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S. Pakvasa

Publications and source records attributed to S. Pakvasa.

At least 37 records · Page 2Linked to original sources
arXiv2003

Neutrino Properties Before and After KamLAND

We review neutrino oscillation physics, including the determination of mass splittings and mixings from current solar, atmospheric, reactor and accelerator neutrino data. A brief discussion is given of cosmological and astrophysical implications. Non-oscillation phenomena such as neutrinoless double beta decay would, if discovered, probe the absolute scale of neutrino mass and also reveal their Majorana nature. Non-oscillation descriptions in terms of spin-flavor precession (SFP) and non-standard neutrino interactions (NSI) currently provide an excellent fit of the solar data. However they are at odds with the first results from the KamLAND experiment which imply that, despite their theoretical interest, non-standard mechanisms can only play a sub-leading role in the solar neutrino anomaly. Accepting the LMA-MSW solution, one can use the current solar neutrino data to place important restrictions on non-standard neutrino properties, such as neutrino magnetic moments. Both solar and atmospheric neutrino data can also be used to place constraints on neutrino instability as well as the more exotic possibility of $CPT$ and Lorentz Violation. Weillustrate the potential of future data from experiments such as KamLAND, Borexino and the upcoming neutrino factories in constraining non-standard neutrino properties.

hep-ph↗
arXiv2002

Yet Another Extension of the Standard Model: Oases in the Desert?

We have searched for conceptually simple extensions of the standard model, and describe here a candidate model which we find attractive. Our starting point is the assumption that off-diagonal CKM mixing matrix elements are directly related by lowest order perturbation theory to the quark mass matrices. This appears to be most easily and naturally implemented by assuming that all off-diagonal elements reside in the down-quark mass matrix. This assumption is in turn naturally realized by introducing three generations of heavy, electroweak-singlet down quarks which couple to the Higgs sector diagonally in flavor, while mass-mixing off-diagonally with the light down-quarks. Anomaly cancellation then naturally leads to inclusion of electroweak vector-doublet leptons. It is then only a short step to completing the extension to three generations of fundamental representations of E(6). Assuming only that the third generation B couples to the Higgs sector at least as strongly as does the top quark, the mass of the B is roughly estimated to lie between 1.7 TeV and 10 TeV, with lower-generation quarks no heavier. The corresponding guess for the new leptons is a factor two lower. Within the validity of the model, flavor and CP violation are ``infrared'' in nature, induced by semi-soft mass mixing terms, not Yukawa couplings. If the Higgs couplings of the new quarks are flavor symmetric, then there necessarily must be at least one ``oasis'' in the desert, induced by new radiative corrections to the top quark and Higgs coupling constants, and roughly at 1000 TeV.

hep-ph↗
arXiv2002

$\bar B^0 \to π^+ X$ in the Standard Model

In this paper we investigate the possibility of studying $B\to π$ form factor using the semi-inclusive decays $\bar B^0 \to π^+ + X_q$. In general $B\to PX$ semi-inclusive decays involve several hadronic parameters. But for $\bar B^0 \to π^+ X_q$ decays we find that in the factorization approximation, the only unknown hadronic parameters are the form factors $F^{B\to π}_{0,1}$. Therefore these form factors can be studied in $\bar B^0 \to π^+ X_q$ decays. Using theoretical model calculations for the form factors the branching ratios for $\bar B^0 \to π^+ X_d(ΔS = 0) $ and $\bar B^0 \to π^+ X_s (ΔS = -1)$, with the cut $E_π > 2.1$ GeV, are estimated to be in the ranges of $(3.1\sim 4.9) \times 10^{-5}(F^{B\to π}_1(0)/0.33)^2$ and $(2.5\sim 4.2)\times 10^{-5}(F_1^{B\to π}(0)/0.33)^2$, respectively, depending on the value of $γ$. The combined branching ratio for $\bar B^0 \to π^+ (X_d+ X_s)$ is about $7.4\times 10^{-5} (F^{B\to π}_1(0)/0.33)^2$ and is insensitive to $γ$. We also discuss CP asymmetries in these decay modes.

hep-ph↗
arXiv2000

CPT-odd Resonances in Neutrino Oscillations

We consider the consequences for future neutrino factory experiments of small CPT-odd interactions in neutrino oscillations. The ν_μ\toν_μand \barν_μ\to\barν_μsurvival probabilities at a baseline L=732 km can test for CPT-odd contributions at orders of magnitude better sensitivity than present limits. Interference between the CPT-violating interaction and CPT-even mass terms in the Lagrangian can lead to a resonant enhancement of the oscillation amplitude. For oscillations in matter, a simultaneous enhancement of both neutrino and antineutrino oscillation amplitudes is possible.

hep-ph↗
arXiv2000

Do Neutrinos Decay?

