The future COMPASS-II Drell-Yan program
M. Alexeev
INFN sez. di Torino.On behalf of the COMPASS collaboration.
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Drell-Yan Process and its Kinematics
Drell-Yan cross section includes a convolution of parton distribution functions.
202
, ,
( ) ( ) ( ) ( ˆ ˆ))( ) (a b aa bq u d
bs
q x q x q x q xd
dx dx Q sdQ
2( ) ( )( ) ,a bs P P
2( ) ( )/ (2 ),a b a bx q P q
,F a bx x x 2 2 2 ,a bM Q q sx x
( )a bTk
a bT T T Tq P k k
( )a bP the momentum of the beam (target) hadron,the total center-of-mass energy squared,the momentum fraction carried by a parton from Ha(b) ,the Feynman variable,the invariant mass squared of the dimuon,the transverse component of the quark momentum,the transverse component of the momentum of the virtual photon.
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At leading order, 3 PDFs are needed to describe the structure of the nucleon in the collinear approximation.
But if one takes into account also the quarks intrinsic transverse momentum kT, 8 PDFs are needed:
Leading Order PDFs
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: the Sivers effect describes the correlation of intrinsic transverse momentum of unpolarized quarks with nucleon transverse polarization.
Transverse Momentum Dependent PDFs
The three TMD PDFs below describe important properties of spin dynamics of nucleon.
21 ( , )T Tf x k
21 ( , )Th x k
21 ( , )T Th x k
: the Boer-Mulders function describes the correlation between the transverse spin and the transverse momentum of a quark inside the unpolarised hadron.
: the Pretzelosity function describes the polarisation of a quark along its intrinsic kT direction making accessible the orbital angular momentum information.
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Single-polarised DY cross-section: Leading order QCD parton model
At LO the general expression of the DY cross-section simplifies to (COMPASS Note 2010-2) :
Thus the measurement of 4 asymmetries (modulations in the DY cross-section):
2
2
2
2cos2
4 2 sin
sin 2
sin
sin sin(2 )
sin
sin(2 )
ˆ 1 cos 2
sin 2
| sin sin(2 )
sin(2
( )
S S
S
LOLO LOemU U
LOL L
LOT T S T S
T
dD A
d qd Fq
S D A
S D
)S
cos2UA
sin STA
sin(2 )STA
sin(2 )STA
- gives access to the Boer-Mulders functions of the incoming hadrons,
- to the Sivers function of the target nucleon,
- to the Boer-Mulders functions of the beam hadron and to , the pretzelosity function of the target nucleon,
1Th
- to the Boer-Mulders functions of the beam hadron and to ,the transversity function of the target nucleon.
1h
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Sivers effect in Drell-Yan processes.M. Anselmino, M. Boglione, U. D'Alesio, S.Melis, F. Murgia, A. ProkudinPublished in Phys.Rev.D79:054010, 2009.
1. TMD PDFs – ALL are sizable in the valence quark region
2. - pT should be small (~ 1 GeV), can be generated by intrinsic motion of quarks and/or by soft gluon emission. This is the region where TMD formalism applies.
QCD Tp Q
Boer-Mulder function for u and d quarks as extracted from p+D data from Zhang et al. Phys.Rev. D77,0504011.
Some indications for the future Drell-Yan experiments
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TMDs universality SIDISDY
The Universality test includes not only the sing-reversal character of the TMDs but also the comparison of the amplitude as well as the shape of the corresponding
TMDs.
The time-reversal odd character of the Sivers and Boer-Mulders PDFs lead to the prediction of a sign change when accessed from SIDIS or from Drell-Yan processes:
Check the predictions:
1 1( ) ( )T Tf DY f SIDIS
1 1( ) ( )h DY h SIDIS
Its experimental confirmation is considered a crucial test of the non-perturbative QCD.
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COmmon Muon Proton Apparatus for Structure and Spectoscopy
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The spectrometer
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Acceptance(360 mrad)
Muons & hadrons beams:
hadron+ => 75% p+, 23% π+, 2% κ+
hadron- => 95% π-, 2-3% κ-, 2-3% p-
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Why Drell-Yan @ COMPASS
1. Large angular acceptance spectrometer.2. SPS M2 secondary beams with the intensity up to 108 particles per second.3. Transversely polarized solid state proton target with a large relaxation time and
high polarization, when going to spin frozen mode.4. a detection system designed to stand relatively high particle fluxes.5. a Data Acquisition System (DAQ) that can handle large amounts of data at large
trigger rates.6. The dedicated muon trigger system.
