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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:

S. Platchkov GDR Dec.15, 2014 8

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