Charm Physics Prospects at Belle II

1.4 MB·

by  A. J. Schwartz

UCHEP–17–01
19January2017
Charm Physics Prospects at Belle II
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A. J. Schwartz∗
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a PhysicsDepartment,UniversityofCincinnati,Cincinnati,Ohio45221USA
J E-mail: [email protected]

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(onbehalfoftheBelleIICollaboration)

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e The Belle II experiment is under construction at the KEK laboratory in Japan. Belle II will
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p studye+e− collisionsatorneartheϒ(4S)resonancewiththegoalofcollecting50ab−1 ofdata,
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whichisalargeincreaseoverthatrecordedbytheBelleandBaBarexperiments. Thisdatawill
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[ providealargesampleofcharmmesondecays,andBelleIIwillhaveaveryactivecharmphysics

1 program.Herewediscusssomehighlightsofthisprogram,focusingonmeasurementsofmixing,
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CPviolation,andleptonicdecays.
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VIIIInternationalWorkshopOnCharmPhysics
5-9September,2016
Bologna,Italy
∗Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/
CharmPhysicsProspectsatBelleII A.J.Schwartz
1. Introduction
There are many strategies to search for new physics beyond the Standard Model. Besides
studyingBmesondecays,forwhichtheBelle,BaBar,andLHCbexperimentsweredesigned,one
canstudyDmesondecays,andallthreeexperimentshaveproducednumerousresultsinthisarea.
WhileahadronexperimentsuchasLHCbproducesamuchlargersampleofcharmedmesonsand
baryonsthandoesane+e−experimentsuchasBelle,studyingcharmdecaysatane+e−experiment
hasadvantages. Forexample,ane+e−experimenthaslowerbackgrounds,highertriggerefficiency,
excellent γ and π0 reconstruction (and thus η, η(cid:48), and ρ+ reconstruction), high flavor-tagging
efficiencywithlowdilution, andnumerouscontrolsampleswithwhichtostudysystematics. Due
to good detector hermeticity and knowledge of the initial state energy and momentum, missing
energyand“missingmass”analysesarestraightforward. Inaddition,absolutebranchingfractions
canbemeasured.
TheBelleexperimentattheKEKlaboratoryinJapanisnowbeingupgradedtotheBelleIIex-
periment. ThegoalofBelleIIistocollectadatasetcorrespondingto50timeswhatBelleobtained.
Here we discuss the charm physics prospects of this large data set, focusing on measurements of
charmmixing,CPviolation,andleptonicdecays.
2. MixingandCPViolation
One emphasis of the Belle II charm physics program will be to make high precision mea-
surements of charm mixing and search for CP violation. These measurements typically depend
on measuring the decay time of D0 mesons, and, due to an improved vertex detector, the decay
time resolution of Belle II is expected to be superior to that of Belle and BaBar. Whereas Belle
usedafour-layersilicon-stripdetector,BelleIIwillusefourlayersofsiliconstripsplustwolayers
of silicon pixels, with the smallest pixel size being 55×50 µm2. These pixel layers will lie only
14mmand22mmfromtheinteractionpoint(IP).
Figure 1 (left) shows the resolution on track impact parameter with respect to the IP as a
function of track momentum, as obtained from Monte Carlo (MC) simulation. Also shown are
corresponding results from BaBar. The figure illustrates that the Belle II resolution should be
half that of BaBar. Figure 1 (right) plots the residuals of decay time for a large sample of MC
D0→π+π− decays. The RMS of this distribution is 140 fs, which is about half the decay time
resolutionofBabar(270fs).
To study the sensitivity of Belle II to D0-D0 mixing parameters x=∆M/Γ and y=∆Γ/2Γ,
and toCP-violating parameters |q/p| and Arg(q/p)=φ, two studies have been performed. The
first study uses “toy MC” to generate D0→K+π− decays [1], and their decay time distribution is
fitted. Thesecondstudyusesatime-dependentDalitzgeneratorbasedonEVTGEN[2]togenerate
multi-bodyD0→K+π−π0 decays,andtheirtime-dependentDalitzdistributionisfitted.