For the moment we consider two step DY program:The program with high intensity pion beam.The program with Radio Frequency separated antiproton beam.
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The main characteristics of the future fixed-target Drell-Yan experiment:
1. Small cross section High intensity hadron beam (up to 10^9 pions per spill) on the COMPASS PT 2. High intensity hadron beam on thick target
1. Hadron absorber to stop secondary particles flux2. Beam plug to stop the non interacted beam 3. Radioprotection shielding around to protect things and people4. High-rate-capable radiation hard beam telescope
190 GeV
π-
DY@COMPASS - set-upπ- p μ- μ X (190 GeV)
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DY@COMPASS – kinematics - valence quark rangeπ- p μ- μ X (190 GeV pion beam)
In our case (π- p μ- μ X) contribution from valence quarks is dominant.
In COMPASS kinematics u-ubar dominance.
<PT> ~ 1GeV – TMDs induced effects expected to be dominant with respect to the higher QCD corrections.
2( ) : 4 9 /M eVR cHM G
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DY@COMPASS - High mass Drell-Yan
Detailed simulations using PYTHIA and GEANT were performed. Results were compared with published cross-sections from past Drell-Yan experiments – good agreement.
Even if the cross-section is low, dimuons with are the ideal sample to study azimuthal asymmetries in Drell-Yan, due to negligible background contamination.
24 9 /M GeV c
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DY Feasibility@COMPASS : Feasibility studies
Beam tests were done in 2007, 2008 and 2009 to study the feasibility of the measurement.
The target temperature does not seem to increase significantly with the hadron beam, long polarization relaxation times measured (2007 beam test).
Reasonable occupancies in the detectors closer to the target can only be achieved if a hadron absorber and beam plug are used (2008 beam test).
Radiation conditions are within safety limits up to a beam intensity of 6x107 π-/second (measurements during all the beam tests).
Physics simulations were validated, within statistical errors (J/ψ peak and combinatorial background, in 2007 and 2009 beam test).
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DY Feasibility@COMPASS: Beam Test 2009 – the most important in a row of three beam tests 2007-2009
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The project.
Mounting and alignment.
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DY Feasibility@COMPASS : Beam Test 2009 (the hadron absorber)
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Installation of the target and of the absorber in the experimental hall.
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2
11
,Q
xPq
2
22
,Q
xP q
1 2,fx x x
DY Feasibility@COMPASS : Kinematic plots for xa and xb
COMPASS acceptance covers the range of valence quarks for both DY and J/ψ.
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DY Feasibility@COMPASS :Kinematic plots for pt and xf
Kinematic distributions for xf and pt of dimuons obtained during the Drell-Yan test run 2009.They correspond to out expectations.
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DY Feasibility@COMPASS : 2009 running
Two CH2 target cells (40+40 cm).
Beam intensity: per spill.78 10
Hadron absorber.
Reconstructed z-vertex position:the two target cells and the absorberare visible.
Mass spectrum of dimuons.
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Expected event rates & Projections IWith a beam intensity Ibeam=6x107 particles/second, a luminosity of L=1.2x1032cm-2s-1 can be obtained:
→ expect 800/day DY events with24 9 /M GeV c
Assuming 2 years of data-taking (140 days/year), one can collect: ≈230000 events in HMR DY.
Possibility to study the asymmetries in several xF bins.
This will translate into a statistical error in the asymmetries:
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Expected statistical error of the Sivers asymmetry for a measurement in three (left) and five (right) bins in xF . The smaller error bar is the statistical only, while the larger one corresponds to the quadratic sum of statistical and systematic errors. The theoretical prediction of the asymmetry from Anselmino et al. is also shown.
Expected statistical error of the Sivers asymmetry in the dimuon mass range 4GeV/c2 ≤ Mμμ ≤ 9GeV/c2, assuming one year of data taking.
