1. 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

2. Jefferson Lab at Newport News, USA
CEBAF
Energy : GeV
Max current : 200mA
Polarization : ~80%
CEBAF
Large
Acceptance
Spectrometer
Hall B
Hall A
Hall C

3. 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

4. 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

5. 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

6. 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

7. Unpolarized and Helicity PDF
Unpolarized DF
WELL KNOWN
Helicity DF
KNOWN

8. Orbital Angular Momentum
SU(6)
RCQM
broken SU(6)
valence quarks: JLab
curves: LO pQCD without/with OAM

9. 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

10. 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

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

12. 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

13. SIDIS Kinematical Plane and ObservablesPT
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 

14. 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

15. 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

16. 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

17. CLAS results

18. Factorization at CLAS energies
ep→e’p0X
DSS (Q2=2.5GeV2)
DSS (Q2=25GeV2)
In the valence region: multiplicityFF
Agreement with FF extraction from world data

19. 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

20. 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

21. 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+

22. Hall A and C results

23. 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

24. 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

25. 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

26. 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

27. 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

28. CLAS12

29. CLAS12 in Hall B CLAS12 Polarimeters Beam monitors, Raster system, ..
Faraday cup,
Beam monitors

30. Kinematic coverage Q2 extending to higher x means lower cross sections
need high luminosity  1035 cm-2 s-1

31. 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

32. 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

33. 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

35. 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
 

36. 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

37. 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

38. 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

39. Single spin asymmetry – new data

40. 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

41. 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