Highlights from the COMPASS experiment at CERN -- Hadron spectroscopy and excitations

1.3 MB·

by  Frank Nerling

EPJWebofConferenceswillbesetbythepublisher
DOI:willbesetbythepublisher
(cid:13)c Ownedbytheauthors,publishedbyEDPSciences,2016
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Highlights from the COMPASS experiment at CERN
2

n
Hadron spectroscopy and excitations
a
J
9 FrankNerling1,a,onbehalfoftheCOMPASSCollaboration
1
1UniversitätMainz,InstitutfürKernphysik,Germany

]
x
e Abstract. The COMPASS experiment at the CERN-SPS studies the spectrum and the
p- structureofhadronsbyscatteringhighenergyhadronsandpolarisedmuonsoffvarious
e fixedtargets. Recentresultsforthehadronprogrammecomprisehighlightsfromdiffer-
h ent topics. A selective overview is given and, among others, the following results are
[ discussed. Theprecisedeterminationofthepionpolarisability, alongstandingpuzzle

thathasbeensolvednow,ispresentedaswellasmeasurementsofradiativewidths. The
1
observationofanewnarrowaxial-vectorstate,thea (1420),aswellasdeeperinsights
v 1
5 intotheexotic1−+-wave,whichisunderstudysincedecadesbyseveralexperiments,are
2 discussed and further, the search for the charmonium-like exotic Zc(3900) state in the
0 COMPASSdataiscovered.
5
0
.
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1 Introduction
6
1
: TheCOMPASSfixed-targetexperiment[1]atCERN-SPSisafacilitytostudyquantumchromody-
v
namics (QCD). It is dedicated to study the perturbative and non-perturbative regime of QCD, and
i
X probethestructureanddynamicsofhadrons. Usingamuonbeam, COMPASSinvestigatesnucleon
r structure by measuring helicity, transversity and general parton distribution functions — the recent
a
highlights from the COMPASS muon programme are summarised in [2]. Hadron excitations and
spectroscopy, includingthesearchforexoticstates, areinvestigatedusinghadronbeamsonaliquid
hydrogen(proton)anddifferentnucleartargets.AselectionofrecentresultsoftheCOMPASShadron
programmearediscussedandsummarisedhere.
Thetwo-stagespectrometer(Fig.1)isequippedwithelectromagneticandhadroniccalorimeters,
providingdetectionofchargedandneutralfinalstateparticleswithahomogeneousacceptanceover
awidekinematicrange,especiallycoveringalargerangeinmomentumtransfer. Atlowestreduced
four-momentum transfers between the beam particle and the target of t(cid:48)<10−3(GeV/c)2, Primakoff
reactions are studied, from which the pion polarisability has been extracted at high experimental
precision,Chiralperturbationtheory(ChPT)istestedandtheradiativedecaywidthsofthea (1320)
2
andtheπ (1670)havebeenmeasuredinpion-photonreactions. Atlowfour-momentumtransfersof
2
0.1(GeV/c)2<t(cid:48)<1(GeV/c)2, the mass spectrum of hadrons is investigated, including searches for
(spin-)exoticmesons.
ae-mail:[email protected]
EPJWebofConferences
COMPASS trackers (upgraded close to beam):
SciFi, Si, MM, GEM, DC, Straw, MWPC
0.4m
liquid H target
2
ECAL2
Beam (0.4°≤ θ ≤ 2°)

(muon or hadron)
ECAL1
(2°≤ θ ≤ 12°)
Recoil detector (RPD)
Electromagnetic calorimeters
to trigger on reactions
inside target
PID using (final states involving neutrals,
RICH & CEDARs e.g. π0, η, or η’ )
Figure1.SchematicdrawingoftheCOMPASStwo-stagespectrometerasusedforthehadronruns2008/09.
2 Results on hadron excitations
FortheunderstandingofQCDandthelow-momentumexpansionChPT,inwhichthepionsareiden-
tified as the Goldstone bosons as a result of the spontaneous chiral symmetry breaking, the pion
properties are a crucial input. Whereas pion-pion scattering has extensively been studied and suc-
cessfullydescribedwithinChPT,experimentalresultsofpion-photonreactionsintermsofthemost
basic process provided still a puzzle: Previous measurements of the pion polarisability, the leading
structure-dependenttermofComptonscattering,providedresultantvaluessignificantlylargerasex-
pectedfromtheory,cf.Fig.2.ApartfromthepionpolarisabilityinPrimakoffreactions(Sec.2.1),also
pion-photoninteractionswithmorethanonepioninthefinalstatearestudied(Secs.2.2and2.3).
