arXiv subjects

Youichi Yamada

Publications and source records attributed to Youichi Yamada.

At least 19 recordsLinked to original sources

arXiv2024

One-loop QCD amplitudes in the Feynman-Diagram gauge

Scattering amplitudes for the massless QCD process, $q\bar{q}\to q^\prime\bar{q}^\prime$, are calculated in the one-loop order in the Feynman-Diagram (FD) gauge, where gluons are quantized on the light cone with opposite direction of the three-momenta. We find non-decoupling of the Faddeev-Popov ghosts and non-conventional UV singularities in dimensional regularization. The known QCD amplitudes with asymptotic freedom are reproduced only after summing propagator and vertex corrections. By quantizing gluons in the Feynman gauge on the FD gauge background, we obtain the one-loop improved FD gauge amplitudes.

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arXiv2023

Long-lived Higgsino pairs decaying within the LHC detectors

Pair-produced long-lived lightest Higgsinos decaying within a few meters in the LHC detectors are considered. The relatively light nearly-pure Higgsino states object of this study are those of a supersymmetric minimal model with gauge-mediated supersymmetry breaking and heavy sfermions and gauginos. The heavier Higgsinos decay instantly into the lightest one, with very little activities. Hence, all pair-produced Higgsinos end up as a pair of the lightest neutral Higgsinos, possibly long lived and decaying into the gravitino and either the Higgs or the Z boson. If they decay inside the detectors, these bosons deposit huge energies in the calorimeters. The cross section $σ_2$ ($σ_1$) for the events with two (at least one) decaying-in-flight lightest Higgsinos is examined. The combination $(σ_1)^2/σ_2$ is found to be almost insensitive to the lightest Higgsino lifetime, thereby providing a good variable to measure its mass. The ratio $σ_1/σ_2$, instead, mildly dependent on its mass, can give good information on its lifetime. It is argued that the lightest Higgsinos in the events are rather slow, as the very relativistic ones escape away. Thus, the initial state radiations tend to be soft and a trigger system appropriate for such events is necessary to detect them.

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arXiv2017

The MSSM without Gluinos; an Effective Field Theory for the Stop Sector

In this article we study the MSSM with stops and Higgs scalars much lighter than gluinos and squarks of the first two generations. In this setup, one should use an effective field theory with partial supersymmetry in which the gluino and heavy squarks are integrated out in order to connect SUSY parameters (given at a high scale) to observables in the stop sector. In the construction of this effective theory, valid below the gluino mass scale, we take into account $O(α_3)$ and $O(Y_{t,b}^2)$ effects and calculate the matching as well as the renormalization group evolution. As a result, the running of the parameters for the stop sector is modified with respect to the full MSSM and SUSY relations between parameters are broken. We show that for some couplings sizable numerical differences exist between the effective field theory approach and the naive calculation based on the MSSM running.

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arXiv2015

Higgs-bosons couplings to quarks and leptons in the supersymmetric Standard Model with a gauge singlet

The loop corrections to the couplings of Higgs bosons to quarks and charged leptons are calculated within supersymmetric versions of the Standard Model, extended by a gauge singlet. The effective couplings of the $SU(2)_L$ doublet and singlet Higgs bosons to quarks and leptons, induced by sfermion loops, are derived. Analytic expressions for the case of generic sfermion flavour mixing, including the complete resummation of all chirally-enhanced contributions are presented. These results are important in scenarios in which the mixing between singlet and doublet components of Higgs bosons is small, and the (pseudo) scalar component of the doublet is light. The calculated loop effects can have important consequences in flavour physics, especially for $ΔF=2$ processes.

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arXiv2009

Electroweak two-loop contribution to the mass splitting within a new heavy SU(2)$_L$ fermion multiplet

New heavy particles in an SU(2)_L multiplet, sometimes introduced in extensions of the standard model, have highly degenerate tree-level mass M if their couplings to the Higgs bosons are very small or forbidden. However, loop corrections may generate the gauge-symmetry-breaking mass splitting within the multiplet, which does not vanish in the large M limit due to the threshold singularity. We calculate the electroweak contribution to the mass splitting for a heavy fermion multiplet, to the two-loop order. Numerically, two-loop electroweak contributions are typically O(MeV).

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arXiv2008

b\to sν\barν decay in the MSSM: Implication of b\to sγat large tan beta

The decay $b\to sν\barν$ is discussed in the minimal supersymmetric standard model with general flavor mixing for squarks, at large $\tanβ$. In this case, in addition to the chargino loop contributions which were analyzed in previously studies, $\tanβ$-enhanced contributions from the gluino and charged Higgs boson loops might become sizable compared with the standard model contribution, at least in principle. However, it is demonstrated that the experimental bounds on the new physics contributions to the radiative decay $b\to sγ$ should strongly constrain these contributions to $b\to sν\barν$, especially on the gluino contribution. We also briefly comment on a possible constraint from the $B_s\toμ^+μ^-$ decay.

