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DRELL-YAN at COMPASS
Stephane Platchkov
Institut de Recherche sur les lois Fondamentales de l’Univers
CEA/IRFU, Saclay, France
GDR NucléonEcole Polytechnique, December 15-17, 2014
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Looked for W... Rapid fall-off: ~Mµµ
-4 , - missed the J/Ψ!
Explanation: 1970 by Drell and Yan:
Model based on the newly invented partons (Feynman, 1968)
Some history
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PRL 25 (1970), 316, 902.
E=30 GeV
AGS@BNL, 1970
DY: first application of the concept of partons after the first SLAC DIS experiment
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Main features of the “Parton annihilation model”
Drell-Yan cross section:
Features (parton model):
Cross section depends on τ = M2/s
Convolution of quark and antiquark PDFs
Can be used to determine PDFs in p, K, p
Transverse momentum of µµ pair is small
No fragmentation process
Confirmed in QCD Assumptions: factorization
S. Platchkov GDR Dec.15, 2014 3
Ito et al. PRD 23(1981)604.(from Kenyon, RPP, 1982)FERMILAB:
non-DY contribution
E=400 GeV
Tung-Mow Yan (SLAC, 1998): “The process has been so well understood that it has become a powerful tool for precision measurements and new physics”
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Nucleon structure studies – different probes
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Drell-Yan
TMD (x,kT) GPD (x,bT)PDF (x)
from Bacchetta from Bacchetta
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Unpolarized DIS vs unpolarized Drell-Yan
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McCaughey, Moss, Peng ARNPS 49 1999) 217.
DIS Drell-Yan
Both DIS and DY probe the parton substructure
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Sivers: correlation between the quark transverse momentum and the nucleon transverse spin (polarized nucleon)
Boer-Mulders: correlation between the quark transverse spin and transverse momentum (unpolarized nucleon)
Transverse-Momentum Dependent (TMD) PDFs
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worm-gear
I R F U Full formalism for two hadrons
Access 4 TMDs – asymmetry modulations (pion beam):
DY (polarized) cross section expansion
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Arnold, Metz and Schlegel, Phys. Rev. D79 (2009) 034005.
Boer-Mulders
Sivers
Pretzelosity
Transversity
Worm-Gear
All five TMDs are also measured in SIDIS
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TMDs in Drell-Yan and SIDIS
Complementary probes SIDIS DRELL-YAN
Assumptions Factorizaton Universality: (unlike PDFs) TMDs can be process dependent Opposite sign in SIDIS and DY processes:
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Collins, Soper, Sterman, Adv. Ser. High En. Phys. 5, 1988.
Crucial test of the QCD factorization approach
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Sivers asymmetry (SIDIS)
Compass and Hermes data on a proton target
pions
kaons
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HERMES: PRL 103 (2009) 152002
COMPASS: PLB 692 (2010) 240PLB 717 (2012) 383+ preliminary
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Sivers TMD - as determined from SIDIS
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. Anselmino et al., Eur.Phys.J.A39 (2009) 89.
Sivers: Good knowledge from SIDIS - never measured in DYOnly possible in a polarized DY experiment
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.
COMPASS setup advantages Beam energy: 100 – 200 GeV Transversely polarized NH3 /6LiD targets Large angular acceptance
CERN beams Negative (pion, kaon, p) or positive hadron beams With a negative pion beam: u/u annihilation Dominated by valence u quarks (x ≥ 0.1)
Polarized Drell-Yan measurements
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Drell and Yan, PRL 25 (1970), 316, 902.
