CERN Accelerating science

COMPASS-‐II: Preparing for the next step

COMPASS (NA58)  is preparing  for the  next  step in its  programme studying  nucleon  structure and   hadron   spectroscopy. From   2014 onward   the focus   will be on transverse momentum dependent    (TMD)    parton   distribution   functions    (PDF)    and   on    generalised   parton   distributions  (GPD). Both  are  related  to  the  intrinsic  transverse  structure  of  the  nucleon  and   the  orbital  angular  momentum  of  quarks   inside   the  nucleon.  Till   today   it   remains   an unanswered  question  how quarks  and  gluons conspire to build up  the nucleon spin  of ½ ℏ and  COMPASS  will continue  to be  a  leading  contributor  to the  field.

Before  we  disembark  for  the  new measurements  after the  accelerator  shutdown,  COMPASS will  measure  in  2012   the  electric  and  magnetic  polarizability   of  the pion  and  collect  for the first  time  data  on  the  kaon  polarizability.  These  polarizabilitites  are   fundamental  properties of   pions/kaons  and can be calculated with high  precision  in  Chiral   Perturbation  Theory –  a   low energy expansion of   QCD. These measurements are difficult   and   experimental results differ far   more   than the   quoted errors. Since neither   a   pion   nor   a   kaon target   exists the measurement   will   make  use  of  the  meson   interaction   with  the   coulomb   field   of   heavy nuclei,  nickel  in  our  case  (Primakoff  scattering).  The  second  part   of  the  2012  beam  time  will be devoted to a  pilot  run  for the  GPD  measurement.

The COMPASS  Collaboration comprises about  230   physicists and the experiment   is located   in  EHN2 (888) in  the North Area. The   highly versatile   M2 beam line at   the SPS   can deliver positive  and   negative   hadron   beams  up  to  about   280 GeV   and  polarized  positive  and   negative   muon  beams   up  to  200 GeV.  Since  2002  precise  measurements  of  the  longitudinal and   transverse   spin-‐dependent   PDF  of   the  nucleon  were  performed  in  deep  inelastic scattering   (DIS)   of   polarized   muons   off   polarized  solid-‐state targets.  In particular  a   large gluon polarization could   be excluded, which   was proposed following the   discovery by the EMC that  the  quark spins  contribute  little  to  the nucleon  spin. The  important  finding  in 2010   of   a   non-‐zero   azimuthal  Sivers  asymmetry    in  single-‐hadron  muo-‐production  from  a   transversely  polarized   proton  target   is  a   prerequisite  for  the   proposed   Drell−Yan  (DY)   measurements  in 2014  (see below).

In 2008 and 2009 a  huge   amount  of data   on hadron  spectroscopy  was taken  with   positive and  negative  hadron  beams  and  liquid  hydrogen  and  a  number  of nuclear targets. While  the positive  beam   contains   many  protons   and  pions,  the  negative   beam  consists  almost   exclusively of pions. Both beams have a   kaon contamination   at   the per cent   level. Beam particles are identified by Cherenkov CEDAR detectors. First   results from 2008/9   are very promising. The   analysis implies partial wave analyses   and acceptance corrections   and it   is particular  difficult  to  obtain  reliable   results for  waves with  small  amplitudes. Nevertheless, as  a  first  result  from  a  pilot  run  in  2004,  the  observation  of  a  resonance  with  exotic  JPC=1−+ quantum   numbers  could  be  published. This resonance  is  consistent  with  the  highly  debated π1(1600).


Two major   spectrometer upgrades/modifications  are needed  for the Phase  II  of  COMPASS. GPDs   provide   a   kind  of  three  dimensional  picture  of   the  nucleon,  sometimes   dubbed nucleon  tomography. For  example the  nucleon’s transverse size  can  be studied  as a  function   of   the  longitudinal   momentum   fraction  of   the   quarks   involved  in  the  scattering.  Key processes  are deeply  virtual  Compton   scattering  (DVCS), µp   → µp,  and  hard   exclusive meson (h) production (HEMP), µp   → µph. These processes imply that   the target   nucleon   stays  intact  and  thus  require  the  detection  of  the  proton  in  the  final  state.  The   large target   recoil   detector   CAMERA (Figure  1)  will  surround a  liquid  a  2.5 m  long  liquid  hydrogen target   presently  being   constructed  by   TE-‐CRG.  The   photomultiplier   signal  of  CAMERA   will   be digitized  with  1 GHz  by  the  Gandalf  board  (Figure  2)

The system will   allow the detection  of   recoil    protons    at   least    down  to  a   momentum    of    300 MeV/c.  The    electromagnetic calorimeter  system   has to  be  modified  to achieve   full  coverage.  A new  ECAL0  located right   downstream   of CAMERA will fully cover   the  enlarged   photon   acceptance  compared  to the previous  measurement  with  the  polarized  target.  The  towers  will  be   read  out  by  multipixel-‐ avalanche-‐photodiodes.  A  number  of  tracking  detector  will   have  to  be  upgraded  and made fit  for another 5+  year  programme. The  CAMERA project  is  let  by Saclay, Mainz  and Freiburg, while  JINR  Dubna  is  leading  the  ECAL0 project.

