how can we understand quark and gluon confinement in ...wqcd/2007/battaglieri.pdfdata were compared...

1) Unconventional baryon and meson spectroscopy at JLAB - M.Battaglieri - INFN Genova

The tool: electromagnetic interaction

- weaker than strong interactions - therefore calculable perturbatively - based on the well-known QED

How can we understand quark and gluon confinement in Quantum Chromodynamics?

Physics goal

The scattering is normally analyzed in term of the One-Photon-Exchange approximation (OPE)

Soft physics Hard physics

?Low Energy, Q2, -t High Energy, Q2, -t

CQM, Phenomen. Models, ... pQCD, Dim. Analysis, ...

s ~ 1 -10 GeV2

Q2 ~ 0-5 GeV2 -t ~ 3-6 GeV2

-qµqµ= Q2 = photon virtualitys = CM total energyt = momentum transfer

The Kinematic regime


Static properties of constituent quarks Baryon spectrum and 'missing resonances'

Physics goal

Exclusive hadronic final states

Exclusive photon scattering in a wide kinematic range

How QCD-partons manifest themself in strong interactions in non-perturbative regime

Dynamic properties of constituent partons Vector meson photoproduction Light scalar meson photoproduction

Beyond the standard quark model Pentaquark searches

High statistics, high resolution low energy exclusive measurement


Emax ~ 6 GeV Imax ~ 200 µADuty Factor ~ 100% σE/E ~ 2.5 10-5

Beam P ~ 80%Eγ ~ 0.8-5.7 GeV

CLAS

Jefferson Lab


The CEBAF Large Acceptance Spectrometer CLAS

Performance L = 1034 cm-2 s-1

∫ B dl = 2.5 T m ∆p/p ~ 0.5-1 % ~ 4π acceptance Best suited for multiparticle final states Bremsstrahlung Photon Tagger (∆Eγ/Eγ~10-3)


The Jefferson Lab and the CLAS detector

Hadron detection efficiency and kinematic coverage

CLAS coverage e p → e' p X

Mx (GeV) 0. 0.2 0.4 0.6 0.8 1.0 1.2 1.4

W (G

eV)

1.2

1

.4

1.

6

1.8

2

.0

2.2

CLAS coverage e p → e' X

W (GeV) 1. 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

Q2 (

GeV

2 ) 1

1.

5

2

2.5

3

3

.5

4

4.5

5

πK

p

Particleidentification

p(GeV)

β

Λ0(1116)

Λ*(1520)

Σ0(1192)

Σ*(1385)

Cou

nts

Mx(γp→K+X ) (GeV)

CLAS resol.γ p → K+ X


500

400

300

200

100

1.0 1.5 2.0

0.5 1.0 2.5 3.0 3.50.0K (GeV)

ECM (GeV)

µb

π N π π N

P π+π−

They are evident in electromagnetic reactions as well

Electroproduction Q2<>0Photoproduction Q2=0

Baryon Spectrum(N* program)

p (GeV/c)

Excited states of the nucleon

were first observed in πN scattering

Total xsec pion-proton

Total photo absorption xsec on proton

Static properties of constituent quarks


Quark model classification of N*

States predicted by symmetric CQM are more than

experimentally observed

With CLASis also possible to

measure multihadronproduction channels

'Missing' resonances?

|q2q>|q3>

1) di-quark model: fewer degrees-of-freedom open question: mechanism for q2 formation?

2) not all states have been found possible reason: decouple from πN-ch and missing states couple to Nππ (∆π, Nρ), Nη, Nω, KY



e p → e' p π+ π-2π electro production

Some resonance structures are visible in CLAS data Simultaneous fit of (pπ+) and (π+π−) invariant masses Data were compared to a phenomenological model including all known resonances

Data show a 'missing' strenght around W~1.7 GeV A good agreement is achieved when:

- the property of the PDG-P13(1720) are changed - a new P13(1720) baryon state is added

M.Ripani et al. Phys. Rev. Lett. 91 022002, 2003

Or



POMERON resolves

in 2 gluons

CORRELATIONSbetween quarks

QUARK EXCHANGE

POMERON exchange

Low t Vector Dominance Diffractive behavior Diff xsec~ βe-α t

High t Small impact parameter B ~ 1/sqrt(t)

Physics motivations

Different qq composition

Sensitivity to different mechanisms

Comparison ofdifferent channels

(Standard)Vector Meson γ-production

Sensitivity to a possible q-diqark structure (ϕ photoproduction) Sensitivity to exchanged quanta structure (ρ and ω photoproduction)


