Overview of Photoproduction Physics at Jefferson Lab

Yordanka Ilieva

University of South Carolina

PHOTON2017, 22 - 26 May 2017, CERN, Geneva

Research supported in part by the U.S. National Science Foundation Thomas Jefferson National Accelerator Facility QCD Machine: Electromagnetic Probes of Hadron Structure

Recirculating Arcs

0.55-GeV Linac

45-MeV Injector

0.55-GeV Linac

Experimental A B C Halls 6-GeV Era: 1995 - 2012 C.W. electron beam: 2-ns wide bunch JLab in Newport News, VA • period, 0.2-ps bunch length Superconducting RF electron linacs • • Polarized Source: Pe ~ 86% with up to 5 times recirculation • Beam energies up to E0 = 6 GeV • Up to 1.1 GeV energy gain per pass • Beam Current up to 200 µA Thomas Jefferson National Accelerator Facility QCD Machine: Electromagnetic Probes of Hadron Structure D 12-GeV Era: 2016 - ... 5.5- pass beam 1.1-GeV Linac 11 GeV

123-MeV Injector

1.1-GeV Linac

5-pass beam 11 GeV A B C • C.W. electron beam: 2-ns wide bunch period, 0.2-ps bunch length • Polarized Source: Pe ~ 85% • Beam energies up to E0 = 11 (12) GeV • Beam Current up to 85 µA Thomas Jefferson National Accelerator Facility Detector Systems CLAS12 Hall B

Hall A

high-resolution spectrometers specialized installations

large acceptance L=1035 s-1cm-2

Hall C GlueX

super-high-momentum spectrometer 9-GeV tagged γ beam L=1038 s-1cm-2 Hall D 4π hermetic detector JLab Photoproduction Scientific Program Probing Hadron Structure and Strong Interaction with Real and Virtual Photons

• The Hadron Spectra as Probes of QCD (Halls B, D) – Baryon and Meson Spectroscopy • The Transverse Structure of Hadrons (Halls A, B, C, D) – Elastic Form Factors – Transition Form Factors • The Longitudinal Structure of Hadrons (Halls A, B, C) – Unpolarized and Polarized Parton Distribution Functions • The 3D Structure of Hadrons (Halls A, B, C) – Generalized Parton Distributions (GPDs) – Transverse Momentum Distributions (TMDs) • Hadrons and Cold Nuclear Matter (Halls A, B, C) – Nucleon Medium Modifications; Quark Hadronization – NN Correlations; Hypernuclear Spectroscopy; Few-Body Experiments – Near Threshold J/ψ Photoproduction • Heavy Photon Search

Hadron Spectroscopy Baryon Spectroscopy RelevantRelevant Degrees degrees ofof Freedomfreedom andand Missingmissing Resonanceresonance Problemproblem N Resonance Spectrum — the low-energy signature of QCD 3000

2700 M ** B 2600 *** 2500

2250 2220 2200 **** **** 2090 ** 2190 2100 2080 * **** 2000 * ** 2000 1986 1990 S ** ** 1900 1897 1895 ** S S

1720 1710 1700

Mass [MeV] 1680 1675 **** 1650 *** **** *** **** **** 1535 1520 1500 **** **** 1440 **** Degrees of freedom

1000 3* or 4* 939 **** J 1/2+ 3/2+ 5/2+ 7/2+ 9/2+ 11/2+ 13/2+ 1/2- 3/2- 5/2- 7/2- 9/2- 11/2- 13/2- LP2T 2J P11 13 FF15 17 HH19 1 11 K1 13 SD11 D13 15 GI17 G19 1 11 I1 13 Quark Models! Constituent Quark Models predict many more of excited states than have been Quark •Models observed; some of the states may only couple weakly to πN.! • Constituent• Quark-Diquark Quark Models Models predict with a tightlymany more bound of diquark excited predict states fewer than states.have been! observed; some• ofQuark the statesand Flux-Tube may only Models couple predict weakly increased to πN. number of states. • Quark-DiquarkCQM: U. Löring, B.C.Models Metsch, andwith H.R. aPetry, tightly Eur. Phys. bound J. A10 395 diquark (2001) predict fewer states. 3 • Quark and Flux-Tube Models predict increased number of states.

