Status report of Hermes

Transcription

1 Status report of Hermes Delia Hasch Physics Research Committee, DESY Oct 27/ Spin physics: finalised and new results on: inclusive, semi-inclusive and exclusive measurements nuclear effects

2 data taking 2004 very successful data taking with transversely polarised H target 2004 transversity and friends (see PRC56,57 reports) high statistic data sets on unpolarised D, Kr, Xe from end-of-fill runs nuclear effects exclusive processes

3 polarised structure function g 1 (x) final data set on g 1 (x) reanalised g 1 p finalised g 1 d new, more rigorous bin to bin unfolding procedure applied: accounts for radiative+acceptance uncorrelates systematic uncertainties (small) statistical correlation known world data: Q 2 < 100 GeV < x < 0.9 for Q 2 >1 GeV 2

4 quark polarisations from sidis first 5-flavour fit to q flavour separation by flavour tagging A h 1 (x) q e q 2 q e 2 q q (x) q (x) dz dz D D h q h q (z) (z) Purities unpolarised pdfs and fragmentation funtions PRC58

5 outlook: FF from multiplicities from high statistic polarised and unpolarised proton data set: π +,π, K +, K multiplicities π + K + extraction of q with Hermes set of fragmentation functions towards NLO analysis of q PRC58

6 meson production: γ * L N Q 2 >>, t<< ( H + E) x dx = Φ J q L t Μ = 1/2 Σ + L z H,E,H,E ~ ~ N asymptotically 1 Q σ 6 for fixed x B and t hunting for L q factorisation theorem for hard exclusive processes: - Műller Generalised Parton Distributions 10-30%(DIS) 4 H H conserve ~ quantum number of final state selects different GPDs: vector mesons DVCS ller (1994) - - Ji & Radyushkin (1996) - pseudoscalar mesons ~ ~ E E flip nucleon helicity

7 cross section for ep e π + n comparison with GPD based model: exclusivity ensured by missing mass technique -Vanderhaeghen, Guichon & Guidal (1999) - σ = σ + tot L ε σ L/T separation not possible but: suppressed by 1/Q 2 Hermes kinematics: 0.80 < ε < 0.96 σ T T at large Q 2 σ L dominates Q2 dependence in general agreement with theoretical expectation power correction (k and soft overlap) calculation overestimate data

8 outlook: transverse target-spin asymmetry target spin asymmetry + γ L + p π + n pion pole contribution quadratic combination of GPDs appears in unpolarised cross section ~ ~ TSA linear dependence on E and H, E and H σ S sin( φ φ ) E ~ H ~ s T S TARGET SPIN ASYMMETRY 0.05 γ γ L * ρl 0 + p0 L + p ρ J u = Q 2 = 2.5 GeV 2 -t = 0.25 GeV 2 J d = particulary sensitive to E PRC58 p x x B

9 exclusive π + π production e p (d) N e' π + p (d) interference between θ S-wave and lower ρ 0 tail m ππ < 0.6 GeV π GPD model predictions: minimum interference π between w/o 2-gluon S-P waves exchange mwith ππ 2-gluon 0.77 GeVexchange + π - sensitive to interference between different π + π isospin states Legendre moments: P (cosθ ) l π+π indication of ρ 0 f 2 interference exchange Odderon m ππ 1.3 x: sensitive to 3-gluon

10 Deeply Virtual Compton Scattering e p e' p' γ kinematics: DVCS << Bethe-Heitler dσ 2 2 * τbh + τdvcs + (τbhτdvcs + τ * DVCS τ BH ) A A LU + / C DVCS-BH interference leads to non-zero azimuthal asymmetry ( φ) Im( τ ( φ) BH τ DVCS Re ( τ BH τ DVCS ) ) PRC58

11 latest news: BCA vs t GPD calculations: - Vanderhaeghen et al., (1999,2001) - t-dependence of BCA: high sensitivity to model assumptions different parametrisations for H: t-dependence factorised Regge-motivated ansatz with and w/o D-term tiny e psample (L~10pb -1 ) BUT HERA: >2004 e

12 what happens in a nuclear medium? reduction of multiplicity of fast hadrons due to both hard partonic and soft hadron interaction h R M = multiplicity A multiplicity D understanding of the space-time evolution of the hadron formation process FF modification

13 hadron separation vs ν Kr first time hadron discrimination in γ * N π + = π = π 0 ~ K -, K + > K p > p, p > π, p > K Hermes energy well suited for study of nuclear effects statistics on Kr doubled first data on Xe! He, N, Ne, Kr, Xe study A-dependence

14 FF modification (parton energy loss) [X.N.Wang et al., PRL89(2002)] 1 free parameter tuned on 14 N (quark-gluon correlation strength inside nuclei) de/dx from HERMES de/dx for

15 Gluon density [X.N.Wang et al., PRL89(2002)] cold hot nuclear matter correlation gluon density in Au+Au order of magnitude higher than in cold matter fixed by HERMES data

16 new tool: double-hadron attenuation disentangling between absorption and energy loss leading hadron: z 1 >0.5 subleading hadron: z 2 < z 1 [T. Falter et al. 2004] R 2h (z 2 ) = N2(z N 1 N2(z N ) ) A D pure absorption model ruled out Kr statistics doubled; first Xe!

17 outlook shutdown finished recoil on track E-Hall ready for taking DATA!

18 π + cross section: Q 2 dependence factorisation theorem: σ 6 L asymptotically for fixed x B and t 1 Q σ = L kine.factor(q fit: 1/Q p p=1.9±0.5 p=1.7±0.6 p=1.5± Q, x) σ Q 2 dependence in agreement with theoretical expectation σ red = M spins Q red

19 Power correction -Vanderhaeghen, Guichon & Guidal (1999) - intrinsic transverse momentum of the active quark (k perp ) (Q2>>) 0 soft overlap contribution no gluon exchange