Rare B decays at the Tevatron

0.18 MB·

by  Masato Aoki

Rare B decays at the Tevatron
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MasatoAoki∗†

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x FermiNationalAcceleratorLaboratory
e E-mail: [email protected]
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[ Studying flavor changing neutral current transitions provides important information that helps

1 searchesforphysicsbeyondthestandardmodel. Inthispaperwereportonrecentmeasurements
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of these transitions using data collected by the CDF and D0 experiments at the Tevatron pp¯
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1 collider, including world-leading limits on the branching fraction of the decay B0 → m +m −,
(s)
0 a forward-backwardasymmetry measurement in B0 →K∗0m +m − and B+ →K+m +m − decays
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. whichisconsistentandcompetitivewithbestB-factoriesresults,andthefirstobservationofthe
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0 decayB0→fm +m −.
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:
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FlavorPhysicsandCPViolation-FPCP2010
May25-29,2010
Turin,Italy
∗Speaker.
†OnbehalfoftheCDFandD0Collaborations.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
RareBdecaysattheTevatron MasatoAoki
1. Introduction
A flavor changing neutral current (FCNC) process is an apparent transition between quarks
of different flavor but equal charge. In the standard model (SM), these processes occur through
higher orderloopdiagrams. TheSMbranching fraction forsuchFCNCdecaysisquitesmall. Itis
interesting tostudysuchraredecaysbecausethecontribution fromphysicsbeyondtheSMmaybe
sizable. Inordertobeabletoobserverareheavyflavordecaysitisessentialtoproduceasufficient
number of B hadrons. At the Tevatron pp¯ collider, various types of B hadrons can be produced
with large production cross sections of O(100 m b) [1]. However, the inelastic pp¯ cross section is
103 times larger than that of B hadron production. Interesting events have to be extracted from a
high track multiplicity environment. Therefore, detectors need to have very good tracking, vertex
resolution, wideacceptance, goodparticleidentification andhighlyselectivetriggers.
In this paper, we report on recent results from studies on rare B decays via FCNCprocesses,
B0 → m +m −, B+ →K+m +m −, B0 →K∗0m +m − and B0 →fm +m −, performed by the CDF and
(s) s
D0collaborations.
2. B0 →m +m −
(s)
TheFCNCBdecayswithdimuoninthefinalstatearehighlysuppressedbythehelicityfactor,
(mm /mB)2. In the SM, the branching fraction of B0s → m +m − decays is (3.6±0.3)×10−9 [2].
B0 → m +m − decay is further suppressed by the ratio of CKM matrix elements, |V /V |2 [3, 4],
td ts
giving a branching fraction in the SM of (1.1±0.1)×10−10 [2]. The decay amplitude can be
enhanced by several orders of magnitude in some extensions of the SM. Since the predicted rate
for these processes in the SM is beyond the current experimental sensitivity at the Tevatron, the
observation of these decays would necessarily imply physics beyond the SM. Improved limits on
the branching fraction can beused toset limits onthe parameter space ofsupersymmetric models
andothernewtheories. Itisalsoimportanttomeasuretheratioofthebranchingfractionsbetween
B0→m +m − andB0→m +m − totesttheMinimalFlavorViolation[2].
s
CDF has performed a search for B0 → m +m − with 3.7 fb−1 of data [5]. Their acceptance
(s)
is increased compared to the previous analysis [6], in addition to increased luminosity, by using
eventswheremuoncandidatescrossthemid-planeofthecentraloutertracker(COT)whereplastic
inserts tomaintainwirespacingcreateazonewithlowertrigger efficiency. Thisadditional trigger
acceptanceincreasesthetotaltriggeracceptanceby12%. Backgroundsfromhadronsmisidentified
asmuonsarenowsuppressedbyselectingmuoncandidatesusingalikelihoodfunction. Thisfunc-
tionteststheconsistency ofelectromagnetic andhadronic energy withthatexpected forminimum
ionizingparticlesandthedifferencesbetweenextrapolatedtracktrajectoriesandmuonsystemhits.
