$\gamma\gamma$ physics with the KLOE experiment

Abstract

The processes $e^+e^-\to e^+e^-X$, with $X$ being either the $\eta$ meson or
$\pi^0\pi^0$, are studied at DA$\Phi$NE, with $e^+e^-$ beams colliding at
$\sqrt{s}\simeq1$ GeV, below the $\phi$ resonance peak. The data sample is from
an integrated luminosity of 240 pb$^{-1}$, collected by the KLOE experiment
without tagging of the outgoing $e^+e^-$. Preliminary results are presented on
the observation of the $\gamma\gamma\to\eta$ process, with both
$\eta\to\pi^+\pi^-\pi^0$ and $\eta\to\pi^0\pi^0\pi^0$ channels, and the
evidence for $\gamma\gamma\to\pi^0\pi^0$ production at low $\pi^0\pi^0$
invariant mass.

Full text

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arXiv:1107.3782v1 [hep-ex] 19 Jul 2011
γγ physics with the KLOE experiment
The KLOE-2 Collaboration
F. Archilli,n,oD. Babusci,fD. Badoni,n,oI. Balwierz,eG. Bencivenni,f
C. Bini,l,mC. Bloise,fV. Bocci,mF. Bossi,fP. Branchini,q
A. Budano,p,qS. A. Bulychjev,gP. Campana,fG. Capon,f
F. Ceradini,p,qP. Ciambrone,fE. Czerwi´ nski,fE. Dan´ e,f
E. De Lucia,fG. De Robertis,bA. De Santis,l,mG. De Zorzi,l,m
A. Di Domenico,l,mC. Di Donato,h,iD. Domenici,fO. Erriquez,a,b
G. Fanizzi,a,bG. Felici,fS. Fiore,l,mP. Franzini,l,mP. Gauzzi,l,m
S. Giovannella,fF. Gonnella,n,oE. Graziani,qF. Happacher,f
B. H¨ oistad,sE. Iarocci,j,fM. Jacewicz,sT. Johansson,sV. Kulikov,g
A. Kupsc,sJ. Lee-Franzini,f,rF. Loddo,bM. Martemianov,g
M. Martini,f,kM. Matsyuk,gR. Messi,n,oS. Miscetti,fG. Morello,f
D. Moricciani,oP. Moskal,eF. Nguyen,p,qA. Passeri,qV. Patera,j,f
I. Prado Longhi,p,qA. Ranieri,bP. Santangelo,fI. Sarra,f
M. Schioppa,c,dB. Sciascia,fA. Sciubba,j,fM. Silarski,eC. Taccini,p,q
L. Tortora,qG. Venanzoni,fR. Versaci,f,uW. Wi´ slicki,tM. Wolke,s
J. Zdebik.e
aDipartimento di Fisica dell’Universit` a di Bari, Bari, Italy.
bINFN Sezione di Bari, Bari, Italy.
cDipartimento di Fisica dell’Universit` a della Calabria, Cosenza, Italy.
dINFN Gruppo collegato di Cosenza, Cosenza, Italy.
eInstitute of Physics, Jagiellonian University, Cracow, Poland.
fLaboratori Nazionali di Frascati dell’INFN, Frascati, Italy.
gInstitute for Theoretical and Experimental Physics (ITEP), Moscow, Russia.
hDipartimento di Fisica dell’Universit` a ”Federico II”, Napoli, Italy.
iINFN Sezione di Napoli, Napoli, Italy.
jDipartimento di Scienze di Base ed Applicate per l’Ingegneria dell’Universit` a “Sapienza”,
Roma, Italy.
kDipartimento di Scienze e Tecnologie applicate, Universit` a ”Guglielmo Marconi”, Roma, Italy.
lDipartimento di Fisica dell’Universit` a ”Sapienza”, Roma, Italy.
mINFN Sezione di Roma, Roma, Italy.
nDipartimento di Fisica dell’Universit` a “Tor Vergata”, Roma, Italy.
oINFN Sezione di Roma Tor Vergata, Roma, Italy.
pDipartimento di Fisica dell’Universit` a “Roma Tre”, Roma, Italy.
qINFN Sezione di Roma Tre, Roma, Italy.
rPhysics Department, State University of New York at Stony Brook, USA.
sDepartment of Nuclear and Particle Physics, Uppsala Univeristy,Uppsala, Sweden.
tA. Soltan Institute for Nuclear Studies, Warsaw, Poland.

