Jusak Tandean - Academia.edu (original) (raw)

Papers by Jusak Tandean

Springer Proceedings in Physics, 2019

As direct searches for new physics (NP) beyond the standard model (SM) at the LHC and other colli... more As direct searches for new physics (NP) beyond the standard model (SM) at the LHC and other colliders have so far come up empty, increasingly attention has been paid to the intensity frontier, where high-precision measurements can probe rare processes which are possibly sensitive to indirect signals of NP. An important tool in the quest for NP effects on rare processes is the null tests of the SM. These are observables which vanish or are very small in the SM and which therefore could receive potentially large contributions from NP. Among such tests are, to be discussed here, asymmetries in the charm-meson decays D → P 1 P 2 + − (P = π, K and = e, μ), direct C P asymmetries in Cabibbo-favored nonleptonic charm-meson decays, muon asymmetries in the hyperon decay Σ + → pμ + μ − , and searches for the rare transitions b → ssd, dds and for nonleptonic B s-meson decays which fully break isospin symmetry. These are under ongoing or near-future investigations in the LHCb and other experiments. Observables which are predicted to be very small or vanish within the standard model (SM) serve, at least in principle, as its (approximate or exact) null tests [1]. They are potentially good places to look for indications of new physics (NP) beyond the SM (BSM), as any unambiguous nonnegligible result of the measurement of such an observable would be a compelling hint of NP BSM. If an experiment performing this kind of test produces no positive result, the null outcome will, on the flip side, translate into a constraint on the NP scenario under consideration. Over the years there have already been numerous efforts to conduct SM null tests with various flavor-changing neutral current (FCNC) transitions. Examples are the searches for processes manifesting charged-lepton-flavor violation, such as μ → eγ and B 0 → μ ± τ ∓ , which have so far come up negative [2]. Since they are forbidden in the SM, their observation would be clear evidence for NP. Another instance of SM null test is the deviation of the ratio R K (*) of the branching fractions of the decays

Journal of High Energy Physics, 2020

The ratio B(K L → π 0 νν)/B(K + → π + νν) of the branching fractions of kaon decays K L → π 0 νν ... more The ratio B(K L → π 0 νν)/B(K + → π + νν) of the branching fractions of kaon decays K L → π 0 νν and K + → π + νν has a maximum of about 4.3 under the assumption that the underlying interactions change isospin by ∆I = 1/2. This is referred to as the Grossman-Nir (GN) bound, which is respected by the standard model (SM) and by many scenarios beyond it. Recent preliminary results of the KOTO and NA62 Collaborations searching for these kaon modes seem to imply a violation of this bound. The KOTO findings also suggest that B(K L → π 0 νν) could be much larger, by nearly two orders of magnitude, than that predicted in the SM. In this work we study the possibility of violating the GN bound in an effective field theory approach with only SM fields. We show that the bound holds, in addition to the original GN scenarios, whether or not the kaon decays conserve lepton number. We demonstrate that the inclusion of ∆I = 3/2 operators can lead to a violation of the GN bound and illustrate with an example of how the KOTO numbers may be reached with a new physics scale of order tens of GeV.

The European Physical Journal C, 2016

We consider the flavor-changing decays of the Higgs boson in a grand unified theory framework whi... more We consider the flavor-changing decays of the Higgs boson in a grand unified theory framework which is based on the SU(5) gauge group and implements the principle of minimal flavor violation. This allows us to explore the possibility of connecting the tentative hint of the Higgs decay h → μτ recently reported in the CMS experiment to potential new physics in the quark sector. We look at different simple scenarios with minimal flavor violation in this context and how they are subject to various empirical restrictions. In one specific case, the relative strengths of the flavorchanging leptonic Higgs couplings are determined mainly by the known quark mixing parameters and masses, and a branching fraction B(h → μτ) ∼ 1% is achievable without the couplings being incompatible with the relevant constraints. Upcoming data on the Higgs leptonic decays and searches for the μ → eγ decay with improved precision can offer further tests on this scenario.

