Hadron Spectroscopy at GlueX and Beyond (3) Justin Stevens - - PowerPoint PPT Presentation

Hadron Spectroscopy at GlueX and Beyond (3)

Justin Stevens

Justin Stevens, HUGS 2018

q q

Confined states of quarks and gluons

2

q q q q q

mesons baryons

q q q

tetraquark pentaquark

q q q q

Λb → J/ψpK−

Observed mesons and baryons well described by 1st principles QCD But these aren’t the only states permitted by QCD

PRL 115, 072001 (2015)

Justin Stevens, HUGS 2018

Do gluonic degrees of freedom manifest themselves in the bound states we observe in nature?

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q q q q q

tetraquark pentaquark

q q q q q q q q q

mesons baryons glueball hybrid meson

q q g g g

Confined states of quarks and gluons

Observed mesons and baryons well described by 1st principles QCD But these aren’t the only states permitted by QCD

Justin Stevens, HUGS 2018

General properties: mass, electric charge, quark flavor Grouped by quantum numbers: Angular momentum: Parity: Invert spatial coordinates Charge conj.: particle ↔ antiparticle Allowed JPC for mesons:

Classifying mesons

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q¯ q ~ J = ~ L + ~ S P = (−1)L+1 C = (−1)L+S

q q

JP C = 0−+, 1−−, 1+−, 0++, 2++...

electric charge strangeness

Spin 1

JP C

HUGS 2018 Justin Stevens,

Excited gluonic field coupled to pair Rich spectrum of hybrid mesons predicted by Lattice QCD Gluonic field with JPC = 1+- and mass = 1-1.5 GeV “Exotic” JPC : not simple from the non-rel. quark model

q¯ q

q¯ q

Hybrid mesons and gluonic excitations

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hybrid meson

q q g

HUGS 2018 Justin Stevens,

Excited gluonic field coupled to pair Rich spectrum of hybrid mesons predicted by Lattice QCD Gluonic field with JPC = 1+- and mass = 1-1.5 GeV “Exotic” JPC : not simple from the non-rel. quark model

q¯ q

q¯ q

JP C = 0+−, 1−+, 2+−...

Hybrid mesons and gluonic excitations

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~ J = ~ L + ~ S P = (−1)L+1 C = (−1)L+S

hybrid meson

q q g

Justin Stevens, HUGS 2018

Lattice QCD

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PRD 88 (2013) 094505

JP C

500 1000 1500 2000

Meson Mass (MeV) Note:

s¯ s

u¯ u + d ¯ d

ω =

• u¯

u + d ¯ d ↵

φ = |s¯ si

π0 =

• u¯

u − d ¯ d ↵

Justin Stevens, HUGS 2018

exotics positive parity negative parity

JP C

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Meson Mass (MeV)

Lattice QCD

PRD 88 (2013) 094505

Justin Stevens, HUGS 2018

exotics positive parity negative parity

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Lightest hybrid mesons

JP C

Meson Mass (MeV)

Lattice QCD

q q g PRD 88 (2013) 094505

CIPANP 2018 Justin Stevens,

Lightest hybrid mesons

exotics positive parity negative parity

JP C

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JP C

exotics

Most experimental searches for hybrids limited to the π1 state

Meson Mass (MeV)

Lattice QCD

s¯ s

u¯ u + d ¯ d

JP C

PRD 88 (2013) 094505 q q g

HUGS 2018 Justin Stevens,

Heavy quarks Light quarks

Light quark experiments

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Electromagnetic probes Hadronic probes

e+e−

¯ pp pp

γp

¯ pp

πp

HUGS 2018 Justin Stevens,

Compass: Diffractive πp scattering

Long history of hadron spectroscopy in pion production experiments Rich structure in π-π+π- spectrum, modeled as intermediate resonances X- and π+π- Isobars

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Compass: PRD 95, 032004 (2017)

HUGS 2018 Justin Stevens,

Compass: Diffractive πp scattering

Long history of hadron spectroscopy in pion production experiments Rich structure in π-π+π- spectrum, modeled as intermediate resonances X- and π+π- Isobars

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Compass: PRD 95, 032004 (2017)

PHYS251: Tour Justin Stevens,

Double Slit Experiment

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Position (x) Physical Model:

PHYS251: Tour Justin Stevens,

Particle physics “many slits”

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Step 1: Create particles a, b and c in some collision Probe: particles a, b and c which can form heavier particles X, Y or Z Goal: determine properties

