\section*{Executive Summary}
%\addcontentsline{toc}{section}{\numberline{~}\bfseries Executive Summary}
\addcontentsline{toc}{section}{\bfseries Executive Summary}
\label{sec:exec_summary}

\noindent
AThe research fields of hadron spectroscopy and hadron structure are 
closely connected since their very beginnings. In 1964, spin-1/2
quarks were conjectured to be the building blocks of baryons and mesons in 
order to explain their quantum numbers observed in hadron spectroscopy. In 
1969, when interpreting data from first direct studies of the structure of 
the proton, partons were hypothesised as its internal constituents and 
soon after identified with quarks. In the early 1970's, Quantum 
Chromodynamics (QCD) became accepted as the theory of strong interactions, 
explaining the observed weakening of the interquark forces at short 
distances or large momentum transfers. QCD not only describes hard 
processes through perturbative expansions, but also the non-perturbative 
dynamics of the strong interaction, down to soft and extremely soft 
processes which are involved in meson spectroscopy and linked to chiral
perturbation theory.

Parton Distribution Functions (PDFs) describe the structure of the nucleon
as a function of the nucleon momentum fraction carried by a parton of a 
certain species. They are studied primarily in Deeply Inelastic Scattering 
(DIS) where the longitudinal momentum structure of the nucleon is explored 
in the collinear approximation, \ie\ neglecting transverse degrees of 
freedom. Up to now, PDFs were investigated 
independently from nucleon electromagnetic form factors that are related to 
ratios of the observed elastic electron--nucleon scattering cross section to 
that predicted for a structureless nucleon. The recently developed 
theoretical framework of Generalised Parton Distributions (GPDs) embodies 
both form factors and PDFs, such that GPDs can be considered as 
momentum-dissected form factors which provide information on the transverse 
localisation of a parton as a function of the fraction it carries of the 
nucleon's longitudinal momentum. Obtaining such a ``3-dimensional picture'' of 
the nucleon is sometimes referred to as ``nucleon tomography''.
In a complementary approach, the subtle 
effects of intrinsic transverse parton momenta are described by 
Transverse-Momentum-Dependent PDFs (TMDs). These effects become visible in 
hadronic Drell--Yan (DY) and Semi-Inclusive DIS (SIDIS) processes. The 
structure of hadrons can not yet be calculated in QCD from first principles. 
However, the deformation of the shape of a hadron in an external 
electromagnetic field, described by polarisabilities, can be predicted by 
chiral perturbation theory which is a low-energy expansion of the QCD 
Lagrangian.

More than 10 years ago, the \compass\ experiment was conceived as ``COmmon 
Muon and Proton apparatus for Structure and Spectroscopy'', capable of 
addressing a large variety of open problems in both hadron structure and 
spectroscopy. As such, it can be considered as a ``QCD experiment''. By now, 
an impressive list of results has been published concerning nucleon 
structure, while the physics harvest of the recent two years of hadron 
spectroscopy data taking is just in its beginnings. The \compass\ 
apparatus has been proven to be very versatile, so that it offers
the unique chance to address in the future another large variety of newly 
opened QCD-related challenges in both nucleon structure and hadron spectroscopy, 
at very moderate upgrade costs. It consists of a high-precision forward 
spectrometer and either an unpolarised, longitudinally or transversely polarised 
target. It is located at the unique \cern\ SPS M2 beam line that delivers 
hadron or naturally polarised $\mu^\pm$ beams in the energy range between 
50~GeV and 280~GeV.

This proposal lays the ground for a new decade of fascinating QCD-related 
studies of nucleon structure and in hadron spectroscopy. It details the 
physics scope and related hardware upgrades for those topics for which 
data taking can be envisaged to start in 2012. This implies mainly studies 
of chiral perturbation theory, ``unpolarised'' generalised parton 
distributions, and transverse-momentum-dependent parton distributions.
More distant projects, 
as the whole complex of future QCD studies using hadron spectroscopy and also 
studies of ``polarised'' GPDs, will be described later in an addendum to this 
proposal. All these studies will significantly expand our knowledge on 
key aspects of hadron structure and spectroscopy which are inaccessible 
to any other facility existing or under construction.