COMPASS is an ideal place for DY studies
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COMPASS – a fixed target experiment
A very versatile setup Several beams available: µ+, µ-, h+, h-, e- => Several physics goals
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50 m
“Minor” changes to the setup – switch between physics programs
Energy: 100 – 200 GeVIntensity: up to 109 /spillLarge acceptance, PID detectorsSeveral particles in the final stateLarge (1.2 m) polarized target
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COMPASS Drell-Yan setup
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Small cross sections – high intensity h beam (~109/spill of 10 sec) Possible use of thin nuclear targets (inside the absorber)
pion beam 190 GeV
Hadron absorber: Tungsten, Alumina
and Stainless steel
Nominal COMPASS setup (minor modifications)Dimuon trigger system
Polarized Target
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Drell-Yan acceptances (two muons)
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COMPASS (2014)E615 (1989)
COMPASS acceptance: an order of magnitude improvement
~4%
~4%
~40%
~40%
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Polarized Drell-Yan – expected results
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Sivers Boer-Mulders
Pretzelosity Transversity
140 days of data 6.108 pions/spill2 x 55 cm NH3 target4 < Mµµ< 9 GeV
First DY experiment with polarization
2000 DY events/day in the mass region 4 < Mµµ < 9 GeV/c2
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Test setup (3 days in 2009) 190 GeV negative pion beam, I ≤ 1.5x107/s (instead of 108/s) “poor-man” hadron absorber ( concrete and steel) two polyethylene target cells preliminary DY trigger
Results Count rate confirmed Mass resolution as expected Good vertex resolution Low background at high masses
Drell-Yan – test data taking
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Drell-Yan region
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In addition...
there is also good physics without polarization...
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Targets for Drell-Yan 2014 and 2015
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Nuclear targets: Al, WRich Drell-Yan physics with unpolarized targets!
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Angular distributions for Drell-Yan
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θ and ϕ: polar and azimuthal angles of the µ+ in the dilepton rest frame
Cross section:
Parton model (naive DY): l = 1, µ = 0, ν = 0
pQCD : 1 – -l 2ν = 0 Lam-Tung, 1978
Valid at O(α1s)
NNLO corrections small Brandenburg et al., PRL 73 (1994) 939.
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Lam-Tung relation: NA10@CERN
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NA10, Z. Phys. 37 (1988) 545.
140 GeV/c 194 GeV/c 286 GeV/c
ν > 0 and ν increases with pT
λ
µ
ν
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Lam-Tung relation
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CERN: NA10, Z. Phys. 37 (1988) 545.
1 –
λ –
2ν
pT (GeV/c) pT (GeV/c) pT (GeV/c)
Clear violation observedA number of explanations proposed...
see Peng and Qiu, PPNP, 76 (2014) 43
FERMILAB: E615, PRD 39 (1989) 92.
252 GeV/c
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Positive/negative hadron beams content
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74.5% 24% 1.5%
2.5%96.5%1%
Negative hadron beam content: π, K, p
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COMPASS estimates (Takahiro Sawada, AS/Taipei)
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Large improvement for π-, NEW data for K- and for p
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Ratio of K- and π- induced DY cross sections
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NA3 experiment, CERN 1980
Ratio proportional to: uK(x)/uπ(x) u(x) distributions in K and π
COMPASS, assuming 140 days data taking
An order of magnitude improvement in statistics
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Flavour-dependent EMC effect
Cloët, Benz, Thomas, PRL 102 (2009):
“For N≠Z nuclei the u and d quarks have distinct nuclear modifications”
Dutta et al., PhysRev C83 (2011): Assuming SU(2):
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With a flavour-dependent EMC effect
uA(x)
dA(x)
Without flavour-dependence
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Predictions for COMPASS with a 160 GeV pion beam
Flavour-dependent EMC in COMPASS
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from Dutta et al., Phys.Rev. C83 (2011)
Important findings expected in COMPASS