2.1 D0→K+π− Decays
For this study we generate an ensemble of 1000 “toy” MC experiments, with each experi-
ment consisting of a sample of D0 decays and a separate sample of D0 decays. The number of
decays in each sample corresponds to 5 ab−1, 20 ab−1, and 50 ab−1 of data. For the full dataset
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CharmPhysicsProspectsatBelleII A.J.Schwartz
(cid:3)(cid:6)(cid:7)(cid:7)(cid:6)(cid:1)(cid:4)(cid:4)(cid:1)(cid:16)(cid:15)(cid:19)(cid:1)(cid:17)(cid:7)(cid:9)(cid:8)(cid:5)(cid:6)(cid:12)(cid:13)(cid:3)(cid:6)(cid:8)(cid:2)(cid:7)(cid:18)(cid:1)
(cid:3)(cid:6)(cid:7)(cid:7)(cid:6)(cid:1)(cid:4)(cid:4)(cid:1)(cid:16)(cid:3)(cid:19)(cid:1)(cid:17)(cid:12)(cid:10)(cid:2)(cid:8)(cid:11)(cid:14)(cid:4)(cid:10)(cid:11)(cid:4)(cid:18)(cid:1)
(cid:3)(cid:2)(cid:3)(cid:2)(cid:5)(cid:1)(cid:16)(cid:15)(cid:19)(cid:1)
(cid:3)(cid:2)(cid:3)(cid:2)(cid:5)(cid:1)(cid:16)(cid:3)(cid:19)(cid:1)
Figure1: Left: BelleIItrackimpactparameterwithrespecttotheIPasafunctionoftrackmomentum,as
obtainedfromMCsimulation. Right: BelleIIresidualsofdecayvertexposition,fromMCsimulation. The
RMSofthisdistributionis140fs.
(50 ab−1), there are 438600 D0→K+π− decays. After generation, the decay times are smeared
by the expected decay time resolution of 140 fs, and the resulting D0 and D0 time distributions
are simultaneously fitted. Backgrounds are not yet included in this study; however, a first look
at backgrounds indicates that when backgrounds are included, the fitted errors increase by only a
smallamount.
Theprobabilitydensityfunctionsusedforthefitaretheconvolutionofthefollowingtheoret-
icalexpressionswithGaussianresolutionfunctions:
D0(t) = e−Γt(cid:40)R +(cid:12)(cid:12)(cid:12)q(cid:12)(cid:12)(cid:12)(cid:112)R (y(cid:48)cosφ−x(cid:48)sinφ)(Γt)+(cid:12)(cid:12)(cid:12)q(cid:12)(cid:12)(cid:12)2x(cid:48)2+y(cid:48)2(Γt)2(cid:41) (2.1)
D (cid:12)p(cid:12) D (cid:12)p(cid:12) 4
D0(t) = e−Γt(cid:40)R +(cid:12)(cid:12)(cid:12)p(cid:12)(cid:12)(cid:12)(cid:113)R (y(cid:48)cosφ+x(cid:48)sinφ)(Γt)+(cid:12)(cid:12)(cid:12)p(cid:12)(cid:12)(cid:12)2x(cid:48)2+y(cid:48)2(Γt)2(cid:41), (2.2)
D (cid:12)q(cid:12) D (cid:12)q(cid:12) 4
where x(cid:48) =xcosδ +ysinδ, y(cid:48) =−xsinδ +ycosδ, and δ is the strong phase difference between
D0→K−π+andD0→K−π+amplitudes. TheparameterR (R )istheratioofamplitudessquared
D D
|A(D0→K+π−)/A(D0→K−π+)|2 (|A(D0→K−π+)/A(D0→K+π−)|2). The preliminary
resultsarelistedinTable1. ThefinalBelleIIsensitivityfor|q/p|is<0.1%,andthatforφ is6◦.
TheseresultsareasignificantimprovementoverthatwhichBelleandBaBarachieved.
Parameter 5ab−1 20ab−1 50ab−1
δx(cid:48) (%) 0.37 0.23 0.15
δy(cid:48) (%) 0.26 0.17 0.10
δ|q/p|(%) 0.20 0.09 0.05
δφ (◦) 16 9.2 5.7
Table1: PreliminaryresultsofatoyMCstudyofD0→K+π−decays: uncertaintyonmixingparametersx(cid:48)
andy(cid:48),andonCP-violatingparameters|q/p|andφ,forthreevaluesofintegratedluminosity.
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CharmPhysicsProspectsatBelleII A.J.Schwartz
2.2 D0→K+π−π0 Decays
For this study we generate an ensemble of 10 experiments using a time-dependent Dalitz
generator based on EVTGEN [2]. Each experiment consists of 225000 D0→K+π−π0 decays,
corresponding to 50 ab−1 of data. The decay times are smeared by a resolution of 140 fs, and the
decaymodelusedtogenerateandfittheDalitzplotistheisobarmodelusedbyBaBar[3]. Possible
CP violation and backgrounds are neglected in this phase of the study. More details are given in
Ref.[4].