2.1 Measurementofpionpolarisability
Therigidityofanextendedobjectagainstdeformationsbyanexternalelectricormagneticfieldare
described by the so-called electric and magnetic polarisabilities α and β, respectively. As the pion
isnotapoint-likeparticle,itissensitivetodeformationsbysuchexternalfields,andthistinyeffect,
calledpionpolarisability,isundertheassumptionofα =−β predictedinChiralPerturbationTheory
π π
(ChPT)tobe5.7×10−4fm3[3].
PolarisabilitiesareusuallymeasuredinComptonscatteringexperiments. Eventhoughpionscan
hardlybeusedasafixed-target,theycanbescatteredofftheCoulombpotentialofaheavynucleus,
like a Nickel (Ni) target. In the corresponding π−γ → π−γ scattering process, the polarisabilities
manifest as a deviation from the Born cross-section of a point-like particle. The measurement of
thecross-sectionofthisprocessallowstoextractthepionpolarisabilitiesviathedeviationfromthe
assumptionforapoint-likeparticle.
Suchameasurementisexperimentallydemandingandthesystematicsneedpreciselytobecon-
trolled. AnimportantadvantageoftheCOMPASSexperimentisthatwecanperformthismeasure-
mentwiththe pionandwiththemuon beam(quicklyswitchablewithinabout twohours), inwhich
thelatteractsasacontrolmeasurementofthepoint-likemuon.
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Figure 2. Left: Compilation of experimental measurements of the pion polarisability α −β including the
π π
newCOMPASSresultandthetheoreticalpredictionfromChPT[3],plottakenfrom[4]. Right: RatioRofthe
measuredcross-sectionoverthesimulatedoneforapoint-likeobjectforthepionbeamdata(top)andforthe
controlmeasurementwiththemuonbeamdata(bottom)[5].
TheCOMPASSdataofscatteringapionbeamoffaNitargetcomprisestheComptonpartofthe
cross-section[5]. Theπ−Ni → π−γNieventsareselectedbyrequiringonenegativelychargedtrack
fromthevertexinsidetheNitarget, ahigh-energeticshowerintheelectromagneticcalorimetersfor
thefinalstatephoton,whereasthephotonexchangeisensuredbyacutonlowphotonvirtualitiesQ2,
energyconservationisappliedtoselectexclusivereactions,i.e.∆E = E −E −E ≈0,neglecting
beam π(cid:48) γ
the (tiny) nuclear recoil energy. In order to stay in the kinematic region of stable trigger and muon
identificationefficiencies,theratioofthefinalstatephotonandtheinitialbeamenergyx = E /E
γ γ beam
is required to be between 0.4 and 0.9. Under the assumption of α = −β that has to be made due
π π
tothelimitedstatisticsatlower x (i.e. thephasespaceregionsensitivetoα +β ),thepolarisation
γ π π
should manifest in terms of a decreasing ratio R of the measured cross-section over the simulated
one for a point-like particle with increasing x . This behaviour has exactly been measured for the
γ
pionbeamdata,i.e. fortheextendedobjectofthepion,asdisplayedinFig.2(right,top). Toextract
the polarisability α from the measured ratio R, the photon energy spectrum is examined according
π
tothefollowingrelation,whichisasimplifiedrelationofthePrimakoffcross-sectioninone-photon
exchangeapproximation:
σ(x ) 3 m 3 x 2
R= γ =1− · π · γ α .
σαπ=0(xγ) 2 α 1−xγ π
Forthemuonbeamdata(Fig.2,right,bottom),thesizeofthefakepolarisabilityof(0.5±0.5 )×
stat
10−4fm3iswithinthestatisticaluncertaintycompatiblewithzero.Thisvalueistakenasanestimateof
thesystematicerrorduetoapparativeimperfections,notdescribedbytheMCsimulation.Forthepion
beamdata(Fig.2,right,top),apionpolarisabilityofα =(2.0±0.6 )×10−4fm3isdeterminedfrom
π stat
thefit. ThispionpolarisabilityvaluemeasuredbyCOMPASSisintensionwithpreviousexperiments
andinagreementwiththepredictionfromChPT(Fig.2,left).