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arXiv2006

Two-loop SUSY QCD correction to the gluino pole mass

We calculate the pole mass of the gluino as a function of the running parameters in the lagrangian, to O(alpha_s^2) in SUSY QCD. The correction shifts the pole mass from the running mass by typically 1-2 %. This shift can be larger than the expected accuracy of the mass determination at future colliders, and should be taken into account for precision studies of the SUSY breaking parameters. The effects of other corrections are breifly commented.

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arXiv2005

Two-loop SUSY QCD correction to the gluino pole mass

The pole mass of the gluino is diagrammatically calculated to the two-loop order in SUSY QCD as a function of the running parameters in the lagrangian, for the gluino and squarks sufficiently heavier than the quarks. The O(alpha_s^2) correction shifts the gluino mass by typically 1-2 %, which may be larger than the expected accuracy of the mass determination at future colliders.

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arXiv2004

Radiative corrections in SUSY phenomenology

We discuss some aspects of the radiative corrections in the phenomenology of the minimal SUSY standard model, by reviewing two recent studies. (1) The full one-loop corrections to the Higgs boson decays into charginos are presented, with emphasis on the renormalization of the chargino sector, including of their mixing matrices. (2) The two-loop O(alpha_s tan beta) corrections to the b -> s gamma decay in models with large tan beta, mainly those to the charged Higgs boson contributions, are discussed. Exact two-loop result is compared to an approximation used in previous studies.

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arXiv2004

Beyond leading-order corrections to bar{B} -> X_s gamma at large tan beta: the charged-Higgs-boson contribution

Among the O(alpha_s tan beta) contributions to the Wilson coefficients C_7 and C_8, relevant for the decay bar{B} -> X_s gamma, those induced by two-loop diagrams with charged-Higgs-boson exchange and squark-gluino corrections are calculated in supersymmetric models at large tan beta. The calculation of the corresponding Feynman integrals is exact, unlike in previous studies that are valid when the typical supersymmetric scale M_SUSY is sufficiently larger than the electroweak scale m_weak (\sim m_W, m_t) and the mass of the charged Higgs boson m_{H^\pm}. Therefore, the results presented here can be used for any value of the various supersymmetric masses. These results are compared with those of an approximate calculation, already existing in the literature, that is at the zeroth order in the expansion parameter (m_weak^2,m_H^2)/M_SUSY^2, and with the results of two new approximate calculations in which the first and second order in the same expansion parameter are retained, respectively. This comparison allows us to assess whether the results of these three approximate calculations can be extended beyond the range of validity for which they were derived, i.e., whether they can be used for m_H^2 \gtap M_SUSY^2 and/or M_SUSY^2 \sim m_weak^2. It is found that the zeroth-order approximation works well even for m_H \gtap M_SUSY, provided M_SUSY^2 >> m_weak^2. The inclusion of the higher-order terms improves the zeroth-order approximation for m_H^2 << M_SUSY^2, but it worsens it for m_H \gtap M_SUSY$.

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arXiv2003

Large tan beta SUSY QCD corrections to B -> X_s gamma

The charged-Higgs boson contributions to the Wilson coefficients C_7 and C_8, relevant for the decay B -> X_s gamma, are discussed in supersymmetric models at large tan beta. These contributions receive two-loop O(alpha_s tan beta) corrections by squark-gluino subloops, which are possibly large and nondecoupling in the limit of heavy superpartners. In previous studies, the relevant two-loop Feynman integrals were approximated by using an effective two-Higgs-doublet lagrangian. However, this approximation is theoretically justified only when the typical supersymmetric scale M_SUSY is sufficiently larger than the electroweak scale m_weak \sim (m_W,m_t) and the mass of the charged-Higgs boson m_{H^\pm}. Here we evaluate these two-loop integrals exactly and compare the results with the existing, approximated ones. We then examine the validity of this approximation beyond the region where it has been derived, i.e. for m_H \gtap M_SUSY and/or M_SUSY \sim m_weak.

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arXiv2003

Towards an exact evaluation of the supersymmetric O(alpha_s tan beta) corrections to \bar{B} -> X_s gamma

The charged-Higgs contributions to the decay \bar{B} -> X_s gamma are discussed in the minimal supersymmetric standard model at large tan beta. These contributions receive two-loop O(alpha_s tan beta) corrections by squark-gluino subloops, which are nondecoupling in the limit of heavy superpartners and possibly large. Their leading parts are already known and were evaluated by using an effective two-Higgs-doublet Lagrangian. Subleading corrections coming from higher-dimension operators in the effective Lagrangian were ignored, although this is not, a priori, justified when m_{H^\pm} is not much smaller than the typical supersymmetric mass M_{SUSY}. Here, we calculate all subleading terms of the O(alpha_s tan beta) corrections up to O((m_t^2,m_{H^\pm}^2/M_{SUSY}^2)^2), as well as all the exact two-loop diagrams with squark-gluino subloops, beyond the effective-Lagrangian approximation. Comments are made on the size of these corrections.

