Observation of New States Decaying into $Lambda_c^+ K^-pi^+$ and $Lambda_c^+ K^0_Spi^-$

a r X i v :h e p -e x /0606051v 1 22 J u n 2006

Observation of New States Decaying into Λ+c K ?π+and Λ+c K 0S π

R.Chistov,11K.Abe,6K.Abe,42I.Adachi,6H.Aihara,44D.Anipko,1V.Aulchenko,1T.Aushev,11

A.M.Bakich,39V.Balagura,11E.Barberio,19A.Bay,16I.Bedny,1K.Belous,10U.Bitenc,https://www.360docs.net/doc/718033346.html,jak,12S.Blyth,22A.Bondar,1A.Bozek,25M.Braˇc ko,6,18,12J.Brodzicka,25T.E.Browder,5M.-C.Chang,43Y.Chao,24

A.Chen,22K.-F.Chen,24W.T.Chen,22

B.G.Cheon,3Y.Choi,38Y.K.Choi,38A.Chuvikov,33S.Cole,39J.Dalseno,19M.Danilov,11M.Dash,50J.Dragic,6A.Drutskoy,4S.Eidelman,1D.Epifanov,1N.Gabyshev,1A.Garmash,33T.Gershon,6A.Go,22G.Gokhroo,40B.Golob,17,12A.Goriˇs ek,12H.Ha,14J.Haba,6T.Hara,30K.Hayasaka,20H.Hayashii,21M.Hazumi,6D.He?ernan,30T.Hokuue,20Y.Hoshi,42S.Hou,22W.-S.Hou,24

Y.B.Hsiung,24T.Iijima,20A.Imoto,21K.Inami,20A.Ishikawa,44R.Itoh,6M.Iwasaki,44Y.Iwasaki,6

J.H.Kang,51N.Katayama,6H.Kawai,2T.Kawasaki,27H.R.Khan,45H.Kichimi,6H.J.Kim,15H.O.Kim,38K.Kinoshita,4S.Korpar,18,12P.Kriˇz an,17,12P.Krokovny,6R.Kumar,31C.C.Kuo,22A.Kuzmin,1Y.-J.Kwon,51

G.Leder,9J.Lee,37T.Lesiak,25S.-W.Lin,24D.Liventsev,11F.Mandl,9D.Marlow,33T.Matsumoto,46

A.Matyja,25S.McOnie,39W.Mitaro?,9K.Miyabayashi,21H.Miyake,30H.Miyata,27Y.Miyazaki,20R.Mizuk,11

G.R.Moloney,19T.Nagamine,43E.Nakano,29M.Nakao,6S.Nishida,6O.Nitoh,47T.Nozaki,6S.Ogawa,41T.Ohshima,20T.Okabe,20S.Okuno,13S.L.Olsen,5Y.Onuki,27H.Ozaki,6P.Pakhlov,11G.Pakhlova,11H.Palka,25H.Park,15K.S.Park,38R.Pestotnik,12L.E.Piilonen,50Y.Sakai,6T.Schietinger,16O.Schneider,16

A.J.Schwartz,4R.Seidl,7,34M.E.Sevior,19M.Shapkin,10H.Shibuya,41

B.Shwartz,1V.Sidorov,1J.B.Singh,31A.Sokolov,10A.Somov,4N.Soni,31S.Staniˇc ,28M.Stariˇc ,12H.Stoeck,39T.Sumiyoshi,46

S.Suzuki,35F.Takasaki,6N.Tamura,27M.Tanaka,6G.N.Taylor,19Y.Teramoto,29X.C.Tian,32I.Tikhomirov,11T.Tsuboyama,6T.Tsukamoto,6S.Uehara,6T.Uglov,11K.Ueno,24S.Uno,https://www.360docs.net/doc/718033346.html,ov,1G.Varner,5S.Villa,16C.C.Wang,24C.H.Wang,23M.-Z.Wang,24Y.Watanabe,45E.Won,14C.-H.Wu,24Q.L.Xie,8B.D.Yabsley,39A.Yamaguchi,43Y.Yamashita,26M.Yamauchi,6L.M.Zhang,36and Z.P.Zhang 36

(The Belle Collaboration)

