Searcharxiv⌕ Search

arXiv subjects

Adam K. Leibovich

Publications and source records attributed to Adam K. Leibovich.

66 records · Page 4Linked to original sources
arXiv1999

Bounds from Primordial Black Holes with a Near Critical Collapse Initial Mass Function

Recent numerical evidence suggests that a mass spectrum of primordial black holes (PBHs) is produced as a consequence of near critical gravitational collapse. Assuming that these holes formed from the initial density perturbations seeded by inflation, we calculate model independent upper bounds on the mass variance at the reheating temperature by requiring the mass density not exceed the critical density and the photon emission not exceed current diffuse gamma-ray measurements. We then translate these results into bounds on the spectral index n by utilizing the COBE data to normalize the mass variance at large scales, assuming a constant power law, then scaling this result to the reheating temperature. We find that our bounds on n differ substantially (δn > 0.05) from those calculated using initial mass functions derived under the assumption that the black hole mass is proportional to the horizon mass at the collapse epoch. We also find a change in the shape of the diffuse gamma-ray spectrum which results from the Hawking radiation. Finally, we study the impact of a nonzero cosmological constant and find that the bounds on n are strengthened considerably if the universe is indeed vacuum-energy dominated today.

astro-ph↗
arXiv1999

The Resummed Rate for B -> X_s gamma

In this paper we investigate the effect of the resummation of threshold logs on the rate for B -> X_s gamma. We calculate the differential rate dGamma/dE_gamma including the infinite set of terms of the form alpha_s^n log^{n+1}(1-x) and alpha_s^n log^n(1-x) in the Sudakov exponent. The resummation is potentially important since these logs turn into log(2E_{cut}/m_b), when the rate is integrated from the lower cut x=2E_{cut}/m_b to 1. The resummed rate is then convolved with models for the structure function to study whether or not the logs will be enhanced due to the fermi motion of the heavy quark. A detailed discussion of the accuracy of the calculation with and without the inclusion of the non-perturbative effects dictated by the B meson structure function is given. We also investigate the first moment with respect to (1-x), which can be used to measure \barΛand lambda_1. It is shown that there are some two loop corrections which are just as large as the alpha_s^2 beta_0 term, which are usually expected to dominate. We conclude that, for the present energy cut, the threshold logs do not form a dominant sub-series and therefore their resummation is unnecessary. Thus, the prospects for predicting the rate for B -> X_s gamma accurately, given the present energy cut, are promising.

hep-ph↗
arXiv1999

Bounds on Heavy-to-Heavy Mesonic Form Factors

We provide upper and lower bounds on the form factors for B -> D, D^* by utilizing inclusive heavy quark effective theory sum rules. These bounds are calculated to leading order in Lambda_QCD/m_Q and alpha_s. The O(alpha_s^2 beta_0) corrections to the bounds at zero recoil are also presented. We compare our bounds with some of the form factor models used in the literature. All the models we investigated failed to fall within the bounds for the combination of form factors (omega^2 - 1)/(4 omega)|omega h_{A2}+h_{A3}|^2.

hep-ph↗
arXiv1998

J/psi Production at LEP: Revisited and Resummed

We present the leading order differential and total rates for J/ψproduction at LEP. By leading order we mean all terms of the form alpha_s[alpha_s log(M_Z^2/M_{psi}^2)]^n and alpha_s^{n+1} log^l(z^2) log^m(M_Z^2/M_{psi}^2), (l+m=2n-1), in the regions z=2E_psi/M_Z ~ O(1) and z << 1, respectively. In the intermediate region we interpolate using the available data. This resummation eliminates the O[alpha_s(M_psi)/alpha_s(M_Z)]~ 2 theoretical uncertainties in previous calculations. The log(z) resummation results in a suppression of the small z region due to coherent gluon emission. Comparing the zeroth moment with the LEP data we find the value for the effective octet matrix element to be <\hat O_8^ψ(^3S_1)>=0.019 GeV^3. The theoretical uncertainties are substantially smaller than those from Tevatron extractions. Using this value of the octet matrix element we make a prediction for the first moment of the differential rate and find that the resummed differential decay rate is in much better agreement with preliminary data than the color singlet result or the unresummed color octet prediction.

hep-ph↗
arXiv1998

Semileptonic Lambda_b decay to excited Lambda_c baryons at order Lambda_{QCD}/m_Q

Exclusive semileptonic Lambda_b decays to excited charmed Lambda_c baryons are investigated at order Lambda_{QCD}/m_Q in the heavy quark effective theory. The differential decay rates are analyzed for the J^π=1/2^- Lambda_c(2593) and the J^π=3/2^- Λ_c(2625). They receive 1/m_{c,b} corrections at zero recoil that are determined by mass splittings and the leading order Isgur-Wise function. With some assumptions, we find that the branching fraction for Lambda_b decays to these states is 2.5-3.3%. The decay rate to the helicity \pm 3/2 states, which vanishes for m_Q \to \infty, remains small at order Lambda_{QCD}/m_Q since 1/m_c corrections do not contribute. Matrix elements of weak currents between a Lambda_b and other excited Lambda_c states are analyzed at zero-recoil to order Lambda_{QCD}/m_Q. Applications to baryonic heavy quark sum-rules are explored.