CQM: U. Löring, B.C. Metsch, and H.R. Petry, Eur. Phys. J. A10 395 (2001) Polarization observablesBaryon Spectroscopy give access to reaction amplitudesExperimental Observables Polarization observables give access to reaction amplitudes !p "N: Four complex amplitudes enhancing sensitivity  describe0  the reaction to weak resonance γ n → K Λ : Four complex amplitudes describe σ in the presence of … the reaction dσ 2 2 2 2 dσ ∝ S21 + S22 + N 2+ D 2 dΩ~| N | + | S1 | + | S2 | + | D | non-resonant! dΩ background Double polarization observable E! + Beam - Target polarizationK Eγ σ γ n strong !Λ ! resonance γ p dσ 2 2 2 2 Cz ~| S2 | − | S1 | − | N | + | D | dΩ 2 2 Eγ dσ 2 2 E ∝ − S1 + S2 + N − D ddσ * * P Ω ~ 2Im(S N − S D ) I.S. Barker, A. Donnachie, J.K. Storrow, Nucl. Phys. B 79, 431 (1974). 2 1 dΩ 5

I.S. Barker, A. Donnachie, J.K. Storrow, Nucl. Phys. B 79, 431 (1974). Baryon Spectroscopy Extracting Nucleon-Resonance Information from Experimental Data

Database of Experimental Observables γ(*)N→ πN, ππN, ηN, η’N, KY, ρN,... πN→πN, ππN, ηN, η’N, KY, ρN,...

Amplitude Dynamical Reaction Analysis Theory

N* Properties

Models LQCD

QCD Baryon Spectroscopy   Examples of New Data (CLAS) 0 γ (n) → K Λ dσ 11 ~ [ b] Cz –Polarization Transfer from ~ to ⇤ Along z–axis CM µ 0 d cosθKDifferential Cross Section of γd→ K Λ(p) Cz K 0 K 0 K 0 1.6 θ θ θ 1.6 -0.70 < cos CM < -0.60 -0.60 < cos CM < -0.50 -0.50 < cos CM < -0.40 Eγ <1.0 1.02.4 0.2 0.2 1 1 1 1 /dcos(

σ 0 0 d 0 0 0 0 0 0 0 0.20 < cos θK < 0.30 0.30 < cos θK < 0.40 0.40 < cos θK < 0.50 0.8 CM CM CM 0.8 −1 −1 −1 −1 0.6 0.6 −0.5 0 0.5 −0.5 0 0.5 −0.5 0 0.5 −0.5 0 0.5 0.4 0.4 PRELIMINARYCM 0.2 0.2 C.Gleason (USC), 2017 cos✓K

0 0 K 0 K 0 K 0 0.8 θ θ θ 0.8 0.50 < cos CM < 0.60 0.60 < cos CM < 0.70 0.70 < cos CM < 0.80 0.7 0.7 Cz data not fit yet PRELIMINARY 0.6 Colin Gleason (USC) March 29, 2017 15 / 30 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0 0 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 W [GeV]

N.Compton et al. (CLAS Collaboration), submitted to PRC,2017 K0 FIG. 12. The g10 (circles) and g13 (squares) di↵erential cross sections as a function of W (GeV) for each cos ✓CM.TheKAON- MAID model (dashed line) is shown, assuming no contributions from the K⇤(892) and K1(1270), along with the di↵erences between the two Bonn-Gatchina K0⇤ fits (shaded curve). The inner error bars represent the statistical uncertainty. The outer error bars are the statistical and systematic uncertainties added in quadrature. these experiments were the magnitude and directionality nature of d K0⇤(p) compared to p K+⇤,where of the magnetic field). The KAON-MAID model is also one has a neutral! exchange in the t-channel! and the other shown, assuming no contributions from the K⇤(892) and a charged exchange, can help di↵erentiate between con- K1(1270). These parameters were chosen for the model tributions from various t-channel exchanges (and the in- as this provided the best agreement with data. From terference between s-channel and t-channel diagrams). this it is seen that these data will be essential to better The cross sections of the data are in good agreement constrain t-channel contributions. The complementary with the PWA fits done by the Bonn-Gatchina group [2] Baryon Spectroscopy Highlights of the Impact of CLAS Data (light-quark states)

Precision data on exclusive photoproduction of light-quark pseudo-scalar and vector mesons ( • Evidence for N(1900)3/2+ (upgraded to *** in PDG).

• N(1710)1/2+ is upgraded to **** in PDG.

• Evidence for Δ(2200)7/2- with mass 2180 (parity partner of Δ(1950)7/2-).

• Evidence for the “missing” N(2000)5/2+

• Other states: N(1880)1/2+, N(1860)5/2+, N(1895)1/2-, N(1875)3/2-, N(2060)5/2-, N(2150)3/2-, Δ(1940)3/2-.

• Spin-parity measurement of Λ(1405)

Resonance ratings in PDG: * (evidence of existence is poor), ** (Evidence of existence is only fair), *** (Existence is very likely, but further confirmation of decay modes is required), **** (Existence is certain and properties are at least fairly well explored). 10

pentaquarks will decay predominantly to the charmedBaryon baryon and charmedSpectroscopy meson.

