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Weak decays of heavy mesons not only provide a unique platform to test the electroweak structures of the Standard Model (SM), but can also shed light on new physics (NP) beyond the SM. Among different species of heavy mesons, the
B+c ① meson, discovered in 1998 by the CDF collaboration [1, 2], is of particular interest in this regard. TheB+c meson has specific production and decay mechanisms, and accordingly the measurement of its mass, lifetime and decay branching ratios would help to probe the underlining quark dynamics and determine SM parameters.Consisting of two heavy quarks of different types, the
B+c meson has three decay categories: 1) b-quark decay with spectator c-quark; 2) c-quark decay with spectator b-quark; 3) annihilation processes (e.g.B+c→τ+ντ,c¯s ). The purely leptonic decay through the annihilation process is sensitive to the decay constantfBc and the CKM matrix element|Vcb| . Such a scheme has been used for the determination of|Vcd| and|Vcs| inD+/D+s→τ+ντ,μ+νμ [3]. For|Vcb| , since theB+c→τ+ντ channel has not been discovered, it is measured using inclusive semileptonicb→c transitions and the exclusive channel of¯B→D∗l¯νl . However, even ifB+c→τ+ντ had been discovered, the decay¯B→D∗l¯νl would still provide a more precise|Vcb| measurement.In recent years a few discrepancies have been found between the SM predictions and different experimental measurements in the bottom sector, especially in tauonic decay modes of B mesons [4-6]. In view of there being no clear signal in the direct searches for NP to date, the implications in low-energy processes are of great importance. Studies of tauonic decay modes of B mesons, mostly
B→D(∗)τν decays, have given some hints of lepton flavor universality violation. While these decay modes are very sensitive to vector/axial-vector type interactions, the (pseudo)scalar type interactions which can be induced in many popular NP models, e.g. the two-Higgs-doublet and leptoquark models, are less constrained by them. Due to the mass hierarchymτ≪mBc that results in helicity suppression forB+c→τ+ντ withV−A interactions in the SM,Bc→τν has a better sensitivity to the (pseudo)scalar NP interactions [7, 8]. Therefore, measurement of the branching ratioB(B+c→τ+ντ) can be a key in the search for NP. As we will show in Sec. II, based on the current state of knowledge, NP can affectB(B+c→τ+ντ) significantly, which highlights the importance of studying this quantity in the future.The recently proposed CEPC (Circular Electron Positron Collider) [9] provides an excellent opportunity to measure
B(B+c→τ+ντ) . It is planned to have a circumference of 100 km and two interaction points. Its primary objective is precision Higgs studies at a center-of-mass-energy(√s ) of 240 GeV, with a nominal production of106 Higgs bosons. In addition, a dedicatedWW threshold scan(√s=158−172 GeV) and the Z factory mode(√s=91.2 GeV) will be operated for electroweak and flavor physics studies. The Z factory will produce up to one trillion Z bosons (Tera-Z) in two years, far exceeding LEP's production [10]. Such a huge data sample will enable high precision tests of the SM and allow the study of many previously unobservable processes. Furthermore, the cleane+e− collision environment and the well-defined initial state compared to hadron colliders are advantages for this analysis at the CEPC. (Super) B factories operating at theΥ (4S) center-of-mass-energy are below the energy threshold forB+c production. A detailed discussion of the various advantages and prospects of flavor studies at the CEPC can be found in Ref. [9].In this paper, we discuss the potential of measuring the processes
B+c→τ+ντ ,τ+→e+νe¯ντ andτ+→μ+νμ¯ντ inZ→b¯b at the CEPC. Important backgrounds are otherZ→c¯c andZ→b¯b processes, especially the decay ofB+→τ+ντ inZ→b¯b events②. BothB+c andB+ have similar masses and event topologies [3]. The main difference is the lifetime (theB+c lifetime is around one third of theB+ lifetime). The L3 experiment at LEP searched forB+→τ+ντ in 1997 with1.475×106 Z→q¯q events [11], and determinedB(B+→τ+ντ)<5.7×10−4 at 90% CL. That study did not consider the contribution fromB+c→τ+ντ . However, Refs. [12, 13] later argued that theB+c→τ+ντ contribution could be comparable to theB+→τ+ντ contribution, and that a similar analysis method could be used to measureB+c→τ+ντ . Understanding theB+→τ+ντ background is crucial in this analysis.We estimate the
B+c/B+→τ+ντ event yield at the CEPC Z pole as follows. The number ofB+→τ+ντ events produced is given by:N(B±→τ±ντ)=NZ×B(Z→b¯b)×2×f(¯b→B+X)×B(B+→τ+ντ),
