18.08.2004 Yu. Guz

Study of the reaction π−p → η’π0n

at the VES spectrometer.

Yu. Guz

(VES collaboration*)

VES collaboration: D. Amelin, Yu. Gavrilov, Yu. Guz, V. Dorofeev, R. Dzhelyadin, A. Ivashin, I. Kachaev, V. Kabachenko, A. Karyukhin, Yu. Khokhlov, A. Konoplyannikov, V. Konstantinov, V. Kostyuhin, V. Matveev, V. Nikolaenko, A. Ostankov, B. Polyakov, D. Ryabchikov, A. Solodkov, O. Solovianov, E. Starchenko, A. Zaitsev, A. Zenin.

*

18.08.2004 Yu. Guz

• The final states ηπ and η’π are mainly interesting for the P-waves, which have exotic quantum numbers JPC=1−+ . They can be produced in π− beam in diffraction-like (η π−, η’π−) and charge exchange (η π0, η’π0) reactions. Three of them were studied before:

• π– A → ηπ–A: (VES, E852) a broad bump in the P+ wave (positive exchange naturality); allows resonance interpretation: π1

–(1400).

• π– A → ηπ0A: (GAMS/NA12, E852) a broad bump of significant intensity in the P+ wave; resonance interpretation is problematic.

• π– A → η’π–A: (VES, E852) the P+ wave is dominant, contains a broad bump in the P+ wave; resonance interpretation is possible: π1

–(1600).

18.08.2004 Yu. Guz

• The P-wave states in η8π and η0π are essentially different: in η0π it is SU(3)f-

octet, while in η8π it can belong only to higher SU(3) representation,

Therefore the π1(1600) is SU(3)f –octet state (can be a hybrid), and π1(1400) is

not. (S.U.Chung, E.Klempt, J.G.Körner, Eur.Phys.J 2002).

• in the reaction π– p → η’π0 n the π10(1600) production in charge exchange

process can be studied.

• The events of the reaction π– p → η’π0 n were selected in the decay mode η’ → π+π–η, η → γγ, π0 → γγ.

• The reaction π– p → ηπ0 n was also studied, for normalizations and comparisons, in the decay mode η → π+π–π0, π0 → γγ.

10 10

18.08.2004 Yu. Guz

The VES setup

VES

Works on secondary π− beam (25-45 GeV) of the U-70 accelerator (IHEP Protvino)

Measures charged particles and photons in final state:

π–(K–) A → n π(K) + m γ A’

18.08.2004 Yu. Guz

η’π0: event selection

Selection criteria:

• 2 tracks, for π+ and π–, and 4 photons;

• the tracks are not electrons: |p|/Ecal<0.8

• 0.105<M(γ1γ2)<0.165 MeV/c2 (π0) ;

• 0.478< M(γ3γ4)<0.618 MeV/c2 (η) ;

• the 4γ should not satisfy the π0π0 hypothesis;

• 0.940<M(π+π–η)<0.976 MeV/c2 (η’) ;

• the side bands around η’ were used for the background evaluation and subtraction

• 1955 “signal” and 809 “background” events were selected for the analysis.

18.08.2004 Yu. Guz

η’π0 : distributions

• The a2(1320) peak is seen in the η’π0 mass spectrum

• The t-distribution peaks at 0

18.08.2004 Yu. Guz

ηπ0 : event selection

Selection criteria:

• 2 tracks, for π+ and π–, and 4 photons;

• the tracks are not electrons: |p|/Ecal<0.8

• 0.105<M(γ1γ2), M(γ3γ4)<0.165 MeV/c2 (π0) ;

• 0.528<M(π+π–π0)<0.568 MeV/c2 (η) ;

• the side bands around η were used for the background evaluation and subtraction

• 18547 “signal” and 8436 “background” events were selected for the analysis.

18.08.2004 Yu. Guz

ηπ0: distributions

• a0(980) and a2(1320) are seen in the mass spectrum

• asymmetry in the cosθGJ distribution

• The φTY distribution shows significant fraction of positive exchange naturality waves

• Agrees with previous studies of the ηπ0

system *.

• D.Alde et al (GAMS collaboration), Phys. At. Nucl. Vol. 62 No. 3 (1999), p 421 ;

• A.R.Dzierba et al, Phys. Rev. D67, 094015 (2003)

*

18.08.2004 Yu. Guz

a2(1320) in ηπ0 and η’π0

0 020 02

( ' )0.047 0.018

( )BR a

RBR a

– agrees with previous measurements (VES, E852)

The mass distributions are acceptance corrected

18.08.2004 Yu. Guz

ηπ0 : PWA

The PWA of the ηπ0 system, with S0, P0, P-, D0, D- waves in the unnatural parity exchange sector and P+ and D+ in the natural parity exchange sector, was performed.