In addition, backgrounds from kaons that penetrate through the calorimeter to the muon system,
or decay in flight outside the COT, are further suppressed by a loose selection based on the mea-
surement of the ionization per unit path length, dE/dx. Afterapplying their baseline selections, a
NeuralNetwork(NN)multivariateclassifierisconstructedusingthefollowingvariablestoachieve
further separation of signal from background: (1) the measured proper decay time, (2) the proper
decay time divided by the estimated uncertainty, (3) the 3D opening angle between the dimuon
flight direction vector and the displacement vector between the interaction point and the dimuon
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RareBdecaysattheTevatron MasatoAoki
vertex, (4) the B-candidate track isolation, (5) the B-candidate transverse momentum, (6) and the
transverse momentum of the lower momentum muon candidate. The NN is trained using back-
ground eventssampledfromthesideband regions andsignaleventsgenerated withaCDFsimula-
tion. TheNNoutputdistributionsofB0signalandsidebandbackgroundeventsareshowninFig.1.
s
Thesearchisperformed inatwodimensional gridindimuonmassmmm andNNspacetoimprove
thesensitivity. Theexpectedbackground isobtainedbysummingcontributions fromthecombina-
torialcontinuumandfromB→h+h′−decays(h+andh′−representachargedkaonorpion),which
peak in thesignal invariant massregion and do not occur inthe sidebands. Thecontribution from
otherheavy-flavor decaysisnegligible. Thecombinatorial background isestimatedbylinearlyex-
trapolating fromthesidebandregiontothesignalregion. TheB→h+h′− contributions areabouta
factor often smaller than the combinatorial background. Thedimuon invariant mass distributions
for three different NN output ranges are shown in Fig. 1. The limit on the branching fraction is
computedbynormalizingtothenumberofreconstructed B+→J/y K+ events. Asampleofabout
19,700 B+→J/y K+ eventsiscollected toserveasanormalization channel usingthesamebase-
line requirements. CDFextracts newupper limitsB(B0→m +m −)<4.3×10−8(3.6×10−8)and
s
B(B0→m +m −)<7.6×10−9(6.0×10−9)atthe95%(90%)C.L.,whicharecurrentlytheworld’s
bestupperlimitsforbothprocesses.
D0 has also performed a search for B0→m +m − using 6.1 fb−1 of data [7]. The event selec-
s
tion criteria are similar to those ofCDF.Themuon selection has been updated with respect to the
previous analysis [8], yielding 10% higher acceptance while keeping the fraction of misidentified
muonsbelow0.5%. Tofurthersuppressthebackground, thefollowingdiscriminatingvariablesare
used: (a)thetransversemomentumoftheB0candidate,(b)the3Dopeningangle,(c)thetransverse
s
decaylengthsignificance,(d)thedecayvertexfitc 2,(e)thesmallerimpactparametersignificance
ofthetwomuons,(f)andthesmallertransverse momentumofthetwomuons. ABayesianNeural
Network(BNN)[9,10]multivariateclassifierwiththeabovevariablesisconstructedtodistinguish
signal events from background. The BNN is trained using background events sampled from the
sideband regions and simulated signal events. The distributions of the BNN output b for the B0
s
signal and the sideband events as well as the B+ →J/y K+ control sample are shown in Fig. 2.
The B0s → m +m − signal region is defined to be 0.9≤ b ≤1.0 and 5.0 GeV ≤mmm ≤5.8 GeV.
