Precision spectroscopy with COMPASS and the observation of a new iso-vector resonance
PPrecision spectroscopy with COMPASS and the observation of a new iso-vector resonance
Stephan Paul on behalf of the COMPASS collaboration
Physik Department E18, Technische Universität München,James Franck Str., D-85748andExcellencecluster "Universe", Boltzmannstr. 2, D-85748 GarchingNovember 14, 2018
Abstract
We report on the results of a novel partial-wave analysis based on · events from the reaction π − + p → π − π − π + + p recoil at 190GeV /c incoming beam momentum using the COMPASS spectrometer.A separated analysis in bins of m π and four-momentum transfer t (cid:48) reveals the interference of resonant and non-resonant particle productionand allows their spectral separation. Besides well known resonanceswe observe a new iso-vector meson a (1420) at a mass of 1420 MeV /c in the f (980) π final state only, the origin of which is unclear. Wehave also examined the structure of the ++ ππ -isobar in the J P C =0 − + , ++ , − + three pion waves. This clearly reveals the various ++ ππ -isobar components and its correlation to the decay of light mesons. Light meson spectroscopy has been performed for about 50 years using varioustools and production mechanisms, each one possessing its own virtue andsensitivity to particular quantum numbers. Breakthroughs have either beenobtained when new mechanisms were opened (e.g. pp annihilation [1] or J/ψ decays [2]) or when the wealth of data allowed new analysis tools to be1 a r X i v : . [ h e p - e x ] D ec eveloped (see e.g. [3]). The COMPASS experiment is a modern high-ratespectrometer 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 piondissociation leading to a π final state. This reaction populates iso-vectorstates with all possible quantum numbers J P , but limited to C = +1 . With10 to100 times more events as compared to previous works we gain newsensitivity to states with very small production cross section, open up thepossibility to study our reactions in terms of t (cid:48) as kinematic variable and takea look inside one of the prominent isobars, namely the ππ S-wave.
The COMPASS experiment [4] is a two-stage magnetic spectrometer, coveringa large solid angle, with precision vertexing and partial particle identificationfor both, incoming and outgoing particles. An incoming π − -beam impingeson a cm-long LH -target surrounded by a two-layer proton recoil detector.The trigger requires a signal from this detector and vetoes events with aforward going particle being within the beam region, thus imposing a minimumvalue for the 4-momentum transfer t (cid:48) = | t | − | t min | ≈ t of 0.08 ( GeV /c ) . Theselection of exclusive π final states requires a matching of incoming andoutgoing momenta as well as transverse momentum balance using the recoilproton. We are left with about · events which cover a mass range up toabout 3 GeV /c and extend to large values of t (cid:48) above GeV /c ) , althoughthe present analysis limits itself to . ≤ t (cid:48) ≤ . GeV /c ) . The analysisfollows the usual scheme of a two-step partial-wave analysis [5] where at firstall events are subject to a series of fits in bins of m π of 20 MeV /c width andin 11 bins of t (cid:48) ( mass-independent fit ), chosen to equal statistics for each t (cid:48) -bin.The PWA model is based on a sequential two-step decay into a π + π − -isobarand a bachelor π − with subsequent decay of the isobar. The population ofthe 5-dimensional phase space is modeled with a set of 88 waves each beingcharacterized by an assumed J P C of the π -state, the properties of the isobar,angular momentum L enclosed by π − and isobar as well as magnetic quantumnumber M and reflectivity ε , which is connected to the naturality of theexchange. The model allows for seven waves with ε = − and one flat-wave representing pure phase space. Both sets add incoherently to the coherentsum of 80 waves with ε = +1 . Only waves with negligible population havebeen omitted from the fits presented here. The basic assumptions of thispartial-wave analysis are founded on the observations shown in fig. 1, depictingthe π -mass spectra at low and high values of t (cid:48) and the correlation of m π c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c / ( G e V t ' S qu a r e d F ou r- M o m e n t u m T r a n s f e r (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ + p - p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y Figure 1:
Left: Correlation of the squared four-momentum transfer t (cid:48) andthe invariant mass of the π -system ( z axis in log scale). The partial-waveanalysis is performed in 20 MeV /c wide bins of m π as well as in bins of t (cid:48) indicated by the horizontal lines. Centre: m π for low values of t (cid:48) ( . ≤ t (cid:48) ≤ .