Budker Institute of Nuclear Physics,Novosibirsk

Chiba University,Chiba

Chonnam National University,Kwangju 4

University of Cincinnati,Cincinnati,Ohio 452215

University of Hawaii,Honolulu,Hawaii 96822

High Energy Accelerator Research Organization (KEK),Tsukuba 7

University of Illinois at Urbana-Champaign,Urbana,Illinois 618018

Institute of High Energy Physics,Chinese Academy of Sciences,Beijing

Institute of High Energy Physics,Vienna 10

Institute of High Energy Physics,Protvino

Institute for Theoretical and Experimental Physics,Moscow

J.Stefan Institute,Ljubljana 13

Kanagawa University,Yokohama 14

Korea University,Seoul

Kyungpook National University,Taegu

Swiss Federal Institute of Technology of Lausanne,EPFL,Lausanne

University of Ljubljana,Ljubljana 18

University of Maribor,Maribor 19

University of Melbourne,Victoria 20

Nagoya University,Nagoya 21

Nara Women’s University,Nara 22

National Central University,Chung-li 23

National United University,Miao Li

Department of Physics,National Taiwan University,Taipei 25

H.Niewodniczanski Institute of Nuclear Physics,Krakow

Nippon Dental University,Niigata 27

Niigata University,Niigata

University of Nova Gorica,Nova Gorica

Osaka City University,Osaka 30

Osaka University,Osaka 31

Panjab University,Chandigarh 32

Peking University,Beijing

Princeton University,Princeton,New Jersey 0854434

RIKEN BNL Research Center,Upton,New York 11973

35Saga University,Saga

36University of Science and Technology of China,Hefei

37Seoul National University,Seoul

38Sungkyunkwan University,Suwon

39University of Sydney,Sydney NSW

40Tata Institute of Fundamental Research,Bombay

41Toho University,Funabashi

42Tohoku Gakuin University,Tagajo

43Tohoku University,Sendai

44Department of Physics,University of Tokyo,Tokyo

45Tokyo Institute of Technology,Tokyo

46Tokyo Metropolitan University,Tokyo

47Tokyo University of Agriculture and Technology,Tokyo

48Toyama National College of Maritime Technology,Toyama

49University of Tsukuba,Tsukuba

50Virginia Polytechnic Institute and State University,Blacksburg,Virginia24061

51Yonsei University,Seoul

We report the?rst observation of two charmed strange baryons that decay intoΛ+c K?π+.The

broader of the two states is measured to have a mass of2978.5±2.1±2.0MeV/c2and a width

of43.5±7.5±7.0MeV/c2.The mass and width of the narrow state are measured to be3076.7±

0.9±0.5MeV/c2and6.2±1.2±0.8MeV/c2,respectively.We also perform a search for the

isospin partner states that decay intoΛ+c K0Sπ?and observe a signi?cant signal at the mass of

3082.8±1.8±1.5MeV/c2.The data used for this analysis was accumulated at or near theΥ(4S) resonance,using the Belle detector at the e+e?asymmetric-energy collider KEKB.The integrated

luminosity of the data sample used is461.5fb?1.

PACS numbers:14.20.Lq,13.30.Eg

Several excitedΛ+c[1],Σc[2]andΞc[3]baryons have already been observed.The most recent examples are an isotriplet of excitedΣc baryons and theΛc(2940)+, reported by Belle and BaBar,respectively[4].The charmed baryon sector o?ers a rich source of states and possible orbital excitations,serving as an excellent lab-oratory to test the predictions of the quark model and other models of bound quarks[5],as well as predictions based on heavy quark symmetry[6].Some of the re-cently discovered baryons are candidates for orbitally ex-cited states[7].Within theΞc system,two candidates for the?rst P-wave orbital excitations were found,the Ξc(2790)andΞc(2815)baryons,which decay intoΞ′cπandΞ?cπ,respectively[8,9].In these decays,the charm and strange quarks of the initial state are inherited by the?nal state baryon.However,nothing is experimen-tally known about charmed strange baryons that decay toΛ+c K?π+.In such a decay processes,the charm and strangeness of the initial state are carried away by two di?erent?nal state particles,a charmed baryon and a strange meson.

The SELEX collaboration has reported the obser-vation of a doubly charmed baryon that decays into theΛ+c K?π+?nal state[10].The SELEX claim has not been con?rmed by other https://www.360docs.net/doc/718033346.html,bined with the measured cross section for double cˉc produc-tion[11],which is an order of magnitude larger than non-relativistic QCD predictions[12],a search for the SELEX doubly charmed baryon is an additional motiva-tion for the examination of theΛ+c K?π+?nal state.

In this Letter we report the results of a search for new baryons decaying intoΛ+c K?π+andΛ+c K0Sπ??nal states.Inclusion of charge conjugate states is implicit unless otherwise stated.The analysis is performed us-ing data collected with the Belle detector at the KEKB asymmetric-energy e+e?collider[13].The data sample corresponds to an integrated luminosity of461.5fb?1col-lected at or near theΥ(4S)resonance.

