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**Overview of recent results from CLAS**

Marco Mirazita I.N.F.N. – Laboratori Nazionali di Frascati for the CLAS Collaboration Meson Production at Intermediate and High Energies - November 10-11, 2011 – Messina

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**Jefferson Lab at Newport News, USA**

CEBAF Energy : GeV Max current : 200mA Polarization : ~80% CEBAF Large Acceptance Spectrometer Hall B Hall A Hall C

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**Performances for charged particles: 8°<q<142° in LAB frame**

The CLAS detector Toroidal magnetic field (6 supercond. coils) Drift chambers (argon/CO2 gas, cells) Time-of-flight scintillators Electromagnetic calorimeters Cherenkov Counters (e/p separation) Performances for charged particles: large acceptance 8°<q<142° in LAB frame 60-80% of f good momentum and angular resolution Dp/p ≤0.5%- 1.5%, Dq ≤ 1 mrad Df ≤ 4 mrad

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**3D imaging of the nucleon Baryon and meson excitation spectrum**

JLab physics program From nuclei to quarks: a laboratory from “strong” to perturbative QCD Distance Energy heavy nuclei few body quarks gluons vacuum quarks gluons Parton D.F. TMD GPD 3D imaging of the nucleon Baryon and meson excitation spectrum Missing resonances Exotics Correlations Eff. NN (+ΛN) force n-radii: N Z Hadrons in-medium CLAS physics program Talk by D. Watts Correlations Eff. NN (+ΛN) force n-radii: N Z Hadrons in-medium Hypernuclei Baryon and meson excitation spectrum Missing resonances Exotics This talk Parton D.F. TMD GPD 3D imaging of the nucleon PV e-scattering Strange FF Start physics program in 1996

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**From baryonic to partonic degrees of freedom**

Hadrons are made by 3 valence quarks, but as the resolution increases, a reach and complicated partonic structure emerges low Q2 high Q2 - only ~2% of the proton mass is from bare quarks, the proton mass is generated dinamically baryonic spectrum? the “spin crisis” - how the proton spin is made up ? How can hadrons be described in terms of quarks and gluons ? parton distribution functions

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**Parton model in DIS ) Q , x ( F ) Q , x ( F W ) Q , x ( g ) Q , x ( g**

e’ = (E’, k’) e = (E, k) p = (M, 0) * = (,q) W Deep Inelastic Scattering (DIS) W2 M2 xB < 1 Q2 >> M2 mn W ) Q , x ( F 2 1 ) Q , x ( F 2 W mn spin ) Q , x ( g 2 1 ) Q , x ( g 2 PDFs have a simple probabilistic interpretation: they encode the distribution of longitudinal momentum and polarization carried by quarks, antiquarks and gluons within a fast moving hadron. x is the fraction of quark momentum in the scaling regime only x-dependence Q2 corrections can be computed

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**Unpolarized and Helicity PDF**

Unpolarized DF WELL KNOWN Helicity DF KNOWN

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**Orbital Angular Momentum**

SU(6) RCQM broken SU(6) valence quarks: JLab curves: LO pQCD without/with OAM

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**From collinear approximation to TMD**

Three PDFs in collinear approximation in DIS partons move collinearly with the nucleon no angular momentum Transverse Momentum Dependent parton distribution functions more complex dist. functions Access to the transverse momentum requires tagging of the leading quark in the final state

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**TMD distributions Parton Distribution Functions**

Parton Fragmentation Functions all functions depend on x and pT of the quark off-diagonal elements from interference between wave functions with different angular momentum 3D picture of quarks inside the nucleon in momentum space

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Accessing TMDs ep → ehX e+e- →h1 h2 X pp → e+e-X pp → hX

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**Universality and TMDs TMDs are universal objects**

- same functions in SIDIS, e+e-, DY, ... Sivers function: - unpolarized quarks in transversely polarized nucleon - correlation between quark transverse momentum and spin of the nucleon Boer-Mulders function: - transversely polarized quarks in unpolarized nucleon - correlation between quark transverse spin and nucleon momentum Non-zero because of initial or final state interaction Sivers and Boer-Mulders change sign form SIDIS to Drell-Yan Crucial test for the gauge structure of QCD

