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Overview of recent results from CLAS
Marco MirazitaI.N.F.N. – Laboratori Nazionali di Frascati
for the CLAS Collaboration
Meson Production at Intermediate and High Energies - November 10-11, 2011 – Messina
Jefferson Lab at Newport News, USA
Hall BHall A Hall C
CEBAF LargeAcceptanceSpectrometer
CEBAF
Energy : 0.8-5.7 GeV Max current : 200mA Polarization : ~80%
The CLAS detector• Toroidal magnetic field (6 supercond. coils)• Drift chambers (argon/CO2 gas, 35000 cells)
• Time-of-flight scintillators • Electromagnetic calorimeters• Cherenkov Counters (e/p separation)
Performances for charged particles:• large acceptance
8°<q<142° in LAB frame60-80% of f
• good momentum and angular resolution
Dp/p ≤0.5%- 1.5%, Dq ≤ 1 mrad Df ≤ 4 mrad
JLab physics program
quarksgluons
From nuclei to quarks: a laboratory from “strong” to perturbative QCD
Start physics program in 1996
Distance
Energyheavynucleifew
body
quarksgluons
vacuum
CorrelationsEff. NN (+ΛN) force
n-radii: N ZHadrons in-medium
Hypernuclei
Baryon and meson excitation spectrumMissing resonances
ExoticsParton D.F.TMDGPD
3D imaging of the nucleon
PV e-scatteringStrange FF
Parton D.F.TMDGPD
3D imaging of the nucleon
Baryon and meson excitation spectrumMissing resonances
Exotics
CorrelationsEff. NN (+ΛN) force
n-radii: N ZHadrons in-medium
CLAS physics program
Talk by D. Watts
This talk
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
How can hadrons be described in terms of quarks and gluons ?
parton distribution functions
low Q2
high Q2
2
1
2
1 30.the “spin crisis”
- how the proton spin is made up ?
qsv LG 2
1
2
1
- only ~2% of the proton mass is from bare quarks, the proton mass is generated dinamically
baryonic spectrum?
Parton model in DISe’ = (E’, k’)
e = (E, k)
p = (M, 0)
* = (,q)
W
WL'E
E
MQd'dE
d4
2
2mnW
)Q,x(g 22)Q,x(g 2
1
spinWWW 0W
mn Wmnspin
)Q,x(F 22)Q,x(F 2
1
Deep Inelastic Scattering (DIS)W2 M2 xB < 1 Q2 >> M2
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.
P
Pxp
q • x is the fraction of quark momentum
• in the scaling regime only x-dependence• Q2 corrections can be computed
M
QBx
QMMW
EEqQ
2
2
22
22
2
2sin'4
22
Unpolarized and Helicity PDFUnpolarized DF
qqxQxF ),( 22
WELL KNOWN
qqQxg ),( 21
Helicity DF
KNOWN
dxg1
Orbital Angular Momentum
SU(6)
RCQMbroken SU(6)
curves: LO pQCD without/with OAM
valence quarks: JLab
ddxuuxF 9
1
9
42 dduug
9
1
9
41
From collinear approximation to TMD
Three PDFs in collinear approximation in DIS
)( ),( ),( xhxgxf 111
• partons move collinearly with the nucleon • no angular momentum
Transverse Momentum Dependent parton distribution functions
P
TpPxp
qTp
more complex dist. functions
),( ),,( ),,( TTT pxhpxgpxf
111
Access to the transverse momentum requires tagging of the leading quark in the final state
TMD distributions
• 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
Parton Distribution Functions
Parton Fragmentation Functions
Accessing TMDs
pp → hX
pp → e+e-X
ep → ehX
e+e- →h1 h2 X
Universality and TMDs
Non-zero because of initial or final state interactionSivers and Boer-Mulders change sign form SIDIS to Drell-Yan
DYTSIDIST ff 11 DYSIDIShh 11
Crucial test for the gauge structure of QCD
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
TMDs are universal objects - same functions in SIDIS, e+e-, DY, ...
PT
SIDIS Kinematical Plane and Observables
Beam helicityTarget polarization
U unpolarized
L long.polarized
T trans.polarized
Extraction of the various terms from moments or asymmetries in
s = sUU + ST sUT sin(f – fS) + l ST sLT cos(f – fS) + ....
UU
UT
TT
TT
SS
SS
11
11
SIDIS cross section
18 structure functions
11 Dg
11 Df
Structure functions decomposition
- leading twist (parton model)- higher twist ~M/Q- only f1 and g1 survive PT integration
FFDFF
Observables in SIDISObservables in SIDIS are the structure function F, not the partonTMD DFs and FFs.
