Measurements of the relative branching fractions of the $B^{+} \to p \bar p K^{+}$ decay channel including charmonium contributions

0.2 MB·English

by  R. Cardinale

Measurements of the relative branching fractions of the B+ → pp¯K+ decay
channel including charmonium contributions
R.Cardinale1,aonbehalfoftheLHCbCollaboration
UniversityofGenovaandINFNGenova,ItalyandCERN,Switzerland
Abstract. Thestudyofthe B+ → pp¯K+ decaychannelatLHCboffersgreatopportunitiestostudydifferent
aspectsoftheStandardModelandpossiblyBeyondStandardModelphysics.Inparticularitcanbeinterestingnot
2
1 onlyforthepossibilitytomeasureCPasymmetrybutalsotostudypossibleintermediateresonances.Theratiosof
0 thebranchingfractionsoftheB+→ pp¯K+decaychannel,ofthecharmlesscomponentwithMpp¯ <2.85GeV/c2
2 andofthecharmoniumcontributionηcrelativetotheJ/ψarepresented.

n
a
J 1 Introduction Moreover, three-body baryonic decays offer a clean en-
vironment to study intermediate states, as charmonium
5
2 ThecurrentknowledgeontheB+ → pp¯K+ decayisbased states, excited Λ baryons and exotic states (glueballs [5],
baryonium,pentaquarks).Infact,the pp¯ finalstatecanbe
on the measurements performed at the B-factories[1,2].
] producedthrougharesonantintermediatestate,acc¯ state
x Thanks to the high statistics available in the next years
e at LHCb, due to the large bb¯ production cross section, orthroughanunknownexoticstate.AtLHCb,beyondthe
- σ = (284±20±49)µb [3] at a centre of mass energy studyofwellknowncharmoniumstates,itcanbeinterest-
p ofbb¯√s = 7TeV, more precise measurements can be per- ingtoobservecharmonium-likestatessuchas X(3872)or
e
new intermediate states in order to study their nature and
h formedandpossiblynewphysicscanberevealed.
properties.
[ LHCb is a single arm forward spectrometer optimised to
AnotherinterestingfeatureobservedbothattheBaBarand
perform precise CP violation measurements and rare de-
1 BelledetectorsinthreebodybaryonicBdecaysisthelow
caysstudyintheheavyflavoursector.
v mass M enhancement. Several explanations have been
2 Three-body baryonic B decays are expected to proceed pp¯
proposed.AstudyofthedistributionofeventsintheDalitz
1 predominantly via b → s penguin diagrams or doubly
plotcanexplaintheoriginofthismassenhancement.[6].
2 Cabibbo suppressed tree diagrams [4] and as such new
A preliminary study of this mode based on the sample
5 heavy virtual particles can give measurable departures
. fromStandardModelexpectations.Thebaryon-antibaryon of events collected by the LHCb detector in the 2010
1
data taking period is presented. The branching ratios of
0 pair can also be produced by a pair of gluons in OZI B+ → pp¯K+, of the charmless component in the range
2 suppressed penguin diagrams, where gluonic resonances
M <2.85GeV/c2 andoftheB± →η K± decaychannel
1 couldbeformed.ThethreeFeynmandiagramsareshown pp¯ c
withη → pp¯relativetotheproductofbranchingfractions
: in Fig. 1. The interference between the penguin diagram c
v B = B(B± → J/ψK±)×B(J/ψ → pp¯), which is well
J/ψ
i
X known[7],havebeenmeasured.
The measurement relies only on the ratios of events,
r
a N(mode), and efficiencies, (cid:15) , with respect to the ref-
mode
erencemode:
B(mode) N(mode) (cid:15)
= × J/ψ (1)
B(J/ψ) N(J/ψ) (cid:15)
mode
whereN(J/ψ)= NB+→J/ψ(→pp¯)K+and(cid:15)J/ψ =(cid:15)B+→J/ψ(→pp¯)K+
2 Study of the B+ → pp¯K+ selection
Fig. 1. Feynman Diagrams: penguin (top left), doubly Cabibbo
suppressedtree(topright),OZIsuppressedpenguin(bottom). Sincethebb¯ crosssectionisapproximately1%ofthetotal
inelasticcrosssectionandtheinterestingchannelbranch-
ingratiosareoftheorderofB∼10−5-10−6,itisextremely
andthedoublyCKMsuppressedtreediagramcanleadto importanttodesignaselectionwithahighrejectionofthe
adirectCPasymmetry. background.Theselectionstrategyisbasedonapreselec-
tionwithverylooseselectioncriteria.Amultivariateanal-
a e-mail:[email protected] ysis using the “Boosted Decision Tree” algorithm of the
EPJWebofConferences
2MeV/c 60 PLHreClibminary M(σB()B =) (=5 N2(17(6B8..)77 = ±± 2 112..354) )± MM 1ee8VV//cc22 2MeV/c 30 PLHreClibminary M(σB()B =) (=5 N2(17(6B7..)07 = ±± 1 110..828) )± MM 1ee2VV//cc22
0 0
nts/1 Ls==376 TpebV 1 nts/1 Ls==376 TpebV 1
Eve 40 Eve 20
20 10
0 0
5100 5200 5300 5400 5100 5200 5300 5400
M(p p K) (MeV/c2) M(p p K) (MeV/c2)
Fig.2.MassdistributionofallselectedB+ → pp¯K+events.The Fig.3.MassdistributionoftheselectedB+ → pp¯K+ eventsfor
continuous line shows the fit result, the dashed line shows the M <2.85GeV/c2.Thecontinuouslineshowsthefitresult,the
pp¯
fittedbackgroundcomponent. dashedlineshowsthefittedbackgroundcomponent.
