Future of COMPASS experiment
Damien Neyret, on behalf of the COMPASS collaboration
DAPNIA/SPhN, CEA Saclay, F-91191 Gif sur Yvette, France
Abstract. The COMPASS physics program covers two domains, 1/ the study of nucleon spin
structure using a muon beam, which includes the measurement of the gluon contribution to the
nucleon spin and of the transversely polarized parton distributions; 2/ the hadron spectroscopy
using hadron beams, in particular the study of charmed hadrons, hadron polarisabilities and
exotic hadrons. The COMPASS spectrometer has been commissioned in 2001 and the physics
data taking has started in 2002. We discuss here the plans for the future data taking and for the
set-up improvements.
PHYSICS OF COMPASS
The COMPASS experiment uses the CERN high energy muon/hadron M2 beam
line. Different targets will be used according to the program: polarized deuterons (6LiD)
or protons (NH3), Carbon, Lead, Copper, and liquid H2. The spectrometer has a wide
angular acceptance for particle characterization and identification [1]. This allows to
study exclusive reactions for several topics in hadron structure and hadron spectroscopy
physics.
Nucleon spin structure
Several nucleon structure topics will be covered by COMPASS. In particular this
experiment will have a large impact on two of them, the gluon contribution to the
nucleon spin (∆G) and the parton transverse polarization measurement.
Measurement of the gluon contribution to the nucleon spin
Previous experiments have shown that the spin contribution of the constituent quarks
amount to only 20-30% to the total nucleon spin. Therefore other contributions like the
gluon contribution ∆G may not be negligible. This value can be measured at
COMPASS by probing polarized nucleons with a polarized muon beam, and selecting
photon-gluon fusion events (γ-g → q q ). In particular, the open charm production of
D0 or D* mesons is a good signature of γ-g fusion events [2, 3] (see Figure 1).
With the high current muon beam used at COMPASS (160 GeV, 2.108 µ/spill), the
following statistical accuracy will be reached after two years and a half of data taking
(i.e. around 225 days): for D0 → K + π events we expect a statistical error on ∆G/G of
0.16, and for D* → K + π + πsoft an error better that 0.15 [1]
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
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µ
D°
µ'
c
G
c
D
K
D ° soft
K soft
p or n
Figure 1: Photon-gluon fusion charm production
The ∆G measurement can also be done by using γ-g fusion events tagged with high
transverse momentum hadrons [4]. This channel is affected by a higher systematical
error due to the QCD Compton background.
Transversity measurement
The transverse spin distribution functions of the partons have never been measured.
The COMPASS experiment can access these distribution functions by measuring the
transverse structure function h x 1 e 2 q x q x . This measurement
T
T
1
q q
2
is done through muon semi-inclusive deep inelastic scattering off a transversely
polarized proton or deuteron target. The asymmetry of the Collins angle ϕC between the
final hadron momentum and the final quark spin distribution is related to h1 [5]. With
30 days of data taking for each target, using a 160 GeV muon beam of 2.108 µ/spill, one
can reach a statistical accuracy better than 6 to 17 % on x.h1(x), depending of the x
ranges.
Other physics topics can be studied with the muon beam, like g1, g2 or Λ baryons
polarization measurements, but they will not been developed here.
Hadron spectroscopy physics
Several topics of hadron spectroscopy physics will be covered by the COMPASS
experiment.
One important goal of COMPASS is to use Primakov scattering events to measure
the polarisability of mesons [6] like pions, with an error level of 5%, or kaons or Σ for
which polarisabilities have never been measured. Primakov scattering is an inverse
Compton scattering: mesons are scattered off heavy nuclei target and the Coulomb field
of the nucleus exposes the beam mesons to a high electromagnetic field which induces
a polarization of the meson.
The COMPASS experiment has also a large program of studies on charmed hadrons:
lifetime measurements, semi-leptonic decays, charm hadroproduction, etc.... There are
also plans to search for double charmed baryon, which have never been seen up to
know, and to measure their characteristics (lifetime, mass, decay modes).
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Exotic hadrons can be also searched for in COMPASS experiment. Glueballs may be
produced in central collisions of a proton beam on a proton target, via a double
pomeron exchange process. They may also be observed in non-diffractive scattering or
on Coulomb excitation, using a heavy nuclei target. Other exotics hadrons like charmed
tetraquarks or pentaquarks will be also searched for.
COMPASS IN 2002
The COMPASS spectrometers
The COMPASS apparatus consists of two spectrometers for small and large angle
scattered particles (Figure 2). High energy muon or hadron beams (up to 300 GeV for
hadrons and 190 GeV for muons) can be scattered off different kind of targets. We
presently use a long polarized 6LiD target (two 60 cm long cells), inserted in the
2.5 Tesla superconducting solenoid magnet from the previous SMC experiment, since
the foreseen large aperture COMPASS solenoid is not yet operating.
