Measurements of Transverse Spin Effects in SIDIS Andrea

Measurements of Transverse Spin Effects in SIDIS Andrea Bressan Torino 01/04/08

Outlook Introduction Asymmetries Transversity DF from Single Hadron from Two Hadrons from Hyperons Sivers DF Other TMD Near/Far Future Conclusions 10/28/2021 Andrea Bressan 2

What you will w/not find Review of all Deuteron Results w/not 2007 proton results They are on their way… ½ PB of data processed Data quality checks ongoing Late Spring 10/28/2021 Andrea Bressan 3

Spectrometer • longitudinally polarised muon beam • longitudinally or transversely polarised target • momentum and calorimetry measurements • particle identification SM 2 E/HCAL SM 1 Polarised Target m beam 10/28/2021 Muon. Wall Muon Wall RICH Beam: • Luminosity • intensity • momentum Andrea Bressan Sci. Fi Silicon Micromegas GEMs Straws SDC MWPC W 45 5. 1032 cm-2 s-1 2 108 µ+/spill (4. 8 s/16. 8 s) 160 Ge. V/c 4

Target solid state target operated in frozen spin mode 3 He – 4 He Dilution refrigerator (T~50 m. K) superconductive Solenoid (2. 5 T) Dipole (0. 5 T) 2002 -2004: 6 Li. D dilution factor f = 0. 38 polarization PT = 50% ~20% of the time transversely polarised two 60 cm long cells with opposite polarisation (systematics) d. N/dz 4000 2000 0 10/28/2021 -1000 0 zvtx (mm) 1000 Andrea Bressan during data taking with transverse polarization • dipole field always • polarization reversal in the 2 cells after ~ 5 days 5

Event Selection DIS cuts: Statistics 2002 - 2004: 8. 5 * 106 positive hadrons 7. 0 * 106 negative hadrons Q 2 > 1 (Ge. V/c)2 0. 1 < y < 0. 9 W > 5 Ge. V/c 2 All hadron selection: z > 0. 20 pt > 0. 1 Ge. V/c Plus for leading hadron: zl > 0. 25 No signals in the CALOs from neutral particles with z > zl 10/28/2021 Andrea Bressan 6

Hadron Identification Hadron identification is based on RICH response: several studies performed on the stability in time of the detector. Cherenkov thresholds: π ~ 2 Ge. V/c K ~ 9 Ge. V/c p ~ 17 Ge. V/c 2 σ π/K separation at 43 Ge. V/c In the leading hadron sample: ~76% pions ~12% kaons positive negative leading 3. 4 M 2. 8 M leading K 0. 7 M 0. 4 M 10/28/2021 Andrea Bressan 7

Kinematics – inclusive 10/28/2021 Andrea Bressan 8

Kinematics - SI 10/28/2021 Andrea Bressan 9

K 0 Reconstruction Z(V 0) – Z(prim. vtx. ) > 10 cm ● angle of target pointing < 0. 01 rad ● |M – MK 0| < 20 Me. V/c² ● pt, arm > 0. 025 Ge. V/c ● pt, K 0 > 0. 1 Ge. V/c K 0 target pointing 10/28/2021 Armenteros Andrea Bressan S/B ~ 15 10

TRANSVERSITY 10/28/2021 Andrea Bressan 11

Transversity DF q=uv, dv, qsea quark with spin parallel to the nucleon spin in a transversely polarised nucleon DTq(x) = q↑↑(x) - q↑↓(x) h 1 q(x), d. Tq(x) Properties: • probes the relativistic nature of quark dynamics • no contribution from the gluons simple Q 2 evolution • Positivity: Soffer bound……………. . Soffer, PRL 74 (1995) • first moments: tensor charge………. • sum rule for transverse spin in Parton Model framework………… Bakker, Leader, Trueman, PRD 70 (04) • it is related to GPD’s • is chiral-odd: decouples from inclusive DIS 10/28/2021 Andrea Bressan 12

