Indirect $\boldsymbol{CP}$ Violation in $\boldsymbol{D^0 \rightarrow h^+ h^-}$ Decays at LHCb

1.1 MB·English

by  Michael Alexander

Indirect CP Violation in D0→ h+h− Decays at LHCb
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1 Michael Alexander∗†
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UniversityofGlasgow
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E-mail: [email protected]
n
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J
6 IndirectCPviolationintheD0 systemcanbeprobedbymeasuringtheparameterAΓ,definedas
2 theCPasymmetryoftheeffectivelifetimeoftheD0mesondecayingtoaCPeigenstate. Thiscan

be significantly enhanced beyond Standard Model predictions by new physics. Measurements
]
x ofA usingD0→K+K− andD0→π+π− decaysreconstructedfromppcollisionscollectedby
Γ
e theLHCbexperiment,correspondingtoanintegratedluminosityof1.0 fb−1,arepresented. The
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p resultsare
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h A (ππ)=(+0.33±1.06±0.14)×10−3,
[ Γ
1 AΓ(KK)=(−0.35±0.62±0.12)×10−3,
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4 where the uncertainties are statistical and systematic, respectively. These are the most precise
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measurementsoftheirkindtodate,andshownoevidenceofCPviolation.
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The15thInternationalConferenceonB-PhysicsatFrontierMachinesattheUniversityofEdinburgh,
14-18July,2014
UniversityofEdinburgh,UK
∗Speaker.
†OnbehalfoftheLHCbcollaboration.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
MeasurementofA atLHCb MichaelAlexander
Γ
2Entries/0.33 MeV/c111000234 LHCb dfscbbbiioatkkkgtmgggna abrDDnl.0s dfifib kpKpgs+-pp++pp-0 2)Entries / (0.02 MeV/c110043 LHCb DFSRDDCiino0+satgfifitdmna.a bKKpl.s- +pbK+kp-gp0+
102
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5 KK mass [MeV/c2] 5 KK deltam [MeV/c2]
Pull 0 Pull 0
-5 1820 1840 1860 1880 m1D900 (0MeV/c2) -5140 145 D1m5 0[MeV/c2]
Figure1: Fitsto(left)theD0invariantmassdistributionand(right)the∆m≡m(D∗+)−m(D0)distribution
forD0→K+K− candidatesfromthedatasubsetwithmagnetpolaritydown, recordedintheearlierofthe
tworunningperiods.
1. Introduction
SimilarlytotheB0andB0systems,themasseigenstatesoftheD0system,|D (cid:105),withmasses
s 1,2
m andwidthsΓ ,aresuperpositionsoftheflavoureigenstates|D (cid:105)= p|D0(cid:105)±q|D0(cid:105),where p
1,2 1,2 1,2
andqarecomplexandsatisfy|p|2+|q|2=1. Thiscausesmixingbetweenthe|D0(cid:105)and|D0(cid:105)states,
and allows for “indirect”CP violation in mixing, and in interference between mixing and decay,
when decaying to aCP eigenstate. IndirectCP asymmetries in the D0 system can be significantly
enhanced beyond Standard Model (SM) predictions by new physics [1]. In decays of D0 mesons
toaCPeigenstate f,indirectCPviolationcanbeprobedusing[2]
Γˆ(D0→ f)−Γˆ(D0→ f) (cid:20)1 (cid:21)
A ≡ ≈η (A +A )ycosφ−xsinφ ,
Γ Γˆ(D0→ f)+Γˆ(D0→ f) CP 2 m d
where Γˆ is the inverse of the effective lifetime of the decay, η is the CP eigenvalue of f,
CP
x≡2(m −m )/(Γ +Γ ), y≡(Γ −Γ )/(Γ +Γ ), A ≡(|q/p|2−|p/q|2)/(|q/p|2+|p/q|2),
2 1 1 2 2 1 1 2 m
(−)
A ≡(|A |2−|A¯ |2)/(|A |2+|A¯ |2), with A the decay amplitude, and φ ≡arg(qA¯ /pA ). The
d f f f f f f f
effective lifetime is defined as the average decay time of a particle with an initial state of |D0(cid:105) or
|D0(cid:105),i.e.thatobtainedbyfittingthedecay-timedistributionofsignalwithasingleexponential.
The LHCb detector at the LHC, CERN, is a forward-arm spectrometer, specifically designed
for high precision measurements of decays of b and c hadrons [3]. During 2011 the experiment
√
collectedppcollisionsat s=7 TeVcorrespondingtoanintegratedluminosityof1.0 fb−1. Due
to the large cc production cross section [4], the decay-time resolution of approximately 50 fs for
D0 decays[5]andtheexcellentseparationofπ andKachievedbythedetector[6], itisverywell
suitedtomeasureA withhighprecision.
Γ
2. Methodology
