18486610
Top anomalous decays in R-parity violating SUSY
The flavor-changing top-quark decay t → ch, where h is the lightest CPeven Higgs boson in the minimal supersymmetric standard model, is examined in the R-parity-violating supersymmetric model. Within the existing bounds on the relevant R-parity-violating couplings, the branching ratio for t→ ch can be as large as about 10−5 in some part of the parameter space, reaching a level accessible at the LHC and at e+e− Linear Colliders with high luminosities. The study of heavy-particle decays via flavor-changing neutral-currents (FCNC) has been playing an important role in testing the standard model (SM) and probing new physics beyond the SM. As the heaviest elementary particle in the SM with a mass at the electroweak scale, the top quark is more likely to be sensitive to new physics. Kinematically it is accessible to many FCNC decay modes, such as t → cV (V = γ,Z, g) and t → ch, where h is a Higgs boson. In the SM these FCNC decay modes are highly suppressed by the GIM mechanism, with typical branching ratios of 10−14 − 10−10 [1, 2], which are too small to be detectable at collider experiments. On the other hand, observation of any of such FCNC top-quark decays would be robust evidence for new physics [3, 4, 5, 6]. Top quarks will be copiously produced at the next generation of colliders. At Tevatron II with an integrated luminosity of 10 fb−1, there will be about 8×104 top quarks produced, ∗Speaker. P r H E P h e p 2 0 0 1 International Europhysics Conference on HEP G. Eilam while there will be about 100 times more at the LHC with the same luminosity. At the LHC with a luminosity of 100 fb−1 there will be more than 108 tops per year, including both tt̄ and single top production, for mh = 120 GeV [7]. At a proposed e +e− Linear Collider with √ s = 500 GeV and luminosity of 300 fb−1, such as TESLA, one expect 1.6× 105 tops [7, 8]. With such large data samples, good sensitivities may be reached for the rare decay channels t → cV [3], and for studying other related processes at hadron colliders [4]. A more recent study showed that the channel t→ ch could also be detectable [5], reaching a sensitivity of Br(t → ch) ∼ 10−5 at the LHC and a few percent at the Tevatron. While these high detection sensitivities are still far above the SM expectation for the rare decay channels [1, 2], in many scenarios beyond the SM the branching ratios of these FCNC top-quark decays could be significantly enhanced [1, 6, 9, 10, 11, 12, 13]. In the minimal supersymmetric (SUSY) standard model (MSSM) with R-parity conservation, it was shown [10] that the possibility for observing the decay channel t → ch could be greatly enhanced (here h is the lightest CP-even Higgs boson). Kinematically this decay mode is always allowed because of the strict theoretical upper bound on the Higgs boson mass [14], and the decay receives dominant contributions from the SUSY QCD (SQCD) loops of flavor-changing interactions [10]. If the gluino and squarks involved in the contributing SQCD loops are both light of order 100 GeV, the branching ratio could be enhanced to a level of 10−5. The branching ratio falls off quickly for heavier sparticles in the loops. In this talk which is basically a short version of [15], we examine the R-parity-violating contributions to t→ ch. It is well-known that R-parity conservation in SUSY theory, which implies the separate conservation of baryon number and lepton number, is put in by hand. R-parity violation ( 6 R) can be made perfectly consistent with other fundamental principles such as gauge invariance, supersymmetry and renormalizability [16]. If 6 R is included in the MSSM, t→ ch will receive new contributions from the loops of 6 R interactions. These can be significant, since the relevant 6 R couplings inducing t→ ch involve the third-generation fermions and are subject to rather weak bounds from existing experiments. We find that the branching ratio for t→ ch from 6 R contributions can be indeed as high as about 10−5 in some part of parameter space, reaching a level accessible at future high luminosity colliders. Assuming that there are only trilinear couplings, one obtains the 6 R Lagrangian Lλ′′ = − 2 λ′′ ijk [ d̃R(ū i R) d j R + d̃ j R(d̄ k R) uR + ũ i R(d̄ j R) dR ] + h.c. (1) Constraints on the couplings have been obtained from various processes [17, 18, 19, 20, 21, 22, 23, 24] and their phenomenology at hadron and lepton colliders has been intensively investigated [24, 25]. For brevity, only the terms with Baryon-number violation ( 6B) are displayed, and the similar Lepton-number violating (6L) terms, i.e. those with λ′’s, are left out. Also, the limits on the 6 L couplings are stricter than for those with 6 B. With 6 B couplings, the decay t → ch can proceed through four simple 1-loop diagrams (see Fig. 1 in [15]). The induced effective tch vertex is given by V = ie λ′′ 2jkλ ′′ 3jk 16π2 2FPL, where PL = 1 2 (1 − γ5). For simplicity and space limitation, we have assumed that all fermion masses can
119233750
Di-photon Higgs signal at the LHC: A comparative study in different supersymmetric models
As the most important discovery channel for a light Higgs boson at the LHC the di-photon signal gg --> h --> gamma gamma is sensitive to underlying physics. In this work we investigate such a signal in a comparative way by considering three different supersymmetric models, namely the minimal supersymmetric standard model (MSSM), the next-to-minimal supersymmetric standard model (NMSSM) and the nearly minimal supersymmetric standard model (nMSSM). Under the current collider and cosmological constraints we scan over the parameter space and obtain the following observation in the allowed parameter space: (i) In the nMSSM the signal rate is always suppressed; (ii) In the MSSM the signal rate is suppressed in most cases, but in a tiny corner of the parameter space it can be enhanced (maximally by a factor of 2); (iii) In the NMSSM the signal rate can be enhanced or suppressed depending on the parameter space, and the enhancement factor can be as large as 7. (C) 2011 Elsevier B.V. All rights reserved.