Two-dimensional (2D) histograms of mmm vs. b , dividing the signal region into several bins, are
prepared to improve the sensitivity relative to using a single bin. The dominant source of back-
ground dimuon events is from decays of heavy flavor hadrons in bb¯ or cc¯ production. To study
this background contribution, inclusive dimuon Monte Carlo samples with PYTHIA [11] generic
QCD processes are generated. The dimuon background events can be categorized by two types:
(i) B(D)→ m +n X,B¯(D¯) → m −n¯X′ double semileptonic decays where the two muons originate
from different b(c) quarks, yielding dimuon masses distributed over the entire signal region, and
(ii) B→ m +n D¯,D¯ → m −n¯X sequential semileptonic decays, resulting in mmm predominantly be-
lowtheBhadronmass. Thesimulateddimuonmassdistributions forbothbackground sourcesare
parametrizedusinganexponentialfunctiontoestimatethenumberofbackgroundeventsinthesig-
nalregionafterfittingthedimuonmassinthedatasidebandregionsineachb bin. Theuncertainty
on this background estimate is dominated by the statistical uncertainty of the sideband sample
(10% ∼35%). The B→h+h′− background contribution is negligible. The limit on the branch-
ing fraction is computed by normalizing to the number of reconstructed B+ →J/y K+ events. A
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Figure1: CDFdata. Left: DistributionsoftheNNoutputforsimulatedB0→m +m − signalandsideband
s
eventsindata. Right: Thedimuoninvariantmassdistributionforeventssatisfyingallselectioncriteriafor
thefinalthreeNNoutputranges.
sample of about 46,800 B+ →J/y K+ events is selected using the same baseline selections. The
observed distributions of dimuon events in the highest sensitivity region are shown in Fig. 2. The
observed number of events is consistent with the background expectations. The resulting limit is
B(B0→m +m −)<5.1×10−8(4.2×10−8)atthe95%(90%)C.L.Thesimulated massresolution
s
oftheD0detector for theB0→m +m − is≈120MeVandistherefore insufficient toreadily sepa-
s
rateB0 fromB0 leptonic decays. Inthisanalysis, itisassumedthattherearenocontributions from
s
B0→m +m − decays, sincethisdecayissuppressed by|V /V |2≈0.04.
td ts
Aside from the background uncertainty, the largest uncertainty of 15% common to CDF and
D0 B0 → m +m − analyses comes from the fragmentation ratio between B+ and B0 [12]. Both
s s
CDF and D0 observe no evidence of physics beyond the SM in B0 → m +m − decays, but they
(s)
significantly improve the existing limits providing tighter constraints for the parameter space of
severalSMextensions.
3. B0 →K∗0m +m −, B+ →K+m +m − and B0 →fm +m −
s
Anothersensitiveprobeofnon-SMphysicsaccessibleattheTevatronistomeasuretheFCNC
b→sℓℓsemileptonic decays. Although thereisnohelicity factor inthese decays, theSMbranch-
ing fraction is still small, of order O(10−6), and there could a sizable contribution to the decay
rate from physics beyond the SM.In addition, the kinematic distributions could be modified. The
lepton forward-backward asymmetryandthedifferential branching fraction asafunction ofdilep-
ton invariant mass in the decays B→K∗ℓℓ differ from the SM expectations in various extended
models. The amplitudes of the effective Wilson coefficients C , C , and C may interfere with
7 9 10
the contributions from non-SM particles, where C , C , and C represent the amplitudes from
7 9 10
the electromagnetic penguin, the vector electroweak, and the axial-vector electroweak contribu-
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(a) DØ, 6.1 fb-1 V 50 (a) DØ, 6.1 fb-1
0.3 Ge 40 0.98 £ b £ 1
Bfi0s m +m - 5
B0 simulation 0 30
0.2 s 0.
