Acoplanarity distributions in KK and PHOTOS

These plots present photon acoplanarity distributions in the
Z->mu mu gamma gamma process.

The following kinematical cuts have been applied:

there must be two photons of p_t >= 1.5 GeV in the event
both photons have to be in the same hemisphere as mu+

Note that in the plots, "SDP=0" has no meaning!
However, it is striking to see that PHOTOS disagrees with KKMC of order O(alpha^1) matrix element CEEX exponentiated.
Only when second order matrix element in KKMC is present, the agreement with PHOTOS is reasonable.

Figure 1: Comparison of distributions produced by KK with full matrix element and KK with O(alpha^1) ME
For pedagogical reason, this plot will be replaced with the plot of comparison KK with O(alpha^1) vs PHOTOS

Figure 2: Comparison of distributions produced by KK with full matrix element and PHOTOS

P_t ordering in PHOTOS.

In principle. each iteration of the kernel for photon emission in PHOTOS is performing exactly the same action. Thus, one could think that photons will be generated in identical, independent way. This is not true, because of Energy-Momentum conservation: the phase space for second (and subsequent) emission is smaller, but also the effect of latter emission change slightly the kinematics of the previous one. There was some freedom of implementing this effect of consecutive kernel actions. This freedom was used to mimic as good as possible distributions of the two-photon matrix-elements. However in 1994 this was not considered to be an important property. We show some "poor-man's" attempt of Shwinger-Dyson - type equation was nonetheless created. This simplification of algorithm to have independent emissions of photons and nonetheless have some p_t-ordering is worth mentioning here. Let us continue with plots quantifying this effects.

We start with a general one, where the effect of p_t distributions on harder/softer photon because of consecutive generation of previous one is quantified. Because everything is piled-together, only very off-diagonal points are visible. These are events usually with two photons in oposite hemisphere than the emitting charge.

In the plots we use SQRT( 1/2 * (1 - p_|| / E_gamma ) ) variable to represent "effective p_t".

Figure 3: Effective p_t (Q) for more energetic photons. PS ROOT
Figure 4: Effective p_t (Q) for less energetic photons. PS ROOT
Figure 5: Difference in effective p_t (Q) caused by second-photon emission - for more energetic photons. PS1 PS2 ROOT
Figure 6: Difference in effective p_t (Q) caused by second-photon emission - for less energetic photons. PS1 PS2 ROOT

Last modified by Piotr Golonka, 7 October 2004, 12:45

Acoplanarity distributions in KK and PHOTOS

Pt ordering in PHOTOS.

P_t ordering in PHOTOS.