(1) where
NZ is the total number of Z bosons produced. The factor two accounts for the quark anti-quark pair. The branching ratiosB(Z→b¯b)=0.1512±0.0005 ,f(¯b→B+X)=0.408±0.007 , andB(B+→τ+ντ)=(1.09±0.24)×10−4 are taken from Ref. [3]. For theBc production, the theoretical result at next-to-leading order inαs givesB(Z→B±cX)=7.9×10−5 [14], and our estimate ofB(B+c→τ+ντ) (see the next section) is(2.36±0.19) %. These numbers giveRBc/B=N(B±c→τ±ντ)N(B±→τ±ντ)=0.28±0.05,
(2) where we use
RBc/B to denote the ratio. Note that the actual uncertainty forRBc/B is larger since we lack the uncertainty forB(Z→B±cX) . We conduct our analysis with109 simulated Z boson decays including(1.3±0.3)×104 B±→τ±ντ events. For simplicity and to give a larger signal dataset for analysis, we assume bothN(B±c/B±→τντ) are equal to1.3×104 and discuss other scenarios at the end, since the results are easily scalable for different values ofRBc/B .The rest of this paper is organized as follows. Section II gives the decay width of
B+c→τ+ντ in the SM and estimates the effects in NP scenarios. Section III introduces the detector, software and the MC-simulated event samples. Section IV presents the analysis method and results. The conclusion is given in Sec. V. -
In the SM, the decay width of the purely leptonic decay
B+c→l+νl is given by:ΓSM(B+c→l+νl)=G2F8π|Vcb|2f2BcmBcm2l(1−m2lm2Bc)2,
(3) where
GF is the Fermi coupling constant,Vcb is the CKM matrix element,fBc is the decay constant, andmBc ,ml are the masses of the meson and the charged lepton, respectively. Due to helicity suppression, theτ final state has the largest branching fraction. The measurement ofB+c→τ+ντ would help to determine the fundamental parameter|Vcb| , once the decay constant is known from first-principle calculations, i.e. lattice QCD. The Feynman diagram forB+c→τ+ντ in the SM is shown in the left-hand panel of Fig. 1.With the decay constant
fBc=(0.434±0.015)GeV [15],τ(Bc)=(0.510±0.009)×10−12s and|Vcb|=(42.2±0.8)×10−3 [3], we obtainB(B+c→τ+ντ)=(2.36±0.19)%,
(4) where the errors from the decay constant and lifetime of the
B+c have been added in quadrature. The uncertainty in theB+c branching fraction is dominated by the decay constant, which might be further reduced in a more accurate lattice QCD calculation in the future. Other theoretical studies on the subject ofB+c decay can be found in Ref. [16].Since the tau lepton has the largest mass compared to the other two species of lepton, the NP coupling might have a more evident effect in tauonic decays of heavy mesons. Two popular types of NP model are the two-Higgs-doublet model (2HDM) with a charged Higgs boson propagator similar to the W boson propagator, and the leptoquark (LQ) models that couple leptons with quarks. The charged Higgs boson in 2HDM can have a significant coupling with the tau, and thereby its contributions to decay widths could be sizable [17, 18].
Theoretical studies of NP contributions can be conducted in two distinct ways. One is to confront the explicit model predictions one by one with available experimental constraints, while the other is to employ an effective field theory (EFT) approach. Integrating out the massive particles, e.g. charged Higgs particle or the LQ in Fig. 1, the NP contributions are incorporated into a few effective operators, with the interaction strengths embedded in Wilson coefficients. A general effective Hamiltonian for the
b→cτν transition can be written asHeff=4GF√2Vcb[(1+CV1)OV1+CV2OV2+CS1OS1+CS2OS2]+h.c.,
(5) where
Oi are four-fermion operators andCi are the corresponding Wilson coefficients. The four-fermion operators are defined asOV1=(ˉcLγμbL)(ˉτLγμνL),OV2=(ˉcRγμbR)(ˉτLγμνL),OS1=(ˉcLbR)(ˉτRνL),OS2=(ˉcRbL)(ˉτRνL),
(6) where
OV1 is the only operator present in the SM. The 2HDM can contribute toOS1 , while the LQs can have more versatile contributions depending on their spin and chirality in couplings.Having Eq. (5) and Eq. (6) at hand, one arrives at
Γeff(B+c→τ+ντ)ΓSM(B+c→τ+ντ)=|1+CV1−CV2+CS1m0Bcmℓ−CS2m0Bcmℓ|2,
(7) where
m0Bc≡m2Bc/(mb+mc) . This expression shows the deviation of decay width ofB+c→τ+ντ compared with the SM.Inspired by the experimental measurements of
B→D(∗)τν and other decays induced byb→cτν , quite a few theoretical analyses of NP contributions have been made in recent years. In this work, we will make use of the results for the Wilson coefficients from Refs. [19, 20]:|1+Re[CV1]|2+|Im[CV1]|2=1.189±0.037,
(8) CV2=(−0.022±0.033)±(0.414±0.056)i,
(9) CS1=(0.206±0.051)+(0.000±0.499)i,
(10) CS2=(−1.085±0.264)±(0.852±0.132)i,
(11) and the masses:
mBc=6.2749GeV,mb=4.18GeV,mc=1.27GeV,mτ=1.77686GeV.