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**SIDIS Kinematical Plane and Observables**

PT U unpolarized L long.polarized T trans.polarized Target polarization Beam helicity s = sUU + ST sUT sin(f – fS) + l ST sLT cos(f – fS) Extraction of the various terms from moments or asymmetries in

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**SIDIS cross section 18 structure functions**

Structure functions decomposition - leading twist (parton model) - higher twist ~M/Q - only f1 and g1 survive PT integration 18 structure functions

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Observables in SIDIS Observables in SIDIS are the structure function F, not the partonTMD DFs and FFs. Unpolarized structure function: momentum conservation kin. factor DF FF TMD PT = z kT + pT Need models to unfold DFs and FFs - gaussian ansatz for the transverse momentum dependence

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**TMD measurements at JLab**

TMDs are studied at JLab through SIDIS scattering on nucleons (and nuclei) with different experimental equipments B large acceptance spectrometer with good resolution lower luminosity 1034 cm-2 s-1 asymmetry measurements over a broad kinematical range Hall A high resolution and small acceptance spectrometers high luminosity 1037 cm-2 s-1 high polarization 3He target (long. or transv.) neutron Hall C high precision cross section measurements

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CLAS results

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**Factorization at CLAS energies**

ep→e’p0X DSS (Q2=2.5GeV2) DSS (Q2=25GeV2) In the valence region: multiplicityFF Agreement with FF extraction from world data

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Double spin asymmetry Same analysis as in the collinear g1 extraction but now focus on TMD smooth dependence for all pions CLAS Calculations using gaussian ansatz 1.0 0.68 0.4 <kT2>g / <kT2>f transverse mom. distribution different for quarks with spin parallel or antiparallel to nucleon spin

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**Target single spin asymmetry**

leading term higher twist p p p0 HT terms can be important at JLab H1 : Collins FF of transverse polarized quark in unpol. hadron h1L : correlation between transverse spin of quarks and longitudinal spin of nucleon

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**Beam single spin asymmetry**

higher twist Non-zero PT dependence dominant contribution from g? g ~ HT correction of Sivers DF no xB dependence same size as p+

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Hall A and C results

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**Unpol. cross section on H and D**

Hall C Similar shape for both pions Smaller slope for D than H data phenomenological fit no sea quark contribution (x>0.3) dominance of favoured FF u p+ d p- gaussian kT shape larger kT width for d quark than for u in DF and FF u and d quarks have different momentum distributions

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**Transverse target SSA on neutron**

Small (zero?) Collins Larger Sivers for p+ than for p- Hall A Collins effect Sivers effect Opposite behaviour with respect to proton data HERMES proton data

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**Summary of experimental results**

the effect of the transverse momentum of quarks can be observed - TMD DFs and FFs are non-zero (Hermes+Compass+JLab+...) - how much does parton angular momentum contribute to the nucleon spin? first information on TMD DFs and FFs - non-zero Collins FF (SIDIS, e+e-) - non zero Sivers and Boer-Mulders DF (Hermes+Compass, JLab for the neutron) - first extraction of transversity (BELLE + HERMES) - possibility to access HT terms at JLab

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**Open issues Need more data, especially on kaons**

strange quark distributions are basically unknown - inconsistency between extractions from DIS and SIDIS experiments - s distributions different from sbar? kaon puzzle - Sivers and Collins for K+ twice as biggere as p+ favoured FF u p+ u K+ TMD extractions largely depend on the gaussian ansatz for the transverse momentum dependences - spin-dependent TMD are differences of probability, they don’t need to be positive analysis of exp. data is complicated due to convolution of DFs and FFs - multidimensional extraction of TMDs - new analysis techniques need to be implemented Need more data, especially on kaons

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**Upgrade magnets and power supplies**