The mixing parameters are x(cid:48)(cid:48) = xcosδ +ysinδ and y(cid:48)(cid:48) = −xsinδ +ycosδ ,
Kππ Kππ Kππ Kππ
whereδ isthestrongphasedifferencebetweenD0→K+π−π0 andD0→K+π−π0 amplitudes
Kππ
evaluatedatm =M . Forthisstudythestrongphaseδ isfixedto10◦,andthefittedparam-
ππ ρ Kππ
eters are x and y directly. The results are shown in Fig. 2. The input values are x=0.0258 and
y=0.0039,andtheRMSofthedistributionsofresidualsareδx=0.057%andδy=0.049%. This
precision is approximately one order of magnitude better than that achieved by BaBar [3]. The
projections of a typical fit (one experiment in the ensemble) are shown in Fig. 3. In practice, to
extract values for x and y requires knowledge of the strong phase δ ; this can in principle be
Kππ
measuredatBESIIIusingCP-taggedD →K+π−π0 decays.
CP
Figure2: Left: preliminaryresultsoffittinganensembleoftenexperiments,witheachexperimentcorre-
sponding to 50 ab−1 of data [4]. Middle and right: projections of the left-most plot. The RMS values of
thesedistributionsareδx=0.057%andδy=0.049%.
Figure3: ProjectionsofthefittoD0→K+π−π0eventsforonetypicalexperimentintheensemble[4].
2.3 Time-integrated(direct)CPViolation
Belle II will have excellent efficiency for reconstructing multi-body final states with very
small detector-based asymmetries. Thus the experiment is ideal for searching for time-integrated
(direct) CP violation in a variety of final states. A listing of D0, D+, and D+ decay modes
s
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CharmPhysicsProspectsatBelleII A.J.Schwartz
that Belle has studied is given in Table 2. The table lists the CP asymmetry A = [Γ(D0)−
CP
Γ(D0)]/[Γ(D0)+Γ(D0)] measured by Belle, and the precision expected for Belle II. The latter is
estimated by scaling the Belle statistical error (σ ) by the ratio of integrated luminosities, and
stat
by dividing the systematic error into those that scale with luminosity such as background shapes
measured with control samples (σ ), and those that do not scale with luminosity such as de-
syst
cay time resolution due to detector misalignment (σ ). The overall error estimate is calculated
irred
(cid:113)
as σ = (σ2 +σ2 )·(L /50ab−1)+σ2 . For most of the decay modes listed, the
BelleII stat syst Belle irred
expecteduncertaintyonACP is<∼0.10%.
Mode L (fb−1) A (%) BelleII50ab−1
CP
D0→K+K− 976 −0.32±0.21±0.09 ±0.03
D0→π+π− 976 +0.55±0.36±0.09 ±0.05
D0→π0π0 966 −0.03±0.64±0.10 ±0.09
D0→K0π0 966 −0.21±0.16±0.07 ±0.03
S
D0→K0η 791 +0.54±0.51±0.16 ±0.07
S
D0→K0η(cid:48) 791 +0.98±0.67±0.14 ±0.09
S
D0→π+π−π0 532 +0.43±1.30 ±0.13
D0→K+π−π0 281 −0.60±5.30 ±0.40
D0→K+π−π+π− 281 −1.80±4.40 ±0.33
D+→φπ+ 955 +0.51±0.28±0.05 ±0.04
D+→ηπ+ 791 +1.74±1.13±0.19 ±0.14
D+→η(cid:48)π+ 791 −0.12±1.12±0.17 ±0.14
D+→K0π+ 977 −0.36±0.09±0.07 ±0.03
S
D+→K0K+ 977 −0.25±0.28±0.14 ±0.05
S
D+→K0π+ 673 +5.45±2.50±0.33 ±0.29
s S
D+→K0K+ 673 +0.12±0.36±0.22 ±0.05
s S
Table2:Time-integrated(direct)CPasymmetriesmeasuredbyBelle,andtheprecisionexpectedforBelleII
in50ab−1ofdata.
3. LeptonicDecays
The low backgrounds of an e+e− experiment allow one to study purely leptonic D− →(cid:96)−ν¯
(s)
decays. The branching fractions of these decays are proportional to either f2|V |2 or f2 |V |2,
D cd D cs
s
where f and f are decay constants, andV andV are Cabibbo-Kobayashi-Maskawa (CKM)
D D cd cs
s
matrix elements. Belle has measured the branching fraction for D−→(cid:96)−ν¯ [5], and inputting the
s
value of f as calculated from lattice QCD [6] results in the world’s most precise determination
D
s
of |V |. The D−→(cid:96)−ν¯ event sample for Belle II will be significantly larger than that for Belle,
cs s
and this will allow for a more precise determination of |V |. In addition, Belle II should measure
cs
D−→µ−ν¯ decays,andfromthisbranchingfractiondetermine|V |to<2%uncertainty.
