New distributions are presented which allow di-photon + jet events to be
clearly separated into three classes of events based on the $p_{T}$ of the
final state particles and their separation $R$ = $\sqrt{(\Delta y)^{2} +
(\Delta \phi)^{2}}$. The analysis used can easily be extended to the case of
di-jet + photon.Comment: 6 pages Revtex, uuencoded postscript file containing 3 figure

Bailey, Bob

English

arXiv

New distributions are presented which allow di-photon + jet events to be clearly separated into three classes of events based on the p_{T} of the final state particles and their separation R = \sqrt{(\Delta y)^{2} + (\Delta \phi)^{2}}. The analysis used can easily be extended to the case of di-jet + photon

Bailey, B

CERN Document Server

EC-HEP-950726
Di-photon + Jet Event Structure
B. Bailey
Department of Physics, Eckerd College, Saint Petersburg, Florida 33733
(July 26, 1995)
Abstract
New distributions are presented which allow di-photon + jet events to be
clearly separated into three classes of events based on the pT of the nal state
particles and their separation R =
p
(y)2 + ()2. The analysis used can
easily be extended to the case of di-jet + photon.
Typeset using REVTEX
1
A recent calculation of di-jet + photon suggested a method of separating these events
into three classes based on whether the photon had the highest, middle or lowest energy
fraction [1]. I refer the interested reader to Ref. [1] and references contained therein. The
analysis here uses dierent observables and distributions. The cuts necessary are simple and
the observables may be constructed from the four-vectors of the nal state particles in the
lab (hadron-hadron center-of-momentum) frame.
The di-photon + jet cross section consists of, at the Born level, qq ! γγg, qg! γγq and
qg ! γγq. In this brief note it will be shown that by ordering events by the pT of the jet, and
looking at experimentally motivated distributions, that di-photon + jet events can be clearly
separated into three classes. These three classes are: (1) pT jet > pTγ1 > pTγ2, (2) pTγ1 >
pT jet > pTγ2, and (3) pTγ1 > pTγ2 > pT jet. The distributions in question are motivated by
the cone algorithm, R =
q
(y)2 + ()2, used to dene jets and isolated photons. Where
y is the magnitude of the dierence in rapidity of two nal state particles and  is the
azimuthal opening angle between the two nal state particles. The distributions which will
be presented in this paper are: (1=) d=d(yγγ), (1=) d=d(γγ) and (1=) d=dRγγ .
The distributions are normalized by the cross section to reduce the sensitivity to Q2 and
dierences in parton distributions. The following inputs are used for this calculation:
p
s =
1800 GeV, CTEQ3L parton distributions [2], the one-loop expression for s(Q2), Q2 = p2Tγ,
and mtop = 180 GeV. Additionally, unless otherwise stated, the following cuts are used:
pTγ > 10 GeV, pT jet > 10 GeV, jyγj < 2.5, jyjetj < 3.0, and R > 0.4.
Fig. 1 shows the results for the yγγ distribution. The dashed curve is class (1), dot-dash
class (2), dotted class (3), and the solid curve is the sum of the three. It can be seen that
the distribution is a rapidly falling function of yγγ and that the three classes are distinct.
It should be noted that the separation can be enhanced by requiring a stier pT cut on the
photons thus enhancing class (3) events at the cost of reducing the total number of events.
Fig. 2 shows the results for the γγ distribution. This distribution has three distinct
features: a \shoulder", a \double-hump", and a \bump". Each is associated with class (1),
class (2) and class (3) type events accordingly. The locations of each peak can be understood
2
easily from the kinematics of each class. In class (1) events both photons have lower pT than
the jet and are thus recoiling against the jet. Therefore class (1) events dominate at low
γγ. The probability to have a class (1) event increases with γγ until γγ reaches

2
and class (2) events \turn on". The \double-hump" or twin-peak structure of class (2) events
can also be understood. In class (2) events the jet pT is intermediate in the pT ordering. The
probability of class (2) events also increases with γγ to peak at
2
3
. These events are the
so-called \Benz" events where each of the nal state particles has nearly equal momentum.
This is also the location of the \turn on" of class (3) events. Both class (2) and class (3)
events are strongly peaked near . For class (2) events this occurs when one of the photons is
soft and in class (3) events when the jet is soft. It is important to remember that although
kinematics determines the location of the various peaks, the matrix elements determine
the relative heights and shapes of the distributions. Also, as in the yγγ distribution, the
separation of the classes can be enhanced by requiring a stier pT cut on the photons thus
enhancing class (3) events at the cost of reducing the total number of events.
Fig. 3 shows the results for the Rγγ distribution. The three classes are once again clearly
separated and retain much of the structure shown in the γγ plots. The \shoulder" from
class (1) events is clearly visible. The \double-hump" of class (2) events has been smeared
out, but the peaking of class (2) and class (3) events at R near three is a remnant of γγ
peaking near . Also, as in the previous distributions, the separation of the classes can be
enhanced by requiring a stier pT cut on the photons thus enhancing class (3) events at the
cost of reducing the total number of events.
From a experimental point of view it should be noted that the requirements on the jet
can be relaxed. In fact the jet need not be directly observed, just inferred from a large
pT imbalance between the photons. This removes the uncertainties associated with jet
denitions, jet momentum and jet triggering eciencies. As mentioned in the text, the
various classes of events can be enhanced by adjusting the pT cut on the photons or the jet.
It should also be noted that the photons can be restricted to more central rapidities at the
cost of a lower cross section. Finally it should be noted the analysis presented here can be
3
carried over to the di-jet + photon process (jets and photon must be directly observed).
ACKNOWLEDGMENTS
This research was supported in part by Eckerd College and is a continuation of research
that was supported by the U. S. Department of Energy under contract number DE-FG05-
87-ER40319. The author would like to thank the Florida State University Department of
Physics and Argonne National Laboratory for the use of computer facilities. Additionally,
the author would like to thank G. Blazey and J. Womersley for stimulating discussions.
4
REFERENCES
[1] S. Keller and J. F. Owens, Phys. Lett. B269, 445 (1991).
[2] CTEQ Collaboration, Phys. Rev. D51, 4763 (1995).
5
FIGURES
Figure 1. d=d(yγγ) vs. y distribution using the cuts described in the text. The dashed
curve is class (1), dot-dash curve is class (2), dotted curve is class (3), the solid curve is the
sum of the three.
Figure 2. d=d(γγ) vs.  distribution using the cuts described in the text. The
dashed curve is class (1), dot-dash curve is class (2), dotted curve is class (3), the solid curve
is the sum of the three.
Figure 3. d=dRγγ vs. R distribution using the cuts described in the text. The dashed
curve is class (1), dot-dash curve is class (2), dotted curve is class (3), the solid curve is the
sum of the three.
6


Di-photon + Jet Event Structure

Abstract

Similar works

Full text

Available Versions

CERN Document Server