12 (
GeV /c ) ). Right: m π for high values of t (cid:48) ( . ≤ t (cid:48) ≤ .
00 (
GeV /c ) ).and t (cid:48) , 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 forthe first time (but had already been addressed by [3] and [6]) can also bederived from fig. 2, where we depict the spectral intensity of four differentbut characteristic partial waves, namely J P C M ε ( isobar ) πL = 1 ++ + ρπS , ++ + ρπD , ++ + ρπG and − + + f (1270) πS for two very different intervalsof t (cid:48) , dubbed low and high t (cid:48) . While we clearly observe the well establishedstates a (1320) and a (2040) with little change of spectral shape at different t (cid:48) ,the structures observed around the a (1260) and π (1670) reveal underlyingdynamics resulting in a t (cid:48) -dependent shape which cannot be attributed solelyto a resonance. As we will see, these issues will be resolved in the second stepof our PWA using a mass-dependent fit. The main result of the aforementioned mass-independent fit is a spin-densitymatrix of size x for each of the 11 bins in t (cid:48) and 100 bins in mass between0.5 and 2.5 GeV /c . In a second step this set of matrices is described by amodel which in turn contains assumptions on resonances (typically describedby dynamic Breit-Wigner functions) and non-resonant contributions . Wehave picked a sub-matrix containing the following waves with ε = +1 and non-resonant contributions are described by two functions of the form F NR , ( m, t (cid:48) ) =( m − m ) c e ( c + c t + c t ) q for ++ , ++ , − + − waves and F NR , ( m ) = e c q for all otherwaves. c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · S p (770) r + ++ - c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · D p (770) r + ++ - c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · S p (1270) f + +- - c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · G p (770) r + ++ - c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · S p (770) r + ++ - c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · D p (770) r + ++ - c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · S p (1270) f + +- - c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) c N u m b e r o f E v e n t s / ( M e V / · G p (770) r + ++ - c / £ t' £ (COMPASS 2008) p - p + p - p fi p - p P r e li m i n a r y Figure 2:
Intensities of four major waves in two different t (cid:48) regions. Upperrow: . ≤ t (cid:48) ≤ .
113 (
GeV /c ) ; lower row: . ≤ t (cid:48) ≤ .
724 (
GeV /c ) . M = 0 and M = 1 for ( a and a ): • ++ + ρπS wave: two resonant terms for the a (1260) and an a (cid:48) • ++ + ρπD wave: two resonant terms for the a (1320) and an a (cid:48) • − + + f (1270) πS wave: two resonant terms for the π (1670) and a π (cid:48) • ++ + ρπG wave: one resonant term for the a (2040) • ++ + f (980) πP wave: one resonant term • − + + f (980) πS wave: one resonant term for the π (1800) It has to be noted that the ++ -isobar is parametrized with the narrow f (980) described by a Flatté distribution [7] and a broad structure, of which thespectral shape follows a parametrization by [8]. While production amplitudesand phases of all components can vary with t (cid:48) , resonance parameters donot. Also the shape of the non-resonant contributions has a predetermined t (cid:48) -dependence . An example for the results of a fit in one bin of t (cid:48) is shownin fig. 3. In detail we show the result for two waves with J P C = 1 ++ . Fig.4 depicts examples for the t (cid:48) -dependence of the a (1260) region. The broadstructure is composed of the a (1260) resonance interfering constructively(low t (cid:48) ) or destructively (high t (cid:48) ) with a broad non-resonant component (likelycaused by the Deck effect [9]). Thus the t (cid:48) -dependence allows for the firsttime to disentangle two components by their respective and very distinct t (cid:48) -dependence. The second wave uses the f (980) as isobar which is considered4 .6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -1
10 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -1
10 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -1
10 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -1
10 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -1
10 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -150-100-500501001500.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 -1 S p (770) r + ++ -