Muon tracking 2
HCAL2
Tracking
SM2 magnet
Muon tracking 1
HCAL1
RICH1
Tracking
SM1 magnet
2nd spectrometer
Polarized target
Beam tracking
s
uon eV
m
,
p
0G
ns, 00-30
o
s
1
me
1st spectrometer
Figure 2: The COMPASS spectrometers
The first spectrometer, used for large angle particles, is composed of a vertical dipole
magnet SM1 surrounded by a tracking system with scintillating fibers detectors,
Micromegas microstrip chambers, drift chambers, GEM microstrip chambers, strawtubes detectors and multi-wires proportional chambers. A RICH placed after the dipole
magnet provides the particle identification of protons, pions and kaons for momentum
between 3 and 30 GeV. It is followed by a hadronic calorimeter, and a muon wall
system for muon tracking and identification.
The second spectrometer is dedicated to small angle particles, it uses the dipole
magnet SM2 with the same tracking system as the first spectrometer, and uses MWPC,
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GEMS and drift chambers after the magnet. An hadronic calorimeter and a muon wall
system completes the second spectrometer but the RICH detector is not yet available.
COMPASS data taking
First physics data have been taken during the three-months run of 2002 (see also
[7]). The 160 GeV polarized muon beam (2.108 µ per spill of 5 seconds each), was
scattered off the 6LiD target; the target polarization reached 50 %, and its direction was
reversed every 8 hours. After one month of detector commissioning, 80 days were
devoted to physics data (taking into account beam off days and hardware problems).
During this period, 19 days have been dedicated to transversity studies, with the
polarized target used in transverse mode. In total, 3.8.109 events were taken in the
longitudinal mode, and 1.2.109 events in the transverse mode.
PLANS FOR COMPASS FUTURE
COMPASS plans for 2003 and 2004
Several improvements are foreseen on COMPASS detectors for the next years.
Straw tubes detectors and large drift chambers will be added to improve the tracking
efficiency at large angle. Electromagnetic calorimeters will be added and fully equipped
in front of the hadronic calorimeters. The COMPASS solenoid magnet should be ready
by 2004 or 2005; it will increase the angular acceptance from 70 to 180 mrad.
In 2003 and 2004, four months per year (about 100 days of full data taking) of muon
beam are foreseen. The 6LiD polarized target will be used in 2003; in 2004 it may be
replaced by the NH3 polarized target. 80% of these data will be taken with a
longitudinal polarization, and the rest with a transverse one.
Tentative plans for 2006 and after
During the year 2005, the SPS accelerator will not run. In 2006 and beyond, a large
part of the data taking will probably be devoted to the hadron beam physics program. In
particular, Primakov reactions and charmed hadron physics may be studied with a set of
data taken during one or two years. Glueballs and exotics programs request more data
taking time.
If necessary, additional data with the polarized muon beam and the upgraded
spectrometer can also been taken.
Several spectrometers upgrades are foreseen. The major one is the installation of a
RICH detector in the second spectrometer in 2007.
Different targets (lead, copper, carbon, liquid H2) and a silicon tracker to be placed
just behind the target, will be prepared for the hadron beam program. In addition,
upgrades in the trigger systems and the data acquisition will be necessary.
Deep Virtual Compton Scattering studies
Some studies [8] show that Deep Virtual Compton Scattering (DVCS) can be studied
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*
x x p
p
Figure 3: Deep Virtual Compton Scattering diagram
at COMPASS if some specific upgrades are made. DVCS (Figure 3) gives access to the
generalized partons distributions in the nucleon [9, 10], which provide new insights on
quark correlation in nucleon structure. A possible goal could be to measure the DVCS
total cross section (which dominates the Bethe-Heitler process at energies of
~ 190 GeV) and the beam charge asymmetry between positive and negative muon
(which is sensitive to the the DVCS - Bethe-Heitler interference at ~ 100 GeV).
The DVCS studies require the following modifications of the COMPASS set-up.
Recoil protons must be detected with a time-of-flight and dE/dX detector for protonpion discrimination at large angle (30 to 80°). The electromagnetic calorimetry
acceptance must be increased to detect photons up to 20°. A liquid H2 target (which
may be common to the hadron program) is necessary. Finally a veto detector must be
added to remove halo events.
CONCLUSION
The COMPASS experiment will cover a vast physics program in nucleon spin
structure and hadron spectroscopy. Physics data taking has started in 2002, with a
160 GeV polarized muon beam scattered on a polarized deuteron target, for the
measurement of ∆G and the transversity. This program will continue until 2004 and
perhaps beyond. No beam is foreseen in 2005. Hadron spectroscopy physics is planed
to be studied from 2006 on, and the opportunity to study Deep Virtual Compton
Scattering is investigated.
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2.
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5.
6.
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G. Altarelli and W.J. Stirling, Particle World Vol. 1, 40 (1989)
M. Glück and E. Reya, Z. Phys. C 39, 569 (1988)
A. Bravar, D. Von Harrach and A. Kotzinian, Phys. Lett. B 421, 349 (1998)
J. Collins, Nucl. Phys. B 396, 161 (1993)
M. Adamovitch et al., Phys. Lett. B305, 402 (1993)
F. Tessarotto for the COMPASS Collaboration, Status of the COMPASS experiment, these proceedings
E. Burtin et al., Outline for a Deeply Virtual Compton Scattering Experiment using COMPASS at CERN, to be
published
9. X. Ji, Phys. Rev. Lett. 78, 610 (1997)
10. M. Vanderhaeghen, these proceedings
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