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arXiv2002

Two-loop renormalization of tan(beta) and its gauge dependence

Renormalization of two-loop divergent corrections to the vacuum expectation values (v_1, v_2) of the two Higgs doublets in the minimal supersymmetric standard model, and their ratio $\tanβ=v_2/v_1$, is discussed for general R_ξ gauge fixings. When the renormalized (v_1, v_2) are defined to give the minimum of the loop-corrected effective potential, it is shown that, beyond the one-loop level, the dimensionful parameters in the R_ξ gauge fixing term generate gauge dependence of the renormalized $\tanβ$. Additional shifts of the Higgs fields are necessary to realize the gauge-independent renormalization of $\tanβ$.

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arXiv2001

Two-loop renormalization of tan beta and its gauge dependence

Renormalization of two-loop divergent corrections to the vacuum expectation values (v_1, v_2) of the two Higgs doublets in the minimal supersymmetric standard model, and their ratio tan beta=v_2/v_1, is discussed for general R_xi gauge fixings. When the renormalized (v_1, v_2) are defined to give the minimum of the loop-corrected effective potential, it is shown that, beyond the one-loop level, the dimensionful parameters in the R_xi gauge fixing term generate gauge dependence of the renormalized tan beta. Additional shifts of the Higgs fields are necessary to realize the gauge-independent renormalization of tan beta.

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arXiv2001

Gauge dependence of the on-shell renormalized mixing matrices

It was recently pointed out that the on-shell renormalization of the Cabibbo-Kobayashi-Maskawa (CKM) matrix in the method by Denner and Sack causes a gauge parameter dependence of the amplitudes. We analyze the gauge dependence of the on-shell renormalization of the mixing matrices both for fermions and scalars in general cases, at the one-loop level. We then show that this gauge dependence can be avoided by fixing the counterterms for the mixing matrices in terms of the off-diagonal wave function corrections for fermions and scalars after a rearrangement, in a similar manner to the pinch technique for gauge bosons. We finally present explicit calculation of the gauge dependence for two cases: CKM matrix in the Standard Model, and left-right mixing of scalar quarks in the minimal supersymmetric standard model.

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arXiv2000

Low-Scale Anomalous U(1) and Decoupling Solution to Supersymmetric Flavor Problem

Supersymmetric standard models where the ultraviolet cut-off scale is only a few orders of magnitude higher than the electroweak scale are considered. Phenomenological consequences of this class of models are expected to be very different from, for example, the conventional supergravity scenario. We apply this idea to a model with an anomalous U(1) gauge group and construct a viable model in which some difficulties of the decoupling solution to the supersymmetric flavor problem are ameliorated.

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arXiv1999

Probing the Nature of Compactification with Kaluza-Klein Excitations at the Large Hadron Collider

It is shown that the nature of compactification of extra dimensions in theories of large radius compactification can be explored in several processes at the Large Hadron Collider (LHC). Specifically it is shown that the characteristics of the Kaluza-Klein (KK) excitations encode information on the nature of compactification, i.e., on the number of compactified dimensions as well as on the type of compactification, e.g., of the specific orbifold compactification. The most dramatic signals arise from the interference pattern involving the exchange of the Standard Model spin 1 bosons ($γ$ and Z) and their Kaluza-Klein modes in the dilepton final state $pp\to l^+l^-X$. It is shown that LHC with 100$fb^{-1}$ of luminosity can discover Kaluza-Klein modes up to compactification scales of $\approx 6$ TeV as well as identify the nature of compactification. Effects of the Kaluza-Klein excitations of the W boson and of the gluon are also studied. Exhibition of these phenomena is given for the case of one extra dimension and for the case of two extra dimensions with $Z_2\times Z_2, Z_3$, and $Z_6$ orbifold compactifications.

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arXiv1999

Neutralino Decays at the CERN LHC

We study the distribution of lepton pairs from the second lightest neutralino decay \tchi^0_2\to\tchi^0_1 l^+l^-. This decay mode is important to measure the mass difference between \tchi^0_2 and the lightest neutralino \tchi^0_1, which helps to determine the parameters of the minimal supersymmetric standard model at the CERN LHC. We found that the decay distribution strongly depends on the values of underlying MSSM parameters. For some extreme cases, the amplitude near the end point of the lepton invariant mass distribution can be suppressed so strongly that one needs the information of the whole m_{ll} distribution to extract m_{\tchi^0_2}-m_{\tchi^0_1}. On the other hand, if systematic errors on the acceptance can be controlled, this distribution can be used to constrain slepton masses and the Z\tchi^0_2\tchi^0_1 coupling. Measurements of the velocity distribution of \tchi^0_2 from samples near the end point of the m_{ll} distribution, and of the asymmetry of the p_T of leptons, would be useful to reduce the systematic errors.

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