arXiv subjects

Alexander Khodjamirian

Publications and source records attributed to Alexander Khodjamirian.

44 records · Page 3Linked to original sources

arXiv2002

Exploring Light-Cone Sum Rules for Pion and Kaon Form Factors

We analyze the higher-twist effects and the SU(3)-flavour symmetry breaking in the correlation functions used to calculate form factors of pseudoscalar mesons in the QCD light-cone sum rule approach. It is shown that the Ward identities for these correlation functions yield relations between twist-4 two- and three-particle distribution amplitudes. In addition to the relations already obtained from the QCD equations of motions, we have found a new one. With the help of these relations, the twist-4 contribution to the light-cone sum rule for the pion electromagnetic form factor is reduced to a very simple form. Simultaneously, we correct a sign error in the earlier calculation. The updated light-cone sum rule prediction for the pion form factor at intermediate momentum transfers is compared with the recent Jefferson Lab data. Furthermore, from the correlation functions with strange-quark currents the kaon electromagnetic form factor and the $K\to π$ weak transition form factors are predicted with $O(m_s)\sim O(m_K^2)$ accuracy.

hep-ph↗

arXiv2001

Counting contact terms in $B\to Vγ$ decays

We clarify the origin and cancellation of contact terms in the weak annihilation amplitudes contributing to $B\to V γ$. It is demonstrated that the photon emission from the final-state quarks vanishes in the chiral limit of massless quarks. The contact terms in the QCD light-cone sum rule evaluation of the weak annihilation amplitudes are also discussed.

hep-ph↗

arXiv2001

Duality in Semi-Exclusive Processes

Semi-exclusive processes like γp \to π^+ Y are closely analogous to DIS, ep \to eX, in the limit where the momentum transfer |t| to the pion and the mass of the inclusive system Y are large but still much smaller than the total CM energy. We apply Bloom-Gilman duality to this semi-exclusive process. The energy dependence of the γp \to π^+ n cross section given by semi-local duality agrees with data for moderate values of |t|, but its normalization is underestimated by about two orders of magnitude. This indicates that rather high momentum transfers are required for the validity of PQCD in the hard subprocess γu \to π^+ d. In the case of Compton scattering γp \to γp the analogous discrepancy is about one order of magnitude. In electroproduction the virtuality of the incoming photon can be used to directly measure the hardness of the subprocess.

hep-ph↗

arXiv2001

QCD Sum Rules for Heavy Flavour Physics

Uses of QCD sum rules for heavy flavoured hadrons are discussed. "Standard" applications such as the determination of the $b$, $c$ quark masses, the calculation of $f_B$, $f_D$ and of the heavy-to-light form factors are overviewed. Furthermore, a new approach to calculate the $B\to ππ$ hadronic matrix elements from QCD light-cone sum rules is described.

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arXiv2001

$B \to ππ$ Decay in QCD

A new method is suggested to calculate the $B\to ππ$ hadronic matrix elements from QCD light-cone sum rules. To leading order in $α_s$ and $1/m_b$, the sum rule reproduces a factorizable matrix element, in accordance with the prediction of the QCD factorization approach. Whereas the QCD factorization can only take into account the nonfactorizable corrections induced by hard-gluon exchanges, the method suggested here also allows a systematic inclusion of soft-gluon effects. In this paper, I concentrate on the latter aspect and present a calculation of the nonfactorizable soft-gluon exchange contributions to $B\to ππ$. The result, including twist 3 and 4 terms, is suppressed by one power of $1/m_b$ with respect to the factorizable amplitude. Despite its numerical smallness, the soft effect is at the same level as the $O(α_s)$ correction to the QCD factorization formula for $B\to ππ$. The method suggested here can be applied to matrix elements of different topologies and operators and to various other B-decay channels. I also comment on the earlier applications of QCD sum rules to nonleptonic decays of heavy mesons.

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arXiv2000

QCD Sum Rules, a Modern Perspective

An introduction to the method of QCD sum rules is given for those who want to learn how to use this method. Furthermore, we discuss various applications of sum rules, from the determination of quark masses to the calculation of hadronic form factors and structure functions. Finally, we explain the idea of the light-cone sum rules and outline the recent development of this approach.

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arXiv1998

Form Factors of $γ^{}ρ\toπ$ and $γ^{}γ\toπ^0$ Transitions and Light-Cone Sum Rules

The method of light-cone QCD sum rules is applied to the calculation of the form factors of $γ^*ρ\to π$ and $γ^*γ\to π^0$ transitions. We consider the dispersion relation for the $γ^*(Q^2)γ^*(q^2) \to π^0$ amplitude in the variable $q^2$. At large virtualities $q^2$ and $Q^2$, this amplitude is calculated in terms of light-cone wave functions of the pion. As a next step, the light-cone sum rule for the $γ^*(Q^2) ρ\to π$ form factor is derived. This sum rule, together with the quark-hadron duality, provides an estimate of the hadronic spectral density in the dispersion relation. Finally, the $γ^*(Q^2)γ\to π^0$ form factor is obtained taking the $q^2=0$ limit in this relation. Our predictions are valid at $Q^2 \geq 1 {GeV}^2$ and have a correct asymptotic behaviour at large $Q^2$.

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arXiv1994

Heavy-to-Light Form Factors from QCD Sum Rules on the Light-Cone

We discuss the calculation of the $B \rightarrow π$ and $D \rightarrow π$ form factors based on an expansion in terms of pion wave functions on the light-cone with increasing twist and QCD sum rule methods. The results are compared with predictions of conventional QCD sum rules and other approaches.

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