Studies of the decays B+→ppˉh+ and observation of B+→Λˉ(1520)p

Original citation:

LHCb Collaboration (Including: Back, John J., Craik, Daniel, Dossett, D., Gershon, Timothy J., Kreps, Michal, Latham, Thomas, Pilar, T., Poluektov, Anton, Reid, Matthew M., Silva Coutinho, R., Wallace, Charlotte, Whitehead, M. (Mark) and Williams, Matthew P.). (2013) Studies of the decays B+→ppˉh+ and observation of B+→Λˉ(1520)p.

Physical Review D (Particles, Fields, Gravitation and Cosmology), Volume 88 (Number 5). Article number 052015. ISSN 1550-7998

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Studies of the decays

B

!

p

ph

and observation of

B

!

ð

1520

Þp

(LHCb Collaboration)

(Received 23 July 2013; published 19 September 2013)

Dynamics and directCPviolation in three-body charmless decays of chargedBmesons to a proton, an antiproton and a light meson (pion or kaon) are studied using data, corresponding to an integrated luminosity of1:0 fb1, collected by the LHCb experiment inppcollisions at a center-of-mass energy of 7 TeV. Production spectra are determined as a function of Dalitz-plot and helicity variables. The forward-backward asymmetry of the light meson in thepprest frame is measured. No significantCPasymmetry inBþ!ppK þ decay is found in any region of the Dalitz plane. We present the first observation of the decay Bþ! ð1520Þð!KþpÞp near the Kþp threshold and measure BðBþ! ð1520ÞpÞ ¼ ð3:9þ1:0

0:9ðstatÞ 0:1ðsystÞ 0:3ðBFÞÞ 107, where BF denotes the uncertainty on secondary branching fractions.

DOI:10.1103/PhysRevD.88.052015 PACS numbers: 13.25.Hw

I. INTRODUCTION

Evidence of inclusive directCPviolation in three-body charmless decays ofBþ mesons1has recently been found in the modes Bþ!Kþþ, Bþ!KþKþK, Bþ ! þþ, and Bþ !KþKþ [1,2]. In addition, very largeCPasymmetries were observed in the lowKþKand þ mass regions, without clear connection to a reso-nance. The localization of the asymmetries and the corre-lation of theCPviolation between the decays suggest that þ $KþK rescattering may play an important role in the generation of the strong phase difference needed for such a violation to occur [3,4]. Conservation of CPT symmetry imposes a constraint on the sum of the rates of final states with the same flavor quantum numbers, providing the possibility of entangled long-range effects contributing to the CP violating mechanism [5]. In con-trast, hþh$pp (h¼ or K throughout the paper) rescattering is expected to be suppressed compared to þ $KþK, and thus is not expected to play an important role.

The leading quark-level diagrams for the modes Bþ!pph þ are shown in Fig. 1. The Bþ!ppK þ mode is expected to be dominated by the b!s loop (penguin) transition while the mode Bþ!ppp þ is likely to be dominated by theb!utree decay, which is Cabibbo-Kobayashi-Maskawa matrix suppressed com-pared to the former. Since the short distance dynamics are similar to that of the Bþ!hþhþh modes, a CP analysis of Bþ!pph þ decays could help to clarify the

role of long-range scatterings in the CP asymmetries of Bþ!hþhþhdecays.

First studies were performed at the B factories on the production and dynamics of Bþ!pph þ decays [6–8]. The results have shown a puzzling opposite behavior of Bþ!ppK þandBþ!pp þdecays in the asymmetric occupation of the Dalitz plane. Charmonium contributions to theBþ!ppK þdecay have been studied by LHCb [9]. This paper reports a detailed study of the dynamics of the Bþ!pph þ decays and a systematic search forCP vio-lation, both inclusively and in regions of the Dalitz plane. The charmless region, defined for the invariant mass mpp<2:85 GeV=c2, is of particular interest. The relevant observables are the differential production spectra of Dalitz-plot variables and the global charge asymmetry ACP, defined as

ACP¼

NðB!fÞ NðBþ !fþÞ

NðB!fÞ þNðBþ !fþÞ; (1)

where f ¼pph . The mode Bþ!J=cð!ppÞKþ serves as a control channel. The first observation of the decayBþ! ð1520Þpis presented. Its branching fraction is derived through the ratio of its yield to the measured yield of theBþ !J=cð!ppÞKþdecay.

II. DETECTOR AND SOFTWARE

The LHCb detector [10] is a single-arm forward spec-trometer covering the pseudorapidity range 2< <5, designed for the study of particles containingborcquarks. The detector includes a high precision tracking system consisting of a silicon-strip vertex detector surrounding theppinteraction region, a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4 Tm, and three stations of silicon-strip detectors and straw drift tubes placed downstream. The combined tracking system has momentum resolution p=p that varies from 0.4% at5 GeV=cto 0.6% at100 GeV=c, and

*Full author list given at the end of the article.

Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri-bution of this work must maintain attridistri-bution to the author(s) and the published article’s title, journal citation, and DOI.

1_{Throughout the paper, the inclusion of charge conjugate}

impact parameter (IP) resolution of20mfor tracks with high transverse momentum. Charged hadrons are identified using two ring-imaging Cherenkov detectors (RICH) [11]. Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire pro-portional chambers.

The trigger [12] consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage which applies a full event reconstruction. Events triggered both on objects indepen-dent of the signal, and associated with the signal, are used. In the latter case, the transverse energy of the hadronic cluster is required to be at least 3.5 GeV. The software trigger requires a two-, three- or four-track secondary vertex with a large sum of the transverse momentum,pT,

of the tracks and a significant displacement from all pri-maryppinteraction vertices. At least one track must have pT>1:7 GeV=c, track fit2 per degree of freedom less

than 2, and an impact parameter2 (2IP) with respect to

any primary interaction greater than 16. The2IPis defined

as the difference between the 2 of the primary vertex reconstructed with and without the considered track. A multivariate algorithm is used to identify secondary vertices [13].

The simulatedppcollisions are generated usingPYTHIA

6.4 [14] with a specific LHCb configuration [15]. Decays of hadronic particles are described by EVTGEN [16] in which final state radiation is generated using PHOTOS

[17]. The interaction of the generated particles with the detector and its response are implemented using theGEANT4

toolkit [18] as described in Ref. [19]. NonresonantBþ ! pph þevents are simulated, uniformly distributed in phase space, to study the variation of efficiencies across the Dalitz plane, as well as resonant samples such asBþ!J=cð! ppÞKþ, Bþ!cð!ppÞKþ, Bþ !cð2SÞð!ppÞKþ, Bþ! ð1520Þð!KþpÞp, andBþ!J=cð!ppÞþ.

III. SIGNAL RECONSTRUCTION AND DETERMINATION

Candidate Bþ!pph þ decays are formed by com-bining three charged tracks, with appropriate mass

assignments. The tracks are required to satisfy track fit quality criteria and a set of loose selection require-ments on their momenta, transverse momenta, 2IP, and

distance of closest approach between any pair of tracks. The requirement on the momentum of the proton candidates, p >3 GeV=c, is larger than for the kaon and pion candidates, p >1:5 GeV=c. The Bþ candidates formed by the combinations are re-quired to have pT>1:7 GeV=c and 2IP<10. The

distance between the decay vertex and the primary vertex is required to be greater than 3 mm, and the vector formed by the primary and decay vertices must align with the Bþ candidate momentum. Particle identification (PID) is applied to the proton, kaon and pion candidates, using combined subdetector information, the main separation power being provided by the RICH system. The PID efficiencies are derived from data calibration samples of kinemati-cally identified pions, kaons and protons originating from the decays Dþ!D0ð!KþÞþ and !p.

Signal and background are extracted using unbinned extended maximum likelihood fits to the mass of the pph þ combinations. The Bþ!ppK þ signal is mod-eled by a double Gaussian function. The combinatorial background is represented by a second-order polyno-mial function. A Gaussian function accounting for a partially reconstructed component from B!ppK de-cays is used. A possible pp þ cross-feed contribution is included in the fit and is found to be small. An asymmetric Gaussian function with power law tails is used to estimate the uncertainties related to the variation of the signal yield.

In the case of theBþ!pp þdecay, the signal yield is smaller and the background is larger. The ranges of the signal and cross-feed parameters are constrained to the values obtained in the simulation within their uncertain-ties. The signal and theppK þcross-feed contribution are modeled with Gaussian functions. The combinatorial background is represented by a third-order polynomial function.

The Bþ!pph þ invariant mass spectra are shown in Fig. 2. The signal yields obtained from the fits are NðppK Þ ¼7029139 and Nðpp Þ ¼65670, where the uncertainties are statistical only.

u _uu

d p d u p b

+

B

d , s u

u

+

π

,

+

K W

u _uu

d p d u p b

+

B

d , s u

u

+

π

,

+

K W

FIG. 1. Leading tree and penguin diagrams forBþ!pph þ decays.

IV. DYNAMICS OFBþ!pph þ DECAYS

To probe the dynamics of the Bþ!pph þ decays, differential production spectra are derived as a function of mpp and cosp, where p is the angle between the charged mesonhand the opposite-sign baryon in the rest

frame of thepp system. Thepph þinvariant mass is fitted in bins of the aforementioned variables and the signal yields are corrected for trigger, reconstruction and selec-tion efficiencies. They are estimated with simulated samples and corrected to account for discrepancies be-tween data and simulation. The signal yields are deter-mined with the fit models described in the previous section, but allowing the combinatorial background parameters to vary. The systematic uncertainties are determined for each bin and include uncertainties related to the PID correction, fit model, trigger efficiency, and the size of the simulated samples. The latter is evaluated from the differences be-tween data and simulation as a function of the Dalitz-plot variables. No trigger-induced distortions are found.

