Precision spectroscopy with COMPASS and the observation of a new iso-vector resonance

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DOI: 10.1051/epjconf/20147303002

C

Owned by the authors, published by EDP Sciences, 2014

Precision spectroscopy with COMPASS and the observation

of a new iso-vector resonance

Stephan Paul1,2,a _{on behalf of the COMPASS collaboration}

1_{Physik Department E18, Technische Universität München, James Franck Str., 85748 Garching,}

Germany

2_{Excellencecluster “Universe”, Boltzmannstr. 2, 85748 Garching, Germany}

Abstract.We report on the results of a novel partial-wave analysis based on 50·106_events

from the reaction−+p→−−++precoil at 190 GeV/cincoming beam momentum

using the COMPASS spectrometer. A separated analysis in bins ofm3and four-momentum

transfert reveals the interference of resonant and non-resonant particle production and

allows their spectral separation. Besides well known resonances we observe a new

iso-vector meson a1(1420) at a mass of 1420 MeV/c2 in the f0(980) final state only, the

origin of which is unclear. We have also examined the structure of the 0++-isobar in

theJP C=0−+, 1++, 2−+three pion waves. This clearly reveals the various 0++-isobar

components and its correlation to the decay of light mesons.

1. Introduction

Light meson spectroscopy has been performed for about 50 years using various tools and production mechanisms, each one possessing its own virtue and sensitivity to particular quantum numbers. Breakthroughs have either been obtained when new mechanisms were opened (e.g.pp annihilation [1] orJ/decays [2]) or when the wealth of data allowed new analysis tools to be developed (see e.g. [3]). The COMPASS experiment is a modern high-rate spectrometer which allows three different mechanisms to be probed, diffractive-, central- as well as quasi-real photo-production using the Primakoff reaction. Various beams are being used but in this work we focus on diffractive pion dissociation leading to a 3 final state. This reaction populates iso-vector states with all possible quantum numbersJP, but limited toC = +1. With 10 to100 times more events as compared to previous works we gain new sensitivity to states with very small production cross section, open up the possibility to study our reactions in terms oftas kinematic variable and take a look inside one of the prominent isobars, namely theS-wave.

2. Analysis scheme

The COMPASS experiment [4] is a two-stage magnetic spectrometer, covering a large solid angle, with precision vertexing and partial particle identification for both, incoming and outgoing particles. An incoming−-beam impinges on a 40 cm-longLH2-target surrounded by a two-layer proton recoil

a_e-mail:

[email protected]

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) 2 c System (GeV/ − π + π − π Mass of

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Figure 1. Left: correlation of the squared four-momentum transfert and the invariant mass of the 3-system

(zaxis in log scale). The partial-wave analysis is performed in 20 MeV/c2_{wide bins of}_m

3as well as in bins oft

indicated by the horizontal lines. Centre:m3for low values oft(0.10≤t≤0.12 (GeV/c)2). Right:m3for high

values oft(0.44≤t≤1.00 (GeV/c)2_).

) 2 c System (GeV/ − π + π − π Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

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2_c

0 0.05 0.1 0.15 0.2 0.25 6 10 × S π (770) ρ + 0 ++ 1 − 1 2 c / 2 0.113 GeV ≤ t' ≤ 0.100 40.45% (COMPASS 2008) p − π + π − π → p − π

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Figure 2.Intensities of four major waves in two differentt regions. Upper row: 0.100≤t≤0.113 (GeV/c)2_;

lower row: 0.449≤t≤0.724 (GeV/c)2_.

detector. The trigger requires a signal from this detector and vetoes events with a forward going particle being within the beam region, thus imposing a minimum value for the 4-momentum transfer

t_{= |}_t_{| − |}_tmin_{| ≈}_t _{of 0.08 (GeV}_/c₎2_{. The selection of exclusive 3}_{final states requires a matching of}

incoming and outgoing momenta as well as transverse momentum balance using the recoil proton. We are left with about 50·106_{events which cover a mass range up to about 3 GeV/c}2_{and extend to large} values oftabove 1 GeV/c)2_{, although the present analysis limits itself to 0.1}_≤_t_≤_{1.0 (GeV/c)}2_{. The} analysis follows the usual scheme of a two-step partial-wave analysis [5] where at first all events are subject to a series of fits in bins of m3of 20 MeV/c2 _{width and in 11 bins of} _t ₍_{mass-independent}