How To the Transversity DF is chiral-odd: survives only by the product with another chiral-odd function can be measured in SIDIS on a transversely polarised target via “quark polarimetry” L N l’ h X Collins Asymmetry (Collins FF) L N l’ L X L polarization (FF of q L) L N l’ hh X Two hadrons asymmetry (Interference FF) 10/28/2021 Andrea Bressan 13

Single hadron asymms. Collins and Sivers terms in SIDIS cross sections depend on different combination of angles: C = h - s’ Collins angle S = h - s Sivers angle h azimuthal angle of the hadron s azimuthal angle of the transverse spin of the initial quark s’ azimuthal angle of the transverse spin of the fragmenting quark 10/28/2021 Andrea Bressan s’ = p - s (spin flip) 14

Collins Effect Collins effect: a quark with an upward (downward) polarization, perpendicular to the motion, prefers to emit the leading meson to the left (right) side with respect to the quark direction i. e. the fragmentation function of a transversely polarized quark has a spin dependent part And the resulting measured asymmetry 10/28/2021 Andrea Bressan 15

Unidentified Hadrons • only statistical errors shown (~1%), systematic errors considerably smaller • small asymmetries compatible with 0 for both + and – hadrons [NP B 765 (2007) 31 -70] 10/28/2021 Andrea Bressan 16

Identified Hadrons 10/28/2021 Andrea Bressan 17

Naïve Interpretation (parton model, valence region) • proton data unfavored Collins FF ~ – favored Collins FF at variance with unpol case u quark dominance (d quark DF ~ unconstrained) • deuteron data F. Bradamante 10/28/2021 some (small) effect expected even if cancellation between Tu (x) and Td (x) access to Td (x) Genova, 27 febbraio 2008 Andrea Bressan 18

Two Hadrons z-axis = virtual photon direction x-z plane = lepton scattering plane R= angle between lepton scattering plane and two-hadron plane S= azimuthal angle of initial quark versus lepton scattering plane S´= π - S (fragmenting quark) R (A. Bacchetta, M. Radici, hep-ph/0407345) (X. Artru, hep-ph/0207309) 10/28/2021 Andrea Bressan RS= R - S´ = R + S - π 19

Azimuthal Asymmetries z=z 1+z 2 Target single spin asymmetry ARS(x, z, Mh 2): and N±( RS): Number of events for target spin up (+) and down (-) f: Dilution factor ≈ 0. 38 D: Depolarisation factor D=(1 -y)/(1 -y+y 2/2) PT: Target polarisation ≈ 0. 5 Under measurement in e+e- (BELLE) expected to depend on the hadron pair invariant mass (X. Artru, hep-ph/0207309) 10/28/2021 Andrea Bressan 20

Selection 10/28/2021 DIS cuts: Hadron selection: • Q 2 > 1 Ge. V 2/c 2 • 0. 1 < y < 0. 9 • W > 5 Ge. V/c 2 • z 1, 2 > 0. 1 (current fragmentation) • x. F 1, 2 > 0. 1 • z 1+z 2 < 0. 9 (exclusive rho) • RICH identification of π, K Andrea Bressan 21

Two hadron asymm. s Expected Small 10/28/2021 (Radici/Bacchetta, PRD 74(2006)114007) Andrea Bressan 22

Identified 2 Hadrons 10/28/2021 Andrea Bressan 23

z-order two identified hadrons Motivations: Hadrons with higher relative energy carry more information about the fragmenting quark polarization For leading hadron pairs signal enhancement is predicted PID with 2003 -04 rich 10/28/2021 Andrea Bressan 24

z-order two identified hadrons 10/28/2021 Andrea Bressan 25

L Polarimetry 10/28/2021 Andrea Bressan 26

Results 10/28/2021 Andrea Bressan 27

SIVERS DF 10/28/2021 Andrea Bressan 28

SIVERS Mechanism The Sivers DF is probably the most famous between TMDs… gives a measure of the correlation between the transverse momentum and the transverse spin Requires final/initial state interactions of the struck quark with the spectator system and the interference between different helicity Fock states to survive time-reversal invariance Time-reversal invariance implies: …to be checked In SIDIS: 10/28/2021 Andrea Bressan 29