The pentaquarks are made of tightly correlated diquarks [23, 24] or colored baryon-like and • 12-GeV Program meson-like constituents [25, 26].

It has been also suggested that at least one of the peaks is not a resonance at all, but rather • 〉 • aStudy kinematical of singularity hybrid due to rescattering[24,excitations 27–29] inin the the decay ⇤light-quarkb J/ pK. baryon spectrum: |qqqg . ! Clearly,E12-16-010. resolving between the models and clarifying the nature of the discovered hidden-charm pentaquark peaks, and possibly searching for similar peaks with other quantum numbers, requires furtherStudy experimental of studies. S=− These2, states−3 baryons: were observed in spin-parity the J/ + p decay mode. measurements, For this search for excited Ω− reason• it is natural to expect that these states can be produced in the photoproduction process + p states,P J/ + newp where and they will missing appear as s-channel Ξ states, resonances at... photon energy around ! c ! 10 GeV [16, 30, 31]. Fig. 4 illustrates the S-channel pentaquark photoproduction process. • Study of charm pentaquark states Pc(4450), Pc(4380) via Pc→ J/ψ+p decay Nature of Pc? J/ J/ charmonium-nucleon bound state? hadronic molecule? Pc kinematic singularity? p p - Hall B: tagged and untagged quasi-real photoproduction techniques (expected

~98FIG. P 4:c Feynman(4450) diagram events/day). for the pentaquark photoproduction Extension process. of E12-12-001.

Such- experimentsHall A: can beuntagged advantageous for real detailed studiesphotoproduction of the production and decay (expected proper- ~70 Pc(4450) events/day). ties of the pentaquarkE12-16-007. resonances in comparison with the LHCb environment. The yield is deter- mined by the branching fraction Br(P + p), and it was shown [16] that this parameter can be c ! expressed in terms of Br(P J/ +p) by a relation similar to vector dominance for the J/ photo- c ! production. Although such dominance cannot be justiﬁed as a general rule, in the situation at hand it can be applied due to arguments based on the heavy quark properties and special kinematics of the processes involved. As a result the peak cross section ( + p P J/ + p) is proportional ! c ! to Br2(P J/ + p) and can reach tens of nanobarns or more, if Br(P J/ + p) 1 10%. c ! c ! ⇠ Such relatively large cross section may allow fairly detailed studies of the pentaquarks and a search JOZEF J. DUDEK PHYSICAL REVIEW D 84, 074023 (2011) 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500

FIG. 5 (color online). Mass spectrum of lightest isovector hybrid supermultiplet along with lightest positive-parity exotic states computed at four pion masses.

There is an exotic 0ÀÀ state that appears at a very heavy and isoscalar states within flavor octets and singlets asso- mass above 3 GeV. Such a state does not arise with a ciated with the isovector states. Kaons have a denser chromomagnetic gluonic excitation combined with qq spectrum described by the more limited JP quantum num- and most likely signals the mass-scale for a different type bers, the flavored states not being eigenstates of charge- PgCg conjugation (or the G-parity extension). An immediate of gluonic excitation, possibly one having Jg 1ÀÀ. P C ¼ consequence of this is that there are no exotic kaons, and We note here that the energy ordering of J g g as 1 < g þÀ hence no smoking gun signature for hybrid kaons. Within 1ÀÀ < ... is the same as is observed for gluelumps, the simple qq models, the spectrum of kaons features states excitations of the gluonic field around a color-octet source constructed from admixtures of opposite C, for example, in in SU 3 Yang-Mills theory[28]. ð Þ the axial (1þ) sector, one would expect two low-lying 3 states constructed from the basis states us P1 1þðþÞ , C. Isoscalar mesons and kaons 1 ð Þ Meson Spectroscopyus P1 1þðÀÞ . Experimentally, two states are found, the PC ð Þ So far we have identified J super multiplets within the K1 1270 , K1 1400 , whose decay properties suggest that isovectorEstablish meson spectrum the Excited but have not Meson considered Spectrum flavor theyð areand strongÞ Searchð admixturesÞ for of the Exotic above basis Mesons states, with a partners in a flavor multiplet. We expect there to be kaonic mixing angle close to 45 [3].

negative parity positive parity exotics

2.5

2.0

1.5

1.0

isoscalar

0.5 isovector

J.J. Dudek, Phys. Rev. D 84, 074023 (2011) YM glueball

LQCD prediction of the light-quark meson spectrum: qq, qqgPC, glueball FIG. 6 (color online). Isoscalar meson spectrum with m 400 MeV labeled by J (reproduced from [15]). The light-strange contentCLAS12 of each state (Hall (cos 2B) , sinand2 GlueX) is given (Hall by the D) fraction will probe of (black, the green) mass and range the mixing 2.0 angle - 2.5 for GeV identiﬁed for pairssuch is states also shown. Horizontalvia squareexclusive braces meson with ellipses photoproduction indicate that additional and statesmulti-particle were extracted final in this states,JPC but were followed not robust. by GreyPartial- boxes indicate the positions of isovector meson states extracted on the same lattice (taken from [14]). Pink boxes indicate the position of glueballs in the quarklessWave Yang-Mills Analysis.. theory [35]. The candidate states for the lightest hybrid meson supermultiplet are indicated by the blue boxes and stars.