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2)MeV/c 89 Cpt' <-OP 0bM.0 fi0P1A GpSe-VpS2- /p2c2+0P0b4 msb] [g 11..02 Cpfr-oOgm Mfi pP p-AP-pSb- Spfi +2 0p0-p4-p +Pb
nts / (5 67 0.8 FLietateddin Cgh OPrTd eInr tCenhsPiTty Prediction
Eve 45 m = 0.72 0.6
3 limp3 0.4
2 0.2 Full Systematic Error
1
Luminosity Uncertainty
0 0.0
0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 0.45 0.5 0.55 0.6 0.65 0.7
m3p [GeV/c2] m3p [GeV/c2]
Figure3. Left: Spectrumofthe3πmassfromπ−Pb → π−π+π−PbinthePrimakoffregionoffour-momentum
transfert(cid:48) < 10−3GeV2/c2. TheregionofinterestinviewofChPTuptom ≈ 0.72GeV/c2 isindicated. At
3π
mK− ≈ 0.49GeV/c2, apeakfromtheadmixtureofincidentbeamkaonsdecayinginto(3π)− isvisiblethatare
used for flux normalisation. Right: First measurement of the photo-production cross-section σ in this mass
γ
range,foundinagreementwiththepredictionfromleadingorderChPT[6].
2.2 Measurementofchiraldynamics
Fromthesamedata,furtherprocessesonchiraldynamicsareaccessible,likee.g. thechiralanomaly
inπ−γ →π−γ,thecorrespondinganalysisisstillunderway,thesameholdsfortheneutralpioncase
of two-pion production π−γ → π−π0π0 at low energy. Based on the 2004 pilot run data (Primakoff
kinematics),thetwo-pionproductionforthechargedcaseπ−γ→π−π+π−hasbeenanalysed[6].
Figure 3 (left) shows the mass spectrum of the outgoing three-pion final state. Of particular in-
terest in view of chiral dynamics is the low mass tail up to three pion masses, as indicated in the
figure. Performingapartial-waveanalysis(PWA)includingamplitudesfromChPTcalculationssub-
stitutingtheisobaricpartialwavesinthismassrange, afterfluxnormalisationontheobservedkaon
decays(usingthekaonadmixtureinthebeam),theabsolutecross-sectionis,withintheexperimental
uncertaintyof20%,foundinagreementwiththeleadingorderChPTprediction.
In the 3π mass spectrum (Fig.3, left), structures from the well-known resonances a (1260) and
1
a (1320) are visible at higher masses — the corresponding radiative couplings have been measured
2
asdiscussedinthenextsection(Sec.2.3).
2.3 Measurementofradiativewidth
ElectromagnetictransitionscanbemeasuredviaradiativedecaysofaresonanceX →πγ. Thedirect
measurementofπγisexperimentallydifficult. Alternatively,theinverseprocessofscatteringapion
offaCoulombfieldproducinganintermediateresonanceX,i.e. thePrimakoffreaction,canbeused.
The Coulomb potential of a heavy nucleus acts as a (quasi-real) photon source, and the Primakoff
production cross-sectionof a resonance X is proportionalto the widthof theradiative decay ofthat
resonance: σPrimakoff(X) ∼ Γ0(X → πγ). Duetotheexchangeparticlebeingaquasi-realphoton,Pri-
makoffproducedresonanceshavepredominantlyspinprojection M=1. Eventhoughstateswithspin
projection M=1canalsobeproduceddiffractively,thecross-sectionhereisproportionaltot(cid:48)|M|e−bt(cid:48),
sothatthediffractiveproductionishighlysuppressedatverysmallvaluesoft(cid:48).
Based on the negative pion beam data on a thin lead (Pb) target, the COMPASS Collabora-
tion studied this reaction and measured the radiative widths of the a (1320) and the π (1670) [7].