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S

π

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1 1−2++1+ρ(770) πD 1−2−+0+ f2(1270) π S 1−4++1+ρ(770) π G 1−1++0+ f0(980) π P 1−0−+ (980) πS

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

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Figure 3.Result of the mass-dependent fit of the spin-density matrix for six waves fort∈[0.189, 0.220] (GeV/c)2_. Rows and columns correspond to the waves depicted in the figure. Distributions on the diagonal show intensities of individual waves, relative phases for pairs of waves are depicted on the off-diagonal. The scheme for the

phase differences iscolumn−row. For better visibility we have shifted all phases such that they lie in the range

[−180◦_,₊₁₈₀◦_{]. The red line represents the result of the mass-dependent fit which is performed using all data points}

in dark blue. Unused data points are drawn in grey. Green: non-resonant contributions; blue: resonant contributions.

Both,a1anda2decaying intoexhibit signatures for excited states around 1.8–1.9 GeV/c2, once appearing as a

small peak, once as dip due destructive interference with a non-resonant contribution.

analysis are founded on the observations shown in Fig.1, depicting the 3-mass spectra at low and high values oftand the correlation ofm3andt, clearly demonstrating the need for a separation of these two variables. The power of this scheme, which can now be exploited in all its beauty for the first time (but had already been addressed by [3] and [6]) can also be derived from Fig.2, where we depict the spectral intensity of four different but characteristic partial waves, namely JP CM(isobar)L=1++0+S, 2++1+D, 4++1+Gand 2−+0+f2(1270)Sfor two very different intervals oft, dubbedlowand

hight. While we clearly observe the well established statesa2(1320) anda4(2040) with little change of spectral shape at differentt, the structures observed around thea1(1260) and2(1670) reveal underlying dynamics resulting in at-dependent shape which cannot be attributed solely to a resonance. As we will see, these issues will be resolved in the second step of our PWA using a mass-dependent fit.

3. Mass-dependent fits

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Non-resonant Term

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π (770) ρ + 0 + + 1 − 1

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Figure 4.1++0+Sintensities in three selected slices ofttogether with the model curve (red). Model components

are shown as colored curves: non-resonant contribution (green) anda1(1260) component (blue).

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used in the mass-dependent fit). Centre: relative phase of the 1++0+f0(980)P wave with respect to 1++1+Sin

the kinematic range 0.100≤t≤0.113 (GeV/c)2_{. The model curve is shown in red. Right: Phase relative to the}

4++1+Gwave.

matrices is described by a model which in turn contains assumptions on resonances (typically described by dynamic Breit-Wigner functions) and non-resonant contributions1_{. We have picked a sub-matrix} containing the following waves with= +1 andM=0 andM=1 for (a2anda4):

• 1++0+Swave: two resonant terms for thea1(1260) and ana₁ • 2++1+Dwave: two resonant terms for thea2(1320) and ana₂ • 2−+0+f2(1270)Swave: two resonant terms for the2(1670) and a₂ • 4++1+Gwave: one resonant term for thea4(2040)

• 1++0+f0(980)P wave: one resonant term

• 0−+0+f0(980)Swave: one resonant term for the(1800).

It has to be noted that the 0++-isobar is parametrized with the narrowf0(980) described by a Flatté distribution [7] and a broad structure, of which the spectral shape follows a parametrization by [8]. While production amplitudes and phases of all components can vary witht, resonance parameters do not. Also the shape of the non-resonant contributions has a predeterminedt-dependence1_{. An example} for the results of a fit in one bin oftis shown in Fig.3. In detail we show the result for two waves with JP C ₌₁++_{. Figure}₄_{depicts examples for the}_t_{-dependence of the}_{a1(1260) region. The broad structure}

is composed of thea1(1260) resonance interfering constructively (lowt) or destructively (hight) with a broad non-resonant component (likely caused by the Deck effect [9]). Thus thet-dependence allows for the first time to disentangle two components by their respective and very distinctt-dependence. The second wave uses the f0(980) as isobar which is considered as a complex object of likely molecular

1_{non-resonant contributions are described by two functions of the form} _F

NR,1(m,t)=(m−m0)c0e(c1+c2t+c3t

2₎_q2 for

1++, 2++, 2−+−wavesandFNR,2(m)=ec1q

2

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+ π − π [ + 0 + − 0 2 ) c 0.20 (GeV/ ≤ t' ≤ 0.10 COMPASS 2008 p − π + π − π → p − π

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+ π − π [ + 0 + + 1 2 ) c 0.20 (GeV/ ≤ t' ≤ 0.10 COMPASS 2008 p − π + π − π → p − π

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Figure 6. 3partial-wave and []_S -isobar correlation for 0−+0+[]_SS, 1++0+[]_SP and 2−+0+[]_SD

for lowt∈[0.1, 0.2] (GeV/c)2_{and high}_t_∈_{[0.2, 1.0] (GeV}_/c₎2_{. The central vertical lines indicate the}_m

3mass

interval used for the Argand diagrams depicted in Fig.7.

