arXiv:0902.3478v2 [nucl-ex] 9 Mar 2009
The High-Acceptance Dielectron Spectrometer HADES
G. Agakishiev
h
, C. Agodi
b
, H. Alvarez-Pol
p
, E. Atkin
s
, E. Badura
d
,
A. Balanda
c,y
, A. Bassi
i
, R. Bassini
i
, G. Bellia
b,v
, D. Belver
p
, A.V Belyaev
f
,
M. Benovic
a
, D. Bertini
d
, J. Bielcik
d
, M. B¨ohmer
l
, C. Boiano
i
, H. Bokemeyer
d
,
A. Bartolotti
i
, J.L. Boyard
n
, S. Brambilla
i
, P. Braun-Munzinger
t,u
,
P. Cabanelas
p
, E. Castro
p
, V. Chepurnov
f
, S. Chernenko
f
, T. Christ
l
,
R. Coniglione
b
, L. Cosentino
b
, M. Dahlinger
d
, H.W. Daues
d
, M. Destefanis
h
,
J. D´ıaz
q
, F. Dohrmann
e
, R. Dressler
e
, I. Dur´an
p
, A. Dybczak
c
, T. Eberl
l
,
W. Enghardt
e
, L. Fabbietti
l
, O.V. Fateev
f
, C. Fernandez
p
, P. Finocchiaro
b
,
J. Friese
l
, I. Fr¨ohlich
g
, B. Fuentes
p
, T. Galatyuk
d
, C. Garabatos
d
,
J.A. Garz´on
p
, B. Genolini
n
, R. Gernh¨auser
l
, C. Gilardi
h
, H. Gilg
l
,
M. Golubeva
j
, D. Gonz´alez-D´ıaz
d
, E. Grosse
e,w
, F. Guber
j
, J. Hehner
d
,
K. Heidel
e
, T. Heinz
d
, T. Hennino
n
, S. Hlavac
a
, J. Hoffmann
d
, R. Holzmann
d
,
J. Homolka
l
, J. Hutsch
e
, A.P. Ierusalimov
f
, I. Iori
i,x
, A. Ivashkin
j
, M. Jaskula
c
,
J. C. Jourdain
n
, M. Jurkovic
l
, B. K¨ampfer
e,w
, M. Kajetanowicz
c
, K. Kanaki
e
,
T. Karavicheva
j
, A. Kastenm¨uller
l
, L. Kidon
c
, P. Kienle
l
, D. Kirschner
h
,
I. Koenig
d
, W. Koenig
d
, H.J. K¨orner
l
, B.W. Kolb
d
, U. Kopf
d
, K. Korcyl
c
,
R. Kotte
e
, A. Kozuch
c,y
, F. Krizek
o
, R. Kr¨ucken
l
, W. K¨uhn
h
, A. Kugler
o
,
R. Kulessa
c
, A. Kurepin
j
, T. Kurtukian-Nieto
p
, S. Lang
d
, J. S. Lange
h
, K.
Lapidus
j
, J. Lehnert
h
, U. Leinberger
d
, C. Lichtblau
h
, E. Lins
h
, C. Lippmann
g
,
M. Lorentz
g
, D. Magestro
d
, L. Maier
l
, P. Maier-Komor
l
, C. Maiolino
b
,
A. Malarz
c
, T. Marek
o
, J. Markert
g
, V. Metag
h
, B. Michalska
c
, J. Michel
g
,
E. Migneco
b,v
, D. Mishra
h
, E. Morini`ere
n
, J. Mousa
m
, M. M¨unch
d
, C. M¨untz
g
,
L. Naumann
e
, A. Nekhaev
k
, W. Niebur
d
, J. Novotny
o
, R. Novotny
h
, W. Ott
d
,
J. Otwinowski
c
, Y. C. Pachmayer
g
, M. Palka
d,c
, Y. Parpottas
m
, V. Pechenov
h
,
O. Pechenova
h
, T. P´erez Cavalcanti
h
, M. Petri
h
, P. Piattelli
b
, J. Pietraszko
d
,
R. Pleskac
o
, M. Ploskon
c
, V. Posp´ısil
o
, J. Pouthas
n
, W. Prokopowicz
c
,
W. Przygoda
c,y
, B. Ramstein
n
, A. Reshetin
j
, J. Ritman
h
, G. Roche
r
,
G. Rodriguez-Prieto
p
, K. Rosenkranz
g
, P. Rosier
n
, M. Roy-Stephan
n
,
A. Rustamov
d
, J. Sabin-Fernandez
p
, A. Sadovsky
j
, B. Sailer
l
, P. Salabura
c
,
C. Salz
h
, M. S´anchez
p
, P. Sapienza
b
, D. Sch¨afer
h
, R.M. Schicker
d
,
A. Schmah
d,l
, H. Sch¨on
d
, W. Sch¨on
d
, C. Schroeder
d
, S. Schroeder
l
,
E. Schwab
d
, P. Senger
d
, K. Shileev
j
, R.S. Simon
d
, M. Skoda
h
,
V. Smolyankin
k
, L. Smykov
f
, M. Sobiella
e
, Yu.G. Sobolev
o
, S. Spataro
h
,
B. Spruck
h
, H. Stelzer
d
, H. Str¨obele
g
, J. Stroth
g,d
, C. Sturm
g
, M. Sudo l
n
,
M. Suk
o
, M. Szczybura
c
, A. Taranenko
o
, A. Tarantola
g
, K. Teilab
g
, V. Tiflov
j
,
A. Tikhonov
o
, P. Tlusty
o
, A. Toia
h
, M. Traxler
d
, R. Trebacz
c
, A.Yu. Troyan
f
,
H. Tsertos
m
, I. Turzo
a
, A. Ulrich
l
, D. Vassiliev
b
, A. V´azquez
p
, Y. Volkov
s
,
V. Wagner
o
, C. Wallner
l
, W. Walus
c
, Y. Wang
g
, M. Weber
l
, J. Wieser
l
,
S. Winkler
l
, M. Wisniowski
c
, T. Wojcik
c
, J. W¨ustenfeld
e
