Observation of New Narrow D_s states
a r X i v :h e p -e x /0309028v 2 2 O c t 2003Physics in Collision -Zeuthen,Germany,June 26-28,2003
Observation of New Narrow D s states Antimo Palano from the B A B A R Collaboration INFN and University of Bari,Italy ABSTRACT
The B A B A R experiment has discovered a new narrow state,D ?sJ (2317)+,near 2.32GeV /c 2
in the inclusive D +s π0invariant mass distribution from e +e ?annihilation data at
energies near 10.6GeV [1].The same experiment has also shown evidence for struc-
ture in the 2.46GeV /c 2region in the D ?s
(2112)+π0mass spectrum.These discover-ies have triggered several experiments in a search for new states coupled to the D +s
meson which con?rmed the existence of D ?sJ (2317)+together with D sJ (2458)+→
D ?s (2112)+π0both in inclusive e +e ?annihilation and in B decays.These two new states are di?cult to explain in terms of potential models.
1Introduction
Experimental information on the spectrum of the c
s states are listed in the last PDG edition[2].The D s1(2536)+has been detected in its D?K decay mode and analysis of the D?decay angular distribution prefers J P=1+.The D?sJ(2573)+was discovered in its D0K+ decay mode and so has natural spin-parity.The assignment J P=2+is consistent with the data,but is not established.
The spectroscopy of c
s states thus can be summarized by stating that experiment has provided good candidates for the two states that theory predicts should be readily observable,but has no candidates for the two states that should be di?cult to observe because of their large predicted widths.
2The B A B A R Experiment.
The B A B A R detector(at the PEP-II asymmetric-energy e+e?storage ring with center-of-mass energy near10.6GeV)is a general purpose,solenoidal,magnetic spectrometer and is described in detail elsewhere[8].The data sample use in this analysis corresponds to an integrated luminosity of91fb?1.
3D+sπ0events selection.
The objective of this analysis is to investigate the inclusively-produced D+sπ0mass spectrum by combining charged particles corresponding to the decay D+s→K+K?π+1 withπ0candidates reconstructed from a pair of photons and performing a one-constraint?t to theπ0mass.A given event may yield several acceptableπ0candi-dates retaining only those candidates for which neither photon belongs to another acceptableπ0candidate.To reduce combinatorial background from the continuum and eliminate background from B-meson decay,each K+K?π+π0candidate was required to have a center-of-mass momentum p?greater than2.5GeV/c.
The upper histogram in Fig.1(a)shows the K+K?π+mass distribution for all candidates.Clear peaks corresponding to D+and D+s mesons are seen.To reduce the background further,only those candidates with K+K?mass within10MeV/c2 of theφ(1020)mass or with K?π+mass within50MeV/c2of the
K?(892)exhibit the expected cos2θh behavior required by conservation of angular momentum,where θh is the helicity angle.The signal-to-background ratio is further improved by requiring|cosθh|>0.5.The lower histogram of Fig.1(a)shows the net e?ect of these additional selection criteria.The D+s signal and sideband regions are shaded. The D+s signal peak,consisting of approximately80,000events,is centered at a mass of(1967.20±0.03)MeV/c2(statistical error only).
Figure1(b)shows the mass distribution for all two-photon combinations associated with the selected events.Theπ0signal and sideband regions are shaded. Candidates in the D+s signal region of Fig.1(a)are combined with the mass-constrainedπ0candidates to yield the mass distribution of Fig.1(c).A clear,nar-row signal at a mass near2.32GeV/c2is seen.The shaded histogram represents the events in the D+s→K+K?π+mass sidebands combined with theπ0candidates.In Fig.1(d)the mass distributions result from the combination of the D+s candidates with the photon pairs from theπ0signal and sideband regions of Fig.1(b)(the side-band distribution is again shaded).In Figs.1(c)and1(d)the2.32GeV/c2signal is absent from the sideband distributions indicating quite clearly that the peak is associated with the D+sπ0system.In order to improve mass resolution,the nominal D+s mass[2]has been used to calculate the D+s energy.