2 2
Performing a partial-wave analysis, the data is decomposed into spin-parity states, and thus also
the M=1 states are identified. To extract the radiative widths of the a (1320) and the π (1670),
2 2
ICNFP2015
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2V/)c 2+ +1 r [D] p p - Pb fi p - p - p + Pb 2V/)c 1.4 2- +1 f2[S] p p - Pb fi p - p - p + Pb
Me 10 s prim/s all = 0.97 t' < 0.001 (GeV/c)2 Me 1.2 s prim/s all = 0.86 t' < 0.001 (GeV/c)2
40 8 40 1.0
-3 / (10 6 G 0(a2 fi p g ) = 358 keV -3 / (10 0.8 G 0(p 2 fi p g ) = 181 keV
(cid:215) y (cid:215) y 0.6
ensit 4 ensit 0.4
nt nt
I 2 I 0.2
0 0.0
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
m3p [GeV/c2] m3p [GeV/c2]
Figure4.Resultsofthemeasuredradiativewidthsofthea (1320)(left)andtheπ (1670)(right)(reprintedwith
2 2
kindpermissionofEPJfrom[7],copyrightSocietàItalianadiFisica/Springer-Verlag2014). Ascomparedto
previousmeasurements,theCOMPASSresultforthea (1320)isthemostpreciseone,whereasfortheπ (1670),
2 2
theCOMPASSvalueisthefirstmeasurementatall.
the two partial waves JPCM(cid:15)(isobar)π = 2++1(cid:15)ρ(770)πD-wave and 2−+1(cid:15)f (1270)πS-wave have
2
been considered, respectively. For the events analysed, namely at t(cid:48)<10−3GeV2/c2, the contribu-
tion of Primakoff production is 97% and 86% of the intensities found in the 2++1(cid:15)ρ(770)πD and
2−+1(cid:15)f (1270)πS-waves, respectively. The intensities for the two different reflectivities (cid:15) = ±1
2
have to be summed up incoherently as the production plane at such small t(cid:48) is not measured suffi-
ciently precise due to limited experimental resolution. That there are further possible decay chan-
nels (than ρ(770)π for the a (1320) and f (1270)π for the π (1670)) has been taken into account
2 2 2
by consideration of the relevant branching fractions. The fits to the data are shown in Fig.4, from
which a width of Γ (a (1320)→πγ)=(358±6±42)keV and Γ (π (1670)→πγ)=(181±11±27)keV
0 2 0 2
havebeenmeasuredforthea (1320)andtheπ (1670),respectively. Bothresultsareconsistentwith
2 2
predictionsbasedontheVMDmodel[8].Comparedtopreviousmeasurements,theCOMPASSresult
forΓ (a (1320)→πγ)isthemostpreciseone,whereasforΓ (π (1670)→πγ)theCOMPASSresult
0 2 0 2
isthefirstmeasurementatall.
3 Results on hadron spectroscopy
There are many states that have been observed so far in the light meson sector, whereas quite some
oftheobservationsstillneedconfirmation. Comparingtheexperimentalsituationtothepredictions
based on the naive quark model, quite some agreement is found, however, not all the predictions
are perfectly consistent with experiment: Some of the predicted states have not yet been observed,
alsothereareobservedstatesthatarenotexpected,seee.g.[9]. Giventherelativelylargewidthsof
many of these states (∼100MeV/c2) together with the high level density of the mesons with (u,d,s)
constituentquarks,therearequitesomeoverlapsandmixing,complicatingtheanalyses.
Ofparticularinterestareexoticmesons,notfittingtheconstituentquarkmodel. QCDallowsfor
andpredictssuchstates,likeglue-balls,hybridsortetraquarksaccordingtovariousmodels.Mesonsof
samespin-parityquantumnumbersmix,andtheunambiguousobservationisthusdifficult. So-called
spin-exoticstateswith JPC quantumnumbersnotaccessiblebyasimpleqq¯ configurationareonthe
otherhandverypromisingtosearchforastheydonotmixwithordinarymesons. Theexperimental
observation of such spin-exotic states would be a fundamental confirmation of QCD, for a recent
overview,seee.g.[10].
The COMPASS data give access to all decay channels spin-exotic hybrid candidates have been
reportedinsofarbydifferentexperiments,likeρ(770)π,η((cid:48))π, f (1285)πandb (1235)π. Thelightest
1 1
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Figure5.Invariantmassspectrumoftheexclusivelyproduced,outgoing(3π)−systemforthecharged(left)and
theneutral(right)decaymodes[14,15].
hybridcandidatewithexotic JPC=1−+,thefamousπ (1600),hasbeensearchedforandisstudiedin
1
twodifferentdecaychannelssimultaneously,namelyinπ−π+π− andπ−π0π0 finalstates[11–16]pro-
ducedindiffractivepiondissociationπ−p → (3π)−p(cid:48). Sincethereconstructiondependsondifferent
partsofthedetector,theobservationofastateinbothchannelsprovidesanindependentconfirmation
withinthesameexperiment,thepossibilityofcross-checksandcontrolofsystematics.