Journal of High Energy Physics, 2016

Direct searches for dark matter (DM) by the LUX and PandaX-II Collaborations employing xenon-base... more Direct searches for dark matter (DM) by the LUX and PandaX-II Collaborations employing xenon-based detectors have recently come up with the most stringent limits to date on the spin-independent elastic scattering of DM off nucleons. For Higgsportal scalar DM models, the new results have precluded any possibility of accommodating low-mass DM as suggested by the DAMA and CDMS II Si experiments utilizing other target materials, even after invoking isospin-violating DM interactions with nucleons. In the simplest model, SM+D, which is the standard model plus a real singlet scalar named darkon acting as the DM candidate, the LUX and PandaX-II limits rule out DM masses roughly from 4 to 450 GeV, except a small range around the resonance point at half of the Higgs mass where the interaction cross-section is near the neutrino-background floor. In the THDM II+D, which is the type-II two-Higgs-doublet model combined with a darkon, the region excluded in the SM+D by the direct searches can be recovered due to suppression of the DM effective interactions with nucleons at some values of the ratios of Higgs couplings to the up and down quarks, making the interactions significantly isospin-violating. However, in either model, if the 125-GeV Higgs boson is the portal between the dark and SM sectors, DM masses less than 50 GeV or so are already ruled out by the LHC constraint on the Higgs invisible decay. In the THDM II+D, if the heavier CP-even Higgs boson is the portal, theoretical restrictions from perturbativity, vacuum stability, and unitarity requirements turn out to be important instead and exclude much of the region below 100 GeV. For larger DM masses, the THDM II+D has plentiful parameter space that corresponds to interaction cross-sections under the neutrino-background floor and therefore is likely to be beyond the reach of future direct searches without directional sensitivity.

We study the |∆I| = 3/2 amplitudes of the octet-hyperon decays B → B ′ π and of the decays Ω − → ... more We study the |∆I| = 3/2 amplitudes of the octet-hyperon decays B → B ′ π and of the decays Ω − → Ξπ in the context of heavy-baryon chiral perturbation theory. For the octet-hyperon decays, we investigate the theoretical uncertainty of the lowest-order predictions by calculating the leading nonanalytic corrections. We find that these corrections are within the expectations of naive power counting and, therefore, that this picture can be tested more accurately with improved measurements. For the Ω − decays, we obtain at leading order two operators responsible for the decays which also contribute at one loop to the octet-hyperon decays. These one-loop contributions are sufficiently large to suggest that the measured ratio Γ(Ω − → Ξ 0 π −)/Γ(Ω − → Ξ − π 0) ≈ 2.7 may be too large.

Phys Rev D, 2000

We present a reanalysis of direct CP violation in the decay K L → π + π − γ. We point out an exis... more We present a reanalysis of direct CP violation in the decay K L → π + π − γ. We point out an existing discrepancy between the theoretical and experimental definitions of ǫ ′ +−γ. Adopting the experimental definition of ǫ ′ +−γ , we estimate that ǫ ′ +−γ /ǫ could be as large as a few times 10 −4 both within the standard model and beyond. We discuss these estimates in detail and we also show how a judicious choice of E * γ cuts can increase the sensitivity of the observable ǫ ′ +−γ to the underlying CP violation.

Physical Review D, 2014

The latest data from the ACME Collaboration have put a stringent constraint on the electric dipol... more The latest data from the ACME Collaboration have put a stringent constraint on the electric dipole moment d e of the electron. Nevertheless, the standard model (SM) prediction for d e is many orders of magnitude below the new result, making this observable a powerful probe for physics beyond the SM. We carry out a model-independent study of d e in the SM with right handed neutrinos and its extension with the neutrino seesaw mechanism under the framework of minimal flavor violation. We find that d e crucially depends on whether neutrinos are Dirac or Majorana particles. In the Majorana case, d e can reach its experimental bound, and it constrains the scale of minimal flavor violation to be above a few hundred GeV or more. We also explore extra CP-violating sources in the Yukawa couplings of the right-handed neutrinos. Such new sources can have important effects on d e .

