Spin structure of the nucleon in COMPASS
C. Quintans, LIP-Lisbon
th 19 December 2010, PASC Workshop COMPASS
Co-financed by:
Spin structure of the nucleon in COMPASS C. Quintans – p.1 Outline
The nucleon spin puzzle Longitudinally polarized DIS ? d A1, g1 and the Bjorken sum rule ? Flavour separation ? Gluon polarization Transversely polarized SIDIS TMD PDFs from polarized Drell-Yan GPDs from DVCS Conclusions
Spin structure of the nucleon in COMPASS C. Quintans – p.2 The COMPASS Experiment at CERN
Studies of the nucleon spin structure (2002 – 2007, 2010 – 2011) ? with polarized muon beam (µ+ at 160 GeV/c, ≈80% polarized) and polarized 6 targets ( LiD and NH3)
Studies of hadron spectroscopy (2008 – 2009) ? with h− beam at 190 GeV/c (95% π−, 4% K−, 1% p¯) and unpolarized targets (liquid H2, Ni, Cu, Pb, W) ? with h+ beam at 190 GeV/c (75% p, 24% π+, 1% K+) and unpolarized liquid H2 target
Future studies on the nucleon structure (2013 – 2015) ? GPDs from DVCS: with polarized muon beam (µ at 100-190 GeV/c and long st nd unpolarized liquid H2 (1 phase) or transversely polarized NH3 target (2 phase) ? TMD PDFs from polarized Drell-Yan: with hadron beam (95% π−) and a polarized NH3 target
Spin structure of the nucleon in COMPASS C. Quintans – p.3 COMPASS spectrometer and target
Polarized target
1m
3 He−Precooler
Superconducting Upstream Downstream Solenoid cell cell
NU ND µ−beam Acceptance + target spin (180mrad) µ beam @160 GeV reversal every 8h
-80% polarization Dilution refrigerator Targets N’ N’ U D 6 LiD or NH3 targets Spin structure of the nucleon in COMPASS C. Quintans – p.4 wn y
small poorl wn unkno kno
The nucleon spin puzzle
Nucleon spin: 1 = 1 ∆Σ + ∆ + 2 2 G LZ
quarks gluons orbital spin spin ang. mom.
In the quark-parton model, including relativistic corrections, expect ∆Σ ≈ 0.6 1988: EMC measured the quarks contribution to the spin of the nucleon to be very small ! Present world data: ∆Σ = 0.30 0.01(stat) 0.02(evol) 7→ COMPASS, PLB 647 (2007) 8-17
Spin structure of the nucleon in COMPASS C. Quintans – p.5 The nucleon spin puzzle
Nucleon spin: 1 = 1 ∆Σ + ∆ + 2 2 G LZ
wn y
quarsmallks gluonspoorl orbitalunkno spin spinwn ang. mom. kno In the quark-parton model, including relativistic corrections, expect ∆Σ ≈ 0.6 1988: EMC measured the quarks contribution to the spin of the nucleon to be very small ! Present world data: ∆Σ = 0.30 0.01(stat) 0.02(evol) 7→ COMPASS, PLB 647 (2007) 8-17
Spin structure of the nucleon in COMPASS C. Quintans – p.5 Spin asymmetries in DIS
For its deep inelastic scattering (DIS) programme, COMPASS uses beam and target polarized to measures double spin asymmetries. 2 2 (E,p) (E',p') Q = −q µ µ 0 q ν = E − E γ* 2 Q x = 2Mν E N X h z = ν
The µ-deuteron/proton asymmetry is measured from the difference between cross-sections from 2 oppositely polarized target cells:
⇒←− ⇐←− µN 1 N − N A = ⇒←− ⇐←− fPT PB N + N
This measured asymmetry relates to the longitudinal and transverse ∗ AµN γ -deuteron/proton asymmetries: D ≈ A1 Spin structure of the nucleon in COMPASS C. Quintans – p.6 Longitudinal polarization: inclusive asymmetries
deuteron (2002-2006 data) proton (2007 data) p p 1 0.7 1
EMC 0.02 0.12 d 1 A d 1 COMPASS A A 0.80.1 SMC 0.8 0.6 0 SMC A 0.08 E143 0.06 E143 0.5 E155 -0.02 0.04 E155 0.60.02 HERMES 0.6 0 CLAS W>2.5 -0.04 HERMES 0.4 -0.02 COMPASS 0.3 0.4 -2 0.4 -0.06 10 x 10-2 x 0.2 0.2 0.2 0.1
0 0 0 -0.1 -2 -1 10-2 10-1 x 1 10 10 x 1 COMPASS, PLB 647 (2007) 330-340. COMPASS, PLB 690 (2010) 466-472.
