Laboratori Nazionali di Frascati INFN PacificSpin2015 October 6 th - - PowerPoint PPT Presentation

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Laboratori Nazionali di Frascati INFN PacificSpin2015 October 6 th - - PowerPoint PPT Presentation

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Silvia Pisano Laboratori Nazionali di Frascati INFN PacificSpin2015 October 6 th , 2015. The interactions of the nucleon The nucleon is sensitive to all the interactions known so far up How the nucleon experiences a specific

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PacificSpin2015 – October 6th, 2015.

Silvia Pisano Laboratori Nazionali di Frascati INFN

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The interactions of the nucleon

The nucleon is sensitive to all the interactions known so far How the nucleon experiences a specific interactions is encoded in a charge β†’ it depends on the nature of the

  • perator describing the interaction

What is the spatial size of the nucleon? And how its charges are distributed in its bulk? What is the orbital angular momentum of the nucleon constituents? And how its description relates to the full-QCD description encoded in lattice-based calculations?

up down up

π‘Ÿ π‘Ÿ π‘Ÿ π‘Ÿ π‘Ÿ π‘Ÿ π‘‡π‘Ÿ 𝐾𝑕 π‘€π‘Ÿ

PacificSpin2015 – October 6th, 2015.

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GPDs & Deeply-Virtual Compton Scattering

Generalized Parton Distributions β†’ transverse spatial images of quarks and gluons as a function of their longitudinal momentum fraction. There are 4 chiral-even + 4 chiral-odd GPDs for any quark flavor

π’š 𝒖 = 𝒒 βˆ’ 𝒒′ πŸ‘ 𝝄 β‰ˆ π’šπ‘ͺ πŸ‘ βˆ’ π’šπ‘ͺ

𝑰𝒓 π’š, 𝟏, 𝟏 = π’ˆπŸ π’š 𝑰 𝒓 π’š, 𝟏, 𝟏 = π’‰πŸ π’š 𝑰𝑼

𝒓 π’š, 𝟏, 𝟏 = π’ŠπŸ π’š

(for π’š > 0; antiquark for π’š<0)

PacificSpin2015 – October 6th, 2015.

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Sensitivity to GPDs in observables - Compton Form Factors

π‘†π‘“π“˜π‘Ÿ = 𝑓2π‘Ÿπ‘„ πΌπ‘Ÿ 𝑦, ΞΎ, 𝑒 βˆ’ πΌπ‘Ÿ(βˆ’π‘¦, ΞΎ, 𝑒)

1 1 ΞΎβˆ’π‘¦ βˆ’ 1 ΞΎ+𝑦 𝑒𝑦

π½π‘›π“˜π‘Ÿ = πœŒπ‘“2π‘Ÿ πΌπ‘Ÿ ΞΎ, ΞΎ, 𝑒 βˆ’ πΌπ‘Ÿ(βˆ’ΞΎ, ΞΎ, 𝑒) Only (ΞΎ, 𝑒) are experimentally accessible, not 𝑦. GPDs will enter in the observables through The two parts will be accessible through

  • bservables sensitive to the imaginary

(𝐡𝑀𝑉, 𝐡𝑉𝑀) or the real part (𝐡𝑀𝑀, π΅πΆπ‘“π‘π‘›π·β„Žπ‘π‘ π‘•π‘“) of the amplitude. The following Compton Form Factors are introduced (experimentally observable):

4

PacificSpin2015 – October 6th, 2015.