The Belle detector is a large-solid-angle magnetic spec-trometer that consists of a silicon vertex detector(SVD), a50-layer central drift chamber(CDC),an array of aerogel threshold Cherenkov counters(ACC),a barrel-like arrangement of time-of-?ight scintillation counters (TOF),and an electromagnetic calorimeter(ECL)com-prised of CsI(Tl)crystals located inside a superconduct-ing solenoid coil that provides a1.5T magnetic?eld.An iron?ux-return located outside of the coil is instrumented to detect K0L mesons and to identify muons(KLM).The detector is described in detail elsewhere[14].Two dif-ferent inner detector con?gurations were used,a2.0cm beam-pipe and a3-layer silicon vertex detector for the ?rst155fb?1,and a1.5cm beam-pipe with a4-layer vertex detector for the remaining306.5fb?1[15].We use a GEANT-based Monte Carlo(MC)simulation to model the response of the detector and determine the e?ciency[16].

Protons,charged pions and kaons are required to orig-inate from the region,dr<1cm,|dz|<3cm.Here, dr and dz are the distances of closest approach to the interaction point in the plane perpendicular to the beam

axis (r ?φplane)and along the beam direction,respec-tively.Charged hadrons are identi?ed using a likelihood ratio method,which combines information from the TOF system and ACC counters with dE/dx measurements in the CDC.To identify charged particles (π,K ,p ),we ap-ply the standard Belle requirements on the corresponding likelihood ratios [14].Neutral kaons are

reconstructed

via

the decay K 0

S →π+π?,requiring M (π+π?)to be within

±10MeV/c 2of the nominal K 0

S mass [7].We require the

displacement of the π+π?

vertex from the interaction point in the r ?φplane to be more than 0.1cm.

We reconstruct the Λ+c via the Λ+c →pK ?π

de-cay channel.All pK ?π+

combinations with an in-variant mass within ±10MeV/c 2(～2.5σ)around 2286.6MeV/c 2are selected as Λ+c candidates.

The mean value of the mass for our Λ+

c signal is 2286.6±0.1(stat.)MeV/c 2,in goo

d agreement with a recent pre-cision measurement by BaBar [17].W

e perform a mass

constrained ?t to the Λ+c vertex and then combine Λ+

c candidates with the remaining K ?π+pairs in the event.The momentum spectra of charme

d hadrons produced in

e +e ?→c ˉc continuum are hard compared to the com-binatorial background.Therefore,we apply the require-ment p ?>3.0GeV/c ,where p ?is the momentum o

f the

Λ+c K ?π

system in the center of mass frame.We also ?t the Λ+c K ?π+

combinations to a common vertex.The resulting invariant mass distribution M (Λ+c K ?π+

)is shown in Fig.1(a).Two peaks are visible in this distribution:a broad one near thresh-old at a mass of about 2980MeV/c 2and a narrower one at a higher mass of about 3077MeV/c 2.We verify that the observed signals are robust and their mass values stable against the variation of particle identi?cation cri-teria,Λ+c mass selection window and the p ?

requirement.Hereafter we denote the observed peaks as Ξcx (2980)+and Ξcx (3077)+as explained below.

Fig.1(b)shows the invariant mass distribution of

the wrong-sign (WS)combinations M (Λ+c K +π?

),which has a smooth structureless behaviour.This demon-strates that the observed peaks in the right-sign (RS)

M (Λ+c K ?π+

)invariant mass distribution are not re?ec-tions due to K ?πmisidenti?cation originating from the four known excited baryons Λc (2593)+,Λc (2625)+,

Λc (2765)+and Λc (2880)+[1,7]that decay into Λ+c π+π?

Re?ections from the Λ+c π+π

decay modes of these states would contribute equally to both the RS and WS distributions.This conclusion is also veri?ed with a MC simulation of Λc (2593)+,Λc (2625)+,Λc (2765)+and Λc (2880)+produced in e +e ?→c ˉc .We generate 104

Λ+c π+π?decays for each excited Λ+c state and recon-struct the MC events with the same selection criteria as the data.The resulting mass distributions exhibit similar behaviour in both RS and WS cases.Simulation shows that the contribution of excited Λ+c baryons is reduced to 1.2%with the above selection https://www.360docs.net/doc/718033346.html,ing the

reconstruction of Λc (2880)+→Λ+c π+π

on the same data set we ?nd that possible contribution of re?ections

is below the statistical sensitivity of our measurement.Fig.1(c)shows an additional check of the

M (Λ+c K ?π+)distribution of data events in the Λ+

c mass sidebands [18].We also check the invariant mass dis-tribution for the Λ+c K ?π

WS combination,shown in Fig.1(d).Both distributions in Fig.1(c)and (d)are featureless near the 2980MeV/c 2and 3077MeV/c 2mass regions.

