arXiv subjects

Tzu Chiang Yuan

Publications and source records attributed to Tzu Chiang Yuan.

At least 19 recordsLinked to original sources

arXiv1996

Quarkonium Production At $Z^0$ and in $Υ(1S)$ Decay

The conventional color-singlet model was challenged by the recent data on quarkonium production. Discrepancies in production rates were observed at the Tevatron, at LEP, and in fixed-target experiments. The newly advocated color-octet mechanism provides a plausible solution to the anomalous quarkonium production observed at the Tevatron. The color-octet mechanism should also affect other quarkonium production channels. In this paper we will summarize the studies of quarkonium production in $Z^0$ and $Υ$ decays.

hep-ph↗

arXiv1996

Color-Octet $J/ψ$ Production in the $Υ$ Decay

The direct production rate of $ψ$ in the $Υ$ decay is shown to be dominated by the process $ Υ\to ggg^*$ followed by $g^* \to ψ$ via the color-octet mechanism proposed recently to explain the anomalous prompt charmonium production at the Tevatron. We show that this plausibly dominant process has a branching ratio compatible with the experimental data. Further experimental study in this channel is important to test the significance of the color-octet component of $c\bar c$ pair inside the $ψ$ system.

hep-ph↗

arXiv1995

Color-Octet Quarkonium Production at the $Z$ Pole

The direct production rate of $J/ψ$ via the color-octet mechanism is calculated at the $Z$ resonance. The color-octet production process $Z\to J/ψq \bar q$ is shown to have a substantial branching ratio as well as a distinctive energy spectrum, which can be used as a powerful tool to distinguish from the color-singlet direct production of the $J/ψ$.

hep-ph↗

arXiv1995

Hadronic Production of S-wave and P-wave Charmed Beauty Mesons via Heavy Quark Fragmentation

At hadron colliders the dominant production mechanism of $(\bar bc)$ mesons with large transverse momentum is due to parton fragmentation. We compute the rates and transverse momentum spectra for production of S-wave and P-wave $(\bar b c)$ mesons at the Tevatron via the direct fragmentation of the bottom antiquark as well as the Altarelli-Parisi induced gluon fragmentation. Since all the radially and orbitally excited $(\bar b c)$ mesons below the $BD$ flavor threshold will cascade into the pseudoscalar ground state $B_c$ through electromagnetic and/or hadronic transitions, they all contribute to the inclusive production of $B_c$. The contributions of the excited S-wave and P-wave states to the inclusive production of $B_c$ are 58 and 23\%, respectively, and hence significant.

hep-ph↗

arXiv1995

Heavy Quark Fragmentation Functions for D-wave Quarkonium and Charmed Beauty Mesons

At the large transverse momentum region, the production of heavy-heavy bound-states such as charmonium, bottomonium, and $\bar bc$ mesons in high energy $e^+e^-$ and hadronic collisions is dominated by parton fragmentation. We calculate the heavy quark fragmentation functions into the D-wave quarkonium and $\bar bc$ mesons to leading order in the strong coupling constant and in the non-relativistic expansion. In the $\bar b c$ meson case, one set of its D-wave states is expected to lie below the open flavor threshold. The total fragmentation probability for a $\bar b$ antiquark to split into the D-wave $\bar b c$ mesons is about $2 \times 10^{-5}$, which implies that only 2\% of the total pseudo-scalar ground state $B_c$ comes from the cascades of these orbitally excited states.

hep-ph↗

arXiv1995

Gluon Fragmentation into Spin-Triplet S-Wave Quarkonium

The leading color-singlet contribution to the fragmentation function for a gluon to split into spin-triplet S-wave quarkonium is presented. In the case of charmonium, we find that this color-singlet term is always negligible compared to the leading color-octet contribution.

hep-ph↗

arXiv1995

Helicity probabilities for heavy quark fragmentation into heavy-light excited mesons

After a brief review on how heavy quark symmetry constraints the helicity fragmentation probabilities for a heavy quark hadronizes into heavy-light hadrons, we present a heavy quark fragmentation model to extract the value for the Falk-Peskin probability $w_{3/2}$ describing the fragmentation of a heavy quark into a heavy-light meson whose light degrees of freedom have angular momentum ${3 \over 2}$. We point out that this probability depends on the longitudinal momentum fraction $z$ of the meson and on its transverse momentum $p_\bot$ relative to the jet axis. In this model, the light degrees of freedom prefer to have their angular momentum aligned transverse to, rather than along, the jet axis. Implications for the production of excited heavy mesons, like $D^{**}$ and $B^{**}$, are briefly discussed.

hep-ph↗

arXiv1994

S-Wave and P-Wave $B_c$ Meson Production at Hadron Colliders by Heavy Quark Fragmentation