hep-ph↗
arXiv1997

Semileptonic B decays to excited charmed mesons

Exclusive semileptonic B decays into excited charmed mesons are investigated at order $Λ_{QCD}/m_Q$ in the heavy quark effective theory. Differential decay rates for each helicity state of the four lightest excited $D$ mesons ($D_1$, $D_2^*$, $D_0^*$, and $D_1^*$) are examined. At zero recoil, $Λ_{QCD}/m_Q$ corrections to the matrix elements of the weak currents can be written in terms of the leading Isgur-Wise functions for the corresponding transition and meson mass splittings. A model independent prediction is found for the slope parameter of the decay rate into helicity zero $D_1$ at zero recoil. The differential decay rates are predicted, including $Λ_{QCD}/m_Q$ corrections with some model dependence away from zero recoil and including order $α_s$ corrections. Ratios of various exclusive branching ratios are computed. Matrix elements of the weak currents between $B$ mesons and other excited charmed mesons are discussed at zero recoil to order $Λ_{QCD}/m_Q$. These amplitudes vanish at leading order, and can be written at order $Λ_{QCD}/m_Q$ in terms of local matrix elements. Applications to $B$ decay sum rules and factorization are presented.

hep-ph↗
arXiv1997

$ψ'$ Polarization due to Color-Octet Quarkonia Production

We calculated the polarization of ψ' due to gg \to Q\bar{Q}[^3P_J^(8)]g \to ψ^λcolor-octet quarkonia production. We find that at low transverse momenta the ψ' is unpolarized due to the contributions proportional to the L=S=0 and L=S=1 color-octet matrix elements. As p_\perp increases, the ψ' mesons become 100% polarized, as predicted by fragmentation calculations. Polarization due to lowest order color-singlet production is also considered, which qualitatively has a similar shape to the color-octet production.

hep-ph↗
arXiv1997

Model independent results for $B\to D_1(2420)\ell\barν$ and $B\to D_2^*(2460)\ell\barν$ at order $Λ_{QCD}/m_{c,b}$

Exclusive semileptonic B decays into $D_1$ and $D_2^*$ mesons are investigated including order $Λ_{QCD}/m_{c,b}$ corrections using the heavy quark effective theory. At zero recoil, the $Λ_{QCD}/m_{c,b}$ corrections can be written in terms of the leading Isgur-Wise function for these transitions, $τ$, and known meson mass splittings. We obtain an almost model independent prediction for the shape of the spectrum near zero recoil, including order $Λ_{QCD}/m_{c,b}$ corrections. We determine $τ(1)$ from the measured $B\to D_1\ell\barν$ branching ratio. Implications for B decay sum rules are discussed.

hep-ph↗
arXiv1997

Perturbative Corrections to a Sum Rule for the Heavy Quark Kinetic Energy

We calculate the perturbative corrections to order α_s^2β_0 to the sum rule derived from the second moment of the time-ordered product of b \to c currents near zero recoil. This sum rule yields a bound on λ_1, the expectation value of the b quark kinetic energy operator inside the B meson. The perturbative corrections significantly weaken the bound relative to the tree level result, yielding λ_1 < -0.15 GeV^2.

hep-ph↗
arXiv1996

Color-Singlet $ψ_Q$ Production at $e^+e^-$ Colliders

We calculate in closed form the complete ${\mathcal O}(α_s^2)$ color-singlet differential cross section for $e^+e^- \to γ^* \to ψ_Q+X$ scattering. The cross section reduces at high energies to a heavy quark fragmentation form. We find that the energy scale at which the approximate fragmentation result becomes reliable exceeds the $ψ_Q$ mass by more than an order of magnitude. We also discuss the color-singlet model's predictions for direct $J/ψ$ angular and energy distributions at CLEO.

hep-ph↗
arXiv1996

Color-octet quarkonia production II

We calculate the lowest order hadronic cross sections for producing colored heavy quark-antiquark pairs in $L=S=0$ and $L=S=1$ configurations. Such $Q\Qbar[{}^1S_0^{(8)}]$ and $Q\Qbar[{}^3P_J^{(8)}]$ states hadronize into $ψ_\Q$ quarkonia at the same order in the NRQCD velocity expansion as previously considered $Q\Qbar[{}^3S_1^{(8)}]$ pairs. Their contributions to prompt Psi and Upsilon production at the Tevatron bring the shapes of theoretical transverse momentum distributions into line with recent CDF measurements. We find that the best fit values for the linear combinations of $Q\Qbar[{}^1S_0^{(8)}]$ and $Q\Qbar[{}^3P_J^{(8)}]$ long distance matrix elements which can be extracted from the data are generally consistent with NRQCD scaling rules.

hep-ph↗