1 D p (770) r + ++ -
1 S p (1270) f + +- -
1 G p (770) r + ++ -
1 P p (980) f + ++ -
1 S p (980) f + +- - ) c N u m be r o f E v en t s / ( M e V / -1 -1 -1 -1 -1 -1 ) c System (GeV/ - p + p - p Mass of C en t e r ed P ha s e ( deg ) -150-100-50050100150-150-100-50050100150-150-100-50050100150-150-100-50050100150-150-100-50050100150 p (COMPASS 2008) - p + p - p fi p - p c / £ t' £ P r e li m i n a r y Figure 3:
Result of the mass-dependent fit of the spin-density matrix for sixwaves for t (cid:48) ∈ [0 . , . GeV /c ) . Rows and columns correspond to thewaves depicted in the figure. Distributions on the diagonal show intensitiesof individual waves, relative phases for pairs of waves are depicted on theoff-diagonal. The scheme for the phase differences is φ column − φ row . Forbetter visibility we have shifted all phases such that they lie in the range [ − ◦ , +180 ◦ ] . The red line represents the result of the mass-dependentfit which is performed using all data points in dark blue. Unused datapoints are drawn in grey. Green: non-resonant contributions; Blue: resonantcontributions. Both, a and a decaying into ρπ exhibit signatures for excitedstates around 1.8-1.9 GeV /c , once appearing as a small peak, once as dipdue destructive interference with a non-resonant contribution.as a complex object of likely molecular structure [10] coupling to both, ππ and KK . Fig. 5 depicts the spectral function integrated over t (cid:48) as well as therelative phase motion w.r.t. two different waves. The data exhibit a strongenhancement at a mass around 1420 MeV /c connected with phase variationof almost ◦ . No such object has previously been observed.5 c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ Mass-independent FitMass-dependent FitResonancesNon-resonant Term p (COMPASS 2008) - p + p - p fi p - p S p (770) r + ++ - P r e li m i n a r y P r e li m i n a r y P r e li m i n a r y Figure 4: ++ + ρπS intensities in three selected slices of t (cid:48) together withthe model curve (red). Model components are shown as colored curves:non-resonant contribution (green) and a (1260) component (blue). ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ Mass of ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · c / £ t' £ System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 ) c N u m be r o f E v en t s / ( M e V / / £ t' £ ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 ) c N u m be r o f E v en t s / ( M e V / · Incoherent Sum
Mass-independent FitMass-dependen (cid:87)
FitResonancesNon-resonant Terms p (COMPASS 2008) - p + p - p fi p - p P p (980) f + ++ - P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 P ha s e ( deg ) -200-150-100-50050100 p (COMPASS 2008) - p + p - p fi p - p c / £ t' £ S)] p (770) r + ++ - P) - (1 p (980) f + ++ - Phase [(1 P r e li m i n a r y ) c System (GeV/ - p + p - p Mass of 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 P ha s e ( deg ) -150-100-50050100150 p (COMPASS 2008) - p + p - p fi p - p c / £ t' £ G)] p (770) r + ++ - P) - (1 p (980) f + ++ - Phase [(1 P r e li m i n a r y Figure 5:
Left: mass spectrum for the ++ + f (980) πP wave (Light-greycoloured data points have not been not used in the mass-dependent fit). Centre:relative phase of the ++ + f (980) πP wave with respect to ++ + ρπS in thekinematic range . ≤ t (cid:48) ≤ .
113 (
GeV /c ) . The model curve is shown inred. Right: Phase relative to the ++ + ρπG wave. ππ S-wave Structure
In order to support the parametrization of the ππ S-wave structure by ourmodel we have developed an alternative PWA-method in which we omit apredetermined parametrization and thus "de-isobar" our J P C M ε ++ π wave.This is done for each bin in m π by replacing the functional isobar descriptiondiscussed above by a series of step-functions, each defined over a small massbin of 40 MeV /c , over a mass range of [2 m π , m π − m π ] . The binning is10 MeV /c around the mass range of the f (980) . As we steeply increasethe effective number of isobars this fit has been performed in two bins of t (cid:48) only. The result is depicted in fig. 6 for three partial-waves, − + , ++ and − + for the π -system. In addition to the mass spectra we also extractthe Argand diagrams which are depicted in fig. 7 for t (cid:48) ∈ [0 . , .