cd
The method used by Belle to reconstruct D−→µ−ν¯ decays is as follows [5]. First, a “tag-
s
side”D0,D+,orΛ+ isreconstructed,nominallyrecoilingagainstthesignalD−. Thedecaymodes
c s
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CharmPhysicsProspectsatBelleII A.J.Schwartz
used for this are listed in Table 3. In addition, tag-side D0 and D+ mesons can be paired with a
π+,π0,orγ candidatetomakeatag-sideD∗+→D0π+,D∗+→D+π0,D∗0→D0π0,orD∗0→D0γ
candidate. Theremainingpions,kaons,andprotonsintheeventaresubsequentlygroupedtogether
into what is referred to as the “fragmentation system” X . The particle combinations allowed
frag
for X are also listed in Table 3. Because the signal decay is D−, to conserve strangeness X
frag s frag
must include a K+ or K0. If the tag side were a Λ+, then X must include a p¯ to conserve
S c frag
baryon number. After X is identified, the event is required to have a µ− candidate nominally
frag
originating from D−→µ−ν¯, and a γ candidate nominally originating from D∗−→D−γ. The
s s s
missing mass squared M2 =(P −P −P −P )2 is calculated and required to be within a
miss CM tag X γ
frag
narrowwindowcenteredaroundM2 . Thesignalyieldisobtainedbyfittingthe“neutrino”missing
Ds
mass distribution M2 =(P −P −P −P −P )2, which should peak at zero. The missing
ν CM tag X γ µ−
frag
massdistributionsforBelle’sD−→µ−ν¯ measurementareshowninFig.4. TheBellesignalyield,
s
andthemuchlargeryieldexpectedforBelleII,arelistedinTable4.
The above method can also be used to search for D−→µ−ν¯ and D0→νν¯ decays [7]. In the
latter case, the D0 is required to originate from D∗+→D0π+, and the daughter π+ momentum is
usedwhencalculatingthemissingmass. RequiringthatM liewithinanarrowwindowcentered
miss
around M results in an inclusive sample of D0 decays. The Belle yield for this sample, and the
D0
expected Belle II yield, are also listed in Table 4. The Belle II yield would allow for a 7× more
sensitivesearchforD0→νν¯ (oranyinvisiblefinalstate)thanthatachievedbyBelle.
Tagside: D0 D+ Λ+
c
K−π+ K−π+π+ pK−π+
K−π+π0 K−π+π+π0 pK−π+π0
K−π+π+π− K0π+ pK0
Decaymode: S S
K−π+π+π−π0 K0π+π0 Λπ+
S
K0π+π− K0π+π+π− Λπ+π0
S S
K0π+π−π0 K+K−π+ Λπ+π+π−
S
K0π+ K0
S S
K0π+π0 K0π0
S S
K0π+π+π− K0π+π− sameasfor
S S
X : K+ K0π+π−π0 D+ tag
frag S
K+π0 K+π− + p¯
K+π+π− K+π−π0
K+π+π−π0 K+π−π+π−
Table3: BelleD−→µ−ν¯ measurementmethod(seetext)[5].
s
4. Acknowledgments
WearegratefultotheorganizersofCHARM2016forawell-organizedworkshopinabeauti-
fullocation,andforexcellenthospitality.
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CharmPhysicsProspectsatBelleII A.J.Schwartz
Belle Preliminary (913 fb )
×103
002 GeV ) 46 IMDDWn*(*scir )0sl→o→-unrse g iDDcv γoπe0nγ 0Dsts rSuicgtneadl signal Xfrag = nothing 2 )01 GeV 60 STCirogumnea bDlins aBtaocrkiaglr Bouanckdground
0. γ from π0 0. 40
Events / ( 2 Events / ( 20
Pull-5050 185 19 195 2 205 Pull-5050 -00.22 00 00.22 00.44 00.66
1.85 1.9 1.95 Mmis2s(DtagKXfra2gγ.0)5 [GeV] M2miss(DtagKXfragγµ) (GeV2)
Figure 4: The Belle missing mass distributions used to measure D− →µ−ν¯ [5]. Left: M =
s miss
(cid:113) (cid:113)
(P −P −P −P)2, which should peak at M . Right: M = (P −P −P −P −P )2,
CM tag Xfrag γ Ds ν CM tag Xfrag γ µ−
whichshouldpeakatM =0.
ν
Mode Belleyield BelleIIyield(50ab−1)
D−→µ−ν¯ 492±26(913fb−1) 27000
s
D−→µ−ν¯ − 1250
D0→νν¯ (695±2)×103 (924fb−1) 38×106
Table4: Belle’sD−→µ−ν¯ [5]andinclusiveD0 [7]signalyields,andtheyieldsexpectedforBelleII.The
s
latterareobtainedbyeitherscalingtheBelleresultsorfromMCsimulationstudies.
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