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uPresent Address: CERN, CH-1211 Geneva 23, Switzerland.
Abstract.
studied at DAΦNE, with e+e−beams colliding at√s ≃ 1 GeV, below the φ resonance peak. The
data sample is from an integrated luminosity of 240 pb−1, collected by the KLOE experiment
without tagging of the outgoing e+e−. Preliminary results are presented on the observation of
the γγ → η process, with both η → π+π−π0and η → π0π0π0channels, and the evidence for
γγ → π0π0production at low π0π0invariant mass.
The processes e+e−→ e+e−X, with X being either the η meson or π0π0, are
1. Introduction
The term “γγ physics” stands for the study of the reaction of order α4e+e−→ e+e−γ∗γ∗→
e+e−X, where X is some arbitrary final state allowed by conservation laws. In particular,
hadronic states with JPC= 0±+;2±+are directly produced through the γ∗γ∗→ X subprocess.
For quasi-real photons the number of produced events can be estimated from the expression
N = Lee
?
dWγγ
dL
dWγγσ(γγ → X)(1)
where Leeis the e+e−luminosity, Wγγthe photon-photon center of mass energy (Wγγ= MX),
dL/dWγγthe photon-photon flux and σ the cross section into a given final state. By neglecting
single powers of ln(E/me), when the scattered leptons are undetected in the final state one has
dL
dWγγ
=
1
Wγγ
?α
2πlnE
me
?2
f(z)z =Wγγ
2E
(2)
with f(z) (Low function) [1, 2] given by
f(z) = (2 + z2)2ln1
z− (1 − z2)(3 + z2)(3)
The γγ processes studied with KLOE are e+e−→ e+e−η and e+e−→ e+e−π0π0.
experiments measuring the γγ cross section for light mesons production, took data at a center
of mass energy 7 <√s < 35 GeV. A low energy e+e−factory, such as DAΦNE, compensates
the small cross section value with the high luminosity.
DAΦNE is an e+e−collider designed to operate at the center of mass energy√s ≃ 1.02 GeV,
namely the φ meson mass. The sample used for the present analyses consists of data taken at
√s = 1 GeV, which allows the reduction of the background from φ decays, with an integrated
luminosity of 240 pb−1.
The KLOE detector consists of a large (3.3 m length and 2 m radius) volume drift
chamber, surrounded by a lead–scintillating fibers calorimeter.
a superconducting coil producing an axial field B=0.52 T. Charged particle momenta are
reconstructed with resolution σp/p ≃ 0.4% (σp/p ≃ 1%) for large (small) angle tracks.
Calorimeter energy clusters are reconstructed with energy and time resolution of σE/E =
5.7%/?E(GeV) and σt= 57 ps/?E(GeV) ⊕ 100 ps.
Data are processed with a dedicated filter asking for at least two prompt photons, clusters not
associated to tracks and propagating with |? r−ct| ∼ 0, with energy E > 15 MeV and polar angle
20◦< θ < 160◦, the most energetic with E > 50 MeV, the fraction of energy carried by photons
R = (?
latter requirement rejects low energy background and the high rate processes e+e−→ e+e−,
e+e−→ γγ.
Past
The detector is inserted in
γEγ)/Ecal> 0.3 and the total energy in the calorimeter 100 < Ecal< 900 MeV. The