, S. Yurevich
d
,
Y.V. Zanevsky
f
, K. Zeitelhack
l
, A. Zentek
g
, P. Zhou
e
, D. Zovinec
d
,
P. Zumbruch
d
a
Institute of Physics, Slovak Academy of Sciences, 84228 Bratislava, Slovakia
b
Istituto Nazionale di Fisica N ucleare - Laboratori Nazionali del Sud, 95125 Catania, Italy
Preprint submitted to Elsevier March 9, 2009
c
Smoluchowski Institute of Physics, Jagiellonian Univ ersity of Krak´ow, 30-059 Krak´ow,
Poland
d
GSI Helmholtzzentrum f¨ur Schwerionenforschung, 64291 Darmstadt, Germany
e
Institut f¨ur Strahlenphysik, Forschungszentrum Dresden-Rossendorf, 01314 Dresden,
Germany
f
Joint Institute of Nuclear Research, 141980 Dubna, Russia
g
Institut f¨ur Kernphysik, Johann Wolfgang Goethe-Universit¨at, 60438 Frankfurt, Germany
h
II. Physikalisches Institut, Justus-Liebig-Universit¨at Gieβ en, 35392 Gieβen, Germany
i
Istituto Nazionale di Fisica Nucleare, Sezione di Milano, 20133 Mi lano, Italy
j
Institute for Nuclear Researc h, Russian Academy of Science, 117312 Moscow, Russia
k
Institute of Theoretical and Experimental Physics, 117218 Moscow, Russia
l
Physik Department E12, Technische Universit¨at M¨unchen, 85748 M¨unchen, Germany
m
Department of Physics, Universit y of Cyprus, 1678 Nicosia, Cyprus
n
Institut de Physique Nucl´eaire (UMR 8608), CNRS/IN2P3 - Universit´e Paris Sud,
F-91406 Orsay Cedex, France
o
Nuclear Physics Institute, Academy of Sciences of Czech Republic, 25068 Rez, Czech
Republic
p
Departamento de F´ısica de Part´ıc ulas, Universidad de Santiago de Compostela,
15706 Santiago de Compostela, Spain
q
Instituto de F´ısica Corpuscular, U ni versidad de Valencia-CSIC, 46971 Valencia, Spain
r
Universit´e Blaise Pascal/Clermont II, 63177 Clermont-Ferrand, France
s
Moscow Engineering Physics Institute (State University), 115409 Moscow, Russia
t
ExtreMe Matter Institute, GSI Helmholtzzentrum f¨ur Schwerionenforschung, D-64291
Darmstadt, Germany
u
Technical University Darmstadt, D-64289 Darmstadt, Germany
v
Also at Dipartimento di Fisica e Ast ronomia, Universit`a di Catania, 95125 Catania, Italy
w
Also at Technische Universit¨at Dresden, 01062 Dresden, Germany
x
Also at Dipartimento di Fisica, Universit`a di Milano, 20133 Milano, Italy
y
Also at Panstwowa Wyzsza Szkola Zawodowa , 33-300 Nowy Sacz, Poland
Abstract
HADES is a versatile magnetic spectrometer aimed at studying dielectron pro-
duction in pion, proton and heavy-ion induced collisions. Its main features
include a ring imaging gas Cherenkov detector for electron-hadron discrimina-
tion, a tracking system consisting of a set of 6 superconducting coils producing
a toroidal field and drift chambers and a multiplicity and electron trigger ar-
ray for additional electron-hadron discrimination and event characterization. A
two-stage trigger system enhances events containing electrons. The physics pro-
gram is focused on the investigation of hadron properties in nuclei and in the hot
and dense hadronic matter. The detector system is characterized by an 85 %
azimuthal coverage over a polar angle interval from 18
◦
to 85
◦
, a single electron
efficiency of 50 % and a vector meson mass resolution of 2.5 %. Identification
of pions, kaons and protons is achieved combining time-of-flight and energy loss
measurements over a large momentum range. This paper describes the main
features and the performance of the detector system.