The D+sπ0mass distribution for p?(D+sπ0)>3.5GeV/c is shown in Fig.2(a). The?t function drawn on Fig.2(a)comprises a Gaussian function describing the 2.32GeV/c2signal and a polynomial background distribution function.The?t
Figure1:B A B A R experiment.(a)The distribution of K+K?π+mass for all can-didate events.Additional selection criteria have been used to produce the lower histogram.(b)The two-photon mass distribution from D+sπ0candidate events.D+s andπ0signal and sideband regions are shaded.(c)The D+sπ0mass distribution for candidates in the D+s signal(top histogram)and K+K?π+sideband regions(shaded histogram)of(a).(d)The D+sγγmass distribution for signal D+s candidates and a photon pair from theπ0signal region of(b)(top histogram)and the sideband regions of(b)(shaded histogram).
yields1267±53candidates in the signal Gaussian with mass(2316.8±0.4)MeV/c2 and standard deviation(8.6±0.4)MeV/c2(statistical errors only).The signal,la-belled as D?sJ(2317)+,is observed in both theφπ+and
Figure2:B A B A R experiment.The D+sπ0mass distribution for(a)the decay D+s→K+K?π+and(b)the decay D+s→K+K?π+π0.
Monte Carlo simulations have been used to investigate the possibility that the D?sJ(2317)+signal could be due to re?ection from other charmed states.This simulation includes e+e?→cˉc events and all known charm states and decays.The generated events were processed by a detailed detector simulation and subjected to the same reconstruction and event-selection procedure as that used for the data.No peak is found in the2.32GeV/c2D+sπ0signal region.
Mass resolution estimates for the K+K?π+π0system are obtained directly from the data using a?t to the mass distribution D+s→K+K?π+π0.The measured width from this mode is consistent with that of the D?sJ(2317)+signal.It can be concluded that the intrinsic width of the D?sJ(2317)+is small(Γ 10MeV).
A search has also been performed for the decay D?sJ(2317)+→D+sγ. Shown in Fig.3(a)is the D+sγmass distribution obtained by combining a D+s candi-date in the signal region of Fig.1(a)with a photon with an energy of at least150MeV that does not belong to aγγcombination in the signal region of Fig.1(b).The re-quirement that the p?of the D+sγsystem be greater than3.5GeV/c is also imposed. There is a clear D?s(2112)+signal,but no indication of D?sJ(2317)+production.The
Figure3:B A B A R experiment.The mass distribution for(a)D+sγand(b)D+sγγafter excluding photons from the signal region of Fig.1(b).(c)The D+sπ0γmass distribution.The lower histograms of(b)and(c)correspond to D+sγmasses that fall in the D?s(2112)+signal region as described in the text.The vertical line indicates the D?sJ(2317)+mass.
