Deeply Virtual Compton Scattering at HERA

3 Pages·0.08 MB·English

by  L. Favart

For Publisher’s use
DEEPLY VIRTUAL COMPTON SCATTERING AT HERA
LAURENT FAVART
On behalf of the H1 and ZEUS Collaborations
Universit´e Libre de Bruxelles, I.I.H.E., Belgium
1
E-mail: [email protected]
0
0
2
ResultsonDeeplyVirtualComptonScatteringatHERAmeasuredbytheH1andZEUSCollaborations
n
arepresented. Thecrosssection, measuredforthefirsttime,isreportedforQ2>2GeV2.
a
J
5 1 Introduction e γ e
2 e
Along these last years, the exclusive vector e e γ
1
meson production (as ρ and J/Ψ) has been γ*
v
6 studied extensively at HERA and has pro- γ*
4 vided very interesting results, in particular, p p p p
0 testing for the domain of applicability and
1 the relevance of perturbative QCD in the Figure2. Bethe–Heitler process.
0
1 field of diffraction (see e.g.1,2). Here, we re-
0 port the first analyses of a similar process,
/ Bethe–Heitler process (Fig. 2).
x theDeeplyVirtualComptonScattering,con-
e sisting in the hard diffractive scattering of
- Sincethevirtualphotonisscatteredonto
p a virtual photon off a proton (Fig. 1). The
e DVCSprocessoffersanewandcomparatively mass shell in the final state it is necessary
h to transfer longitudinal momentum from the
clean way to study diffraction at HERA. In
:
v photon to the proton, i.e. forcing a nonfor-
comparison to vector meson production it
i ward kinematic situation3,4. In the picture
X avoids large uncertainties on the theoretical
of the two gluonexchange the fractionalmo-
r predictions due to the meson wave-function.
a mentumofthegluonsarethereforenotequal,
The largest interest comes from the access it
which implies the involvement of the skewed
gives to the skewed parton distributions of
the proton3. (or nonforward) parton distribution.
InpresenceofahardscaletheDVCSpro-
e cess can be completely calculated in pertur-
e bative QCD. In the present case, the photon
γ* γ virtuality Q2, above a few GeV2, insures the
presence of a hard scale. LO QCD calcula-
tions exist, based on the two gluon exchange
model5. The factorization theorem in per-
p p turbativeQCDhavingbeendemonstrated4,6,
the DVCS process provides a unique way to
Figure1. DVCSprocess.
extractthesedistributionsfromexperimental
data through the measurement of the asym-
TheDVCSprocesscontributesto the re- metryofthephotonazimuthalangledistribu-
actione+p→e+γp,whosetotalcrosssection tion due to the interference with the Bethe–
is dominated by the purely electromagnetic Heitler process7.
For Publisher’s use
is observed. The LO calculation including
From an experimental point of view, the DVCS and the Bethe–Heitler processes
HERA kinematic enables the DVCS process achives a good description of the experimen-
tobestudiedinalargerangeinQ2,W andt taldata. AclearDVCSsignalisstillseenaf-
andoffersthepossibilitytostudythisdiffrac- ter the photon energy cut is increased. Also
tive mechanism in detail. a shower shape analysis of the calorimetric
clusters was performed that shows that the
2 Analysis strategy signal originates from photons and not from
π0 background.
Around the interaction region both exper-
ZEUS 1996/97 Preliminary
iments, H1 and ZEUS, are equipped with
γN
tracking devices which are surrounded by 140 γ candidates
calorimeters. Since the proton escapes the
120
main detector through the beam pipe only
100
the scattered electron and photon are mea-
sured. Therefore the event selection is based 80
on demanding two electromagnetic clusters, 60
one in the backward and one in the central
40
or forwardpart of the detector (θ<∼140o - the
20
backward direction (θ = 0) is defined as the
directionoftheincomingelectron). Ifatrack 00 0.25 0.5 0.75 1 1.25 1.5 1.7θ5 (r2adia2.n25s)
can be reconstructed it has to be associated 2