Unidentified hadrons • only statistical errors shown (~1%), systematic errors considerably smaller • small asymmetries compatible with 0 for both + and – hadrons [NP B 765 (2007) 31 -70] 10/28/2021 Andrea Bressan 30

Identified Hadrons 10/28/2021 Andrea Bressan 31

Naïve Interpretation • proton data asymmetry for + > 0, asymmetry for - ≈ 0 Sivers DF for d-quark ≈ - 2 Sivers DF for u-quark 2 • deuteron data preliminary 2002 -2004 the measured asymmetries compatible with zero suggest F. Bradamante 10/28/2021 Andrea Bressan 32

Small SIVERS on D: hint for small Lg the measured asymmetry on deuteron compatible with zero has been interpreted as Evidence for the Absence of Gluon Orbital Angular Momentum in the Nucleon S. J. Brodsky and S. Gardner, PLB 643 (2006) 22 10/28/2021 Andrea Bressan 33

Other TMD’s 10/28/2021 Andrea Bressan 34

Sidis Xsection From A. Bacchetta et al. , JHEP 0702: 093, 2007. e-Print: hep-ph/0611265 10/28/2021 Andrea Bressan 35

Results - LO g 1 T is the only parton DF which is chiral-even, T-even, leading twist function in addition to the unpolarised DF and to the helicity DF 10/28/2021 Andrea Bressan 36

Results - LO 10/28/2021 Andrea Bressan 37

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Transverse Target SSA for exclusive r (Q 2>1) Motivations: Hard exclusive meson production (HEMP) is a way, complementary to DVCS, to access GPDs Vector mesons Transverse Target Single Spin Asymmetry AUT( h, S) connected to GPD E Ji Sum Rule E allows flip of proton helicity, while quark helicity is not flipped → overall helicity is not conserved angular momentum conservation implies transfer of orbital angular momentum 10/28/2021 Andrea Bressan 40

Selection Besides standard DIS cuts: • Q 2 > 1 Ge. V 2/c 2 • 0. 1 < y < 0. 9 • W > 5 Ge. V/c 2 10/28/2021 Exclusive r selection: • only 3 outgoing particles m, +, • 0. 01 < p. T 2 < 0. 5 [(Ge. V/c)2] • missing energy -2. 5 Ge. V< Emiss<2. 5 Ge. V • Inv. Mass -0. 3 Me. V/c 2 < M – Mr < 0. 3 Me. V/c 2 Andrea Bressan 41

Results 10/28/2021 Andrea Bressan 42

2007 RUN 10/28/2021 Andrea Bressan 43

Target in 2007 solid state target operated in frozen spin mode 2007: NH 3 dilution factor f = 0. 14 polarization PT = 90% 2 3 cells Coupling hole Microwave power Beam line 10/28/2021 Cavity 1 Thermal screen Andrea Bressan Cavity 2 Cavity 3 44

Projected errors for full statistics (45 x 1012 m) 10/28/2021 Andrea Bressan 45

Summary precise deuteron data from COMPASS are now available Collins and Sivers asymmetries h±, p±, K± + other 6 TMDs [sin(3 h- S), sin(2 h- S), cos(2 h- S), cos( h)] Two hadron asymmetries pp, p. K, KK Transverse Lamba polarization all the measured deuteron asymmetries are very small, and compatible with zero COMPASS data on deuteron allows to determine the d-quark contribution present phenomenological studies can describe at the same time the BELLE (FF), the HERMES (proton) and COMPASS (deuteron)…and have allowed a first extraction of the transversity distribution DTq At the same way COMPASS and HERMES data have allowed a first extraction of the Sivers DF 10/28/2021 Andrea Bressan 46

Coming Soon q Deuteron q Cahn and Boer-Mulders asymmetries unpol q g 2 q Proton q Collins + Sivers… q Little further in time… all the rest Future programs q Drell-Yan [see … M. Chiosso talk] q Precision measurement … for better flavour separation and measurements of first moments… 10/28/2021 Andrea Bressan 47

THAT’S ALL… (FOR NOW!) 10/28/2021 Andrea Bressan 48