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Results from HERMES in view of futurepp experiments Michael Düren Universität Gießen —pp-QCD Workshop, ECT* Trento, July 3-7, 2006 — Outline HERMES physics: Modest aims and rich harvest HERMES technology: polarization and novel techniques Physics results: – Spin structure of the nucleon – Hard exclusive reactions – Quarks in nuclei Future prospects for pp Conclusions Some transparencies are stolen from Aschenauer, Hasch, Nowak, Ji, … M. Düren, Univ. Giessen 2 HERMES physics Why HERMES? A historical review In 1987 we discovered in the EMC the “spin puzzle”: – Violation of the Ellis Jaffe sum rule – A large negative strange sea polarization Both results were based on inclusive DIS data and validity of SU(3)f Original physics program of HERMES: In 1989 we decided to solve the “spin puzzle” with a semi-inclusive DIS experiment doing a flavor decomposition of the quark spin Re-measure inclusive polarized DIS on proton and neutron: – Ellis-Jaffe sum rule – Bjorken sum rule – g 1, g 2 Measure semi-inclusive polarized DIS on p and n: – u(x), d(x), s(x) 4 Why HERMES? A historical review Results (in short): up Ellis-Jaffe sum rule: is really broken Bjorken sum rule: is fine (fortunately) g1, g2 are well known nowadays u(x) is large and positive d(x) is smaller and negative s(x) is approx. zero i.e. – SU(3)f is broken and – only ~30% of the spin of the nucleon is due to the spin of the quarks down M. Düren, Univ. Giessen u d strange 5 Spin of the nucleon Latest results! Q( x ) u( x ) u( x ) d ( x ) d ( x ) 1 0.02 Q dx 0.29 0.03 0.01 ~30% up and down quark contribution in valence region S ( x ) s( x ) s( x ) 1 0.02 S dx 0.006 0.029 0.007 <3% strange quark contribution M. Düren, Univ. Giessen 6 Physics program at HERMES today: Flavor decomposition of quark spin (done) Fragmentation process in vacuum and nuclear medium Gluon spin contribution (first result but large errors) Spin matrix elements of vector meson production Transversity, Collins and Sivers functions (first non-zero results) ... Orbital angular momentum of quarks (first results, but model dependent) Generalized parton distributions (first results on BCA, BSA, TTSA,…) Positive Pentaquark signal In many areas HERMES contributed as a pioneering experiment M. Düren, Univ. Giessen 7 HERMES technology Polarization and novel techniques HERMES requires: Large polarization of beam and target; pure target Relatively large luminosity (Much larger than EMC) Relatively high beam energy (Q²>1 GeV²; larger than Jlab) Relatively large acceptance (Much larger than SLAC) Strangeness identification (kaons) Recoil protons (for exclusive reactions) Solution: High HERA beam polarization (pushed by HERMES) and ABS Storage cell technique (new at that time) HERA fixed target Standard open spectrometer RICH upgrade in 2000 (well working RICH) Recoil upgrade in 2006 (for GPD program) M. Düren, Univ. Giessen 9 Polarized target M. Düren, Univ. Giessen 10 The spectrometer M. Düren, Univ. Giessen 11 Recoil detector M. Düren, Univ. Giessen 12 Scintillating fiber detector M. Düren, Univ. Giessen 13 Guide to success: Novel technologies (at time of proposal) Unique facility (energy, luminosity, precision, …) Polarization (that is where one can falsify models) Flexibility for upgrades Electromagnetic processes that can be directly related to QCD My message: Panda at Fair will also measure completely different things compared to what is in the proposal today! M. Düren, Univ. Giessen 14 Outline HERMES physics: Modest aims and rich harvest HERMES technology: polarization and novel techniques Physics results: – Spin structure of the nucleon – Hard exclusive reactions – Quarks in nuclei Future prospects for pp Conclusions M. Düren, Univ. Giessen 15 Transverse spin distribution of quarks The transverse polarization of quarks in a transversely polarized proton is not identical with the longitudinal polarization of quarks in a longitudinally polarized proton (Rotation and boost do not commute) There are three leading-twist structure functions: Quark density Helicity distribution Transversity M. Düren, Univ. Giessen See talk by Anselmino 16 Transverse spin distribution of