Already in 2015
Need data taking with a π+ beam
With flavour dependence
Without flavour dependence
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Estimated number of DY events
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Available statistics improved by an order of magnitude
Beam hadron dependence study: π-, K-, p Target hadron dependence study: NH3, AL, W
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Planned Polarized Drell-Yan Experiments (Lorenzon, 2014)
experiment particles energy xb or xt Luminosity timeline
COMPASS(CERN) p± + p↑ 160 GeV
s = 17.4 GeV xt = 0.2 – 0.3 2 x 1033 cm-2 s-1 2015, 2018*
PAX(GSI) p↑ + pbar
colliders = 14 GeV xb = 0.1 – 0.9 2 x 1030 cm-2 s-1 >2017
PANDA(GSI) pbar + p
↑ 15 GeVs = 5.5 GeV xt = 0.2 – 0.4 2 x 1032 cm-2 s-1 >2016
NICA(JINR) p↑ + p
colliders = 20 GeV xb = 0.1 – 0.8 1 x 1030 cm-2 s-1 >2018
PHENIX(RHIC) p↑ + p
colliders = 500 GeV xb = 0.05 – 0.1 2 x 1032 cm-2 s-1 >2018
RHIC internaltarget phase-1 p↑ + p
250 GeVs = 22 GeV xb = 0.25 – 0.4 2 x 1033 cm-2 s-1 >2018
RHIC internaltarget phase-1 p↑ + p
250 GeVs = 22 GeV xb = 0.25 – 0.4 6 x 1034 cm-2 s-1 >2018
SeaQuest (unpol.)(FNAL) p + p 120 GeV
s = 15 GeVxb = 0.35 – 0.85xt = 0.1 – 0.45 3.4 x 1035 cm-2 s-1 2012 - 2015
Pol tgt DY‡ (E1039)(FNAL) p + p↑ 120 GeV
s = 15 GeV xt = 0.1 – 0.45 3.4 x 1035 cm-2 s-1 2016
Pol beam DY§ (E1027)(FNAL) p↑ + p
120 GeVs = 15 GeV xb = 0.35 – 0.85 2 x 1035 cm-2 s-1 2018
‡ 8 cm NH3 target § L= 1 x 1036 cm-2 s-1 (LH2 tgt limited) / L= 2 x 1035 cm-2 s-1 (10% of MI beam limited)
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Summary: DY in COMPASS
An ideal place for DY studies Only laboratory today with antiquark beams beam dependence: using K- and p beams target dependence: NH3, Al, W targets
Address several outstanding physics issues Test of QCD factorization theorems (Sivers, BM sign-change) Determine the BM function of the pion Better understanding of the violation of the Lam-Tung sum rule Flavor-dependence of the EMC effect? Comparison between u(x) PDFs in kaons and pions
Long term Possibility for a RF separated K- and p beams
Aim at 1 to 2 orders of magnitude improvement for K- and p29
S. Platchkov GDR Dec.15, 2014
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Thank you!
30S. Platchkov GDR Dec.15, 2014
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Estimated number of DY events
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from T. Sawada 4 < Mµµ < 9 GeV/c2; p/k/p = 96.5/2.5/1.0%140 days of data takingCorrected for exp. efficiencies
Available statistics improved by an order of magnitude
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Next-to-Leading order DY contributions
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Next-to-leading order diagrams complicate the picture
These diagrams are responsible for 50% of the measured cross section
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COMPASS Drell-Yan setup
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Upgrade of the spectrometer• Beam telescope (Sci-Fi)• Thick hadron absorber/beam dump• Vertex detector • Polarized target moved 2.2 m upstream
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Lam-Tung relation and BM function
Possible explanation (among others): cos2φ modulation
Boer (PRD60, 1999, 01402): the cos2 dependence could be due to the BM
function.
(BM function: correlation between kT and sT
in an unpolarized nucleon )
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νData: NA10, 194 GeV/c
BM explains the ν dependence
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Drell-Yan: parameter ν vs pT
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E866 (2007) : p + p
E866 (2009) : p + d NA10
(1988) : π- + W
Fits with parametrizations from Boer, PRD60 (99) 014012.
Effect is larger for pions (valence vs sea antiquarks)
from Peng and Qiu, PPNP, 2014
pion beam
proton beam
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Drell-Yan acceptances (COMPASS vs NA10)
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NA10
COMPASS
5%
38%
Nearly an order of magnitude improvement
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Drell-Yan experiments (from Grosse-Perdekamp, SPIN 2014)
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Expected statistical accuracy
Assumptions: Ibeam = 108p/s, L = 2.3x1033, P=90%, f = 0.22, t = 140 days,
spill length = 10 s, every 34 s
► ~ 2000 DY events/day in the mass region 4 < Mµµ < 9 GeV/c2
J/Ψ cross section: about 50 times largerS. Platchkov GDR Dec.15, 2014 38
Boer-Mulders
Sivers
Pretzelosity
Transversity
First DY experiment with polarization