Theinvariantmassofthediffractivelyproduced(3π)− systemisshownforthechargedandneu-
tral ρπ decay modes in Fig.5 (left) and (right), respectively. In the range of four-momentum trans-
fer 0.1(GeV/c)2<t(cid:48)<1.0(GeV/c)2 used for PWA, about 50 million and 3.5 million exclusive (3π)−
events have been reconstructed from the 2008 data (of negative pion beam impinging on a proton
target)inthemassrangeof0.5and2.5GeV/c2forthechargedandtheneutralmode,respectively. By
theinteractionwiththetarget, thebeampionisexcitedtoanintermediateresonance X− thatsubse-
quentlydecaysintoadi-pionresonance,theso-calledisobardecayingintotwopions,andabachelor
pion with the relative orbital angular momentum L between them. For the results presented here,
theρ(770), f (980), f (1270), f (1500),ρ (1690)andabroad(ππ) componenthavebeenused. The
0 2 0 3 s
setofpartialwavescontains87partialwavesupto L=6andanincoherentwave,theso-called“flat
wave”,havinganisotropicangulardistribution,representingeventsofthreeuncorrelatedpions.
Giventheobserveddependenceont(cid:48) andthelargestatistics,thePWAmethodhasbeenextended
with respect to the scheme of a two-step PWA as applied previously, for a complete description in-
cludingalldetails,see [17]. Thedatahasbeendividedinto11and8binsoft(cid:48) suchthataboutequal
statisticsiscontainedineachbin,and(aspreviously)into20MeV/c2 and40MeV/c2 widem bins
3π
for the charged and the neutral mode data, respectively. The first step analysis, the so-called mass-
independentPWA,iscarriedoutindependentlyforeachmassbinandindependentlyforthedifferent
rangesoft(cid:48).Thismaximumlikelihood(rank-1)fitofthemodeltothedataextractstheproductionam-
plitudes,takingintoaccountthedetectoracceptance. Finally,theanalysisiscompletedbythesecond
stepanalysis, theso-calledmass-dependentfit. Inthisχ2 fitofBreit-Wigneramplitudestoasubset
ofthespindensitymatrix,resonanceparametersareextracted,forwhichnotonlytheintensitiesbut
alsotheinterferencesaretakenintoaccount. Alsoittakesintoaccounttheobservedt(cid:48) dependencies
byperformingasimultaneousoptimisationoftheresonantparametersinallt(cid:48)regions.
ICNFP2015
2)intensity (per 40 MeV/c 11122212460248·103 COM0P.1A0pS0-S p< P02 pt0'0r (10<,ie +8n p+1 cl0-(.oipp+0 hm-0-fpep00r(+fi e9i G(n8ns(et0c3 Vasap) ul2)rpe-/mcdyp P2))) 2)intensity (per 40 MeV/c 345000·14p0(P0i+.3-n1+prc100eo+0p hl r0<e,i( r 7mpte'7- n<p0ti -)s1n pup.+0ma G0()srC0c yOaGlMeedVP)2A/cS2S 2008 (p-pfi (3p)-p) phase (degree)1200000 f(1++0+ fC0(O9M80P) ApS PS -2 040++81 +(p r-(p7fi70()3 pp) -Gp))
10
20 Preliminary
8 -100
6
4 10 -200 0.100 < t' < p0-.1p10p6 0G, peV-p2-/pc+2
2 0.100 < t' < 0.113 GeV2/c2
0 0
0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
· 103 m(3p)- (GeV/c2) · 103 m(3p)- (GeV/c2) m(3p)- (GeV/c2)
2)c20 MeV/ 2205 10((12+.1))+ 0Ma<1+ (ot 1'fd 04<e(2 9l10 c8.)u0 0rr e()vG sepoen VPa/ncc)e2 2)c20 MeV/ 2205 40((12.+1))+ Ma1<4+ (o t2'rd 0<e(4 l71 0c7.)u0 0rr (ev)Gs epoenV Ga/ncc)e2 Phase [deg]120000 1++0+ f0(980) p P - 4++1+ r(770) p G
y / ( 15 (3) Non-resonant term y / ( (3) Non-resonant term
nsit (2) nsit 15 0
e e (1)
Int 10 Int 10 (2) - 100
(1)
5 (3) 5 (3) - 200 000...114064049 <<< ttt''' <<< 000...117182394 (((GGGeeeVVV///ccc)))222
0 1 1.2 1.4 1.6 1.8 2 2.2 0 1 1.2 1.4 1.6 1.8 2 2.2 1 1.2 1.4 1.6 1.8 2 2.2
m3p [GeV/c2] m3p [GeV/c2] m3p [GeV/c2]
Figure 6. Top: First step PWA result, charged versus neutral decay mode [14]: The fitted intensity for the