A. Invariant mass of thepp system

The yields and total efficiency forBþ!pph þinmpp bins are shown in TablesIandII. The charmonium contri-butions originate from the decaysBþ!J=cð!ppÞKþ, Bþ!cð!ppÞKþ and Bþ!cð2SÞð!ppÞKþ for the Bþ!ppK þ mode, and Bþ!J=cð!ppÞþ for theBþ!pp þ mode. Before deriving the distribu-tions, the charmonium contributions are unfolded by

TABLE II. FittedBþ!pp þsignal yield, including theJ=c mode,Bþ!ppK þcross-feed yield, signal efficiency, and relative systematic uncertainty in bins ofpp invariant mass.

mpp ½GeV=c2 Bþ!pp þyield Bþ!ppK þ cross-feed Efficiency (%) Systematics (%)

<2:85 56461 11462 1:310:10 7.6

<2 14026 6426 1:340:15 11

[2, 2.2] 26131 1029 1:300:10 7.9

[2.2, 2.4] 9530 039 1:330:09 7.1

[2.4, 2.6] 4828 1430 1:350:09 6.4

[2.6, 2.85] 2120 3523 1:260:07 5.9

[2.85, 3.15] 7219 1218 1:280:07 5.5

[3.15, 3.3] 1911 03 1:240:08 6.7

[3.3, 4] 07 023 1:240:06 4.7

>4 2321 5723 0:940:05 4.9

TABLE I. FittedBþ!ppK þsignal yield, including the char-monium modes, efficiency and relative systematic uncertainty, in bins ofppinvariant mass. The error on the efficiency includes all the sources of uncertainty.

mpp ½GeV=c2Bþ!ppK þyield Efficiency (%) Systematics (%)

<2:85 331583 1:740:04 2.9

<2 44632 1:800:08 8.1

[2, 2.2] 100142 1:770:05 4.4

[2.2, 2.4] 73239 1:770:03 4.0

[2.4, 2.6] 55035 1:670:03 3.4

[2.6, 2.85] 58034 1:670:02 2.9

[2.85, 3.15] 276858 1:610:02 2.6

[3.15, 3.3] 12518 1:570:03 3.8

[3.3, 4] 58537 1:470:01 2.2

>4 23332 1:220:01 2.3

]

2

[GeV/c ± K p p

m

5.1 5.2 5.3 5.4 5.5

)

2

Candidates / (0.01 GeV/c

0 500 1000 1500 2000 2500

LHCb

]

2

[GeV/c ±

π

p p

m

5.1 5.2 5.3 5.4 5.5

)

2

Candidates / (0.01 GeV/c

0 200 400 600 800

1000 _LHCb

FIG. 2 (color online). Invariant mass distributions of (left)ppK þand (right)pp þcandidates. The points with error bars represent data. The solid black line represents the total fit function. Blue dashed, purple dotted, red long-dashed and green dashed-dotted curves represent the signal, cross-feed, combinatorial background and partially reconstructed background, respectively.

performing two dimensional extended unbinned maxi-mum likelihood fits to the pph þ and pp invariant masses. The J=c and cð2SÞ resonances are modeled by Gaussian functions and the c resonance is modeled by a convolution of Breit-Wigner and Gaussian func-tions. The nonresonantpp component and the combina-torial background are modeled by polynomial shapes. Table III shows the yields of contributing charmonium modes. The results are consistent with those reported in Ref. [9].

After unfolding, the efficiency-corrected differential dis-tributions are shown in Fig.3. An enhancement is observed at lowppmass both forBþ!ppK þandBþ!pp þ, with a more sharply peaked distribution forBþ!pp þ. This accumulation of events at lowmpp is a well known feature that has also been observed in different contexts such as ð1SÞ !pp [20],J=c !pp [21] andB0 !DðÞ0pp [22] decays. It appears to be caused by proton-antiproton rescattering and is modulated by the particular kinematics of the decay from which thepp pair originates [23].

B. Invariant mass squared of theKpsystem The Bþ!ppK þ signal yield as a function of the Dalitz-plot variablem2_Kp is considered, whereKpdenotes the neutral combinationsKporKþp. TableIVshows the

yields and efficiencies, after the charmonium bands have been vetoed in the ranges mpp 2 ½2:85;3:15 GeV=c2 and ½3:60;3:75GeV=c2. The differential spectrum derived after efficiency correction is shown in Fig. 4. Contrary to the situation formpp, the data distribution is in reasonable agree-ment with the uniform phase space distribution, with some discrepancies in the regionm2Kp2 ½4;12 ðGeV=c2Þ2.

C. Helicity angle of thepp system

TheBþ!pph þsignal yields are considered as a func-tion of cosp. Tables VandVI show the corresponding yields and efficiencies. The differential distributions are shown in Fig.5.

