arXiv subjects

Sean Fleming

Publications and source records attributed to Sean Fleming.

At least 55 records · Page 3Linked to original sources

arXiv2001

An effective field theory for collinear and soft gluons: heavy to light decays

We construct the Lagrangian for an effective theory of highly energetic quarks with energy Q, interacting with collinear and soft gluons. This theory has two low energy scales, the transverse momentum of the collinear particles, p_perp, and the scale p_perp^2/Q. The heavy to light currents are matched onto operators in the effective theory at one-loop and the renormalization group equations for the corresponding Wilson coefficients are solved. This running is used to sum Sudakov logarithms in inclusive B -> X_s gamma and B -> X_u \ell nu decays. We also show that the interactions with collinear gluons preserve the relations for the soft part of the form factor for heavy to light decays found by Charles et al., establishing these relations in the large energy limit of QCD.

hep-ph↗

arXiv2001

Resumming the color-octet contribution to radiative Upsilon decay

At the upper endpoint of the photon energy spectrum in Upsilon -> X gamma, the standard NRQCD power counting breaks down and the OPE gives rise to color-octet structure functions. Furthermore, in this kinematic regime large Sudakov logarithms appear in the octet Wilson coefficients. The endpoint spectrum can be treated consistently within the framework of a recently developed effective field theory of collinear and soft particles. Here we show that within this approach the octet structure functions arise naturally and that Sudakov logarithms can be summed using the renormalization group equations. We derive an expression for the resummed energy spectrum and, using a model lightcone structure function, investigate the phenomenological importance of the resummation.

hep-ph↗

arXiv2000

Power counting and effective field theory for charmonium

We hypothesize that the correct power counting for charmonia is in the parameter Lambda_QCD/m_c, but is not based purely on dimensional analysis (as is HQET). This power counting leads to predictions which differ from those resulting from the usual velocity power counting rules of NRQCD. In particular, we show that while Lambda_QCD/m_c power counting preserves the empirically verified predictions of spin symmetry in decays, it also leads to new predictions which include: A hierarchy between spin singlet and triplet octet matrix elements in the J/psi system. A quenching of the net polarization in production at large transverse momentum. No end point enhancement in radiative decays. We discuss explicit tests which can differentiate between the traditional and new theories of NRQCD.

hep-ph↗

arXiv2000

NRQCD Analysis of Bottomonium Production at the Tevatron

Recent data from the CDF collaboration on the production of spin-triplet bottomonium states at the Tevatron p \bar p collider are analyzed within the NRQCD factorization formalism. The color-singlet matrix elements are determined from electromagnetic decays and from potential models. The color-octet matrix elements are determined by fitting the CDF data on the cross sections for Upsilon(1S), Upsilon(2S), and Upsilon(3S) at large p_T and the fractions of Upsilon(1S) coming from chi_b(1P) and chi_b(2P). We use the resulting matrix elements to predict the cross sections at the Tevatron for the spin-singlet states eta_b(nS) and h_b(nP). We argue that eta_b(1S) should be observable in Run II through the decay eta_b -> J/psi + J/psi.

hep-ph↗

arXiv2000

NNLO Corrections to Nucleon-Nucleon Scattering and Perturbative Pions

The 1S0, 3S1, and 3D1 nucleon-nucleon scattering phase shifts are calculated at next-to-next-to-leading order (NNLO) in an effective field theory. Predictions for the 1P1, 3P0, 3P1, 3P2, 1D2, 3D2, and 3D3 phase shifts at this order are also compared with data. The calculations treat pions perturbatively and include the NNLO contributions from order $Q_r^3$ and $Q_r^4$ radiation pion graphs. In the 3S1, 3D1, 3P0, and 3P2 channels we find large disagreement with the Nijmegen partial wave analysis at NNLO. These spin triplet channels have large corrections from graphs with two potential pion exchange which do not vanish in the chiral limit. We compare our results to calculations within the Weinberg approach, and find that in some spin triplet channels the summation of potential pion diagrams seems to be necessary to reproduce the observed phase shifts. In the spin singlet channels the nonperturbative treatment of potential pions does not afford a significant improvement over the perturbative approach.

nucl-th↗

arXiv2000

The NN scattering 3S1-3D1 mixing angle at NNLO

The 3S1-3D1 mixing angle for nucleon-nucleon scattering, epsilon_1, is calculated to next-to-next-to-leading order in an effective field theory with perturbative pions. Without pions, the low energy theory fits the observed epsilon_1 well for momenta less than $\sim 50$ MeV. Including pions perturbatively significantly improves the agreement with data for momenta up to $\sim 150$ MeV with one less parameter. Furthermore, for these momenta the accuracy of our calculation is similar to an effective field theory calculation in which the pion is treated non-perturbatively. This gives phenomenological support for a perturbative treatment of pions in low energy two-nucleon processes. We explain why it is necessary to perform spin and isospin traces in d dimensions when regulating divergences with dimensional regularization in higher partial wave amplitudes.