quarks The azimuthal angular distributions of hadrons from a transversely polarized target show two effects: – Collins asymmetry in +s Target spin – Sivers asymmetry in -s M. Düren, Univ. Giessen 17 Transverse spin distribution of quarks The azimuthal angular distributions of hadrons from a transversely polarized target show two effects: – Collins asymmetry in +s Product of the chiral-odd transversity distribution h1(x) and the chiral-odd fragmentation function H1(z) related to the transverse spin distribution of quarks First results from Belle Target spin – Sivers asymmetry in -s M. Düren, Univ. Giessen 18 Transverse spin distribution of quarks The azimuthal angular distributions of hadrons from a transversely polarized target show two effects: – Collins asymmetry in +s Product of the chiral-odd transversity distribution h1(x) and the chiral-odd fragmentation function H1(z) related to the transverse spin distribution of quarks First results from Belle Target spin – Sivers-Asymmetrie in -s: Product of the T-odd distribution function f1T(x) and the ordinary fragmentation function D1(z) related to the orbital angular momentum of quarks M. Düren, Univ. Giessen See talk by Metz 19 Collins asymmetries at HERMES (H) significantly positive/negative for +/ at COMPASS (D) compatible with zero 20 Sivers asymmetries at HERMES (H) positive/zero for +/ at COMPASS (D) compatible with zero The results are unexpected 21 but not inconsistent Collins and Sivers Kaon asymmetries at HERMES (H) and at COMPASS (D) are similar except for the Sivers K+ asymmetry M. Düren, Univ. Giessen 22 Perspectives for futurepp experiments: Drell Yan processes Parton distributions accessible similar (complementary) to DIS Asymmetries of polarized beam and/or target experiments give access to transversity – if the energy is sufficient (energy scale ~√(x1x2s)) – if the beam polarizations are sufficient No knowledge of polarized fragmentation functions needed! (Contrary to DIS) M. Düren, Univ. Giessen qq * pp X See talk by Anselmino 23 Hard exclusive reactions Quantum phase-space „tomography“ of the nucleon Quantum phase-space Wigner distribution A classical particle is defined by its coordinate and momentum (x,p): phase-space A state of classical identical particle system can be described by a phase-space distribution f(x,p). The time evolution of f(x,p) obeys the Boltzmann equation. In quantum mechanics, because of the uncertainty principle, the phase-space distributions seem useless, but… Wigner introduced the first phase-space distribution in quantum mechanics (1932) Wigner function: M. Düren, Univ. Giessen 25 Wigner function When integrated over x, one gets the momentum density. When integrated over p, one gets the probability density. Any dynamical variable can be calculated from it! The Wigner function contains the most complete (one-body) info about a quantum system. ! A Wigner operator can be defined that describes quarks in the nucleon The reduced Wigner distribution is related to Generalized parton distributions (GPDs) M. Düren, Univ. Giessen 26 What is a GPD? A proton matrix element which is a hybrid of elastic form factor and Feynman distribution Depends on x: fraction of the longitudinal momentum carried by parton t=q2: t-channel momentum transfer squared ξ: skewness parameter There are 4 important GPDs (among others): ~ ~ H q ( x, , t ), E q ( x, , t ), H q ( x, , t ), E q ( x, , t ) Limiting cases: t0: Ignoring the impact parameters leads to ordinary parton distributions q( x) H q ( x,0,0) ~ q( x) H q ( x,0,0) Integrating over x: Parton momentum information is lost, spatial distributions = form factors remain F 1 (t ) H q ( x, , t ) dx q F2 (t ) E q ( x, , t ) dx q 27 3-D contours of quark distributions for various Feynman x values z bx by Fits to the known form factors and parton distributions M. Düren, Univ. Giessen 28 with additional theoretical constraints (e.g. polynomiality) and model assumptions Conclusions: Quarks in the quantum mechanical phase-space Elastic form factors charge distribution (space coordinates) Parton distributions momentum distribution of quarks (momentum space) Generalized parton distributions (GPDs) are reduced Wigner functions