CHL-2 Upgrade magnets and power supplies 12 GeV CEBAF add Hall D (and beam line) 6 GeV CEBAF End physics 6 GeV in 2012 Enhance equipment in existing halls Beam Power: 1MW Beam Current: 90 µA Max Pass energy: 2.2 GeV Max Enery Hall A-C: 10.9 GeV Max Energy Hall D: 12 GeV May Accelerator Commissioning starts October 2013 Hall Commissioning starts

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CLAS12

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**CLAS12 in Hall B CLAS12 Polarimeters Beam monitors, Raster system, ..**

Faraday cup, Beam monitors

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**Kinematic coverage Q2 extending to higher x means lower cross sections**

need high luminosity 1035 cm-2 s-1

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**PID in CLAS12 no kaon ID RICH detector to replace LTCC**

TOF scintillators charged particle radiator Photodetectors Proximity gap RICH detector to replace LTCC - good PID of kaons over the whole kinematics range - challenging project because of the large area for photodetectors need mirrors to reduce the area Low Threshold Cerenkov High Threshold Cerenkov GeV/c 1 2 3 4 5 6 7 8 9 10 p/K p/p K/p TOF LTCC HTCC no kaon ID

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**Experiments for the first 5 years of data taking already approved**

Physics program with CLAS12 Experiments for the first 5 years of data taking already approved 32

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Conclusions Study of TMDs is one of the main items in the JLab physics program They provide a novel insight into the rich nucleon structure The first generation of experiments have shown evidence of sizeable effects due to TMDs but also open questions A new generation of experiments is in preparation at JLab with higher luminosity and improved detectors to test fundamental properties of TMDs universality test of gauge structure of QCD

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**Hall A L[cm-2s-1] = 1039 Hall B-CLAS Hall C L[cm-2s-1] = 1034**

Pol. 3He (neutron) target. <PHe>=0.5 Longitudinal, transverse pol. Hall B-CLAS L[cm-2s-1] = 1034 Pol. NH3, ND3 targets <PH> =0.8, <PD>=0.3 Longitudinal polarization High Momentum Spectrometer (HMS) Short Orbit (SOS) Hall C L[cm-2s-1] = 1039 Pol. NH3, ND3 targets <PH> =0.8, <PD>=0.3 Longitudinal, transverse polarization

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**Structure of the nucleon**

The complex structure of the nucleon can be described through a large variety of functions PDF longitudinal momentum distributions of partons inclusive scattering elastic FF charge and current distributions elastic scattering transition FF inelastic scattering TMD longitudinal and transverse momentum distributions of partons semi-inclusive scattering GPD longitudinal momentum distributions at a given transverse point exclusive reactions JLab main program: determination of multi-dimensional parton distribution functions in a large kinematics range

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**z-dependence of SIDIS proton g1/F1**

CLAS 5.7 GeV PRELIMINARY No significant z-dependence for 0.3<z<0.7 Good agreement with leading order calculation

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**Double spin asymmetry on the neutron**

Evidence for non zero g1T opposite sign between p+ and p- consistent in sign with models but larger effect

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**Single spin asymmetry – new data**

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**p multiplicities in SIDIS**

ep→e’pX Hall-C M.Aghasyan DSS (Q2=2.5GeV2) DSS (Q2=25GeV2) Tests of factorization performed at Hall-C (charged pions) and Hall-B (neutral pions) indicate that cross sections and multiplicities are consistent with partonic picture and factorization used to calculate curves in both plots. p+/- multiplicities at large z diverge from SIDIS predictions p0 multiplicities less affected by higher twists 0.4<z<0.7 kinematical range, where higher twists are expected to be small

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**JLab Physics Program @ 12 GeV**

Hall A – form factors, SRC, GPDs & TMDs , Low-energy tests of the SM and Fund. Symmetry Exp. Hall B - understanding 3-D nucleon structure via GPDs & TMDs - Search of new form of hadronic matter via Meson Spectroscopy Hall C – precision determination of valence quark properties in nucleons and nuclei Hall D - exploring origin of confinement by studying exotic mesons using real photons

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