light exotic hadrons at compass · 2020. 9. 30. · spin-exotic π 1 (1600): (re-) observed by...

Bernhard KetzerRheinische Friedrich-Wilhelms-Universität Bonn

Workshop on Light Quark Exotics (Snowmass 2021 Planning)

30 September 2020

Light Exotic Hadrons

at COMPASS

Light Meson Spectrum

B. Ketzer

[B. Ketzer et al., Prog. Part. Nucl. Phys. 113, 103755 (2020)]Relativistic Quark Model: [D. Ebert et al., Phys. Rev. D 79, 114029 (2009)]

Light Exotic Hadrons at COMPASS 2

The COMPASS Experiment

B. Ketzer Light Exotic Hadrons at COMPASS 3


SM1

SM2

Beam

MuonWall

MuonWall

E/HCAL

E/HCAL

RICHTarget

[COMPASS, P. Abbon et al., NIM A 779, 69 (2015)]Light Exotic Hadrons at COMPASS 5

Two-stage spectrometerDipole magnetsTracking detectorsRICHEl.-mag. calorimeterHadronic calorimeterMuon identification

B. Ketzer

𝟑𝟑𝟑𝟑 Final State - PWA


Total intensity 1++ Waves

• Largest wave-set to date: 88 waves• Independent fits in 100 bins (20 MeV) of 𝑚𝑚3𝜋𝜋 and 11 bins of 𝑡𝑡𝑡

[COMPASS, C. Adolph et al., PRD 95, 032004 (2017)]


B. Ketzer



New a1(1420)


[C. Adolph et al., COMPASS, PRL 115, 082001 (2015)]

• Data described well by Breit-Wigner and non-resonant background

• Parameters for BW:𝑀𝑀0 = 1414−13+15 MeV/cΓ0 = 153−23+8 MeV/𝑐𝑐

• Not an artefact of analysis (↗ freed isobar fit)

B. Ketzer

Interpretations


• Tetraquark state [Z.-G. Wang (2014), H.-X.Chen et al. (2015), T. Gutsche et al. (2017)]

B. Ketzer

𝑞𝑞 𝑞𝑞

�𝑞𝑞 �𝑞𝑞𝑞𝑞𝑞𝑞 8 �𝑞𝑞�𝑞𝑞 8

Interpretations


• Tetraquark state [Z.-G. Wang (2014), H.-X.Chen et al. (2015), T. Gutsche et al. (2017)]• 𝐾𝐾∗ �𝐾𝐾 molecule [T. Gutsche et al. (2017)]

B. Ketzer

�𝑞𝑞 �𝑞𝑞𝑞𝑞 𝑞𝑞

𝜋𝜋,𝜎𝜎𝜌𝜌,𝜔𝜔 𝑞𝑞�𝑞𝑞 0 𝑞𝑞�𝑞𝑞 0

Interpretations


• Tetraquark state [Z.-G. Wang (2014), H.-X.Chen et al. (2015), T. Gutsche et al. (2017)]• 𝐾𝐾∗ �𝐾𝐾 molecule [T. Gutsche et al. (2017)]• Interference of Deck 𝜌𝜌𝜋𝜋 𝑆𝑆 and 𝑓𝑓0𝜋𝜋 𝑃𝑃-wave [J.-L. Basdevant et al. (2015)]

,𝑓𝑓0

B. Ketzer

Interpretations


• Tetraquark state [Z.-G. Wang (2014), H.-X.Chen et al. (2015), T. Gutsche et al. (2017)]• 𝐾𝐾∗ �𝐾𝐾 molecule [T. Gutsche et al. (2017)]• Interference of Deck 𝜌𝜌𝜋𝜋 𝑆𝑆 and 𝑓𝑓0𝜋𝜋 𝑃𝑃-wave [J.-L. Basdevant et al. (2015)]• Triangle singularity [M. Mikhasenko et al., PRD 91, 094015 (2015), F. Aceti, PRD 94, 096015 (2016)]

− Decay of wide 𝑎𝑎1 1260 → 𝐾𝐾∗�𝐾𝐾 above threshold− Final-state rescattering of 𝐾𝐾�𝐾𝐾 to 𝑓𝑓0 980

logarithmic singularity of amplitude if particles close to mass shell

B. Ketzer

Triangle Amplitude


Scalar case:

• dispersive approach• all particles treated as scalars• finite width of K* included

Take into account spins:

• partial-wave projection method

Comparison TS - BW


• Similar 𝜒𝜒red2 for both fits (slightly better for triangle) • No new free parameters for a1(1420) signal by triangle mechanism

[COMPASS, M.G. Alexeev et al., subm. to PRL, arXiv:2006.05342]

Comparison TS - BW


• Phase motion of pure triangle diagram is only ~ 90°• Observed phase motion close to 180° produced by shift due to background

B. Ketzer


Contribution of other Graphs


• Why not include direct decay 𝑎𝑎1 1260 → 𝑓𝑓0 980 𝜋𝜋?• What about contributions from other triangles, e.g. 𝜌𝜌𝜋𝜋𝜋𝜋?