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Accessing GPDs through DVCS observables

  • 1. Beam-Spin Asymmetry:

βˆ†πœπ‘€π‘‰βˆ sin πœ’ 𝐽𝑛 𝐺

1π“˜ + ΞΎ 𝐺 1 + 𝐺 2 π“˜

+ 𝑙𝐺

2𝓕 π‘’πœ’

  • 2. Target-Spin Asymmetry:

βˆ†πœπ‘‰π‘€βˆ sin πœ’ 𝐽𝑛 𝐺

1π“˜

+ ξ 𝐺

1 + 𝐺 2 π“˜ + 𝑙𝐺 2𝓕 π‘’πœ’

  • 3. Double-Spin Asymmetry:

βˆ†πœπ‘€π‘€βˆ (𝐡 + 𝐢cos πœ’) 𝑆𝑓 𝐺

1π“˜

+ ξ 𝐺

1 + 𝐺 2

π“˜ +

𝑦𝐢 2 𝓕

π‘’πœ’

  • 4. Transverse Target-Spin Asymmetry:

βˆ†πœπ‘‰π‘ˆβˆ sin πœ’ 𝐽𝑛 𝑙(𝐺

2π“˜ βˆ’ 𝐺 1𝓕) + … π‘’πœ’

Different observables are sensitive to different combinations of Compton Form Factors and electromagnetic Form Factors: 𝜏 = |𝐢𝐼|2 + 𝐽 𝐢𝐼 βˆ™ πΈπ‘Šπ·π‘‡ + |πΈπ‘Šπ·π‘‡|2 Access to LINEAR combinations of GPDs (instead of bilinear) thanks to the presence of Bethe-Heitler Asymmetries identified as modulations in πœ’, the angle between the leptonic and the hadronic plane

5

PacificSpin2015 – October 6th, 2015.

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Thomas Jefferson National Accelerator Facility

The CEBAF (Continuous Electron Beams Accelerator Facility) operates in the Thomas Jefferson National Accelerator Facility (Newport News, VA, USA). The Cebaf:

  • provides a continuous electron beam with a duty

factor ~ 100%;

  • with a beam energy up to 6 GeV;
  • has a good energy resolution (

𝜏𝐹 𝐹 ~10βˆ’5);

  • and the beam has a polarization ~ 85%

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PacificSpin2015 – October 6th, 2015.

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The three experimental Halls@JLab

Hall-A: High-resolution spectrometers (πœΊπ’’ 𝒒 ~πŸπŸβˆ’πŸ“), measurements with well- defined kinematics at very- high luminosity

NIM A 522, 294 (2004)

The CEBAF provides longitudinally-polarized electrons to 3 experimental Halls, characterized by different and complementary characteristics.

Hall-B: high luminosity, Large acceptance, Multi- particle final state measurements

NIM A 503, 513 (2003)

Hall C: High momentum spectromer and Short Orbit Spectrometerβ€”well- controlled acceptance for precise cross section measurements

PRC 78, 045202 (2008)

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PacificSpin2015 – October 6th, 2015.

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The 12-GeV upgrade

4 experimental halls with a longitudinally-polarized electron beam of πΉπ‘“βˆ’ up to 12 GeV.

High Resolution Spectrometer (HRS) pair and specialized large installation experiments CLAS12: large acceptance, high luminosity Super High Momentum Spectrometer (SHMS) at high luminosity and forward angles 8

SoLID RICH for CLAS12

PacificSpin2015 – October 6th, 2015.

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Experiments and phase-space coverage

Different experiments (will) explore (-ed) different regions of the phase space …ranging from the gluon-dominated domain of HERA to the quark valence region of JLab Fixed-target experiments in the past:

  • HERMES@Desy: 𝑓± beam (𝐹𝑓 = 27π»π‘“π‘Š)
  • Hall-A, CLAS@JLab: π‘“βˆ’ beam (𝐹𝑓 = 6π»π‘“π‘Š)

Future experiments:

  • Hall-A, CLAS12@JLab12: π‘“βˆ’ beam (𝐹𝑓 =

12π»π‘“π‘Š)

  • COMPASSII@CERN: 𝜈± beam (𝐹𝑓 = 160π»π‘“π‘Š)

PacificSpin2015 – October 6th, 2015.

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DVCS on the proton in Hall-A (E00-110)

PacificSpin2015 – October 6th, 2015.

