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V. Barger

Publications and source records attributed to V. Barger.

At least 145 records · Page 8Linked to original sources
arXiv1996

Particle Physics Opportunities at $μ^+μ^-$ Colliders

We discuss the capabilities of future muon colliders to resolve important particle physics questions. A collider with c.m. energy $\sqrt s = 100$ to 500~GeV offers the unique opportunity to produce Higgs bosons in the $s$-channel and thereby measure the Higgs masses, total widths and several partial widths to high precision. At this same machine, $t\bar t$ and $W^+W^-$ threshold studies would yield superior precision in the determination of $m_t$ and $m_W$. A multi-TeV $μ^+μ^-$ collider would open up the realm of physics above the 1 TeV scale, allowing, for example, copious production of supersymmetric particles up to the highest anticipated masses or a detailed study of the strongly-interacting scenario of electroweak symmetry breaking.

hep-ph↗
arXiv1996

Baryonic $Z'$ connection of LEP $R_{b,c}$ data with Tevatron $(W,Z,γ)b\bar b$ events

The mixing of a new $Z'$ boson with the $Z$ significantly improves the fit to the LEP precision electroweak data, provided that the $Z'$ couples mainly to quarks. If $M_{Z_2}<200$~GeV, the $s$-channel $Z_2$ production and $(W,Z,γ)Z_2$ pair production cross sections at the Tevatron give an excess above QCD of $b\bar b$ and $(W,Z,γ)b\bar b$ events, respectively, with invariant mass $m(b\bar b)\approx M_{Z_2}$, which provide viable signals for detection of the $Z_2$. The interference of the $Z_2$ with $γ,Z_1$ in $e^+e^-\to \bar bb(\bar cc)$ at LEP\,1.5 energies is correlated with $R_b(R_c)$ and may be observable.

hep-ph↗
arXiv1996

Higgs Boson Physics in the $s$-channel at $μ^+μ^-$ Colliders

Techniques and strategies for discovering and measuring the properties of Higgs bosons via $s$-channel production at a $μ^+μ^-$ collider, and the associated requirements for the machine and detector, are discussed in detail. The unique feature of $s$-channel production is that, with good energy resolution, the mass, total width and partial widths of a Higgs boson can be directly measured with remarkable accuracy in most cases. For the expected machine parameters and luminosity the Standard Model (SM) Higgs boson $\hsm$, with mass $\lsim 2\mw$, the light $\hl$ of the minimal supersymmetric Standard Model (MSSM), and the heavier MSSM Higgs bosons (the CP-odd $\ha$ and the CP-even $\hh$) can all be studied in the $s$-channel, with the heavier states accessible up to the maximal $\sqrt s$ over a large fraction of the MSSM parameter space. In addition, it may be possible to discover the $\ha$ and $\hh$ by running the collider at full energy and observing excess events in the bremsstrahlung tail at lower energy. The integrated luminosity, beam resolution and machine/detector features required to distinguish between the $\hsm$ and $\hl$ are delineated.

hep-ph↗
arXiv1995

Thresholds in $α_s$ evolution and the $p_T$ dependence of jets

We point out that high-mass thresholds in the evolution of the strong-interaction coupling parameter $α_s$, due to gluinos, squarks and possible new heavy quarks, could introduce appreciable corrections to the transverse momentum dependence of jet production at the Tevatron. If the new thresholds were near scale $μ= 200$~GeV, then within the limits of asymptotic freedom they could introduce up to $11\%$ increase in $α_s(μ)$ (and hence $23\%$ increase in jet production) at scale $μ=500$ GeV, compared to Standard Model extrapolations.

hep-ph↗
arXiv1995

Renormalization group evolution of R-parity-violating Yukawa couplings

We study the evolution of $R$-parity-violating (RPV) couplings in the minimum supersymmetric standard model, between the electroweak and grand unification scales, assuming a family hierarchy for these coupling strengths. Particular attention is given to solutions where both the $R$-conserving and $R$-violating top quark Yukawa couplings simultaneously approach infrared fixed points; these we analyse both algebraically and with numerical solutions of the evolution equations at one-loop level. We identify constraints on these couplings at the GUT scale, arising from lower limits on the top quark mass. We show that fixed points offer a new source of bounds on RPV couplings at the electroweak scale. We derive evolution equations for the CKM matrix, and show that RPV couplings affect the scaling of the unitarity triangle. The fixed-point behaviour is compatible with all present experimental constraints. However, fixed-point values of RPV top-quark couplings would require the corresponding sleptons or squarks to have mass $\agt m_t$ to suppress strong new top decays to sparticles.