074023-8

Transition Form Factors Transition Form Factors N→N* Dynamical constituents of the nucleon and its excited states at Dynamical Constituentsvarying of thedistance Ground scales and Excited Nucleons

quark/gluon confinement Emergence of dressed dynamical quarks:

D.Q. C. Roberts, arXiv:1606.03909 (2016)

N/N* states as the bound systems of dressed dynamical quarks: LQCD data D.Q.

D.Q.

D.Q. • more than 98% of dressed quark (N/N*) masses as well as their dynamical structure are More than 98% of the constituent-quark (N/N*) mass is generated non-perturbatively generated non-perturtbatively through dynamical chiral symmetry breaking (DCSB). The Higgsthrough mechanism its coupling accounts to a low-momentum for less than 2%pion of cloud. the nucleon The Higgs and mechanismN* mass. accounts for less than 2% of the N (N*) mass. • Momentum the momentum dependence dependence of the of theconstituent-quark dressed quark mass mass reflects reflects the the transition transition from from quark/gluonconfinement confinement to asymptotic to asymptotic freedom. freedom.

V.I.Mokeev, UGM 2014, Jefferson Lab, Newport News, VA, June 2-4, 2014 Transition Form Factors with CLAS Q2 Evolution of γ*NN*(Q2) electro-couplings

Measured: differential, total cross sections, polarizations

Decomposition: σL, σT, σTT, σLT, σLT’ 7

Reaction models used: obtain γ*NN*(Q2) electro-couplings

Nπ loops to model MB cloud: running quark mass in N(1440)½+ N(1535)½- Light-Front (LF) Relativistic QM. I. G. Aznauryan, V.D. Burkert PR C 85 (2012) 055202. LF RQM LC SR (NLO) Nσ loops to model MB cloud: frozen constituent-quark mass in LF RQM. I. T. Obukhovsky et al. PRD 89, (2014) 0140032.

Quark-core contributions from DSE/QCD J. Segovia et al. PRL 115 (2015) 171801.

Meson Baryon cloud inferred from CLAS data as the difference between data and the quark-core evaluation in DSE/QCD. V. Mokeev et al., PR C 93 (2016) 025206.

CLAS12 Program: E12-09-003, E12-06-108A, E12-16-010, E12-16-010A 1 + Fig. 8 Left panel: The transverse helicity amplitudes A1/2 for the Roper resonance N(1440) 2 . Data are from CLAS compared to two LF RQM with fixed quark masses (dashed) and with running quark mass (solid red), and with projections from the DSE/QCD approach. The magenta dotted line with error band indicates non 3-quark contributions obtained from a the di↵erence of the DSE curve and the CLAS data. The right 1 panel shows the same amplitude for the N(1535) 2 compared to LF RQM calculations (solid line) and QCD computation within the LC Sum Rule approach. exchanged photon can be varied to probe the spatial structure (Fig. 7). Electroproduction of final states with pseudoscalar mesons (e.g. N⇡, p⌘, K⇤) have been employed with CLAS, leading to new insights into the scale dependence of e↵ective degrees of freedom, e.g. meson-baryon, constituent quark, and dressed quark contributions. Several excited states, shown in Fig. 7 assigned to their primary + SU(6) O(3) supermultiplets have been studied. The N(1232) 3 transition is now well measured ⌦ 2 in a large range of Q2 [36; 37; 38; 40]. Two of the prominent higher mass states, the Roper resonance + 1 1 N(1440) 2 and N(1535) 2 are shown in Fig. 8 as representative examples [39; 40] from a wide program at JLab [41; 42; 43; 44; 45; 46]. For these two states advanced relativistic quark model calculations [47] and QCD calculations from Dyson-Schwinger Equation [48] and Light Cone sum rule [49] have recently become available, for the first time employing QCD-based modeling of the excitation of the quark core. There is agreement with the data at Q2 > 1.5 GeV2. The calculations deviate significantly from the data at lower Q2, which indicates significant non quark core e↵ects. For the Roper resonance such contributions have been described successfully in dynamical meson-baryon models [50] and in e↵ective field theory [51]. Knowledge of the helicity amplitudes in a large Q2 allows for the determination of the transition charge densities on the light cone in transverse impact parameter space (bx,by) [52]. Figure 9 shows + 1 1 the comparison of N(1440) 2 and N(1535) 2 . There are clear di↵erences in the charge transition densities between the two states. The Roper state has a softer positive core and a wider negative outer cloud than N(1535) and develops a larger shift in by when the proton is polarized along the bx axis.