𝑦𝐢 = 0.37, 𝑅2 = 2.36 π»π‘“π‘Š2, βˆ’π‘’ = 0.32 π»π‘“π‘Š2

  • Significan contribution from π“€πΈπ‘Šπ·π‘‡ 2 (πœ’-

independent) and π“€π‘—π‘œπ‘’

  • Clear deviation from BH-only behaviour around

πœ’ = 180Β°

  • Helicity-dependent cross-section twist-2 dominated
  • no 𝑅2 dependence visible in the CFFs (evolution

effects negligible for the present 𝑅2 lever arm)

  • M. Defurne et. al., hep-ex:1504.05453
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DVCS on the proton in Hall-A (E00-110)

PacificSpin2015 – October 6th, 2015.

  • Both Double-Distribution based models

(VGG&KMS12) overestimate the helicity- dependence cross-section

  • KMS12 tuned on vector-meson data at low-to-

very-low 𝑦𝐢

  • KM10a shows good agreement β†’ model

parameters already constrained from CLAS (Hall-B) asymmetry data on the same kinematical region.

  • KM10a underestimates DVCS contribution

around πœ’ = 180Β°

  • Lack of strength around πœ’ = 180Β° partly

compensates by Target-Mass Corrections (TMS)

  • Need a refit of KMS12 including valence data
  • M. Defurne et. al., hep-ex:1504.05453
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Hall-A@11 GeV: E12-06-114

Beam-polarized and unpolarized cross sections with high precision at three electron-beam energies to get:

  • increased kinematic coverage
  • Test of scaling β†’ 𝑅2

dependence of π‘’πœ at fixed 𝑦𝐢

βˆ†πœπ‘€π‘‰βˆ sin πœ’ 𝐽𝑛 𝐺

1π“˜ + ΞΎ 𝐺 1 + 𝐺2 π“˜

+ 𝑙𝐺2𝓕 π‘’πœ’ Large π‘ΉπŸ‘ region explored with high statistics

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PacificSpin2015 – October 6th, 2015.

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Hall-B: DVCS cross-section on the proton in Hall-B (E01-113)

  • H. S. Jo et. al., hep-ex:1504.02009, accepted by PRL

Extraction in a LARGE kinematic domain

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  • ---- KMS

𝑒4πœπ‘“π‘žβ†’π‘“β€²π‘žβ€²π›Ώ 𝑒𝑅2π‘’π‘¦πΆπ‘’π‘’π‘’πœ’ PacificSpin2015 – October 6th, 2015.

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PacificSpin2015 – October 6th, 2015.

Hall-B: DVCS cross-section on the proton in Hall-B (E01-113)

  • VGG model
  • 𝐡𝑓𝑐𝑒

𝐡, 𝑐 increases with 𝑦𝐢 β†’ the partonic content of the nucleon increases when probing smaller 𝑦𝐢

  • H. S. Jo et. al., hep-ex:1504.02009, accepted by PRL
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First CLAS DVCS devoted experiment on unpolarized 𝐼2

𝑩𝑴𝑽

𝐭𝐣𝐨 𝝌

  • F. X. Girod et al., Phys. Rev. Lett. 100, 162002 (2008).

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Mapping GPDs: Beam-spin asymmetries - ℋ𝑱𝒏

PacificSpin2015 – October 6th, 2015.

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Comparing charge distributions: 𝑩𝑴𝑽 ∝ ℋ𝑱𝒏, 𝑩𝑽𝑴 ∝ β„‹ 𝑱𝒏

High statistics extraction of Single and Double-Spin Asymmetries β†’ simultaneous CFF extraction from three observables in a common kinematics

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𝟏. πŸ“ 𝟏. πŸ“ 𝟏. πŸ‘ 𝟏. πŸ‘ 𝟏. πŸ“ 𝟏. πŸ“ 𝟏. πŸ‘ 𝟏. πŸ‘ 𝟏. 𝟏 𝟏. πŸ” 𝟐. 𝟏

𝑩𝑴𝑽 𝑩𝑽𝑴 𝑩𝑴𝑴

  • E. Seder et al, Phys. Rev. Lett. 114, 032001 (2015)

S.P. et al, Phys. Rev. D 91, 052014 (2015) 𝑱𝒏 β„‹ , 𝑱𝒏 β„‹ 𝑰𝒓 π’š, 𝟏, 𝟏 = π’ˆπŸ π’š 𝑰 𝒓 π’š, 𝟏, 𝟏 = π’‰πŸ π’š (for π’š > 0; antiquark for π’š<0)

β†’ axial charge is more concentrated in the nucleon centre than the electric charge

PacificSpin2015 – October 6th, 2015.

