[PPT] - Measurement of the Polarised Drell-Yan process at COMPASS M arcia PowerPoint Presentation

SLIDE 1

COMPASS

Measurement of the Polarised Drell-Yan process at COMPASS

M´ arcia Quaresma, LIP - Lisbon

n behalf of the COMPASS Collaboration

2nd June 2014, MESON 2014 - Krakow Co-financed by:

2nd June 2014, MESON 2014 - Krakow 1 / 16

SLIDE 2

COMPASS

Transverse Momentum Dependent Parton Distribution Functions - TMD PDFs

The nucleon structure in leading order QCD, taking into account kT , is described by 8 PDFs for each quark flavour. Sivers, Boer-Mulders, transversity and pretzelosity are accessible via either the single polarised Drell-Yan measurement or the transversely polarised SIDIS.

QUARK NUCLEON

unpolarized longitudinally pol. transversely pol. unpolarized longitudinally pol. transversely pol.

kT kT kT kT kT

quark nucleon

number density

f

Sivers

f

helicity

g g h

Boer−Mulders

h h

pretzelosity transversity

h

1 1T 1 1 1L 1T 1L 1T

2nd June 2014, MESON 2014 - Krakow 2 / 16

SLIDE 3

COMPASS

Polarised Drell-Yan process

Quark-antiquark annihilation, with dilepton production

Ha(Pa) X γ∗(q) l−(l) l+(l′) Hb(Pb, S) ¯ u(ka) u(kb)

Pa(b), beam (target) hadron momentum s = (Pa + Pb)2, centre of mass energy squared xa(b) = q2/(2Pa(b) · q), momentum fraction carried by the quark from Ha(b) xF = xa − xb, Feynman x Q2 = q2 = Mµµ2 = sxaxb, dimuon invariant mass squared kT a(b), quark intrinsic transverse momentum This process is an excellent tool to access TMD PDFs: No fragmentation functions involved, but the convolution of two PDFs. The use of a negative pion beam allows the annihilation between the valence quark ¯ u from π− with a valence quark u from proton to be dominant. ֒ → All the TMD PDFs are expected to be sizeable in the valence quark region The QCD TMD approach is valid in the region Q (Mµµ > 4 GeV/c2) ≫ pT ∼ 1 GeV/c A drawback of this process is its very low cross-section (fraction of nb for Mµµ > 4 GeV/c2) ֒ → Imposing an experiment with high luminosity

2nd June 2014, MESON 2014 - Krakow 3 / 16

SLIDE 4

COMPASS

Azimuthal Asymmetries

Considering an unpolarised π− beam and a transversely polarised proton target the σDY at LO can be written as: dσ d4qdΩ = α2

em

Fq2 ˆ σU{(1 + D[sin2 θ] Acos 2φ

U

cos 2φ) + |− → S T |[ Asin φS

T

sin φS + D[sin2θ]( Asin(2φ+φS )

T

sin(2φ + φS) + Asin(2φ−φS )

T

sin(2φ − φS))]} Each angular modulation present in the DY cross-section has an amplitude that contains the convolution of two TMD PDFs. These amplitudes are accessed via the measurement of the angular azimuthal asymmetries between the two oppositely transversely polarised target cells. Each asymmetry relates to: Acos 2φ

U

Boer-Mulders h⊥

1 (π) ⊗ Boer-Mulders h⊥ 1 (p)

Asin φS

T

unpolarised PDF f1(π) ⊗ Sivers f ⊥

1T (p)

Asin(2φ+φS )

T

Boer-Mulders h⊥

1 (π) ⊗ pretzelosity h⊥ 1T (p)

Asin(2φ−φS )

T

Boer-Mulders h⊥

1 (π) ⊗ transversity h1(p)

2nd June 2014, MESON 2014 - Krakow 4 / 16

SLIDE 5

COMPASS

COMPASS @ CERN

COmmon Muon Proton Apparatus for Structure and Spectroscopy

Fixed target experiment at the end of M2 SPS beam line Around 240 collaborators from 13 countries and 23 institutes

