Marco Contalbrigo, 27 September 2005 The HERMES experiment Marco Contalbrigo, 27 September 2005

Search the carriers of proton spin Three possible sources: quarks: valence quarks sea quarks gluons orbital momentum ½ = ½ Sq + Lq + Jg ~ 30% ? EMC (85): q ~ 10% HERMES: reliable multi-purpose detector system !

The HERMES experiment 27.6 GeV e+/e- 0.002 < x < 0.9 0.1 < Q2 < 15 GeV2 p/p ~ 1-2%  < 0.6 mrad Lepton ID with e~98% Hadron contamination <0.5%

Particle identification Dual radiator RICH: C4F10 Aerogel P p K Detection efficiencies 24.08.2004

The HERMES experiment e+ BEAM ~1.5m EM Calorimeter TRD RICH Magnet Target area e+ BEAM ~1.5m

Polarized Hera e-beam 27.5 GeV (e+/e-) self-polarised electrons: e <Pb>~ 53±2.5 %

HERMES target e- p ABS S Atomic Beam Source Pz+ = |1> + |4> Pz- = |2> + |3> mj = +1/2 mj = -1/2 mi=-1/2 mi=+1/2 1 |1> |2> |3> |4> e- p Hyperfine states Stern-Gerlach separation

HERMES target Target Cell Cell x 100 areal density (vs jet target) Pure nuclear-polarised atomic gas Flipped at 90s time intervals

HERMES target Diagnostic system TGA BRP Dz Dzz PT = 0.845 ± 0.028 Longitudinal Field (B=335 mT) Diagnostic system

Data taking since 1996 1996-2000 2002 - 2005

Hermes kinematics inclusive: Q2 > 0.1 GeV2 : semi-inclusive: 0.002 < x < 0.9 with y<0.91 W2>3.24 GeV2 semi-inclusive: Q2 > 1.0 GeV2 : 0.02 < x < 0.6 with y<0.85, W2>10 GeV2 0.2<z<0.8

Model-independent unfolding detector smearing QED radiative effects smearing within acceptance radiative effects detector smearing kinematic migration inside acceptance for each spin state j=0 bin: kinematic migration into the acceptance systematic correlations between bins fully unfolded resulting (small) statistical correlations known

g1/F1 of the proton and deuteron Q2 > 0.1 GeV2 0.002 < x < 0.9

Structure function g1d(x) Moments: 0.02<x<0.9 Q02=5 GeV2 Proton 0.1246 ± 0.0032 ± 0.0074 Deuteron 0.0452 ± 0.0015 ± 0.0017 0.03<x<0.8 Q02=5 GeV2 Hermes E143 p 0.1174 ± 0.0027 0.117 ± 0.003 ± 0.006 d 0.0433 ± 0.0013 0.043 ± 0.003 ± 0.002

Structure function b1d(x) Quark densities: Structure functions:

Structure function b1d(x) [hep-ex/0506018]

Q 2 dependence of F2(x) and g1(x)

Polarized parton density from inclusive data Next-to-Leading-Order analysis of g1(x)-data

Polarized parton density from inclusive data Duv and Ddv well determined: Duv >0, Ddv <0 Dq and DG weakly constraint by inclusive data Ds <0 could generate violation of Ellis-Jaffe SR more, direct probes needed

Polarized Parton Distribution Functions ! Flavour decomposition of spin Key issue: role of sea quarks in nucleon spin In semi-inclusive deep inelastic scattering the hadron tags the flavour of struck quark Derive purity of tag from unpolarized data (fragmentation functions) Hadron asymmetries (measured) Polarized Parton Distribution Functions ! Known quantities (from other data)

Purities @Hermes probability that g* hit a quark of flavour q when hadron of type h was detected

Measured asymmetries Just like in short dq paper: pi,K for d all sea objects Just like in short dq paper: pi,K for d Consistent with but more precise than the undifferentiated SMC hadron asymmetries