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JLab 12 GeV data: impact on β„‹

  • M. Guidal, H. Moutarde, M. Vanderhaeghen: hep-ph > arXiv:1303.6600

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PacificSpin2015 – October 6th, 2015.

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JLab 12 GeV data: impact on β„‹

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  • M. Guidal, H. Moutarde, M. Vanderhaeghen: hep-ph > arXiv:1303.6600

PacificSpin2015 – October 6th, 2015.

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Quark orbital angular momentum

πΎπ‘Ÿ = π‘€π‘Ÿ + π‘‡π‘Ÿ

π‘‡π‘Ÿ β†’ accessible through Inclusive Deep-Inelastic Scattering Quark Orbital Angular Momentum can be extracted To access 𝐹𝑣&𝐹𝑒 both πΉπ‘ž&πΉπ‘œ are needed so to perform a flavor separation 𝑰, 𝑭 𝒗 𝝄, 𝝄, 𝒖 = 𝟘 πŸπŸ” πŸ“ 𝑰, 𝑭 𝒒 𝝄, 𝝄, 𝒖 βˆ’ 𝑰, 𝑭 𝒐 𝝄, 𝝄, 𝒖 𝑰, 𝑭 𝒆(𝝄, 𝝄, 𝒖) = 𝟘 πŸπŸ” πŸ“ 𝑰, 𝑭 𝒐 𝝄, 𝝄, 𝒖 βˆ’ 𝑰, 𝑭 𝒒 𝝄, 𝝄, 𝒖 Neutron GPD 𝑭𝒐: 𝐡𝑀𝑉 on the neutron Proton GPD 𝑭𝒒: cos πœ’ modulation in πœπ‘‰π‘ˆ on proton

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π‘‡π‘Ÿ 𝐾𝑕 π‘€π‘Ÿ

PacificSpin2015 – October 6th, 2015.

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Hall-B@12 GeV: Quark orbital angular momentum & GPD 𝑭

To access 𝐹𝑣&𝐹𝑒 both πΉπ‘ž&πΉπ‘œ are needed.

𝑰, 𝑭 𝒗 𝝄, 𝝄, 𝒖 = 𝟘 πŸπŸ” πŸ“ 𝑰, 𝑭 𝒒 𝝄, 𝝄, 𝒖 βˆ’ 𝑰, 𝑭 𝒐 𝝄, 𝝄, 𝒖 𝑰, 𝑭 𝒆(𝝄, 𝝄, 𝒖) = 𝟘 πŸπŸ” πŸ“ 𝑰, 𝑭 𝒐 𝝄, 𝝄, 𝒖 βˆ’ 𝑰, 𝑭 𝒒 𝝄, 𝝄, 𝒖

E12-12-010 𝑩𝑽𝑼 on proton E12-11-003 𝑩𝑴𝑽 on neutron Neutron GPD 𝑭𝒐: 𝐡𝑀𝑉on the neutron Proton GPD 𝑭𝒒: cos πœ’ modulation in πœπ‘‰π‘ˆ on proton

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PacificSpin2015 – October 6th, 2015.

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Exploring 𝑰𝒐 in the valence region

ℋ𝒐𝑱𝒏 ℋ𝒐𝑺𝒇

β†’ to be (re-)submitted to PAC44

PacificSpin2015 – October 6th, 2015.