1++0+ f (980)πP-waveshowsanarrowstructureatabout1.4GeV/c2,itissimilarlyobservedforboth,neutral
0
and charged (3π)− mode (left). In the resultant intensity for the 4++1+ρ(770)πG-wave, the known a (2040)
4
is similarly observed (centre). The relative phase of the two shows a clean rapid phase motion in the signal
region. Bottom:SecondstepPWAresult,chargeddecaymode[18]:Completefitresultintermsoftheresultant
1++0+ f (980)πPintensity(incoherentsumovert’ranges)(left),thefirststepresultisgivenbytheblackdata
0
pointsandtheresonancemodelfitoverlaidinredconsistsoftheBWdescribingthenarrowstructure,thatwecall
thea (1420)(bluecurve),andthenon-resonantbackgroundcontribution(greencurve).Thecorrespondingresult
1
isshownforthea (2040)observedinthe4++1+ρ(770)πG-wave(centre). Therelativephaseofthea (1420)
4 1
against the a (2040) (right) is shown for three different t’ ranges, consistently showing a clean rapid phase
4
rotationinthesignalregion.
3.1 Observationofanewaxial-vectorstatea (1420)
1
Figure 6 (top/left) shows the first step PWA result for one out of the 87 partial waves, namely the
1++0+f (980)πP-wave, intermsofthefittedintensityforbothdecaymodes. Indeed, theincoherent
0
sumoverthedifferentt’rangesisshownhere,andthechargedisnormalisedtotheneutralmodedata
using the integral. Good agreement on the fit result is found for the two channels. For both decay
modes,the1++0+f (980)πP-waveintensity,containingonlyaverysmallfractionofthetotalobserved
0
intensityofabout0.25%[18],showsaclearsignalslightlyabove1.4GeV/c2. Rapidphaserotations
withrespecttoknownresonances,likee.g.thea (2040)(Fig.6,top/centre),areobservedinthesignal
4
region(Fig.6,top/right),asitisexpectedforaresonance. Aphasedoesnotdependontheamountof
amplitude,andthephasemotionisobservedindependentoft(cid:48).
In the second analysis step, a Breit-Wigner resonance model with a minimal set of three waves
isused,namelythe2++1+ρ(770)πD,4++1+ρ(770)πGand1++0+ f (980)πPwaves. Forthecharged
0
modedata, thecompletedPWAresultisshowforthe1++0+ f (980)πP-waveinFig.6(bottom/left)
0
and for the 4++0+ρ(770)πG-wave in Fig. 6 (bottom/centre), the phase difference between them is
giveninFig.6(bottom/right). Fromthisfit,weobtainamassofm = 1414+15MeV/c2 andawidth
−13
EPJWebofConferences
· 103 COMPASS 2008 (p -pfi (3p )-p) COMPASS 2008 (p -pfi (3p )-p)
2)V/c 10 p -p 0p 0,1 p-+-1p+- rp (+7 (7s0ca) lped P) gree)100 f (1-+1+ r (770) p P - 1++0+ r (770) p S)
Me 0.100 < t' < 0.116 GeV2/c2 de
per 40 8 0.100 <P t'r <e 0l.i1m13 iGneVa2r/cy2 phase ( 500
y (
sit -50
n 6
e
nt -100
i
4 -150 Preliminary
-200
2 -250 p -p 0p 0, p -p -p +
0.100 < t' < 0.116 GeV2/c2
-300
0 0.100 < t' < 0.113 GeV2/c2
-350
0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
m (GeV/c2) m (GeV/c2)
(3p )- (3p )-
Figure7. FirststepPWAresultfortheexotic1−+1+ρ(770)πP-wavein(3π)−finalstates,chargedversusneutral
modedata. Theresultantfittedintensitiesarefoundingoodagreementforthetwochannels(left). Shownare
theresultsforthelowestrangeoffour-momentumtransfert(cid:48),wherethesignalontopofthebroadbumpatabout
1.6GeVc2hasthelowestevidenceascomparedtothehighert’regions,inwhichthesignaltobackgroundratio
improveswithincreasingt(cid:48). Therelativephaseswithrespecttothe1++0+ρ(770)πS-wave, inwhichthewell-
knowna (1260)resonanceisobserved,isdisplayed(right),showingacleanphasemotionintherangeofabout
1
1.4and1.8GeV/c2consistentlyforthetwodecaymodes.