(12) Equation (8) directly implies that the branching fraction of
B+c→τ+ντ can be affected by(18.9±3.7) % if only the SM-likeV−A operatorOV1 is included. IfOV2 is considered, the contributions to(Γeff−ΓSM)/ΓSM are shown in Fig. 2. The red shaded areas in this figure correspond to the global fitted results of data on B meson decays induced byb→cτν , as shown in Eq. (9). In this figure and the following ones, we do not consider the correlation between the real and imaginary part in the Wilson coefficients. Two branches are found due to the ambiguous sign in the imaginary part ofCV2 . From this figure, one can infer that the NP contributions range from about 10% to 30%. In these two scenarios, branching fractions ofB+c→τ+ντ are mildly affected due to helicity suppression.Figure 2. (color online) Sensitivities of
(Γeff−ΓSM)/ΓSM (100%) toCV2 . The SM lies at the origin withRe[CV2]=Im[CV2]=0 . Labels (in units of 100%) on contours denote the modification of branching ratios (decay widths) with respect to the SM values. The red shaded areas correspond to the global fitted results of available data onb→cτν decays, as shown in Eq. (9). These areas deviate from the SM predictions by about a fewσ .If we switch to
OS1 , the results are shown in Fig. 3, and again the red shaded area corresponds to the global fitted results shown in Eq. (10). Similar results are shown in Fig. 4 forOS2 . In these two figures, one can clearly see thatΓ(B+c→τ+ντ) is dramatically affected by NP contributions. At this stage the errors do not allow a very conclusive result on the existence of NP, and accordingly measurements of this width at CEPC would help to confirm or rule out these NP scenarios.Figure 3. (color online) Sensitivities of
(Γeff−ΓSM)/ΓSM (100%) toCS1 . The SM lies at the origin withRe[CS1]=Im[CS1]=0 . Labels (in units of 100%) on contours denote the modification of branching ratios (decay widths) with respect to the SM values. The red shaded area corresponds to the global fitted results of available data onb→cτν decays, as shown in Eq. (10).Figure 4. (color online) Similar to Fig. 3, with red shaded areas as parameter spaces of
CS2 given in Eq. (11).Next, let us consider the
|Vcb| measurement in the SM scenario. Its uncertainty can be derived from the relative uncertainty of the signal strengthσ(μ)/μ . The signal strengthμ is the ratio between the measured effective cross section and the corresponding SM prediction, andσ(μ) is its uncertainty. Therefore it is straightforward that:σ(μ)μ=σ(N(B±c→τντ))N(B±c→τντ)=σ(B(Z→B±cX)B(B+c→τ+ντ))B(Z→B±cX)B(B+c→τ+ντ)=σ(B(Z→B±c)ΓSM(B+c→τ+ντ)/Γ(B+c))B(Z→B±c)ΓSM(B+c→τ+ντ)/Γ(B+c),
(13) where
Γ(B+c) is the total width of theB+c . Substituting Eq. (3) into the above equation, we have:(σ(μ)μ)2=(σ(B(Z→B±cX))B(Z→B±cX))2+4(σ(|Vcb|)|Vcb|)2+4(σ(fBc)fBc)2+(σ(Γ(B+c))Γ(B+c))2+Cov.+O(10−6),
(14) where Cov. refers to the covariances between variables. The
σ(fBc)/fBc andσ(Γ(B+c))/Γ(B+c) are both atO (1%) level. Section IV shows thatσ(μ)/μ is also likely at 1% level at Tera-Z. This leaves the error terms to be dominated by theB+c production term, which has a much bigger uncertainty, and will determine the uncertainty of|Vcb| . If theB+c production term can be determined toO (1%) level in the future and the covariances are also around the same level or less,|Vcb| could be determined toO (1%) level as well. -
The CEPC CDR (Conceptual Design Report) [9] provides a detailed description of the detector setup and the software infrastructure. These are both inspired by the International Large Detector (ILD) of the International Linear Collider (ILC) and offer comparable performances. The general flow of software is as follows: 1) create simulated event samples using Pythia [21] and Whizard [22]; 2) MokkaPlus [23], a GEANT4 [24] based simulation tool, simulates the interaction with the detector; 3) the reconstruction framework mimics the electronics responses and employs Arbor [25] and LICH [26] for physics object creation and lepton identification. Upon completing the standard procedures, two more software packages are used for further analysis. One is LCFIPlus [27], an ILC software package which can perform jet clustering and flavor tagging operations to separate different quark flavors in
