Status of the COMPASS Experiment
F. Tessarotto
CERN (CH) and INFN-Trieste (I)
on behalf of the COMPASS Collaboration
Abstract. The COMPASS Experiment at CERN has a broad physics program aimed at the study
of nucleon spin structure and hadron spectroscopy. It has an outstanding fixed-target apparatus,
mostly commissioned in 2001, presently consisting of a solid 6LiD polarised target and a two
stage spectrometer with high resolution tracking, particle identification and calorimetry, capable
of standing high event rates.
This paper describes the apparatus and its performances during the run of 2002, when 260 TB of
polarised muon nucleon scattering data have been collected. First physics signals from the analysis
and projections for the expected accuracy of the measurement of the gluon polarisation AG/G from
photon gluon fusion are presented too.
THE COLLABORATION
In 1996 two communities which had presented the CHEOPS [1] and HMC [2] Letters of
Intent for fixed target experiments at CERN merged in the COMPASS (COmmon Muon
and Proton Apparatus for Structure and Spectroscopy) Collaboration and submitted a
Proposal [3] which obtained approval in 1997.
COMPASS has a broad physics program with different beams and targets, extending
over a decade; the Collaboration consists of about 220 physicists from 27 Institutes.
Its spin program with a polarised muon beam and a polarised target aims to provide
a direct measurement of the gluon polarisation AG/G via the asymmetry of the photongluon fusion process, accessed by detecting open charm and high-pr correlated hadron
pairs [4]; it also aims to determine the transversity structure function hv to perform accurate flavour decomposition of the quark helicity distributions and to measure polarised
fragmentation functions.
Using hadronic beams COMPASS will study n and K polarisabilities from Primakoff
reactions and test predictions from chiral perturbation theory; perform extensive meson
spectroscopy to investigate the presence of exotic states; collect large samples of semileptonic decays of charmed mesons and baryons, determine form factors and probe
predictions from Heavy Quark Effective Theory; perform a systematic study of charm
hadroproduction cross sections and investigate doubly charmed baryon states, among
which the E+, recently claimed [5] by SELEX.
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. L Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
509
THE COMPASS APPARATUS
In 2002 COMPASS used the CERN ±i+ beam with an energy of 160 (± 5) GeV.
Muons were generated by decay of secondary n (and K) mesons and had a polarisation
PB « —80%. Typical beam intensity was 2.1 • 108 ^+ per spill, with 4.8 s long spills
of 16.8 s period. The track of each muon was measured upstream of the target and its
momentum determined from the track measurement.
The polarised target
The target consisted of two cylindrical cells (60 cm long, 3 cm diameter, separated by
10 cm) filled with solid 6LiD, which has a dilution factor f w 0.5.
The luminosity was 5 • 1032 cm~2s~l.
A 3He-4He dilution refrigerator cooled the target at T < 100 mK, while a solenoid
provided a highly homogeneous 2.5 T magnetic field along the beam axis. Since the
COMPASS solenoid (600 mm internal diam.) is not yet available, the solenoid of the
SMC experiment (255 mm internal diam.) was used instead: the resulting loss in angular
acceptance has a moderate impact on the measurement of AG/G, but it significantly
affects other physics items.
A dipole magnet (0.5 T) was used to reverse the target spin orientations every 8 hours
and to provide transverse polarisation: in this case the target was kept in frozen spin
mode (with a relaxation time measureed to be > 1000 h). The two target cells were
dynamically polarised (and kept) in opposite directions by microwave irradiation, with
frequency modulation. The needed paramagnetic centres were produced by previous
intense irradiation of the material by e~ beam.
Maximum polarisation values as high as PT = (—0.49,+0.57) have been obtained,
while typical values during the run were around —0.45 and +0.52. About one day was
needed to reach PT ~ 0.40. The polarisations were constantly measured in longitudinal
mode via 10 NMR coils with an accuracy around 3%.
