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1 First Measurement of the Structure Function b 1 on Tensor Polarized Deuteron Target at HERMES A.Nagaitsev Joint Institute for Nuclear Research, Dubna for the collaboration 1. The tensor asymmetry and structure function b 1 2. HERMES Polarized Target 3. HERMES Spectrometer 4. Extraction of Tensor asymmetry 5.The results 6. Conclusions

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2 The tensor asymmetry and structure function b 1 In QPM the quark distribution for spin-1 target: q + - 0 is the probability to find parton(quark) with momentum fraction x and the same (+) and opposite (-) helicity with respect to the nucleon Proton Deuteron F 1 1/2 f e 2 (q + + q - ) 1/3 f e 2 (q + + q - + q 0 ) g 1 1/2 f e 2 (q + - q - ) 1/2 f e 2 (q + - q - ) b 1 ---- 1/3 f e 2 (2q 0 - q - - q + ) F 2, b 2 F 2 =2x(1+R)F 1 /(1+ ) b 2 =2x(1+R)b 1 /(1+ ) g 2, b 3, b 4 higher twist contribution to the DIS cross section x – Bjorken variable, Q 2 / 2

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3 The tensor asymmetry and structure function b 1 The inclusive differential cross section for the DIS off spin-1 target: p + p p 0 p + p p 0 p + 0 where p + 0 are the probabilities of the corresponding projections and are the cross sections for the target states. Vector polarization: V = p + - p - Tensor polarization: T = p + + p - - 2p 0 With definitions of Vector and Tensor polarizations and Beam Polarization (P B ): U (1+ P B VA // U (1+ P B VA // +1/2TA T ) U /3 Unpolarized cross section: U /3 Tensor polarization asymmetry: A T = / U b 1 / F 1 ) A T = / U b 1 / F 1 ) Vector polarization asymmetry: A // = ~ g 1 / F 1, 1/2 T A T - contamination of A // A // = ~ g 1 / F 1, 1/2 T A T - contamination of A //

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4 HERMES Polarized/Unpolarized Target ABS: ABS: to provide nuclear polarized hydrogen or deuterium atoms for the HERMES target Cell: Cell: open-ended, thin walls, areal density 10 14 at/cm 2 TGA: TGA: target gas analyser, to measure atomic and molecular abundances BRP: BRP: Breit-Rabi Polarimeter measures nuclear polarization The energy diagram of the hyperfine states of hydrogen and deuterium as function of the magnetic holding field Polarization Injected state V T Vector + |1> +|6> N + 1 1 Vector - |3> +|4> N - -1 1 Tensor +/- |3> +|6> N +/- 0 1 Tensor 0 |2> +|5> N 0 0 -2

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5 HERMES Spectrometer Tracking Particle Identification Lumi Monitor Tracking Particle Identification Lumi Monitor Drift and Proportional TRD, Preshower,RICH 2 Calorimeters chambers Calorimeter p/p ~ 2%, ~ 1% e PID eff. ~ 98-99% Bhabha, Moller p/p ~ 2%, ~ 1% e PID eff. ~ 98-99% Bhabha, Moller x/x ~ 3-8%, Q 2 /Q 2 ~1-2% Hadron contam. ~ 1-2 % scattering x/x ~ 3-8%, Q 2 /Q 2 ~1-2% Hadron contam. ~ 1-2 % scattering

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6 Extraction of Tensor Asymmetry Data taking during 1 month (flip vector/tensor target polarization): 1.5 M selected events with tensor polarized target Kinematical cuts: 0.002 0.1 GeV 2, 0.1 3.24 GeV 2 6 bins over x Tensor asymmetry: N + - 0 are the number of DIS events in the kinematical bin, L + - 0 are dead-time corrected luminosity, is the tensor polarization =0.83+/- 0.03 No depolarization factor, no beam polarization, because b 1 is in symmetric part of the hadronic vertex tensor

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7 The results on A T A T is small : less that 2 % Contamination on A // is found to be less than 1% (1/2TA T ) Radiative corrections are calculated with POLRAD 2.0 (elastic tail only) The systematic uncertainties dominated by target polarization: 3%

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8 The results on b 1 The structure function b 1 was calculated: b 1 =-3/2 A T F 1 D from NMC fit, NP B371(1992) from ALLM97 fit, Abramowicz and A.Levy, hep-ph/9712415 The structure function R= L Whitlow et. al., Phys.Lett. B250(1990) No higher twist effects considered b 1 deviates from zero at low x

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9 The results on b 2 Comparison with Bora-Jaffe prediction: K.Bora and R.Jaffe Phys.Rev. D57(1998) The electron-nucleon cross section at the starting point for Q 2 evolution by vector-meson-dominance (VMD). Shadowing due to the double-scat- tering of vector mesons, along with the presence of a d-state admixture in ground state deuteron wave function gives rise to a non vanishing contribution to b 1 (x, Q 2 ).

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10 Conclusions The tensor asymmetry and structure functions b 1, b 2 are measured for the first time on tensor polarized atomic deuteron target at HERMES with 27.6 GeV positron beam. The measured tensor asymmetry A T is small : less than 2 % Contamination on A // is found to be less than 1% The structure function b 1 deviates from zero at low x, x<0.1 The obtained structure function b 2 is in qualitatively agreement with Bora - Jaffe prediction

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