2nd June 2014, MESON 2014 - Krakow 5 / 16

SLIDE 6

COMPASS

Experimental setup

L a r g e A n g l e S p e c t r

m

e t e r ( L A S ) S m a l l A n g l e S p e c t r

m

e t e r ( S A S )

Beam π− @ 190 GeV/c Polarised target, NH3 dilution factor 22% polarisation up to 90% Large angular acceptance (±180 mrad) Two target cells (NH3) with opposite polarisations transverse to the beam

π

Nu

every few days Target spin reversal

N Nd

u

Nd 2nd June 2014, MESON 2014 - Krakow 6 / 16

SLIDE 7

COMPASS

Experimental setup - Hadron absorber and beam plug

A hadron absorber made of alumina will be placed downstream of the target to stop the hadrons and with a beam plug in the centre to stop the non-interacting beam. The hadron absorber will introduce multiple scattering

n muons and there will be a degradation of the
resolutions. To partially solve this problem a vertex

detector is introduced in the first part of the absorber. In parallel to the polarised DY measurements, unpolarised DY measurements will also be performed using nuclear targets, W and some thin lighter materials:

2nd June 2014, MESON 2014 - Krakow 7 / 16

SLIDE 8

COMPASS

TMDs (Non-)Universality – DY <> SIDIS

There is a theoretical prediction that Sivers (f ⊥

1T ) and Boer-Mulders (h⊥ 1 ) functions must change

sign when accessed from DY or SIDIS due to the fact that these functions are time-reversal odd functions. DY SIDIS

µ µ p

γ*

X h

f ⊥

1T (x, kT )|DY = −f ⊥ 1T (x, kT )|SIDIS

h⊥

1 (x, kT )|DY = −h⊥ 1 (x, kT )|SIDIS

The experimental confirmation of this sign change is considered a crucial test of the QCD TMD approach.

2nd June 2014, MESON 2014 - Krakow 8 / 16

SLIDE 9

COMPASS

Sivers asymmetry - COMPASS vs HERMES

x

−2

10

−1

10 p Siv

A

−0.05 0.05 0.1 z 0.5 1 −0.05 0.05 0.1 ) c (GeV/

h T

p 0.5 1 1.5 −0.05 0.05 0.1

p Siv

A

−0.05 0.05 0.1 0.5 1 −0.05 0.05 0.1 0.5 1 1.5 −0.05 0.05 0.1 COMPASS 2010 proton data

<0.032 x COMPASS positive hadrons >0.032 x COMPASS positive hadrons PRL 103 (2009)

+

π HERMES <0.032 x COMPASS negative hadrons >0.032 x COMPASS negative hadrons PRL 103 (2009)

π

HERMES

The Q2 coverage between the 2 experiments is different: COMPASS: x > 0.032, ❁Q2❃= 8.7 GeV/c2 (PLB 717 2012) HERMES: x > 0.032, ❁Q2❃= 2.4 GeV/c2 (PRL 103 2009) For h− the asymmetry is zero, for h+ the asymmetry is positive and slightly different between the two experiments, being the difference assigned to the Q2 coverage.

2nd June 2014, MESON 2014 - Krakow 9 / 16

SLIDE 10

COMPASS

Q2 vs x domain in COMPASS

In COMPASS we have the opportunity to access the TMD PDFs from both DY and SIDIS processes.

3

10

2

10

1

10 1 10

2

10 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

vs. x at COMPASS

2

Q Drell-Yan (MC) SIDIS (2010 proton data)

2

(GeV/c)

2

Q x

Sivers asymmetry from SIDIS - h+ and h− There is a phase space overlap between the two measurements. However to properly compare the extracted TMDs, their Q2 evolution must be taken into account. Recently the SIDIS analysis was performed in 4 Q2 bins, one of the bins being Q2 > 16 (GeV/c)2, the so-called DY range. δAsin(φh−φS )

UT

≈ 0.01 for both h+ and h− in SIDIS for Q2 > 16 (GeV/c)2, same statistical error as expected for Sivers from DY.