The forward-backward asymmetries are derived by com-paring the yields forcosp>0andcosp<0, accounting for the weighted-average efficiencies in each region

AFB¼

Npos pos Nneg neg Npos pos þ Nneg neg

¼NposfNneg

NposþfNneg; (2)

where pos¼ðcosp>0Þ and neg¼ðcosp<0Þ are the averaged efficiencies, f¼pos=neg and TABLE III. Yields, efficiencies and relative systematic

uncer-tainties of the charmonium modes from the combined ðmpph þ; mppÞ fits for the regions mpp 2 ½2:85;3:15GeV=c2

(for both Bþ!ppK þ and Bþ!pp þ) and

½3:60;3:75GeV=c2(forBþ!ppK þ).

Mode Yield

Efficiency (%)

Systematics (%)

Bþ!J=cð!ppÞKþ 141340 1:6240:005 1.8

Bþ!cð!ppÞKþ 72236 1:6600:005 2.0

Bþ!cð2SÞð!ppÞKþ 13216 1:4750:011 1.5

Bþ!J=cð!ppÞþ 5911 1:3280:011 4.2

TABLE IV. Fitted Bþ!ppK þ yields after subtracting the charmonium bands, efficiencies and relative systematic uncer-tainties in bins of Kpinvariant mass squared.

m2Kp ½ðGeV=c2Þ2

Bþ!ppK þ yield

Efficiency (%)

Systematics (%)

<4 45437 1:400:02 3.3

[4, 6] 52236 1:430:02 2.5

[6, 8] 79737 1:450:01 2.6

[8, 10] 70242 1:510:01 2.6

[10, 12] 44532 1:530:01 2.8

[12, 14] 52634 1:660:01 2.8

[14, 16] 33829 1:670:02 3.4

>16 30528 1:660:02 3.5

] 2 [GeV/c p p m

2 3 4 5

] 2 c -1 [GeV p p dm dN 0 50 100 150 200 250 300 3 10 × LHCb ] 2 [GeV/c p p m

2 3 4 5

] 2 c -1 [GeV _p p dm dN 0 20 40 60 80 100 120 3 10 × LHCb

FIG. 3 (color online). Efficiency-corrected differential yield as a function ofmpp for (left)Bþ!ppK þ and (right)Bþ!pp þ. The data points are shown with their statistical and total uncertainties. For comparison, the solid lines represent the expectations for a uniform phase space production, normalized to the efficiency-corrected area.

Npos ¼Nðcosp>0Þ, Nneg¼Nðcosp<0Þ. The values obtained are AFBðppK þÞ ¼0:3700:018ðstatÞ

0:016ðsystÞ and AFBðpp þÞ ¼ 0:3920:117ðstatÞ

0:015ðsystÞ, where the systematic uncertainties are eval-uated from the uncertainties on the efficiencies listed in TablesVandVI, taking into account the relative weights of the bins.

A clear opposite angular correlation between Bþ ! ppK þ and Bþ !pp þ decays is observed; the light

meson htends to align with the opposite-sign baryon for B!ppK while it aligns with the same-sign baryon for the B !pp mode. A quark level analysis suggests that the meson should align with the same-sign baryon, since the opposite-sign baryon has larger momentum, being formed by products from the decaying quark[24]. This is in agreement with the angular spectrum of Bþ!pp þ but not forBþ!ppK þdecays.

D. Dalitz plot

From the fits to theB-candidate invariant mass, shown in Fig. 2, signal weights are calculated with the sPlot tech-nique [25] and are used to produce the signal Dalitz-plot distributions shown in Fig.6. To ease the comparison, the cospcurves corresponding to the boundaries of the eight bins used to make the angular distributions in Fig. 5 are superimposed.

With the exception of the charmonium bands [c,J=c, cð2SÞ forBþ!ppK þ, and J=c for Bþ!pp þ], the structure of the lowppmass enhancement is very different betweenBþ!ppK þandBþ!pp þ. TheBþ!ppK þ events are distributed in the middle and lower m2_Kp half, exhibiting a possible pp band structure near 4 GeV2=c4. An enhancement at lowmKpis also observed and is caused to a large extent by að1520Þsignal, as will be shown in the next section. The Bþ!pp þ events are mainly clustered in the upper m2p half, with also a few events

TABLE V. FittedBþ!ppK þyields, efficiencies and relative systematic uncertainties in bins ofcosp.

cosp range Bþ!ppK þ yield Efficiency (%) Systematics (%)

½1;0:75 50834 1:540:01 2.7

½0:75;0:5 49731 1:510:02 3.0

½0:5;0:25 30927 1:480:01 2.9

½0:25;0 38128 1:490:01 2.6

[0, 0.25] 64046 1:510:01 2.9

[0.25, 0.5] 79942 1:520:01 2.2

[0.5, 0.75] 97641 1:560:01 2.8

[0.75, 1] 134651 1:550:01 2.7

TABLE VI. FittedBþ!pp þ signal yields, efficiencies and relative systematic uncertain-ties in bins ofcosp.