0] (
GeV /c ) ,in the mass region of the resonances observed, namely π (1800) , a (1420) and π (1880) . It should be noted that the phases are measured w.r.t. the6 c (GeV/ - p + p - p Mass of 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 ) c ( G e V / + p - p M a ss o f S - p * S ] + p - p [ + +- ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y ) c (GeV/ - p + p - p Mass of 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 ) c ( G e V / + p - p M a ss o f P - p * S ] + p - p [ + ++ ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y ) c (GeV/ - p + p - p Mass of 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 ) c ( G e V / + p - p M a ss o f D - p * S ] + p - p [ + +- ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y ) c (GeV/ - p + p - p Mass of 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 ) c ( G e V / + p - p M a ss o f S - p * S ] + p - p [ + +- ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y ) c (GeV/ - p + p - p Mass of 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 ) c ( G e V / + p - p M a ss o f P - p * S ] + p - p [ + ++ ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y ) c (GeV/ - p + p - p Mass of 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 ) c ( G e V / + p - p M a ss o f D - p * S ] + p - p [ + +- ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y Figure 6: π partial-wave and [ ππ ] S -isobar correlation for − + + [ ππ ] S πS , ++ + [ ππ ] S πP and − + + [ ππ ] S πD for low t (cid:48) ∈ [0 . , .
2] (
GeV /c ) and high t (cid:48) ∈ [0 . , .
0] (
GeV /c ) . The central vertical lines indicate the m π massinterval used for the Argand diagrams depicted in fig. 7. ++ + ρπS wave and contain the sum of both, production and decay phases.The strong dependence of the distributions in fig. 6 on t (cid:48) is striking and revealsthat much of the structure observed in the unresolved distributions is due tonon-resonant processes with steeply falling t (cid:48) dependence. Thus we clearlydisentangle resonant and non-resonant components and identify the dominantrole of the iso-scalar resonances f (980) and f (1500) in the decay processof the three π -states (pair of white lines in fig. 7). The resonance structureis also reflected in the circles observed in the Argand diagrams. Here, theabsence of phase motion in the ππ -System related to a (1420) is very distinct(no circle in the Argand diagram visible) and hints to production and decayphase having opposite sign and similar magnitude. We thus conclude thatthe a (1420) coupling to f (980) is genuine and not an artifact of the isobarparametrization and no strong coupling to the broad component in ππ S-wavechannel can be observed. More details, however, require a mass-dependent fitrelating the π + π − ++ mass and phase spectra to the π systems. Using the worlds largest data set on diffractive pion dissociation we havedeveloped new analysis tools. This allows to disentangle resonant and non-7 (A. U.) S pr + ++ Real Part relative to the 1-6 -5 -4 -3 -2 -1 0 1 2 · ( A . U . ) S pr + ++ I m ag i na r y P a r t r e l a t i v e t o t he -3-2-1012345 · S - p * S ] + p - p [ + +- c £ p m £ ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y (A. U.) S pr + ++ Real Part relative to the 10 2 4 6 8 10 12 14 16 18 20 · ( A . U . ) S pr + ++ I m ag i na r y P a r t r e l a t i v e t o t he -6-4-2024681012 · P - p * S ] + p - p [ + ++ c £ p m £ ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y (A. U.) S pr + ++ Real Part relative to the 1-12 -10 -8 -6 -4 -2 0 2 4 6 8 · ( A . U . ) S pr + ++ I m ag i na r y P a r t r e l a t i v e t o t he -10-50510 · D - p * S ] + p - p [ + +- c £ p m £ ) c £ t' £ COMPASS 2008 p - p + p - pfi p - p P r e li m i n a r y Figure 7:
Argand diagrams for π + π − ++ -isobar extracted for m π at the π (1800) , a (1420) and π (1880) resonances for high t (cid:48) ∈ [0 . , .
0] (
GeV /c ) (see white lines in fig. 6).resonant processes contributing to multi-particle production using the reaction π − + p → π − π − π + + p recoil . We have extracted the a (1260) resonance andobserved hints for excited states of the a and a around − MeV /c .In particular we have observed a new iso-vector meson a (1420) decayinginto f (980) π with no clear signs of concurring decay modes. A full phasemotion of ◦ characteristic to resonance production has been observed.The origin of this object with a width of about MeV /c is yet unclear.The similarity in mass and width to the f (1420) strongly coupling to KK ∗ is striking. At present we cannot exclude re-scattering effects involving acoupling of KK ∗ and f (980) π as outlined in [11]. A detailed coupled channelanalysis including the study of the KK final state would be necessary. Inaddition we have studied the iso-scalar structure of the ππ isobar in thedecay of J P C = 0 − + , ++ , − + . A clear coupling of a (1420) to f (980) andof π (1800) and π (1880) to both, f (980) and f (1500) , has been observed inaddition to a broad ππ S-wave component, exhibiting a strong t (cid:48) dependence. References [1] A. Abele et al. (CBAR), Phys. Lett.
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