• intensity of other graphs much smaller than K triangle (scaled down 20×), because only K triangle produces log. singularity on 2nd Riemann sheet, all others on 3rd sheet or higher ⇒ effect at threshold small

• no sharp phase motion at 1.4 GeV


Contribution of other Graphs


Summary for a1(1420)• Peak and phase motion are not unique sign of

a resonance!• a1(1420) signal can be fully described with

a1(1260) as source and rescattering via triangle diagram

• Old theoretical concept, now data allow toclearly observe this!

• Intensity of signal ~1%, in agreement withexperiment

• Cannot completely exclude additional pole due to 𝐾𝐾∗ �𝐾𝐾 resonance

Hybrids: Lattice QCD

[J. Dudek et al., Hadron Spectrum Collaboration, Phys. Rev. D 88, 094505 (2013)]

negative parity

Light Exotic Hadrons at COMPASS 20B. Ketzer

positive parity

exotic

Hybrids:• excitation of gluonic degrees of freedom• angular momentum in flux tube• hybrids also predicted by models

1−+ Partial Wave


Bad description of data without 1−+ resonance 𝜋𝜋1(1600) needed to describe data

[M. Aghasyan et al. (COMPASS), Phys. Rev. D 98, 092003 (2018)]

𝑀𝑀0 = 1600−60+110 MeV/𝑐𝑐2

Γ0 = 580−230+100 MeV/𝑐𝑐2

ηπ− / η’π− Final States


[C. Adolph (COMPASS), Phys. Lett. B 740, 303 (2015) ]

Forward 𝜂𝜂(′) Backward 𝜂𝜂(′)

ηπ− / η’π− Final States


[C. Adolph (COMPASS), Phys. Lett. B 740, 303 (2015) ]

P wave P wave

Extraction of Poles


[A. Rodas et al. (JPAC), Phys. Rev. Lett. 122, 042002 (2019)]

o Data from

• Analytical model based on S-matrix theory• Test case: 𝜂𝜂𝜋𝜋 𝐷𝐷-wave• Unitarity

Extraction of Poles


[A. Rodas et al. (JPAC), Phys. Rev. Lett. 122, 042002 (2019)]

• only a single pole needed todescribe both ηπ and η′π peaks

• consistent with π1(1600)

Summary for 𝟑𝟑𝟏𝟏


• Resonant nature of signal in 𝐽𝐽𝑃𝑃𝑃𝑃 = 1− + established from COMPASS 3𝜋𝜋data

• Coupled-channel analysis for 𝜂𝜂𝜋𝜋 and 𝜂𝜂′𝜋𝜋 using a unitary model onlyrequires one single pole to describe P-wave peaks at 1.4 and 1.6 GeV

• Fit allows to extract pole position of lightest hybrid meson for first time

Conclusions

Hadron spectroscopy is entering precision eraExtremely large data samples with 𝜋𝜋 and 𝜇𝜇 beams from COMPASSVery small statistical uncertainties for dominating resonances systematic model uncertainties dominate multi-dimensional PWA in bins of 𝑚𝑚𝑋𝑋 and 𝑡𝑡′

14 new PDG entries (Jan. 2019) based on COMPASS data (3 w/ JPAC)Small signals and effects can be studied for the first timeSpin-exotic π1(1600): (re-) observed by COMPASS resonant nature established one single pole sufficient to describe peaks at 1.4 and 1.6 GeV

New axial vector signal observed in 𝑎𝑎1(1420) → 𝑓𝑓0(980)𝜋𝜋 has all features of a genuine resonance data can be described by triangle singularity

Light Exotic Hadrons at COMPASS 27B. Ketzer

Outlook


Strongly coupled QCD still far from being understood

Identify (exotic) multiplets and measure decay patterns

Need large data samples for

• complementary production mechanisms

• different final states

Advanced analysis methods

• simple BW fits may be misleading

• reaction models satisfying principles of S-matrix theory

Advances in Lattice QCD (multi-particle scattering states)