• f X, Y, and Z

Physical System Under Study: intermediate particles X, Y, and Z Detector: Measure mass of abc system for each collision Step 2: For each collision record the mass of the abc system

X Y Z X Y Z

Mass of abc system

I(M) =

• X

i=X,Y,Z

ViAi(M)

• 2

Physical Model: a b Intermediate Particles c

Justin Stevens, HUGS 2018

Amplitude Analysis

Goal: Identify JPC of X → π+π-π+ Model the intensity of events at the level

• f QM amplitudes (allow for interference)

5-dimensional problem: two new angles at each decay step (X and I)

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Example Intensity:

X(1++) → ρπ− (S wave)

ρ → π+π−

ρπ+ (S wave) X → ρπ+

Justin Stevens, HUGS 2018

Amplitude Analysis

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X(1++) → ρπ− (S wave) → ρπ− (D wave) X(2++)

ρ → π+π−

Example Intensities:

Expand set of possible amplitudes over many X and I, and determine Vα via maximum likelihood fit Good angular acceptance critical for disentangling JPC

ρπ+ (D wave) ρπ+ (S wave) X → ρπ+

HUGS 2018 Justin Stevens,

Resonance phase motion expectations

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Assuming a Breit-Wigner mass-dependent amplitude:

Matt Shepherd

Justin Stevens, HUGS 2018

Early evidence for 1-+ exotics: π1(1600)

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PRL 104, 241803 (2010)

COMPASS: π−p → π−π+π−p

1-+ intensity

π1(1600) Background

t’

Justin Stevens, HUGS 2018

Compass: PRD 95, 032004 (2017)

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Compass: PRL 104, 241803 (2010)

Compass πp amplitude analysis

Unprecedented statistics: ~50M exclusive events

Decompose into 88 amplitudes representing X-, Isobars, L, and M Extract amplitude intensity and phase in bins of π-π+π- mass

a2(1320) → ρπ

Justin Stevens, HUGS 2018

Compass: PRD 95, 032004 (2017)

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Compass: PRL 104, 241803 (2010) Unprecedented statistics: ~50M exclusive events

Decompose into 88 amplitudes representing X-, Isobars, L, and M Extract amplitude intensity and phase in bins of π-π+π- mass

a2(1320) → ρπ

Compass πp amplitude analysis

Justin Stevens, HUGS 2018 22

Compass πp: example amplitudes

t’

arXiv:1802.05913

X(1++) → ρπ− (S wave) → ρπ− (D wave) X(2++)

Interference

Justin Stevens, HUGS 2018

a2(1320) → ρπ

Compass: 1509.00992

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Compass: PRL 104, 241803 (2010) Unprecedented statistics

Compass πp amplitude analysis

Justin Stevens, HUGS 2018

a2(1320) → ρπ

Compass: 1509.00992

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Compass: PRL 104, 241803 (2010) Unprecedented statistics

Compass πp amplitude analysis

t’

Also bin in t’

Justin Stevens, HUGS 2018 25

Compass: 1512.03599

Evidence for 1-+ π1(1600) in ρπ

Low t’ High t’

t’

Need to work with theorists on better models for resonant and non-resonant: Joint Physics Analysis Center

arXiv:1802.05913

Justin Stevens, HUGS 2018 26

PLB 740 (2015) 303

COMPASS: π−p → η(0)π−p

Evidence for 1-+ exotics: π1(1600)

η(0)

η’

η

Justin Stevens, HUGS 2018 27

1-+ intensity

η’ η η η’

COMPASS: π−p → η(0)π−p

η(0)

Evidence for 1-+ exotics: π1(1600)

Need to work with theorists on better models to describe broad structures: Joint Physics Analysis Center

PLB 740 (2015) 303

HUGS 2018 Justin Stevens,

Tensor mesons with JPAC

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η(0)

Need to work with theorists on better models to describe broad structures: Joint Physics Analysis Center

PLB779 (2018) 464

π−p → ηπ−p : 2++ Intensity

a2(1320)

a0

2(1700)

Fit with single a2 resonances Find additional a’2 is required

HUGS 2018 Justin Stevens,

Tensor mesons with JPAC

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η(0)

Need to work with theorists on better models to describe broad structures: Joint Physics Analysis Center

PLB779 (2018) 464

HUGS 2018 Justin Stevens,

Produce vector charmonium in e+e- CLEO, BESIII, etc. Study decay products through amplitude analysis

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Exotics from charmonium decays

