arXiv subjects

P. Singer

Publications and source records attributed to P. Singer.

At least 19 recordsLinked to original sources

arXiv2004

New-Physics scenarios in $ΔS = 2$ decays of the B_c meson

The $ΔS = 2$ transition $b \to s s \bar d$ is extremely small in the standard model, proceeding only via box diagrams. Such decays are thus an appropriate ground for searching new physics. We present a study of several two-body and three-body decays of the B_c meson belonging to this class. Our calculation covers the minimal supersymmetric model with and without R-parity and a two-Higgs-doublet model. The current limits on the parameters of these models allow for rather large branching ratios for several two and three-body hadronic decay modes of B_c, up to the 10^{-4} range in the minimal supersymmetric model with R-parity violation.

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arXiv2002

The Radiative Leptonic Decays D^0 -> e^+ e^- gamma, mu^+ mu^- gamma in the Standard Model and Beyond

We present a calculation of the rare decay modes D^0 -> e^+ e^- gamma and D^0 -> mu^+ mu^- gamma in the framework of Standard Model. For the short distance part, we have derived QCD corrections to the Wilson coefficients involved, including C_9. The latter is found to be strongly suppressed by the corrections, leading to diminished values for the c -> u l^+l^- branching ratios in the 10^(-10) range. Within SM the exclusive decays are dominated by long distance effects. Nonresonant contributions are estimated using heavy quark and chiral symmetries to be at the level of 10%, compared to the contributions arising from D^0-> V gamma->l^+ l^- gamma, with V=rho, omega, phi. The total SM branching ratio is predicted to be in the range (1-3) x 10^(-9). We also consider contributions coming from MSSM with and without R parity conservation. Effects from MSSM are significant only for the R-parity violating case. Such contributions enhance the branching ratio D^0-> mu^+ mu^- gamma to 0.5 x 10^(-7), based on appropriately allowed values for C_9 and C_{10}. This selects D^0->mu^+ mu^- gamma as a possible probe of new physics.

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arXiv2002

The rare decays of D mesons

The flavor changing transitions in the c->u gamma, c->u gamma gamma and c->u l+ l- offer the possibility to search for new physics in the charm sector. We investigate dominant decay mechanisms in the radiative decays D->V gamma, D->P(V) l+ l-, D->gamma gamma and we discuss chances to see physics beyond the standard model in these decays. In addition, we analyze Cabibbo allowed D->K pi gamma decays with nonresonant K pi, and we probe the role of light vector mesons in these decays.

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arXiv2001

Constraints on heavy Z' couplings from ΔS = 2 B^- -> K^- K^- π^+ decay

The heavy Z' boson with family nonuniversal couplings can introduce flavour changing effects. Constraints on nondiagonal Z' couplings coming from the μ-e conversion in a muonic atom, K^0 - \bar{K^0} and B - \bar{B} mixing, epsilon and epsilon'/ εCP- violating coefficients have been already established. By using the OPAL upper bound of the branching ratio for the B^- -> K^- K^- π^+ decay, we indicate additional constraints on the Z' couplings. We comment also on the constraints of Z' couplings coming from the b->d d \bar{s} transition. The constraint obtained here from the upper bound of the B^- -> K^- K^- π^+ decay involves a different combination of couplings than those previously presented, but is much weaker.

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arXiv2001

The Rare Decay D^0 -> gamma gamma

We present a calculation of the rare decay mode D^0 -> gamma gamma, in which the long distance contributions are expected to be dominant. Using the Heavy Quark Chiral Perturbation Theory Lagrangian with a strong g coupling as recently determined by CLEO from the D^* -> D pi width, we consider both the anomaly contribution which relates to the annihilation part of the weak Lagrangian and the one-loop pi, K diagrams. The loop contributions which are proportional to g and contain the a_1 Wilson coefficient are found to dominate the decay amplitude, which turns out to be mainly parity violating. The branching ratio is then calculated to be (1.0+-0.5)x10^(-8). Observation of an order of magnitude larger branching ratio could be indicative of new physics.

hep-ph↗

arXiv2001

Rare charm meson decays D->Pl^+l^- and c->ul^+l^- in SM and MSSM

We study the nine possible rare charm meson decays D->Pl^+l^- (P=pi,K,eta,eta') using the Heavy Meson Chiral Lagrangians and find them to be dominated by the long distance contributions. The decay D^+ -> pi^+l^+l^- with the branching ratio 1*10^(-6) is expected to have the best chances for an early experimental discovery. The short distance contribution in the five Cabibbo suppressed channels arises via the c->ul^+l^- transition; we find that this contribution is detectable only in the D->pi l^+l^- decay, where it dominates the differential spectrum at high-q^2. The general Minimal Supersymmetric Standard Model can enhance the c->ul^+l^- rate by up to an order of magnitude; its effect on the D->Pl^+l^- rates is small since the c->ul^+l^- enhancement is sizable in low-q^2 region, which is inhibited in the hadronic decay.