of Γ = 153+8 MeV/c2. The interpretation of this new isovector state is still unclear. The proper-
−23
ties of the a (1420) suggest it to be the isospin partner of the f (1420), especially supported by the
1 1
strongcouplingtothe f (980)(andnottoρ)thatisinterpretableasaKK¯ molecule. Thea (1420)and
0 1
the f (1420)maypossiblybethefirstobservedisospinpartnersforaKK¯πmoleculartypeexcitation,
1
which suggests further studies of the KK¯π final state. A first, very promising look at the f (1420)
1
observeddecayingtoKK¯πinthe(same)COMPASS2008datasethasalreadybeenprovided[19],in-
cludingfirstverypreliminaryPWAconfirming(duetospinassignment)thestateintheKK¯πinvariant
massindeedtobethe f (1420)andnottheη(1295).
1
3.2 Statusofthesearchforthespin-exoticπ (1600)resonance
1
Inthe2004pilotrundata(π−beam,Pbtarget),asignificantJPC spin-exoticsignalhasbeenobserved
at 1660±10+0 MeV/c2. It shows a clean phase motion against well-known resonances and is con-
−64
sistentwiththedisputedπ (1600)[20]. Thehighstatisticsofthe2008protontargetdataallowsthe
1
search for exotic states in different decay modes in the same experiment [11]. Employing the same
PWAmethodasin[20],theresultsobtainedforthe(1−+)1+ρ−π0 and(1−+)1+ρ0π− intensityandrela-
tivephasearesimilarandconsistentwiththepreviousobservations[12,13]. Apartoftheestablished
resonancesa (1260),a (1320),π (1670),alsoπ(1800)anda (2040),anexoticsignalinthe1−+wave
1 2 2 4
ataround1.6GeV/c2hasbeenobserved,thatshowsacleanphasemotionwithrespecttowell-known
resonances.Theseresultsareconsistentlyobtainedforbothρπdecaymodes,neutralandcharged.The
extensiontoanadditionalt(cid:48) binningandperformingthePWAindependentlyalsoindifferentranges
oft(cid:48) [17]allowsfordisentanglingresonantfromnon-resonantparticleproduction(e.g. dynamically
producedcomponentscausedbytheDeckeffect[21]).
ICNFP2015
Figure8.FirststepPWAresultfortheintensityoftheexotic1−+1+ρ(770)πP-waveforthechargeddecaymode
(blue)overlaidwithaprojectionofasimulatedDeck-effectinthiswave(green),shownfortherangeoflowest
(left)andhighest(right)valuesoft(cid:48)[16].
A dependency of the exotic 1−+1+ρ(770)πP wave intensity on the squared four-momentum t(cid:48)
is observed. Whereas a significant signal is observed at around 1.6GeV/c2 on top of a moderate
background at larger values of t(cid:48), the signal to background ratio decreases with decreasing t(cid:48). The
fittedintensityoftheexotic1−+1+ρ(770)πPwaveisshownfortherangeofsmallestt(cid:48)forthecharged
andneutraldecaymodesinFig.7(left).Apartfromthegoodagreementbetweenthetwochannels,we
observemainlyabroadbump. Eventhoughthereishardlyevidenceforasignalatabout1.6GeV/c2
in this plot, i.e. for this t(cid:48) range of 0.100<t(cid:48)<0.113GeV/c2, the corresponding relative phase with
respect to the 1++0+ρ(770)πS-wave, in which the well-known a (1260) is observed, a clean phase
1
motion is consistently observed for the two different decay modes in the range of about 1.4 and
1.8GeV/c2.Thisisexactlythemassrangewhereasignificantexotic1−+signalisobservedforranges
oflargert(cid:48) (seee.g. Fig.8, right). Therelativephaseoftworesonancesisindependentofthegiven
absoluteamplitudes,andaconsistentphasemotionisobservedinallbinsoft’.