cTiMenVcAy. Tpahcekvagareia[b8l]esisuusseedditnotohpetismeliescetitohnepsreoleccetdiounreeeffix-- 2MeV/c 25 PLHreClimbinary L=36 pb 1
0 N(J/ψ) = 48.3 ± 7.4 s=7 TeV
pdtrleaocniastyvksei.nrTseemhmraetoeimcchaenanrdtgateoadrpepoalroertgqiciucliearesldcin.hTatrhhaeecfittehnrraiesletsicfitasntoealfwsbti-athhtealdaprraogrne- Events/1 1250 NMσMΓ ((((=ηJηη c/(cc)8)ψ)= .==)2 ( =833( 2 54±(93.. 23170 .69±±1 5 4±71 ) ±.3 67M ))1 MeM.3Vee)/ VcVM/2/ce c2V2 /c2
ticleshavetocomefromthesamedisplacedsecondaryver- 10
tex requiring a good vertex fit and a significant distance
5
between the two vertices. Moreover it is required a small
maximumdistanceofclosestapproachbetweenthetracks 0
2700 2800 2900 3000 3100 3200 3300
andnonzeroimpactparameters.Andthemomentadirec- M(p p) (MeV/c2)
tion of the three daughters has to be compatible with the
flight direction of the B. Furthermore the particle identi- Fig. 4. Mass distribution of pp¯ in the J/ψ-ηc range for events
fication information, provided by two RICH detectors in having|M(pp¯K)−M(B)|<50MeV/c2.Thesolidlineshowsthe
LHCb, is a fundamental ingredient of the selection in or- result of the fit, the dashed line corresponds to the fitted back-
groundcontribution.
der to identify correctly protons and kaons candidates. A
signal efficiency of more than 70% with respect to prese-
lected events has been obtained using the BDT algorithm
for each considered channel (all the events, events with
withasignaltobackgroundratioofS/B∼1.
M < 2.85GeV/c2 and η events) with respect to the
pp¯ c
J/ψ:(cid:15) /(cid:15) .
mode J/ψ
Theoverallefficiencyratios,productofreconstruction,trig-
3 Signal Extraction gerandeventselectionefficiencyasafunctionof M in-
pp¯
variant mass have been calculated using simulated events
ThenumberofB+ → pp¯K+signaleventsisextractedfrom inordertocorrectforvaryingefficiencyovertheM range.
pp¯
an unbinned maximum likelihood fit to the Mpp¯K invari- TheratioswiththeiruncertaintiesarereportedinTab.1.
ant mass distribution of the selected events (Fig. 2). The
fit procedure has been applied also on the subsample of
eventshavingMpp¯ <2.85GeV/c2toextractthenumberof 5 Evaluation of systematics
eventsinthecharmlessregion(Fig.3).Thesignalproba-
bilitydensityfunctionhasbeenparametrizedwithaGaus-
Sincethefinalstatefortheconsideredmodesandtherefer-
sian function while for the background a linear function
enceoneisthesame,mostofthesystematicuncertainties
hasbeenused.
cancel.
FortheB+ → J/ψ(→ pp¯)K+ andB+ → η (→ pp¯)K+,the
c Possiblesystematicuncertaintieshavebeenconsideredon
yield has been extracted with a fit to the Mpp¯ distribution theeventyielddeterminationandontheefficiencyratios.
for the B events having M invariant mass compatible
pp¯K Thesystematicuncertaintyontheeventyieldextractionis
within 2.5σ with the known B+ mass (Fig. 4). Since the estimated using different fit ranges, different parametrisa-
J/ψmesonhasanaturalwidthmuchsmallerthanthede- tion for signal and background. Half of the largest differ-
tector resolution while the natural width of the ηc is not enceinthecentralvalueobtainedwithdifferentfitconfig-
negligible,theJ/ψhasbeenparametrizedwithaGaussian
urations is taken as systematic uncertainty on the number
functionwhilefortheη theconvolutionofaBreit-Wigner
c of signal events. The event yields for each mode and the
with a Gaussian function with the same resolution of the
correspondingsystematicuncertaintiesarelistedinTab.1.