correlation in phase-space e.g. the orbital momentum of quarks: L r p Angular momentum of quarks can be extracted from GPDs: Ji sum rule: Jq 1 1 2 1 xdx H q ( x, ,0) Eq ( x, ,0) GPDs provide a unified theoretical framework for various experimental processes M. Düren, Univ. Giessen 29 Hard exclusive reactions at Kinematic coverage M. Düren, Univ. Giessen 31 Experimental Access to GPDs QCD handbag diagram Deeply virtual Compton scattering (DVCS) Hard exclusive meson production (HEMP) M. Düren, Univ. Giessen 32 Deeply virtual Compton scattering (DVCS) DVCS is the cleanest way to access GPDs: *N N Factorization theorem is proven! Handbag diagram separates hard scattering process (QED & QCD) and non-pertubative structure of the nucleon (GPDs) x+: longitudinal momentum fraction of the quark x B 2: exchanged longitudinal momentum fraction 1 2 1 xB / 2 t :squared momentum transfer GPDs = probability amplitude for N to emit a parton (x+) and for N’ to absorb it (x-) M. Düren, Univ. Giessen 33 DVCS and BH Interference (epe‘p) FF Bethe-Heitler (BH) Kinematics: dσ | τ DVCS |2 | τ BH |2 (τ τ * BH DVCS τ DVCS τ ) * DVCS BH DVCS-BH interference I gives non-zero azimuthal asymmetry x[-1,1] xB/(2- xB) t=(q-q‘)2 Q2=-q2 Use BH as a vehicle to study DVCS. 34 Laser and nucleon holography (Belitsky/Mueller) M. Düren, Univ. Giessen 35 Azimuthal asymmetries in beam spin and beam charge Fourier decomposition of interference term: 2 I 3 I I c0 cn cos( n ) snI sin( n ) n 1 n 1 charge spin Access to real and imaginary part of helicity conserving amplitude M1,1 (GPDs enter in linear combinations in amplitudes) • beam spin asymmetry (BSA) dσep dσep s1I sin sin Im M 1,1 • beam charge asymmetry (BCA) 1,1 dσ e p dσ e- p c1I cos M.Düren, cosUniv. Re M Giessen HERMES is the only experiment which measures BCA 36 Results: BSA and BCA on proton 1 N ( ) N ( ) ALU ( ) | PB | N ( ) N ( ) BSA: Significant sin() dependence dσep dσep s1I sin sin Im M 1,1 N ( ) N ( ) AC ( ) N ( ) N ( ) BCA: Significant cos() dependence dσ e p dσ e- p c1I cos cos Re M 1,1 37 BCA and BSA on proton HERMES results and projected error-bars of the recoil running M. Düren, Univ. Giessen 38 DVCS: Transverse Target Spin Asymmetry sin( s )cos( ) AUT ~ Im( F2 H F1 E ) cos( s )sin( ) AUT ~ Im( F2 H F1E ) first model dependent extraction of Ju possible Code: VGG Jd assumed to be zero M. Düren, Univ. Giessen 39 HERMES can constrain the total angular momentum of up and down quarks (Ju+Jd)! same statistics with electron beam on tape independent data set to constrain (Ju+Jd) M. Düren, Univ. Giessen 40 Generalized EMC-Effect in nuclear DVCS Beam spin asymmetries in nuclear DVCS (Neon) More to come: 2H,4He,14N,20Ne,82-86Kr,129-134Xe: A-dependence of coherent DVCS processes to study quarks in nuclei M. Düren, Univ. Giessen 41 First results for Hard Exclusive Meson Production (HEMP) from HERMES: Pseudoscalar Mesons σ L Sσ L S||σ LT A sinφ UL sinφ ~ ~ σ | ST | sinφ E H Pion form factor HEMP cross section ep e’n + q GPD Model: 42 Vanderhaeghen, Guichon, Guidal First results for HEMP from HERMES: Vector Mesons (r0,,w) -Clean signal without background subtraction; -DIS-background well described by MC; HEMP target spin asymmetry Univ. e’pGiessen r0 M. ep Düren, 43 M. Düren, Univ. Giessen 44 Quarks in nuclei Study fragmentation in nuclei understanding of the space-time evolution of the hadron formation process K N h (z) N e A R hM (z) N h (z) N e D p M. Düren, Univ. Giessen 47 2-hadron production in nuclei disentangle absorption and energy loss models leading hadron: z1 0.5 subleading hadron: z2 z1 N 2 (z 2 ) N1 A R 2h (z 2 ) N 2 (z 2 ) N1 D M. Düren, Univ. Giessen 48 Hard exclusive reactions and GPDs in futurepp experiments Perspectives for futurepp experiments For exclusive processes, „crossing“ allows to measure the same matrix elements with completely different experiments and probes Instead of having an initial electromagnetic process, a final state electromagnetic process is selected Example: Compton scattering Annihilation M. Düren, Univ. Giessen p p pp 50 Perspectives for futurepp experiments Experimental difficulty: the total cross section of a hadronic beam is typically much larger than the one of the exclusive