arXiv subjects

Thomas G. Rizzo

Publications and source records attributed to Thomas G. Rizzo.

At least 145 records · Page 8Linked to original sources

arXiv1998

Gauge Kinetic Mixing and Leptophobic $Z'$ in $E_6$ and SO(10)

We examine the influence of gauge kinetic mixing on the couplings of a TeV scale $Z'$ in both $E_6$ and SO(10) models. The strength of such mixing, which arises due to the existence of incomplete matter representations at low scale, can be described by a single parameter, $δ$. The value of this parameter can significantly influence the ability of both hadron and lepton colliders to detect a $Z'$ using conventional search techniques. In addition, $δ\neq 0$ also adds to the complexities involved in separating $E_6$ $Z'$ models from those arising from alternative scenarios. Employing a reasonable set of assumptions we have determined the allowed range for this parameter within a wide class of models via an RGE analysis. In particular, given the requirements of Standard Model gauge coupling unification, anomaly freedom and perturbativity up to the GUT scale, we demonstrate that the necessary condition for exact leptophobia in $η$ type $E_6$ models, $δ=-1/3$, is impossible to achieve in this scenario. Furthermore we show that the allowed range for $δ$ is rather restricted for arbitrary values of the mixing between the $U(1)_χ$ and $U(1)_ψ$ type couplings. The SO(10) $Z'$ model $χ$ is discussed as a separate case since it requires special attention.

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arXiv1998

Right-Handed Currents in B Decay Revisited

We critically re-examine the case for and against a sizeable right-handed component in the $b\to c$ charged current coupling with a strength $ξ$ relative to the conventional left-handed current. Using data from CLEO on the decay $B\to D^*\ell ν$, as well as our knowledge of $V_{cb}$ extracted from both inclusive and exclusive processes, we are able to determine the presently allowed parameter space for $ξ$ via HQET. We then identify several observables which could be measured at $B$ factories to either strengthen these constraints or otherwise observe right-handed currents. This parameter space region is found to be consistent with the low degree of $Λ_b$ polarization as determined by ALEPH as well as the measurements of the charged lepton and neutrino energy spectra from $b$ decay made by L3. We discuss how future measurements of semileptonic decay distributions may distinguish between exotic $Λ_b$ depolarization mechanisms and the existence of right-handed currents. Within the parameter space allowed by CLEO, using the Left-Right Symmetric Model as a guide, we perform a detailed search for specific sub-regions which can lead to a reduction in both the $B$ semileptonic branching fraction as well as the the average yield of charmed quarks in $B$ decay. The results provide a concrete realization of an earlier suggestion by Voloshin but may lead to potential difficulties with certain penguin mediated decay processes.

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arXiv1998

Failure of JoAnne's Global Fit to the Wilson Coefficients in Rare B Decays: A Left-Right Model Example

In the Standard Model and many of its extensions, it is well known that all of the observables associated with the rare decays $b\to sγ$ and $b\to s\ell^+\ell^-$ can be expressed in terms of the three Wilson coefficients, $C_{7L,9L,10L}(μ\sim m_b)$, together with several universal kinematic functions. In particular it has been shown that the numerical values of these coefficients can be uniquely extracted by a three parameter global fit to data obtainable at future $B$-factories given sufficient integrated luminosity. In this paper we examine if such global fits are also sensitive to new operators beyond those which correspond to the above coefficients, i.e., whether is it possible that new operators can be of sufficient importance for the three parameter fit to fail and for this to be experimentally observable. Using the Left-Right Symmetric Model as an example of a scenario with an extended operator basis, we demonstrate via Monte Carlo techniques that such a possibility can indeed be realized. In some sense this potential failure of the global fit approach can actually be one of its greatest successes in identifying the existence of new physics.

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arXiv1997

Don't Stop Thinking About Leptoquarks: Constructing New Models

We discuss the general framework for the construction of new models containing a single, fermion number zero scalar leptoquark of mass $\simeq 200-220$ GeV which can both satisfy the D0/CDF search constraints as well as low energy data, and can lead to both neutral and charged current-like final states at HERA. The class of models of this kind necessarily contain new vector-like fermions with masses at the TeV scale which mix with those of the Standard Model after symmetry breaking. In this paper we classify all models of this type and examine their phenomenological implications as well as their potential embedding into SUSY and non-SUSY GUT scenarios. The general coupling parameter space allowed by low energy as well as collider data for these models is described and requires no fine-tuning of the parameters.

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arXiv1997

Leptoquarks at Future Lepton Colliders

In this talk I summarize the capability of future lepton colliders to discover leptoquarks and to determine their electroweak quantum numbers. This analysis is an updated discussion based on the results presented in the Snowmass 1996 New Phenomena Working Group report as well as some more recent work that has appeared in the literature as a result of the HERA high-$Q^2$ excess.