cosprange Bþ!pp þ yield Efficiency(%) Systematics (%)

½1;0:75 15031 1:230:02 5.5

½0:75;0:5 8527 1:150:02 5.5

½0:5;0:25 10424 1:190:02 5.5

½0:25;0 7723 1:190:02 5.5

[0, 0.25] 4321 1:140:02 5.5

[0.25, 0.5] 2420 1:160:02 5.5

[0.5, 0.75] 1012 1:190:02 5.5

[0.75, 1] 9326 1:190:02 5.2

] 4 /c 2 [GeV 2 Kp m

5 10 15 20

]

4

c

-2

[GeV

2 Kp

dm

dN

0 5000 10000 15000 20000 25000 30000

LHCb

FIG. 4 (color online). Efficiency-corrected differential yield as a function ofm2KpforBþ!ppK þ. The data points are shown with their statistical and total uncertainties. The solid line represents the expectation for a uniform phase space production, normalized to the efficiency-corrected area, for comparison.

on the doubly charged top diagonal ðpÞþþ (near the cosp¼ 1boundary). These distributions of events are consistent with the angular distributions and asymmetries reported earlier.

V. MEASUREMENT OFA_CPFOR Bþ _!ppK þ _DECAYS

The raw charge asymmetry is obtained by performing a simultaneous extended unbinned maximum likelihood fit to theBandBþsamples. TheByields are defined as a function of the total yieldNand the raw asymmetry,Araw,

byN¼Nð1ArawÞ=2.

The CP asymmetry is then derived after correcting for the B production asymmetry APðBÞ and the kaon

detection asymmetryADðKÞ

ACP¼ArawAPðBÞ ADðKÞ: (3)

The correctionA¼APðBÞ þADðKÞis measured from

data with the decayB!J=cð!ppÞKwhich is part of the data sample

A¼ArawðJ=cð!ppÞKÞ ACPðJ=cKÞ; (4)

whereACPðJ=cKÞ ¼ ð17Þ 103 [26].

Another correction has been applied to account for the proton-antiproton asymmetry, which exactly cancels for J=cð!ppÞK but not necessarily in the full phase space of ppK events. This effect has been estimated in simu-lation studying the difference in the interactions of pro-tons and antipropro-tons with the detector material between J=cð!ppÞK and ppK events generated uniformly over phase space. We obtained a m2Kp-dependent bias, up to 3% for the highest bin, for Araw.

To measureArawfor charmonium modes, and in particular

J=cð!ppÞK, a two-dimensionalðmB; mppÞsimultaneous fit to theBþ andB samples is performed. The systematic uncertainties are estimated by varying the fit functions and splitting the data sample according to trigger requirements or magnet polarities, and recombining the results from the sub-samples. The procedure is applied to obtain a global value of ACPas well as the variation of the asymmetry as a function of the Dalitz-plot variables. The results areACP¼ 0:022 0:031ðstatÞ 0:007ðsystÞfor the fullppK spectrum, and ACP¼ 0:0470:036ðstatÞ 0:007ðsystÞ for the region mpp<2:85 GeV=c2. Figure 7shows the variation of ACP as a function of the Dalitz-plot variables.

For the charmonium resonances, the values are ACPðcKÞ ¼0:0460:057ðstatÞ 0:007ðsystÞ and ] 2 ) 2 [(GeV/c 2 p p m

5 10 15 20

] 2₎ 2 [(GeV/c 2 Kp m 2 4 6 8 10 12 14 16 18 20 ) 8 /c 4

Signal yield /(0.16 GeV

0 10 20 30 40 50 60 70 LHCb ] 2 ) 2 [(GeV/c 2 p p m

5 10 15 20 25

]

2₎

2

[(GeV/c

2 pπ

m 0 2 4 6 8 10 12 14 16 18 20 ) 8 /c 4

Signal yield /(0.20 GeV

0 2 4 6 8 10 12 14 16 18 20 22 LHCb

FIG. 6 (color online). Signal weighted Dalitz-plot distributions for (left)Bþ!ppK þand (right)Bþ!pp þ. Also shown are the iso-cosplines corresponding to thecospbin boundaries;cosp¼ 1(þ1) is the uppermost (lowermost) line. The distributions are not corrected for efficiency.

p θ

cos

-1 -0.5 0 0.5 1

p θ cos d dN 0 50 100 150 200 250 300 350 3 10 × LHCb p θ cos

-1 -0.5 0 0.5 1

p θ cos d dN 0 10000 20000 30000 40000 50000 60000 LHCb

FIG. 5. Efficiency-corrected differential yields as functions ofcospfor (left)Bþ!ppK þand (right)Bþ !pp þmodes, after subtraction of the charmonium contributions. The data points are shown with their statistical and total uncertainties.

