Polarized Drell-Yan Physics at
COMPASS
IhnJea Choi
University of Illinois at Urbana-Champaign
For the COMPASS collaboration
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Outline
 TMD PDFs

Sivers asymmetry results from Semi-Inclusive DIS (SIDIS)

Sign change of Sivers functions between Drell-Yan (DY) and SIDIS

Polarized DY process

COMPASS experiment

Pion-induced transversely polarized DY experiment

DY Simulation with COMPASS spectrometer

Beam test results for DY feasibility and estimation at COMPASS

Summary
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Proton Spin Structure, 3 PDFs
NUCLEON Spin
NUCLEON,
Transverse Pol.
Un Pol.
Longitudinal Pol.
Long. Pol.
Unpolarized f1 (x)
−
Helicity g1(x)
−
Trans. Pol.
Quark,
Quark Spin
Un Pol.
Transversity h1(x)
 At leading twist, structure of nucleon can be described by three PDFs.
 However, perturbative QCD in collinear approximation cannot explain measurements
of large azimuthal spin asymmetries from high energy collision experiments.
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Proton Spin Structure, 3 PDFs
Taking account the intrinsic transverse momentum of quark (kT )
NUCLEON Spin
NUCLEON,
Long. Pol.
−
Helicity g1(x)
−
Trans. Pol.
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Transverse Pol.
Un Pol.
Longitudinal Pol.
Unpolarized f1 (x)
Quark,
Quark Spin
Un Pol.
Transversity h1(x)
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Proton Spin Structure, 3 PDFs +5 TMDs
Taking account the intrinsic transverse momentum of quark (kT )
NUCLEON Spin
NUCLEON,
Un Pol.
Longitudinal Pol.
Transverse Pol.
Sivers
kT
−
kT
Long. Pol.
Unpolarized f1 (x)
−
Trans. Pol.
kT
−
kT
Helicity g1(x)
Quark,
Quark Spin
Un Pol.
−
kT
−
kT
kT
−
kT
h1(x)
Transversity h1(x)
kT
−
kT
 8 PDFs are needed for a full description of the nucleon, Transverse Momentum dependent PDFs
 Subtle quark transvers momentum of parton embedded into the PDFs (x)  (x, kT2)
 Provide essential non-perturbative information on the parton sub-structure of nucleon
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Sivers in Semi-Inclusive DIS(SIDIS)
Semi-Inclusive IDIS
Sivers Function
+ P- = '+ h + X
kT
−
kT
 Influence of the transverse hadron spin
onto the quark transverse momentum
Sivers asymmetry
AUT ® sin(fh - fs )
 Lepton and produced hadron used a probe access to quark kT and transverse polarization
 TMDs appears in azimuthal asymmetries in convolution of PDFs and fragmentation
function (FFs)
 If FFs known, TMDs are accessible for different flavor from SIDIS measurements of
identified hadrons
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Sivers measurement from SIDIS
 COMPASS 160 GeV, HERMES 27.6 GeV
 Non zero Sivers asymmetries measured in SIDIS at COMPASS and HERMES
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Sign Change of Sivers Functions between SIDIS and DY
SIDIS
DY
VS.
J.C. Collins, Phys. Lett. B536 (2002) 43
S.J. Brodsky, D.S. Hwang, I.Schmidt, Nucl. Phys. B642 344
Direction of the gauge-link of the kT dependent PDF is process-dependent and it changes to
the opposite in SIDIS w.r.t DY
Crucial test of non-perturbative QCD!
arXiv:0805.2677v2
[hep-ph] 9 Jan 2009
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M.Anselmino et al,
PRD 79 054010
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Drell-Yan Process
 Quark and antiquark annihilation process.
 DY, no fragmentation process required, is an excellent tool to study TMDs as it
accesses directly to initial parton structure
 Complementary measurement to SIDIS measurement
Arnold, Metz and sion Phys. Rev. D79 (2009) 034005
H a (Pa )+ H b (Pb ) ® g * (q)+ X ® l - (l)+ l + (l - )
Pa Hadron beam momentum
Pb = 0, (fixed target)
ι Lepton momenta
S target polarization
If the quarks intrinsic transverse momentum ≠ 0,
The dilepton has also qT = kTa + kTb
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Definition of azimuthal angles in DY
1) Target rest frame (TF)
FS
Angle between target spin (ST) and transverse
component of outgoing lepton pair momentum
Beam hadron
2) Rest frame of the virtural photon (CS : Collins-Soper frame)
F
Angle between planes defined by hadron and
lepton momenta
Beam hadron
q
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Polar angle of lepton pair in the CF frame
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Pol. Drell-Yan cross section
DY cross section for transversely polarized nucleon target
S. Arnold et al.,
Phys. Rev. D79 (2009) 034005.
A. Kotzinian,
COMPASS Note 2010-2,
February 10, 2010,
F: Flux of incoming hadrons, D[] depolarisation factor
ϕs : TF frame
ϕ, θ, CS frame
DY cross section (LO QCD)
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Pol. Drell-Yan cross section
All these asymmetries (modulations) are expected to be sizable in the
valence quark range and can be measured at COMPASS.
At LO QCD
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COMPASS
COmmon Muon Proton Apparatus for Structure and Spectroscopy
SPS M2 Beam line
COMPASS II
(Proposal approved by CERN on 2010)
- Generalized Parton Distributions
- TMDs in SIDIS
- TMDs in pion induced Drell Yan
 Fixed target experiment located at CERN SPS M2 beam line since 2002
 Muon and Baryon spectroscopy with high energy
 25 Institutes, ~250 people
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COMPASS Experimental Hall
Beam
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The COMPASS Spectrometer
 Muon/hadron beam: 160/190 GeV
 High beam intensity (2x109 / spill)
 Polarized proton target
MuonWall
SM2
E/HCAL
E/HCAL
SM1
MuonWall
Target
Small Angle
Spectrometer(SAS)
RICH
Beam
Large Angle
Spectrometer (LAS)
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Simulation for Beam Choice
 Choice of beam (π-)
p - (ud)+ p­(uud) ®
+ -
X
Valence u-quark and anti-u quark dominant in Drell-Yan process
TMDs are sensitive at valence quark regime (x > 0.1)
 Choice of beam energy (190GeV)
(PYTHIA, 4 < Mμμ< 9 GeV/c)
xp
~ 0.1
xp -
The value s = 357 GeV2, corresponding to a pion beam momentum of 190 GeV/c,
seems to be a good choice. -> As the energy increased, the phase space coverage
extends to the low-x non-valence region
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Simulation for DY Acceptance
Polar angle distribution of muon pairs
from Drell-Yan events
COMPASS Spectrometer
LAS
SAS
Large Angle Spectrometer(LAS) < 140 mrad
Small Angle Spectrometer(SAS) < 15 mrad
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Simulation for DY Kinematics
(High Di-muon mass region, 4 < Muu < 9 GeV/c2)
M. Anselmino .. Phys.Rev.D79:054010, 2009
 Valence quark range (x > 0.1) for both quarks (pure u-ubar annihilation)
(PT dimuon about 1GeV/c where TMD effects are dominant)
 Acceptance is largest where the Sivers function is expected to be largest
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Simulation for DY Kinematics
(Low Di-muon mass region, 2 < Muu < 2.5 GeV/c2)
M. Anselmino .. Phys.Rev.D79:054010, 2009
 Maximal expected asymmetry value nearly correspond to the maximal acc.
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Simulation, SIDIS vs. DY kinematic
Mean value comparison