4 Conclusions and Outlook

Over the past ﬁve years eight baryon states in the mass range from 1.85 to 2.15 GeV have been either discovered or evidence for the existence of states has been signiﬁcantly strengthened. To a large degree this is the result of adding very precise photoproduction data in open strangeness channels to the data base that is included in multi-channel partial wave analyses, especially the Bonn-Gatchina PWA. The possibility to measure polarization observables in these processes has been critical [53]. In the mass range above 2 GeV more complex processes such as vector mesons or ⇡ may have sensitivity to states with higher masses but require more complex analyses techniques to be brought to bear. Precision data in such channels have been available for a few years but remain to be fully incorporated in multi-channel partial wave analyses processes. The light-quark baryon spectrum is likely also populated with hybrid

Elastic Form Factors Nucleon Elastic Form Factors Proton electric-charge distribution is different from its magnetization distribution

Rosenbluth Separation

2 2 2 2 dσ dσ ⎛ GE (Q ) +τGM (Q ) 2 2 2 θ ⎞ = ⎜ + 2τGM (Q )tan ⎟ dΩ dΩMott ⎝ 1+τ 2⎠ Q2 e' τ = e * 4m2 γ p p p

Recoil Polarization Ratio   ep e p → ′ Pl, Pt: recoil-proton polarizations longitudinal G τ (1+ ) P E = − t and transverse to proton G 2 P M l momentum in the 2 −1 scattering plane  = (1+ 2(1+τ )tan (θ / 2))

Ch. Perdrisat, AIP Conference Proceedings 1441, 153 (2012) FIGURE 1. On the left all recoil polarization results for µpGEp/GMp; also included are selected Rosenbluth results (empty circles). On the right all polarization results µnGEn/GMn; also included are older Rosenbluth separation results. In both ﬁgures the solid and dashed lines are the results of the VMD calculations of Lomon and Bijker, Refs. [9, 10]

FIGURE 2. The perturbative QCD behavior of the Fermi and Dirac form factor ratio for proton on the left, neutron on the right; a slow down of the rise is visible for the proton; the data is too limited to decide for the neutron.

u,d u,d and with the further assumption of isospin symmetry for the corresponding u and d quark F1p,n and F2p,n form factors, implying: d u u d F1n = F1p and F1n = F1p, and similar relations for F2, the ﬂavor separated u and d quark form factors in the nucleons are linear combinations of the measured form factors F1p,n and F2p,n : u d F1 = 2F1p + F1n and F1 = F1p + 2F1n, u d F2 = 2F2p + F2n and F2 = F2p + 2F2n,

154 Two Photon Exchange

 Proton form factor measurements – ComparisonTwo-Photon-Exchange of precise Rosenbluth andMechanism Polarization measurements of 2 ProposedGEp/GMp toshow explain clear the discrepancydifference betweenat high proton Q FF ratio from cross-section and from polarization data  Two-photon exchange corrections believed to explain the discrepancy TPEP.A.M.Guichon Corrections and M.Vanderhaeghen, PRL 91, 142303 (2003) • sizeable for the cross-section data (Rosenbluth separation) • very small (≲3% at 5.5 GeV2) for the polarization-ratio data J. Arrington et al. / Progress in Particle and Nuclear Physics 66 (2011) 782–833 811

 Have only limited direct evidence of effect on cross section – Active experimental, theoretical withTPE & high programno TPE to fully understandwithTPE Q2 correction TPE effects P. G. Blunden et al, PRC 72 (2005) 034612 A.V. Afanasev et al, PRD 72 (2005) 013008 JA, et al, PRC 76 (2007) 035205 J. Arrington, P.G. Blunden, W. Melnitchouk, Prog. Part. Nucl. Phys. 66, 782 (2011) Fig. 26. Comparison ofJ. polarization Carlson, M. measurements Vanderhaeghen, (filled diamonds) and LT separations (open circles) with no TPE corrections (left), TPE corrections from Ref. [6] (center), and with the additional high-Q 2 correction applied in Ref. [129] (right). M.K.Jones, et al., PRL 84, 1398 (2000) Ann. Rev. Nucl. Part. Sci. 57 (2007) 171 O.Gayou, et al., PRL 88, 092301 (2003) etc… I.A.Qattan, et al., PRL 94, 142301 (2005)

