> I am trying to look at the behaviour of the cross section of a g g > b
> b~ a a process using the heft model. Specifcally I am interested in the
> so-called triangle diagram; the one containing a single ggh coupling. To
> this end I have toyed with the coupling orders in the heft model in
> order to eliminate all diagrams except one where we have g g > h > h h >
> b b~ a a.
>
> This diagram was generated using: g g > b b~ a a and then a list of
> coupling orders (which I added myself in order to switch off unwanted
> diagrams, i.e. it is a modified version of the MG heft one) set to zero.
> I use a dR cut on the b's and photons of 0.4 (so on (b,a), (a,a) and
> (b,b)), in addition I use an eta cut of 2.5 and an pT cut of 10 GeV on
> the b's and photons.
>
> If I disable pdfs and set the total energy to any energy (including
> those below the double Higgs production threshold of 250 GeV (the Higgs
> mass was set to 125 GeV)) I get a cross section which corresponds very
> well to the theoretical result.
>
> However if I include pdfs, set the total energy to 14 TeV, generate
> events, and then later on look at the Ecm distribution (using
> MadAnalysis as well as my own root analysis of the .lhe output), I see
> no events below 250 GeV. This is a bit strange; especially since without
> pdfs there was no problem generating below 250 GeV, and I have not used
> the decay chain formalism whilst generating the diagram, so off-shell
> Higgses should be allowed.
>
> At first I thought that there was perhaps some setting forcing the
> Higgses to be on-shell, but if I lower the total energy to something
> lower like 400 GeV, I do get events with an Ecm below 250 GeV. The
> threshold also moves with the Higgs mass, so setting the Higgs mass to
> something like 25 GeV makes it so that no events are generated below 50
> GeV for a total energy of 14 TeV.
>
> Am I missing some setting or something? I would very much like to access
> the Ecm regime below 250 GeV using pdfs.
>
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