The COMPASS SIDIS and DY experimental
There is overlap in the phase-space accessed by the 2
measurements.
When comparing the extracted TMDs, QCD
measurements have an overlapping kinematic region.
evolution of the TMDs must be taken into account.
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DY Feasibility – Test Setup





Pion induced DY test runs at COMPASS, year 2007, 2008, 2009 (3 days)
π- 190 GeV/c beam
Transversely polarized target simulated by two cylinders targets of polyetylene (CH2) -> NH3
Hadron absorber installed
Beam intensity : 8 x107 pions/spill (spill length 9.6s)
COMPASS Detector
π- beam
Transvers Pol. target
Two 40cm length of Polyethylene(CH2) separated by 20cm
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Hadron absorber and beam plug (stop non-interacting beam)
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Vertex & Di-muon Distributon
Preliminary on Nov 2013
After all track quality cuts included
From absorber
cell1
cell2
 Both target cells (40cm + 40cm, gap between cells 20cm) are visible in the distribution
(to cancel out systematic uncertainties from different location of polarized target)
 Requires improved beam and vertex tracking
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Dimuon Mass Distributon
For 3 days of data taking in 2009
Preliminary on Nov 2013
 6787 number of J/ψ reconstructed for 3 days of data taking (a factor two increased from the
previous released data, 3170 # J/ψ)
 Collected number of J/ψ is agreement with the expectations from MC simulation.
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Dimuon Transverse Momentum Distribution
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xp vs. xπ, and xF Distributions
xp
xp -
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Di-muon Background
 Combinatorial muon BG is negligible under the J/Psi region
 Beam intensity in this test was 8.3*106 pions/s, which is around 10 times lower than the beam intensity
expected in the future
- controllable by optimizing hadron absorber and beam plug
 Open-charm semi-leptonic decays
- Seen to negligible in simulation Muu>2GeV in PYTHIA
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Expected DY events at COMPASS
Previous COMPASS Proposal
Current estimation
Beam intensity of Ibeam = 6 x 108 pion/s
Spill 9.6s
Beam intensity of Ibeam = 108 pion/s
SPS super-cycle of 33.6s
892 DY events / day
2 year runs (280days)
2034 DY events / day
1 year run (140days)
0.2 Million DY events estimated
For dimuon high mass region
(4 < Muu < 9 GeV/c2)
0.285 Million DY events estimated
For dimuon high mass region,
(4 < Muu < 9 GeV/c2)
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Sivers Asymmetry Prediction
(Dimuon Mass Region, 4 < Mμμ < 9 GeV/c2)
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Asymmetries Prediction
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Detector Upgrades
• COMPASS Polarised target:
– New target holder (2x55 cm, 20 cm gap)
– Old/modified Micro-Wave cavity (2 cells target)
– PT Pump system refurbishing
• COMPASS PT has to be moved by ~2.2 meters upstream in order to release
space for the Hadron Absorber
• Hadron absorber (Alumina Al2O3) and beam plug (tungsten)
• Radio-Protection screen (stainless steel & borated polyeth.)
• New SciFi-based beam telescope
• H1 trigger hodoscope modification (central hole size adjustment)
• New vertex detector (SciFi based)
• New Large Area tracking station in the LAS
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Summary
• The Sivers asymmetry has been measured in SIDIS at COMPASS
and at HERMES.
• COMPASS II will test the expected sign change of Sivers and
Boer-Mulders PDFs in DY
• Feasibility of Drell-Yan measurement was demonstrated
with 2009 test beam
• Expected statistical precision of asymmetry measurement
is 1~2%
• First ever pion induced transversely polarized Drell-Yan
experiment will start in this year (2014).
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