The spectrometer
To achieve high-resolution tracking over a wide angular and dynamical range the
COMPASS spectrometer comprised two magnetic stages: the first (Large Acceptance
Spectrometer: LAS) had an acceptance of ±180 mrad and a wide aperture dipole magnet with 1 Tm bending power; the second (Small Acceptance Spectrometer: SAS), covered ±30 mrad and used a 4.4 Tm dipole to analyse high momentum particles.
Tracking
Tracking in the beam region was provided by 9 stations of scintillating fibres hodoscopes, with 0.5 mm (1.0 mm) diameter fibres read by multi-anode PMs capable
of withstanding rates above 5 MHz/channel. They provided a total of 21 coordinates
with efficiency > 99%, better than 500 ps time and 130 jum (250 ,um) space resolution.
Silicon microstrip detectors covering 50x70 mm2 area provided four coordinates before
510
POt
FIGURE 1. Schematic view of the COMPASS setup for the run of 2002.
the target, with analog readout, resolutions of 14 /im and 2.5 ns and ^99% efficiency.
The high flux area around the beam region was covered by micro-pattern detectors:
Micromegas and GEMs [6] . The COMPASS Micromegas [7] contained a thin micromesh foil separating an ionization volume from a high field (40 KV/cm, 100 /ira gap)
amplification region; they had typical efficiencies of 98% and a space resolution of
70 jUm. 12 planes of 40 x 40 cm2 active area were used.
The COMPASS GEMs (Gas Electron Multiplier) [8] were made of kapton foils having
Cu layers on both sides and a large (104/cm2) number of 60 /im diameter holes with the
high field inside the holes providing electrons amplification. Each GEM-chamber used
three cascaded GEM-foils and provided two-dimensional projective readout, for a total
of 40 coordinates. They had 50 /im space resolution, typical efficiencies around 96%,
time resolution in the range of 15 ns and covered 31 x3lcm2
Both Micromegas and GEMs did operate very smoothly during the 2002 run.
Tracking in the lower flux regions of the LAS was performed by 3 Drift Chambers
(with 8 coord. each, 1.2 x 1.2 m2 active area, 170 /im res.) and by 9 large Straw Tube
detectors [9] covering 3.2 x 2.4 m2 with double (staggered) layers of 6 mm diameter
tubes (10 mm in the outer region), having « 300 /im res.
In the SAS a set of old MWPC's refurbished and equipped with fast electronics
provided 34 coordinates with 600 jUm resolution, while the very external region was
covered by drift chambers with 2.6 x 5.2 m2 active area.
Calorimetry and Particle Identification
Hadron calorimeters were present on both spectrometers and used at the trigger
level too. Resolutions were similar for the two calorimeters HCALl and HCAL2, both
made of Fe scintillator sandwiches with planar WLS: a/E w 6% 0 60%/\/£ for pions
and a /E « 0.6%024%/\/£ for electrons. Electro-magnetic calorimetry was only
511
marginally implemented in the SAS, with LG blocks having <3/E = 2.3% 0 5.8%/V#Muons were identified with high efficiency by large sets of streamer detector planes
before and after a 60 cm thick iron absorber for the LAS and by drift tube planes for the
SAS.
Hadron identification was provided in the first spectrometer only, by a Ring Imaging
Cherenkov Detector (RICH1) [10], designed to perform n-K separation up to 60 GeV/c
over the entire acceptance.
RICH1 consisted of a 3 m long C4F10 radiator at atmospheric pressure, a wall of 116
spherical mirrors (3.3 focal length) covering an area of >20 m2 and two sets of far UV
photon detectors placed above and below the acceptance region. Cherenkov photons in
the range between 160 and 210 nm were detected using MWPCs equipped with Csl
photocathodes [11] and covering 5.3 m2. The photocathodes were segmented in pads
of 8 x 8 mm2 from where the induced signals were readout by a system of front-end
boards [12] with local intelligence. A total of 83000 channels were digitized by 10 bit
ADCs and compared to their individual threshod values at each event.