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Projections II (COMPASS II proposal )
2( ) : 4 9 /M eVR cHM G 2( ) : 2 2.5 /M eV cIMR G
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Summary
The polarized Drell-Yan measurement is part of the COMPASS-II new physics proposal.
The proposal for a first period of 3 years (2014 to 2016) including 1 year of Drell-Yan data tacking was recommended for approval by the SPSC/CERN and approved by the CERN research board.
The feasibility of the measurement was confirmed by the 3 beam tests performed in 2007-2009.
The expected statistical accuracy reached in 2 years (6 10∙ 7 π-/sec) of data taking should allow to check the theory predictions and to extract TMD PDFs, namely Sivers and Boer-Mulders, as well as the transversity PDF.
Sivers and Boer-Mulders PDFs sign change when measured in Drell-Yan versus SIDIS will be checked. This is considered to be a fundamental test of the TMDs approach.
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experiment particles energy x1 or x2 luminosity timeline
COMPASS(CERN) p± + p↑ 160 GeV
s = 17.4 GeVx2 = 0.2 – 0.3x2 ~ 0.05 (low mass) 2 x 1033 cm-2 s-1 2014
PAX(GSI) p↑ + ppar
colliders = 14 GeV x1 = 0.1 – 0.9 2 x 1030 cm-2 s-1 >2017
PANDA(GSI) ppar + p↑ 15 GeV
s = 5.5 GeV x2 = 0.2 – 0.4 2 x 1032 cm-2 s-1 >2016
J-PARC p↑ + p 50 GeVs = 10 GeV x1 = 0.5 – 0.9 1 x 1035 cm-2 s-1 >2015 ??
NICA(JINR) p↑ + p collider
s = 20 GeV x1 = 0.1 – 0.8 1 x 1030 cm-2 s-1 >2014
PHENIX(RHIC) p↑ + p collider
s = 500 GeV x1 = 0.05 – 0.1 2 x 1032 cm-2 s-1 >2018
RHIC internaltarget phase-1 p↑ + p 250 GeV
s = 22 GeV x1 = 0.25 – 0.4 2 x 1033 cm-2 s-1 >2018
RHIC internaltarget phase-1 p↑ + p 250 GeV
s = 22 GeV x1 = 0.25 – 0.4 6 x 1034 cm-2 s-1 >2018
AnDYRHIC (IP-2) p↑ + p 500 GeV
s = 32 GeV x1 = ?? ?? cm-2 s-1 2013
SeaQuest (unpol.)(FNAL) p + p 120 GeV
s = 15 GeV x1 = 0.3 – 0.9 3.4 x 1035 cm-2 s-1 2011
pol. SeaQuest(FNAL) p↑ + p 120 GeV
s = 15 GeV x1 = 0.3 – 0.9 1 x 1036 cm-2 s-1 >2014
DY around the worldSlide taken from Wolfgang Lorenzon’s talk at the Transversity2011 Workshop
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Coordinate systems
TF
Collins-Soper
25
26
STMD DFs
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Quark polarization
U L T
Nucleon Polarization
U
L
T
21 ( , )q
Tf x k
12( , )qT
TT f x kM
k S
21 ( , )q
L L TS g x k
21 ( , )qTT
TTg x k
Mk S
21 ( , )q
T T Th x kS
21 ( , )T qTT
TT
T h x kM M
k Sk
21 ( , )q
L L TTS h x kM
k
21 ( , )q
ij jT T
Thk
x kM
ò
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COMPASS & AnDY
500s GeV
pp
e e
Different (but overlapping) kinematic ranges.Different background conditions.
COMPLEMENTARY MEASUREMENTS
01.11.2011
19s GeV
p
High level of feasibility studies
COMPASS AnDY
28
SIDIS: CFR
01.11.2011 EINN 2011, Pafos.
0Fx
1
( , ) ( , ) ( ) ( ) ( , ) ( , ) ( ) ( , )
, / , ,2 2 2
Nl N P S l h P X l q k s l q k sh
q s N S q sS T
d df D
dxdQ d dzd P dQ
1 2 2, 1 1
'( , ) ( , ) ( , )h T T
q s T T Th
D z D z p H z pm
p s
p