ψ(2S) → γχc1

• Phys. Rev. D 84, 112009

HUGS 2018 Justin Stevens,

Exotics from charmonium decays

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ψ(2S) → γχc1

χc1 → Xπ

χc1 → Xη0

χc1 → η0ππ

• Phys. Rev. D 84, 112009

HUGS 2018 Justin Stevens,

Exotics in other decay modes

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π−p → ωπ0π−p

X → ωπ0π−

ω → π+π−π0

ω π0 E852

PRL 94 (2005) 032002

HUGS 2018 Justin Stevens,

Exotics in other decay modes

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π−p → ωπ0π−p

ω π0 E852

PRL 94 (2005) 032002

1-+(b1π) 4++(ωρ) Phase motion: 1-+(b1π) - 4++(ωρ)

Justin Stevens, HUGS 2018

Summary

Hybrid mesons provide unique access to gluonic degrees of freedom in bound states Lattice QCD predicts a rich spectrum of hybrid mesons, some with exotic JPC Significant evidence for exotic JPC = 1-+ in the literature, but interpretation is murky The search for a coherent pattern of gluonic excitations continues…

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1-+ intensity

η’ η

q q g

HUGS 2018 Justin Stevens,

Further Reading

Light isovector resonances in 
 Compass Collaboration [arXiv:1802.05913] Searching for the rules that govern hadron construction
 Matt Shepherd, Jozef J. Dudek, and Ryan Mitchell Nature 534 (2016) 487 Hybrid mesons
 Curtis A. Meyer and Eric S Swanson, Progress in Particle and Nuclear Physics 82, 21-58 (2015)

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π−p → π−π−π+

HUGS 2018 Justin Stevens,

Backup

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HUGS 2018 Justin Stevens,

Exotic JPC decays

Predictions for the spectrum of hybrids from lattice, but decay predictions are model dependent Candidates for π1 state observed at multiple experiments (COMPASS, E852, etc.) Recent review by Meyer and Van Haarlem (arXiv:1004:5516) Mapping the hybrid spectrum requires: large statistics samples of many particle final states in strange and non-strange decay modes

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• C. A. Meyer and E. S. Swanson,

Progress in Particle and Nuclear Physics B82, 21, (2015)

1-+ channels observed

πρ → πππ πη0 → ηπππ πb1 → ωππ

HUGS 2018 Justin Stevens,

Exotic JPC decays

Predictions for the spectrum of hybrids from lattice, but decay predictions are model dependent Candidates for π1 state observed at multiple experiments (COMPASS, E852, etc.) Recent review by Meyer and Van Haarlem (arXiv:1004:5516) Mapping the hybrid spectrum requires: large statistics samples of many particle final states in strange and non-strange decay modes

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• C. A. Meyer and E. S. Swanson,

Progress in Particle and Nuclear Physics B82, 21, (2015)

1-+ channels observed Some additional 1-+ channels

πρ → πππ πη0 → ηπππ πb1 → ωππ ηf1 → ηηππ πa2 → ηππ KK∗ → KKπ KK1(1270) → KKππ

HUGS 2018 Justin Stevens,

π1(1400) → ηπ Not likely a hybrid: dynamical origin or 4-quark state? π1(1600) → πππ, η’π, b1π, etc. Not observed in ɣp → n π+π-π+ at CLAS: charge vs neutral exchange? Clear evidence for JPC=1-+ partial waves, but interpretation unclear

Evidence for 1-+ exotics

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Compass: PLB 740 (2015) 303

1-+ wave

η’ η

HUGS 2018 Justin Stevens,

π1(1400) → ηπ Not likely a hybrid: dynamical origin or 4-quark state? π1(1600) → πππ, η’π, b1π, etc. Not observed in ɣp → n π+π-π+ at CLAS: charge vs neutral exchange? Clear evidence for JPC=1-+ partial waves, but interpretation unclear

Evidence for 1-+ exotics

81

Compass: PRL 104, 241803 (2010)

HUGS 2018 Justin Stevens,

Evidence for 1-+ exotics

π1(1400) → ηπ Not likely a hybrid: dynamical origin or 4-quark state? π1(1600) → πππ, η’π, b1π, etc. Not observed in ɣp → n π+π-π+ at CLAS: charged vs neutral exchange? Clear evidence for JPC=1-+ partial waves, but interpretation not conclusive

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Found no exotic when using a larger set of partial waves (ie. “high wave”) than previous analysis

E852: PRD 73 (2006) 072001