The observed dependency of the signal to background ratio on t(cid:48) could be caused by the inter-
ference of a resonance with a large non-resonant background contribution, possibly introduced and
tobeexplainedbytheDeckeffect[21]. Inordertofurtherinvestigatethisexplanation,pseudodata
were generated according to a dedicated model [22]. This data has been analysed as the real data,
includingthedecompositionintopartialwaves,inordertostudytheDeck-amplitudecontributionto
thedifferentpartialwavesincludedinthePWA.Figure8showsthepromisingoutcomeofthestudy
for the exotic 1−+ partial wave, exemplary for the ranges of smallest and largest t(cid:48) for the charged
decay mode. The Deck amplitude intensities are normalised to the total intensity (summed over all
mass and t(cid:48) bins) in the given wave. A large part of the exotic 1−+ wave intensity at small values
of t(cid:48) can be described as a contribution by the non-resonant Deck process (Fig.8, left), whereas the
contributionatlarget(cid:48)isalmostnegligibleaccordingtothisansatz.
Forcompleteness,furtherrelevantresultsandongoinganalysesthatarenotdicussedindetailhere
arethePWAresultsofdiffractive(ηπ)− and(η(cid:48)π)− production[23]andpreliminaryPWAresultsfor
centralproduction(i.e. viadoublepomeronexchange)ofπ+π− andK+K− fromthe2009COMPASS
protonbeamdata[24].
Search
 for
 charmonium-­‐like
 (exo3c)

EPJstWaetbeo
 fZCo(n3fe9re0n0ce)s

c
a)
  b)

J/ψ
γ J/ψ
pomeron
π+
p
n
Exclusive
 produc3on
 channel
 for
 COMPASS:

Figure 9. Left: The charmonium-like exotic state Z (3900) decaying to J/ψπ± as observed by BESIII [26].
c
Right: In the COMPASS muon data, the Z (3900) should be produced via photoproduction according to the
c
processgivenundera),themainbackgroundoriginatesfrompomeronexchangereactions,b).
3.3 SearchforthemanifestlyexoticZ (3900)state
c
Even though the COMPASS muon beam data is mainly dedicated to study (spin-dependent) parton
distributionfunctions,ahadronspectroscopyresulthasbeenobtainedbasedonthefullsetofCOM-
PASSdatacollectedwithamuonbeambetween2002and2011, namelyanupperlimitonthepho-
toproductionoftheZ (3900)[25]. Thechargedcharmonium-likeexoticstateZ (3900)wasobserved
c c
decayingto J/ψπ± bytheBESIII[26]CollaborationandconfirmedbyBelle[27]in2013,seeFig.9
(left)fortheobservationplot,lateralsotheneutralpartnerwasconfirmed[28].
As the photon may act as a J/ψ according to the VDM, the Z (3900) should be produced via
c
interaction of the incoming photon with the virtual pion from the nucleon target according to the
diagraminFig.9(right,a),andthecross-sectionforthisprocessshouldbesizable[29].
The reconstructed exclusive production channel is µ+N → µ+Z±(3900)N → µ+J/ψπ±N, with
c
J/ψ decaying to µ+µ−, means the final state to detect is µ+µ+µ−π±N. The event selection of the
exclusively produced µ+J/ψπ± is straight forward: A vertex is reconstrcuted having exactly three
outgoingmuonsandapion,toidentifytheJ/ψ,amasscutonthereconstructedµ+µ−invariantmassis
appliedasindicatedinFig.10(left),energybalanceisrequestedand,inordertosuppressbackground
frompomeronexchangereactions(Fig.10,right,b),amomentumcut p >2GeV/cforthepionsis
π±
applied. In addition, in order to make use of the measurement of the cross-section σ(γN→J/ψN)
provided by the NA14 experiment [30] for absolute normalisation, the exclusive µ+J/ψ sample is
selected as well using the same selection criteria. The ratio of acceptances for both samples equals
in first approximation about the acceptance for the additional pion that we know to be 0.5±0.1 ,
syst
averagedoverallsetupandtargetconfiguartions.
TheresultantCOMPASSplotforthesearchfortheZ (3900)isgiveninFig.10(right),wherethe
c
reconstructed J/ψπ invariant mass is plotted and the expectation region for a signal of the Z (3900)
c
isindicated. NosignalofexclusivephotoproductionoftheZ±(3900)stateandthedecayinto J/ψπ±
c
has been found. Thus, an upper limit was determined on the product of the cross-section and the
relativeZ±(3900)→ J/ψπ±decayprobabilitynormalisedtothecross-sectionofincoherentexclusive
c