Fig. 27. Proton G (left) and G (right) form factors from the global fit in Ref. [129], scaled by the dipole form factor (37). Note that above Q 2 6 GeV2 M E = there were no direct measurements of GE and GM was extracted from the cross section data with an additional uncertainty related to the uncertainty in 2 GE . The solid line is the final fit, while the points are from direct extractions from data in small Q bins. The short-dashed line shows the fit from the cross sections with no TPE corrections applied. finds 0.867 0.020 fm [23] from a fit combining previous cross section measurements with new low-Q 2 polarization measurements± [23,25]. The latter analysis includes hadronic TPE corrections for all cross section measurements, while the former includes the Coulomb distortion correction of Ref. [39], corresponding to the Q 2 0 limit of the soft-photon approximation. This is an overestimate of the correction at all measured Q 2 values, and entirely= neglects the Q 2 dependence of the correction, which is important in the extraction of the magnetic radius. An estimate of the impact of a more complete TPE correction [134] suggests that this could have a large impact on the form factor and magnetic radius extracted in Ref. [81].

5.5. Normal asymmetries

The TPE exchange process gives rise to a nonzero contribution to the elastic cross section for a recoil proton polarized normal to the scattering plane. By time reversal invariance this is also equivalent to scattering (unpolarized) electrons from a target polarized normal to the scattering plane [133]. Normal polarization observables vanishes in the Born approximation, and their measurement directly accesses the imaginary part of the TPE amplitude. A detailed discussion of the formalism and experimental status of normal asymmetry measurements can be found in Ref. [110]. The target normal asymmetry AN is defined as [110,133,135]

" # AN , (63) = " # ( ) where " # is the cross section for unpolarized electrons scattering from a proton target with spin parallel (antiparallel) to 2 the direction normal to the scattering plane, defined by the spin vector ⇣N k k0/ k k0 (see Eq. (12)). At order ↵ in the electromagnetic coupling, the target normal asymmetry is given by [133=] ⇥ | ⇥ |

2 Im M⇤ M AN , (64) = M 2 | | providing a direct measure of the imaginary part of M.

2 Equivalently, by time reversal invariance, AN would be the asymmetry for scattering unpolarized electrons from an unpolarized proton target, with the recoil proton polarized normal to the scattering plane. Two-Photon-Exchange Correction Directly Accessed Experimentally in e+p vs e-p Elastic Scattering

e+ p σ 1+δ even −δ 2 −δ e.p.brems 1− 2(δ 2 +δ e.p.brems ) R γ γ , R 1 2 = e− p ≈ ≈ 2γ ≈ − δ 2γ σ 1+δ even +δ 2γ +δ e.p.brems 1+δ even 17