Physical Review Letters, 2006

The HyperCP collaboration has observed three events for the decay Sigma^+ -> p mu^+ mu^- which... more The HyperCP collaboration has observed three events for the decay Sigma^+ -> p mu^+ mu^- which may be interpreted as a new particle of mass 214.3 MeV. However, existing data from kaon and B-meson decays severely constrain this interpretation, and it is nontrivial to construct a model consistent with all the data. In this letter we show that the ``HyperCP

Journal of High Energy Physics, 2013

Physics Letters B, 2011

The recent measurement of the like-sign dimuon charge asymmetry in semileptonic b-hadron decays b... more The recent measurement of the like-sign dimuon charge asymmetry in semileptonic b-hadron decays by the D0 Collaboration is about three sigmas away from the standard-model prediction, hinting at the presence of CP-violating new physics in the mixing of B s mesons. We consider the possibility that this anomalous result arises from the contribution of a light spin-1 particle. Taking into account various experimental constraints, we find that the effect of such a particle with mass below the b-quark mass can yield a prediction consistent with the anomalous D0 measurement within its one-sigma range.

Physics Letters B, 2005

The HyperCP Collaboration has recently reported the observation of three events for the decay Σ +... more The HyperCP Collaboration has recently reported the observation of three events for the decay Σ + → pµ + µ − with an invariant mass m µ + µ − for the muon-antimuon pair of ∼214 MeV. They suggest that a new particle state X may be needed to explain the observed m µ + µ − distribution. Motivated by this result, we study the properties of such a hypothetical particle. We first use K + → π + µ + µ − data to conclude that X cannot be a scalar or vector particle. We then collect existing constraints on a pseudoscalar or axial-vector X and find that these possibilities are still allowed as explanations for the HyperCP data. Finally we assume that the HyperCP data is indeed explained by a new pseudoscalar or axial-vector particle and use this to predict enhanced rates for K

Physics Letters B, 1993

In the presence of some forms of global anomalies, the equivalence theorem, which relates the int... more In the presence of some forms of global anomalies, the equivalence theorem, which relates the interactions of longitudinal gauge bosons to those of the Goldstone bosons, is not always valid. This can occur when the Goldstone sector contains an anomaly which is canceled in the gauge currents by the effects of a different sector of the theory. The example of the Standard Model without Higgs particles is used to illustrate this phenomena.

Physical Review D, 2013

The CDMS II experiment has observed three events which may have arisen from weakly interacting ma... more The CDMS II experiment has observed three events which may have arisen from weakly interacting massive particle (WIMP) dark matter (DM) with mass of order 9 GeV colliding with nuclei. Although the implied WIMP parameter region seems to be excluded by limits from the XENON experiments, it is interesting that most of this tension can go away if the WIMP-nucleon interaction violates isospin. This motivates us to explore some of the implications for models in which a real gauge-singlet scalar particle, the darkon, serves as the WIMP, taking into account the recent discovery of a Higgs boson at the LHC and Planck determination of the DM relic density. In the simplest scenario, involving only the standard model plus a darkon, the Higgs boson is largely invisible due to its decay into a pair of darkons having the WIMP mass suggested by CDMS II and hence cannot be identified with the one found at the LHC. We find, on the other hand, that a two-Higgs-doublet model supplemented with a darkon has ample parameter space to accommodate well both the new potential DM hint from CDMS II and the Higgs data from the LHC, whether or not the darkon-nucleon interaction conserves isospin.

Physical Review D, 2010

We consider a scalar dark matter model, the SM4+D, consisting of the standard model with four gen... more We consider a scalar dark matter model, the SM4+D, consisting of the standard model with four generations (SM4) and a real gauge-singlet scalar called darkon, D, as the weakly interacting massive particle (WIMP) dark-matter (DM) candidate. We explore constraints on the darkon sector of the SM4+D from WIMP DM direct-search experiments, including CDMS II and CoGeNT, and from the decay of a B meson into a kaon plus missing energy. We find that a sizable portion of the darkon parameter space is still compatible with the experimental data. Since the darkon-Higgs interaction may give rise to considerable enhancement of the Higgs invisible decay mode, the existence of the darkon could lead to the weakening or evasion of some of the restrictions on the Higgs mass in the presence of fourth-generation quarks. In addition, it can affect the flavor-changing decays of these new heavy quarks into a lighter quark and the Higgs boson, as the Higgs may subsequently decay invisibly. Therefore we also study these flavor-changing neutral transitions involving the darkon, as well as the corresponding top-quark decay t → cDD, some of which may be observable at the Tevatron or LHC and thus provide additional tests for the SM4+D.