The concept of GPDs attracted much attention 
after it was shown that the total angular momentum of a given parton species, 
$J^f$ for quarks ($f=u,d$ or $s$) or $J^g$ for gluons, is related to the second
moment of the sum of the two GPDs $H$ and $E$. As of today, it is 
by far not fully understood how the nucleon spin $\frac{1}{2}$ is shared 
between the contributions of intrinsic and orbital angular momenta of quarks 
of various flavors and gluons. Constraining quark GPDs experimentally by 
measuring exclusive Deeply Virtual Compton Scattering (DVCS),
$\mu \,p \rightarrow \mu \, \gamma \, p$, or Deeply Virtual Meson ($M$) 
Production (DVMP), $\mu \,p \rightarrow \mu \, M \, p$, is the only known 
way to constrain the quark components of 
the nucleon's spin budget $\frac{1}{2}=\sum_{f=u,d,s}J^f + J^g$. 
Such data will also be very important to experimentally validate GPD moments 
calculated from first principles through QCD calculations on the lattice.
In order to ensure exclusivity of DVCS and DVMP events, a new recoil detector 
will surround a 2.5~m long liquid hydrogen target. The kinematic domain
accessible with 160~GeV muon beams cannot be explored by any other facility 
in the near future. The DVCS cross section will be determined as a function of 
both the momentum transfer between initial and final nucleons and the fraction 
of the longitudinal nucleon momentum carried by the struck parton. A new
electromagnetic calorimeter (ECAL0) will provide coverage of substantially 
higher values of this fraction as compared to the existing calorimeters
ECAL1 and ECAL2. One key result 
will be the first, model-independent answer on the question how the 
transverse nucleon size varies gradually from the gluon/sea-quark region to 
that dominated by valence quarks. Only \compass\ can explore the kinematic
region between the \hone/\zeus\ collider range and the \hermes/\jlab\ 
fixed-target range, so that particularly important results can be expected from
3-dimensional nucleon ``tomography'' within this kinematic domain. The transverse
structure of the nucleon in the \compass\ kinematic range is considered to be 
important input for background simulations in proton-proton collisions at LHC. 
The second key result is information on the GPD $H$, obtained by separating
the real and imaginary parts of the DVCS amplitude. This will be accomplished 
by combining data from positive and negative muon beams. The azimuthal
dependence of the cross section will be used to isolate the contribution of
the GPD $H$, which is of particular importance for the evaluation of the spin 
sum rule. The measurements with the liquid hydrogen target will mainly 
constrain $H$. An extension of the programme is envisaged using a transversely 
polarised target, mainly to constrain $E$. This will be subject of an addendum 
to this proposal. After completion of data taking, the combined DVCS and DVMP 
data set of \hone, \zeus, \compass, \hermes\ and \jlab\ will constrain the 
nucleon-helicity-conserving $u$ and $d$ quark GPDs
over a wide kinematic range in parton longitudinal momentum versus parton 
transverse localisation, and virtual-photon resolution scale. It is expected 
that ongoing activities towards global fits of GPDs will lead to a reliable 
determination of total and also orbital quark angular momenta.

Simultaneously with the GPD programme, high-statistics data will be recorded 
on unpolarised semi-inclusive deep inelastic scattering,
 $\mu \, p \rightarrow \mu \, h \, X$. The pion and kaon multiplicities 
will be used to extract at leading 
order $\alpha_s$ (LO) the unpolarised strange quark distribution function 
$s(x)$ as well as fragmentation functions describing how a quark fragments
into a hadron. Presently, the poor knowledge of these quantities is the 
limiting factor in the determination of the polarisation of strange quarks 
from SIDIS data. These multiplicities will also represent important input to 
future global analyses beyond LO. 

