Charm CP Violation Research Articles

A U-spin anomaly in charm CP violation

Recent LHCb data shows that the direct CP asymmetries of the decay modes D0 → π+π− and D0 → K+K− have the same sign, violating an improved U-spin limit sum rule in an unexpected way at 2.1σ. From the new data, we determine for the first time the imaginary part of the CKM-subleading, U-spin breaking ∆U = 1 correction to the U-spin limit ∆U = 0 amplitude. The imaginary part of the ∆U = 0 amplitude is determined by Delta {a}_{textrm{CP}}^{textrm{dir}} . The corresponding strong phases are yet unknown and could be extracted in the future from time-dependent measurements. Assuming mathcal{O}(1) strong phases due to non-perturbative rescattering, we find the ratio of U-spin breaking to U-spin limit contributions to the CKM-subleading amplitudes to be left({173}_{hbox{--} 74}^{+85}right)% . This highly exceeds the Standard Model (SM) expectation of ~ 30% U -spin breaking, with a significance of 1.95σ. If this puzzle is confirmed with more data in the future, in the SM it would imply the breakdown of the U -spin expansion in CKM-subleading amplitudes of charm decays. The other solution are new physics models that generate an additional ∆U = 1 operator, leaving the U-spin power expansion intact. Examples for the latter option are an extended scalar sector or flavorful Z′ models.

Mixing and CP violation in charm decays at LHCb

We present an overview of four recent measurements of charm decays performed at the LHCb experiment. These include the first observation of the non-zero mass difference between neutral charm-meson eigenstates, the most precise measurements of ∆YKK and ∆Yππ, and timeintegrated ACP of eight two-body charm decays. The precision of the measurements is mainly limited by statistical uncertainty, so further improvement is expected with the upcoming LHC Run 3.

Enhancement of charm CP violation due to nearby resonances

Quantitative understanding of CP violation is extremely important as naturalness reasoning strongly suggests that new physics should be accompanied by beyond the Standard Model CP-odd phases. In 2019 LHCb made the first 5σ discovery of CP violation in the charm system, leading to the new world average ΔaCPdir=−0.00161±0.00028. While some calculations have found this observation as requiring new physics, we suggest that scalar resonances nearby to mD0, in particular, f0(1710) and/or f0(1790) cause enhancements for CP violation within the SM. Thereby, our calculations based on the Standard Model suggest compatibility with the LHCb observations for now. However, experimental information especially on f0(1790) is rather sparse limiting our accuracy and further data on these resonances is strongly advocated.

The charm of 331

We perform a detailed analysis of flavour changing neutral current processes in the charm sector in the context of 331 models. As pointed out recently, in the case of Z′ contributions in these models there are no new free parameters beyond those already present in the Bd,s and K meson systems analyzed in the past. As a result, definite ranges for new Physics (NP) effects in various charm observables could be obtained. While generally NP effects turn out to be small, in a number of observables they are much larger than the tiny effects predicted within the Standard Model. In particular we find that the branching ratio of the mode D0→ μ+μ−, despite remaining tiny, can be enhanced by 6 orders of magnitude with respect to the SM. We work out correlations between this mode and rare Bd,s and K decays. We also discuss neutral charm meson oscillations and CP violation in the charm system. In particular, we point out that 331 models provide new weak phases that are a necessary condition to have non-vanishing CP asymmetries. In the case of ∆ACP, the difference between the CP asymmetries in D0→ K+K− and D0→ π+π−, we find that agreement with experiment can be obtained provided that two conditions are verified: the phases in the ranges predicted in 331 models and large hadronic matrix elements.

Simple model for large CP violation in charm decays, B-physics anomalies, muon g \u2212 2 and dark matter

We present a minimal extension of the Standard Model that can simultaneously account for the anomalies in semi-leptonic B meson decays and the muon g − 2, give large CP violation in charm decays (up to the value recently measured by LHCb), and provide thermal-relic dark matter, while evading all constraints set by other flavour observables, LHC searches, and dark matter experiments. This is achieved by introducing only four new fields: a vectorlike quark, a vectorlike lepton, and two scalar fields (a singlet and a doublet) that mix due to the electroweak symmetry breaking and provide the dark matter candidate. The singlet-doublet mixing induces chirally-enhanced dipole transitions, which are crucial for the explanation of the muon g − 2 discrepancy and the large charm CP violation, and allows to achieve the observed dark matter density in wide regions of the parameter space.

CP violation in beauty and charm at LHCb

Precision measurements of CP violating observables in the decays of b and c hadrons are powerful probes to search for physics beyond the Standard Model. The most recent results on CP violation in the decay, mixing and interference of both b and c hadrons obtained by the LHCb Collaboration with Run I and years 2015-2016 of Run II are presented, including the first observation of CP violation in the charm system. In particular world best constraints and world first measurements are provided for CKM elements, unitarity angles and charm parameters. Prospects for future sensitivities are also discussed.