D+sγγmass distribution for p?(D+sγγ)>3.5GeV/c,excluding any photon that be-longs to theπ0signal region of Fig.1(b),is shown as the upper histogram of Fig.3(b). No signal is observed near2.32GeV/c2.The shaded histogram corresponds to the subset of combinations for which either D+sγcombination lies in the D?s(2112)+re-gion,de?ned as2.096 that,because of the small widths of both the D?s(2112)+and D?sJ(2317)+mesons, produces a narrow peak in the D+sπ0γmass distribution that survives a D?s(2112)+ selection. If the peak in the D+sπ0γmass distribution of Fig.3(c)were due to the pro-duction of a narrow state with mass near2.46GeV/c2decaying to D?s(2112)+π0,the kinematics are such that a peak would be produced in the D+sπ0mass distribution at a mass near2.32GeV/c2.Such a D+sπ0mass peak,however,would have a root-mean-square of～15MeV/c2,which is signi?cantly larger than that obtained for the D?sJ(2317)+signal.In addition,Monte Carlo studies indicate that if the apparent signal at2.46GeV/c2were due to a state that decays entirely to D?s(2112)+π0,it would produce only one-sixth of the observed signal at2.32GeV/c2. The B A B A R experiment was somewhat coutious in claiming the discovery of this second state.“Although we rule out the decay of a state of mass2.46GeV/c2 as the sole source of the D+sπ0mass peak corresponding to the D?sJ(2317)+,such a state may be produced in addition to the D?sJ(2317)+.However,the complexity of the overlapping kinematics of the D?s(2112)+→D+sγand D?sJ(2317)+→D+sπ0 decays requires more detailed study,currently underway,in order to arrive at a de?nitive conclusion[1].” (2317)+and observation of D sJ(2458)+by other 4Con?rmation of D? sJ experiments. Using13.5and78fb?1integrated luminosity,the CLEO[9]and Belle[11]experi-ments readily con?rmed the existence of D?sJ(2317)+(shown in?g4).In terms of ?m=m(K+K?π+π0)?m(K+K?π+),CLEO reported a value of?m=350.3±1.0 MeV/c2with231±30events.Belle reports?m=348.9±0.5MeV/c2with643±50events.These values are in good agreement with the B A B A R measurement of ?m=384.4±0.4MeV/c2. In addition,both the CLEO and Belle Collaborations have analyzed the D?s(2112)+π0mass distribution?nding evidence for structure in the2.46GeV region (see?g.5).De?ning now?m=m(K+K?π+π0γ)?m(K+K?π+γ),they reported the following parameters for this state:?m=351.2±1.7MeV/c2with41±12events (CLEO)and?m=344.1±1.3MeV/c2with79±18events(Belle). Belle also reported evidence for both states[10]in the B decays to B→DD?sJ(2317)+and B→DD sJ(2458)+.In addition,they reported an observation of D sJ(2458)+→D+sγin both B decays and continuum e+e?annihilations.The angular analysis shows the expected behaviour for a J P=1+state. M(D s π0) (GeV/c 2)E v e n t s /5M e V /c 2GeV/c 2 D s (2317)/5M e V 050100 150200250300Figure 4:D +s π 0and ?m from CLEO and Belle respectively showing the signal of D ?sJ (2317)+.(a) (b)M(D s γπ0) - M(D s γ) (GeV/c 2)E v e n t s / 5 M e V /c 2010 20 010200.10.20.3 0.40.50.6GeV/c 2 D s (2459)/6M e V 020*********Figure 5:?m (D ?+s π0)from CLEO and Belle Collaborations showing the evidence for D sJ (2458)+.Data are from continuum e +e ?annihilations.The distribution from D ?+s sidebands is also shown. No evidence has been found for narrow structures in D+sπ±or D+sππ?nal states. 5The observation of D sJ(2458)+by the B A B A R experiment. In order to study the D+sπ0γsystem,each D+s candidate is combined with all com-binations of accompanyingπ0candidates with momentum greater than300MeV/c and photon candidates of energy greater than100MeV/c2.To suppress background, photon candidates that belong to anyπ0candidate are excluded and it is required that the combined momentum p?in the e+e?center-of-mass mass frame of each D+sπ0γcombination be greater than3.5GeV/c.The D+sπ0γinvariant mass dis-tribution is shown in Fig.6.A small peak is observed near2.46GeV/c2.The background underneath this peak is from several sources,which can be described in terms of mass di?erences de?ned as: ?m(D+sγ)≡m(K?K+π+γ)?m(K?K+π+)(1) ?m(D?