arXiv subjects

Ekaterina Christova

Publications and source records attributed to Ekaterina Christova.

At least 19 recordsLinked to original sources

arXiv2018

Consistency tests for the extraction of the Boer-Mulders and Sivers functions

At present, the Boer-Mulders (BM) function for a given quark flavour is extracted from data on semi-inclusive deep inelastic scattering (SIDIS) using the simplifying assumption that it is proportional to the Sivers function for that flavour. In a recent paper we suggested that the consistency of this assumption could be tested using information on so-called difference asymmetries, i.e. the difference between the asymmetries in the production of particles and their anti-particles. In this paper, using the SIDIS COMPASS deuteron data on the $\langle\cosϕ_h\rangle$, $\langle\cos 2 ϕ_h\rangle$ and Sivers difference asymmetries, we carry out two independent consistency tests of the assumption of proportionality, but here applied to the sum of the valence-quark contributions. We find that such an assumption is compatible with the data. We also show that the proportionality assumptions made in the existing parametrizations of the BM functions are not compatible with our analysis, which suggests that the published results for the Boer-Mulders functions for individual flavours are unreliable. The $\langle\cosϕ_h\rangle$ and $\langle\cos 2 ϕ_h\rangle$ asymmetries receive contributions also from the, in principle, calculable Cahn effect. We succeed in extracting the Cahn contributions from experiment (we believe for the first time) and compare with their calculated values, with interesting implications.

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arXiv2017

Tests for the extraction of Boer-Mulders functions

At present, the Boer-Mulders (BM) functions are extracted from asymmetry data using the simplifying assumption of their proportionality to the Sivers functions for each quark flavour. Here we present two independent tests for this assumption. We subject COMPASS data on semi-inclusive deep inelastic scattering on the $\langle\cosϕ_h\rangle$, $\langle\cos 2ϕ_h\rangle$ and Sivers asymmetries to these tests. Our analysis shows that the tests are satisfied with the available data if the proportionality constant is the same for all quark flavours, which does not correspond to the flavour dependence used in existing analyses. This suggests that the published information on the BM functions may be unreliable. The $\langle\cosϕ_h\rangle$ and $\langle\cos 2 ϕ_h\rangle$ asymmetries receive contributions also from the, in principle, calculable Cahn effect. We succeed in extracting the Cahn contributions from experiment (we believe for the first time) and compare with their calculated values, with interesting implications.

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arXiv2016

The problem of kinematic mass corrections for unpolarized semi-inclusive deep inelastic scattering

Miraculously, target mass corrections for inclusive deep inelastic scattering can be calculated exactly. On the contrary, there does not exist a consistent derivation of kinematic hadron mass corrections for semi-inclusive deep inelastic scattering (SIDIS). Recently this has become of topical interest, since there is a significant difference between the measured HERMES and COMPASS pion and kaon multiplicities, which cannot be explained as a consequence of evolution in $Q^2$, and it has been suggested that the difference can be understood if kinematic hadron mass corrections are taken into account. We explain why this argument is incorrect.

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arXiv2015

Tests for Sivers, Boer-Mulders and transversity distributions in difference cross sections in SIDIS

A major experimental program is presently underway to determine the Sivers, Boer-Mulders and transversity distributions, vital for understanding the internal structure of the nucleon. To this end we consider the Sivers, Boer-Mulders and transversity azimuthal asymmetries of the difference cross sections of hadrons with opposite charges in SIDIS reactions with unpolarized and transversely polarized target l+N\to l'+h+X, h=π^\pm, K^\pm, h^\pm. We show that on deuteron target these asymmetries are particularly simple and determine the sum of the valence-quark Q_V=u_V+d_V transverse momentum dependent distributions without any contributions from the strange or other sea-quark functions. At present, data on these asymmetries are presented for the integrated asymmetries i.e. the x_B- and z_h-dependent asymmetries. If data are available in small bins in Q^2, so that Q^2-dependence can be neglected, these expressions simplify dramatically leading to remarkably simple and powerful tests of the simplifying assumptions used in extracting these functions from the data.

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arXiv2015

Tests of the extraction of the Sivers, Boer-Mulders and transversity distributions in SIDIS reactions

A major experimental program is presently underway worldwide to determine the fundamental non-perturbative functions, the Sivers, Boer-Mulders and transversity distributions, which are vital for an understanding of the internal structure of the nucleon. However, at present, many simplifying assumptions are used in extracting these functions from the data. We demonstrate that if the binning of the data in Q^2 is small enough so that Q^2-evolution can be neglected inside a bin, then one can obtain stringent tests of these assumptions. Failure to satisfy these tests implies that the presently extracted non-perturbative functions are unreliable. To this end we consider the measurement of the Sivers, Boer-Mulders and transversity difference asymmetries for hadrons with opposite charges in SIDIS reactions with unpolarized and transversely polarized deuteron and proton targets: l+N\to l'+h+X, h=π^\pm ,K^\pm ,h^\pm. Utilizing only charge and isospin invariance, and applying the above mentioned simplifying assumptions we obtain several testable relations amongst the measured asymmetries. If these tests are satisfied then the measured asymmetries determine two different combinations of the valence-quark transverse momentum dependent distributions, which can be determined separately without any contributions from the strange and other sea-quarks.