We compute model-independently the production rates and transverse momentum spectra for the $B_c$ mesons in various spin-orbital states ($n\,^1S_0$, $n\,^3S_1$, $n\,^1P_1$, and $n\,^3P_J\,(J=0,1,2)$ ) at hadron colliders via the direct fragmentation of the bottom antiquark and via the Altarelli-Parisi-induced gluon fragmentation. Since all the radially and orbitally excited states below the $BD$ flavor threshold will decay, either electromagnetically, hadronically, or a combination of both, into the pseudoscalar ground state $1\,^1S_0$, they all contribute significantly to the inclusive $B_c$ meson production.

hep-ph↗

arXiv1994

Helicity Probabilities For Heavy Quark Fragmentation Into Excited Mesons

In the fragmentation of a heavy quark into a heavy meson whose light degrees of freedom have angular momentum $3/2$, all the helicity probabilities are completely determined in the heavy quark limit up to a single probability $w_{3/2}$. We point out that this probability depends on the longitudinal momentum fraction $z$ of the meson and on its transverse momentum $p_\bot$ relative to the jet axis. We calculate $w_{3/2}$ as a function of scaling variables corresponding to $z$ and $p_\bot$ for the heavy quark limit of the perturbative QCD fragmentation functions for $b$ quark to fragment into $(b \bar c)$ mesons. In this model, the light degrees of freedom prefer to have their angular momentum aligned transverse to, rather than along, the jet axis. Implications for the production of excited heavy mesons, like $D^{**}$ and $B^{**}$, are discussed.

hep-ph↗

arXiv1994

Perturbative QCD Fragmentation Functions for Production of P-wave Mesons with Charm and Beauty

We calculate the leading order QCD fragmentation functions for the production of $P$-wave charmed beauty mesons. Long-distance effects are factored into two nonperturbative parameters: the derivative of the radial wavefunction at the origin and a second parameter related to the probability for a $(\bar b c)$ heavy quark pair that is produced in a color-octet $S$-wave state to form a color-singlet $P$-wave bound state. The four $2P$ states and those $3P$ states which lie below the $BD$ flavor threshold eventually all decay into the $1S$ ground state $B_c$ through hadronic cascades or by emitting photons. The total fragmentation probabilities for production of the $1S$ ground state $B_c$ from the cascades of the $2P$ and $3P$ states are about $1.7 \times 10^{-4}$ and $2.3 \times 10^{-4}$ respectively. Thus the direct production of the $P$-wave states via fragmentation may account for a significant fraction of the inclusive production rate of the $B_c$ at large transverse momentum in high energy colliders. Our analytic results for the $P$-wave fragmentation functions disagree with those obtained earlier in the literature.

hep-ph↗

arXiv1994

Spin Alignment in the Production of Vector Mesons with Charm and/or Beauty via Heavy Quark Fragmentation

We calculate the process-independent fragmentation functions for a $\bar b$ antiquark to fragment into longitudinally and transversely polarized $B_c^*$ ($^3S_1$) mesons to leading order in the QCD strong coupling constant. In the special case of equal quark mass we recover previous results for the fragmentation of $c\to ψ$ and $b\toΥ$. Various spin asymmetry parameters are defined as measures of the relative population of the longitudinally and transversely polarized vector meson states. In the heavy quark mass limit $m_b \to \infty$ our polarized fragmentation functions obey heavy quark spin symmetry, we therefore apply them as a model to describe the fragmentation of charm and bottom into heavy-light mesons like $D^*$ and $B^*$. The spin asymmetry parameter, $α(z)$, is consistent with the existing CLEO data for $D^*$. The scaling behavior of $\langle z \rangle$ is studied in detail. We find excellent agreement between the predictions of $\langle z \rangle$ from our fragmentation functions and the experimental data for $D^*$ and $B^*$ from the LEP, CLEO, and ARGUS detectors. Finally, we also point out that the spin asymmetry depends significantly on the transverse momentum $p_\perp$ of the vector mesons relative to the fragmentation axis.

hep-ph↗

arXiv1994

Gluon Fragmentation into P-wave Heavy Quarkonium

The fragmentation functions for gluons to split into P-wave heavy quarkonium states are calculated to leading order in the QCD coupling constant. Long-distance effects are factored into two nonperturbative parameters: the derivative of the radial wavefunction at the origin and a second parameter related to the probability for a heavy-quark-antiquark pair that is produced in a color-octet S-wave state to form a color-singlet P-wave bound state. The fragmentation probabilities for a high transverse momentum gluon to split into the P-wave charmonium states $χ_{c0}$, $χ_{c1}$, and $χ_{c2}$ are estimated to be $0.4 \times 10^{-4}$, $1.8 \times 10^{-4}$, and $2.4 \times 10^{-4}$, respectively. This fragmentation process may account for a significant fraction of the rate for the inclusive production of $χ_{cJ}$ at large transverse momentum in $p \bar p$ colliders.