Z→q¯q . The other is TMVA [28], a multi-variable analysis tool for BDT (boosted decision tree) training.The simulated sample consists of
Z→q¯q,B+→τ+ντ andB+c→τ+ντ . The latter two are additionalZ→q¯q events that contain the corresponding processes. In order to save time, only a fraction of theq¯q (not includingB+c/B+→τ+ντ ) events that are sufficient for analysis are actually simulated. The data are then scaled to reach the sample size corresponding to109 Z boson decays. For theB+c/B+→τ+ντ , we simulated one million events each, and the final numbers and histograms are correspondingly scaled down. All of the scaling factors are shown in Table 1 and Table 2.B±c→τντ (0.013)B±→τντ (0.013)d¯d(15) +u¯u(12) +s¯s(15) c¯c(4.8) b¯b(3.25) τ→eν¯ν excl. τ→eν¯ν τ→eν¯ν excl. τ→eν¯ν All events 2,303 10,691 2,270 10,633 419,928,342 119,954,033 151,286,603 b-tag > 0.6 1,611 7,463 1,547 7,151 2,134,617 7,344,014 116,723,067 Energy asymmetry > 10 GeV 1,425 6,184 1,389 5,801 486,762 1,609,771 30,064,030 Has electron in signal hemisphere 1,273 1,300 1,243 1,132 143,595 625,670 15,905,613 Electron is the most energetic particle 915 116 859 93 8,490 79,190 4,587,248 EB>20 GeV909 112 852 88 981 34,147 3,203,073 1st BDT score > 0.99390 12 259 4 — 48 910 2nd BDT score > 0.4199 12⋆ 73 4⋆ — 48⋆ 33 Table 1. The cut chain for the electron final state for
109 Z bosons. The numbers in parentheses are corresponding scale factors. In the final row, the numbers with stars mean the corresponding channels are not used in the second BDT training in order to avoid possible overfitting. Instead, we make a conservative assumption that all of the events which pass the first BDT cut survive the second BDT cut.B±c→τντ (0.013)B±→τντ (0.013)d¯d(15) +u¯u(12) +s¯s(15) c¯c(4.8) b¯b(3.25) τ→μν¯ν excl. τ→μν¯ν τ→μν¯ν excl. τ→μν¯ν All events 2,250 10,745 2,213 10,698 419,928,342 119,954,033 151,286,603 b-tag > 0.6 1,576 7,499 1,505 7,199 2,134,617 7,344,014 116,723,067 Energy asymmetry > 10 GeV 1,387 6,222 1,348 5,848 486,762 1,609,771 30,064,030 Has muon in signal hemisphere 1,175 2,204 1,168 2,233 244,752 813,083 19,569,212 Muon is the most energetic particle 882 222 838 171 9,777 89,290 4,943,760 EB>20 GeV877 216 832 166 1,713 39,583 3,516,717 1st BDT score > 0.99394 48 306 28 — 76 1,125 2nd BDT score > 0.4192 13 68 5 — 76⋆ 59 Table 2. The cut chain for the muon final state for
109 Z bosons. The numbers in parentheses and the stars in the final row have the same meaning as in Table 1.Since we are looking for leptonic final states, it is helpful to demonstrate the lepton identification performance of the CEPC. Figure 5 shows the generated energy spectrum of the signal and background electrons from
1.76×105 B+c→τ+ντ,τ+→e+νe¯ντ events (corresponding to one millionB+c→τ+ντ events based onB(τ+→e+νe¯ντ) ; the histograms are scaled down to match1.3×104 B+c→τ+ντ events). The signal electrons are those fromB+c→τ+ντ,τ+→e+νe¯ντ . We define the efficiency as the fraction of correctly identified electrons with respect to the total number of electrons. The electron mis-identification rate is defined as the rate of hadrons which are identified as electrons③. The overall lepton identification efficiency and mis-identification rate at energies above 2 GeV are better than 95% and 1%, respectively. For more details, see Ref. [26]. -
The characteristic event topology of
B+c/B+→τ+ντ,τ+→e+/μ+ν¯ν inZ→b¯b is shown in Fig. 6. The event can be divided into two hemispheres by the plane normal to the thrust. The thrust is the unit vectorˆn , which maximizesFigure 6. (color online)