Significant improvement of precision in the low x region. Large asymmetry observed at high x, in agreement with other experiments.
Spin structure of the nucleon in COMPASS C. Quintans – p.7 Longitudinal spin-dependent g1 PDF
g1 is obtained from the asymmetry A1, for proton or deuteron:
F2 g1 = A1 2x(1 + R)
L T F2 → SMC parameterization; R = σ /σ → SLAC parameterization
0.08
0.04 d 1 p SMC 1 0.07 EMC E143 SMC xg xg 0.03 E155 0.06 E143 deuteron E155 proton HERMES 0.05 0.02 CLAS W>2.5 HERMES COMPASS 0.04 CLAS W>2.5 COMPASS 0.03 0.01
0.02
0 0.01
0 -0.01 -2 -1 -2 -1 10 10 x 1 10 10 x 1
st d From the 1 moment of g1 (x): 2 2 ∆Σ = a0 = 0.33 0.03(stat) 0.05(syst) at Q = 3 (GeV/c) 1 2 ∆s + ∆s¯ = 3 (a0 − a8) = −0.08 0.01 0.02 at Q → ∞
Spin structure of the nucleon in COMPASS C. Quintans – p.8 Bjorken sum rule
Combining deuteron and proton data, one can access the NS p n non-singlet spin structure function g1 = g1 − g1 .
NS According to the Bjorken sum rule the first moment of g1 is proportional to the ratio of axial and vector coupling constants gA/gV :
1 NS 2 1 gA NS 2 dx g1 (x, Q ) = | | C1 (Q ) Zxmin 6 gV
1 NS ∫ gNS(x)dx 92% of the first moment of g comes from the 1 1 xmin 0.2 Bjorken sum rule measured region 0.18 0.16 At the lowest accessible x in COMPASS, 0.14 1 NS 0.12 R0.004 dx g1 = 0.180 0.009 0.1 0.08 The value expected from the Bjorken sum rule is COMPASS data g 0.06 A 0.188, taking | g | from neutron β decay 0.04 V 0.02 Validity of the Bjorken sum rule confirmed with 5% sta- 0 10-2 10-1 1 tistical precision. xmin
Spin structure of the nucleon in COMPASS C. Quintans – p.9 Flavour separation
From a semi-inclusive DIS analysis, using charged kaons and pions identified in the RICH detector, with both deuteron and proton polarized targets, full quarks flavour decomposition (in LO) can be done:
0.4 x∆u 0.4 x∆d Using the DSS fragmentation functions 0.2 0.2 (D.Florian, Sassot, Stratmann, Phys.Rev. 0 0 D75 (2007) 114010) -0.2 -0.2
-2 -1 -2 -1 The curves are predictions from the ∆ 10 10 x 10 10 x 0.05 x u x∆d 0.05 DSSV fit calculated at NLO 0 0 (D.Florian, Sassot, Stratmann, -0.05 -0.05 Vogelsang, Phys.Rev.D 80 (2009) 10-2 10-1 -2 -1 0.02 x∆s x 10 10 x 034030) 0 -0.02 The flavour asymmetry of the sea helicity ¯ -0.04 distributions ∆u¯ − ∆d is slightly positive,
-2 -1 10 10 x compatible with zero. PLB 693 (2010) 227-235
Spin structure of the nucleon in COMPASS C. Quintans – p.10 µ µ
γ∗ q High pT hadron pairs g q ? Large statistics available N ? Very dependent on Monte-Carlo
Gluon polarization
The direct measurement of ∆G is of crucial importance for the understanding of the spin puzzle. ,→ Access it via the photon-gluon fusion (PGF) process. PGF events are selected by analysing: Open Charm production µ µ ? Very clean: almost no physics γ∗ c background ? Low statistics g c N
Spin structure of the nucleon in COMPASS C. Quintans – p.11 µ µ
γ∗ c g c N
Gluon polarization
The direct measurement of ∆G is of crucial importance for the understanding of the spin puzzle. ,→ Access it via the photon-gluon fusion (PGF) process. PGF events are selected by analysing: Open Charm production ? Very clean: almost no physics µ µ background ? Low statistics γ∗ q High pT hadron pairs g q ? Large statistics available N ? Very dependent on Monte-Carlo
Spin structure of the nucleon in COMPASS C. Quintans – p.11 ∆G from open-charm
5 channels with Do production are studied (data 2002-2007): Do → Kπ (untagged) D∗ tagged : Do → Kπ, Kππo, Kπππ, sub(K)π