2nd June 2014, MESON 2014 - Krakow 10 / 16

SLIDE 11

COMPASS

Feasibility of the experiment

)

2

(GeV/c

µ µ

M

1 2 3 4 5 6 7 8

)

2

Events/(20 MeV/c

−1

10 1 10

2

10

3

10

4

10

5

10

109 ± = 6787 ψ # J/

2

0.004 GeV/c ± M = 3.042

2

0.003 GeV/c ± = 0.217

M

σ Total ψ J/ Continuum ’ ψ

preliminary

COMPASS DY 2009

z (cm)

250
200
150
100
50

50 100

Events/(1 cm)

100 200 300 400 500 600 700 2

>2.5 GeV/c

µ µ

M

COMPASS DY 2009

preliminary

In 2009 a 3 days data taking beam test was done using a hadron absorber prototype, two polyethylene target cells and a negative pion beam at 190 GeV/c with an intensity of 1.5 × 107π/s. A double trigger based on calorimeter signals was also used. The analysis confirmed the expectations. The J/ψ yields were confirmed considering the low efficiencies involved. The mass and the mass resolution were in agreement with the MC simulations. In the future the trigger will be based on hodoscopes with a high efficiency, purity and target pointing capability. The two target cells and the beam plug are distinguishable even if the absorber was not

ideal. For the future the Zvtx resolution will

be better because of the better absorber and of the inclusion of the vertex detector.

2nd June 2014, MESON 2014 - Krakow 11 / 16

SLIDE 12

COMPASS

2014/2015 Geometrical acceptance

The dimuons geometrical acceptance in the HMR (Mµµ > 4 GeV/c2) is 39%.

)

2

M (GeV/c

4 5 6 7 8 9 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

Dimuon mass acceptance

(GeV/c)

T

p

0.5 1 1.5 2 2.5 3 3.5 4 4.5 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

acceptance

T

Dimuon p

F

x

1
0.5

0.5 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

acceptance

F

x

CS

θ cos

1
0.5

0.5 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

acceptance

CS

θ cos

CS

φ

3
2
1

1 2 3 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

acceptance

CS

φ

S

φ

3
2
1

1 2 3 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

acceptance

S

φ

For the extraction of the asymmetries the differential acceptance must be taken into account and to be well known.

2nd June 2014, MESON 2014 - Krakow 12 / 16

SLIDE 13

COMPASS

Event rates and statistical accuracy

For a π− beam at 190 GeV/c, Ibeam = 108π/s and L = 2.3 × 1033 cm−2 s−1 the DY rate in the mass region 4 < Mµµ < 9 GeV/c2 is: 2000 events/day considering 9.6 s of beam spill and a 34 s SPS super cycle. 285000 events after one year of data taking (≈ 140 days) The expected statistical errors of the asymmetries, considering 285000 events, are: Asymmetry Statistical error (4 < Mµµ < 9 GeV/c2) δ Acos 2φ

UU

0.005 δ Asin φS

UT

0.013 δ Asin(2φ+φS )

UT

0.027 δ Asin(2φ−φS )

UT

0.027

2nd June 2014, MESON 2014 - Krakow 13 / 16

SLIDE 14

COMPASS

Asymmetries precision projections

2

−x

1

=x

F

x

−0.2 −0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 −0.05 0.05 0.1 0.15

S

φ sin UT

A

COMPASS Drell-Yan MC (140 days)

2

< 9 GeV/c

µ µ

4 < M

2

−x

1

=x

F

x

−0.2 −0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 −0.05 0.05 0.1 0.15

φ cos 2 UU

A

COMPASS Drell-Yan MC (140 days)

2

< 9 GeV/c

µ µ

4 < M

2

−x

1

=x

F

x

−0.2 −0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 −0.05 0.05 0.1 0.15

)

S

φ + φ sin (2 UT

A

COMPASS Drell-Yan MC (140 days)

2

< 9 GeV/c

µ µ

4 < M

2

−x

1

=x

F

x

−0.2 −0.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 −0.05 0.05 0.1 0.15

)

S

φ

φ

sin (2 UT

A

COMPASS Drell-Yan MC (140 days)