The decay chain D∗+→D0π+ is used to determine the flavour of the D0 candidates at pro-
s
duction, via the charge of the π meson. The CP-even K+K− and π+π− final states are used to
s
calculateA [7]. ThepredominantcandidateselectioncriteriarequiretheK+K− orπ+π− tracksto
Γ
havelargeimpactparameter(IP),largetransversemomentum(p ),invariantmasswithin50 MeV
T
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MeasurementofA atLHCb MichaelAlexander
Γ
Entries / (0.02 ps)111000243 LHCb DFSSPSDDCiiereo0+satgoccfifitmnmao. anrbKKpnld.t -d+a bprK.rk+ npypg-dps0.+ ps Entries / (0.05) 111000243 DFSSPSDDCiiereo0+satgoccfifitmnmao. anrbKKpnld.t -d+a bprK.rk+ npypg-dps0.+ ps LHCb
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5 5
Pull 0 Pull 0
-5 2 4 t [ps6] -5-10 -5 0 5 ln(c2)
IP
Figure 2: Fits to (left) the D0 decay-time distribution and (right) the ln(χ2) distribution for D0→K+K−
IP
candidatesfromthedatasubsetwithmagnetpolaritydown,recordedintheearlierofthetworunningperiods.
of the world average D0 mass, and for the vector sum of their momenta to point closely back to
thepositionoftheppcollision. Usingdatacorrespondingtoanintegratedluminosityof1.0 fb−1,
4.8MD0→K+K− candidatesand1.5MD0→π+π− candidatesareselected. Thedataaredivided
byD0 flavour,thepolarityoftheLHCbdipolemagnet,andtwoseparaterunningperiods. Combi-
natorial and partially reconstructed backgrounds are discriminated using a simultaneous fit to the
distributions of D0 mass and ∆m≡m(D∗+)−m(D0). Examples of these fits are shown in Fig. 1
forD0→K+K− candidates,fordatarecordedwiththemagnetpolaritydownduringtheearlierof
thetworunningperiods.
A fit to the decay-time distribution of the candidates is then used to determine the effective
lifetimes of the D0 and D0 signal. Only candidates for which the D∗+ is produced directly at the
ppcollisionareconsideredassignal. ThebackgroundfromB→D∗+Xdecaysisdiscriminatedby
simultaneouslyfittingthedistributionsofthedecaytimeandthenaturallogarithmoftheχ2 ofthe
hypothesis that the D0 candidate originates directly from the pp collision (ln(χ2)). The selection
IP
efficiency as a function of decay time is obtained from data using per-candidate acceptance func-
tions,asdescribedindetailinRef. [8]. Thedecay-timeandln(χ2)distributionsforcombinatorial
IP
andspecificbackgroundsareobtainedfromthedatausingthediscriminationprovidedbythemass
and ∆m fits to employ the Weights technique [9] with kernel density estimation [10]. Figure 2
s
shows fits to the distributions of decay time and ln(χ2) for D0→ K+K− candidates, using the
IP
same data subset as Fig. 1. Inaccuracies in the fit model are examined as a source of systematic
uncertainty,asdiscussedinthefollowingsection.
3. Resultsandsystematics
Thefitsdetailedintheprevioussectionfind
A (ππ)=(+0.33±1.06±0.14)×10−3,
Γ
A (KK)=(−0.35±0.62±0.12)×10−3,
Γ
where the uncertainties are statistical and systematic, respectively. These are the most precise
measurementsoftheirkindtodate,andshownoevidenceofCPviolation. Thedominantsystematic
uncertainties arise from the modelling of the selection efficiency as a function of decay time, and
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MeasurementofA atLHCb MichaelAlexander
Γ
Figure3: Theworldaveragesof(left)A and(right)directvs. indirectCPviolationinD0→h+h− decays,
Γ
reproducedfrom[11].
themodellingofthebackgroundfromB→D∗+Xdecays. Figure3(left)showstheworldaverage
of A , which is dominated by these measurements and is consistent with zero. Figure 3 (right)
Γ
showsthecombinedfittomeasurementsofdirectandindirectCPviolationinD0→h+h− decays,
whichyieldsap-valueforzeroCPviolationof5.1%[11].
The precision of these measurements will be improved by the addition of 2.1 fb−1 of data
alreadycollectedduring2012. TogetherwithdatatoberecordedinrunII,and,intime,following
the LHCb upgrade, measurements with precisions of approximately 1×10−4 are possible, giving
greatpotentialforthediscoveryofindirectCPviolationintheD0 system.
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