nucl-th↗

arXiv1999

Associated Production of Upsilon and Weak Gauge Bosons at the Tevatron

We calculate the rate of production of W+Upsilon and Z+Upsilon at the Tevatron. We find the cross sections at a center-of-mass energy of 1.8 TeV to be roughly 450 pb for W+Upsilon and 150 pb for Z+Upsilon. The dominant production mechanism involves the binding of a color-octet b-bbar pair into a P-wave bottomonium state which subsequently decays into Upsilon. The purely leptonic decay modes of Upsilon, W, and Z provide signatures with small backgrounds. In Run I of the Tevatron, the number of events in the purely leptonic decay channels before allowing for detector acceptances and efficiencies should be about 500 for W+Upsilon and about 60 for Z+Upsilon. We conclude that W+Upsilon events may be observable in the Run I data.

hep-ph↗

arXiv1998

Photoproduction of h_c

Using the NRQCD factorization formalism, we calculate the total cross section for the photoproduction of h_c mesons. We include color-octet and color-singlet mechanisms as well as next-to-leading order perturbative QCD corrections. The theoretical prediction depends on two nonperturbative matrix elements that are not well determined from existing data on charmonium production. For reasonable values of these matrix elements, the cross section is large enough that the h_c may be observable at the E831 experiment and at the HERA experiments.

hep-ph↗

arXiv1997

Photoproduction of J/$ψ$ in the forward region

We study the phenomenology of fixed-target elastic $J/ψ$ photoproduction in the NRQCD factorization formalism. Our the goal is to test an essential feature of this formalism --- the color-octet mechanism. We obtain an order-of-magnitude estimate for a certain linear combination of NRQCD color-octet matrix elements. Our estimate is consistent with other empirical determinations and with the $v$-scaling rules of NRQCD.

hep-ph↗

arXiv1997

Leptoproduction of J/psi

We study leptoproduction of $J/ψ$ at large $Q^2$ within the nonrelativistic QCD (NRQCD) factorization formalism. The cross section is dominated by color-octet terms that are of order $α_s$. The color-singlet term, which is of order $α^2_s$, is shown to be a small contribution to the total cross section. We also calculate the tree diagrams for color-octet production at order $α^2_s$ in a region of phase space where there is no leading color-octet contribution. We find that in this regime the color-singlet contribution dominates. We argue that non-perturbative corrections arising from diffractive leptoproduction, higher twist effects, and higher order terms in the NRQCD velocity expansion should be suppressed as $Q^2$ is increased. Therefore, the color-octet matrix elements $< {\cal O}_8^ψ(^1S_0)>$ and $<{\cal O}_8^ψ(^3P_0)>$ can be reliably extracted from this process. Finally, we point out that an experimental measurement of the polarization of leptoproduced $J/ψ$ will provide an excellent test of the NRQCD factorization formalism.

hep-ph↗

arXiv1996

Determining Color-Octet psi-Production Matrix Elements from gamma p and e p Processes

We calculated, within the NRQCD factorization formalism, the leading color-octet contributions to psi production in photon-nucleon and electron-nucleon collisions. The expressions obtained depend on the NRQCD matrix elements and . These matrix elements can be determined by fitting to experimental data. The color-octet contribution to psi photoproduction is in the forward region of phase space, where there may be large corrections to the NRQCD result from higher twist terms. As to psi leptoproduction we point out that the theoretical uncertainties plaguing the photoproduction calculation vanish in the large momentum transfer limit. In this region of phase space the NRQCD formalism should be valid, making psi leptoproduction an ideal laboratory for testing the theory.

hep-ph↗

arXiv1996

NRQCD Prediction for the Polarization of the J/psi Produced from b-decay

Presented at the Meeting of the Division of Particles and Fields, August 1996, in Minneapolis. The work presented here is based upon a recent paper (hep-ph/9608413) done in collaboration with Oscar F. Hernandez, Ivan Maksymyk, and Helene Nadeau. The NRQCD predictions for the polarization of the J/psi produced in b->J/psi+X, as well as the helicity-summed production rate are presented.