Bc/B→τν,τ→e/μν¯ν inZ→b¯b event topology. The extension of the lepton track passes close by the thrust axis, but does not need to intersect it.T=Σi|pi⋅ˆn|Σi|pi|,
(15) where
pi is the momentum of theith final state particle. We let the thrust point towards the hemisphere with less total energy. The axis where the thrust lies is the thrust axis. The hemisphere in which theB+c/B+→τ+ντ,τ+→e+/μ+ν¯ν decay occurs is the signal hemisphere and the other is the tag hemisphere. The main event topology features are: 1) a b-jet in the tag hemisphere; 2) a single energetic e orμ with relatively large impact parameter along the thrust axis; 3) large energy imbalance between the signal and the tag hemispheres due to missing neutrinos in the signal hemisphere; and 4) some soft fragmentation tracks are also present in both hemispheres. Based on the above definitions and features, it is clear that the thrust axis will mostly point towards the signal hemisphere. The impact parameter is defined as follows. The point on the thrust axis that is closest to the track is found. The impact parameter is the signed distance from this point to the interaction point. If the point lies in the signal hemisphere, then the impact parameter is positive; otherwise it is negative. Therefore, the signal lepton's impact parameter characterizes the sum of the decay length of the B meson and theτ . The main difference betweenB+ andB+c events is the impact parameter, due to the difference between their lifetimes. The general analysis strategy is:1. Employ a cut chain which exploits the main features of the event topology to reduce most of the backgrounds from Z decays to light flavor jets.
2. Use a BDT to separate jets with
B+c/B+→τ+ντ ,τ+→e+/μ+ν¯ν from other heavy flavor jets. In this case both theBc and B events are considered as signal.3. Use another BDT to separate the
Bc events from the B and the remainingb¯b events.Using two BDTs allows us to maximize the separation power of the final state lepton's impact parameter in the second BDT, where it will be used as an additional parameter. We begin with the electron final state and later apply the same method to the muon final state, as they are highly similar. The first stage cut chain is described in the following:
1. The b-tagging score (ranging from zero to unity) has to be greater than 0.6. This reduces most non-
b¯b q¯q backgrounds.2. The energy asymmetry, defined as the total energy in the tag hemisphere subtracted by the total energy in the signal hemisphere, has to be larger than 10 GeV. This step significantly reduces all
q¯q events again, while preserving most of theB+/B+c events.3. The signal hemisphere needs to have at least one electron. In the case of multiple electrons, the most energetic one is selected for analysis. Most of the signal electrons have sufficient momenta to hit the electromagnetic calorimeter and meet the requirement.
4. The electron is the most energetic particle in the signal hemisphere.
5. The nominal B meson energy is greater than 20 GeV. The quantity is defined as: EB = 91.2 GeV - all visible energy except the signal electron .
Table 1 shows the numbers of events during the cut chain. We have eliminated most of the light flavor backgrounds. Although their total number is comparable to the signal, considering the corresponding scale factors, they are likely to be eliminated by the following process, and hence we ignore the events onwards.
After the first stage cut chain, we choose several variables for the BDT to eliminate
b¯b andc¯c backgrounds. Some of the variables were used in the L3 analysis [11]. They are listed as following:– Nominal B meson energy.
– Maximum neutral cluster energy inside a 30 degree cone around the thrust axis in the signal hemisphere.
– The largest impact parameter along the thrust axis in the signal hemisphere besides the selected electron. After the cut chain, in most events the signal electron has the largest impact parameter in the signal hemisphere.
– Energy asymmetry.
