Update on studies of
𝛄
𝐅𝟐
B. Krupa, T. Lesiak, B. Pawlik, T. Wojtoń, L. Zawiejski
Institute of Nuclear Physics Polish Academy of Sciences
29th FCAL Collaboration Workshop
16 – 20 September 2016, Tel Aviv University
Photon structure function & its measurement
e+e- → e+e- γγ → e+e- X
The single-tag process
tag
𝑑𝜎(𝑒𝛾 → 𝑒𝑋)
𝑑𝑥𝑑𝑄2
2𝜋𝛼 2
=
∙ 1+ 1−𝑦
𝑥𝑄4
2
𝛾
𝛾
𝐹2 𝑥, 𝑄2 − 𝑦 2 𝐹𝐿 (𝑥, 𝑄2 )
𝑄2 = 4𝐸𝑏 𝐸 ′ 𝑠𝑖𝑛2 𝜃/2
x
𝑦
𝐸′
𝜃
2
= 1 − 𝑐𝑜𝑠
𝐸𝑏
2
– fraction of parton momentum with
respect to the target photon
y
𝑥
𝑄2
= 2
𝑄 + 𝑊 2 + 𝑃2
– energy lost by the inelastically
scattered electrons
𝐸𝑏 (𝐸 ′ ) – energy of the beam electrons
(the scattered electrons)
2
𝑊2 =
𝐸ℎ
2
−
𝑝ℎ
anti-tag
𝐸ℎ (𝑝ℎ ) – energies (momenta) of final
state particles
2
Expected values
of kinematic
variables –
generation level
LumiCal
31 – 78 mrad
ILC
BeamCal
5.8 – 43.5 mrad
LumiCal
38 – 110 mrad
CLIC
BeamCal
10 – 40 mrad
3
Polar angles of scattered electrons
For higher beam energy electrons are scattered
in smaller angles
more events are lost
ILC
CLIC
LumiCal
31 – 78 mrad
BeamCal
5.8 – 43.5 mrad
LumiCal
38 – 110 mrad
BeamCal 10 – 40 mrad
4
Expected values
of kinematic
variables –
generation level,
various structure
functions
5
Energy distribution of tagged electrons
LumiCal, ILC(500), Twogam
At the reconstruction level events with small values of Etag are observed.
→ particles which pass through the detector leaving in it only part of their energies
→ particles which were scattered
6
PYTHIA
Q2 = 16.5 GeV 2
Q2 = 113.4 GeV 2
Photon structure function
Examples of results for generation and
reconstruction level – ILC (500 GeV)
For generation were used various
generators
TWOGAM Q2 = 15.4 GeV 2
Q2 = 114.6 GeV 2
Scattered electrons tagged at the LumiCal
and BeamCal
Only statistical uncertainties
HERWIG Q2 = 23.1 GeV 2
Q2 = 111.0 GeV 2
old results
(without using
FCALClusterer)
7
Background – first results
Distribution of the shape
of background against
the shape of signal
Expected dominant background :
e+e-  e+e- +Z0 /   hadrons
8
Summary and Outlook
● At ILC/CLIC it will be possible to extend the available kinematical range for the
measurement of the photon structure functions.
● Information from forward detectors can be used to study the photon structure
functions. However, at higher energies tagging of the scattered electrons is difficult.
● The comparison of the PYTHIA generator level results with predictions of other
Monte Carlo generators (HERWIG, TWOGAM) using various structure functions (SAS,
GRV-LO, GS, DG) has been made.
● Results for photon structure functions were obtained at generation and
reconstruction levels.
● First steps towards estimating the background were made.
● It is necessary to consider systematic effects and the determined kinematical
variables should be corrected due to the detector effects.
9