Measurements of $\phi_s$ at the LHCb experiment

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by  Greig A. Cowan

Measurements of φ at the LHCb experiment
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Greig A. Cowan∗†
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UniversityofEdinburgh,UK
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a E-mail: [email protected]
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3 These proceedings present the current status of measurements of theCP-violating phase φ by
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] the LHCb collaboration, reviewing the measurements in channels such as B0s →J/ψφ, B0s →
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J/ψπ+π− andB0→ψ(2S)φ. TheobservationoftheB0→η φ decaymodeispresentedforthe
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- firsttime,whichcanbeusedtomeasureφ withlargerdatasamplesthatwillbecollectedoverthe
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e coming years by the LHCb experiment. Finally, the expected increase in precision from LHCb
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measurementsofφ overthenextdecadeispresented.
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9thInternationalWorkshopontheCKMUnitarityTriangle
28November-3December2016
TataInstituteforFundamentalResearch(TIFR),Mumbai,India
∗Speaker.
†OnbehalfoftheLHCbcollaboration
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/
Measurementsofφ atLHCb GreigA.Cowan
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Candidates / (0.2 ps) 111100001243 LHCb Candidates / 0.05112233505050500000000000000 LHCb Candidates / 0.05112233505050500000000000000 LHCb p rad)Candidates / (0.05112233050505500000000000000 LHCb
10-1 5 De1c0ay time [ps] 0-1 -0.5 0 0.5 cosqm1 0-1 -0.5 0 0.5 cosqK1 0 -2 0 2j h [rad]
Figure 1: Projections of B0 →J/ψφ data onto the decay time and three helicity angles. The projection
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of the total fit model is shown (blue) along with the individualCP-odd (green),CP-even (red) and S-wave
(purple)contributions.
1. Introductionandmotivation
AkeyobservabletobemeasuredintheB0 mesonsystemistheCP-violatingphase,φ ,which
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arises due to the interference between B0 meson mixing and decay processes. It is defined as
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(cid:16) (cid:17)
φ ≡−arg(λ )≡−arg qAf ,whereq,parecomplexeigenvaluesrelatedtoB0mixingandA (A )
s f pAf s f f
arethecomplexamplitudesforB0(B0)mesondecaytofinalstate f. Globalfitstoexperimentaldata
s s
giveaprecisepredictionforφ intheStandardModelof−0.0376±0.0008rad[1]. Anydeviation
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from this prediction would be a clear sign for non-Standard Model physics, strongly motivating
the need for precise experimental measurements of this quantity. In this article I will review the
measurements of this observable from the LHCb collaboration and discuss new measurements of
B0 meson decay channels that can be used to measure φ in the future. All measurements shown
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here use 3fb−1 of data collected by the LHCb experiment [2] in pp collisions at the LHC during
2011and2012.
2. State-of-the-artofφ measurements
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2.1 φ fromB0→J/ψφ andB0→J/ψπ+π−
s s s
The so-called “golden mode" for measuring φ is using a flavour-tagged time-dependent an-
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gular analysis of the B0 → J/ψφ decay, where J/ψ → µ+µ− and φ → K+K−. This b → ccs
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mediateddecayhasahighbranchingfractionandthepresenceoftwomuonsinthefinalstateleads
to a high trigger efficiency. The angular analysis is necessary to disentangle the interfering CP-
oddandCP-evencomponentsinthefinalstate,whichariseduetotherelativeangularmomentum
between the two vector resonances. In addition, there is a small (∼2%) CP-odd K+K− S-wave
contribution that must be accounted for. The LHCb detector has excellent time resolution (∼45
fs [3]) and tagging power (∼4% [4]), both of which are crucial to the measurement. In Run 1,
the LHCb collaboration used a sample of ∼96000 B0 →J/ψφ decays to measure φ , the width
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difference between the light and heavy B0 mass eigensates (∆Γ ), the average decay time (Γ ),
s s s
mixing frequency (∆m ) and direct CP violation parameter (|λ|). Figure 1 shows the results of
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thisanalysis,whichgaveφ =−0.058±0.049±0.006rad,∆Γ =0.0805±0.0091±0.0032ps−1
s s
and Γ =0.6603±0.0027±0.0015 ps−1 [5]. These are the most precise determinations of these
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parameters to date and are consistent with SM predictions [1,6]. The dominant systematic uncer-
taintiesinthesemeasurementarisefromknowledgeaboutthedecaytimeandangularefficiencies.