hep-ph↗

arXiv1996

NRQCD matrix elements in polarization of J-Psi produced from b-decay

We present the non-relativistic QCD (NRQCD) prediction for the polarization of the J-Psi produced in b to J-Psi + X, as well as the helicity-summed production rate. We propose that these observables provide a means of measuring the three most important color-octet NRQCD matrix elements involved in J-Psi production. Anticipating the measurement of the polarization parameter alpha, we determine its expected range given current experimental bounds on the color-octet matrix elements.

hep-ph↗

arXiv1995

Hadronic $ψ$ production calculated in the NRQCD factorization formalism

The NRQCD factorization formalism of Bodwin, Braaten, and Lepage prescribes how to write quarkonium production rates as a sum of products of short-distance coefficients times non-perturbative long-distance NRQCD matrix elements. We present, in the true spirit of the factorization formalism, a detailed calculation of the inclusive cross section for hadronic $ψ$ production. We find that in addition to the well known {\it color-singlet} production mechanisms, there are equally important mechanisms in which the $c\bar{c}$ pair that forms the $ψ$ is initially produced in a {\it color-octet} state, in either a ${}^3S_1$, ${}^1S_0$, ${}^3P_0$ or ${}^3P_2$ angular-momentum configuration. In our presentation, we emphasize the ``matching'' procedure, which %is the method that allows us to determine the short-distance coefficients appearing in the factorization formula. We also point out how one may systematically include relativistic corrections in these calculations.

hep-ph↗

arXiv1995

Can light Goldstone boson loops counter the `S-argument' against Technicolor?

We examine the oblique correction phenomenology of one-family Technicolor with light pseudo-Goldstone bosons. From loop calculations based on a gauged chiral lagrangian for Technicolor, we are lead to conclude that even though loops with light Goldstone bosons give a negative contribution to $S$ measured at the $Z$-pole, this effect is not sufficiently large to unambiguously counter the `S-argument' against one-family Technicolor. This result cannot be guessed \apriori, but must be explicitly calculated. Our analysis entails an extended version of the $STU$ oblique parametrization of Peskin and Takeuchi. In principle, this extended formalism ($STUVWX$) must be used when there are light new particles in loops.

hep-ph↗

arXiv1995

QCD Radiative Corrections to the Leptonic Decay Rate of the B_c Meson

The QCD radiative corrections to the leptonic decay rate of the $B_c$ meson are calculated using the formalism of nonrelativistic QCD (NRQCD) to separate short-distance and long-distance effects. The $B_c$ decay constant is factored into a sum of NRQCD matrix elements each multiplied by a short-distance coefficient. The short-distance coefficient for the leading matrix element is calculated to order $α_s$ by matching a perturbative calculation in full QCD with the corresponding perturbative calculation in NRQCD. This short-distance correction decreases the leptonic decay rate by approximately $15\%$.

hep-ph↗

arXiv1994

Color-Octet Fragmentation and the psi' Surplus at the Tevatron

The production rate of prompt $ψ'$'s at large transverse momentum at the Tevatron is larger than theoretical expectations by about a factor of 30. As a solution to this puzzle, we suggest that the dominant $ψ'$ production mechanism is the fragmentation of a gluon into a $c \bar c$ pair in a pointlike color-octet S-wave state, which subsequently evolves nonperturbatively into a $ψ'$ plus light hadrons. The contribution to the fragmentation function from this process is enhanced by a short-distance factor of $1/α_s^2$ relative to the conventional color-singlet contribution. This may compensate for the suppression by $v^4$, where $v$ is the relative momentum of the charm quark in the $ψ'$. If this is indeed the dominant production mechanism at large $p_T$, then the prompt $ψ'$'s that are observed at the Tevatron should almost always be associated with a jet of light hadrons.

hep-ph↗

arXiv1994

Fragmentation production of J/psi and psi' at the Tevatron

We present a calculation of the charm and gluon fragmentation contributions to inclusive $J/ψ$ and $ψ'$ production at large transverse momentum at the Tevatron. For $ψ$ production, we include both fragmentation directly into $ψ$ and fragmentation into $χ_c$ followed by the radiative decay $χ_c \to ψ+ γ$. We find that fragmentation overwhelms the leading-order mechanisms for prompt $ψ$ production at large $p_T$, and that the dominant contributions come from fragmentation into $χ_c$. Our results are consistent with recent data on $ψ$ production from the CDF and D0 experiments. In the case of prompt $ψ'$ production, the dominant mechanism at large $p_T$ is charm fragmentation into $ψ'$. We find serious disagreement between our theoretical predictions and recent $ψ'$ data from the Tevatron.

hep-ph↗