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2. Observation of γγ → η, with η → π+π−π0
The selection of these events asks for two photons compatible with a π0decay and two tracks with
opposite curvature coming from the collision point. The charged pion mass is assigned to the two
tracks and a least squares function based on Lagrange multipliers imposes that π+π−π0come
from an η decay. Therefore most background events are suppressed, except for the irreducible
process e+e−→ ηγ → π+π−π0γ, with the monochromatic photon, Eγ= 350 MeV, lost in the
beam pipe. For these background events, however, the correlation between the squared missing
mass, M2
for the signal. Further criteria are applied to suppress processes with photons and e+e−in the
final state. The Monte Carlo, MC, generator used for the signal [3] allows the full phase space
generation of e+e−→ e+e−γ∗γ∗→ e+e−η events. Fig. 1 (left) shows the M2
data fitted with the superposition of MC shapes for signal and background. An independent
fit is performed with the π+π−π0longitudinal momentum. Both fits show the same yields for
the background processes and more than 600 signal events. The cross section of the irreducible
process e+e−→ ηγ → π+π−π0γ is evaluated using the same sample of data and asking for
three photons in the final state. A kinematic fit is performed, requiring energy and momentum
conservation, and the improved variables are used to fit in the distribution of the recoil photon
energy (Fig. 1, right). The preliminary result is σe+e−→ηγ(1 GeV) = 0.866(9)stat(93)systnb,
where the systematic uncertainty is given by residual e+e−→ ωπ0→ π+π−π0π0background.
The cross section σγγ→η→π+π−π0(1 GeV) is under evaluation.
miss, and the longitudinal momentum, pL, of the π+π−π0system is rather different than
missdistribution for

0
20
40
60
80
100
-0.15-0.1 -0.0500.05
miss(GeV2)
0.10.150.2 0.25
M2
0
500
1000
1500
2000
2500
50100150200250300350400
Eγ (MeV)
Figure 1. Left: fit of the M2
are: e+e−γ (green) at negative M2
(red), signal events (ligt blue). Right: fit of the recoil photon energy spectrum for the e+e−→ ηγ
analysis. The ηγ peak (blue) and the ωγ peak (light blue) are visible; the broad distribution
(green) is due to ωπ0events.
missdistribution for the e+e−→ e+e−η analysis. Main contributions
missvalues due to the pion mass assigned to e+e−tracks, ηγ
3. Observation of γγ → η, with η → π0π0π0
The main backgrounds for this analysis are annihilation processes with at least four prompt
photons in the final state, e+e−→ ηγ, KSKL, ωπ0, for which accidental or split calorimeter
clusters increase the photon multiplicity. Events with six and only six prompt photons in the
final state and with no tracks in the drift chamber are selected. The photons are paired choosing

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the combination which minimizes the χ2-like variable
χ2
6γ=(mπ0 − mij)2
σ2
ij
+(mπ0 − mmn)2
σ2
mn
+(mπ0 − mkl)2
σ2
kl
,(4)
where mijis the invariant mass of each pair of photons and σijthe resolution. Then, a kinematic
fit is performed asking for the six photons invariant mass to be equal to the mass of the η meson.
A cut is then applied on the energy of the most energetic photon, to reject e+e−→ ηγ events in
which the monochromatic photon is detected. The squared missing mass, M2
well described by the γγ → η → π0π0π0and η(→ π0π0π0)γ MC shapes. Fig. 2, left, shows the
fit in M2
miss.
miss, distribution is
Entries
Mean
RMS
ALLCHAN
200
0.7382E-01
0.9458E-01
2725.
Mmiss2(GeV2)
0
-0.15 -0.1 -0.05
20
40
60
80
100
120
140
00.050.1 0.150.20.250.3 0.35
E(MeV)
σ (nb)
0
950
0.5
1
1.5
2
2.5
3
3.5
4
960970 98099010001010 1020
Figure 2. γγ → η → π0π0π0analysis. Left: squared missing mass distributions for data (points
whith error bars) and backgrounds (histograms) normalized according to the fit results. Colour
code: red = e+e−→ ηγ, light blue = signal, blue = e+e−→ ωπ0. Right: preliminary KLOE
value for σ(e+e−→ ηγ) at√s = 1GeV (red point), and SND results [4] for several values of√s.
From the fit we obtain about 900 γγ → η → 3π0events and a preliminary value for the cross
section, measured with the η → π0π0π0channel, σe+e−→ηγ(1 GeV) = 0.875(18)stat(35)syst, in
agreement with that measured through the η → π+π−π0channel. This is shown in Fig. 2 (right)
among other experimental results [4] as a function of√s. We are finalizing the evaluation of
the σγγ→η→π0π0π0(1 GeV) cross section.
4. Search for γγ → π0π0
The main goal of this analysis is to investigate the low π0π0invariant mass region, just above
the production threshold, where a contribution from the σ(600) scalar meson [7, 8, 9] as an
intermediate state is expected.
The main backgrounds are annihilation processes with four or more prompt photons in the
final state: e+e−→ KSKL,ηγ,ωπ0,f0(980)(→ π0π0)γ. In addition, due to the possibility of
cluster splitting, the e+e−→ γγ process is also considered as a source of background. Selected
events have no tracks in the drift chamber and four prompt photons in the final state with
polar angle 23o< ϑ < 157oand energy Eγ > 15 MeV. The photons are paired choosing the
combination which minimizes the χ2-like variable of eq.(4) in the case of four photons coming
from two neutral pions, χ2
4γ> 4 are rejected: the effect of this selection
is shown in Fig. 3 (left).To reject e+e−→ KSKL events, where a large amount of non-
prompt energy is released in the detector a tighter cut on the energy fraction carried by photons,
4γ. Events with χ2