Results of the ?t to the M (Λ+c K ?π+

)distribution are shown by the solid curve in Fig.1(a).The simulated

mass resolution of Ξcx (2980)→Λ+c K ?π

is found to be 1.2MeV/c 2,which is much smaller than the observed signal width.Therefore,the broad signal near the thresh-old is modeled by a Breit-Wigner function only.To de-scribe the Ξcx (3077)+resonance we use a Breit-Wigner convolved with a Gaussian detector resolution function.The width of the Gaussian (σ)is ?xed from MC to be 2.0MeV/c 2.The background is described by a thresh-old function atan(

√?2ln(L 0/L max ).Here,L 0and L max are the values of the likelihood function with the corresponding signal ?xed to zero and at the best ?t value,respectively.We ?t the WS mass distri-bution using the same parametrization,with parameters describing the shape of the signal ?xed to the above val-ues.The ?t yields ?34.8±19.6(?78.2±54.6)events for the higher (lower)mass peak,consistent with zero.

To provide more information on the origin of the states found in the present analysis,we perform a search

for their neutral isospin partners in the Λ+c K 0S π

?nal state.In this case the selection criteria are the same

as for the Λ+c K ?π

?nal state with one exception:a tighter momentum requirement,p ?>3.5GeV/c ,is ap-plied for the Λ+c K 0S π

system.The resulting invariant mass distribution,M (Λ+c K 0S π?

),is shown in Fig.2(a),where a clear signal near 3077MeV/c 2is observed.A broad enhancement near the threshold can also be iden-ti?ed.Fig.2(b)shows the WS M (Λ+c K 0S π+

)distribu-tion,which is featureless.To describe the narrow signal,which we denote as Ξcx (3077)0,we use a Breit-Wigner function convolved with a Gaussian detector resolution function.The width of the Gaussian is ?xed from MC to be σ=2.4MeV/c 2.To describe the broad signal near the threshold which we denote as Ξcx (2980)0we use a Breit-Wigner function with the width ?xed to that of Ξcx (2980)+,Γ=43.5MeV/c 2.The background is described by a threshold function multiplied by a third-order polynomial.The signal yields and parameters of the two Breit-Wigner functions determined from the ?t are given in Table I.

We also vary order of the polynomial for the back-

TABLE I:Summary of the parameters of the new states in the Λ+c K ?π+and Λ+c K 0S π+

?nal states:masses,widths,yields and statistical signi?cances.

New State

Mass (MeV/c 2)Width (MeV/c 2)

Yield (events)Signi?cance (σ)

Ξcx (2980)02977.1±8.8±3.543.5(?xed)42.3±23.8 2.0Ξcx (3077)0

3082.8±1.8±1.55.2±3.1±1.867.1±19.9 5.1

M(Λc + K -π+) (GeV/c 2

)

E v e n t s / 2.5 M e V /c 2

M(Λc + K +π-) (GeV/c 2

)

E v e n t s / 2.5 M e V /c 2

255075100125150175

2002.9

2.95

3.05

3.1

3.15

3.2

3.25

020406080100120140

2.9

2.953

3.05 3.1 3.15 3.2 3.25

M(Λc + K -π+) (GeV/c 2

)

E v e n t s / 2.5 M e V /c 2

M(Λc + K -π-) (GeV/c 2

)

E v e n t s / 2.5 M e V /c 2

4060801001201402.9

2.953

3.05 3.1 3.15 3.2 3.25

FIG.1:(a)M (Λ+c K ?π+

)distribution together with the overlaid ?tting curve.Points with errors represent the data,dashed line is the background component of the ?tting function described in the text,and the solid curve is the sum of the background and

signal.(b)The WS combination mass distribution M (Λ+c K +π?

