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Ignazio Scimemi

Publications and source records attributed to Ignazio Scimemi.

70 records · Page 4Linked to original sources
arXiv2010

The T-Wilson Line

We argue that soft-collinear effective theory, as is currently formulated, lacks an ingredient which is essential for obtaining a gauge invariant definitions of certain non-perturbative matrix elements relevant for semi-inclusive high-energy processes. Those processes and the companion cross-sections are to be analyzed and measured at the Large Hadron Collider. The missing feature is a gauge link constructed only from the transverse component of the gluon field denoted by the "$T$-Wilson line"'. It is neither soft nor collinear. As such it is a new feature of the theory.

hep-ph↗
arXiv2009

R-evolution: Improving perturbative QCD

Perturbative QCD results in the MSbar scheme can be dramatically improved by switching to a scheme that accounts for the dominant power law dependence on the factorization scale in the operator product expansion. We introduce the ``MSR scheme'' which achieves this in a Lorentz and gauge invariant way. The MSR scheme has a very simple relation to MSbar, and can be easily used to reanalyze MSbar results. Results in MSR depend on a cutoff parameter R, in addition to the mu of MSbar. R variations can be used to independently estimate i) the size of power corrections, and ii) higher order perturbative corrections (much like mu in MSbar). We give two examples at three-loop order, the ratio of mass splittings in the B*-B and D*-D systems, and the Ellis-Jaffe sum rule as a function of momentum transfer Q in deep inelastic scattering. Comparing to data, the perturbative MSR results work well even for Q ~ 1 GeV, and the size of power corrections is reduced compared to those in MSbar.

hep-ph↗
arXiv2009

The R-evolution of QCD matrix elements

Perturbation series in QCD are generally asymptotic and suffer from so-called infrared renormalon ambiguities. In the context of the standard operator product expansion in MS-bar these ambiguities are compensated by matrix elements of higher dimension operators, but the procedure can be difficult to control due to large numerical cancellations. Explicit subtractions for matrix elements and coefficients, depending on a subtraction scale R, can avoid this problem. The appropriate choice for R in the Wilson coefficients can widely vary for different processes. In this talk we discuss renormalization group evolution with the scale R, and show that it sums large logarithms in the difference of processes with widely different R's. We also show that the solution of the R-evolution equations can be used to recover the all order asymptotic form of the singularities in the Borel transform of the perturbative series. For the normalization of these singularities we obtain a quickly converging sum rule that only needs the known perturbative coefficients as an input. This sum rule can be used as a novel test for renormalon ambiguities without replying on the large-beta_0 approximation.

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arXiv2008

Infrared Renormalization Group Flow for Heavy Quark Masses

A short-distance heavy quark mass depends on two parameters, the renormalization scale mu controlling the absorption of ultraviolet fluctuations into the mass, and a scale R controlling the absorption of infrared fluctuations. 1/R can be thought of as the radius for perturbative corrections that build up the mass beyond its point-like definition in the pole scheme. Treating R as a variable gives a renormalization group equation. We argue that the sign of this anomalous dimension is universal: increasing R to add IR modes decreases m(R). The flow improves the stability of conversions between mass schemes, allowing us to avoid large logs and the renormalon. The flow in R can be used to study IR renormalons without using bubble chains, and we use it to determine the coefficient of the LambdaQCD renormalon ambiguity of the pole mass with a convergent sum-rule.

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arXiv2008

Two-loop Jet-Function and Jet-Mass for Top Quarks

We compute the two-loop heavy quark jet-function in the heavy quark limit. This is one of the key ingredients in next-to-next-to-leading order (NNLO) and next-to-next-to-leading-log order (NNLL) computations of the invariant mass distribution of top-jets at a future e+e- collider. The shape of the top invariant mass distribution is affected by large logs which we compute at NNLL order. Exploiting the non-abelian exponentiation theorem, a definition of the top jet-mass is given which is transitive and whose renormalization group evolution is determined by the cusp-anomalous dimension to all orders in perturbation theory. Relations of the jet-mass to the pole, MSbar, and 1S masses are presented at two-loop order.