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arXiv2001

The Search for New Physics in D^0 --> gamma gamma Decay

We present main results of the investigation of the rare decay mode D^0 -->gamma gamma, in which the long distance contributions are expected to be dominant. Using the Heavy Quark Chiral Lagrangian we have considered the anomaly contribution which relates to the annihilation part of the weak Lagrangian and the one - loop pi, K diagrams. The loop contributions which are proportional to g and contain the a_1 Wilson coefficient are found to dominate the decay amplitude. The branching ratio is then calculated to be (1.0+-0.5)x 10^{-8}. Observation of an order of magnitude larger branching ratio could signal new physics.

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arXiv2001

$ΔS = 2$ decays of $B^-$ meson in MSSM and two Higgs doublet model

In view of the extreme smallness of $ΔS = 2$ transitions of $B$ meson in the Standard Model, we consider their occurence in several extensions of it. Thus, we analyze the three - body $B^- \to K^- K^- π^+$ and two - body $B^- \to K^{*-} \barK^{*0}$, $B^- \to K^- \bar K^{0}$, $B^- \to K^{*-} \bar K^{0}$, $B^- \to K^{-} \bar K^{*0}$ decay modes both in the Standard Model and in the Minimal Supersymmetric Model with and without ${\cal R}$ parity conservation and in two Higgs doublet models. All five modes are found to have a branching ratio of the order of $10^{-13}$ in the Standard Model, while the expected branching ratio in the different extensions vary between $10^{-9} - 10^{-6}$, for a given reasonable choice of parameters.

hep-ph↗

arXiv2000

Search for new physics in $ΔS = 2$ two - body (VV, PP, VP) decays of the B^- - meson

The $ΔS = 2$ $b \to \bar d ss$ transition proceeds via the box - diagram in the Standard Model with a branching ratio calculated to be below $10^{-11}$, thus providing an appropriate testing ground for physics beyond the Standard Model. We analyze the $ΔS = 2$ two - body $B^- \to K^{*-} \bar K^{*0}$, $B^- \to K^- \bar K^{0}$, $B^- \to K^{*-} \bar K^{0}$ and $B^- \to K^{-} \bar K^{*0}$ exclusive decays which are driven by the $b \to \bar d ss$ transition, both in the Standard Model and in several extensions of it. The models considered are the minimal supersymmetric model with and without ${\cal R}$ parity conservation and two Higgs doublet models. All four modes are found to have a branching ratio of the order of $10^{-13}$ in the Standard Model, while the expected branching ratio in the different extensions vary between $10^{-9} - 10^{-6}$.

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arXiv2000

A Possible Arena for Searching New Physics - the $Γ(D^0 \to ρ^0 γ)/Γ(D^0 \to ωγ)$ Ratio

We propose to investigate flavour changing neutral currents in the $c \to u γ$ transition through the measurement of the difference between $Γ(D^0 \to ρ^0 γ)$ and $Γ(D^0 \to ωγ)$. This is based on the observation that $D^0 \to (d \bar d) γ$ is due to long distance physics while $D^0 \to (u \bar u) γ$ arises from the $c \to u γ$ transition. The effect of $ρ- ω$ mixing is included. A difference in the decay widths of more than about 30% would be indicative of new physics.

hep-ph↗

arXiv2000

Long distance contribution to $B^- \to K^- K^- π^+$, - a searching ground mode for new physics

The decay $B^- \to K^- K^- π^+$ has been sugested as a test for minimal supersymmetric standard model and for supersymmetric models with R-parity violating couplings, in view of its extreme smallnesss in the standard model. We calculate two long distance contributions to this decay, that associated with $DD$ and $Dπ$ intermediate states and that induced by virtual $D$, $π$ mesons. The branching ratio due to these contributions is $6 \times 10^{-12}$, which is somewhat smaller than the standard model short distance result, leaving this decay free for the search of new physics.

hep-ph↗

arXiv1999

FCNC transition $c\to uγ$ in $B_c\to B_u^*γ$ decay

We propose the $B_c\to B_u^*γ$ decay as the most suitable probe for the flavour changing neutral transition $c\to uγ$. We estimate the short and long distance contributions to this decay within the standard model and we find them to be comparable; this is in contrast to radiative decays of $D$ mesons, that are completely dominated by the long distance contributions. Since the $c\to uγ$ transition is very sensitive to the physics beyond the standard model, the standard model prediction $Br(B_c\to B_u^*γ)\sim 10^{-8}$ obtained here opens a new window for future experiments. The detection of $B_c\to B_u^*γ$ decay at branching ratio well above $10^{-8}$ would signal new physics.