Physical Review D, 2010

We evaluate the rare radiative kaon decays K L,S → 3γ. Applying the requirements of gauge invaria... more We evaluate the rare radiative kaon decays K L,S → 3γ. Applying the requirements of gauge invariance and Bose symmetry, we derive a general form of the decay amplitude, including both parity-conserving and parity-violating contributions. We employ a chiral-Lagrangian approach combined with dimensional analysis arguments to estimate the branching ratios of these decays in the standard model, obtaining values as large as B K L → 3γ ∼ 1 × 10 −14 and B K S → 3γ ∼ 2 × 10 −17 , which exceed those found previously by a few orders of magnitude. Measurements on the branching ratios which are significantly larger than these numbers would likely hint at the presence of new physics beyond the standard model.

Physical Review D, 2011

Recent LHC searches have not found a clear signal of the Higgs boson h of the standard model (SM)... more Recent LHC searches have not found a clear signal of the Higgs boson h of the standard model (SM) with three or four families in the mass range m h = 120-600 GeV. If the Higgs had an unexpectedly large invisible branching ratio, the excluded m h regions would shrink. This can be realized in the simplest weakly interacting massive particle dark matter (DM) model, which is the SM plus a real gauge-singlet scalar field D as the DM, via the invisible mode h → DD. Current data allow this decay to occur for D-mass values near, but below, m h /2 and those compatible with the light DM hypothesis. For such D masses, h → DD can dominate the Higgs width depending on m h , and thus sizable portions of the m h exclusion zones in the SM with three or four families may be recovered. Increased luminosity at the LHC may even reveal a Higgs having SM-like visible decays still hiding in the presently disallowed regions. The model also accommodates well the new possible DM hints from CRESST-II and will be further tested by improved data from future DM direct searches.

Springer Proceedings in Physics, 2019

As direct searches for new physics (NP) beyond the standard model (SM) at the LHC and other colli... more As direct searches for new physics (NP) beyond the standard model (SM) at the LHC and other colliders have so far come up empty, increasingly attention has been paid to the intensity frontier, where high-precision measurements can probe rare processes which are possibly sensitive to indirect signals of NP. An important tool in the quest for NP effects on rare processes is the null tests of the SM. These are observables which vanish or are very small in the SM and which therefore could receive potentially large contributions from NP. Among such tests are, to be discussed here, asymmetries in the charm-meson decays D → P 1 P 2 + − (P = π, K and = e, μ), direct C P asymmetries in Cabibbo-favored nonleptonic charm-meson decays, muon asymmetries in the hyperon decay Σ + → pμ + μ − , and searches for the rare transitions b → ssd, dds and for nonleptonic B s-meson decays which fully break isospin symmetry. These are under ongoing or near-future investigations in the LHCb and other experiments. Observables which are predicted to be very small or vanish within the standard model (SM) serve, at least in principle, as its (approximate or exact) null tests [1]. They are potentially good places to look for indications of new physics (NP) beyond the SM (BSM), as any unambiguous nonnegligible result of the measurement of such an observable would be a compelling hint of NP BSM. If an experiment performing this kind of test produces no positive result, the null outcome will, on the flip side, translate into a constraint on the NP scenario under consideration. Over the years there have already been numerous efforts to conduct SM null tests with various flavor-changing neutral current (FCNC) transitions. Examples are the searches for processes manifesting charged-lepton-flavor violation, such as μ → eγ and B 0 → μ ± τ ∓ , which have so far come up negative [2]. Since they are forbidden in the SM, their observation would be clear evidence for NP. Another instance of SM null test is the deviation of the ratio R K (*) of the branching fractions of the decays