Sum rules for CP asymmetries of charmed baryon decays in the SU(3)_F limit

Motivated by the recent LHCb observation of CP violation in charm, we study CP violation in the charmed baryon decays. A simple method to search for the CP violation relations in the flavor SU(3) limit, which is associated with a complete interchange of d and s quarks, is proposed. With this method, hundreds of CP violation sum rules in the doubly and singly charmed baryon decays can be found. As examples, the CP violation sum rules in two-body charmed baryon decays are presented. Some of the CP violation sum rules could help the experiment to find better observables. As byproducts, the branching fraction of Xi ^+_crightarrow pK^-pi ^+ is predicted to be (1.7pm 0.5)% in the U-spin limit and the fragmentation–fraction ratio is determined to be f_{Xi _b}/f_{Lambda _b}=0.065pm 0.020 using the LHCb data.

Charm Mixing and CP violation at LHCb

The LHCb experiment has collected the world’s largest sample of charmed hadrons. This sample is used to measure mixing and to search for direct and indirect CP violation, and its sensitivity is now approaching the Standard Model expectations. In this document, the most recent LHCb results on CP violation in charm sector and on charm mixing are reported.

CPviolation inD0→KSKS

The direct CP asymmetry $a_{CP}^{\mathrm{dir}}(D^0\rightarrow K_SK_S)$ involves exchange diagrams which are induced at tree level in the Standard Model. Since the corresponding topological amplitude $E_{KK}$ can be large, $D^0\rightarrow K_SK_S$ is a promising discovery channel for charm CP violation. We estimate the penguin annihilation amplitude with a perturbative calculation and extract the exchange amplitude $E_{KK}$ from a global fit to $D$ branching ratios. Our results are further used to predict the size of mixing-induced CP violation. We obtain $\vert a_{CP}^{\mathrm{dir}}(D^0\rightarrow K_SK_S)\vert \leq 1.1\%$ (95\% C.L.). The same bound applies to the nonuniversal part of the phase between the $D\!-\overline{\!D}\,$ mixing and decay amplitudes. If future data exceed our predictions, this will point to new physics or an enhancement of the penguin annihilation amplitude by QCD dynamics. We briefly discuss the implications of these possibilities for other CP asymmetries.

Charm CP violation and mixing at Belle

We present charm CP violation and mixing measurements at Belle. They are the first observation of D0 − D̄0 mixing in e+e− collisions from D0 → K+π− decays, the most precise mixing and indirect CP violation parameters from D0 → K0Sπ+π− decays, and the timeintegrated CP asymmetries in D0 → π0π0 and D0 → K0Sπ0 decays. Our mixing measurement in D0 → K+π− decays excludes the no-mixing hypothesis at the 5.1 standard deviation level. The mixing parameters x = (0.56 ± 0.19+0.03+0.06−0.09−0.09)%, y = (0.30 ± 0.15+0.04+0.03−0.05−0.06)% and indirect CP violation parameters |q/p| = (0.90+0.16+0.05+0.06−0.15−0.04−0.05)%, arg(q/p) = (−6 ± 11 ± 3+3−4)° measured from D0 → K0Sπ+π− decays, and the time-integrated CP asymmetries AD0→π0π0CP = (−0.03 ± 0.64 ± 0.10)% and AD0→K0Sπ0CP = (−0.21 ± 0.16 ± 0.07)% are the most precisemeasurements to date. Our measurements here are consistent with predictions of the standard model.

Charm mixing and CP violation at LHCb

The LHCb experiment has collected the world's largest sample of charmed hadrons. Using data corresponding to an integrated luminosity of 3.0 fb−1 recorded in 2011 and 2012, measurements of direct and indirect CP violation in the charm sector and of D0 mixing parameters were performed. Results from several decay modes are presented with complementary time-dependent and time-integrated analyses.

Review of recent results on charm mixing and CP violation

The establishment of [Formula: see text] mixing in 2007 generated a renaissance of interest in charm physics. The field gained even more strength with the commissioning of the LHC, which opened a new era of high sensitivity measurements. In this paper, we briefly review the basic formalism and experimental techniques related to charm mixing and to CP violation searches in D-meson decays and then report on recent experimental results obtained by the B-factories, CDF and LHCb.

CHARM MIXING AND CP VIOLATION AT B-FACTORIES

CP violation in charm decays is expected to be very small in the Standard Model, at the level of 0.1% or less. A sizable excess of CP violation with respect to the Standard Model predictions could be a signature of new physics. We report on recent searches for CP violation in charm meson decays at BABAR and Belle experiments. In particular we report a lifetime ratio analysis of D0 → K+K−, π+π− with respect to D0 → K−π+ decays, which is sensitive to [Formula: see text] mixing and CP violation. We report also on searches for CPV in the 3-body D+ → K+K−π+ decay and for decay modes with a [Formula: see text] in the final state, such as [Formula: see text].