ACPðcð2SÞKÞ ¼ 0:0020:123ðstatÞ 0:012ðsystÞ. All results indicate no significantCPasymmetries.

VI. OBSERVATION OF THE Bþ_{! ð1520ÞpDECAY}

In theppK þspectrum, near the threshold of the neutral Kpcombination, a peak in invariant mass at1:52 GeV=c2 is observed, as shown in Fig.8, corresponding to theuds resonance ð1520Þ. The possible presence of higher and resonances may explain the enhancement in the range of½1:6;1:7GeV=c2.

To identify the ð1520Þsignal, theBþsignal is analyzed in the regionmKp2 ½1:44;1:585GeV=c2. Figure9shows theBsignal weightedKpinvariant mass, and the expected ð1520Þshape obtained from a model based on an asym-metric Breit-Wigner function derived from anEVTGEN[16] simulation of the decay Bþ! ð1520Þp, convolved with a Gaussian resolution function, and a second-order polynomial function representing the tail of the non-ð1520ÞBþ !ppK þ decays.

These shapes are then used in a two-dimensional ðmppK þ; mKpÞ extended unbinned maximum likelihood fit

to obtain the Bþ! ð1520Þp yield. The fit results in NðBþ! ð1520ÞpÞ ¼47þ1211with a statistical significance

of 5.3 standard deviations, obtained by comparing the like-lihood at its maximum for the nominal fit and for the background-only hypothesis. Figure10shows the projections of the fit for theKpandppK þinvariant masses.

To test the robustness of the observation, different representations of the Kp background have been used, combining first- or second-order polynomials and a contribution modeled by a Breit-Wigner function, for which the mean () and width () are allowed to vary within the known values of the ð1600Þ baryon (2 ½1:56;1:7 GeV=c2, 2 ½0:05;0:25GeV=c2). Fits in a wider mKp range were also considered. In all cases the yield was stable with a statistical significance similar to the nominal fit case.

The branching fraction for the decayBþ! ð1520Þpis derived from the ratio

BðBþ ! ð1520Þð!KþpÞpÞ

BðBþ!J=cð!ppÞKþÞ

¼Nð1520Þ!Kp NJ=c!pp

gen

J=c!pp gen_ð1520Þ!Kp

sel_J=c!pp

selð1520Þ!Kp ; (5)

]

2

[GeV/c

p p

m

2 3 4 5

CP

A

-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

LHCb

]

2

)

2

[(GeV/c

2 Kp

m

5 10 15 20

CP

A

-0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25

LHCb

FIG. 7. Distribution ofACPfor the Dalitz-plot projections onmppandm2KpforB!ppK events. In thempp projection (left), the bin½2:85;3:15GeV=c2contains only the value of the charmless ppK after subtraction of thecJ=c contribution. Them2Kp projection (right) has been obtained after removing the charmonia bands.

]

2

[GeV/c

Kp

m

1.4 1.6 1.8 2 2.2 2.4

)

2

Candidates/(0.01 GeV/c

0 10 20 30 40

50 LHCb

FIG. 8. Invariant mass mKp for the Bþ !ppK þ candidates near threshold.

] 2 [GeV/c Kp m

1.45 1.5 1.55

)

2

Signal yield /(0.01 GeV/c

-5 0 5 10 15

20 _LHCb

FIG. 9 (color online). Fit to the B signal weighted mKp distribution.

whereNiis the yield of the decay chaini, andgendenotes the efficiency after geometrical acceptance and simulation requirements. The global selection efficiencyselincludes the reconstruction, the trigger, the offline selection, and the particle identification requirements. The ratio of branching fractions obtained is

BðBþ! ð1520Þð!KþpÞpÞ

BðBþ!J=cð!ppÞKþÞ ¼0:041þ0:011

0:010ðstatÞ 0:001ðsystÞ:

The systematic uncertainties include effects of the Kp background model, the particle identification, the limited simulation sample size, and the uncertainties on the relative trigger efficiencies, and they are summarized in TableVII. Convolving the systematic uncertainty with the statistical likelihood profile, the global significance is 5.1 standard deviations.

Using BðBþ !J=cKþÞ ¼ ð1:0160:033Þ 103,

BðJ=c !ppÞ ¼ ð2:170:07Þ 103 [26], and

Bðð1520Þ !KpÞ ¼0:2340:016[27], the branching fraction is

BðBþ! ð1520ÞpÞ

¼ ð3:9þ10::09ðstatÞ 0:1ðsystÞ 0:3ðBFÞÞ 107:

The last error corresponds to the uncertainty on the sec-ondary branching fractions. This result is in agreement

with the upper limit set in Ref. [6],BðBþ! ð1520ÞpÞ<

1_:5106_{. Considering the separate}

B signals in the range mKp2 ½1:44;1:585GeV=c2, the yields are NðBÞ ¼5012andNðBþÞ ¼2711.