tooneoftheclustersanddeterminestheelec- Figure3. Distribution(uncorrected)ofthepolaran-
gle of the photon candidate with an energy above
tron candidate. To enhance the DVCS con-
2GeV. Datacorrespondtothefullcircles. Thepre-
tributionin comparisonto the Bethe–Heitler dictionfor the Bethe–Heitler process isindicated by
process the phase space has to be restricted the open triangles. The prediction of Frankfurt et
al. basedoncalculationsincludingtheBethe–Heitler
by demanding the photon candidate in the
andtheDVCSprocessisshownbytheopencircles.
forward part of the detector.
The H1 analysis selects more specifically
the elastic component by using, in addition, 3.2 H1
detectors which are placed close to the beam
In the H1 analysis, the DVCS cross sec-
pipe and which are used to identify parti-
tion is measured in the kinematic region:
clesoriginatingfromprotondissociationpro-
2 < Q2 < 20GeV2, |t| < 1GeV2 and
cesses.
30 < W < 120GeV. The proton disso-
ciation background has been estimated at
3 Results
around 10% and subtracted statistically as-
suming the same W and Q2 dependence as
3.1 ZEUS
for the elastic component. The acceptance,
The first observation of the DVCS process initialstateradiationofrealphotonsandde-
was reported by the ZEUS collaboration in tector effects have been estimated by MC to
19998. In the analysis a photon virtuality extract the elastic cross section.
Q2 > 6GeV2 is demanded. In Fig. 3 the
polarangulardistributionofthe photoncan- InFig.4the differentialcrosssections as
didates is shown. A clear signal above the a function of Q2 and of W are shown. The
expectations for the Bethe–Heitler process data are compared with the Bethe–Heitler
For Publisher’s use
predictionaloneandwiththe fullcalculation 4 Conclusion
including Bethe–Heitler and DVCS. The de-
The DVCS process has been observedby the
scription of the data by the calculations is
H1 and ZEUS Collaborations. Cross section
good, in shape and in absolute normaliza-
tion when a t slope is chosen between 7 and measurements have been presented for the
10 GeV−2. first time. The experimental results are well
described by the calculations of Frankfurt et
al.
Itisimportanttonoticethat,attheLO,
the interference term cancels when integrat-
ingovertheazimuthalangleofthefinalstate References
photon(asinthedifferentialcrosssectionsin
Q2 and of W). 1. P.Schleper, these proceedings.
2. P.Marage, “Hard Diffraction in Vector
Meson Production at HERA”, Review
2
10 presented at the XXVIII International
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2
V H1 Preliminary Symposium on Multiparticle Dynamics,
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[ 30 < W < 120 GeV 5524 (1997).
2 | t | < 1 GeV2
Q 4. J.C.Collins, A.Freund, Phys. Rev.
d
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σ
d 1 5. L.L.Frankfurt, A.Freund, M.Strikman
Phys. Rev. D58, 114001 (1998) and
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10 6. X.-D. Ji and J.Osborne, Phys. Rev.
5 10 15 20
D58, 094018 (1998).
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7. A.Freund Phys. Lett. B472, 412
(2000).
]

V H1 Preliminary 8. P.R.B.Saull, Proceedings of the Int.
e 3
G BH + DVCS (FFS) Europhysics Conf. on High En-
/
b BBHH ergy Physics, Tampere, Finland, 15-
p
[ 2 < Q2 < 20 GeV2 21 July 1999, edited by K. Huitu,

W 2 | t | < 1 GeV2
H. Kurki-Suonio and J.Maalampi (hep-
d
/ ex/0003030).
σ
d
1
0
40 60 80 100 120
W [ GeV ]
Figure 4. The measured cross sections of the reac-
tion e+p → e+γp as a function of Q2 and W are
shownandcomparedtotheoretical predictions. The
uncertaintyinthetheoretical prediction,shownhere
asashadedbandisdominatedbytheunknownslope
of the t-dependence of the DVCS part of the cross
section,assuming7<b<10GeV−2