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Measurementsofφ atLHCb GreigA.Cowan
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2)cCandidates / (2.5 MeV/1024680000000000 LHCb andidates / ( 0.274 ps )110012 LHCb Candidates / ( 0.1 )11223505050000000 LHCb
0 5350 m(y (2S)K5+4K00-) [MeV/c2] C10-1 5 10 t [ps] 0-1 -0.5 0 0.5 cosq K1
Candidates / ( 0.1 )112205050000 LHCb dates / ( 0.314 rad )112205050000 LHCb
50 di 50
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0-1 -0.5 0 0.5 1 Ca 0 -2 0 2
cosq m j [rad]
Figure 2: Distribution of ψ(2S)φ invariant mass of selected B0 →ψ(2S)φ candidates and projections of
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B0→ψ(2S)φ dataandfitmodel(seeFigure1forlegend).
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ItispossiblethatduetounknownhadroniceffectsorbeyondtheSMphysics,thevaluesofφ
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and|λ|couldbedifferentforeachofthefourpolarisationstates[7,8]. Forthefirsttime,theLHCb
collaborationrelaxedthisassumptionintheanalysis, findingthatnopolarisationdependencewas
visiblewithintheavailablestatisticalprecision.
TheLHCbcollaborationhasalsousedasimilaranalysisofB0→J/ψπ+π−decaystomeasure
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φ [9]. Here,thefullπ+π− massspectrumisusedinthemeasurement,whichhaspreviouslybeen
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studied and found to be >97.7% completelyCP-odd [10], dominated by the f (980) component.
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Withthistime-dependentamplitudeanalysis,φ wasmeasuredtobe0.070±0.068±0.008rad,the
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dominant systematic uncertainty coming from knowledge about the composition of resonances in
theπ+π− spectrum. SincethefinalstateisalmostallCP-odd,asimplifiedtaggedfittoonlytheB0
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decaytimedistributionyieldscompatibleresults. CombiningtheB0→J/ψφ andB0→J/ψπ+π−
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resultsgivesφ =−0.010±0.039rad.
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2.2 φ fromB0→ψ(2S)φ
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OtherB0decaymodeswithb→ccstransitionscanbeusedtomeasureφ . InRef.[11],LHCb
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studiedtheB0→ψ(2S)φ (withψ(2S)→µ+µ−)decaysforthefirsttimeusingthesameanalysis
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techniquesasRef.[5]. Figure2shows∼4500signaldecaysinRun1data,selectedusingaboosted
decisiontreethathasbeentrainedusingsimulatedsignaleventsandabackgroundsamplefromthe
high-masssideband. Figure2alsoshowstheprojectionsofthedataandfitontothedecaytimeand
helicity angles, demonstrating a good fit to the data. In addition to ∆Γ and Γ , φ was measured
s s s
tobe0.23+0.29±0.02rad. Forthefirsttimethemagnitudeofthetransversityamplitudesandtheir
−0.28
phasesweremeasuredforthisdecay,whicharedifferenttothoseinB0→J/ψφ asexpected[12].
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2.3 Globalcombination
The global combination of φ and ∆Γ measurements from the Heavy Flavour Averaging
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Group[13]isshowninFigure3,usingmeasurementsfromtheLHCbcollaborationdiscussedhere
along with those from the CDF [14], D0 [15], ATLAS [16] and CMS [17] collaborations. They
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Measurementsofφ atLHCb GreigA.Cowan
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Figure3: HFAGcombination[13]ofφ and∆Γ fromseveralexperiments.
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find ∆Γ =0.085±0.006ps−1 and φ =−0.030±0.033 rad. The results are dominated by those
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fromtheLHCbcollaborationandareconsistentwiththeSMpredictions. Thereremainsspacefor
new physics contributions at the ∼20% level, however, as the experimental precision improves,
it is essential that there is good control over hadronic effects (so-called “penguin pollution") that
couldmimictheeffectfrombeyond-the-SMphysics.
2.4 φsss fromB0→φφ
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ArelatedCP-violatingphase, φsss, canbemeasuredbyapplyingsimilarmethodsasaboveto
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B0 mesondecaysthatgoviaab→ssstransition. TheLHCbcollaborationhasperformedsuchan
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analysis using B0 →φφ [18], measuring φ =−0.17±0.15±0.03 rad, which is consistent with
s s
the Standard Model predictions, all of which are very close to zero [19–21]. An upcoming study
ofB0→K+π−K+π− decayswillprovideanotheravenueformeasuringthisquantity[22].
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3. Futureprospectsformeasuringφ
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The measurement of φ using B0 →J/ψφ decays has so far restricted to using the region of
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K+K− phasespaceneartheφ(1020)resonance. AfullamplitudeanalysisoftheB0→J/ψK+K−
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system was performed in Ref. [23], indicating a significant contribution from other K+K− reso-
nancessuchasthe f(cid:48)(1525)thatcanbeusedwhenmeasuringφ toincreasethestatisticalprecision.
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ThisapproachwillrequiretheapplicationofthesameanalysisformalismasinRef[9]. Similarly,
therecentlyobservedB0→φπ+π− decay[23]couldbeusedwithfuturedatasamplesfromRun2
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andbeyondtomeasureφsss,againwithaflavour-tagged,decay-timedependentamplitudeanalysis,
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includingallappropriateπ+π− resonances.