Despite non-optimal conditions of some photon detectors and sometimes of the radiator gas, RICH1 has been operational during most of the running time,
Trigger and data acquisition
Coincidences between elements of hodoscope planes at different positions along the
beam selected scattered muons for triggering purposes: 2 ns wide coincidence matrices
filtered signals from 500 trigger channels accepting target pointing "tracks" in the
kinematical region of interest. Presence of hadron shower signal was required to provide
more selective triggering.
Typical trigger rate was 5 kHz and the overall dead time was w 7%.
COMPASS used custom designed, parallel, readout electronics with local pre event
building for a total of ~ 190 k channels. A pipelined acquisition system transferred the
data via S-Links to 16 PCs used for buffering during spills; network swithches received
the data through Gigabit Ethernet and distributed them to 12 Event Builder PCs.
With an event size of ~ 44 kB the typical data flow was 220 MB/s during spill and
the average data recording rate was almost 3 TB/day.
A Detector Control System based on PVSS monitored and archived all values of
voltage, temperature and other parameters, and handled a complex alarms system. Online data monitoring was provided by COOOL, a C++ program built on ROOT libraries
which sampled events from the DAQ farm and shared the decoding with the off-line
software. A flexible Electronic Log-book system was used: edited by the shift crew
and accessible via www it also contained many automatically transferred informations,
including COOOL plots for each run.
After a setting-up period COMPASS took physics data during about 80 days, 19
of which in transverse polarisation mode. The average combined efficiency of SPS,
polarised target, spectrometer and DAQ was almost 70 % and a total of 260 TB of data,
corresponding to > 5 G events have been collected.
512
FIGURE 2. Invariant mass and Armenteros plots from a very preliminary analysis of asubsample of
2002 data: K°, A (A) and 0(1020) signals are clearly seen.
THE ANALYSIS
For both data recording and reconstruction COMPASS used a farm of 100 dual processor
PCs at CERN and the CASTOR file system, on which the 260 TB of data were stored in
about 260000 files.
The analysis is performed through a program called CORAL, fully object oriented,
with a modular architecture, written in C++, from scratch, by members of the Collaboration.
In a very preliminary analysis on a subsample of the 2002 data, secondary vertices
have been fitted to the reconstructed tracks; the distribution of invariant masses for
opposite charged pairs with the n+n~ hypothesis is shown on the left top of fig. 2:
the K® peak appears clearly and has a width similar to the one expected from Monte
Carlo simulations; a shoulder corresponding to the p meson is present too. For vertices
reconstructed behind the target the invariant mass for the n~p hypothesis is shown on the
top centraljplot of fig. 2, the Armenteros plot in the left bottom: here too the resolution
for A and A is compatible with expectations.
A first look at the RICH information provided promising hints: the right part of
fig. 2 shows the invariant mass of opposite charged tracks with the K+K~ hypothesis
when none (top), at least one (central) and both (bottom) tracks are asked to have been
identified as kaons by the RICH. The peak corresponding to the 0(1020) meson appears
unambiguously when the particle identification information is used.
513
THE MEASUREMENT OF AG
The gluon helicity distribution AG(rj) is poorly known: NLO QCD analyses of polarised
structure functions suggest large positive values, and so does a comparison between
high-/?r data and Monte Carlo simulations [13] performed by HERMES.
A major step forward is expected in the incoming years: COMPASS at CERN, STAR
and PHENIX at RtflC are planning to provide accurate data for AG in the next runs, and
Experiment E161 has proposed a high statistics measurement at SLAC.
The gluon polarisation AG/G is measured in COMPASS from the asymmetry of two
processes: open charm production and correlated high pT hadron pair production.
Open charm is predominantly produced by photon gluon fusion: y*g —> cc, at a scale
s > 4ra^; the useful cross section for a muon beam of 160 GeV/c is about 3 nb.