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arXiv1997

Leptoquarks: Pride and Prejudice

Attempts to understand the recent observation of an excess of events in the neutral and charged current channels at high$-Q^2$ at HERA has provided an excellent example of how experiments at both low and high energies can be used to simultaneously constrain scenarios which predict new physics beyond the Standard Model. In this talk I will discuss this subject from the point of view of the construction of new models of leptoquarks.

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arXiv1997

Much Ado About Leptoquarks: A Comprehensive Analysis

We examine the phenomenological implications of a 200 GeV leptoquark in light of the recent excess of events at HERA. Given the relative predictions of events rates in e^+p versus e^-p, we demonstrate that classes of leptoquarks may be excluded, including those contained in E_6 GUT models. It is shown that future studies with polarized beams at HERA could reveal the chirality of the leptoquark fermionic coupling and that given sufficient luminosity in each e^\pm_{L,R} channel the leptoquark quantum numbers could be determined. The implications of 200-220 GeV leptoquarks at the Tevatron are examined. While present Tevatron data most likely excludes vector leptoquarks and leptogluons in this mass region, it does allow for scalar leptoquarks. We find that while leptoquarks have little influence on Drell-Yan production, further studies at the Main Injector are possible in the single production channel. We investigate precision electroweak measurements as well as the process e^+e^-\to q\bar q at LEP II and find they provide no further restrictions on these leptoquark models. We then ascertain that cross section and polarization asymmetry measurements at the NLC provide the only direct mechanism to determine the leptoquark's electroweak quantum numbers. The single production of leptoquarks in γe collisions by both the backscattered laser and Weisacker-Williams techniques at the NLC is also discussed. Finally, we demonstrate that we can obtain successful coupling constant unification in models with leptoquarks, both with or without supersymmetry. The supersymmetric case requires the GUT group to be larger than SU(5) such as flipped SU(5)\times U(1)_X.

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arXiv1997

Searching For Anomalous $τνW$ Couplings

The capability of current and future measurements at low and high energy $e^+e^-$ colliders to probe for the existence of anomalous, CP conserving, $τνW$ dipole moment-type couplings is examined. At present, constraints on the universality of the tau charged and neutral current interactions as well as the shape of the $τ\to \ell$ energy spectrum provide the strongest bounds on such anomalous couplings. The presence of these dipole moments are shown to influence, e.g., the extraction of $α_s(m_τ^2)$ from $τ$ decays and can lead to apparent violations of CVC expectations.

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arXiv1996

Extended Gauge Sectors at Future Colliders: Report of the New Gauge Boson Subgroup

We summarize the results of the New Gauge Boson Subgroup on the physics of extended gauge sectors at future colliders as presented at the 1996 Snowmass workshop. We discuss the direct and indirect search reaches for new gauge bosons at both hadron and lepton colliders as well as the ability of such machines to extract detailed information on the couplings of these particles to the fermions and gauge bosons of the Standard Model. [To appear in the Proceedings of the 1996 DPF/DPB Summer Study on New Directions for High Energy Physics-Snowmass96, Snowmass, CO, 25 June-12 July, 1996.]

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arXiv1996

An Exploration of Below Threshold $Z'$ Mass and Coupling Determinations at the NLC

We examine of the capability of the Next Linear Collider to determine the mass as well as the couplings to leptons and $b$-quarks of a new neutral gauge boson, $Z'$, below direct production threshold. By using simulated data collected at several different values of $\sqrt s$, we demonstrate how this can be done in a model-independent manner via an anonymously case approach. The importance of beam polarization to the success of this program is discussed. The procedure is shown to be easily extended to the case of top and charm quark couplings.

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arXiv1996

Using Final State Gluons as Probes of Anomalous Top Quark Couplings at the NLC

The rate and corresponding gluon jet energy distribution for the process $e^+e^- \to t\bar tg$ are sensitive to the presence of anomalous dipole-like couplings of the top to the photon and $Z$ at the production vertex as well as to the gluon itself. For sizeable anomalous couplings substantial deviations in the shape and magnitude of the gluon spectrum from the expectations of the Standard Model are anticipated. We explore the capability of the Next Linear Collider to either discover or place bounds on these types of top quark couplings through measurements of the gluon energy distribution. The resulting constraints are found to be quite complementary to those obtained using other techniques. [To appear in the Proceedings of the 1996 DPF/DPB Summer Study on New Directions for High Energy Physics-Snowmass96, Snowmass, CO, 25 June-12 July, 1996.]

hep-ph↗

arXiv1996

The Polarization Asymmetry in $γe$ Collisions at the NLC and Triple Gauge Boson Couplings

The capability of the NLC in the $γe$ collider mode to probe the CP-conserving $γWW$ and $γZZ$ anomalous couplings through the use of the polarization asymmetry is examined. When combined with other measurements, very strong constraints on both varieties of anomalous couplings can be obtained. We show that these bounds are complementary to those that can be extracted from data taken at the LHC. [To appear in the Proceedings of the 1996 DPF/DPB Summer Study on New Directions for High Energy Physics-Snowmass96, Snowmass, CO, 25 June-12 July, 1996.]