The GPD   pilot-‐run in   2012   aims   to providing   first   physics data   on   the DVCS process. The measurement   profits from   the   DVCS  interference  with  the Bethe−Heitler  process,   which enhances the  cross-‐section  of  the  interference term  and  provides  a  natural  normalisation  in   kinematic regions  where  it  dominates.  The  measurement  proceeds  via  the  determination  of   azimuthal  cross-‐section   asymmetries   for  positive    and  negative  muons  with   opposite polarization. The  latter is  a  unique   feature  of  muon  beams  arising from pion  decay.  Details can  be found  in  the  COMPASS-‐II  proposal  chap.  1,  which  was  approved  in  December   2010 in  parallel  to the  GPD   programme   data   will  be  taken  for  semi-‐inclusive DIS  in   order   to further constrain the   favour decomposition of the   spin-‐averaged quark distributions. For these measurements particle identification   by the RICH   is mandatory. An   upgrade of   the RICH  photon  detection  is planned  by the  Trieste group.


During   the   2013  shutdown  the   COMPASS  target   region  will   be  modified  for  the   first   polarized   Drell–Yan  measurements  with  a   negative  pion  beam  in  2014.  This  process  is dominated by the  annihilation of  a  valence  anti-‐u-‐quark  of  the  pion a  with  a  valence u  quark of   the  target   proton   yielding   a   µ+µ−  pair.  Apart   from  the  study  of  TMD  PDFs, which   is interesting   by itself, one   of the   main goals is to test   the QCD prediction   that   certain   TMD   PDFs  show  a  restricted  universality: they should  change  sign  when  measuring  the  same  TMD   PDF   in DIS on  one   hand  and  in Drell–Yan  reactions  on  the  other.  The  reason  for this  is   that   for  time-‐reversal  odd   TMD   PDFs,  like  the  Sivers  function,  the   involved  gauge  link  has opposite sign  for   initial  (DIS) and final  state (DY) interactions. A violation  of this prediction   would  have   far  reaching  consequences   for  the  calculation  of  cross-‐sections  in  QCD.  In  2010 COMPASS  established  that  the   Sivers  asymmetry  is  non-‐zero  for  the  proton,  an  observation  made  earlier by the  HERMES  Collaboration.  However, only  at  COMPASS  energies one can  be reasonably sure that   higher-‐twist   effects are not   the   origin   of   this effect. The   next   step   is now   to measure   the   corresponding azimuthal asymmetry   in polarized   Drell–Yan reactions. Efforts  for similar measurements are under way at  RHIC  and JPARC.

The heart   of the setup   is the large COMPASS polarized target   used up   to now   only   with   muon   beam. To suppress background and to achieve highest   luminosities   a   sophisticated   hadron  absorber   designed  at  Turin  will be  placed  downstream   of  the  polarized  target. This in   turn requires a   complete  restructuring of  the target   area, since the  target   requires a   complicated   infrastructure   like   the   pump system   in   an annex building. The   new   layout   is sketched   in   Figure  3  and  Figure  4.  The   absorber   has  been  dimensioned   to  achieve acceptable  radiation  and  background levels  up  to a  beam  intensity of 109pions  per 10 s  spill.   The main signal region is  in  the muon  pair mass  region  4  GeV  <  M  µµ<  9  GeV  beyond  the J/ψ resonance.  Detailed   event   rate  simulations   can  be   found  in   the   COMPASS-‐II   proposal, chap. 3. A  determination of  the  above  mentioned  relative   sign  change   between  DIS  and  DY reactions  will  be  achieved  in  2014.  To  obtain  more  information  about  the  shape of  the  TMD   PDF  more measurements  will  be required.

The  COMPASS  schedule after   the   2013  shutdown   starts with   DY  measurements  using   a   negative   pion beam  and  the polarized  target  in  2014  followed by  two  years  of  GPD-‐related   measurements   with muon   beams and   a  liquid hydrogen   target. Beyond 2016   we consider further  DY  measurements  and    GPD-‐related  measurements  with    a    polarized    target. Depending on the findings  from the spectroscopy   data   it   might   be highly desirable  to also   perform more  detailed experiments in  this sector.

COMPASS  will  enrich  the  CERN  non-‐LHC programme  also  in  the  years  to  come  thanks  to  the support   of our funding agencies   and CERN, in   particular from   the TE, EN,   DGS-‐RP and PH   departments and groups.