Charm Production, Mixing and CP Violation

Charm Production, Mixing and CP Violation

Direct CP violation in charm and flavor mixing beyond the SM

We discuss the impact of physics beyond the Standard Model on direct CP violation in charm. On general grounds, models in which the primary source of flavor violation is linked to the breaking of chiral symmetry (left-right flavor mixing) are natural candidates to explain this effect, via enhanced chromomagnetic operators. In the case of super-symmetric models, we identify two motivated scenarios: disoriented A-terms and split families. Non-supersymmetric models with Z-mediated and scalar-mediated FCNC are also discussed.

Recent results on CP violation in charm and τ decays from BABAR

In the Standard Model CP violation in charm decays is expected to be very small, at the level of 0.1% or below, while CP violation in τ decays is predicted to be negligible. A significant excess of CP violation with respect to the Standard Model predictions would be a signature of new physics. We report on recent searches for CP violation in charm meson decays and in τ lepton decays at BABAR, using a data sample corresponding to an integrated luminosity of about 470 fb−1. In particular, we report on searches for CPV in the decay modes with a K0S in the final state, such as D+ → K0SK+, D+s → K0SK+, D+s → K)Sπ+ and τ+ → π+K0Sv̄τ, and in the 3-body D+ → K+K−π+ decay. A lifetime ratio analysis of D0 → K+K−, π+π− with respect to D0 → K−π+ decays, which is sensitive to D0-D̄0 mixing and CP violation, is also presented here.

Charm mixing and CP violation at LHCb

In pp collisions at the LHC the LHCb experiment has collected the world's largest sample of charmed hadrons. Using data of an integrated luminosity of 1.0 fb−1 recorded in 2011 a measurement of direct and indirect CP violation in the charm sector and of D0 mixing parameters was performed. Results from several decay modes are presented with complementary time-dependent and time-integrated analyses.

Effect of direct CP violation in charm on γ extraction from B ±→DK ±, $D\to K^{0}_{S}\pi^{+}\pi^{-}$ Dalitz plot analysis

A possible effect of direct CP violation in $D\to K^{0}_{S}\pi^{+}\pi^{-}$ decay on the γ measurement from B ±→DK ±, $D\to K^{0}_{S}\pi^{+}\pi^{-}$ Dalitz plot analysis is considered. Systematic uncertainty of γ coming from the current limits on direct CP violation in $D\to K^{0}_{S}\pi ^{+}\pi^{-}$ is estimated, and a modified model-independent procedure of B ±→DK ±, $D\to K^{0}_{S}\pi^{+}\pi^{-}$ Dalitz plot analysis is proposed that gives an unbiased γ measurement even in presence of direct CP violation in charm decays. The technique is applicable to other three-body D decays such as $D^{0}\to K_{S}^{0}K^{+}K^{-}$ , D 0→π + π − π 0, etc.

Charm CP violation and the electric dipole moments from the charm scale

The reported CP asymmetry in D->K^+K^- / pi^+pi^- is argued to be too large to naturally fit the SM. If so, a new source of CP violation is implied in the Delta C=1 sector with a milliweak strength. CP-odd interactions in the flavor-diagonal sector are strongly constrained by the EDMs placing severe limitations on the underlying theory. While the largest effects usually come from the New Physics energy scale, they are strongly model-dependent. Yet the interference of the CP-odd forces manifested in D decays with the conventional CP-even Delta C=1 weak interaction generates at the charm scale a background level. It has been argued that the d_n in the SM is largely generated via such an interference, with mild KM-specific additional suppression. The reported CP asymmetry is expected to generate d_n of 30 to 100 times larger than in the SM, or even higher in certain model yet not quite natural examples. In the SM the charm-induced loop-less |d_n| is expected around 10^{-31}e*cm. On the technical side, we present a compact Ward-identity--based derivation of the induced scalar pion-nucleon coupling in the presence of the CP-odd interactions, which appears once the latter include the right-handed light quarks.

B±→DK±with directCPviolation in charm

We investigate the implications of direct $CP$ violation (CPV) on the determination of the unitarity triangle angle $\ensuremath{\gamma}$ from $B\ensuremath{\rightarrow}DK$ decays. We show that $\ensuremath{\gamma}$ can still be extracted even with the inclusion of direct CPV in charm if (i) at least one of the $D$ decays has negligible $CP$ violation; and (ii) data from a charm factory at threshold are used. If approximate expressions without direct $CP$ violation in charm are used, this can result in a shift in $\ensuremath{\gamma}$ that is $\mathcal{O}({r}_{D}/{r}_{B})$. It is modest for $B\ensuremath{\rightarrow}DK$ but can be $\mathcal{O}(1)$ for $B\ensuremath{\rightarrow}D\ensuremath{\pi}$. We illustrate the size of the shift using an example of the Gronau-London-Wyler method.