Journal of High Energy Physics, 2020

The ratio B(K L → π 0 νν)/B(K + → π + νν) of the branching fractions of kaon decays K L → π 0 νν ... more The ratio B(K L → π 0 νν)/B(K + → π + νν) of the branching fractions of kaon decays K L → π 0 νν and K + → π + νν has a maximum of about 4.3 under the assumption that the underlying interactions change isospin by ∆I = 1/2. This is referred to as the Grossman-Nir (GN) bound, which is respected by the standard model (SM) and by many scenarios beyond it. Recent preliminary results of the KOTO and NA62 Collaborations searching for these kaon modes seem to imply a violation of this bound. The KOTO findings also suggest that B(K L → π 0 νν) could be much larger, by nearly two orders of magnitude, than that predicted in the SM. In this work we study the possibility of violating the GN bound in an effective field theory approach with only SM fields. We show that the bound holds, in addition to the original GN scenarios, whether or not the kaon decays conserve lepton number. We demonstrate that the inclusion of ∆I = 3/2 operators can lead to a violation of the GN bound and illustrate with an example of how the KOTO numbers may be reached with a new physics scale of order tens of GeV.

The European Physical Journal C, 2016

We consider the flavor-changing decays of the Higgs boson in a grand unified theory framework whi... more We consider the flavor-changing decays of the Higgs boson in a grand unified theory framework which is based on the SU(5) gauge group and implements the principle of minimal flavor violation. This allows us to explore the possibility of connecting the tentative hint of the Higgs decay h → μτ recently reported in the CMS experiment to potential new physics in the quark sector. We look at different simple scenarios with minimal flavor violation in this context and how they are subject to various empirical restrictions. In one specific case, the relative strengths of the flavorchanging leptonic Higgs couplings are determined mainly by the known quark mixing parameters and masses, and a branching fraction B(h → μτ) ∼ 1% is achievable without the couplings being incompatible with the relevant constraints. Upcoming data on the Higgs leptonic decays and searches for the μ → eγ decay with improved precision can offer further tests on this scenario.

Journal of High Energy Physics, 2016

Direct searches for dark matter (DM) by the LUX and PandaX-II Collaborations employing xenon-base... more Direct searches for dark matter (DM) by the LUX and PandaX-II Collaborations employing xenon-based detectors have recently come up with the most stringent limits to date on the spin-independent elastic scattering of DM off nucleons. For Higgsportal scalar DM models, the new results have precluded any possibility of accommodating low-mass DM as suggested by the DAMA and CDMS II Si experiments utilizing other target materials, even after invoking isospin-violating DM interactions with nucleons. In the simplest model, SM+D, which is the standard model plus a real singlet scalar named darkon acting as the DM candidate, the LUX and PandaX-II limits rule out DM masses roughly from 4 to 450 GeV, except a small range around the resonance point at half of the Higgs mass where the interaction cross-section is near the neutrino-background floor. In the THDM II+D, which is the type-II two-Higgs-doublet model combined with a darkon, the region excluded in the SM+D by the direct searches can be recovered due to suppression of the DM effective interactions with nucleons at some values of the ratios of Higgs couplings to the up and down quarks, making the interactions significantly isospin-violating. However, in either model, if the 125-GeV Higgs boson is the portal between the dark and SM sectors, DM masses less than 50 GeV or so are already ruled out by the LHC constraint on the Higgs invisible decay. In the THDM II+D, if the heavier CP-even Higgs boson is the portal, theoretical restrictions from perturbativity, vacuum stability, and unitarity requirements turn out to be important instead and exclude much of the region below 100 GeV. For larger DM masses, the THDM II+D has plentiful parameter space that corresponds to interaction cross-sections under the neutrino-background floor and therefore is likely to be beyond the reach of future direct searches without directional sensitivity.

We study the |∆I| = 3/2 amplitudes of the octet-hyperon decays B → B ′ π and of the decays Ω − → ... more We study the |∆I| = 3/2 amplitudes of the octet-hyperon decays B → B ′ π and of the decays Ω − → Ξπ in the context of heavy-baryon chiral perturbation theory. For the octet-hyperon decays, we investigate the theoretical uncertainty of the lowest-order predictions by calculating the leading nonanalytic corrections. We find that these corrections are within the expectations of naive power counting and, therefore, that this picture can be tested more accurately with improved measurements. For the Ω − decays, we obtain at leading order two operators responsible for the decays which also contribute at one loop to the octet-hyperon decays. These one-loop contributions are sufficiently large to suggest that the measured ratio Γ(Ω − → Ξ 0 π −)/Γ(Ω − → Ξ − π 0) ≈ 2.7 may be too large.