Quark polarisations first 5-flavour extraction [PRL92, 012005, 2004; PRD71, 012003, 2005] Quark polarisations first 5-flavour extraction no significant sea quark polarisation  results perfectly consistent with inclusive fits: c2/dof =0.6…1.6 for BB(SU3-sym) and GRSV-valence in measured x-range (0.023-0.6):

Isoscalar method and Ds Polarization of strange quarks in the proton could explain small DS Alternative Ds extraction Two isoscalar component analysis: 2) Only isoscalar deuteron data: inclusive and charged kaon yields 3) Two component purity matrix

Extraction of kaon FF K++ K- multiplicities From high statistic polarised and unpolarised deuteron data set: K++ K- multiplicities Fragmentation Functions for Purities: without resort to other experiments does not require MC fragmentation model

Isoscalar method and Ds Not-strange Strange Inclusive asymmetry Charged Kaon asymmetry 5parameters and isoscalar methods in agreement both disfavour significative negative Ds

Light sea flavour asymmetry Unpolarized data: Strong breaking of flavour symmetry no significant breaking of polarised SU(2)-flavour symmetry cQSM asymmetric sea disfavoured: c2/dof=17.6/7 Polarized data

Outlook: FF from multiplicities from high statistic polarised and unpolarised proton data set: p+ ,p- , K+, K- multiplicities p+ K+ extraction of Dq with Hermes set of fragmentation functions

Hunting for Lq Factorisation theorem for hard exclusive processes: Generalised Parton Distributions - Műller (1994) - - Ji & Radyushkin (1996) - N g*L t N’ M F Q2>>, t<< H,E,H,E ~ ~ 10-30%(DIS) ( H + E) x dx = Jq = 1/2 DS + Lz 4 GPDs @twist-2: H H conserve nucleon helicity E E flip nucleon helicity ~ Quantum number of final state selects different GPDs ~

Experimental access to GPDs Exclusive meson electroproduction: Vector mesons (0): Pseudoscalar mesons (): Deeply virtual Compton scattering (DVCS): DVCS Bethe-Heitler

Experimental access to DVCS Dirac and Pauli Form Factors calculabe in QED F1, F2 Compton Form Factors (convolution of GDPs and hard scattering amplitude) GPDs amplitudes from interference term At HERMES kinematics tBH »tDVCS

Experimental access to DVCS

Beam-Spin Asymmetries in DVCS Proton Deuteron

Beam-Charge Asymmetries in DVCS t-dependence of BCA can be used to constrain GPD models Proton

Beam-Charge Asymmetries in DVCS Proton Limited by e-p sample (L ~10 pb-1) HERMES is running with e- beam in 2005

Experimental access to DVCS

Transverse Target-Spin Asymmetries in DVCS Sensitive to Ju More data are coming (about same statistics in 2005)

Allows recoil proton detection Recoil detector Dedicated to exclusive processes Allows recoil proton detection background free DVCS

Summary HERMES results on nucleon spin: Precise determination of g1p,g1d,g1n First measurement of b1d First separation of polarized quark flavour Quark sea → unpolarized First exploration of GPDs Asymmetries in exclusive reactions Alternative, refined extractions New polarized data Recoil detector

Summary HERMES results on nucleon spin: Precise determination of g1p,g1d,g1n First measurement of b1d First separation of polarized quark flavour Quark sea → unpolarized First indication of gluon polarization Gluons → polarized // proton First exploration of GPDs Asymmetries in exclusive reactions Alternative, refined extractions Deuterium data New polarized data Recoil detector

How to probe the quark polarization? Polarized deep inelastic electron scattering Parallel electron & quark spins Anti-parallel electron & quark spins Measure yield asymmetry: In the Quark-Parton Model: Spin-dependent Structure Function

Gluon polarization Photoproduction of high pT –hadron pairs Contributing diagrams: Corresponding asymmetries:

Data and plans for G/G Outlook: DG/G from polarized Deuterium data Asymmetry for high-pT hadron pairs production: ±0.18±0.03 Extraction strongly model dependent GeV2 Outlook: DG/G from polarized Deuterium data

Structure function g1(x)