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Conclusions

  • Our knowledge of the nucleon structure has

become richer in the last years thanks to GPD formalism and the experimental results of DVCS

  • combined measurement of several DVCS
  • bservables in a vast kinematic space 𝑒𝑝

disentangle the contributions of the various GPDs and their complex kinematic dependences

  • Extracting both proton and neutron data is

paramount if we want to ultimately perform a flavor decomposition of the GPDs

  • Such a flavor separation is critical to access

the elementary degrees of freedom of QCD and to connect them to macroscopic hadron properties such as mass or orbital angular momentum

22

  • X. Ji Phys. Rev. Lett. 78 (1997) 610

PacificSpin2015 – October 6th, 2015.

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backup

PacificSpin2015 – October 6th, 2015.

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Generalized Parton Distributions

through DVCS & DVMP

Hall-A: feasibility test at JLab kinematics & handbag description Hall-B: Pioneering single-spin asymmetry observations Hall-B: DVCS & DVMP cross-section measurements in a large kinematic domain Hall-B: High-statistics extraction of Single- and Double Spin Asymmetries Hall-B Orbital Angular Momentum through GPDs

PRL97: 262002 (2006), C. Munoz Camacho et al. (Hall A collaboration), E12-06-114 𝑩𝑴𝑽: S. Stepanyan et al., Phys. Rev. Lett. 87, 182002 (2001) 𝑩𝑽𝑴: S. Chen et al., Phys. Rev. Lett. 97, 072002 (2006) E01-113, H. Jo et al., soon to be published

  • I. Bedlinskiy et al., PRL109:112001 (2012)

𝑩𝑴𝑽 for π†πŸ on π‘°πŸ‘: R. de Masi et al., PRC77:042201 (2008) 𝑩𝑴𝑽 on π‘°πŸ‘: PRL100: 162002 (2008) F.X. Girod et al., E12-06-119 DVCS & DVπ†πŸP 𝑩𝑴𝑽, 𝑩𝑽𝑴, 𝑩𝑴𝑴 on π‘Άπ‘°πŸ’: soon to be published E12-12-010, 𝑩𝑽𝑼 on proton E12-11-003, 𝑩𝑴𝑽 on neutron 24

PacificSpin2015 – October 6th, 2015.

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DVπ†πŸP cross-section in Hall-B@JLab – tensor charge

  • I. Bedlinskiy et al., PRL109:112001 (2012)

𝜌0 electroproduction β†’ sensitivity to transversity GPDs β„Ž1 𝑦 is related to the nucleon tensor charge β†’ possible test of beyond-SM interactions of the nucleon (effects on beta decay)

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PacificSpin2015 – October 6th, 2015.

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Quark orbital angular momentum

πΎπ‘Ÿ = π‘€π‘Ÿ + π‘‡π‘Ÿ π‘‡π‘Ÿ β†’ accessible through Inclusive Deep-Inelastic Scattering Quark Orbital Angular Momentum can be extracted

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PacificSpin2015 – October 6th, 2015.

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Electron-Ion Collider

  • A collider is needed to reach the gluon-saturated domain
  • electron probe will provide the unmatched precision of

the electromagnetic probes

  • dynamical interplay between sea quarks & gluons

through their distributions

  • change of distributions when going from small to large 𝑦,

to relate sea and valence quarks

PacificSpin2015 – October 6th, 2015.

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DVCS on the proton in Hall-A (E00-110)

E00-110 β†’ accurate cross section measurements at: 1. different 𝑅2 (1.5 Γ· 2.3 π»π‘“π‘Š2) 2. fixed 𝑦𝐢 = 0.36

πŸ”. πŸ–πŸ” 𝑯𝒇𝑾 electron beam, 𝑸π‘ͺ~πŸ–πŸ•% 15-cm-long hydrogen target

πŸπŸπŸ’πŸ–π’…π’βˆ’πŸ‘π’•βˆ’πŸ

(𝒇, 𝜹) 1-𝜹 π†πŸ π†πŸ-subtracted (𝒇, 𝜹) (𝒇, 𝜹, 𝒒) MC

Residual contamination < 3%

PacificSpin2015 – October 6th, 2015.