VII. SUMMARY

Based on a data sample, corresponding to an integrated luminosity of 1:0 fb1, collected in 2011 by the LHCb experiment, an analysis of the three-body Bþ !pph þ decays (h¼K or ) has been performed. The dynamics of the decays has been probed using differential spectra of Dalitz-plot variables and signal-weighted Dalitz plots. The charmless Bþ!ppK þ decay populates mainly the low m2pp and lower m2Kþp-half regions whereas the Bþ!pp þ decay has a similar enhancement at low m2pp but with an upper m2þp-half occupancy. From the occupation pattern of the Dalitz plots, it is likely that the Bþ!ppK þdecay is primarily driven bypp rescattering with a secondary contribution from neutralKprescattering while the Bþ!pp þ decay is also dominated by pp rescattering but with a secondary contribution from doubly chargedðpÞþþrescattering, along the lines of the rescat-tering amplitude analysis performed in Ref. [28]. This difference of behavior is reflected in the values of the forward-backward asymmetry of the light meson in the pp rest frame

AFBðppK þÞ ¼0:3700:018ðstatÞ 0:016ðsystÞ;

AFBðpp þÞ ¼ 0:3920:117ðstatÞ 0:015ðsystÞ:

CP asymmetries for the Bþ!ppK þ decay have been measured and no significant deviation from zero ob-served: ACP¼ 0:0470:036ðstatÞ 0:007ðsystÞ for the charmless regionmpp<2:85 GeV=c2,ACPðcKÞ ¼ 0_:0460_:057ðstatÞ 0_:007ðsystÞ and ACPðcð2SÞKÞ ¼ 0:0020:123ðstatÞ 0:012ðsystÞ. These measure-ments are consistent with the current known values, ACPðB!ppK ; mpp<2:85 GeV=c2Þ ¼ 0:160:07 [26], ACPðcKÞ ¼ 0:160:08ðstatÞ 0:02ðsystÞ [8], ]

2

[GeV/c

Kp

m

1.45 1.5 1.55

)

2

Candidates / (0.01 GeV/c

0 20 40 60 80

100 LHCb

]

2

[GeV/c ± K p p

m

5.1 5.2 5.3 5.4 5.5

)

2

Candidates / (0.01 GeV/c

0 10 20 30 40

50 LHCb

FIG. 10 (color online). Projections of (left)mKpand (right)mppK þ of the two-dimensional fit used to obtain theBþ! ð1520Þp signal yield.

TABLE VII. Systematic uncertainties for the BðBþ!

ð1520Þð!KþpÞpÞ=BðB þ!J=cð!ppÞK þÞbranching

frac-tion ratio. The total uncertainty is the sum in quadrature of the individual sources.

Source Uncertainty (%)

Kpbackground 2.1

PID 1.7

Simulation sample size 0.5

Trigger 1.0

Total 2.9

andACPðcð2SÞKÞ ¼ 0:0250:024[26]. The absence of any significant charge asymmetry, contrary to the situ-ation forBþ!hþhþhdecays [1,2], may be due to differ-ent long-range behavior. Final state interactions in the Bþ!pph þcase do not change the nature of the particles, such as pp !pp or ph!ph, while Bþ !hþhþh modes can be affected byþ$KþKscattering.

Finally, the observation of the decayBþ ! ð1520Þpis reported, with the branching fraction

BðBþ! ð1520ÞpÞ ¼ ð3_:9þ1:0

0:9ðstatÞ 0:1ðsystÞ 0:3ðBFÞÞ 107

in agreement with the current existing upper limit [6].

ACKNOWLEDGMENTS

We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of

the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 and Region Auvergne (France); BMBF, DFG, HGF and MPG (Germany); SFI (Ireland); INFN (Italy); FOM and NWO (The Netherlands); SCSR (Poland); MEN/IFA (Romania); MinES, Rosatom, RFBR and NRC ‘‘Kurchatov Institute’’ (Russia); MinECo, XuntaGal and GENCAT (Spain); SNSF and SER (Switzerland); NAS Ukraine (Ukraine); STFC (United Kingdom); NSF (USA). We also acknowledge the support received from the ERC under FP7. The Tier1 computing centers are supported by IN2P3 (France), KIT and BMBF (Germany), INFN (Italy), NWO and SURF (The Netherlands), PIC (Spain), GridPP (United Kingdom). We are thankful for the computing resources put at our disposal by Yandex LLC (Russia), as well as to the communities behind the multiple open source software packages that we depend on.

