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Xavier Garcia i Tormo

Publications and source records attributed to Xavier Garcia i Tormo.

30 records · Page 2Linked to original sources
arXiv2010

Factorization of low-energy gluons in exclusive processes

We outline a proof of factorization in exclusive processes, taking into account the presence of soft and collinear modes of arbitrarily low energy, which arise when the external lines of the process are taken on shell. Specifically, we examine the process of e^+e^- annihilation through a virtual photon into two light mesons. In an intermediate step, we establish a factorized form that contains a soft function that is free of collinear divergences. In contrast, in soft-collinear effective theory, the low-energy collinear modes factor most straightforwardly into the soft function. We point out that the cancellation of the soft function, which relies on the color-singlet nature of the external hadrons, fails when the soft function contains low-energy collinear modes.

hep-ph↗
arXiv2010

Factorization in exclusive quarkonium production

We present factorization theorems for two exclusive heavy-quarkonium production processes: production of two quarkonia in e^+e^- annihilation and production of a quarkonium and a light meson in B-meson decays. We describe the general proofs of factorization and supplement them with explicit one-loop analyses, which illustrate some of the features of the soft-gluon cancellations. We find that violations of factorization are generally suppressed relative to the factorized contributions by a factor v^2m_c/Q for each S-wave charmonium and a factor m_c/Q for each L-wave charmonium with L>0. Here, v is the velocity of the heavy quark or antiquark in the quarkonium rest frame, Q=sqrt{s} for e^+e^- annihilation, Q=m_B for B-meson decays, sqrt{s} is the e^+e^- center-of-momentum energy, m_c is the charm-quark mass, and m_B is the B-meson mass. There are modifications to the suppression factors if quantum-number restrictions apply for the specific process.

hep-ph↗
arXiv2009

The QCD static energy at NNNLL

We compute the static energy of QCD at short distances at next-to-next-to-next-to leading-logarithmic accuracy in terms of the three-loop singlet potential. By comparing our results with lattice data we extract the value of the unknown piece of the three-loop singlet potential.

hep-ph↗
arXiv2008

Factorization theorems for exclusive heavy-quarkonium production

We outline the proofs of the factorization theorems for exclusive two-body charmonium production in B-meson decay and e^+e^- annihilation to all orders in perturbation theory in quantum chromodynamics. We find that factorized expressions hold up to corrections of order m_c/m_b in B-meson decay and corrections of order m_c^2/s in e^+e^- annihilation, where m_c is the charm-quark mass, m_b is the bottom-quark mass, and root-s is the e^+e^- center-of-momentum energy.

hep-ph↗
arXiv2007

Extraction of alpha_s from radiative Upsilon(1S) decays

We improve on a recent determination of alpha_s from Gamma(Upsilon(1S)-->X gamma)/Gamma(Upsilon(1S)-->X) with CLEO data by taking into account color octet contributions and avoiding any model dependence in the extraction. We obtain alpha_s(M_Upsilon(1S))= 0.184+0.015-0.014, which corresponds to alpha_s(M_Z)=0.119+0.006-0.005.

hep-ph↗
arXiv2007

The logarithmic contribution to the QCD static energy at NNNNLO

Using pNRQCD and known results for the field strength correlator, we calculate the ultrasoft contribution to the QCD static energy of a quark-antiquark pair at short distances at NNNNLO in alpha_s. At the same order, this provides the logarithmic terms of the singlet static potential in pNRQCD and the log alpha_s terms of the static energy.

hep-ph↗
arXiv2007

Inclusive radiative decays of charmonium

We discuss the theoretical status of inclusive radiative decays of charmonium, with a particular emphasis on the QCD description of the photon spectrum, where progress has occurred in recent years. We also comment on the possible extraction of alpha_s (M_J/psi) and on the possibility to gain important information on the nature of J/psi and psi (2S).

hep-ph↗
arXiv2006

Radiative decays and the nature of heavy quarkonia

We argue that the photon spectra in radiative decays of various heavy quarkonium states provide important information on their nature. If two of these states are in the strong coupling regime, we are able to produce a parameter-free model independent formula, which holds at next-to-leading order and includes both direct and fragmentation contributions. When the formula is checked against recent CLEO data it favors Upsilon (2S) and Upsilon (3S) in the strong coupling regime and disfavors Upsilon (1S) in it.

hep-ph↗
arXiv2005

Semi-inclusive radiative decays of Upsilon 1S

We discuss in detail the photon spectrum of radiative Upsilon 1S decays taking into account a number of results that have recently appeared in the literature. In particular, we show how to consistently combine expressions which are valid in the upper end-point region, where NRQCD factorization breaks down, with those of the central region, where NRQCD factorization holds. An excellent description of data is achieved, but theoretical errors are large.

hep-ph↗
arXiv2004

Octet contributions in radiative upsilon decays

We analyze the end-point region of the photon spectrum in semi-inclusive radiative decays of very heavy quarkonium (m alpha_s^2 >> Lambda_QCD). The S- and P-wave octet shape functions are calculated. When they are included in the analysis of the photon spectrum of the Upsilon (1S) system the agreement with data becomes excellent.

hep-ph↗
arXiv2004

Soft, collinear and non-relativistic modes in radiative decays of very heavy quarkonium

We analyze the end-point region of the photon spectrum in semi-inclusive radiative decays of very heavy quarkonium (m alpha_s^2 >> Lambda_QCD). We discuss the interplay of the scales arising in the Soft-Collinear Effective Theory, m, m(1-z)^{1/2} and m(1-z) for z close to 1, with the scales of heavy quarkonium systems in the weak coupling regime, m, m alpha_s and m alpha_s^2. For 1-z \sim alpha_s^2 only collinear and (ultra)soft modes are seen to be relevant, but the recently discovered soft-collinear modes show up for 1-z << alpha_s^2. The S- and P-wave octet shape functions are calculated. When they are included in the analysis of the photon spectrum of the Upsilon (1S) system, the agreement with data in the end-point region becomes excellent. The NRQCD matrix elements <1^3S_1| O_8(^1S_0) | 1^3S_1> and <1^3S_1| O_8(^3P_J) | 1^3S_1> are also obtained.

hep-ph↗