Saturating Regge TrajectoriesPhenomenology

p-p elastic scattering CIM: -t→∞ |α(t)|→ fixed value(process dependent) CIM: the same Reggeons exchanged at low |-t|gives

the asymptotic behavior: -t→∞ |α(t)|→ fixed value Dual model: -t→∞ |α(t)| log

Extracted effective trajectoryfor p-p elastic scattering

D.Coon et al. Phys.Rev. D18 (1978) 1451P.Collins and P.Kearney Z. Phys C22 (1984) 277

Photoproduction

-t→∞ |α(t)| → fixed value in γp→pπ0

gives the right s-3 behavior at fixed -t

Extensive studies:

-t→∞ |α(t)| → fixed value in γp→pπ+

M.Shupe et al. Phys.Rev. D19 (1979) 1921

M.Guidal et al. Nucl. Phys. A627 (1997) 645

Linear Saturated


Linear vs. NON-linear trajectories

M.Brisudova et al. Phys.Rev. D61 (2000) 054013L.Burakovsky hep-ph/9904322

Linear Originally in the Veneziano Model Linear confining potential String model

(α(t) = -1 -t > 3 GeV2)Saturating Regge trajectories: -1 when -t → ∞

NON-Linear From data analysis

αΝ(t) = -0.4 + 0.9 t + 0.125 t2

αP(t) = 1.1 + 0.25 t + 0.5 (0.16±0.02) t2

From theory (Froissart bound) String model with variable tension + flux tube braking

Lattice calculations show a good agreement with non-linear traject.

QCD motivated q-q potential

V(r) = -4/3 αs 1/r + κr + V

0

M.Sergeenko Z.Phys. C64 (1994) 315

Different shapes: α(t) ~ -sqrt(-t) α(t) ~ -log(-t) α(t) ~ fixed value

From trajectory -> qq potential Smooth interp. between f/b peaks

CONFINING

PERTURBATIVE


γ p → p ϕ

3.6 GeV CLAS

u-exchange

The ϕ photoproduction POMERON ⇔ 2- gluons Nucleon wave function Correlations between quarks u-exchange around -t

max

The ρ photoproduction P+ f

2+σ exchange at low -t

Quark-exchange ( 'Saturated Regge traj.')

The ω photoproduction P+ f

2+ σ + π exch. at low -t

Quark-exchange No free parameters

The CLAS results

γ p → p ϕ → p k+ k- γ p → p ρ → p π+ π− γ p → p ω → p π+ π− π0

VM photoproductionA coherent picture of vector mesons photoproduction

F.Cano and J.-M. Laget Phys.Rev. D65 074022 (2002)

E.Anciant et al. Phys.Rev.Lett. 85 4682 (2000) M.Battaglieri et al. Phys.Rev.Lett. 87 172002 (2001) M.Battaglieri et al. Phys. Rev. Lett. 90 022002 (2003)


Meson spectroscopy with CLAS

CLAS is a 4π detector but with limited forward coverage and ϕ uniformity

One needs to prove that PWA is feasible

In our kinematic background from associated baryon resonance production

One needs to show that wave truncations does not affect meson mass spectrum

γ p → p π π


PWA with CLAS (with A.Szczepaniak) We started a comprehensive analysis of ππ and KK photoproduction data γ p → p π π reaction: π+π− spectrum below 1.5 GeV:

P wave: ρ meson S wave: σ, f0(980) and f0(1320) D wave: f2(1270)

Production mechanisms are related to the the resonance nature e.g. short range (QCD) vs long range (hadron) dynamics

Data analysis strategy:1) Extract moments of the angular distribution (correcting for the CLAS acceptance) by log-likelihood fit

2) Describe moments in terms of partial waves

3) Parametrize partial waves in term of known ππ phase shift and unknown coefficients

4) Derive partial wave cross sections to compare with models


Quarks are confined inside colorless hadronsQuarks combine to 'neutralize' color force

q

qqq q

Mystery remains:Of the many possibilities for combining quarks with color into colorless hadrons, only two configurations were found, till now...

What are pentaquarks? Minimum quark content is 4 quarks and 1 antiquark 'Exotic' pentaquarks are those where the antiquark has

a different flavor than the other 4 quarks Quantum numbers cannot be defined by 3 quarks alone

Example: uudss, non-exoticBaryon number = 1/3 + 1/3 + 1/3 + 1/3 - 1/3 = 1Strangeness = 0 + 0 + 0 - 1 + 1 = 0

_Example: uudds, exotic

Baryon number = 1/3 + 1/3 + 1/3 + 1/3 - 1/3 = 1Strangeness = 0 + 0 + 0 + 0 + 1 = 1