3.1 ObservationofB0→η φ
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AtthisconferencetheLHCbcollaborationannouncedapreliminaryobservationoftheB0 →
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η φ decay mode, with η →K+K−π+π−,π+π−π+π−, K+K−K+K−,pp [24]. This decay is an-
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otherb→ccstransitionthatcouldbeusedtomeasureφ . Figure4showstheinvariantmassofthe
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B0systemintheppmodealongwiththeppspectrum,withtheη andJ/ψ charmoniumresonances
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clearlyvisible. Asimultaneousamplitudefitisperformedusingallmodesandincludingcontribu-
tionsfrominterferingnon-resonantcomponents. Thebranchingfractionisextractedrelativetothe
J/ψ channelandfoundtobeB(B0→η φ)=(5.01±0.53(stat)±0.27(syst)±0.63(B))×10−4.
s c
FirstevidenceoftheB0→η π+π− decaywasalsopresented.
s c
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Measurementsofφ atLHCb GreigA.Cowan
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2)Candidates/(6.0 MeV/c 123456789000000000 LHCb DTBBBBCCBos0s0000ooa ttmmfififififiaalbb PpppppiinnDpppppaafKKKfFttoo+++rrKKpiiaa---ll ppppKf+K- 2)Candidates / (7.0 MeV/c11010-0121 LHDTBBos0s0aC ttfifiaal b PhJD/cy(Ffi(fi p pp)pf)f
0 10-2
5200 5300 5400 5500 2.9 3 3.1
m(ppK+K-) [MeV/c2] m(pp) [GeV/c2]
Figure4: InvariantmassdistributionsforselectedK+K−pp(left)and pp(right)candidates.
2)cCandidates/ (12 MeV/123450000000000 LHCb Candidates/ (0.202 ps)11100023 LHCb
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5200 5400 5600 2 4 6 8 10
M(J/y h ) [MeV/c2] t [ps]
Figure 5: Distributions of J/ψη invariant mass and decay time for selected B0 →J/ψη decays. Combi-
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natorialbackground(green),partiallyreconstructedbackground(orange)andbackgroundfromB0→J/ψη
decays(blue)areshown.
3.2 B0→J/ψη effectivelifetime
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TheLHCbcollaborationhasrecentlyobservedtheB0→J/ψη(→γγ)decay[25]anduseditto
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measuretheB0 effectivelifetime. AsthismodeisaCP-eveneigenstatetheeffectivelifetimegives
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a measurement of Γ . The final state is challenging, containing only two charged tracks and the
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invariantmassresolutionis∼48MeV/c2 (seeFigure5),comparedto∼8MeV/c2 forB0→J/ψφ
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decays. Using ∼ 3000 signal candidates, the lifetime was measured to be τ = 1.479±0.034±
0.011ps, consistent with other measurements of the CP-even lifetime [26,27]. In the future the
B0→J/ψη modecanbeusedtomeasureφ fromaflavour-taggedfittothedecaytimedistribution.
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An update of the HFAG averages of ∆Γ and Γ was presented, showing good consistency
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between all measurements and the SM predicitions [6]. The ∆Γ prediction has an uncertainty
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morethanthreetimeslargerthantheexperimentalaverage.
4. Summary
TheLHCbcollaborationhasmadeleadingmeasurementsoftheCP-violatingphaseφ andB0
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mesonlifetimesusingRun-1data. Sofarallmeasurementsareconsistentwithpredictionsfromthe
StandardModel. Newb→ccsdecaymodeshavebeeninvestigatedandmeasurementsperformed
to either measureCP violating effects or make preparations for such measurements in the future.
Figure 6 shows how the precision on φ and φsss will reduce as a function of time for key decay
s s
channels discussed in these proceedings. The precision is expected to reach ∼ 0.01rad at end
of Run 3 [28] (the LHCb upgrade era) which is further discussed in Ref. [29]. As the precision
improves it will be essential to control hadronic effects that can hide small contributions from
non-StandardModelphysics[30].
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Measurementsofφ atLHCb GreigA.Cowan
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d]
stat [ra)s00..1124 BBB0s0s0s fififi JJf//fyy LpfH pLC HLbCHbCb
f( 0.1 SM upper limit on |fssss| B0s fi ff
s 0.08 SM s(fcscs) B0s fi J/yf
0.06
0.04
0.02
0
2020 2030
Year
Figure6: Projectionofhowprecisiononφ fromLHCbmeasurementswillscaleasafunctionoftimefor
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differentdecaymodes. InformationtakenfromRef.[28].
5. Acknowledgements
The author thanks the organisers of the CKM2016 meeting and acknowledges the support of
theScienceandTechnologyFacilitiesCouncil(UK)grantST/K004646/1.
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