A new QCD facility is proposed at the M2 beamline of CERN SPS from 2022 onwards

COMPASS++ / AMBER

B. Ketzer

https://arxiv.org/abs/1808.00848

Proton radius measurement using muon-proton elastic scatteringHard exclusive reactions using a muon beam and a transversely polarised targetDrell-Yan and charmonium productionMeasurement of antiproton production cross sections for Dark Matter SearchSpectroscopy with low-energy antiprotonsSpectroscopy of kaonsStudy of the gluon distribution in the kaon via prompt-photon productionLow-energy tests of QCD using Primakoff reactionsProduction of vector mesons and excited kaons off nuclei


Kaon Beam: RF Separation

B. Ketzer

Panofsky-Schnell-System with two cavities (CERN 68-29)

• Particle species: same momenta but different velocities• Time-dependent transverse kick by RF cavities in dipole mode• RF1 kick compensated or amplified by RF2• Selection of particle species by selection of phase difference

∆Φ = 2π (L f / c) (β1-1 – β2-1)• For large momenta: β1-1 – β2-1 = (m12-m22)/2p2


Kaon Excitation Spectrum

B. Ketzer

• 25 kaon states listed by PDG (𝑀𝑀 < 3.1 GeV), 13 of those need confirmation• many predicted quark-model states still missing• some hints for supernumerary states


𝑲𝑲−𝟑𝟑−𝟑𝟑+ Final State

B. Ketzer

Study reaction 𝐾𝐾− + 𝑝𝑝 → 𝐾𝐾−𝜋𝜋−𝜋𝜋+ + 𝑝𝑝 by tagging beam kaons (2.4%)

⇒ access to all kaon states: 𝐾𝐾𝐽𝐽,𝐾𝐾𝐽𝐽∗

⇒ world‘s largest data set so far: 720 000 exclusive events


𝑲𝑲−𝟑𝟑−𝟑𝟑+ Final State - PWA


• 1+0+ 𝐾𝐾∗ 892 𝜋𝜋 𝑆𝑆 wave most dominant (~ 30% of total intensity)• 𝐾𝐾1(1270) and 𝐾𝐾1 1400 visible, but large syst. effects for 𝑚𝑚 < 1.5 GeV

(due to limited PID)• 𝐾𝐾1(1270) also visible in 1+0+ 𝜌𝜌𝐾𝐾 𝑆𝑆 wave, plus shoulder at ~1.6 GeV

(𝐾𝐾1 1650 ? )

𝑲𝑲−𝟑𝟑−𝟑𝟑+ Final State - PWA


• Broad peak and shoulder in 2−0+ 𝐾𝐾2∗ 1430 𝜋𝜋 𝑆𝑆 wave: 𝐾𝐾2(1770), 𝐾𝐾2(1820), 𝐾𝐾2 2250 ?

• Various structures in 2−0+𝐾𝐾∗ 892 𝜋𝜋 𝐹𝐹 and 2−0+ 𝜌𝜌𝐾𝐾 𝐹𝐹 waves• need PWA to clarify nature of observed signal

Goal for AMBER: collect 20 × 106 exclusive 𝐾𝐾−𝜋𝜋−𝜋𝜋+ events in one year

Light Exotic Hadrons�at COMPASSLight Meson SpectrumThe COMPASS ExperimentThe COMPASS ExperimentThe COMPASS Experiment𝟑𝝅 Final State - PWALight Meson SpectrumLight Meson SpectrumNew a1(1420)InterpretationsInterpretationsInterpretationsInterpretationsTriangle AmplitudeComparison TS - BWComparison TS - BWContribution of other GraphsContribution of other GraphsLight Meson SpectrumHybrids: Lattice QCD1−+ Partial Wavehp- / h’p- Final Stateshp- / h’p- Final StatesExtraction of PolesExtraction of PolesSummary for 𝝅 𝟏 ConclusionsOutlookCOMPASS++ / AMBERCOMPASS++ / AMBERKaon Beam: RF SeparationKaon Excitation Spectrum 𝑲 − 𝝅 − 𝝅 + Final State 𝑲 − 𝝅 − 𝝅 + Final State - PWA 𝑲 − 𝝅 − 𝝅 + Final State - PWA

light exotic hadrons at compass · 2020. 9. 30. · spin-exotic π 1 (1600): (re-) observed by...

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