hep-ph↗

arXiv1999

Probing $c\to uγ$ in $B_c\to B_u^*γ$ decay

Flavour changing neutral current (FCNC) transitions $c\to uγ$ and $c\to ul^+l^-$ are very rare in the standard model and present an interesting probe for the physics beyond it. We study long distance contamination to these short distance processes in different hadron decays. As the most suitable probe for $c\to uγ$ transition we propose $B_c\to B_u^*γ$ decay. Its detection at the branching ratio well above $10^{-8}$ would signal new physics. The $c\to ul^+l^-$ transition may be probed at high $m_{ll}$ in $D\to πl^+l^-$. The decays $D\to Vγ$ and $D\to Vl^+l^-$ ($V$ is a light vector meson) are less suitable, since they are dominated by the long distance effects.

hep-ph↗

arXiv1999

FCNC transitions $c\to uγ$ and $s\to dγ$ in $B_c\to B_u^γ$ and $B_s\to B_d^γ$ decays

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arXiv1998

Resonant and nonresonant contributions to the weak $D\to Vl^+l^-$ decays

The Cabibbo suppressed decays $D\to Vl^+l^-$ (V is light vector meson) present in principle the opportunity to observe the short distance FCNC transition $c\to ul^+l^-$, which is sensitive to physics beyond the Standard Model. We analyze these as well as the Cabibbo allowed $D \to V l^+ l^-$ decays within the Standard Model, where in addition to the short distance dynamics also the long distance dynamics is present. The long distance contribution is induced by the effective nonleptonic weak Lagrangian accompanied by the emission of a virtual photon, which occurs resonantly via conversion from a vector meson $ρ^0, ω$ or $ϕ$ or nonresonantly as direct emission from a $D$ meson. We calculate the branching ratios for all $D\to Vl^+l^-$ decays using the model, which combines heavy quark symmetry and chiral perturbation theory. The short distance contribution due to $c\to ul^+l^-$ transition, which is present only in the Cabibbo suppressed decays, is found to be three orders of magnitude smaller than the long distance contribution. The branching ratios well above $10^{-7}$ for Cabibbo suppressed decays could signal new physics. The most frequent decays are the Cabibbo allowed decays, which are expected at the rates, that are not much lower than the present experimental upper limit: $D_s^+\to ρ^+μ^+μ^-$ is expected at the branching ratio of approximately $3\cdot 10^{-5}$, while $D^0\to\bar K^{*0}μ^+μ^-$ is expected at $1.7\cdot 10^{-6}$.

hep-ph↗

arXiv1998

Long distance contributions in $D \to V γ$ decays

Using the factorization scheme for the nonleptonic $D \to V V_0$ weak amplitudes, we classify all diagrams which arise in $D \to V γ$ decays and calculate them with the help of the hybrid model which combines the heavy quark effective theory and the chiral Lagrangian approach. Thus we determine the long distance contribution to the amplitudes of Cabibbo allowed and Cabibbo suppressed $D \to Vγ$ decays. The calculation of the expected range of the branching ratios of nine different $D \to V γ$ channels is compared with results of other approaches. The present work establishes an increase of the parity violating contribution in these decays in comparison with previous analyses.

hep-ph↗

arXiv1998

Radiative decays of D mesons

The short-distance contribution due to the $c\to uγ$ penguin as well as long-distance contributions are considered in an analysis of $D\to Vγ$ decays, the latter being dominant. The matrix elements for nine $D\to Vγ$ transitions are calculated using a hybrid model which combines heavy quark effective theory and the chiral Lagrangian. We present the expected range of the branching ratios for these decays, the most frequent ones $D^0\to \bar{K}^{*0}γ$ and $D^+_s \to ρ^+γ$, being a few times $10^{-4}$.

hep-ph↗

arXiv1998

Can FCNC transition $c\to ul^+l^-$ be seen in $D\to Vl^+l^-$ decays?

The decays $D\to Vl^+l^-$ present in principle the opportunity to observe the short distance FCNC transition $c\to ul^+l^-$, which is sensitive to the physics beyond the Standard Model. We analyze the $D \to V l^+ l^- $ decays within the Standard Model, where in addition to the short distance dynamics also the long distance dynamics is present. The short distance contribution due to $c\to ul^+l^-$ transition, which is present only in the Cabibbo suppressed decays, is found to be three orders of magnitude smaller than the long distance contribution. The branching ratios well above $10^{-7}$ for Cabibbo suppressed decays could signal new physics. The most frequent decays are the Cabibbo allowed decays $D_s^+\toρ^+μ^+μ^-$ and $D^0\to\bar K^{*0}μ^+μ^-$, which are expected at the branching ratios of $3\cdot 10^{-5}$ and $1.7\cdot 10^{-6}$, respectively. These rates are not much lower than the present experimental upper limit.

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