download report - Sapienza

2
2
2
Scientific Report 2007-2009
Particle physics
P22. The ZEUS experiment at the HERA collider
The Zeus experiment (see Fig 1) was taking data at
the electron-proton collider HERA at Desy, Hamburg,
from 1992 to 2007. At present, data analysis is going
on and will continue for several years. This general purpose
detector was built by a large international collaboration,
involving more than 40 experimental teams from
18 countries, for a total of about 400 physicists. There
was a large Italian participation (funded by INFN, Istituto
Nazionale di Fisica Nucleare), and the Rome group
was responsible, together with groups from Bologna and
Padova, of the muon detectors.
Since the famous ‘Rutherford experiment’ scattering has
proven to be a very powerful tool to study the structure
of atomic, nuclear and subnuclear matter. In particular,
deep inelastic scattering (DIS) experiments at the
end of the 60’s, with a resolution power well below the
the radius of the proton, were able to directly probe the
elementary constituents of the nucleons. Deep inelastic
scattering of leptons off nucleons has proved to be a key
process in the understanding of the structure of the proton
and testing of the Standard Model (SM). Neutral
current (NC) DIS is mediated by the photon and the Z
boson and is sensitive to all quark flavours. However, at
leading order only up-type quarks and down-type antiquarks
contribute to ep charged current (CC) DIS, mediated
by a W ± boson. Thus this process is a powerful
probe of flavour specific parton distribution functions.
The e ± -p collider HERA, unique of its kind and financed
performed extensive studies with polarized beams [1].
The high energy particle beams of HERA allowed the
exploration of a significant extension of the kinematic
phase space in deep inelastic scattering and provided a
very clean way of measuring the structure of the proton.
The double differential charged current cross sections for
lepton-nucleon scattering can be given in terms of three
structure functions, F 2 , F L and xF 3 . The longitudinal
structure function, F L , stems from the exchange of longitudinally
polarised gauge bosons. The parity violating
structure function, xF 3 , arises from the interference between
the vector and axial-vector couplings of the weak
interaction. Results on the measurements of F 2 were
extensively published in the past and are now part of
particle physics textbooks. The term xF 3 could be evaluated
by combining electron-proton and positron-proton
scattering data[2]. More recently first measurements of
F L were made possible, taking data at different centerof-mass
energies (Fig 2)[3].
Many other studies were performed with Zeus data, resulting
in more than 200 published papers.
& F
F L
1.5
1.0
0.5
0.0
ZEUS
2
2
Q = 24 GeV
1.5
1.0
0.5
0.0
2
2
Q = 32 GeV
& F
F L
1.5
Q 2 = 45 GeV 2
1.5
Q 2 = 60 GeV 2
1.0
1.0
0.5
0.5
0.0
0.0
& F
F L
1.5
1.0
0.5
2
2
Q = 80 GeV
1.5
1.0
0.5
F 2
F L
ZEUS-JETS
2
2
Q = 110 GeV
0.0
0.0
-3
10
-2
10
x
-3
10
-2
10
x
Figure 1: Schematic overview of the ZEUS detector (longitudinal
cut)
with a substantial Italian contribution, became operational
in 1992 and collided 27.5 GeV e ± against 920 GeV
p, thus providing an unprecedented resolution for probing
the structure of the proton down to 1/1000th of its
radius. In the second part of it’s life (2002 - 2007) the
accelerator allowed also for the use of polarised lepton
beams. Due to the chiral nature of the weak interaction,
the SM predicts a linear dependence of the CC
cross section on the degree of longitudinal polarisation
of the electron beam. The cross section is expected to
be zero for a right-handed electron beam. Zeus has
Figure 2: F L and F 2 at 6 values of Q 2 as a function of x. .
References
1. S .Chekanov et al., Eur.Phys. J. C 61, 223-235 (2009)
2. S. Chekanov et al., Eur.Phys. J. C 62, 625-658 (2009)
3. S. Chekanov et al., Phys.Lett.B 682, 8-22 (2009)
Authors
G. D’Agostini, A. Nigro
http://www-zeus.roma1.infn.it/
Sapienza Università di Roma 129 Dipartimento di Fisica