The CLAS TPE Experiment

FIG. 21. (Color online) Reduced cross sections divided by the 2 Q2 square of the dipole form factor, GD = 1+ 0.71 ,plottedas afunctionof". The black triangles show⇣ the original⌘ measurements from Andivahis et al. [2] and the red circles show the TPE corrected measurements. The dashed black and solid red lines show the corresponding linear fits where the slope is 2 2 proportional to GE and the intercept is proportional to GM . Extracted correction applied to e-p cross section (Q2=1.75 2 GeV ) , followed by Rosenbluth separation 1026 no-TPE hypothesis at the 4.4 level, and rule out the 1027 point-proton result at the 24 level. The point-proton µpGE/GM= 0.910±0.060, uncorrected PRELIMINARY ⇠ • 1028 result has sometimes been used to approximate TPE cor- 2 1029 rections at low Q values [58]. The fit includes a varia- • µpGE/GM= 0.837±0.066, TPE corrected; consistent with 1030 tion of the normalization uncertainty associated with the polarization-ratio value of 0.789±0.042 1031 model dependence of the radiative corrections, which in- 1032 creases all of the CLAS ratios by roughly 0.3% for the fit D. Rimal et al. (CLAS Collaboration), Measurement of two-photon exchange effect by comparing elastic e+/- p cross sections, accepted in Phys. Rev.1033 to the hadronic calculation and decreases it by a similar FIG. 20. (Color online) Same as Fig. 19 except as a function C (2017). 1034 amount for the point-like comparison. of Q2 at " 0.45 (top) and 0.88 (bottom). Also included is our the CLAS⇡ 2013 result (black open square), which has been averaged to a single point at " =0.893. The open green circles show the previous world data at 0.2 " 0.7and1035 V. CONCLUSIONS 0.7 " 0.95 in the top and bottom plots, respectively  [32].   1036 Our results, along with recently published results from 1037 VEPP-3, rule out the zero TPE e↵ect hypothesis at the 1038 99.4% confidence level and are in excellent agreement 1019 to account for the model-dependence of the high-" nor- 2 1039 (⌫ =1.03 to 1.09) with the calculations [21, 31] that 1020 malization procedure. The comparison of the CLAS plus1040 include TPE e↵ects and largely reconcile the form-factor 1021 VEPP-3 data (16 data points total) to the various mod-1041 discrepancy. The combined world data are consistent 2 1022 els is summarized in Table IV. The data are in good1042 with an increase in R2 with decreasing " at Q 0.85 2 ⇡ 1023 agreement with the hadronic calculations of Ref. [21, 31]1043 and 1.45 GeV . A slight, non-statistically significant, 2 1024 but of insucient precision to make any definitive dis-1044 increase in R2 with Q is seen. Extracting the "- 1025 tinction between them. However, the data exclude the1045 dependent TPE correction factor, 2 (") from our results 2 2 1046 for R2 at Q 1.45 GeV and applying it to the extrac- ⇡ 2 2 1047 2 tion of µpGE/GM at Q =1.75 GeV from the Ref. [2] TPE calculation ⌫ Conf. Level 1048 reduced cross-section data brings it into good agreement Blunden (N)[21] 1.06 38.6% 2 1049 with the polarization transfer measurement at Q =1.77 Zhou & Yang (N)[31] 1.09 35.7% 2 Zhou & Yang (N + )[31]1.03 42.0% 1050 GeV by Punjabi et al. [7]. 1051 Our data, together with those of VEPP-3, show that 2 =0(NoTPE) 2.10 0.62% 14 Point-proton calculation 6.96 2.4 10 % 1052 TPE e↵ects are present and are large enough to explain 2 ⇥ 1053 the proton electric form factor discrepancy up to Q 2 2 ⇡ TABLE IV. Comparison of the 16 CLAS and VEPP-3 data1054 GeV . We look forward to the OLYMPUS results, which points to various TPE calculations showing the reduced -1055 will check the existing measurements and extend them to squared value and the confidence level. 1056 slightly higher momentum transfer. However, the form 1057 factor discrepancy is small at the low momentum trans-

Near-Threshold J/ψ Photoproduction Near-Threshold J/ψ Photoproduction at 12 GeV Probing Non-Perturbative Gluon Fields in the Proton

2 • Eγ,thr=8.20 GeV, |tmin|=1.7 GeV : small transverse-size probe (r⊥~1/mc=0.13 fm), small impact parameter (b~1/|t|1/2~0.2 fm).

• Reaction cross section sensitive to short-distance dynamics in the proton 8 J/ψ region:and multi-gluon Photoproduction exchange mechanisms; J/ψ-N cross near section threshold 13 10 2 hard J /ψ two-gluon exchange Vigurous Program: 10 Cornell 75 Proton Target: x x 1 2 elastic) nb SLAC 75 • Kinematics near threshold → Talk Meziani

CLAS12: E12-12-001

J/ 1 2

Hall A: PR12-12-006 ? Large |tmin|,upto2.2GeV N

-1 ( 10 Hall C: E12-16-007 three-gluon Nuclear Targets: Large longit. momentum transfer x1 − x2 = ζ -2 exchange t 10 Hall C: PR12-07106 Planned d target: -3 10 4 S.J. Brodsky, E. Chudakov, P. Hoyer, CLAS12: LOI2017 J.M. Laget,• Phys.Reaction Lett. B 498 , 23 mechanism near threshold

] -4 (2001) -2 10 8 10 12 14 16 18 20 22 2

threshold GPD-based description at t ∼ 1–2 GeV E GeV 2 and large skewness: Two-gluon form factor FIG. 2: Variation of the J/ photoproduction cross section nearFrankfurt, threshold. Solid Strikman line: two gluon exchange. 02 Dashed line: three gluon exchange [10]. Slope B [GeV

low mass mesons and baryon, which would be very dicultHard to explain scattering if these states consist of mechanism, the cf. high–t FFs 0 Brodsky, Chudakov, Hoyer, Laget 01 0 5 10 light quarks 15 only. So the 20 interpretation of these structures as pentaquark with hidden charm looks very reasonable. 100 SLAC 75 CERN 87 There are severalFNAL 82 models available that attempt to describeCan the be internal tested structure of the with pen- JLab 12 GeV! Cornell 75 taquarksFNAL with hidden93 charm.