0 0 COMPASS 2007 DKπ (D* tagged) COMPASS 2007 DKππ0 (D* tagged)
0 ) 1200 ) 2 2 COMPASS 2007 DKπ
) 0 2 N(D ) = 3100 Preliminary Preliminary 800 1000 ππ0 40000 Preliminary K N/10 (MeV/c N/10 (MeV/c N/5 (MeV/c 800 600 0 30000 N(D ) = 4600
600 400 20000 N(D0) = 19000 400
200 10000 200
0 0 0 -400 -300 -200 -100 0 100 200 300 400 -600 -400 -200 0 200 400 600 -600 -400 -200 0 200 400 600 2 2 2 M - M 0 (MeV/c ) M - M 0 (MeV/c ) M - M 0 (MeV/c ) Kπ D Kπ D Kπ D
0 COMPASS 2007 Dsub(K)π (D* tagged)
) 0 2 COMPASS 2007 DKπππ (D* tagged) ) Preliminary 2 400 Preliminary 1000 N/10 (MeV/c N(D0) = 1100 N/10 (MeV/c sub-threshold Kaons 300 S ∆G 0 Araw = PtPbfaLL + 500 N(D ) = 1000 200 S B G
100
0 0 -400 -300 -200 -100 0 100 200 300 400 -400 -300 -200 -100 0 100 200 300 400 2 2 M - M 0 (MeV/c ) M - M 0 (MeV/c ) Kπ D Kπ D
Weighted events: all factors between Araw and ∆G/G enter in the weight. The method implies simultaneous extraction of the background asymmetry: 1 Abkg = B Araw PtPbfD S+B
Spin structure of the nucleon in COMPASS C. Quintans – p.12 ∆G from high pT hadron pairs
In LO and Q2 > 1 (GeV/c)2, besides PGF 2 background processes must be considered: leading processes (LP) and QCD-Compton (QCDC). Generically, the higher
the hadrons pT , the larger is the PGF fraction expected.
µ µ µ µ µ µ g γ∗ q γ∗ γ∗ + + q g q q
xg xQCDC xBj PGF QCD−C LP
The fractions R and the analysing powers aLL of each process are calculated from Monte-Carlo. They are parameterized on an event basis by a Neural Network trained on MC.
Data 2002-2006 were analysed
Analysis in 3 bins of xg
The present analysis requires leading hadron with pT > 0.7 and sub-leading with pT > 0.4 GeV/c, and uses fractions R and aLL to weight the events.
Spin structure of the nucleon in COMPASS C. Quintans – p.13 Results on ∆G/G
Open-charm analysis: ∆G/G = −0.08 0.21(stat) 0.03(syst) 2 2 at hxgi = 0.11 and hµ i = 13 GeV .
High pT analysis: ∆G/G = 0.125 0.060(stat) 0.063(syst) 2 2 2 at hxgi = 0.094 and hµ i = (hQ i) = 3.4 GeV .
New COMPASS, high p , Q2>1 (GeV/c)2, prel., 02-06 T COMPASS, high p , Q2<1 (GeV/c)2, prel., 02-04 T COMPASS, open charm, prel., 02-07 0.6 SMC, high p , Q2>1 (GeV/c)2 T HERMES, high p , all Q2 T 2 2 g/g COMPASS global fit with ∆G>0, µ =3(GeV/c) 0.4 2 2 ∆ COMPASS global fit with ∆G<0, µ =3(GeV/c)
0.2
0
-0.2
-0.4 COMPASS Preliminary
-0.6 -2 -1 10 10 xg Spin structure of the nucleon in COMPASS C. Quintans – p.14 Transverse momentum dependent PDFs
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: quark
nucleon NUCLEON unpolarized longitudinally pol. transversely pol.
f1 f1T
kT number density Sivers unpolarized
g1L g1T kT
helicity QUARK longitudinally pol.
h1 h1 transversity kT k kT T 1T Boer−Mulders h1L h pretzelosity transversely pol.
Spin structure of the nucleon in COMPASS C. Quintans – p.15 Interpreting the PDFs
⊥ 2 Sivers: the f1T (x, kT ) function describes the distortion of the probability distribution of a non-polarized quark when inside a transversely polarized nucleon.
⊥ 2 Boer-Mulders: the h1 (x, kT ) function describes the correlation between the transverse spin and the transverse momentum of a quark in an unpolarized hadron.
2 2 Transversity: results from the integration over kT of h1(x, kT ).
The last 2 are chiral-odd – as a consequence, they can only be observed convoluted with another chiral-odd object. ,→ in SIDIS, the PDF is convoluted with a fragmentation function.
