The light meson spectroscopy program

Elton S. Smith MENU2013 Sep 30-Oct 4, 2013 1

The light meson spectroscopy program

Elton S. Smith, Jefferson Lab13th International Conference on Meson-Nucleon Physics and the

Structure of the Nucleon

Landscape Experiments BES III (J/y decays) COMPASS (p− beam) CLAS (g beam)

Outlook


Light Mesons

Quark model mesons (u, d, s quarks) qq

Glueballs (gluons and no quarks)

Multiquarks (quark-antiquark pairs)

qq

q q

Hybrids (quarks and gluons, which contribute to static properties)

qq

Call these the “ABC” statesKlempt Zaitsev Phys Rep 454 (2007) 1


Normal Mesons – qq color singlet bound statesSpin/angular momentum configurations & radial excitations generate

the known spectrum of light quark mesons.

Starting with u - d - s we expect to find mesons grouped in nonets - eachcharacterized by a given J, P and C.

JPC = 0– – 0+ – 1– + 2+ – …

Not-allowed: exotic

JPC = 0– + 0++ 1– – 1+ – 2++ …Allowed combinations

K+

p−

K−

p+p0 h’h

K0

K0 K*

r

K*

wf

Spin 0 Spin 1

S = S1 + S2J = L + SP = - (-1)L

C = (-1)L+S

S1

S2

L


Nonets of Scalar (0++) Mesons

Jaffe Phys Rev D15 (1977) 267Crede Meyer Prog Part Nucl Phy 63 (2009) 74

qq

q q

I3


BESIII: e+e- in the charmonium regionOperating since 2008, √s = 2 − 4.2 GeV

Wei Moriond QCD and High Energy Interactions 2013


BES III: pp mass threshold enhancement

Possible interpretations Conventional meson ppbar bound state / multi-quark glueball

Chinese Phys C 34 (2010) 421

Confirmation of enhancement first observed by BES II

M ~ 1.83-1.86 GeV


BES III: pp mass threshold PWA

Breit-Wigner and S-wave FSI describes structure M = 1832 ± 19 MeV G = 13 ± 39 MeV Spin-parity of 0-+

strongly favored by PWA fit

Chinese Phys C 34 (2010) 421 Ablikim PRL 108 (2012) 112003


Angular distribution of X(1835) consistent with 0-+ assignment

Pure glue lattice expects lowest 0-+ glueball mass ~ 2.5 GeV

X(2370) M = 2376 ± 9 MeV G = 83 ± 17 MeV

X(2120) M = 2122 ± 7 MeV G = 83 ± 16 MeV

X(1835) M = 1837 ± 3 MeV G = 190 ± 9 MeV

BES III: X(1835) structures in J/y->gh’p+p-

Ablikim PRL 106 (2011) 072002


BES III: X(1870) in J/y->wX, X->hp+p-

Identification of X(1870)? Spin-parity 0-+? Is it the X(1835)? h2(1870)? Need PWA!

w

Ablikim PRL 107 (2011) 182001

X(1870) M = 1877 ± 6 MeV G = 57 ± 12 MeV X-> a0(980)p, a0->hp

f1(1285)h(1405)

X(1870)


BES III: Summary and prospects Huge data samples collected for charmonium decays at BES

III Data based on 200 M J/y events Confirms of the ppbar mass threshold enhancement at 1832

MeV Confirms of the X(1835)->h’p+p- and finds two new structures:

X(2120) and X(2370) Observes the new structure X(1870) in J/y -> wX(hp+p-) 1.2 billion J/y and 500 million y events already collected

Expect rich physics results in the coming years from BES III


Spectrum of 1−+ exotic signals

Meyer Van Haarlem Phys Rev C82 (2010) 025208

Expectation from LQCD

Unlikely HybridDynamical origin?

qq

q q

I3


COMPASS: spectroscopy with pion beams Hadron Setup 2008/09

Courtesy of F. Nerling

2004 Pilot Run (4 days 190 GeV, Pb tgt, p-)2008/09 (high statistics, 190 GeV, LH2 tgt, p±, K±, p±)

190 GeV


COMPASS: Exotic 1-+ p1(1600)->p+p-p-

~0.4 M events Intensity =1.7% of total


Amplitude analysis 3p system in isobar model

Partial waves (in the reflectivity basis): JPCMe [isobar] L

g (beam)

n (recoil)p (target)

HybridJPCMe

e = + naturale = − unnaturalparity exchange

pa

p (bachelor)

pbRpp

spin S

L1

2

The decay of the 3p system at point 1 to the di-pion resonance Rpp and a bachelor pion is described in the Gottfried-Jackson frame.

The decay of Rpp→ papb at point 2 is described in the helicity frame.


Toy example: resonant phases

Phase motionBlack – Blue

2 waves


COMPASS: Diffractive dissociation into 3p

PWA ~ 1M events PWA ~ 24M events

38% of 2008 data


COMPASS: reproducing known resonances

Phase motion

Intensity

Acceptance corrected53 waves included

a1(1260)->(pp)s p


COMPASS: reference and exotic signals

Phase motion

Intensity

1-+ Exotic

Mass (GeV)

Acceptance corrected53 waves included


COMPASS: Primakoff production of p+p-p-

No intensity exotic 1-+ wave near 1.6 GeV Pilot run for Primakoff reaction, t’ < 0.001 GeV2

Photoproduction in the Coulomb field of heavy nucleus

1-+ Exotic

Grube CLAS12 Workshop, Glasgow

t’ < 0.001 GeV2


COMPASS: Summary and prospects Diffractive production of 3p states

Analysis of 50x statistics of initial pilot run Results for p1(1600)-> p-p-p+ and p1(1600)->p-p0p0

Confirmation of exotic state Ongoing analysis

Pilot Primakoff data sees no exotic in quasi-photoproduction Exotic signals also seen in hp and h’p channels Search for p1(2000) in the KKpp final state

COMPASS has acquired large statistics on many reactions Diffractive dissociation of p, p-, K- on various targets Central production of (pp) in pp reactions, which can be used for

glueball searches. p-g and K-g Primakoff reactions on heavy targets


CLAS: Photoproduction at 6 GeV

Ie = 60-65 nA Eg = 3.584 – 5.453 GeV 26.2 billion triggers Integrated luminosity

of 68 pb-1

G12 Data Run Conditions

Nozar PRL 102 (2009) 102002


CLAS: PWA extraction of known resonances


CLAS: Small non-resonant 1-+ exotic wave

M=1+ and 1-Degenerate wavesfor unpolarized g

M=0Expect Zero For pion exchange


CLAS: PWA 1−+ − 2−+ Phase Motion


CLAS: Summary and prospects No evidence for 1−+ exotic resonance in photoproduction

Confirmation of earlier limits with higher sensitivity Intensity of exotic wave < 2% of total Phase of exotic wave consistent with non-resonant signal

Other analyses Study of exotic signal in gp->hp−D++ Searches for strangeonium in gp->pfh[Spec1: D. Schott]

Forward Tagger for CLAS12

[L. Elouadrhiri] GlueX in Hall D at 12 GeV

Commissioning Fall 2014

[P. Eugenio]

[Future Facilities: R. Mckeown]


Summary

The spectrum of “ABC” mesons is being investigated in both collider and fixed target experiments

BES III has identified structures with masses ~ 1.8−2.4 GeV Their internal structure is under study and may shed light on

the nature of gluonic interactions. The 1−+ exotic p1(1600)->3p is confirmed by COMPASS in p−

interactions Upper limits are set on the 1−+ exotic wave in CLAS

photoproduction, which are consistent with non-resonant background

Documents

The light meson spectroscopy program