If neutrinos have masses and mixings, in general some will decay. In models with new particles and new couplings, some decay modes can be fast enough to be of phenomenological interest.

hep-ph↗
arXiv1999

Fermion Dipole Moments in Supersymmetric Models with Explicitly Broken R-parity

We present a simple analysis that allows us to extract the leading mass dependence of the dipole moment of matter fermions that might be induced by new physics. We present explicit results for the supersymmetric model with broken R-parity as an illustration. We show that the extra contributions to the electric dipole moment (edm) of fermions from R-parity violating interactions can occur only at two loop level, contrary to claims in the literature. We further find that unlike the generic lepto-quark models, the extra contributions to the dipole moments of the leptons can only be enhanced by m_b/m_l and not by m_t/m_l relative to the expectations in the Standard Model. An interesting feature about this enhancement of these dipole moments is that it does not involve unknown mixing angles. We then use experimental constraints on the electric dipole moments of electron and neutron to obtain bounds on (the imaginary part of) products of R-parity violating couplings, and show that bounds claimed in the literature are too stringent by many orders of magnitude.

hep-ph↗
arXiv1999

Neutrino Anomalies without Oscillations

I review explanations for the three neutrino anomalies (solar, atmospheric and LSND) which go beyond the ``conventional'' neutrino oscillations induced by mass-mixing. Several of these require non-zero neutrino masses as well.

hep-ph↗
arXiv1999

Measuring $β$ in $B \to D^{*+}D^{*-}K_s$ Decays

We consider the possibility of measuring both $\sin (2 β)$ and $\cos (2 β)$ in the KM unitarity triangle using the process $B^0 \to D^{*+}D^{*-}K_s$. This decay mode has a higher branching fraction (O(1%)) than the mode $B^0 \to D^{*+}D^{*-}$. We use the factorization assumption and heavy hadron chiral perturbation theory to estimate the branching fraction and polarization. The time dependent rate for $B^0(t) \to D^{*+} D^{*-} K_s$ can be used to measure $\sin (2 β)$ and $\cos(2 β)$ . Furthermore, examination of the $D^{*+} K_s$ mass spectrum may be the best way to experimentally find the broad $1^+$ p-wave $D_s$ meson.

hep-ph↗
arXiv1999

Neutrino Decay and Atmospheric Neutrinos

We reconsider neutrino decay as an explanation for atmospheric neutrino observations. We show that if the mass-difference relevant to the two mixed states ν_μand ν_τis very small (< 10^{-4} eV^2), then a very good fit to the observations can be obtained with decay of a component of ν_μto a sterile neutrino and a Majoron. We discuss how the K2K and MINOS long-baseline experiments can distinguish the decay and oscillation scenarios.

hep-ph↗
arXiv1999

Exotic Explanations for Neutrino Anomalies

I review explanations for the three neutrino anomalies (solar, atmospheric and LSND) which go beyond the ``conventional'' neutrino oscillations induced by mass-mixing. Several of these require non-zero neutrino masses as well.

hep-ph↗
arXiv1999

$Λ_s (1405)$ and Negative Parity Baryon States

After a brief historical background on the $\Ls$, we revisit the 25 year old controversy on whether $\Ls$ is a $qqq$ (L=1) three quark state or a $\K$ quasi virtual bound state. This work is stimulated by the recent suggestion of Isgur that s be treated as a heavy quark in heavy quark effective theory HQET. We re-examine the empirical evidence for minimal mixing amongst singlets and octets in negative parity baryon states, with a possible dynamical origin in the opening of inelastic threshold channels. Finally, we suggest that $\Ls$ belongs to a class of hadrons which are described simultaneously as $qqq$ or $q \bar{q}$ states and as hadronic bound states.

hep-ph↗
arXiv1998

Bi-Maximal Mixing of Three Neutrinos

We show that if the solar and atmospheric data are both described by maximal vacuum oscillations at the relevant mass scales then there exists a unique mixing matrix for three neutrino flavors. The solution necessarily conserves CP and automatically implies that there is no disappearance of atmospheric $ν_e$, consistent with indications from the Super-Kamiokande experiment. We also investigate the consequences for three-neutrino mixing if the solar and atmospheric oscillations exhibit mixing that is large but not maximal. For non-maximal mixing $ν_e\leftrightarrowν_τ$ and $ν_e\leftrightarrowν_μ$ oscillations are predicted that may be observable in future long-baseline experiments.

hep-ph↗