In agreement with results of similar analysis of GAMS and E852:

a0(980) in the S0 wave

a2(1320) in the D+ wave

broad bump in P+

Arg(D+/P+): only phase variation corresponding to the a2(1320) production

18.08.2004 Yu. Guz

Comparison of the mass spectra of η’π0 and η’π−

η’π– : dominated by the π1(1600) production, a2(1320) is less

η’π0 : π1(1600) is absent, only a2(1320) is visible

π1(1600)

a2(1320)

18.08.2004 Yu. Guz

η’π– : PWA

G.M.Beladidze et al (VES collaboration), Phys.Lett.B313(1993) 276

The exotic wave is dominant in the natural parity exchange sector (P+ wave). It forms a bump at ~1.6 GeV/c2, π1(1600). It is most intensive in the mass range above a2(1320), at 1.45<M<1.9 GeV/c2.

π1(1600)

18.08.2004 Yu. Guz

ηπ0 at 1.45<M<1.9 GeV/c2

These spectra show that significant P+ is present and positive exchange naturality waves dominate:

• significant asymmetry in cosθGJ

• φTY distribution is almost sin2

JPC=1-+ is produced mainly in the P+ wave (ρ-exchange)

18.08.2004 Yu. Guz

η’π0 at 1.45<M<1.9 GeV/c2

• The φTY spectrum is almost flat;

• some asymmetry in cosθGJ

JPC=1-+ is not produced in the P+ wave (π1(1600) not coupled to ρπ)

18.08.2004 Yu. Guz

The PWA of both ηπ0 and η’π0 was performed for events in the whole interval of 1.45…1.9 GeV : no rapid phase variations expected. The waves S0, P0, P-, D0, D-, P+, D+were included into the analysis.

PWA in “large bin” (1.45…1.9 GeV): η’π0

Here η’π0 is shown.

• The S0, P0 and D0 waves are significant

• P+ and D+ are small

18.08.2004 Yu. Guz

PWA in “large bin” (1.45…1.9 GeV): ηπ0

In ηπ0 the P+ and D+ are dominant.

18.08.2004 Yu. Guz

0

0

2'

20.1 0.1P

P

T

T

The ratio of matrix elements squared for the P+ production in ηπ0 and η’π0 at 1.45...1.9 GeV:

P+ wave in η(η’)π0 systems is mainly η8π, i.e. not octet state

in ηπ– / η’π– it is opposite: the P+ wave is mainly octet (G.M.Beladidze et al, 1993)

1-+ in ηπ0 and η’π0

18.08.2004 Yu. Guz

1-+ production in ρ-exchange

The a2(1320) gives main contribution to the D+ wave in ηπ0; let us suppose that the P+ wave in η’π0 contains only π1(1600). Both are produced via ρ-exchange. Comparing their production intensities, we can conclude that the coupling of π1(1600) to ρπ is weak.

In the assumption of the same dependence of the couplings of a2 and π1 to ρπ on the ρ virtuality:

01 2

0 12

2' 1 1

22 2

'P

a

Da

I m m BR BR

m BR a BR aI m

which gives

31 1 ' 3 10BR BR

18.08.2004 Yu. Guz

π1(1600): known decay modes

π1(1600) was seen in η’π, f1(1285)π and b1(1235)π,

with relative intensities of approx. 1 : 1.1±0.3 : 1.0±0.3.

It was not seen in f2π and K*K.

The η’π decay mode is big: certainly BR(π1(1600) → η’π) > 0.1

This gives BR(π1(1600) → ρπ) < 0.03

The suppression of the decay into ρπ is predicted in many models for 1−+ hybrids does not contradict to the hypothesis that π1(1600) is a hybrid.

18.08.2004 Yu. Guz

Direct search of π1(1600) → ρπ

π+π−π− total intensity

1−1+ ρπ, tight model

1−1+ ρπ, loose model

- the results are ambiguous:

π1(1600) peak was observed in ρπ in the PWA of the reaction π– A → π+π–π–A, in the wave JPMη = 1−1+

Its intensity depends on the PWA model: it is high when the waves are assumed coherent, and is much lower when the coherency is not assumed. (mainly leakage from π2(1670) ?)

Even in the latter case, the fit gives for π1(1600) ρπ : η’π ≈ 1.5±0.5 : 1 (A.Zaitsev, contribution to HADRON 97)

Possibilities for the bump in 1−1+ ρπ “loose model” : still mainly leakage a physical object different from π1(1600) of η’π– or π1(1600) branchings are not constant (this is not a true resonance)

18.08.2004 Yu. Guz

Conclusion

The reactions π−p → η’π0 n and π−p → ηπ0 n were studied at the VES experiment.

• The ratio of branchings for the a20 meson was determined: Ra2=0.047±0.018, in agreement

with measurements in the η’π– and ηπ– final states.

• The P waves were studied in η’π0 and ηπ0 systems in the region of π1(1600) meson, 1.45…

1.9 GeV. It was found that in this mass interval in the η’π0 system the P+ wave production is small, and, in particular, much smaller than in the ηπ0 system. This means that π1(1600) is not produced in ρ-exchange. The model-dependent constraint is obtained: BR(π1(1600) → ρπ) < 0.03.