Key words: Spectrometer, Electron-positron pairs, Relativistic heavy-ion
collisions, Hadron properties
PACS: 21.65, 24.85, 25.75, 29.30, 29.40
2
1. Introduction
1.1. Physics motivation
A central topic of contemporary hadron physics is the investigation of had-
ronic matter. Theoretical models based on non-perturbative Quantum Chromo-
Dynamics indicate that the properties of hadrons are modified, if the particles
are embedded in a strongly interacting medium (for a theory overview see [1]).
The High-Acceptance DiElectron Spectrometer (HADES) in operation at the
GSI Helmholtzzentrum f¨ur Schwerionenforschung has been specifically designed
to study medium modifications of the light vector mesons ρ, ω, φ [2]. Experi-
mentally, these probes are well suited for two reasons. The vector mesons are
short-lived with lifetimes comparable to the duration of the compression phase
of relativistic heavy-ion reactions in the 1 to 2 AGeV regime of the heavy-ion
synchrotron SIS18. Equally important is their electromagnetic decay branch
into e
+
e
−
pairs. This channel is not subject to strong final state interaction
and thus provides an undistorted signal of the matter phase. The goal of the
HADES experiments is to measure the spectral properties of the vector mesons
such as their in-medium masses and widths.
The HADES heavy-ion program is focused on incident kinetic energies from
1 to 2 AGeV. Above about 0.7 AGeV these nucleus-nucleus reactions become
increasingly complex as new particles - predominantly mesons - are produced
which induce secondary reactions [3]. Some of these elementary reactions are not
well known and need to be explored as well. While relativistic heavy-ion colli-
sions produce hadronic matter at a few times normal nuclear matter density and
elevated temperature, pion or proton induced reactions embed vector mesons
into normal nuclear matter. A dedicated physics program including heavy ions,
deuteron, proton and pion beams has been proposed for the HADES detector
[4, 5].
Dilepton decays of vector mesons at SIS energies are rare events and their
observation presents a challenge for the detector design. Thus, HADES has
been equipped with a hadron-blind ring imaging Cherenkov counter, a tracking
system and a multiplicity and electron trigger array. A two-stage trigger system
selects events containing electron candidates in real time. With its much larger
solid angle and improved resolution, HADES continues and has the capability
to complete the physics program which was pioneered by the DLS spectrometer
at the BEVALAC [6].
1.2. Detector overview
HADES features six identical sectors defined by the superconducting coils
producing the toroidal geometry magnetic field. The spectrometer has 85 %
azimuthal acceptance and covers polar angles between θ = 18
◦
and θ = 85
◦
.
The angular and momentum acceptance has been optimized for the detection
of dielectron decays of hadrons produced in the SIS energy regime. A section of
the detector in the vertical plane containing the beam axis is shown in fig. 1.
3
beam
RICH
MDCI/II
MDCIII/IV
TOF
TOFINO
Pre-Shower
target
START
Magnet
Figure 1: Schematic layout of the HADES detector. A RICH detector with gaseous radiator,
carbon fiber mirror and UV photon detector with solid CsI photocathode is used for electron
identification. Two sets of Mini-Drift Chambers (MDCs) with 4 modules per sector are placed
in front and behind the toroidal magnetic field to measure particle momenta. A time of flight
wall (TOF/TOFINO) accompanied by a Pre-Shower detector at forward angles is used for
additional electron identification and trigger purposes. The target is placed at half radius off
the centre of the mirror. For reaction time measurement, a START detector is located in front
of the target. A few particle tracks are depicted too.
