The Exotic η′π− Wave in 190 GeV π

−p → η′π−p

at COMPASS

Tobias Schluterfor the COMPASS collaboration

Ludwig-Maximilians-Universitat Munchen

June 16, 2011

πη′ in diffractive scattering

Possible quantum numbers for the πη′ system:

L S-wave P-wave D-wave F -wave G -wave · · ·JPC 0++ 1−+ 2++ 3−+ 4++ · · ·

Hence: P-wave resonant → exotic meson.

This system has been studied by the following experiments:

experiment beam momentum reaction year publishedVES 37 GeV/c π−N → η′π−N 1993, 2005E852 18 GeV/c π−p → η′π−p 2001

They all see a very strong P-wave.

πη′ in diffractive scattering

Possible quantum numbers for the πη′ system:

L S-wave P-wave D-wave F -wave G -wave · · ·JPC 0++ 1−+ 2++ 3−+ 4++ · · ·

Hence: P-wave resonant → exotic meson.

This system has been studied by the following experiments:

experiment beam momentum reaction year publishedVES 37 GeV/c π−N → η′π−N 1993, 2005E852 18 GeV/c π−p → η′π−p 2001

COMPASS 190 GeV/c π−p → η′π−p 2012 (?)

They all see a very strong P-wave.

Previous πη′ results – VES

Results from VES (Be target, 37 GeV):

◮ VES sees the a2(1320) (peak in D+-wave)

◮ VES says: “there may be an a2(1700)”explaining the broad structure in theD+-wave

◮ VES says: “there may be an exoticπ1(1600)”

Note the jump in the relative P+ − D+ phasenear 2 GeV

Previous πη′ results – BNL E852

Results from BNL E852 (proton target, 18 GeV):

◮ they see the a2(1320) (peak in D+-wave)

◮ they add a G+-wave to the fit, gives: a4(2040)

◮ they explain the broad D+-wave with an a2(1700) and the P+-wavewith an exotic resonance π1(1600)

◮ they find an unusual t-slope

Note the various jumps at 2 GeV

Remarks on PWA formalism

For a given mass, two-body states in the reflectivity basis (ǫ, ℓ, m) havethe form (θ, φ Gottfried-Jackson angles)

Aǫlm ∝ Y m

ℓ (θ, 0)

{

sin(mφ) ǫ = +1 (m > 0)

cos(mφ) ǫ = −1 (m ≥ 0)

The observed intensity is then proportional to

I (θ, φ) =

∣

∣

∣

∣

∣

∑

lm

T+1,ℓmA+1,ℓm(θ, φ)

∣

∣

∣

∣

∣

2

+

∣

∣

∣

∣

∣

∑

ℓm

T−1,ℓmA−1,ℓm(θ, φ)

∣

∣

∣

∣

∣

2

where the production amplitudes Tǫℓm were introduced.Important observations: only negative reflectivity (= unnatural exchange)contributes to intensity at φ = 0; all positive reflectivity waves withm = 1 have the same φ dependency. Negative reflectivity compatiblewith zero in VES, BNL, COMPASS analyses.

The COMPASS spectrometer

◮ fixed target experiment at CERN’s SPS accelerator

◮ variety of beams available (pos/neg muon, pos/neg hadron)

◮ variety of targets (polarized targets, LH2, solid state)

◮ diverse physics program

◮ 2008, 2009 : hadron beam runs with various targets

In this talk: 2008 data, negative pion beam at 191 GeV, LH2 target

Data selection

Final state selected: exclusive 3 tracks, 2 photons

)γγm(0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

entr

ies

/ 0.5

MeV

0

20

40

60

80

100

310×COMPASS 2008

pγγ+π-π-π →p -π

preliminary

) [GeV]η+π-πm(1 1.5 2 2.5 3 3.5 4 4.5

entr

ies

[MeV

] (2

entr

ies

per

even

t)

0

2

4

6

8

10

12

310×COMPASS 2008

pη+π-π-π →p -πw/o acceptance correction

preliminary

◮ select exclusive events with 3 tracks + recoil proton, 2 good ECALclusters

◮ select η → 2γ (left)

◮ select η′ → π−π+η (right)

Data selection

Final state selected: exclusive 3 tracks, 2 photons

)γγm(0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

entr

ies

/ 0.5

MeV

0

20

40

60

80

100

310×COMPASS 2008

pγγ+π-π-π →p -π

preliminary

) [GeV]η+π-πm(1 1.5 2 2.5 3 3.5 4 4.5

entr

ies

[MeV

] (2

entr

ies

per

even

t)

0

2

4

6

8

10

12

310×COMPASS 2008

pη+π-π-π →p -πw/o acceptance correction

preliminary

’) [GeV]ηπm(1.5 2 2.5 3 3.5 4 4.5 5

entr

ies

/ 20

MeV

0

100

200

300

400

500

600COMPASS 2008

)pγγ+π-π’(η-π →p -πw/o acceptance correction

preliminary

Result:

◮ 18 000 events withm(η′π) < 2 GeV/c2, 35 000total

◮ mass reach beyond 2 GeV/c2

◮ additionally, about 3 000 eventsin πη′, η → 3π channel

First look at the data: t slopes

We find indication for a continuous transition between differentproduction mechanisms, fitting the t distribution in several areas

mass bin fit with A exp(−B|t|) fit with A|t| exp(−B|t|)m < 1.5 5.5 8.2

1.5 < m < 1.9 5.1 7.51.9 < m < 2.2 4.8 7.12.2 < m < 3 4.6 6.9

(BNL fitted with a simple exponential between 0.25 < |t| < 1.0 GeV/c2,they found B = 2.93/GeV

2)We find: higher mass → broader slopeand: clear contradicition with BNL

Input to the PWA

Like previous analyses, we used all waves with ℓ ≤ 2, m ≤ 1 andadditionally the ǫ = +1, ℓ = 4, m = 1. I.e.:

ǫ = +1 P+ D+ G+

ǫ = −1 S0 P0 P− D0 D−

As expected from Pomeron (i.e., natural) exchange, the negativereflectivity waves turn out compatible with zero (below 2 GeV).Ambiguities are bounded by the size of the negative waves, i.e. they arenot a problem.

PWA results – P+ and G+ waves

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

2 e

vent

s / 4

0 M

eV/c

0

1000

2000

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5000

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7000

8000

preliminary

COMPASS 2008’pη-π →p -π

) intensity-+ = 1PC (J+P

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

) [d

eg]

+ /

P+

arg

(D

-180-160-140-120-100-80-60-40-20

020

preliminary

COMPASS 2008’pη-π →p -π

+ - P+

phase D

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

2 e

vent

s / 4

0 M

eV/c

0

1000

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preliminary

COMPASS 2008’pη-π →p -π

) intensity++ = 2PC (J+D

Intensity P+ Phase D+ − P+

Intensity D+

Clear phase-motion from a2(1320), jump in phase near 2 GeV, slowphase-motion in range of P+-wave intensity peak.

PWA results – can the G+-wave clarify the picture?

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

2 e

vent

s / 4

0 M

eV/c

0

1000

2000

3000

4000

5000

6000

7000

8000

preliminary

COMPASS 2008’pη-π →p -π

) intensity-+ = 1PC (J+P

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

) [d

eg]

+ /

P+

arg

(D

-180-160-140-120-100-80-60-40-20

020

preliminary

COMPASS 2008’pη-π →p -π

+ - P+

phase D

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

2 e

vent

s / 4

0 M

eV/c

0

1000

2000

3000

4000

5000

6000

preliminary

COMPASS 2008’pη-π →p -π

) intensity++ = 2PC (J+D

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

) [d

eg]

+ /

P+

arg

(G

300

400

500

600

700

800

preliminary

COMPASS 2008’pη-π →p -π

+ - P+

phase G

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

) [d

eg]

+ /

D+

arg

(G

50

100

150

200

250

300

350

400

preliminary

COMPASS 2008’pη-π →p -π

+ - D+

phase G

m [GeV]1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

2 e

vent

s / 4

0 M

eV/c

0

200

400

600

800

1000

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1400

1600

preliminary

COMPASS 2008’pη-π →p -π

) intensity++ = 4PC (J+G

Int. P+

∆Φ(D+ − P+) ∆Φ(G+ − P+)

Int. D+

∆Φ(G+ − D+)

Int. G+

Clear phase-motion in G+-wave relative to D+ wave, compatible witha4(2040). Again: jump at 2 GeV in phase relative to P+ wave. But:unlike between P+ and G+ no rapid phase jump between D+ and G+

waves at 2 GeV

Transition between different production processes?

m [GeV]1.5 2 2.5 3 3.5 4 4.5 5

GJ

θco

s

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0

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COMPASS 2008’pη-π →p -π

w/o acceptance correction

preliminary

Depicted: cos θGJ of the η′ in the π−η′ GJ restframe vs. m(πη′).Low masses show P and D wave interference, a4 near 2 GeV/c2, abovethat strong forward/backward peaking indicative of central production.Question: How does the forward/backward peaking at high masses affectthe interpretation at low masses?

Comparison to ηπ

We also selected the ηπ− final state along the same lines. No PWA yet,for comparison, here’s the same plot as on the previous slide, but for theπη:

m [GeV]1 1.5 2 2.5 3 3.5 4 4.5

GJ

θco

s

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

COMPASS 2008pη-π →p -π

w/o acceptance correction

preliminary

Depicted: cos θGJ of the η in the π−η GJ restframe vs. m(πη).Dominated by a2(1320), structures due to a4(2040) visible, againforward/backward peaking at high masses.

Conclusions

◮ COMPASS can confirm previous observations of a strong P-wave inη′π and in addition finds confirming evidence for thea4(2040) → π−η′

◮ the t distribution shows a decreasing slope with increasing m(η′π−)and the slope disagrees with the findings of E852

◮ resonant interpretation of the P-wave cannot be confirmed (at thispoint)

The road ahead:

◮ Primary Objective: clarify what happens in the transition betweenthe regimes below and above ≈ 2 GeV/c

2

◮ Secondary Objective: use this to gain clearer understanding of thenature of π−η′ P-wave

Thank you!