2

< 9 GeV/c

µ µ

4 < M

2nd June 2014, MESON 2014 - Krakow 14 / 16

SLIDE 15

COMPASS

Asymmetries theoretical predictions

Different theory predictions for the spin asymmetries in COMPASS are available. Two predictions for the Sivers asymmetry are shown:

F

x

−0.2 0.2 0.4 0.6 0.8 0.05 0.1 0.15 0.2

S

φ sin UT

A

< 2 GeV/c

T

COMPASS DY MC (140 days), p

P. Sun & F. Yuan, PRD 88, 114012

Echevarria et al, arXiv: 1401.5078 xF = xp − xπ

2nd June 2014, MESON 2014 - Krakow 15 / 16

SLIDE 16

COMPASS

Final remarks

The Pilot Drell-Yan run will start in mid-October 2014 for 2 months. This pilot run will be the opportunity to test and check everything before next year physics data taking. The Sivers function sign change is expected to be checked based on the COMPASS SIDIS and DY results. The nuclear targets will give the opportunity to perform some unpolarised DY studies such as the flavour dependence EMC effect. Dedicated J/ψ studies will be performed. The possibility to have a 2nd year of DY data taking after 2017 was proposed, but this still requires approval. We are looking forward to have the first ever DY polarised data.

2nd June 2014, MESON 2014 - Krakow 16 / 16

SLIDE 17

COMPASS

BACKUP SLIDES

2nd June 2014, MESON 2014 - Krakow 17

SLIDE 18

COMPASS

Update on expected Drell-Yan rates

The expected DY rate for the future is given by: R = L σπN dspill nspill εtot = 2.3 · 1033 × 2.136 · 10−34 × 9.6 × 2464 × 0.175 = = 2034/day being the Luminosity given by: L = Ibeam × Leff ρ × Ff × NA Amol A = 108 × 89.85 0.85 × 0.5 × 6.022 · 1023 17 17 = = 2.3 · 1033 cm−2 s−1 and the effective target length for a target of 55+55 cm of NH3: Leff = λint ρ Ff (1 − exp(−Lρ/λint)) = 89.85 cm

2nd June 2014, MESON 2014 - Krakow 18

SLIDE 19

COMPASS

Update on expected Drell-Yan rates (cont.)

The expected DY rate for the future is given by: R = L σπN dspill nspill εtot = 2.3 · 1033 × 2.136 · 10−34 × 9.6 × 2464 × 0.175 = = 2034/day The cross-section for pion nucleon is obtained taking into account the pion-proton and pion-neutron cross-sections from PYTHIA: σπN = 10 17 σπp + 7 17 σπn = 2.316 · 10−34cm2 The duration of the spill is: dspill = 9.6s The number of spills is: nspill = 23 × 60 × 60 33.6 = 2464 And the expected total efficiency is: εtot = Ω εrec εtrig εSPS εspec = 0.387 × 0.8 × (0.952 × 0.92) × 0.8 × 0.85 = = 0.175

2nd June 2014, MESON 2014 - Krakow 19

SLIDE 20

COMPASS

Flavour dependent EMC effect

The EMC effect corresponds to the modification of the quark distributions in nuclei. Several models try to explain this effect. Some of them considering a flavour dependence. For a nucleus with different number of protons and neutrons u and d quarks will have different nuclear effects. One way to study the flavour dependence is via the A dependence, where the ratios proton/neutron and so u/d are different. The existing data are not sufficiently accurate to draw any firm conclusion. PHYSICAL REVIEW C 83, 042201 (R) (2011)

2nd June 2014, MESON 2014 - Krakow 20

SLIDE 21

COMPASS

J/ψ studies

Measurement of the J/ψ cross-section Measurement of the J/ψ polarisation J/ψ production mechanisms: In case of duality DY ↔ J/ψ (q¯ q → γ∗/J/ψ → µ+µ−X): Possibility to extract the TMD PDFs with much larger statistics If gg production mechanism is dominating (gg → J/ψ → µ+µ−X): Possibility to extract the gluon Sivers TMD (related with gluons OAM)

2nd June 2014, MESON 2014 - Krakow 21