Spin structure of the nucleon in COMPASS C. Quintans – p.16 Transverse Polarization
The transverse momentum dependent (TMD) PDFs of the nucleon carry important information about the nucleon spin dynamics, that can be revealed from SIDIS in a transversely polarized target.
=⇒ 8 azimuthal asymmetries that can be measured at once.
φh: azimuthal angle of the outgoing hadron momentum
φS : azimuthal angle of the initial quark spin
Spin structure of the nucleon in COMPASS C. Quintans – p.17 Transversity PDF
lN ↑ → l0hX Collins effect: a quark moving "horizontally" and polarized "upwards" emits the leading meson preferentially on the "left" side of the jet (Collins FF).
The Collins asymmetry is proportional to the convolution of the transversity PDF h1(x) with the Collins FF.
Collins angle: φC = φh + φS − π
COMPASS 2007 proton data Identified hadron Collins asymmetries p Coll
A 0.1 0.1 0.1 in agreement with theory predictions.
0 0 0 Agreement with previous −0.1 −0.1 −0.1 measurements from HERMES and π −0.2 + −0.2 −0.2 preliminary BELLE – both transversity and Collins 10−2 10−1 0.5 1 0.5 1 1.5 p Coll x A 0.1 0.1 0.1 FF are non-zero!
0 0 0 Same asymmetries measured in COM- −0.1 −0.1 −0.1 COMPASS 2007 proton data PASS with deuteron target are zero – in M.Anselmino et al. π Nucl.Phys.Proc.Suppl.191,2009. −0.2 - −0.2 −0.2 deuteron there is cancellation between u 10−2 10−1 0.5 1 0.5 1 1.5 x p h (GeV/c) z T and d quark contributions.
Spin structure of the nucleon in COMPASS C. Quintans – p.18 Sivers PDF
Another azimuthal modulation that can be extracted from SIDIS is the Sivers effect, measured to be non-zero by HERMES
Sivers angle: φSiv = φh − φS COMPASS 2007 proton data positive hadrons p Siv 0.1 0.1 negative hadrons 0.1 preliminary A
0 0 0
−0.1 −0.1 −0.1
10−2 10−1 0.5 1 0.5 1 1.5 p h (GeV/c) x z T
Non-zero Sivers asymmetry for h+, measured on proton target. Sivers asymmetry measured on deuteron is compatible with zero. HERMES measures larger asymmetries – COMPASS result still to be understood. 2010 data on proton target to be analysed soon.
Spin structure of the nucleon in COMPASS C. Quintans – p.19 Accessing PDFs
TMD PDFs, like Sivers, can be accessed not only from SIDIS but also from the Drell-Yan process (DY).
SIDIS DY µ µ
µ+ γ* q γ*
q µ−
p X
The spin asymmetry is given by the The spin asymmetry is proportional to a convolution of structure function with product of structure functions, one for beam fragmentation function. and one for target.
Because Sivers is T-odd, it is expected that: ⊥ ⊥ f1T (DY ) = −f1T (SIDIS)
The experimental check of this relation is a crucial test of QCD. The same is true for Boer-Mulders.
Spin structure of the nucleon in COMPASS C. Quintans – p.20 The future: COMPASS-II
The COMPASS programme is expected to be completed in 2011, a year for data-taking with muon beam on longitudinally polarized proton target.
A new proposal for further spin studies in COMPASS was delivered to the SPS committee in May 2010 (CERN-SPSC-2010-014).
The proposal was recommended for approval in September 2010.
Spin structure of the nucleon in COMPASS C. Quintans – p.21 Drell-Yan @ COMPASS
COMPASS has the unique opportunity to study, with the same spectrometer, TMD PDFs both from SIDIS and from polarized DY processes, thanks to a multipurpose spectrometer: Availability of both muon and pion beams
Unique polarized NH3 target, well suited for transversity studies Wide angular acceptance and a muon detection system Measurement requires large hadron absorber downstream of the target, and a dimuon trigger.
π−p↑ → µ+µ−X
The COMPASS phase-space covers the proton quarks valence region (xp > 0.1), for 2 high mass Drell-Yan events (4. < Mµµ < 9. GeV/c ).
COMPASS has sensitivity to measure Sivers, Boer-Mulders, transversity and pretzelosity PDFs from Drell-Yan.