40224148
Virtual effects of split-SUSY in Higgs productions at linear colliders
In split supersymmetry, gauginos and higgsinos are the only supersymmetric particles possibly accessible at foreseeable colliders like the CERN Large Hadron Collider (LHC) and the International Linear Collider (ILC). In order to account for the cosmic dark matter measured by WMAP, these gauginos and higgsinos are stringently constrained and could be explored at the colliders through their direct productions and/or virtual effects in some processes. The clean environment and high luminosity of the ILC render the virtual effects at percent level meaningful in unraveling the new physics effects. In this work we assume split supersymmetry and calculate the virtual effects of the WMAP-allowed gauginos and higgsinos in the Higgs productions e+e-→Zh and e+e-→νeν̄eh through WW fusion at the ILC. We find that the production cross section of e+e-→Zh can be altered by a few percent in some part of the WMAP-allowed parameter space, while the correction to the WW fusion process e+e-→νeν̄eh is below 1%. Such virtual effects are correlated with the cross sections of chargino pair productions and can offer complementary information in probing split supersymmetry at the colliders.
15720470
Supersymmetric electroweak corrections to single top quark production at the Fermilab Tevatron
We calculate the O({alpha}{sub ew}M{sub t}{sup 2}/M{sub W}{sup 2}) supersymmetric (SUSY) electroweak corrections to single top quark production via q{bar q}{sup {prime}}{r_arrow}t{bar b} at the Fermilab Tevatron in the minimal supersymmetric model. The supersymmetric electroweak corrections to the cross section are at most a few percent for tan{beta} {gt} 1, but can exceed 10{percent} for tan{beta} {lt} 1. The combined effects of SUSY electroweak corrections and the Yukawa corrections can exceed 10{percent} for favorable parameter values, which might be observable at a high-luminosity Tevatron. {copyright} {ital 1997} {ital The American Physical Society}
118505781
A SM-like Higgs near 125 GeV in low energy SUSY: a comparative study for MSSM and NMSSM
A bstractMotivated by the recent LHC hints of a Higgs boson around 125 GeV, we assume a SM-like Higgs with the mass 123-127 GeV and study its implication in low energy SUSY by comparing the MSSM and NMSSM. We consider various experimental constraints at 2σ level (including the muon g − 2 and the dark matter relic density) and perform a comprehensive scan over the parameter space of each model. Then in the parameter space which is allowed by current experimental constraints and also predicts a SM-like Higgs in 123-127 GeV, we examine the properties of the sensitive parameters (like the top squark mass and the trilinear coupling At) and calculate the rates of the di-photon signal and the V V* (V = W, Z) signals at the LHC. Our typical findings are: (i) In the MSSM the top squark and At must be large and thus incur some fine-tuning, which can be much ameliorated in the NMSSM; (ii) In the MSSM a light stau is needed to enhance the di- photon rate of the SM-like Higgs to exceed its SM prediction, while in the NMSSM the di-photon rate can be readily enhanced in several ways; (iii) In the MSSM the signal rates of pp → h → V V* at the LHC are never enhanced compared with their SM predictions, while in the NMSSM they may get enhanced significantly; (iv) A large part of the parameter space so far survived will be soon covered by the expected XENON100(2012) sensitivity (especially for the NMSSM).
119516872
Muon anomalous magnetic moment in technicolor models
Contributions to the muon anomalous magnetic moment are evaluated in the technicolor model with scalars and topcolor assisted technicolor model, In the technicolor model with scalers, the additional contributions come from the loops of scalers, which were found sizable only for a very large f/f' disfavored by the experiment of h --> s gamma. The topcolor effect is also found to be large only for an unnaturally large tan theta', and thus the previously evaluated loop effects of extended technicolor bosons, suppressed by m(mu)(2)/M-ETC(2), must be resorted to account for the E821 experiment. So, if the E821 experiment result persists, it would be a challenge to technicolor models. (C) 2001 Published by Elsevier Science B.V.