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(LHCb Collaboration)

1_{Centro Brasileiro de Pesquisas Fı´sicas (CBPF), Rio de Janeiro, Brazil} 2_{Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil}

3_{Center for High Energy Physics, Tsinghua University, Beijing, China} 4_{LAPP, Universite´ de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France}

5_{Clermont Universite´, Universite´ Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France} 6_{CPPM, Aix-Marseille Universite´, CNRS/IN2P3, Marseille, France}

7_{LAL, Universite´ Paris-Sud, CNRS/IN2P3, Orsay, France}

8_{LPNHE, Universite´ Pierre et Marie Curie, Universite´ Paris Diderot, CNRS/IN2P3, Paris, France} 9_{Fakulta¨t Physik, Technische Universita¨t Dortmund, Dortmund, Germany}

10_{Max-Planck-Institut fu¨r Kernphysik (MPIK), Heidelberg, Germany}

11_{Physikalisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany} 12

School of Physics, University College Dublin, Dublin, Ireland

13_{Sezione INFN di Bari, Bari, Italy} 14_{Sezione INFN di Bologna, Bologna, Italy}

15_{Sezione INFN di Cagliari, Cagliari, Italy} 16_{Sezione INFN di Ferrara, Ferrara, Italy}

17_{Sezione INFN di Firenze, Firenze, Italy}

18_{Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy} 19_{Sezione INFN di Genova, Genova, Italy}

20_{Sezione INFN di Milano Bicocca, Milano, Italy} 21_{Sezione INFN di Padova, Padova, Italy}

22_{Sezione INFN di Pisa, Pisa, Italy} 23

Sezione INFN di Roma Tor Vergata, Roma, Italy

24_{Sezione INFN di Roma La Sapienza, Roma, Italy}

25_{Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Krako´w, Poland} 26_{Faculty of Physics and Applied Computer Science, AGH-University of Science and Technology, Krako´w, Poland}

27_{National Center for Nuclear Research (NCBJ), Warsaw, Poland}

28_{Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania} 29_{Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia}

30_{Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia} 31_{Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia} 32_{Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia} 33_{Budker Institute of Nuclear Physics (SB RAS) and Novosibirsk State University, Novosibirsk, Russia}

34_{Institute for High Energy Physics (IHEP), Protvino, Russia} 35_{Universitat de Barcelona, Barcelona, Spain}

36_{Universidad de Santiago de Compostela, Santiago de Compostela, Spain} 37_{European Organization for Nuclear Research (CERN), Geneva, Switzerland}

38_{Ecole Polytechnique Fe´de´rale de Lausanne (EPFL), Lausanne, Switzerland} 39

Physik-Institut, Universita¨t Zu¨rich, Zu¨rich, Switzerland

40_{Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands}

41_{Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands} 42_{NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine}

43_{Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine} 44_{University of Birmingham, Birmingham, United Kingdom}

45_{H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom} 46_{Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom} 47_{Department of Physics, University of Warwick, Coventry, United Kingdom}

48_{STFC Rutherford Appleton Laboratory, Didcot, United Kingdom}

49_{School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom} 50_{School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom}

51_{Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom} 52_{Imperial College London, London, United Kingdom}

53_{School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom} 54_{Department of Physics, University of Oxford, Oxford, United Kingdom}

55_{Massachusetts Institute of Technology, Cambridge, Massachusetts, USA} 56_{University of Cincinnati, Cincinnati, Ohio, USA}

57_{University of Maryland, College Park, Maryland, USA} 58_{Syracuse University, Syracuse, New York, USA}

59_{Pontifı´cia Universidade Cato´lica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil associated to Universidade Federal}

do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil

60_{Institut fu¨r Physik, Universita¨t Rostock, Rostock, Germany associated to Physikalisches Institut,}

Ruprecht-Karls-Universita¨t Heidelberg, Heidelberg, Germany

61_{Celal Bayar University, Manisa, Turkey associated to European Organization for Nuclear Research (CERN),}

Geneva, Switzerland

a_{Also at Universita` di Firenze, Firenze, Italy.}

b_{Also at Universita` della Basilicata, Potenza, Italy.}

c_{Also at Universita` di Modena e Reggio Emilia, Modena, Italy.}

d_{Also at Universita` di Padova, Padova, Italy.}

e_{Also at Universita` di Milano Bicocca, Milano, Italy.}

f_{Also at LIFAELS, La Salle, Universitat Ramon Llull, Barcelona, Spain.}

g_{Also at Universita` di Bologna, Bologna, Italy.}

h_{Also at Universita` di Roma Tor Vergata, Roma, Italy.}

i_{Also at Universita` di Genova, Genova, Italy.}

j_{Also at Universita` di Ferrara, Ferrara, Italy.}

k_{Also at Universita` di Cagliari, Cagliari, Italy.}

l_{Also at Scuola Normale Superiore, Pisa, Italy.}

m_{Also at Hanoi University of Science, Hanoi, Viet Nam.}

n_{Also at Universita` di Bari, Bari, Italy.}

o_{Also at Universita` di Roma La Sapienza, Roma, Italy.}

p_{Also at Universita` di Pisa, Pisa, Italy.}

q_{Also at Institute of Physics and Technology, Moscow, Russia.}

r_{Also at Universita` di Urbino, Urbino, Italy.}

s_{Also at P.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia.}