Spin structure of the nucleon in COMPASS C. Quintans – p.22 TMD PDFs from DY@COMPASS
DY 4. − 9. GeV/c2 ,-./φ 23,/( + φ 1 14 #$(* 0 #$(*
#$( #$( Boer-
#$)* #$)* Mulders Sivers #$) #$) B. Zhang et al, #$#* #$#* M. Anselmino et al, Phys.Rev.D77 # #
PRD79 (2009) 054010 #$#* #$#* (2008)054011
#$) #$) #$% #$& #$' #$( # #$( #$' #$& #$% #$% #$& #$' #$( # #$( #$' #$& #$%
"! π ↑ "! π ↑
,-./7(φ 6/φ 5 ,-./7(φ /φ 5 1 + 1 + #$(* 0 #$(* 0
#$( #$( BM ⊗ #$)* #$)*
BM ⊗ pretzelosity #$) #$) transversity
A.N.Sissakian et al, #$#* #$#* A.Efremov et al, Phys.Par t.Nucl.41: # # in preparation #$#* #$#* 64-100,2010 #$) #$) #$% #$& #$' #$( # #$( #$' #$& #$% #$% #$& #$' #$( # #$( #$' #$& #$% ! π ↑ " "! π ↑
Spin structure of the nucleon in COMPASS C. Quintans – p.23 TMDs and GPDs
TMD PDFs: adding information about intrinsic transverse momentum dependence or GPDs: adding information about the transverse distance of the constituent quark are 2 "complementary" ways to improve our knowledge about the nucleon structure.
They are related via Fourier transform. They both allow to access information about the quarks orbital angular momentum – although their interpretation is difficult.
4 GPDs: H, E, H˜ and E˜ for each quark flavour and gluons. They depend on 3 variables: x, ξ = xB and t. 2−xB GPDs have no direct probabilistic interpretation.
Spin structure of the nucleon in COMPASS C. Quintans – p.24 DVCS with unpolarized target
Using µ+↓ and µ−↑ beams on a long (2.5m) unpolarized liquid H2 target, one can do transverse imaging (tomography) of the nucleon. ,→Access the GPD H from Deeply Virtual Compton Scattering (DVCS).
µp → µ0pγ (µ0pρ)
dσDV CS/dt ≈ exp(−Bt)
B ≈ hr2i/2
Measurement requires a large recoil proton detector, and large coverage electromagnetic calorimetry. Main priority is DVCS. Meson production will be studied in parallel. 2 competing processes: DVCS and Bethe-Heitler.
Low xB : BH; High xB : DVCS; intermediate xB : interference DVCS-BH. BH is well-known: used as reference process.
Spin structure of the nucleon in COMPASS C. Quintans – p.25 DVCS in COMPASS
In a 2nd phase of the measurement: use a transversely polarized target (NH3), inside a large Recoil Proton Detector. ,→ Access GPD E: from measured azimuthal asymmetries
Phase 2
) 8 -2
6 Phase 1 B (GeV α' = 0.125 4 α' = 0.26 ZEUS < Q2 > = 3.2 GeV2 H1-HERA I < Q2 > = 4 GeV2 2 H1-HERA II < Q2 > = 8 GeV2
COMPASS < Q2 > = 2 GeV2 0 280 days at 160 GeV
-2
-4 -3 -2 -1 10 10 10 10 Spin structure of the nucleon in COMPASS C. Quintans – p.26 Present conclusions
COMPASS takes data since 2002, for studies both on the nucleon spin structure and on hadron spectroscopy.
COMPASS results on spin physics give an important contribution to our understanding:
The most precise data on A1 at the low xBj region, with impact in the d p determination of g1 and g1 . A direct measurement of the gluon polarization, with a result compatible with zero, within errors, in the measured xg range. Collins and Sivers asymmetries on deuteron target compatible with zero – understood now to be due to cancellation between u and d quark contributions. Sizable Collins asymmetry on proton target – access to the transversity PDF. Positive Sivers asymmetry for h+ on proton target, but difficult systematics – still to be understood.
=⇒ Data from 2010 (transverse pol.) and 2011 (longitudinal pol.) still to be analysed!
Spin structure of the nucleon in COMPASS C. Quintans – p.27 COMPASS future
A proposal for new spin studies in COMPASS was recommended for approval by the CERN Scientific Committee in September 2010:
Study of TMD PDFs from polarized Drell-Yan – It can be the first ever polarized Drell-Yan experiment.
Drell-Yan COMPASS phase-space (in the proton’s valence quarks region) seems to be very favorable to access TMDs, according to theory predictions.
Study of GPDs from DVCS – the COMPASS measurement will be in an intermediate x range, not covered by any other experiment in the near future.
DVCS process using unpolarized proton target in a 1st phase, to constrain GPD H, and do transverse imaging of the nucleon. Use of a transversely polarized proton target in a 2nd phase, to constrain GPD E.
Thank you!
Spin structure of the nucleon in COMPASS C. Quintans – p.28