Momentum reconstruction is carried out by measuring the deflection angle
of the particle trajectories derived from the 4 hit positions in the planes of the
Mini-Drift Chambers (MDC) located before and after the magnetic field region.
Electron identification is performed with the hadron-blind gas Ring Imaging
Cherenkov detector (RICH) together with the Multiplicity and Electron Trig-
ger Array (META) consisting of time-of-flight scintillator walls (TOF/TOFINO)
and electromagnetic shower detectors (Pre-Shower). A powerful two-stage trig-
ger system is employed to select events within a predefined charged particle
multiplicity interval (first-level trigger LVL1), as well as electron candidates
(second-level trigger LVL2).
In the following, a detailed description of the main spectrometer components
is given: magnet (sect. 2.1), RICH (sect. 2.2), tracking system (sect. 2.3), META
(sects. 2.4 and 2.5) and beam detectors (sect. 2.6). The detector description is
4
followed by a discussion of the data acquisition and trigger system (sect. 3). The
data analysis framework and the detector performance are discussed in sect. 4.
2. Major spectrometer components
2.1. Magnet
2.1.1. Basic design considerations
The purpose of the magnet is to provide a transverse kick to charged particles
in order to obtain their momenta with sufficient resolution being of the order
of σ
p
/p = 1.5 - 2 % for electrons. On the other hand, electron identification
with the RICH detector requires a nearly field free region around the target.
Furthermore, a large momentum range of p = 0.1 - 2 GeV/c should be accepted
simultaneously within a large solid angle (θ = 18
◦
− 85
◦
, as close as possible to
full azimuthal coverage). Simulations of reactions in the SIS18 energy regime
have shown that these requirements call for a non-focusing spectrometer with
a transverse momentum kick p
k
of about 0.05 to 0.1 GeV/c, where p
k
is the
momentum difference between the incoming and outgoing momentum vectors
in the plane perpendicular to the field. The p
k
is proportional to the product of
magnetic field strength B and path length L. Assuming a magnetic field path
length of L ≃ 0.4 m, in order to keep the spectrometer compact, the respective
magnetic field strength stays below B = 0.9 T.
For such a design, the required momentum resolution can be obtained only
by keeping multiple scattering in the region of large magnetic field as small as
possible (i.e. allowing no detector material in this region). For high momentum
electrons (p ∼ 1 GeV/c), p
k
= 0.1 GeV/c also puts constraints on the position
resolution of the particle detectors (MDCs) in front and behind the field region.
For example, at p = 1 GeV/c and θ = 20
◦
, the deflection angle ∆θ
k
amounts to
5.7
◦
for p
k
= 0.1 GeV/c. A simple model calculation assuming two sets of two
detectors each spaced by d = 0.3 m shows that for this case a position resolution
of better than 150 µm is required to keep the corresponding contribution to the
momentum resolution below 1 %.
2.1.2. Field geometry
The toroidal field geometry provides a field free region around the target
and inside the active volume of the RICH. Since the shadow of the coils can
be aligned with the detector frames, no additional loss of solid angle is caused
by the coils. Although the field strength is rather low, superconducting coils
are necessary in order to obtain a compact coil construction. An additional
advantage is the low operating cost.
2.1.3. Superconducting coils
The system consists of 6 coils surrounding the beam axis. Each coil is sep-
arately contained in its individual vacuum chamber. The latter ones are con-
nected to a support ring located upstream of the target. Figure 2 shows a side
and a back view of the magnet including the support structure of the coil cases.
5
![Figure 35: Left: Integrated momentum resolution for the 6 sectors of HADES. Right: Momentum resolution as a function of momentum. A full featured GEANT simulation assuming the drift cell resolution obtained from Garfield simulations [59] (black open squares) and a four times worse resolution to account for uncertainties originating from calibration and alignment (blue open triangles) is also shown. In both figures, 3.5 GeV proton-proton elastic events were used. Elastic scattering laboratory angles corresponding to the momentum range shown are indicated.](/figures/figure-35-left-integrated-momentum-resolution-for-the-6-1vwtjgnh.webp)