Phys Rev D, 2000

We present a reanalysis of direct CP violation in the decay K L → π + π − γ. We point out an exis... more We present a reanalysis of direct CP violation in the decay K L → π + π − γ. We point out an existing discrepancy between the theoretical and experimental definitions of ǫ ′ +−γ. Adopting the experimental definition of ǫ ′ +−γ , we estimate that ǫ ′ +−γ /ǫ could be as large as a few times 10 −4 both within the standard model and beyond. We discuss these estimates in detail and we also show how a judicious choice of E * γ cuts can increase the sensitivity of the observable ǫ ′ +−γ to the underlying CP violation.

Physical Review D, 2014

The latest data from the ACME Collaboration have put a stringent constraint on the electric dipol... more The latest data from the ACME Collaboration have put a stringent constraint on the electric dipole moment d e of the electron. Nevertheless, the standard model (SM) prediction for d e is many orders of magnitude below the new result, making this observable a powerful probe for physics beyond the SM. We carry out a model-independent study of d e in the SM with right handed neutrinos and its extension with the neutrino seesaw mechanism under the framework of minimal flavor violation. We find that d e crucially depends on whether neutrinos are Dirac or Majorana particles. In the Majorana case, d e can reach its experimental bound, and it constrains the scale of minimal flavor violation to be above a few hundred GeV or more. We also explore extra CP-violating sources in the Yukawa couplings of the right-handed neutrinos. Such new sources can have important effects on d e .

Physical Review Letters, 2006

The HyperCP collaboration has observed three events for the decay Sigma^+ -> p mu^+ mu^- which... more The HyperCP collaboration has observed three events for the decay Sigma^+ -> p mu^+ mu^- which may be interpreted as a new particle of mass 214.3 MeV. However, existing data from kaon and B-meson decays severely constrain this interpretation, and it is nontrivial to construct a model consistent with all the data. In this letter we show that the ``HyperCP

Journal of High Energy Physics, 2013

Physics Letters B, 2011

The recent measurement of the like-sign dimuon charge asymmetry in semileptonic b-hadron decays b... more The recent measurement of the like-sign dimuon charge asymmetry in semileptonic b-hadron decays by the D0 Collaboration is about three sigmas away from the standard-model prediction, hinting at the presence of CP-violating new physics in the mixing of B s mesons. We consider the possibility that this anomalous result arises from the contribution of a light spin-1 particle. Taking into account various experimental constraints, we find that the effect of such a particle with mass below the b-quark mass can yield a prediction consistent with the anomalous D0 measurement within its one-sigma range.

Physics Letters B, 2005

The HyperCP Collaboration has recently reported the observation of three events for the decay Σ +... more The HyperCP Collaboration has recently reported the observation of three events for the decay Σ + → pµ + µ − with an invariant mass m µ + µ − for the muon-antimuon pair of ∼214 MeV. They suggest that a new particle state X may be needed to explain the observed m µ + µ − distribution. Motivated by this result, we study the properties of such a hypothetical particle. We first use K + → π + µ + µ − data to conclude that X cannot be a scalar or vector particle. We then collect existing constraints on a pseudoscalar or axial-vector X and find that these possibilities are still allowed as explanations for the HyperCP data. Finally we assume that the HyperCP data is indeed explained by a new pseudoscalar or axial-vector particle and use this to predict enhanced rates for K

Physics Letters B, 1993

In the presence of some forms of global anomalies, the equivalence theorem, which relates the int... more In the presence of some forms of global anomalies, the equivalence theorem, which relates the interactions of longitudinal gauge bosons to those of the Goldstone bosons, is not always valid. This can occur when the Goldstone sector contains an anomaly which is canceled in the gauge currents by the effects of a different sector of the theory. The example of the Standard Model without Higgs particles is used to illustrate this phenomena.