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arXiv2008

The top quark jet-function at two loops

Far above threshold the production process e^+e^-to t t-bar can be analyzed using effective field theories. In this talk we consider the invariant mass distribution of top-jets and report about our computation of the two-loop heavy quark jet-function. This is a key part of a next-to-next-to-leading order analysis, and already allows for a resummation of all large logs which effect the shape of the top-invariant mass distribution at next-to-next-to-leading log order. A top-mass scheme is defined which is suitable for measurements involving jets, and whose anomalous dimension is determined by the cusp-anomalous dimension to all orders in perturbation theory.

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arXiv2006

K -> 3 pi Final State Interactions at NLO in CHPT and Cabibbo's Proposal to Measure a_0-a_2

We present the analytical results for the K -> 3 pi final state interactions at next-to-leading order (NLO) in CHPT. We also study the recent Cabibbo's proposal to measure the pi-pi scattering lenghts combination a_0-a_2 from the cusp effect in the pi^0-pi^0 energy spectrum at threshold for K^+ -> pi^0 pi^0 pi^+ and K_L -> pi^0 pi^0 pi^0$, and give the relevant formulas to describe it at NLO. For that, we use the NLO CHPT expression to fit the real part of K -> 3 pi to data while the pi-pi scattering lenghts are treated non-perturbatively. Using them, we make a quantitative estimate of the theoretical uncertaintity of the a_0-a_2 determination at NLO in our approach and obtain that it is not smaller than 5 % if added quadratically and 7 % if linearly for K^+ -> pi^0 pi^0 pi^+. One gets similar theoretical uncertainties if the neutral K_L -> pi^0 pi^0 pi^0 decay data below threshold are used instead. For this decay, there are very large theoretical uncertainties above threshold due to cancellations and data above threshold cannot be used to get the scattering lenghts. All the numbers we present are in the isospin limit apart of two-pion phase space factors which are physical. We compare our results for the cusp effect with Cabibbo and Isidori's results and discuss the differences and agreements. We also comment on the apperance of the singularity at the K -> 3 pi pseudo-threshold s=(m_K-m_pi)^2 in the discontinuity that defines the cusp.

hep-ph↗
arXiv2006

Measuring the a0-a2 pion scattering lengths through K to 3 pi decays

We discuss the recent Cabibbo's proposal to measure the pion-pion scattering lengths combination a0-a2 from the cusp effect in the pi0-pi0 energy spectrum at threshold for K+ to (pi0 pi0 pi+) and KL to (pi0 pi0 pi0). We estimate the theoretical uncertainty of the a0-a2 determination at NLO in our approach and obtain that it is not smaller than 5% for K+ to (pi0 pi0 pi+). One gets similar theoretical uncertainties if the neutral KL to (pi0 pi0 pi0) decay data below threshold are used instead. For this decay, there are very large theoretical uncertainties above threshold due to cancellations and data above threshold cannot be used to get the scattering lengths.

hep-ph↗
arXiv2005

Charged Kaon K^+ --> 3 pi CP Violating Asymmetries vs epsilon'_K / epsilon_K

We present the next-to-leading order full results in Chiral Perturbation Theory for the charged Kaon K --> 3 pi slope g CP violating asymmetries. We discuss the constraints that a measurement of these asymmetries would impose on the Standard Model results of epsilon_K' and search for new physics. We also study the kind of information that such measurement can provide on Im G_8, Im (e^2 G_E) and higher order weak couplings.

hep-ph↗
arXiv2004

Charged Kaon K -> 3 pi CP Violating Asymmetries

First full next-to-leading order analytical results in Chiral Perturbation Theory for the charged Kaon K -> 3 pi slope g and decay rates CP-violating asymmetries are presented. We discuss the constraints that a measurement of these asymmetries would impose on the Standard Model calculations of epsilon_K' and the kind of information it can provide on Im G_8, Im (e^2 G_E) and higher order weak couplings.

