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