hep-ph↗

arXiv1993

$B_c$ Meson Productions Via Induced Gluon Fragmentation

Gluons cannot directly fragment into $B_c$ mesons until at order $α_s^3$. However, the Altarelli-Parisi evolution of the $\bar b$-quark fragmentation functions $D_{\bar b \to B_c}(z)$ and $D_{\bar b \to B_c^*}(z)$ from the heavy quark mass scales up to the collider energy scale $Q$ can induce the gluon fragmentation functions $D_{g \to B_c}(z)$ and $D_{g \to B_c^*}(z)$, respectively, through the gluon splitting $g\to \bar b$ at the order $α_s^3 \, {\rm log} (Q/m_{b,c})$. We will determine these induced gluon fragmentation functions. The $B_c$ meson productions due to the induced gluon fragmentation will be evaluated and compared to the productions by the direct $\bar b$-quark fragmentation. The contribution from the induced gluon fragmentation is found to be a significant portion of the total $B_c$ and $B_c^*$ meson productions, and therefore cannot be ignored.

hep-ph↗

arXiv1993

Perturbative QCD Fragmentation Functions for $B_c$ and $B_c^*$ Production

The dominant production mechanism for ${\bar b} c$ bound states in high energy processes is the production of a high energy ${\bar b}$ or $c$ quark, followed by its fragmentation into the ${\bar b} c$ state. We calculate the fragmentation functions for the production of the S-wave states $B_c$ and $B_c^*$ to leading order in the QCD coupling constant. The fragmentation probabilities for ${\bar b} \rightarrow B_c$ and ${\bar b} \rightarrow B_c^*$ are approximately $2.2 \times 10^{-4}$ and $3.1 \times 10^{-4}$, while those for $c \rightarrow B_c$ and $c \rightarrow B_c^*$ are smaller by almost two orders of magnitude.

hep-ph↗

arXiv1993

Higgs-Boson Decay to Four Fermions Including a Single Top Quark Below $t \bar t$ Threshold

The rare decay modes Higgs $\rightarrow$ four light fermions, and Higgs $\rightarrow$ single top-quark + three light fermions for $m_t 100\GeV$ makes the second amplitude competitive or dominant for most $M_H,m_t$ values. Thus the Higgs decay rate to single top directly probes the SM universal mechanism generating both gauge boson and fermion masses, and offers a means to infer the Higgs-$t \bar t$ Yukawa coupling when $H\rightarrow t \bar t$ is kinematically disallowed. We find that the modes $pp\rightarrow Xt\bar t(H\rightarrow t\bar b W^{(*)})$ at the SSC, and $e^+ e^-\rightarrow Z\,or\,ν\barν + (H\rightarrow t\bar b W^{(*)})$ at future high energy, high luminosity colliders, may be measureable if $2m_t$ is not too far above $M_H$. We classify non-standard Higgses as gaugeo-phobic, fermio-phobic or fermio-philic, and discuss the Higgs$\rightarrow$ single top rates for these classes.

hep-ph↗

arXiv1993

Gluon Fragmentation into Heavy Quarkonium

The dominant production mechanism for heavy quark-antiquark bound states in very high energy processes is fragmentation, the splitting of a high energy parton into a quarkonium state and other partons. We show that the fragmentation functions $D(z,μ)$ describing these processes can be calculated using perturbative QCD. We calculate the fragmentation functions for a gluon to split into S-wave quarkonium states to leading order in the QCD coupling constant. The leading logarithms of $μ/m_Q$, where $μ$ is the factorization scale and $m_Q$ is the heavy quark mass, are summed up using Altarelli-Parisi evolution equations.

hep-ph↗

arXiv1993

Z^0 Decay into Charmonium via Charm Quark Fragmentation

In decays of the $Z^0$, the dominant mechanism for the direct production of charmonium states is the decay of the $Z^0$ into a charm quark or antiquark followed by its fragmentation into the charmonium state. We calculate the fragmentation functions describing the splitting of charm quarks into S-wave charmonium states to leading order in the QCD coupling constant. Leading logarithms of $M_Z/m_c$ are summed up using Altarelli-Parisi evolution equations. Our analytic result agrees with the complete leading order calculation of the rate for $Z^0 \rightarrow ψc {\bar c}$. We also use our fragmentation functions to calculate the production rate of heavy quarkonium states in $W^\pm$, top quark, and Higgs decays.

hep-ph↗

arXiv1992

P-Wave Charmonium Production in B-Meson Decays

We calculate the decay rates of $B$ mesons into P-wave charmonium states using new factorization formulas that are valid to leading order in the relative velocity of the charmed quark and antiquark and to all orders in the running coupling constant of QCD. We express the production rates for all four P states in terms of two nonperturbative parameters, the derivative of the wavefunction at the origin and another parameter related to the probability for a charmed-quark-antiquark pair in a color-octet S-wave state to radiate a soft gluon and form a P-wave bound state. Using existing data on $B$ meson decays into $χ_{c1}$ to estimate the color-octet parameter, we find that the color-octet mechanism may account for a significant fraction of the $χ_{c1}$ production rate and that $B$ mesons should decay into $χ_{c2}$ at a similar rate.

hep-ph↗