(A. U.) S π ρ + 0 + + Real Part relative to the 1 -6 -5 -4 -3 -2 -1 0 1 2

3 10 × (A. U.) S πρ +0 + +

Imaginary Part relative to the 1

-3 -2 -1 0 1 2 3 4 5 3 10 × 0.3 0.7 0.965 1.065 1.42 S − π * S ] + π − π [ + 0 + − 0 2 c 1.82 GeV/ ≤ π 3 m ≤ 1.80 2 ) c 1.00 (GeV/ ≤ t' ≤ 0.20 COMPASS 2008 p − π + π − π → p − π

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0.7 (A. U.) S π ρ + 0 + + Real Part relative to the 1 0 2 4 6 8 10 12 14 16 18 20

3 10 × (A. U.) S πρ +0 + +

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0 (A. U.) S π ρ + 0 + + Real Part relative to the 1 -12 -10 -8 -6 -4 -2 0 2 4 6 8

3 10 × (A. U.) S πρ +0 + +

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P

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Figure 7.Argand diagrams for +− 0++-isobar extracted for m3 at the (1800), a1(1420) and 2(1880)

resonances for hight∈[0.2, 1.0] (GeV/c)2_{(see white lines in Fig.}₆_).

structure [10] coupling to both, andKK. Figure 5depicts the spectral function integrated over t as well as the relative phase motion w.r.t. two different waves. The data exhibit a strong enhancement at a mass around 1420 MeV/c2 _{connected with phase variation of almost 180}◦_{. No such object has}

previously been observed.

4. Analysis of

S-wave structure

(6)

2(1880). It should be noted that the phases are measured w.r.t. the 1++0+S wave and contain the sum of both, production and decay phases. The strong dependence of the distributions in Fig.6ontis striking and reveals that much of the structure observed in the unresolved distributions is due to non-resonant processes with steeply fallingt dependence. Thus we clearly disentangle resonant and non-resonant components and identify the dominant role of the iso-scalar resonancesf0(980) andf0(1500) in the decay process of the three 3-states (pair of white lines in Fig.7). The resonance structure is also reflected in the circles observed in the Argand diagrams. Here, the absence of phase motion in the-System related toa1(1420) is very distinct (no circle in the Argand diagram visible) and hints to production and decay phase having opposite sign and similar magnitude. We thus conclude that the a1(1420) coupling tof0(980) is genuine and not an artifact of the isobar parametrization and no strong coupling to the broad component inS-wave channel can be observed. More details, however, require a mass-dependent fit relating the+−0++mass and phase spectra to the 3systems.

5. Conclusions

Using the worlds largest data set on diffractive pion dissociation we have developed new analysis tools. This allows to disentangle resonant and non-resonant processes contributing to multi-particle production using the reaction−+p→−−++precoil. We have extracted thea1(1260) resonance and observed hints for excited states of thea1anda2around 1800−1900 MeV/c2_{. In particular we have observed a} new iso-vector mesona1(1420) decaying intof0(980)with no clear signs of concurring decay modes. A full phase motion of 180◦characteristic to resonance production has been observed. The origin of this object with a width of about 140 MeV/c2_{is yet unclear. The similarity in mass and width to the}_f1(1420) strongly coupling to KK∗ is striking. At present we cannot exclude re-scattering effects involving a coupling ofKK∗andf0(980)as outlined in [11]. A detailed coupled channel analysis including the study of theKK final state would be necessary. In addition we have studied the iso-scalar structure of theisobar in the decay ofJP C =0−+, 1++, 2−+. A clear coupling ofa1(1420) tof0(980) and of

(1800) and2(1880) to both,f0(980) andf0(1500), has been observed in addition to a broadS-wave component, exhibiting a strongtdependence.

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