Unpolarized structure function:
11, DfF TUU C
a
Ta
Ta
TThTTTTa kzDpxfkpwzPkpkdpdexwfD ,,,/2222 C
momentum conservation
kin. factorDF FF
1. TMD PT = z kT + pT
2. Need models to unfold DFs and FFs - gaussian ansatz for the transverse momentum dependence
zDxfDf 1111 C
TMD measurements at JLab
• CLAS@Hall Blarge acceptance spectrometer with good resolutionlower luminosity 1034 cm-2 s-1 asymmetry measurements over a broad kinematical range
• Hall Ahigh resolution and small acceptance spectrometershigh luminosity 1037 cm-2 s-1 high polarization 3He target (long. or transv.) neutron
• Hall Chigh resolution and small acceptance spectrometershigh luminosity 1037 cm-2 s-1 high precision cross section measurements
TMDs are studied at JLab through SIDIS scattering on nucleons (and nuclei) with different experimental equipments
CLAS results
Factorization at CLAS energies
ep→e’p0X
In the valence region: multiplicityFFAgreement with FF extraction from world data
DSS (Q2=2.5GeV2)
DSS (Q2=25GeV2)
smooth dependence for all pions
CLAS
Double spin asymmetryXepe '
1
11
f
g
NN
NN
PPyfDA
tBLL
Same analysis as in the collinear g1 extraction but now focus on TMD
Calculations using gaussian ansatz f
T
T
k
k
T exfkxf2
2
11 ,
gT
T
k
k
T exgkxg2
2
11 ,
1.0
0.68
0.4
<kT2>g / <kT
2>f
transverse mom. distribution different for quarks with spin parallel or antiparallel to nucleon spin
Xepe '
leading termhigher twist
NN
NN
fPA
tUL
1
sin2sin sin2sinULULUL FF
Target single spin asymmetry
112sin HhF LUL
H1 :
Collins FF of transverse polarized quark in unpol. hadronh1L
: correlation between transverse spin of quarks and longitudinal spin of nucleon
p+ p- p0
HT terms can be important at JLab
Xepe 0'
higher twist
NN
NN
PA
beamLU
1
Beam single spin asymmetry
• Non-zero• PT dependence
1Hedominant contribution from g?g ~ HT correction of Sivers DF
• no xB dependence • same size as p+
Hall A and C results
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
Unpol. cross section on H and D
• Similar shape for both pions• Smaller slope for D than H data
Hall CXeep '
• Small (zero?) Collins • Larger Sivers for p+ than for p-
Hall AXeHee ' 3 Collins effect
11sin HhF shUT
Sivers effect
11
sin DfF TUTsh
Opposite behaviour with respect to proton data
HERMES proton data
Transverse target SSA on neutron
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
Open issues• 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
End physics program @ 6 GeV in 2012
6 GeV CEBAF
CHL-2
Upgrade magnets and power supplies
12 GeV CEBAF
Enhance equipment in existing halls
add Hall D (and beam line)
Beam Power: 1MWBeam Current: 90 µAMax Pass energy: 2.2 GeVMax Enery Hall A-C: 10.9 GeVMax Energy Hall D: 12 GeVMay 2013
Accelerator Commissioning starts
October 2013 Hall Commissioning starts
R. De Vita, INFN – Genova Workshop sulle prospettive di fisica adronica a Jefferson Lab Genova, 27 Febbraio 2008
CLAS12
R. De Vita, INFN – Genova Workshop sulle prospettive di fisica adronica a Jefferson Lab Genova, 27 Febbraio 2008
CLAS12 in Hall B
PolarimetersBeam monitors,Raster system, .. Faraday cup,
Beam monitors
CLAS12
Q2
Kinematic coverage
extending to higher x means lower cross sectionsneed high luminosity 1035 cm-2 s-1
GeV/c 1 2 3 4 5 6 7 8 9 10
p/K
p/p
K/p
TOF
TOF
LTCCHTCC
HTCC
LTCC
TOF
LTCC
TOF scintillatorsLow Threshold Cerenkov
High Threshold Cerenkov
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
PID in CLAS12
no kaon ID
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
Hall A L[cm-2s-1] = 1039
Pol. 3He (neutron) target. <PHe>=0.5
Longitudinal, transverse pol.
Hall B-CLASL[cm-2s-1] = 1034
Pol. NH3, ND3 targets <PH> =0.8, <PD>=0.3
Longitudinal polarization
High Momentum Spectrometer
(HMS)Short Orbit Spectrometer
(SOS)
Hall CL[cm-2s-1] = 1039
Pol. NH3, ND3 targets <PH> =0.8, <PD>=0.3
Longitudinal, transverse polarization
Structure of the nucleonThe complex structure of the nucleon can be described through a large variety of functions
elastic FFcharge and current
distributionselastic scattering
transition FFinelastic scattering
GPDlongitudinal momentum distributions at a given
transverse pointexclusive reactions
PDFlongitudinal momentum distributions of partonsinclusive scattering
TMDlongitudinal and transverse momentum distributions of partonssemi-inclusive scattering
JLab main program: determination of multi-dimensional parton distribution
functions in a large kinematics range
z-dependence of SIDIS proton g1/F1
37
CLAS 5.7 GeV
PRELIMINARY
• No significant z-dependence for 0.3<z<0.7• Good agreement with leading order calculation
Double spin asymmetry on the neutron
XeHee '3
Evidence for non zero g1T
opposite sign between p+ and p-consistent in sign with models but larger effect
Single spin asymmetry – new data
p multiplicities in SIDIS ep→e’pX
p+/- multiplicities at large z diverge from SIDIS predictionsp0 multiplicities less affected by higher twists0.4<z<0.7 kinematical range, where higher twists are expected to be small
DSS (Q2=2.5GeV2)
DSS (Q2=25GeV2)
M.AghasyanHall-C
JLab Physics Program @ 12 GeV
Hall A – form factors, SRC, GPDs & TMDs ,Low-energy tests of the SM and Fund. Symmetry Exp.
Hall C – precision determination of valence quark properties in nucleons and nuclei
Hall B - understanding 3-D nucleon structure via GPDs & TMDs - Search of new form of hadronic matter via Meson Spectroscopy
Hall D - exploring origin of confinement by studying exotic mesons using real photons