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arXiv1996

Constraints on $q\bar q γγ$ Contact Interactions at Future Hadron Colliders

We explore the capability of the Tevatron and LHC as well as other future hadron colliders to place limits on the possible existence of flavor-independent $q \bar q γγ$ contact interactions which can lead to an excess of high $p_t$ diphoton events with large invariant masses. Constraints on the corresponding $e^+e^-γγ$ contact interaction already exist from LEP. In the case of hadron colliders, strong constraints on the scale associated with such interactions are achievable in all cases, e.g., of order 0.9(3) TeV at TeV33(LHC). [To appear in the Proceedings of the 1996 DPF/DPB Summer Study on New Directions for High Energy Physics-Snowmass96, Snowmass, CO, 25 June-12 July, 1996.]

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arXiv1996

Constraints on Anomalous Top Quark Couplings at the LHC

Measurements of distributions associated with the pair production of top quarks at the LHC can be used to constrain (or observe) the anomalous chromomagnetic dipole moment($κ$) of the top. For example, using either the $t\bar t$ invariant mass or the $p_t$ distribution of top we find that sensitivities to $|κ|$ of order 0.05 are obtainable with 100 $fb^{-1}$ of integrated luminosity. This is similar in magnitude to what can be obtained at a 500 GeV NLC with an integrated luminosity of 50 $fb^{-1}$ through an examination of the $e^+e^- \to t\bar tg$ process. [To appear in the Proceedings of the 1996 DPF/DPB Summer Study on NewDirections for High Energy Physics-Snowmass96, Snowmass, CO, 25 June-12 July, 1996.]

hep-ph↗

arXiv1996

Searches for Scalar and Vector Leptoquarks at Future Hadron Colliders

The search reaches for both scalar($S$) and vector($V$) leptoquarks at future hadron colliders are summarized. In particular we evaluate the production cross sections of both leptoquark types at TeV33 and LHC as well as the proposed 60 and 200 TeV colliders through both quark-antiquark annihilation and gluon-gluon fusion: $q \bar q, gg \to SS,VV$. Experiments at these machines should easily discover such particles if their masses are not in excess of the few TeV range. [To appear in the Proceedings of the 1996 DPF/DPB Summer Study on New Directions for High Energy Physics-Snowmass96, Snowmass, CO, 25 June-12 July, 1996.]

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arXiv1996

Searches for New Gauge Bosons at Future Colliders

The search reaches for new gauge bosons at future hadron and lepton colliders are summarized for a variety of extended gauge models. Experiments at these energies will vastly improve over present limits and will easily discover a $Z'$ and/or $W'$ in the multi-TeV range. [To appear in the Proceedings of the 1996 DPF/DPB Summer Study on New Directions for High Energy Physics-Snowmass96, Snowmass, CO, 25 June-12 July, 1996.]

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arXiv1996

Determination of the $Z'$ Mass and Couplings Below Threshold at the NLC

We investigate the capability of the NLC to indirectly determine both the mass as well as the couplings to leptons and $b$-quarks of a new neutral gauge boson below direct production threshold. By using data collected at several different values of the collider center of mass energy, we demonstrate how this can be done in an anonymous and model-independent manner. The procedure can be easily extended to the top and charm quark couplings.[To appear in the Proceedings of the 28th International Conference on High Energy Physics, Warsaw, Poland, 25-31 July 1996]

hep-ph↗

arXiv1996

Probing Weak Anomalous Top Quark Couplings with Final State Gluons at the NLC

The rate and corresponding gluon jet energy distribution for the process $e^+e^- \to t\bar tg$ are sensitive to the presence of anomalous dipole-like couplings of the top to the photon and $Z$ at the production vertex. For sizeable anomalous couplings of this type substantial deviations from the expectations of the Standard Model are likely. We explore the capability of the NLC to discover or place bounds on these types of top quark couplings. The resulting constraints are found to be quite complementary to those which arise from direct probes of the top quark production vertex. [To appear in {\it Physics and Technology of the Next Linear Collider}, eds. D.\ Burke and M.\ Peskin, reports submitted to Snowmass 1996.]

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