02 Data sideband s/ 20
y / 0.0.1 Event 100
t 4.5 5 5.5 6 6.5
bili 0 mm m [GeV]
a (b) DØ, 6.1 fb-1
Prob0.2 B+fi J/y (m +m -)K+ 01110200 (b) DØ, 6.1 fb5-1.2 GeV £ mm m £ 5.5 GeV
B+ data 0. 80
0.1 B+ simulation nts/ 60
e 40
v
E 20
0 0
0 0.2 0.4 0.6 0.8 1 0.8 0.85 0.9 0.95 1
b b
Figure2: D0data. Left:Distributionsofb :(a)B0→m +m −signalandsidebandevents,(b)B+→J/y K+
s
dataandsimulation. Right: Thedistributionofmmm inthehighestsensitivityb region(a),andthedistribu-
tion of b in the highestsensitivitymmm region(b) fordata (dotswith uncertainties),expectedbackground
distribution (solid line), and the SM signal distributionmultiplied by a factor of 100 (dotted-dashedline).
The dimuonbackgroundcontributionsfromtheB(D)→m +n X,B¯(D¯)→m −n¯X′ decays(dashedline)and
theB→m +n D¯,D¯ →m −n¯X decays(dottedline)arealsoshown.
tions, respectively. Amongmanyb→sm +m − decays, theexclusivechannels B+→K+m +m − and
B0 →K∗0m +m − have been observed and studied at Belle [13] and BaBar [14]. Atthe Tevatron it
isalsopossible tosearchforanalogous decaysB0→fm +m − andL 0→L m +m −.
s b
CDF reports new results on measurements of B+ → K+m +m −, B0 → K∗0m +m − and B0 →
s
fm +m − decays [15]. Event selection starts from selecting two oppositely charged muon candi-
dates with a transverse momentum greater than 1.5 GeV or 2.0 GeV depending on the trigger
selection. B → hm +m − candidates are then reconstructed by adding h = K+, K∗0, or f candi-
dates to the dimuon pair. The K∗0 is reconstructed in the mode K∗0 → K+p − and the f is re-
constructed as f →K+K−. The kaon and pion candidates are required to be consistent with the
time-of-flight (TOF) and dE/dx combined log-likelihood probability of each particle hypothesis.
The muon candidates are further purified by the muon-likelihood. To enhance separation of sig-
nal from background, anArtificial Neural Network multivariate classifier is constructed. Figure 3
shows the invariant mass distribution for each rare decay. By performing an unbinned maximum
log-likelihoodfitoftheBinvariantmassdistribution,120±16eventsforB+→K+m +m −,101±12
events for B0 →K∗0m +m −, and 27±6 events for B0 →fm +m − are found, with 8.5s , 9.7s and
s
6.3s statistical significance, respectively. Thisisthefirstobservation ofB0→fm +m − decay. Us-
s
ing the corresponding B→ J/y h modes as a reference, CDF determines the following absolute
branching fractions: B(B+ →K+m +m −)=[0.38±0.05±0.03]×10−6, B(B0 →K∗0m +m −)=
[1.06±0.14±0.09]×10−6 and B(B0→fm +m −)=[1.44±0.33±0.46]×10−6, where the first
s
uncertainty is statistical and the second is systematic. The differential branching fractions with
respect to q2 =m2mm c2 for B0 →K∗0m +m − and B+ →K+m +m − are also calculated as shown in
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CDF Run II Preliminary L=4.4fb-1 CDF Run II Preliminary L=4.4fb-1 CDF Run II Preliminary L=4.4fb-1
2)MeV/c60 YMiaeslds::150218 –4 –1 23 (M10e2V e/cx2pected) 2)MeV/c100 YMiaeslds::152207 7– –1 63 (M14e2V /ecx2pected) 2)MeV/c2202 YMiaeslds::2573 6–5 6 – ( 351 M eexVp/ecc2ted)
Events / (20 4500 B0DTSfioiagttan aKla Fl*i0tm +m - Events / (20 80 B+DTSfioiagttan aKla Fl+imt+m - Events / (20 111468 B0sTSDfioiagfttan ala mFli+tm -
Background 60 Background Background
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30 10
40
8
20
6
20 4
10
2
0 0 0
5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7
Mm(m K*) (GeV/c2) Mm(m K) (GeV/c2) Mm (m f) (GeV/c2)
Figure 3: CDF data. The B invariant mass of B0 →K∗0m +m − (left), B+ →K+m +m − (middle) and
B0→fm +m − (right).