The transverse momentum of partons is a central element in understanding 
the 3-dimensional structure of the nucleon. From the  measured azimuthal 
asymmetries of hadrons produced in unpolarised SIDIS and DY processes a 
sizable transverse momentum was derived.
When this intrinsic transverse momentum is taken into account, several 
new functions are required to describe the structure of the nucleon.
Transverse spin, in fact, couples naturally to intrinsic transverse 
momentum, and the resulting correlations are encoded in various 
transverse-momentum-dependent parton distribution and fragmentation functions.
The SIDIS cross section contains convolutions of these two types of functions,
while the convolutions in the DY cross section comprise only PDFs and/or TMDs.
In spite of the widespread interest in this approach which goes
beyond collinear QCD, the field is still in its infancy and only
data can sort out which correlations are appreciably different 
from zero and relevant.
Of particular interest are the correlations between quark
transverse momentum and nucleon transverse spin, and between quark
transverse spin and its transverse momentum in an unpolarised nucleon, 
which are encoded in the so-called Sivers and Boer--Mulders functions, 
both (na{\"\i}vely) $T$-odd.
The Boer--Mulders function contributes to the 
azimuthal modulations in the cross sections of unpolarised SIDIS and 
DY processes which have been observed since many years. 
We intend to accurately measure such modulations both in DY
and in SIDIS (this last measurement in parallel to the GPD programme).
%
Much attention in the recent years has been devoted to the Sivers function 
originally proposed to explain the large single-spin 
asymmetries observed in hadron--hadron scattering. From $T$-invariance 
arguments, for a long time it was believed to be zero.
One of the main theoretical achievements of the recent years was
the discovery that the Wilson-line structure of parton distributions,
which is necessary to enforce gauge invariance of QCD, implies
a sign difference between the $T$-odd distributions measured in
SIDIS and the same distributions measured in DY.
According to this ``restricted universality'', the Sivers function can
be different from zero but must have opposite sign in SIDIS and DY.
There is a keen interest in the community to test this prediction
which is rooted in fundamental aspects of QCD, and many laboratories
are planning experiments just to test it. The Sivers function was recently 
measured by \hermes\ and \compass\ in SIDIS off transversely polarised 
targets and shown to be different from zero and measurable. In order to test 
its sign change, DY experiments with transversely polarised hadrons are 
required, but none were performed so far. The main goal of our DY programme 
is to measure for the first time on a transversely polarised target the 
process $\pi^- p^{\uparrow} \rightarrow \mu^+ \mu^- X$. This will be a unique
measurement as at \compass\ energies the virtual photon originates mainly 
from the fusion of a $\bar{u}$ quark from the pion and a $u$ quark from the 
nucleon, both in valence-like kinematics. In two years of data taking with 
the 190~GeV $\pi^-$ beam and the \compass\ spectrometer with the NH$_3$ 
transversely polarised target, the fundamental prediction for the sign of the 
$u$ quark Sivers function can be tested for the first time.

Measurements of exclusive final states produced by incoming high-energy
pions at very small momentum transfer to the recoiling nucleus,
explore the Primakoff region where the cross section is dominated by the
exchange of a quasi-real photon. The initial $\pi^-\gamma^*$ system may 
scatter into $\pi^-\gamma$ (Compton reaction), $\pi^-\pi^0$,
$\pi^-\pi^0\pi^0$, $\pi^-\pi^+\pi^-$, or final states containing
more pions. In QCD, chiral Perturbation Theory (ChPT) predicts the low-energy
behaviour for all these reactions at small intermediate-state masses
$m^2_{\pi\gamma}$, from threshold to a few pion masses. The chiral expansion
of the cross section contains several low-energy constants which describe
important physical properties of the pion.
For the Compton reaction, the ChPT calculations result in deviations from the 
QED bremsstrahlung cross section that is exactly calculable for a point-like 
particle. The first term in the expansion in $m_{\pi\gamma}$ 
originates from the electric and  magnetic
dipole polarisabilities of the charged pion, $\alpha_\pi$ and $\beta_\pi$,
and is proportional to their difference  $\alpha_\pi-\beta_\pi$.
In order to resolve these two polarisabilities independently, \ie\ to
also determine  $\alpha_\pi+\beta_\pi$, it is necessary to measure the
cross section differential in the centre-of-momentum scattering angle
$\theta_{cm}$, in which the two contributions have a complementary functional
dependence. At that level of precision, it is possible (and necessary)
to also account for the most relevant combination of the pion quadrupole
polarisabilities, $\alpha_2-\beta_2$. Its effect has a similar $\theta_{cm}$ 
dependence as that of $\alpha_\pi-\beta_\pi$ but is proportional
to $m_{\pi\gamma}^4$ instead of $m_{\pi\gamma}^2$. The planned measurements 
will also allow for the determination of the two combinations 
$\alpha_\pi+\beta_\pi$ and $\alpha_2-\beta_2$, for the first time.
The neutral (electromagnetic) trigger permits at the same time
the precise measurement of final states containing one or more $\pi^0$.
The threshold behaviour of $\pi^-\pi^0$ determines the chiral
anomaly constant $F_{3\pi}$, for which the new data set will allow
a new level of experimental precision beyond that of the theoretical 
prediction of about 1\%.

The physics programme described in this proposal covers a period of
five years, one year for the tests of chiral perturbation theory
and two years each for the GPD and DY programmes. The tentative 
schedule for the first three years is as follows:
\bi
\item 2012: Tests of chiral perturbation theory,
\item 2013: GPD programme,
\item 2014: Drell--Yan programme.
\ei
On the basis of the results from the 2008 and 2009 hadron runs,
an addendum to this proposal aiming at further hadron spectroscopy
measurements will be submitted in due time. 
The schedule of these measurements will be considered together with that of the
remaining parts of the proposed GPD and DY programmes, possibly taking into
account extensions of the latter as sketched in the proposal.