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Projected quark densities in impact parameter

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Contribution of E Contribution of H+E

  • Fourier transform of GPDs

to access quark densities in impact parameter space.

  • GPD E probes the u- and

d-quark separation in impact parameter space. Transversely polarized proton shows flavor dipole.

  • M. Burkardt

F.X. Girod

PacificSpin2015 – October 6th, 2015.

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DVCS on the neutron in Hall-A (E03-106)

(𝑓 , 𝑓′, 𝛿) reaction on neutron off a deuterium target β†’ decomposed into elastic (d-DVCS) and quasi-elastic (p-DVCS and n-DVCS) contributions. 𝑬 𝒇, 𝒇′, 𝜹 𝒀 = 𝒆 𝒇, 𝒇′, 𝜹 𝒆 + 𝒐 𝒇, 𝒇′, 𝜹 𝒐 + 𝒒 𝒇, 𝒇′, 𝜹 𝒒 + β‹― From 𝐸 𝑓 , 𝑓′, 𝛿 π‘Œ neutron events are obtained after the subtraction of the measured π‘ž 𝑓 , 𝑓′, 𝛿 π‘Œ

  • n hydrogen.

β†’ Only twist-2 contributions are considered Sum of the coherent deuteron and incoherent neutron contributions

PacificSpin2015 – October 6th, 2015.

  • M. Mazouz et. al., PRL 99 242501 (2007)
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DVCS on the neutron in Hall-A - results

First experimental constraint on the parametrization of πΉπ‘Ÿβ†’ it is translated, whitin a model, in a constraint on the quark orbital angular momentum

PacificSpin2015 – October 6th, 2015.

  • M. Mazouz et. al., PRL 99 242501 (2007)
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Experimental access to GPDs

𝜏 = |𝐢𝐼|2 + 𝐽 𝐢𝐼 βˆ™ πΈπ‘Šπ·π‘‡ + |πΈπ‘Šπ·π‘‡|2

𝑱 π‘ͺ𝑰 βˆ™ 𝑬𝑾𝑫𝑻 gives rise to spin asymmetries, which can be connected to combinations of GPDs Two processes contribute to the same (𝑓, π‘ž, 𝛿) final state: Bethe-Heitler and Deeply-Virtual Compton Scattering.

32

PacificSpin2015 – October 6th, 2015.

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SLIDE 33

Test of the formalism: scaling in JLab/Hall-A (E00-110)

4-fold extraction at 𝑦𝐢 = 0.36 of 𝜏 (< 𝑅2> = 2.3 π»π‘“π‘Š2) & 𝜏+ βˆ’ πœβˆ’ (< 𝑅2> = 1.5, 1.9, 2.3 π»π‘“π‘Š2) in 4 βˆ’π‘’ bins β†’ 𝑱𝒏(𝓓𝑱(𝓖)) independent of 𝑅2: no higher-order corrections enter β†’ perturbative QCD scaling in DVCS

PRL97: 262002 (2006), C. Munoz Camacho et al. 33

Significant deviation from pure BH β†’ DVCS contribution to the cross section not negligible

PacificSpin2015 – October 6th, 2015.

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SLIDE 34

Hall-B/CLAS: First observations of 𝑩𝑴𝑽& 𝑩𝑽𝑴

  • S. Stepanyan et al., Phys. Rev. Lett. 87, 182002 (2001).

β†’ signal of the Bethe-Heitler and DVCS interference observed already at CLAS6 kinematics.

  • S. Chen et al., Phys. Rev. Lett. 97, 072002 (2006).

𝐹𝑐𝑓𝑏𝑛 = 4.25 π»π‘“π‘Š 𝐹𝑐𝑓𝑏𝑛 = 5.7 π»π‘“π‘Š

34

PacificSpin2015 – October 6th, 2015.

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𝑩𝑴𝑽 on π‘Άπ‘°πŸ’

S.P. et al, Phys. Rev. D 91, 052014 (2015)

PacificSpin2015 – October 6th, 2015.

