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