+sπ0)≡m(K?K+π+π0γ)?m(K?K+π+γ).(2) A scatter plot of these quantities is plotted in Fig.6b.Two signals are clearly visible: D?s(2112)+→D+sγdecay combined with unassociatedπ0candidates,which appears as a horizontal band,and D?sJ(2317)+→D+sπ0decay combined with unassociated γcandidates,which appears as a band that is almost vertical. An enhancement is evident in the vicinity of the overlap of these two bands with a D+sπ0γmass near2.46GeV/c2.The upper histogram of Fig.6c shows events in the D?s(2112)+signal region,and the shaded histogram shows those in the two D?s(2112)+sidebands.One can conclude that a state with decay to D+sπ0γis seen to exist over a background from D?sJ(2317)+combined with aγ.This background is peaked at a mass slightly higher than the signal.In Fig.6d the subtracted plot shows the narrow signal?tted to a Gaussian shape on a second order polynomial background at?m(D+sγ)=346.2±0.9MeV/c2(statistical errors only). The signal,which is labelled D sJ(2458)+,may decay either through D+sπ0γ, D?s(2112)+π0or D?sJ(2317)+γ.To disentangle these decay modes and extract the most information from the data,a binless-maximum likelihood?t to the D+sπ0γsystem has been developed.The?t determines a D sJ(2458)+mass of(2458.0±1.0)MeV/c2and a measured Gaussian width equal to(8.5±1.0)MeV/c2.This mass value(yielding?m=345.6±1.0MeV/c2)agrees with that obtained by BELLE but di?ers from that obtained by CLEO. Figure6:B A B A R experiment.(a)The mass distribution for all selected D+sπ0γcombinations.The shaded region is from D+s sidebands de?ned by1.912< m(K?K+π+)<1.933,1.999 The shape of the D+sπ0and D+sγdistributions in the D sJ(2458)+mass re-gion can be used in order to distinguish the two possible D sJ(2458)+→D?s(2112)+π0 and D sJ(2458)+→D?sJ(2317)+γdecay modes.These shapes are in?uenced by the allowed kinematic range for D sJ(2458)+decay,as shown in Fig.7a.Figs.7b–7c show the D sJ(2458)+background subtracted D+sπ0and D+sγmass projections compared with MC simulations of the two hypotheses.The decay D sJ(2458)+→D?s(2112)+π0 is clearly favored. The distribution of the angle?h of the decay D?s(2112)+→D+sγin its center-of-mass with respect to the D sJ(2458)+can be used investigate the spin-parity of the D sJ(2458)+.The resulting e?ciency-corrected cos?h distribution is Figure7:B A B A R experiment.(a)The D+sπ0versus D+sγmass distribution for all D+sπ0γcombinations.The curves indicate the kinematically allowed re-gion for D sJ(2458)+decay.(b)Sideband subtracted D+sγmass distribution with(line)Monte Carlo simulation for D sJ(2458)+→D?s(2112)+π0and(dashed) D sJ(2458)+→D?sJ(2317)+γ.(c)A similar plot for the D+sπ0mass distribution.(d) The e?ciency corrected yield as a function of?h(statistical errors only).The solid (dashed)histogram corresponds to the best?t of a sin2?h(1+cos2?h)distribution. shown in Fig.7d(statistical errors only).This distribution is not consistent with a sin?h distribution,which rules out a D sJ(2458)+spin-parity assignment of J P=0?. MC simulations have been used to measure the detector resolution,lead-ing to the conclusion that the intrinsic width of the D sJ(2458)+is small(Γ 10MeV/c2). 6Conclusions The decay of any c The low mass and the absence of a D?sJ(2317)+→D+sγfavors J P=0+ for D?sJ(2317)+.The mass of D?sJ(2317)+lies below the DK threshold,the mass of D sJ(2458)+lies above DK and below D?K thresholds. Di?erent interpretations for these states have been proposed.In ref.[13,14] models in terms of baryonia or molecules have been proposed.Ref.[15]provides an explanation in terms of relativistic vector and scalar exchange forces.Ref.[16]uses HQET plus chiral symmetry to predict parity doubling,i.e.the expectation is that the mass splitting between the1+and1?states should be the same as for the0+ and0?states. Since a cˉs mesons with these masses contradicts current models of charm meson spectroscopy[5,6,7],most likely these models need modi?cation. 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