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SLIDE 36

𝑩𝑽𝑴 on π‘Άπ‘°πŸ’

  • E. Seder et al, Phys. Rev. Lett. 114, 032001 (2015)

S.P. et al, Phys. Rev. D 91, 052014 (2015)

PacificSpin2015 – October 6th, 2015.

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SLIDE 37

𝑩𝑴𝑴 on π‘Άπ‘°πŸ’

S.P. et al, Phys. Rev. D 91, 052014 (2015)

PacificSpin2015 – October 6th, 2015.

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SLIDE 38

Hall-B/CLAS@12 GeV: High-statistics 𝑩𝑴𝑽&𝑩𝑽𝑴- E12-06-119

βˆ†πœπ‘€π‘‰βˆ sin πœ’ 𝐽𝑛 𝐺

1π“˜ + ΞΎ 𝐺 1 + 𝐺 2 π“˜

+ 𝑙𝐺

2𝓕 π‘’πœ’

βˆ†πœπ‘‰π‘€βˆ sin πœ’ 𝐽𝑛 𝐺

1π“˜

+ ξ 𝐺

1 + 𝐺 2 π“˜ + 𝑙𝐺 2𝓕 π‘’πœ’ 38

PacificSpin2015 – October 6th, 2015.

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SLIDE 39

DVCS on neutron and the GPD 𝑭𝒐

E12-11-003, 𝑩𝑴𝑽 on neutron

Beam-Spin Asymmetry on the neutron highly sensitive to quark angular momentum Different curves: VGG at 𝐹𝑓 = 11 π»π‘“π‘Š, 𝑦𝐢 = 0.17, 𝑅2 = 2 π»π‘“π‘Š2, βˆ’π‘’ = 0.4π»π‘“π‘Š2 for βˆ’ 𝐾𝑣 = 0.3, 𝐾𝑒 = 0.1

  • --- 𝐾𝑣 = βˆ’0.5, 𝐾𝑒 = 0.1
  • βˆ™ - 𝐾𝑣 = 0.3, 𝐾𝑒 = 0.8
  • --- 𝐾𝑣 = 0.3, 𝐾𝑒 = βˆ’0.5

PacificSpin2015 – October 6th, 2015.

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SLIDE 40

HERMES measurements

hydrogen deuterium hydrogen pure

PacificSpin2015 – October 6th, 2015.

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SLIDE 41

Nucleon transverse size

The distance 𝑠βŠ₯2 between the struck quark and the spectator c.m. is given by the 𝑒-slope of the DVCS cross-section. Extracting it for different 𝑦𝐢 values provides a tomographic picture of the nucleon, i.e. how its shape changes with 𝑦𝐢

π‘’πœ0πΈπ‘Šπ·π‘‡ 𝑒𝑒 ∝ π‘“π‘¦π‘ž βˆ’πΆ(𝑦𝐢)|𝑒| 𝐢 𝑦𝐢 = 𝐢0 + 2π›½β€²π‘šπ‘π‘• 𝑦0 𝑦𝐢

COMPASS pilot run 2012

PacificSpin2015 – October 6th, 2015.

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SLIDE 42

Mapping GPDs: Beam-charge asymmetries - ℋ𝑺𝒇

Different beam charge in COMPASS and HERMES provides access to the Beam- Charge Asymmetry β†’ mostly sensitive to the real part of β„‹ Asymmetry found significantly non zero in HERMES Strong dependence on -𝑒 COMPASSII measurement will extend HERMES measurement to lower-𝑦𝐢

  • --- Fits by Kumericki, Mueller

JHEP 07 (2012) 032

PacificSpin2015 – October 6th, 2015.

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SLIDE 43

Exploring 𝑭𝒒 in a wide kinematic range

Coefficient accessible through π΅π‘‰π‘ˆ modulations sensitive to πΉπ‘ž β†’ observed significantly non- zero @HERMES Different 𝐾𝑣 values tested (with 𝐾𝑒 = 0) βˆ’π‘’ (π»π‘“π‘Š2) 𝑦𝐢 𝑅2 (π»π‘“π‘Š2)

E12-12-010 JHEP 06 (2008) 066

PacificSpin2015 – October 6th, 2015.