Physical Review D, 2013

The CDMS II experiment has observed three events which may have arisen from weakly interacting ma... more The CDMS II experiment has observed three events which may have arisen from weakly interacting massive particle (WIMP) dark matter (DM) with mass of order 9 GeV colliding with nuclei. Although the implied WIMP parameter region seems to be excluded by limits from the XENON experiments, it is interesting that most of this tension can go away if the WIMP-nucleon interaction violates isospin. This motivates us to explore some of the implications for models in which a real gauge-singlet scalar particle, the darkon, serves as the WIMP, taking into account the recent discovery of a Higgs boson at the LHC and Planck determination of the DM relic density. In the simplest scenario, involving only the standard model plus a darkon, the Higgs boson is largely invisible due to its decay into a pair of darkons having the WIMP mass suggested by CDMS II and hence cannot be identified with the one found at the LHC. We find, on the other hand, that a two-Higgs-doublet model supplemented with a darkon has ample parameter space to accommodate well both the new potential DM hint from CDMS II and the Higgs data from the LHC, whether or not the darkon-nucleon interaction conserves isospin.

Physical Review D, 2010

We consider a scalar dark matter model, the SM4+D, consisting of the standard model with four gen... more We consider a scalar dark matter model, the SM4+D, consisting of the standard model with four generations (SM4) and a real gauge-singlet scalar called darkon, D, as the weakly interacting massive particle (WIMP) dark-matter (DM) candidate. We explore constraints on the darkon sector of the SM4+D from WIMP DM direct-search experiments, including CDMS II and CoGeNT, and from the decay of a B meson into a kaon plus missing energy. We find that a sizable portion of the darkon parameter space is still compatible with the experimental data. Since the darkon-Higgs interaction may give rise to considerable enhancement of the Higgs invisible decay mode, the existence of the darkon could lead to the weakening or evasion of some of the restrictions on the Higgs mass in the presence of fourth-generation quarks. In addition, it can affect the flavor-changing decays of these new heavy quarks into a lighter quark and the Higgs boson, as the Higgs may subsequently decay invisibly. Therefore we also study these flavor-changing neutral transitions involving the darkon, as well as the corresponding top-quark decay t → cDD, some of which may be observable at the Tevatron or LHC and thus provide additional tests for the SM4+D.

Physical Review D, 2010

We evaluate the rare radiative kaon decays K L,S → 3γ. Applying the requirements of gauge invaria... more We evaluate the rare radiative kaon decays K L,S → 3γ. Applying the requirements of gauge invariance and Bose symmetry, we derive a general form of the decay amplitude, including both parity-conserving and parity-violating contributions. We employ a chiral-Lagrangian approach combined with dimensional analysis arguments to estimate the branching ratios of these decays in the standard model, obtaining values as large as B K L → 3γ ∼ 1 × 10 −14 and B K S → 3γ ∼ 2 × 10 −17 , which exceed those found previously by a few orders of magnitude. Measurements on the branching ratios which are significantly larger than these numbers would likely hint at the presence of new physics beyond the standard model.

Physical Review D, 2011

Recent LHC searches have not found a clear signal of the Higgs boson h of the standard model (SM)... more Recent LHC searches have not found a clear signal of the Higgs boson h of the standard model (SM) with three or four families in the mass range m h = 120-600 GeV. If the Higgs had an unexpectedly large invisible branching ratio, the excluded m h regions would shrink. This can be realized in the simplest weakly interacting massive particle dark matter (DM) model, which is the SM plus a real gauge-singlet scalar field D as the DM, via the invisible mode h → DD. Current data allow this decay to occur for D-mass values near, but below, m h /2 and those compatible with the light DM hypothesis. For such D masses, h → DD can dominate the Higgs width depending on m h , and thus sizable portions of the m h exclusion zones in the SM with three or four families may be recovered. Increased luminosity at the LHC may even reveal a Higgs having SM-like visible decays still hiding in the presently disallowed regions. The model also accommodates well the new possible DM hints from CRESST-II and will be further tested by improved data from future DM direct searches.