The identification is done by reconstructing D° and D°: on average 1.2 neutral D
mesons are produced per open charm event; they are primarily detected through their
golden decay channel: D° —> K~n+, D° —» K+n~ with a branching ratio of « 4%; cuts
on the K direction in the D° rest frame: \cos(6^) \ < 0.5 and on the D° energy fraction
ZD = ED/E * > 0.25 are needed to reduce background contamination.
A much cleaner charm sample is expected from identifying the D*± —>• DQn^ decay
chain from its unique kinematics. Also, the D° statistics will be increased by including
D0-^^+7r"7r° decays.
The gluon polarization will be determined from the measured charm production asymmetry: Ameas- = (N^ -ArJJ)/(7VjJ- -hJVjJ) = PBPTfDA£N where WjJ(A^) represents
the number of charm events with target spin parallel (anti-parallel) to the \i helicity and
D is the y* depolarisation factor, with < D >& 0.66 for COMPASS. Using the equation:
one averaged value for AG(rj) will be derived, for rj « 0.1. All other terms in the above
equation are known, in particular Acr(1si)y<g~>cc has been calculated in QCD at NLO [14].
An alternative methode uses the large analysing power of the photon gluon fusion production of light quarks: in order to discriminate this process from the leading order one
(7*# ~^ #)» events with hadron pairs having correlated high transverse momentum are
selected. The kinematics of these events allows the reconstruction of rj, probing AG(rj)
in different r] bins in the range 0.04 < r] < 0.2. The abundant production rate provides
high statistical accuracy but QCD compton scattering (y*g —> qg) contributes to these
events: the selection of K+K~ pairs will reduce its contribution but the interpretation of
the measurement will remain to some extent model dependent.
The expected statistical errors for 80 days of run for the two methods are presented in
fig. 3 together with three parametrisations of AG/G from Gehrmann and Stirling [15].
CONCLUSIONS
The COMPASS Experiment at CERN has a state-of-the-art fixed target apparatus with
high resolution tracking, particle identification and calorimetry, capable of standing high
event rates; it collected 260 TB of polarised jU-N scattering data in 2002.
514
Ag/g Set B
Ag/g Set C
Open Charm
High pT (K+,K')
FIGURE 3. Expected statistical errors for AG(rj)/G(rj) from 80 days of COMPASS data: the open
charm asymmetry measurement will provide one point with low systematic error, high pT kaon pairs will
access different 77 regions with high statistical accuracy. Not shown are systematical uncertainties
The gluon polarisation AG/G will probably be known with an accuracy around 10%
in few years, from direct measurements by COMPASS, STAR, PHENIX and E161.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Alexandrov, Y., et al., CHEOPS Letter of Intent (1995), CERN/SPSLC 95-22.
Nappi, E., et al., HMC Letter of Intent (1995), CERN/SPSLC 95-27.
The COMPASS Collaboration, COMPASS Proposal (1996), CERN/SPSLC 96-14.
Bravar, A., et al., Phys. Lett. B, 421, 349 (1998).
The SELEX Collaboration (2002), FERMILAB-Pub-02/183-E.
Sauli, E, Nucl Instrum. andMeth. A, 386, 531 (1997).
Thers, D., et al., Nucl. Instrum. and Meth. A, 469, 133 (2001).
Altunbas, C., et al., Nucl. Instrum. andMeth. A, 490, 177 (2002).
Bychkov, V. N., et al., Particles and Nuclei, Letters No. 2, 111, 64 (2002).
Baum, G., et al., Nucl. Instrum. andMeth. A, 333, 207 (1999).
The RD26 Collaboration, RD26 Status Report (1996), CERN/DRDC 96-20.
Baum, G., et al., Nucl. Instrum. andMeth. A, 333, 426 (1999).
Airapetian, A., et al., Z Phys. Rev. Lett., 6584,2584 (2000).
Bojak, I., and M.Stratmann, Nucl Phys. B, 540, 345 (1999).
Gehrmann, T., and Sterling, W. J., Z. Physics C, 65,461 (1994).
515