The HERMES experiment 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 q + L q + Jg ~ 30% ? EMC (85): q ~ 10% ? HERMES: reliable multi-purpose detector system !

The HERMES experiment 27. 6 Ge. V e+/e- 0. 002 < x < 0. 9 p/p ~ 1 -2% 0. 1 < Q 2 < 15 Ge. V 2 < 0. 6 mrad Lepton ID with e~98% Hadron contamination <0. 5%

Particle identification • Dual radiator RICH: Aerogel Detection efficiencies K P C 4 F 10 24. 08. 2004 K

The HERMES experiment EM Calorimeter Magnet Target area e+ BEAM ~1. 5 m RICH TRD

Polarized Hera e-beam 27. 5 Ge. V (e+/e-) self-polarised electrons: e <Pb>~ 53± 2. 5 %

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

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

HERMES target Diagnostic system Dz Longitudinal Field (B=335 m. T) PT = 0. 845 ± 0. 028 Dzz TGA BRP HERMES

Data taking since 1996 -2000 2002 - 2005

Hermes kinematics inclusive: Q 2 > 0. 1 Ge. V 2 : 0. 002 < x < 0. 9 with y<0. 91 W 2>3. 24 Ge. V 2 semi-inclusive: Q 2 > 1. 0 Ge. V 2 : 0. 02 < x < 0. 6 with y<0. 85, W 2>10 Ge. V 2 0. 2<z<0. 8

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

g 1/F 1 of the proton and deuteron proton Q 2 > 0. 1 Ge. V 2 0. 002 < x < 0. 9

Structure function g 1 d(x) Moments: 0. 02<x<0. 9 Proton Q 02=5 Ge. V 2 0. 1246 ± 0. 0032 ± 0. 0074 Deuteron 0. 0452 ± 0. 0015 ± 0. 0017 0. 03<x<0. 8 Hermes Q 02=5 Ge. V 2 E 143 p 0. 1174 ± 0. 0027 0. 117 ± 0. 003 ± 0. 006 d 0. 0433 ± 0. 0013 0. 043 ± 0. 002

Structure function b 1 d(x) Quark densities: Structure functions:

Structure function b 1 d(x) [hep-ex/0506018]

Q 2 dependence of F 2(x) and g 1(x)

Polarized parton density from inclusive data Next-to-Leading-Order analysis of g 1(x)-data

Polarized parton density from inclusive data • uv and dv well determined: uv >0, dv <0 • q and G weakly constraint by inclusive data • s <0 could generate violation of Ellis-Jaffe SR more, direct probes needed

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 Hadron asymmetries (measured) Derive purity of tag from unpolarized data (fragmentation functions) 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 Consistent with but more precise than the undifferentiated SMC hadron asymmetries all sea objects

[PRL 92, 012005, 2004; PRD 71, 012003, 2005] Quark polarisations first 5 -flavour extraction • no significant sea quark polarisation results perfectly consistent with inclusive fits: c 2/dof =0. 6… 1. 6 for BB(SU 3 -sym) and GRSV-valence • in measured x-range (0. 023 -0. 6):

Isoscalar method and s Polarization of strange quarks in the proton could explain small Alternative s extraction 1) Two isoscalar component analysis: 2) Only isoscalar deuteron data: inclusive and charged kaon yields 3) Two component purity matrix

Extraction of kaon FF 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 s Not-strange Inclusive asymmetry Strange Charged Kaon asymmetry 5 parameters and isoscalar methods in agreement both disfavour significative negative s

Light sea flavour asymmetry Unpolarized data: Polarized data • no significant breaking of polarised SU(2)-flavour symmetry • c. QSM asymmetric sea disfavoured: c 2/dof=17. 6/7 Strong breaking of flavour symmetry

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

Hunting for Lq Factorisation theorem for hard exclusive processes: - Műller (1994) Generalised Parton Distributions - Ji & Radyushkin (1996) - g*L M Q 2>>, t<< F ( H + E) x dx = Jq = 1/2 DS + Lz ~~ H, E, H, E N N’ 10 -30%(DIS) t 4 GPDs @twist-2: H E ~ H ~ E conserve nucleon helicity 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 GPDs amplitudes from interference term At HERMES kinematics t. BH » t. DVCS Compton Form Factors (convolution of GDPs and hard scattering amplitude) F 1, F 2 Dirac and Pauli Form Factors calculabe in QED

Experimental access to DVCS

Beam-Spin Asymmetries in DVCS Proton Deuteron

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

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)

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

Summary • HERMES results on nucleon spin: – Precise determination of g 1 p, g 1 d, g 1 n – First measurement of b 1 d – First separation of polarized quark flavour Quark sea → unpolarized Alternative, refined extractions – First exploration of GPDs Asymmetries in exclusive reactions New polarized data Recoil detector

Summary • HERMES results on nucleon spin: – Precise determination of g 1 p, g 1 d, g 1 n – First measurement of b 1 d – First separation of polarized quark flavour Quark sea → unpolarized Alternative, refined extractions – First indication of gluon polarization Gluons → polarized // proton Deuterium data – First exploration of GPDs Asymmetries in exclusive reactions 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 p. T –hadron pairs • Contributing diagrams: • Corresponding asymmetries:

Data and plans for G/G • Asymmetry for high-p. T hadron pairs production: ± 0. 18± 0. 03 Extraction strongly model dependent Ge. V 2 Outlook: DG/G from polarized Deuterium data

Structure function g 1(x)