– Second largest track momentum in the signal hemisphere.
– Electron energy.
– Electron impact parameter along the thrust axis.
We then apply cuts on the outputs of the two BDTs as described before. In the first BDT, we use all but the electron impact parameter along the thrust axis. This parameter is then added in the second BDT.
-
The first BDT scores are shown in Fig. 7. They range from -1 to 1, of which we show the rightmost part in the figure. The presence of the signal is apparent at large BDT scores. We apply a cut on the BDT score at 0.99 and only use
Bc/B→τντ,τ→eν¯ν andZ→b¯b for the second BDT. Ignoring the non-electronτ decay andZ→c¯c channels will avoid the possibility of overfitting attributed to these channels; besides, the numbers are already small anyway. We then make a conservative assumption that all of the ignored events survive the second BDT cut, except the light flavor events. The second BDT scores are shown in Fig. 8 and we cut at 0.4. The cuts on the BDT scores are chosen to maximize the final signal strength accuracy. Numbers from two BDT results are shown in Table 1.Figure 7. (color online) The first BDT score. Here the notation
Bc/B means the combination of the two data.Now we can compute the relative accuracy of the signal strength:
σ(μ)/μ=√NS+NB/NS,
(16) where
NS andNB denote the numbers of signal and background events that pass all selection cuts, respectively. For the electron final states, we haveσ(μe)/μe=9.7 %. We can repeat the entire process for the muon final state. Here we will include the non-muonτ decay channels in the second BDT, since the numbers of events are significantly larger. The results are shown in Table 2, andσ(μμ)/μμ=10.6 %. Combining the two final states, we haveσ(μ)/μ=7.2 %. It is now straightforward to calculateσ(μ)/μ for bothB+c/B+→τ+ντ at Tera-Z at variousRBc/B . For theB→τν,τ→e/μν¯ν analysis, all we need to do is repeat the second BDT after switching the signal and background status between it and theBc . Figure 9 shows their relationship withRBc/B . Here, the yieldN(B±→τ+ντ) is fixed at1.3×104 per one billion Z. The projectedσ(μ)/μ s at Tera-Z are aroundO(0.1)∼O (1)% level for bothB+c→τ+ντ andB+→τ+ντ . At theRBc/B value given in Eq. (2), where the yieldN(B±c→τ+ντ) is around3.6×103 per one billion Z, we need around109 Z boson decays to achieve fiveσ significance. In Sec. II we discussed the|Vcb| measurement, and with our current results we argue that the accuracy could reach up toO (1)% level with certain improvements. -
As we have shown in Sec. II, based on the current results on NP in
b→cτν ,Γ(B+c→τ+ντ) tends to deviate from SM predictions, but the statistical importance is not significant. From Fig. 9, one can see that at the CEPC,σ(μ)/μ forB+c→τ+ντ can reach about 1% level. This includes the constraint in both the production ofB+c and the decay intoτ+ντ . If the production mechanism is well understood, the result forσ(μ)/μ would also imply that the uncertainties inΓ(B+c→τ+ντ) are reduced to the percent level. Furthermore, in the future one could also useB(B+c→J/ψπ+) as a calibration mode. In theory, lattice QCD can calculate theBc→J/ψ transition form factors while the perturbative contributions are well under control in perturbation theory.One can use such results for
Γ(B+c→τ+ντ) to probe NP to a high precision. In Fig. 10, we show the constraints onRe[CV2] andIm[CV2] . If the central values in Eq. (9) remain the same while the uncertainty inΓ(B+c→τ+ντ) is reduced to 1%, the allowed region forCV2 shrinks to the dark-blue regions, where the deviation from the SM is greatly enhanced.Figure 10. (color online) Constraints on the real and imaginary parts of
CV2 . The red shaded area corresponds to the current constraints using available data onb→cτν decays. If the central values in Eq. (9) remain while the uncertainty inΓ(B+c→τ+ντ) is reduced to 1%, the allowed region forCV2 shrinks to the dark-blue regions.Similar results can be obtained for the NP coefficients
CS1 andCS2 , but as we have demonstrated in Sec. II, both scenarios will induce dramatic changes toΓ(B+c→τ+ντ) . These NP effects are so large that they would already be verified or ruled out before entering into the high-precision era of the CEPC. Thus it is less meaningful to present the constraints for these two coefficients.
Analysis of Bc → τντ at CEPC
- Received Date: 2020-09-08
- Available Online: 2021-02-15
Abstract: Precise determination of the