The P states are interpreted as a composite of the charmonium state and the proton or • c 10 exited nucleon states [16] similar to the known resonances N(1440) or N(1520). In this case we expect a sizable branching ratio Br(P J/ + p) laying in the range from 1% to 10%. •c !Theoretical questions

[nb] It was pointed out in [16] that in this model one can expect the decays of the pentaquarks σ P J/ + p + ⇡ and P J/ + p + ⇡ + ⇡ should at least compete in the total rate with 1 c ! c ! the observed J/ + p channel. Behavior of two–gluon form factor?

Correlations in nucleon LCWF? 0.1 0 5 10 15 20 W [GeV] Near-Threshold J/ψ Photoproduction at 12 Photoproduction of J/ close to threshold with The very first J/ψ Detected at JLab with GlueX • Exclusive reaction p → p J/J/ →e+ e- •Exclusive measurement of γp→J/ψ+p, J/ψ→e+e- (all final-state particles detected) • Kinematic fit using tagged energy; e+ e- PID using calorimeters •Kinematic fit using tagged-photon energy; e+ e- PID using calorimeters • •DataData from from first first (engineering) (engineering) run in 2016 run in 2016



J/100 events  9 ± 1 MeV

L. Pentchev, Private Communication, 2017 Near-Threshold J/ψ Photoproduction at 12 Photoproduction of J/ close to threshold with The very first J/ψ Detected at JLab with GlueX • First time cross-section measurements down to the threshold, •First time cross-section measurements down to the threshold, sensitive to gluons at high xsensitive to gluons at high x •Can• Canset limits set limitson pentaquark on pentaquark Pc(4450) Pyieldc(4450) (6 MeV yield mass (6 resolution)MeV mass resolution)

Pc(4450)

2-gluon exchange arbitrary normalization Brodsky et al. PL 2001 2-gluon exchange normalized to data Brodsky et al. PL 2001

L. Pentchev, Private Communication, 2017 Summary JLab Photoproduction Scientific Program

• Baryon Resonances – CLAS has been providing precision observables on exclusive meson photoproduction off the nucleon leading to the discovery of several new states and the confirmation of poorly known states. – Exclusive electro-production data have provided new insight into the scale dependence of effective degrees of freedom of resonances. – Extensive program for studies of hybrid, S=-2, -3 resonances at 12 GeV • Meson Spectroscopy – Studies of hybrid and exotic mesons with GlueX and CLAS12 • Elastic Proton Form Factors – Are a highlight of the 6-GeV program; Show importance of 2-photon exchange mechanisms; Extended studies at 12 GeV. • Near Threshold J/ψ Photoproduction off proton, deuteron, and heavier targets: non-perturbative gluon dynamics, J/ψ-N cross section, charm pentaquarks.

Backup Slides Baryon Spectroscopy ResonanceResonance spectrum Spectrum in Lattice in Lattice QCD QCD Hadron-spectrumHadron-spectrum collaboration

2.0

1.8

1.6

1.4

1.2 2 3 2 1 4 5 3 1 1.0 1 1 2 2 1 (1232) 0.8 N(938) mπ = 396 MeV 0.6

LQCD predicts states with the same quantum numbers as CQMs with underlying LQCD predicts states with the same quantum numbers as CQMs with underlying SU(6) x O(3) symmetry; more states than have been identified experimentally. SU(6) x O(3) symmetry; more states than have been identiﬁed experimentally.

R.G. Edwards,R.G. Edwards, J.J. Dudek, J.J. D.G.Dudek, Richards, D.G. Richards, and S.J. Wallace, and S.J. Phys. Wallace, Rev. DPhys. 84, 074508 Rev. D (2011)84, 074508 (2011) 4 Near-Threshold J/ψ Production at JLab 12 Approved Experiments

Experiment Beam Target Detector Detect Days

PR12-12-001 γ (QR) p CLAS12 e+, e -, p 120 e+, e -, p, e’ PR12-12-006 e- p SoLID 60 e+, e -, e’ PR12-16-007 γ (R) p Hall C e+, e - 11

p, d, Be, e+, e -, PR12-07-106 γ (QR) Ca, Al, Hall C CA μ+,μ- Cu, Ag, Au CLAS12 Detector Overview

• General - 11 GeV polarized electron beam - Luminosity: 1035 cm-2s-2 • Forward Detector (toroidal spectrometer) Charged Tracks Neutral Tracks

Ang. Acceptance: 5° - 35° Ang. Acceptance: 5° - 40° (neutron)

0.5% - 1% for 5 GeV track Energy resolution: < 0.1/ E

1 mrad for the electron track < 4 mrad • Central Detector (solenoid magnet): momentum range: 0.3 - 1.3 GeV/c Charged Tracks Neutral Tracks

Ang. Acceptance: 35° - 125° 40° - 125° (neutron)

5% for 1 GeV track Energy resolution: < 5%

5 - 10 mrad < 10 mrad