J/ψhasbeenused. The uncertainty on the efficiency ratios described in Sec-
tion4isconsideredasasystematicuncertainty.
Most data-simulation discrepancies are independent from
4 Estimation of efficiencies the Dalitz plot variables and cancel in the ratio. In some
cases, in particular for particle identification, the discrep-
ThecalculationofthebranchingfractionsbasedonEq.1 ancies are larger for low momentum tracks. A conserva-
requires the determination of the ratio of the efficiency tive10%systematicuncertaintyduetomomentumdepen-
2011HadronColliderPhysicssymposium(HCP-2011),Paris,France,November14-182011
dent data-simulation discrepancy on the efficiency ratios Table4.BranchingratiosmeasuredattheB-factories.Allvalues
hasbeenassumedforthetotalB(B+ → pp¯K+)andforthe areinunits10−6.
charmlessB(B+ → pp¯K+) branchingratios.
Mpp¯<2.85GeV/c2
Mode BaBar Belle
[1] [2]
6 Branching ratio measurements ALL - 10.76+0.36±0.70
−0.33
M <2.85GeV/c2 5.3±0.4±0.3 5.0+0.24±0.32
pp¯ −0.22
Usingthenumberofeventsextractedforeachconsidered η 1.8+0.3±0.2 1.42±0.11+0.16
c −0.2 −0.20
modeandthecorrespondingratiosofefficiencieswithre-
specttotheB+ → J/ψ(→ pp¯)K+thepreliminarymeasure-
mentsofratiosofbranchingfractionshavebeenextracted Theresultsarecompatiblewiththeaveragevaluesre-
andarereportedinTab.2. ported in the PDG and measured at the B-factories as re-
portedinTab.4,evenwiththelimitedstatisticsusedinthis
analysis.
Table1.YieldsofB+ → pp¯K+eventsineachmode,ratioofef- Thecurrentsystematicuncertaintyislargelyofstatis-
ficienciesofeachmodewithrespecttotheB+→J/ψ(→ pp¯)K+. tical origin, so both statistic and systematic uncertainties
willdecreaseassoonastheavailablestatisticsincrease.
Mode Yield (cid:15)/(cid:15) In2011LHCbhascollectedmorethan1fb−1.Thelarge
J/ψ
±stat±syst available statistics collected in 2011 is a factor almost 10
J/ψ 48±7±2 1 morewithrespecttothestatisticscollectedattheB-factories.
ALL 225±18±5 1.01±0.10 Themostaccurateavailablemeasurementshavebeenper-
M <2.85GeV/c2 102±12±2 0.96±0.10 formedbytheBaBarandBelleCollaborationsusingabout
pp¯
η 34±8±3 0.994±0.003 1000 fully reconstructed exclusive decays. An improve-
c
ment on the precision of the branching fractions is ex-
pected in particular for B(B± → η K±) × B(η → pp¯).
c c
Moreover the study of more rare charmonium contribu-
tionswillbepossible,suchasη (2S)andX(3872),observ-
c
Table 2. Ratios of branching fractions with statistical and sys- ingorputtinganupperlimitonB(B+ →cc¯K+)×B(cc¯ →
tematicuncertaintiesforthedifferentmodesconsidered. pp¯)∼10−7.
Mode Bratio
LHCb References
J/ψ 1
ALL 4.6±0.6±0.5 1. BABARCollaboration,Phys.Rev.D72(2005)051101
M <2.85GeV/c2 2.21±0.41±0.24 2. BELLECollaboration,Phys.Lett.B659(2008)80
pp¯
η 0.71±0.20±0.07 3. LHCbCollaboration,Phys.Lett.B694(2010)209
c
4. H. Y. Cheng, K. C. Yang, Phys. Rev. D66, 014020
(2002).
5. C. K. Chua, W. S. Hou, S. Y. Tsai, Phys. Lett. B 544
(2002)139
6. J.L.Rosner,Phys.Rev.D68,014004(2003)
7 Conclusions and perspectives with the
7. K. Nakamura et al. [Particle Data Group Collabora-
full 2011 data set tion],J.Phys.GG37,075021(2010).
8. A. Hoecker et al.PoS A CAT 040 (2007)
The preliminary measurements of the ratios of branching [physics/0703039].
fractions for the B+ → pp¯K+ decays have been reported.
From the measured ratios, using the known values of the
B(B+ → J/ψK+) and B(J/ψ → pp¯) the corresponding
branchingfractionshavebeenderivedandreportedinTab.3.
Table 3. Derived LHCb preliminary branching ratios with sta-
tistical error, systematic uncertainty and the uncertainty on the
knowledge of the B(B+ → J/ψ(→ pp¯)K+). All values are in
units10−6.
Mode LHCbB
(preliminary)
ALL 10.2±1.4±1.1±0.5
M <2.85GeV/c2 4.87±0.91±0.54±0.22
pp¯
η 1.57±0.43±0.15±0.07
c