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

2. 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 !

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

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

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

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

7. 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

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

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

10. Data taking since 1996

11. Hermes kinematics inclusive: Q2 > 0.1 GeV2 : semi-inclusive:
0.002 < x < 0.9
with y< 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

12. 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

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

14. Structure function g1d(x)
Moments:
0.02<x< Q02=5 GeV2
Proton
± ±
Deuteron
± ±
0.03<x< Q02=5 GeV2
Hermes
E143 p ±
0.117 ± ± 0.006 d ±
0.043 ± ± 0.002

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

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

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

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

19. 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

20. 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)

21.
probability that g* hit a quark of flavour q when hadron of type h was detected

22. 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

23. Quark polarisations first 5-flavour extraction
[PRL92, , 2004; PRD71, , 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 ( ):

24. 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

25. 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

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

27. 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

28. 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

29. 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
H H conserve nucleon helicity
E E flip nucleon helicity ~
Quantum number of final state
selects different GPDs ~

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

31. 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

32. Experimental access to DVCS

33. Beam-Spin Asymmetries in DVCS
Proton
Deuteron

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

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

36. Experimental access to DVCS

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

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

39. 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

40. 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

41. 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

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

43. 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

44. Structure function g1(x)