hep-ph↗
arXiv2003

Hadronic processes and electromagnetic corrections

The inclusion of electromagnetism in a low energy effective theory is worth further study in view of the present high precision experiments (muon g-2, pi_0->2 gamma, tau decays, etc.). In particular in many applications of chiral perturbation theory, one has to purify physical matrix elements from electromagnetic effects. The theoretical problems that I want to point out here are following: the splitting of a pure QCD and a pure electromagnetic part in a hadronic process is model dependent: is it possible to parametrise in a clear way this splitting? What kind of information (scale dependence, gauge dependence,..) is actually included in the parameters of the low energy effective theory? I will attempt to answer these questions introducing a possible convention to perform the splitting between strong and electromagnetic parts in some examples.

hep-ph↗
arXiv2003

Charged Kaon K \to 3 pi CP Violating Asymmetries at NLO in CHPT

We give the first full next-to-leading order analytical results in Chiral Perturbation Theory for the charged Kaon K \to 3 pi slope g and decay rates CP-violating asymmetries. We have included the dominant Final State Interactions at NLO analytically and discussed the importance of the unknown counterterms. We find that the uncertainty due to them is reasonable just for Δg_C, i.e. the asymmetry in the K^+ \to pi^+ pi^+ pi^- slope g; we get Δg_C = -(2.4 +- 1.2) 10^{-5}. The rest of the asymmetries are very sensitive to the unknown counterterms. In particular, the decay rate asymmetries can change even sign. One can use this large sentivity to get valuable information on those counterterms and on Im(G_8) coupling --very important for the CP-violating parameter epsilon'_K-- from the eventual measurement of these asymmetries. We also provide the one-loop O(e^2 p^2) electroweak octet contributions for the neutral and charged Kaon K \to 3 pi decays.

hep-ph↗
arXiv2003

CP-Violating Asymmetries in K^+ -> 3 pi in the Standard Model

We update the CP-violating asymmetries in K^+ -> 3 pi. In particular we study Delta g_C and Delta g_N -the asymmetries in the slope g. We emphasize its complementarity to the direct CP-violation parameter epsilon_K' and the large sensitivity to the size of the imaginary part of the octet coupling Im G_8 at lowest order in CHPT. We also give the prospects of a full calculation at next-to-leading order of this and other CP-violating asymmetries which will be presented elsewhere.

hep-ph↗
arXiv2000

CP Violation in rare semileptonic B decays and supersymmetry

We study the effect of new flavor changing SUSY phases arising in the squark mass matrix in semileptonic decays B->Xs ll and B->K(*) ll (l=e,mu). SUSY effects are parametrized using the mass insertion approximation formalism. Constraints on SUSY contributions coming from other processes (e.g. b->s gamma, B->K* ll) are taken into account. Chargino and gluino contributions to photon and Z-mediated decays are included and non-perturbative corrections are considered. We study the correlation between the CP (and forward--backward) asymmetries and the expected value of the inclusive branching ratio. Several possible scenarios are distinguished and discussed according to the mass insertion that is considered.

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arXiv1999

The Supersymmetric Flavor Problem for Heavy First-Two Generation Scalars at Next-to-Leading Order

We analyze in detail the constraints on SUSY-model parameters obtained from K-Kbar mixing in the hypothesis of a splitted SUSY spectrum. FCNC contributions from gluino-squark-quark interactions are studied in the so-called mass insertion approximation. We present boundaries on mass insertions and on SUSY mass scales. We improve previous results by including the NLO-QCD corrections to Delta S=2 effective Hamiltonian and the complete set of B-parameters for the evaluation of hadronic matrix elements. A full set of magic-numbers, that can be used for further analyses of these models, is also given. We find that the inclusion of NLO-QCD corrections and the B-parameters change the results obtained at LO and in the Vacuum Insertion Approximation by an amount of about 25-35%.

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