)?tted with the same function including the signal components where the masses and widths of the signals are ?xed to the values from the ?t to the RS distribution.Two additional cross-checks are shown (c)the invariant mass distribution of the right-sign Λ+c K ?π

combinations but using appropriately scaled sidebands of the Λ+c signal and (d)the invariant mass distribution of the other WS Λ+c K ?π?

combinations.No structures are visible in the signal regions near 2980MeV/c 2and 3077MeV/c 2.

ground function and the widths of the detector resolu-tion within their errors.The resulting systematic un-certainties are given in Table I.None of the variations reduces the signi?cances of the Ξcx (3077)+,Ξcx (2980)+and Ξcx (3077)0to less than 9σ,6σand 5σ,respectively.The SELEX Collaboration reported the observation of a doubly charmed baryon with a mass of 3520MeV/c 2

in the Λ+c K ?π

?nal state [10].We extend the range of the M (Λ+c K ?π+

)search to include the region sur-rounding 3520MeV/c 2(Fig.3).To compare the yield to the inclusive production of Λ+c ,we modify the mo-mentum requirement p ?

>2.5GeV/c only for the Λ+c baryon.We ?nd no evidence for a signal either at this mass or in a wide range around it.The overlaid curve in Fig.3is the result of the ?t.To describe a possi-

ble signal we use a Gaussian resolution function with the width ?xed to the signal MC value of 4.9MeV/c 2.The background is parameterized by a third-order polyno-mial function.From the ?t,we obtain an upper limit of 69.1events at 90%con?dence level (C.L.)When the same selection criteria are applied for the inclu-sive Λ+c (p ?

>2.5GeV/c )production,we reconstruct (83.5±1.4)×104Λ+c decays.Taking into account the ratio of the total reconstruction e?ciencies,we derive an upper limit on the ratio of production cross sections with

p ?(Λ+c )>2.5GeV/c ,σ(Ξcc (3520)+)×B (Ξcc (3520)+

→

Λ+c K ?π+)/σ(Λ+c )<1.5×10

at 90%C.L.Recently,the BaBar collaboration has also performed an extensive search for doubly charmed baryons.They set an upper limit of 2.7×10?4at 95%C.L.[19]for the same de-

cay process taking the account the e?ciencies of the p ?requirement.

In conclusion,we report the ?rst observation of two charged baryons Ξcx (2980)+and Ξcx (3077)+decaying

into Λ+c K ?π+

.We also search for neutral isospin re-lated partners in Λ+c K 0S π

??nal state and observe a signal for the Ξcx (3077)0

.The statistical signi?cance of each of these signals is more than 5σ.The masses and widths of all the observed states are summarized in Table I.Taking into account the presence of s and c quarks in the ?nal state and the observation of an isospin

5101520253035

402.9

2.95

3.05

3.1

3.15

3.2

3.25

M(Λc + K S 0π-) (GeV/c 2)

E v e n t s / 2.5 M e V /c 2

M(Λc + K S 0π+

) (GeV/c 2)

E v e n t s / 2.5 M e V /c 2

0510152025

2.9

2.953

3.05 3.1 3.15 3.2 3.25

FIG.2:(a)M (Λ+c K 0S π?

)distribution together with the over-laid ?tting curve.The ?tting function is the same as in the

Λ+c K +π

case (see the text).(b)The WS combination mass distribution M (Λ+c K 0S π+

M(Λc + K -π+) (GeV/c 2

)

E v e n t s / 5 M e V /c

FIG.3:The M (Λ+c K ?π+)distribution near 3520MeV/c 2

(indicated by an arrow),the mass of a possible doubly charmed baryon candidate [10].

partner near 3077MeV/c 2in the Λ+c K 0S π

?nal state,the most natural interpretations of these states are that they are excited charmed strange baryons,Ξc .In con-trast to decays of known excited Ξc states the observed baryons decay into separate charmed (Λ+c )and strange (K )hadrons.Further studies of the properties of the observed states are ongoing.We have also searched for the doubly charmed baryon state at 3520MeV/c 2re-ported by the SELEX collaboration in the Λ+c K ?π+

?nal state [10],and extract an upper limit on its production cross section relative to the inclusive Λ+c yield.

We thank the KEKB group for excellent operation of the accelerator,the KEK cryogenics group for e?cient solenoid operations,and the KEK computer group and the NII for valuable computing and Super-SINET net-work support.We acknowledge support from MEXT and JSPS (Japan);ARC and DEST (Australia);NSFC (con-tract No.10175071,China);DST (India);the BK21pro-gram of MOEHRD,and the CHEP SRC and BR (grant No.R01-2005-000-10089-0)programs of KOSEF (Ko-rea);KBN (contract No.2P03B 01324,Poland);MIST (Russia);MHEST (Slovenia);SNSF (Switzerland);NSC and MOE (Taiwan);and DOE (USA).

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