119099687
Pad\'e expansion and nucleon-nucleon scattering in coupled channels
Since the significant effective field theory (EFT) approach to nucleon systems[1, 2], there has been creating controversies about the consistent renormalization and EFT power counting in nonperturbative regime[3, 4]. The main difficulty is due to the nontrivial prescription dependence developed in nonperturbative regime, where some wisdoms about renormalization established within perturbative regimes cease to apply directly[5]. Therefore, the nonperturbative prescription dependence must be removed through imposing appropriate boundary conditions which are usually implemented through various forms of data fitting either explicitly or implicitly[6, 7, 8, 9, 10, 11, 12, 13]. In more recent literature, nonperturbative counter terms, or equivalently, nonperturbative parametrization of the renormalization prescription has been the main focus[14, 15]. That is, due to the difficulty in treating the issues in nonperturbative regimes, the key issue is to find more efficient parametrization of nonperturbative prescription. In this regard, we have performed an analysis and treatment of the nonperturbative prescription dependence basing on Padé approximant of an important nonperturbative factor of T -matrix for nucleon-nucleon (NN) scattering in uncoupled channles[16, 17, 18]. In this report, we will extend our analysis to coupled channels. Some general theoretical and technical issues associated with coupled channels will be addressed first, then we will illustrate our method in D3−G3. The object under consideration is the T -matrix for nucleon-nucleon scattering processes at low energies. According to Weinberg’s proposal[19], this T -matrix should be solved from Lippmann-Schwinger (LS) equation with the potential to be systematically constructed using χPT as the low energy effective theory of QCD. For coupled channels, such LSE’s read,
45314613
Numerical evaluation of a two loop diagram in the cutoff regularization
Dimensional regularization [1] has become an efficient regularization scheme for theoretical calculations in high energy physics. It makes analytical results more available, and its gauge invariance almost decisively defeated the other schemes (say cutoff scheme) in QCD and other gauge theories. However, things are not so simple. It is very natural to anticipate that DR might fail sooner or later somewhere as it is just a special regularization but never a true theory of the short distance physics. Such event is now known: the MS scheme does not provide a physical prediction for low energy nucleons scattering in case of long scattering lengths [2] in nonperturbative context. To yield meaningful results, unconventional subtraction is devised and now known as PDS [2]. Since this is obtained with hindsight, we should not discard the cutoff scheme without further exploiting its virtues. There have recently been some further investigations over the relations between the divergences in DR and cutoff schemes and the associated limitations [4,5]. The conventional virtues of the DR scheme were shown to be due to an implicit subtraction [4]. In nonperturbative contexts involving multiloop corrections the DR might even make renormalization infeasible [5]. Traditionally, it is very hard to obtain analytical results for multiloop amplitudes within the cutoff scheme. In this report we show that one could achieve this goal numerically at a pretty good precision. We will make use of the dynamical symmetry breaking of λφ discussed in [6] to illustrate the numerical evaluation of the finite constant term in the two loop sunset diagram. Thus the report is organized as follows. First we quote some results of Ref. [6] in Sec. II as preparation. In Sec. III we present our strategy and procedures for doing the numerical analysis of the problem and its possible prospective significance. The result is also given there. In Sec. IV we revisit the problem introduced in Sec. II to show the significance of the new result obtained in Sec. III. The summary will also be given there.
10526710
Trace and chiral anomalies in QED and their underlying theory interpretation
Parametrizing the possible underlying theory or new physics' decoupling effects in the most general way we reexamined the validity of canonical trace relation and chiral symmetry in certain one-loop two-point functions. The anomalies and the relation between chiral and trace Ward identities are investigated and interpreted in the perspective of a complete underlying theory or new physics instead of regularization, with some new phenomena found as by-products.
10538446
Why have Higgs particles not been found yet
It is suggested that within the effective potential approach, convex-ity and spontaneous symmetry breaking(SSB) make the Higgs bosons bizarre and experimentally nonexistent, a possible resolution of the Higgs puzzle. This observation might have quite some nontrivial consequences for the Standard Model (SM) and particle physics. By now, the experimentally nonexistence of Higgs particles predicted by the SM has become a puzzle in spite that the SM has been deemed as well established by the community of high energy physics (HEP), especially given that the estimated mass range of Higgs bosons has already become accessible [1]. There can be found a number of attempts to explain this puzzling situation. There is even modeling work trying to provide masses for intermediate gauge bosons without introducing Higgs model [2]. In this report, We attempt to suggest an alternative way of looking at this problem still within the SM theory. We will adopt the effective potential 1
123028444
Study on Spin Precession of Polarized Slow Positrons in Solids
We have investigated spin precession mechanisms of polarized slow positrons in materials caused by multiple Coulomb scatterings, Bhabha scatterings and bremsstrahlungs. It is found that the degree of depolarization is strongly dependent on the atomic number of materials, and the depolarizations caused by the Bhabha scatterings and the bremsstrahlungs are negligible compared with those caused by multiple Coulomb scatterings. By incorporating the theoretical formulations of spin precession due to the three processes mentioned above into a Monte-Carlo simulation program, we can successfully describe depolarizations of polarized slow-positron beams in materials, i.e. a target, an absorber, and a moderator which are utilized for slow-positron production facilities. The magnitudes of the polarizations thus estimated are in good agreement with experimental measurements.