_


Soliton model for barions D.Diakonov et al. Z. Phys A359, 1997, 305 In SU(3)-flavor, the multiplets are: [8] Spin = 1/2 [10] Spin = 3/2 [10] Spin = ½ NEW

_ As in the CQM

Θ +I, SP = 0, 1/2+

Strangeness = +1Mass ~ 1.530 MeV Γ ~ 15 MeV

First clear evidence of exotic configurations (light and narrow) New kind of particle will influence our understanding of baryons structure 5-quark states are predicted in many other theoretical models:Skyrme model, MIT bag model, CQM, Lattice QCD, Clustered CQM, QCD Sum rules

Diakonov and collaborators paper had more than 620 citations

Introduction to pentaquarks

Θ+(1539)

N(1650-1690)

Σ(1760-1810)

Ξ+(1862)Ξ5−− Ξ0

5

Oneexperiment

12 observations

Nullresults?

Why pentaquarks are so important?


LEPS @ Spring-8 (Osaka) Compton backscattering photon beam, E

γ~1.5 - 2.4 GeV on Carbon target

γ C → γ n X → Θ + K- X → K+ K- n X Θ + → K+ n Neutron reconstructed using missing-mass technique Fermi motion of struck neutron corrected comparing to other known final states (Λ, Σ) Background:

Red: detected particles

Θ+ should show-up as a peak in the K- missing mass Background comes mainly from

γ p/n → ϕ p/n → K+ K- p/n γ p → Λ*(1520) K+ → K+ K- pγ p/n → K+ K- p/n

19 +/- 2.8 over bg~17Mass = 1540 +/- 10 MeVWidth < 25 MeV @ 90% CL

T.Nakano et al. PRL 012002(2003)(~730 citations)

γ p/n → K+ K- p/n

Experimental evidences First evidence for a possible Θ+(1540)


SAPHIR

JLab-p

HERMES

ITEP

pp → Σ+Θ+.

COSY-TOF

DIANA

SVD/IHEP

JLab-d

ZEUSCERN/NA49

H1

... corroborated by several experiments using different probes and targets Experimental evidences

NEW SPRING8

All experiments with low energy beams (but ZEUS)


FOCUS

HyperCP

BABARBES

CDF

FOCUS

SPHINX

CDF DELPHI

HERA-B

CDF

Θ0c

Ξ--

Ξ--

+ more

High-energy experiments did not like pentaquark!


Is it enough to prove that pentaquark DOES NOT exist?Hadro-production

in e+ e-

slope forpseudo-scalars

slope forbaryons

slope forpentaquarks

(?)

Slopes:pseudo-scalars ~10-2/GeV need to generate qq pair

Baryons ~10-4/GeV need to generate 2 pairs

Pentaquarks (?) ~10-8/GeV need to generate 4 pairs

Pentaquark production in e+e- likely requires orders of magnitudes higher luminosity!

Νο Θ+

e+

e-

e

e

q

q


POSITIVE vs. NEGATIVE

10 high-energy experiments did not find any signal

12 (almost-all) 'low-energy' experiments found evidence of a possible pentaquark state

high statistics set upper limit

Production mechanisms? Background reactions ? Different reactions/kinematics Spectra affected by different acceptance

Different probes/targets Different Labs Some have high statistical significance

Structures have few counts in peaks Mass difference Background shape not known Strong cuts to enhance the signal Kinematical reflections Some of them do not tag Strangeness

To solve the controversy about the existence of the Θ+(1540) pentaquarkis needed definitive confirmation from dedicated low energy experiments

high statistics high resolution

Search for Pentaquarks at JLabin photoproduction on p and d


the K0 is detected via its KS component decaying into π+ π−

final state is identified using the missing mass technique strangeness is tagged detecting the K+

using the full statistics (70 pb-1) a total of ~350K events are selected

n

The reaction γ p → K0 Θ+ → π+ π- K+ (n)

KS

M.Battaglieri et al. Phys.Rev.Lett.96:042001,2006

Background of known hyperons decaying in the same final state is rejected

γ p → Λ*(1520) K+

Λ*(1520) → n K0

γ p → Σ+ (-) π - (+) K+

Σ + (-) → n π + (-)

Σ+(1190) Σ-(1197)

Λ*(1520)


nK+ Mass Spectrum

M(nK+)(GeV)

Cou

nts/

4 M

eV

Θ+(1540) ?

no structure is observed at a mass of ~1540 MeV the nK+ mass spectrum is smooth

The reaction γ p → K 0 Θ + → π + π - K + (n)


one K0 is detected via its KS component decaying into π+ π−

final state is identified using the missing mass technique

Detected KS can be either from the Θ decay or from the K0KS

Kmiss

Λ(1115)

Background of known hyperons decaying in the same final state is rejected

ϕ

γ p → Λ(1115) π+ X

Λ → p π -

γ p → p ϕ ϕ → K

L K

S

The reaction γ p → K0 Θ+ → π+ π- p (K0)R. DeVita et al. Phys.Rev.D74:032001,2006.


pKS Mass SpectrumThe reaction γ p → K 0 Θ + → π + π - p (K 0)

no structure is observed at a mass of ~1540 MeV the pK

S mass spectrum is smooth

Θ+(1540) ?