electromagnetic process The experiment needs a huge background suppression factor as the majority of events have a hadronic final state Example: Compton scattering Annihilation M. Düren, Univ. Giessen p p pp 51 Turning Matter into Light: pp annihilation at large energies and angles What is the dominant mechanism? Large pT in this exclusive reaction defines hard scale: parton picture Emission of a single photon from one (quasi-free) parton is suppressed Emission of two photons from the same parton is allowed At intermediate energies (s~10 GeV) the hard interaction of one parton can be separated from the soft part which is parameterized by General Distribution Amplitudes: handbag diagram See talk by Kroll Quasi-free partons cannot emit single photons (suppressed) Handbag diagram: one quark emits both photons (allowed) Soft part of annihilation M. Düren, Univ. Giessen 52 Further processes of interest: Reactions with the QCD handbag diagram hard gluon , r, , ... Hard exclusive meson production (large pT) Q2 small: like Crossedchannel wide angle Compton scattering (large pT) Q2 large: additional degrees of freedom e-, e+, M. Düren, Univ. Giessen 53 Further processes of interest: Another „distribution amplitude“ •No handbag diagram •Here the photons and the pion are produced in forward direction! •Measure „Transition distribution amplitudes“ pp * explores the pion cloud pp * r explores the r cloud pp * explores the photon cloud M. Düren, Univ. Giessen (Study next to lowest Fock state of the proton) See talk by 54 B. Pire and L. Szymanowski Turning light into matter..... Compare pp with the time inverted process pp Results e.g. from Belle: Asymmetric e+e- collider High luminosity machine Available at e+e- machines: quasi real photons s 10.6 GeV L 1034 /cm 2s e e e e * * e e pp M. Düren, Univ. Giessen 55 Recent results from Belle Energy dependence pp Angular dependence M. Düren, Univ. (GPD Giessen curve from Kroll/Schäfer) 56 Processes of interest: hard gluon -process is not only background but also signal! , r, , ... Hard exclusive meson production (large pT) •Much larger cross section (compared to ) makes it easier to access! •3- final state M. Düren, Univ. Giessen 2 E760 results 57 (curve from Kroll/Schäfer) More physics potential at PANDA: Time-like proton Formfactors at large q2 wide kinematic range accessible at PANDA (up to q²~20 GeV²) q2>0 high statistical precision possible separate GM and GE q2<0 M. Düren, Univ. Giessen 58 A few words on polarization: Target polarization PAX wants to use a polarized H,D storage cell target (like HERMES) PANDA could (in principle) use a polarized 3He storage cell target (like HERMES in 1995) where the polarized 3He is injected from outside the solenoid into the cell A more attractive option is to run HESR in collider mode (to obtain high enough energy especially for Drell Yan) with both beams polarized The PANDA detector would be suitable for the asymmetric collider mode, so (part of) the PAX and ASSIA program could be done in an extended PANDA detector (“PANDAX”) M. Düren, Univ. Giessen 59 A few words on polarization: Polarization of the final state The polarization of the final state can be determined in some special cases: Using parity violation of weak decays (e.g. of ´s) by observing angular distributions of decay products Angular distribution of decay products without parity violation (e.g. decay of vector mesons or of virtual photons) New idea: angular distribution of the nuclear interaction of proton in calorimeter can be used to determine the transverse proton spin (from the azimuthal distribution of the shower profile in the calorimeter) M. Düren, Univ. Giessen 60 Some spin physics possible with out polarized target! M. Düren, Univ. Giessen 61 Conclusions HERMES has done and still does unique and pioneering measurements in the fields of – Spin structure of the nucleon – Exclusive reactions and GPDs – Hadronization studies in nuclei – … Part of the program can be completed by the measurement of complementary reactions inpp annihilation experiments at PANDA, PAX and ASSIA An upgrade of HESR to a polarizedpp-collider seems ideal to follow up the HERMES program. M. Düren, Univ. Giessen 62