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arXiv2015

Once more on the W-loop contribution to the Higgs decay into two photons

The imaginary part of the Feynman amplitude of the $W$-loop contribution to the Higgs decay into two gammas (viewed as a function of the square of the off shell Higgs momentum) is finite and unambiguous. It is presented as the product of an invariant amplitude ${\cal A}$ times a bilinear in the components of the (on shell) photon momenta factor which takes the Ward identity into account. The unsubtracted dispersion integral of ${\cal A}$ is convergent and reproduces the amplitude computed by R. Gastmans, S.L. Wu and T.T. Wu [GWW]. In particular, the decoupling theorem, criticized as an unjustified assumption in a subsequent paper [SVVZ12], is obtained as a corollary. By contrast with the currently used value (computed in [SVVZ]) our calculation provides a smaller Higgs decay rate into 2 photons than the currently observed. If accepted as a true prediction of the Standard Model, it would be a first indication for the need of a New Physics.

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arXiv2014

The difference cross sections of unpolarized SIDIS with transverse momentum dependence

Previously we showed that, based only on C and SU(2) invariance, the difference cross sections of hadrons with opposite charge in SIDIS e+N\to l+h+X is expressed solely in terms of the valence-quark densities and certain non-singlet combinations of FFs. This allowed to determine these quantities in a model independent way. Now we extend this approach to processes when the transverse momentum of the final hadron is measured as well. We show that the difference cross sections of unpolarized SIDIS on proton and deuterium targets, d\s_N^{h^+-h^-}, d\s_N^{π^+-π^-} and d\s_N^{K^+-K^-}, are expressed solely in terms of the TMD unpolarized valence quark densities and FFs, and the valence-quark Boer-Mulders and Collins functions. This allows to determine them separately and study flavour dependence of the quark transverse momentum. Measurements on deuterium target, d\s_d^{h^+-h^-}, d\s_d^{π^+-π^-} and d\s_d^{K^+-K^-}, provide 3 independent measurements for the sum of the TMD valence quark densities and Boer-Mulders functions: (u_{1,V}+d_{1,V}) and (h_{1,uV}^\perp + h_{1,dV}^\perp).

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arXiv2013

Polarization of the final nucleon in quasi-elastic neutrino scattering and the axial form factor of the nucleon

Measurements of the polarization of the final proton in elastic e-p scattering drastically changed our knowledge about the electromagnetic form factors of the proton. Here we present our results of the calculation of the polarization of the final nucleon in charged current quasi-elastic neutrino nucleon scattering. Relations which connect the axial form factor with the polarization, the cross section and the electromagnetic form factors of the nucleon are derived. Measurements of the polarization of the nucleon in the high-statistics short baseline neutrino experiments (or in near detectors of long baseline experiments) could provide important information on the axial form factor of the nucleon.

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arXiv2008

CP violation in associated production of a charged Higgs boson and a top quark at the LHC

We study effects of CP violation in an associated production of a charged Higgs boson and a top quark at the LHC: $pp \to tH^\pm +X$. We calculate the CP violating asymmetry between the total cross section for $H^+$ and $H^-$ production at next-to-leading order in the MSSM, and perform a detailed numerical analysis. In the production only the asymmetry is of the order of 20%. The asymmetry in the production and any subsequent decay of an on-shell charged Higgs boson is to a good approximation the sum of the asymmetry in the production and the asymmetry in the decay. We consider subsequent decays of $H^\pm $ to $tb$, $ν_ττ^\pm$ and $Wh^0$. In the case of subsequent $H^\pm \to tb$ decay, the $W^\pm - H^\pm$ self energy contributions from the production and the decay cancel. However, the remaining effect, mainly due to CP violating box graphs with gluino can go up to $\sim 13$%.

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arXiv2008

CP violation in charged Higgs boson decays

CP violation in H^\pm decays into the three possible decay modes into ordinary particles, 1) H^\pm \to tb, 2) H^\pm \to ντand 3) H^\pm \to W^\pm h^0 is considered. Analytic expressions and numerical results for the CP violating decay rate asymmetries in the MSSM are obtained. Increasing \tanβthe asymmetries for the fermionic decays, H^\pm \to tb and H^\pm \to ντ, decrease and it increases for H^\pm \to W^\pm h^0. The asymmetry of H^\pm \to tb is most sensitive to the phase of $A_t and can go up to 20%, the asymmetries of 2) and 3) depend mainly on the phases of A_τand M_1. The asymmetry of 2) is smaller than 0.5% and of 3) can reach up to 2%.