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SLIDE 44

Higher-twist GPDs: 𝑩𝑽𝑴

sin πŸ‘πŒ

HERMES Collaboration JHEP06 (2010) 019 red: S.P. et al, PRD 91 052014 (2015) black: 𝒕𝒋𝒐 πŸ‘πŒ CLAS preliminary

Higher-twist modulations of the longitudinal Target-Spin Asymmetry could provide access to the quark orbital angular momentum

PacificSpin2015 – October 6th, 2015.

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SLIDE 45

Longitudinal Target-Spin Asymmetry: E12-06-119

Dynamically- polarized π‘Άπ‘°πŸ’ target

βˆ†πœπ‘‰π‘€βˆ sin πœ’ 𝐽𝑛 𝐺

1π“˜

+ ξ 𝐺

1 + 𝐺2 π“˜ + 𝑙𝐺2𝓕 π‘’πœ’

45

PacificSpin2015 – October 6th, 2015.

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SLIDE 46

DVCS on the neutron in Hall-A - results

First experimental constraint on the parametrization of πΉπ‘Ÿβ†’ it is translated, whitin a model, in a constraint on the quark orbital angular momentum

46

PacificSpin2015 – October 6th, 2015.

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SLIDE 47

The 12-GeV upgrade

4 experimental halls with a longitudinally-polarized electron beam of πΉπ‘“βˆ’ up to 12 GeV.

High Resolution Spectrometer (HRS) pair and specialized large installation experiments CLAS12: large acceptance, high luminosity Super High Momentum Spectrometer (SHMS) at high luminosity and forward angles 47

SoLID RICH for CLAS12

PacificSpin2015 – October 6th, 2015.

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SLIDE 48

Hall-A 1Β° cross section

PacificSpin2015 – October 6th, 2015.

1. solid lines β†’ total fit 2. dot-dash line β†’ higher-twist contribution 3. dot-dot-dashed line β†’ BH 4. short-dashed lines β†’ fitted 𝑱𝒏(𝓓𝑱(𝓖)) and 𝑺𝒇(𝓓𝑱(𝓖)) 5. long-dashed line β†’ fitted 𝑺𝒇(𝓓𝑱 + βˆ†π““π‘±)(𝓖) 6. dot-dashed curves β†’ fitted 𝑱𝒏(𝓓𝑱(π“–π’‡π’ˆπ’ˆ)) and 𝑺𝒇(𝓓𝑱(π“–π’‡π’ˆπ’ˆ))

β†’ twist-3 contributions smaller than twist-2 β†’ DVCS contribution to the cross-section not negligible β†’ 𝑱𝒏(𝓓𝑱(𝓖)) independent of 𝑅2: no higher-order corrections enter β†’ perturbative QCD scaling in DVCS

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SLIDE 49

Conclusions 2

  • Past experiments, both fixed target (JLab, HERMES, COMPASS) or active in colliders

(ZEUS, H1), played a crucial role in proving the feasibility of a nucleon tomography through the formalism of the Generalized Parton Distributions

  • Deeply-Virtual Compton Scattering emerged as the cleanest process to access GPDs

(CFFs) through specific observables

  • First constraints of the CFFs β„‹, β„‹

through DVCS 𝐡𝑀𝑉, 𝐡𝑉𝑀, 𝐡𝑀𝑀and cross-sections

  • A good mapping of GPDs will describe how the different charges describing nucleon

interactions are distributed inside its volume

  • The observables can be compared to Lattice results β†’ connection to pure QCD
  • The (bright?) future will see a wide investigation ranging from the gluon/sea regime

explored at COMPASSII to the valence region explored at JLab12 β†’ final goal (together with the TMDs): wide-coverage, high-statistics mapping of the 5D nucleon structure

49

PacificSpin2015 – October 6th, 2015.