s
CDF Run II Preliminary L=4.4fb-1 CDF Run II Preliminary L=4.4fb-1
22)/cGeV1.21 22)/cGeV00..56
-7/0 -7/00.4
10.8 1
2 (q 2 (q0.3
d0.6 d
Br/ Br/
d0.4 d0.2
0.2 0.1
00 2 4 6 8 10 12 14 16 18 00 2 4 6 8 10 12 14 16 18 20 22
q2 (GeV2/c2) q2 (GeV2/c2)
Figure4: CDFdata. DifferentialbranchingfractionofB0→K∗0m +m −(left)andB+→K+m +m −(right).
The hatched regions are vetoed to prevent contributions from charmonium. The solid lines are the SM
expectation, which use maximum- and minimum- allowed form factor. The dashed line is the averaged
theoreticalcurveineachq2bin.
Fig. 4. The forward-backward asymmetry (A ) and K∗0 longitudinal polarization (F ) are ex-
FB L
tracted from cosq m and cosq K distributions, respectively, where q m is the helicity angle between
m + (m −) direction and the direction opposite to the B (B¯) direction in the dimuon rest-frame, and
q istheanglebetweenthekaondirectionandthedirectionoppositetotheBmesonintheK∗0rest
K
frame. The fit results of F and A for B0 →K∗0m +m − are shown in Fig. 5, as well as A for
L FB FB
B+→K+m +m −. BothF andA areconsistent withtheSMexpectation, andalsowithanexam-
L FB
ple ofaSUSYmodel [16]. These results are also consistent and competitive withthe B-factories’
measurements [13,14].
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FL1.21CDF Run II PrelimiFnLa(Bry0 Lfi= 4KD.4*a0fmtba+-m1-) AFB1.25CDF Run II PrelimiAnFaBr(yB L0 =fi4D .K4af*tb0am-1+m -) AFB1.52CDF Run II PrelimiAnaFBr(yB L+ =fi4. 4KDf+bam-t+1am -)
SM SM
0.8 C7=-CS7M 1 C7=-CS7M 1 SM
0.6
0.4 0.5 0.5
0.2 0 0
0
-0.5 -0.5
-0.20 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10121416182022
q2 (GeV2/c2) q2 (GeV2/c2) q2 (GeV2/c2)
Figure 5: CDF data. F (left) and A (middle) fit results as a function of q2 for B0 →K∗0m +m −, and
L FB
A (right)asafunctionofq2 forB+→K+m +m −. Thepointsshowdata. Solid(dotted)curveistheSM
FB
(an exampleof SUSY) expectation[16]. Thedashed line is the averagedexpectationin each q2 bin. The
hatchedregionsarevetoedtopreventcontributionsfromcharmonium.
4. Conclusion
CDFreportsthefirstobservationofthedecayB0→fm +m −,resultsontheforward-backward
s
asymmetry measurements in B0→K∗0m +m − and B+ →K+m +m − decays using 4.4 fb−1 of inte-
grated luminosity, andupperlimitsonthebranching fraction B(B0→m +m −)<4.3×10−8(3.6×
s
10−8), B(B0 →m +m −)<7.6×10−9(6.0×10−9) at the 95% (90%) C.L. using 3.7 fb−1 of inte-
grated luminosity. D0 also reports a new upper limit B(B0 → m +m −)<5.1×10−8(4.2×10−8)
s
at the 95% (90%) C.L.using 6.1 fb−1 of integrated luminosity. There is not a significant discrep-
ancy from theSMexpectation inthe FCNCBdecays sofar. Allthese results are done withabout
halforlessthestatistics expected inayearfromnowandtheanalyses arecontinuously improved,
therefore wecouldexpectlargeimprovementsinthenearfuture.
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