hep-ph↗
arXiv1995

Top pair production with an extra gluon at the Tevatron

We calculate top pair production and decay at the Tevatron $p\bar p$ collider, with the emission of an extra gluon, and study the corresponding $W+5$\,jet top signals including full spin correlations in the $W \to \ellν$ leptonic and $W\to jj$ hadronic decays. We study the feasibility of reconstructing $W+5$\,jet top events with a single $b$-tag, including realistic energy resolution. Our suggested basic procedure based on kinematic fitting achieves about 74\% reconstruction efficiency, with 74\% of the reconstructed events correctly classified (purity); this improves to 82\% efficiency with 77\% purity in double-$b$-tagged events. We suggest possible refinements, based on virtuality criteria, that give higher purity at the cost of lower reconstruction efficiency.

hep-ph↗
arXiv1995

Double gluon fragmentation to $J/ψ$ pairs at the Tevatron

It has been proposed that the large cross sections for prompt $ψ$, $ψ'$, and $χ_c$ production at the Fermilab Tevatron $p\bar p$ collider can be explained by a dominant color-octet term in the fragmentation function for a gluon to split into quarkonium. We show that this mechanism makes testable predictions for double-quarkonium $ψψ$, $ψψ'$, $ψχ_c$, $ψΥ$ and $ψχ_b$ production, as well as for $W ψ$ production, using color-octet matrix elements previously determined from charmonium production data. The $ψψ$ signal would already be measurable at the Tevatron, while the $ψχ_c$ and $Wψ$ signals would be on the edge of present detectability.

hep-ph↗
arXiv1995

Possible sneutrino-pair signatures with R-parity breaking

If sneutrinos are the lightest supersymmetry partners and $R$-parity is not conserved, the process $e^+e^- \to \tilde {\barν} \tilde ν$ can have striking signatures due to the decay modes $\tildeν\to\ell^+\ell'^-$ or $\tildeν\to q \bar q'$. We present cross section formulas and discuss event rates and detection at the upgraded $e^+e^-$ collider LEP. Four-lepton signals should be detectable up to sneutrino mass $\tilde {m}_ν= 80$ GeV and maybe beyond; four-jet signals should be detectable up to $\tilde {m}_ν= 70$ GeV, but would probably be obscured thereafter by $WW$ background.

hep-ph↗
arXiv1995

LHC Analysis of the Strongly Interacting $WW$ System: Gold-Plated Modes

We study the gold-plated purely leptonic signal and background rates at the LHC for the $ZZ,$ $W^{+}W^-,$ $W^\pm Z$ and $W^\pm W^\pm$ final states associated with strongly interacting electroweak symmetry breaking. We work at an energy of $\sqrt s = 14$ TeV, and develop a combination of back-to-back leptonic, central-jet-vetoing and forwaring cuts that suppresses the Standard-Model backgrounds. We find that the LHC with an annual luminosity of 100 fb$^{-1}$ will achieve a reasonably good sensitivity to the physics of strongly interacting electroweak symmetry breaking.

hep-ph↗
arXiv1995

Remarks on the KARMEN Anomaly

A recently reported anomaly in the time structure of signals in the KARMEN neutrino detector suggests the decay of a new particle $x$, produced in $π^+ \to μ^+ x$ with mass $m_x=33.9$ MeV. We discuss the constraints and difficulties in interpreting $x$ as a neutrino. We show that a mainly-sterile neutrino scenario is compatible with all laboratory constraints, within narrow limits on the mixing parameters, although there are problems with astrophysical and cosmological constraints. This scenario predicts that appreciable numbers of other $x$-decay events with different origins and time structures should also be observable in the KARMEN detector. Such $x$-decay events should also be found in the LSND experiment and may be relevant to the search for $\barν_μ\to\barν_e$ oscillations.

hep-ph↗
arXiv1995

Possible pre-LEP200 SUSY threshold signals

If $R$-parity is not conserved, the SUSY-threshold production process $e^+e^- \to χ_1^0χ_1^0$ could be detectable with relatively low luminosity. Hence an interesting mass range for the lightest SUSY particle $χ_1^0$ could be explored at the CERN LEP collider during its intermediate energy development, even before the full LEP200 upgrade is completed. We present cross section formulas and discuss event rates and detection for the three distinct decay options: $χ_1^0\to 2\, {\rm charged \; leptons}\, +\, {\rm neutrino}$, $χ_1^0\to {\rm lepton}\, +2\, {\rm quarks}$, and $χ_1^0\to 3\, {\rm quarks}$.

hep-ph↗
arXiv1995

$s$-Channel Higgs Boson Production at a Muon-Muon Collider

High luminosity muon-muon colliders would provide a powerful new probe of Higgs boson physics through $s$-channel resonance production. We discuss the prospects for detection of Higgs bosons and precision measurements of their masses and widths at such a machine.