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1
10
102
103
Mγγ (MeV)
Mγγ (MeV)
1
10
102
103
Mγγ (MeV)
Mγγ (MeV)
0
25
50
75
100
125
150
175
200
225
250
0255075 100125 150175200225250
0
25
50
75
100
125
150
175
200
225
250
0 255075100125150175200225250
data
Entries
Mean
RMS
ALLCHAN
8090
559.2
170.9
8090.
M4γ (MeV)
dN/dM4γ (10 MeV)-1
0
50
100
150
200
250
300
350
2003004005006007008009001000
Figure 3. γγ → π0π0analysis. Left: scatter plots in two photons pairs invariant masses before
(up) and after (down) rejecting bad χ2
4γevents; selected events have both photons pairs invariant
masses centered around π0mass value. Right: 4γ invariant mass spectrum for data (points with
error bars) and backgrounds. Colour code: light blue = e+e−→ KSKL, blue = e+e−→ ωπ0,
violet = e+e−→ f0(980)γ, green = e+e−→ ηγ, red = e+e−→ γγ.
R = (?
data sample is shown in Fig. 3 (right) together with the normalized background Monte Carlo
simulations. The excess of events with respect to the expected annihilation processes, in the low
invariant mass region, is an indication of the γγ → π0π0production. The determination of the
differential cross section dσγγ→π0π0/dM4γis in progress.
γEγ)/Ecal> 0.75, is applied. The four photon invariant mass spectrum for the selected
5. Conclusions
From an integrated luminosity of 240 pb−1of data collected at DAΦNE, operating at√s ≃ 1
GeV, the following preliminary results in γγ analyses are achieved:
• unambiguous signature of both γγ → η and γγ → π0π0events, without any e±tagger;
• γγ → η events are observed through both η → π+π−π0and η → π0π0π0channels, with
independent systematics;
• from the same data sample, an exploratory research shows a structure at small values of
the M4γspectrum, where the process e+e−→ e+e−π0π0is expected.
As a by-product, we determined the cross section σe+e−→ηγat√s = 1 GeV, with accuracy better
than the closer data points. These results are encouraging also in view of the forthcoming data
taking campaign of the KLOE-2 project [10], when both low and high energy e±tagging devices
will be available.
Acknowledgments
We thank the DAFNE team for their efforts in maintaining low background running conditions
and their collaboration during all data-taking.
G. F. Fortugno and F. Sborzacchi for their dedication in ensuring efficient operation of the
KLOE computing facilities; M. Anelli for his continuous attention to the gas system and detector
safety; A. Balla, M. Gatta, G. Corradi and G. Papalino for electronics maintenance; M. Santoni,
G. Paoluzzi and R. Rosellini for general detector support; C. Piscitelli for his help during major
We want to thank our technical staff:

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maintenance periods. This work was supported in part by EURODAPHNE, contract FMRX-
CT98-0169; by the German Federal Ministry of Education and Research (BMBF) contract 06-
KA-957; by the German Research Foundation (DFG), ’Emmy Noether Programme’, contracts
DE839/1-4; by the EU Integrated Infrastructure Initiative HadronPhysics Project under
contract number RII3-CT-2004-506078; by the European Commission under the 7th Framework
Programme through the ’Research Infrastructures’ action of the ’Capacities’ Programme, Call:
FP7-INFRASTRUCTURES-2008-1, Grant Agreement N. 227431; by the Polish Ministery of
Science and Higher Education through the Grant No. 0469/B/H03/2009/37.
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