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arXiv2008

Towards a model independent approach to fragmentation functions

We show that the difference cross sections in unpolarized SIDIS e+N\to e+h+X and pp hadron production p+p\to h+X determine independently in a model independent way, in any order in QCD, the two FFs: D_u^{h-\bar h} and D_d^{h-\bar h}, h=π^\pm, K^\pm or a sum over charged hadrons. If both K^\pm and K_s^0 are measured, then e+e-\toK+X, e+N\to e+K+X and p+p\to K+X present independent measurements of just one FF: D_{u-d}^{K^++K^-}. The above results allow to test the existing parametrizations, obtained with various different assumptions about the FFs, and to test the $Q^2$ evolution and factorization.

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arXiv2008

About S-\bar s, ΔS-Δ\bar s and D_d^{K+ - K-} in K^\pm Production in Sidis

We consider semi-inclusive unpolarized DIS for the production of charged Kaons and the different possibilities, both in LO and NLO, to test the conventionally used assumptions s-\bar s=0, Δs-Δ\bar s=0 and D_d^{K+ - K-}=0. The considered tests have the advantage that they do not require any knowledge of the fragmentation functions.

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arXiv2007

On Kaon production in e+e- and Semi-inclusive DIS reactions

We consider semi-inclusive unpolarized DIS for the production of charged kaons and the different possibilities to test the conventionally used assumptions s-\bar=0 and D_d^{K^+-K^-}=0. The considered tests have the advantage that they do not require any knowledge of the fragmentation functions. We also show that measurements of both charged and neutral kaons would allow the determination of the kaon FFs D_q^{K^++K^-} solely from SIDIS measurements, and discuss the comparison of (D_u-D_d)^{K^+-K^-} obtained independently in SIDIS and e+e- reactions. All analysis are performed in LO and NLO in QCD. The feasibility of the tests to HERMES SIDIS data is considered.

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arXiv2007

On Non-singlets in Kaon Production in Semi-inclusive DIS reactions

We consider semi-inclusive unpolarized DIS for the production of charged kaons and the different possibilities, both in LO and NLO, to test the conventionally used assumptions $s-\bar s=0$ and $D_d^{K^+-K^-}=0$. The considered tests have the advantage that they do not require any knowledge of the fragmentation functions.

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arXiv2005

Non-singlets in Semi-inclusive DIS and inclusive e+e- annihilation

We show that non-singlets in semi-inclusive DIS with pi^\pm determine without assumptions Delta u_V, Delta d_V and D_u^{π+ - pi-}. Non-singlets in SIDIS and inclusive e+e- -annihilation with K^\pm determine (s-bar s) and (Delta s-Delta bar s), but an assumption is necessary -- we choose as most natural D_d^{K+ - K-}=0. Measurements with charged K^\pm and neutral K0+bar K0 in semi-inclusive DIS and e+e--annihilation determine D_{u,d,s}^{K+ + K-} without any assumptions and allow tests of LO approximation in QCD.

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arXiv2004

A Model Independent Approach to Semi-Inclusive Deep Inelastic Scattering

We present a method for extraction of detailed information on polarized quark densities from semi-inclusive deep inelastic scattering l+N -> l+h+X, in both LO and NLO QCD without any assumptions about fragmentation functions and polarized sea densities. The only symmetries utilised are charge conjugation and isotopic spin invariance of strong interactions.

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arXiv2001

A Strategy for the Analysis of Semi-Inclusive Deep Inelastic Scattering

We present a strategy for the systematic extraction of a vast amount of detailed information on polarized parton densities and fragmentation functions from semi-inclusive deep inelastic scattering l+N -> l+h+X, in both LO and NLO QCD. A method is suggested for estimating the errors involved in the much simpler, and therefore much more attractive, LO analysis. The approach is based upon a novel interplay with data from inclusive DIS and from e+e- -> hX, and leads to a much simplified form of the NLO expressions. No assumptions are made about the equality of any parton densities and the only symmetries utilised are charge conjugation invariance and isotopic spin invariance of strong interactions.

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arXiv1999

Semi-inclusive π^\pm production - tests for independent fragmentation and for polarized quark densities

We show that measurements of semi-inclusive $π^\pm$ asymmetries on $p$ and $n$ with polarized and unpolarized target and beams allow, without any knowledge of the polarized parton densities 1) to test independent fragmentation and SU(2) symmetry for the polarized sea, and 2) to determine separately the polarized strange and valence quark densities, and the ratio of the fragmentation functions $D_s^{π^+ + π^-}/D_u^{π^+ +π^-}$

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