Combined Upper Limit for the reaction γ p → Θ+ K0

Differential Cross Sectiondσ/dcosθcm

Total Cross Section

Upper limit (95% CL) σ γ p → Θ+ K0 < 0.4 - 1.0 nb

Upper limits from Θ+→nK+ and Θ+→pKs are combined since the event samples are independent The two decay modes were combined taking the weigthed average

γ p → Θ + K0

p

γΘ+

K+ (K0)

n (p)

π+

π−

K0

K0 → KS0 → π+ π−

Θ +→ n K+ → p K0

R. DeVita et al. Phys.Rev.D74:032001,2006.


Comparison with SAPHIR results

cosθCM(K0) > 0.5

cosθCM(K0) > 0.5

cosθCM(K0) > 0.5

cosθCM(K0) > 0.5

Λ(1520)

SAPHIR

g11@CLAS

Θ+(1540) ?

M(nK+) (GeV)

Cou

nts

Cou

nts

Cou

nts

Cou

nts

M(nK+) (GeV)M(nK0) (GeV)

M(nK0) (GeV)

Observed YieldsSAPHIR N(Θ+)/N(Λ*) ~ 63/630 ~ 10%CLAS N(Θ+)/N(Λ*) <100/53000 <0.2% (95%CL)

Cross SectionsSAPHIR σγ p → Θ+ K0 ~ 300 nb reanalysis 50 nbCLAS σγ p → ΘK0 < 0.8 nb

“Battaglieri et al. (CLAS Collaboration)basically repeat with greatly increased statistics the photoproduction measurements of Barth et al. (SAPHIR Collaboration) using the reaction γp → K0K+n. Whereas the SAPHIR Group had reported a 4.8 σ signal in the K+n mass spectrum, the new CLAS experiment shows no signal at all. Indeed the upper limit on the ratio of Θ+ to Λ(1520) production from CLAS is more than a factor of 50 lower than the value claimed by the SAPHIR group. This result completely negates what appeared to be one of the strongest of the positive observations. Combined with the other negative reports, it leaves the reality of the Θ+ in great doubt.”

From PDG 2007 'Pentaquark update'


pK+ Invariant Mass Spectrum

Θ++ ?Eve

nts/

2 M

eV

The reaction γ p → K- Θ++ → p K+ (K-)

Searching for the Θ+ isospin partner: Θ++ V.Kubarovsky et al. Phys.Rev.Lett.97:102001,2006

Differential Cross Sectiondσ/dcosθcm

Total Cross Section

Upper limit (95% CL)σ γ p → Θ++ K- < 0.1 - 0.3 nb


g2 experiment(OLD)

g10 experiment(NEW, 10x stat)

CLAS resultson deuteron target

Pentaquark at Jefferson Lab B. McKinnon et al. Phys.Rev.Lett.96:212001,2006


Statistical significance of g2 peak was overestimated Background shape from g10 reduces stat sig from 5.5σ to 3σ Background shape shows wide structures fluctuating in a narrow bump

How was it possible ?

cosθCM

Upper limit (95% CL) σ γ d → Θ+ K-p < 0.3 - 0.5 nb

σ γ n → Θ+ K- < 3 - 5 nb


Better understanding of nucleon structure and nuclear dynamics Progress in understanding confinement in QCD and the role of

constituent quark and gluons to describe the non-perturbative regime

Conclusions New precise and abundant data from CLAS@Jefferson Lab

Static properties of constituent quarks Exclusive reactions reveal the baryon complexity beyond quark model

'Missing' resonances in multi-hadron final states

Dynamic properties of constituent partonsInteracting partons in meson photoproduction

Production mechanisms help to understand confinement

Beyond the standard quark modelSearch for exotic configurations (pentaquarks, S=+1)

New high statistics, high precision, low energymeasurementResults for reactions γ p→ Θ+ K0 (Θ+→nK+ and Θ+→pK0) and γ n→ Θ+ K- show

no indication of a narrow resonanceAn upper limit of 0.75nb (3.0nb) was set for Θ+ production on proton (neutron)

how can we understand quark and gluon confinement in ...wqcd/2007/battaglieri.pdfdata were compared...

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