hep-ph↗
arXiv1995

Physics Goals of a $μ^+μ^-$ Collider

This working group report focuses on the physics potential of $μ^+μ^-$ colliders beyond what can be accomplished at linear $e^+e^-$ colliders and the LHC. Particularly interesting possibilities include (i)~$s$-channel resonance production to discover and study heavy Higgs bosons with $ZZ$ and $WW$ couplings that are suppressed or absent at tree-level (such as the $H$ and $A$ Higgs bosons of supersymmetric models), (ii)~study of the strongly interacting electroweak sector, where higher energies give larger signals, and (iii)~measurements of the masses and properties of heavy supersymmetric particles.

hep-ph↗
arXiv1995

Quark Singlets: Implications and Constraints

Quarks whose left- and right-handed chiral components are both singlets with respect to the SU(2) weak-isospin gauge group, offer interesting physics possibilities beyond the Standard Model (SM) already studied in many contexts. We here address some further aspects. We first collect and update the constraints from present data on their masses and mixings with conventional quarks. We discuss possible effects on $b\to sγ$ and $Z\to b\bar b$ decays and give fresh illustrations of CP asymmetries in $B^0$ decays differing dramatically from SM expectations. We analyse singlet effects in grand unification scenarios: $d$-type singlets are most economically introduced in ${\bf 5+5^*}$ multiplets of $SU(5)$, with up to three generations, preserving gauge coupling unification with perturbative values up to the GUT scale; $u$-type singlets can arise in ${\bf 10+10^*}$ multiplets of $SU(5)$ with at most one light generation. With extra matter multiplets the gauge couplings are bigger; we give the two-loop evolution equations including exotic multiplets and a possible extra $U(1)$ symmetry. Two-loop effects can become important, threatening unification (modulo threshold effects), perturbativity and asymptotic freedom of $α_3$. In the Yukawa sector, top-quark fixed-point behaviour is preserved and singlet-quark couplings have infrared fixed points too, but unification of $b$ and $τ$ couplings is not possible in a three-generation $E_6$ model.

hep-ph↗
arXiv1995

Probing Strongly-interacting Electroweak Dynamics through $W^+W^-/ZZ$ Ratios at Future $e^+e^-$ Colliders

We point out that the ratio of $W^+ W^- \to W^+ W^-$ and $W^+W^- \to ZZ$ cross sections is a sensitive probe of the dynamics of electroweak symmetry breaking, in the CM energy region $\sqrt {s_{\rm ww}^{}} \agt 1$~TeV where vector boson scattering may well become strong. We suggest ways in which this ratio can be extracted at a 1.5 TeV $e^+e^-$ linear collider, using $W^\pm,Z \to jj$ hadronic decays and relying on dijet mass resolution to provide statistical discrimination between $W^\pm$ and $Z$. $WW$ fusion processes studied here are unique for exploring scalar resonances of mass about 1 TeV and are complementary to studies via the direct channel $e^+ e^- \rightarrow W^+W^-$ for the vector and non-resonant cases. With an integrated luminosity of 200 fb$^{-1}$, the signals obtained are statistically significant. Comparison with a study of $e^-e^- \rightarrow ννW^-W^-$ process is made. Enhancements of the signal rate from using a polarized electron beam, or at a 2 TeV $e^+e^-$ linear collider and possible higher energy $μ^+μ^-$ colliders, are also presented.

hep-ph↗
arXiv1994

Minijet veto: a tool for the heavy Higgs search at the LHC

The distinct color flow of the $qq\to qqH,\; H\to W^+W^-$ process leads to suppressed radiation of soft gluons in the central region, a feature which is not shared by major background processes like $t\bar t$ production or $q\bar q \to W^+W^-$. For the leptonic decay of a heavy Higgs boson, $H\to W^+W^- \to \ell^+ν\ell^-\barν$, it is shown that these backgrounds are typically accompanied by minijet emission in the 20--40 GeV range. A central minijet veto thus constitutes a powerful background rejection tool. It may be regarded as a rapidity gap trigger at the semihard parton level which should work even at high luminosities.

hep-ph↗
arXiv1994

Further ways to distinguish single-lepton top quark signals from background at the Tevatron

Further kinematical variables are suggested, in which to compare the putative leptonic $W$ plus 4-jet top quark signal with the QCD background. We show that the lepton rapidity asymmetry, the $p_T$-ranking of the tagged $b$-jet, the double-tag probability, the reconstructed $t+\bar t$ invariant mass, and the lepton energy in the parent top quark rest-frame, all display interesting differences between signal and background.

hep-ph↗