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R. Aleksan

Publications and source records attributed to R. Aleksan.

33 records · Page 2Linked to original sources
arXiv2013

TLEP, first step in a long-term vision for HEP

The discovery of H(126) has renewed interest in circular e+e- colliders that can operate as Higgs factories, which benefit from three unique characteristics: i) high luminosity and reliability, ii) the availability of several interaction points, iii) superior beam energy accuracy. TLEP is an e+e- storage ring of 80-km circumference that can operate with very high luminosity from the Z peak (90 GeV) to the top quark pair threshold (350 GeV). It can achieve transverse beam polarization at the Z peak and WW threshold, giving it unparalleled accuracy on the beam energy. A preliminary study indicates that an 80 km tunnel could be constructed around CERN. Such a tunnel would allow a 100 TeV proton-proton collider to be established in the same ring (VHE-LHC), offering a long term vision.

physics.acc-ph↗
arXiv2013

TLEP: A High-Performance Circular e+e- Collider to Study the Higgs Boson

The recent discovery of a light Higgs boson has opened up considerable interest in circular e+e- Higgs factories around the world. We report on the progress of the TLEP concept since last year. TLEP is an e+e- circular collider capable of very high luminosities in a wide centre-of-mass (ECM) spectrum from 90 to 350 GeV. TLEP could be housed in a new 80 to 100 km tunnel in the Geneva region. The design can be adapted to different ring circumference (e.g. LEP3 in the 27 km LHC tunnel). TLEP is an ideal complementary machine to the LHC thanks to high luminosity, exquisite determination of ECM and the possibility of four interaction points, both for precision measurements of the Higgs boson properties and for precision tests of the closure of the Standard Model from the Z pole to the top threshold.

physics.acc-ph↗
arXiv2006

Towards the European strategy for particle physics: the Briefing Book

This document was prepared as part of the briefing material for the Workshop of the CERN Council Strategy Group, held in DESY Zeuthen from 2nd to 6th May 2006. It gives an overview of the physics issues and of the technological challenges that will shape the future of the field, and incorporates material presented and discussed during the Symposium on the European Strategy for Particle Physics, held in Orsay from 30th January to 2nd February 2006, reflecting the various opinions of the European community as recorded in written submissions to the Strategy Group and in the discussions at the Symposium.

hep-ph↗
arXiv2003

Measuring 2beta+gamma with color-allowed B0 -> D+- Ks pi-+ decays

We present a method to measure the weak phase 2beta+gamma in the three-body decay B0 -> D+- Ks pi-+. These decays are mediated by interfering amplitudes which are color-allowed and hence relatively large. In addition, the three-body decay helps to resolve discrete ambiguities present in measurements of the weak phase. The experimental implications are discussed, and the sensitivity of the method is evaluated using a simulation.

hep-ph↗
arXiv2003

Phenomenological discussion of $B\to P V$ decays in QCD improved factorization approach

Trying a global fit of the experimental branching ratios and CP-asymmetries of the charmless $B\to PV$ decays according to QCD factorization, we find it impossible to reach a satisfactory agreement, the confidence level (CL) of the best fit is smaller than .1 %. This failure reflects the difficulty to accommodate several large experimental branching ratios of the strange channels. Furthermore, experiment was not able to exclude a large direct CP asymmetry in $\bar {B}^0\toρ^+ π^-$, which is predicted very small by QCD factorization. Proposing a fit with QCD factorization complemented by a charming-penguin inspired model we reach a best fit which is not excluded by experiment (CL of about 8 %) but is not fully convincing. These negative results must be tempered by the remark that some of the experimental data used are recent and might still evolve significantly.

hep-ph↗
arXiv2003

Testing QCD factorisation and charming penguins in charmless ${\boldsymbol{B\to PV}}$

We try a global fit of the experimental branching ratios and CP-asymmetries of the charmless $B\to PV$ decays according to QCD factorisation. We find it impossible to reach a satisfactory agreement, the confidence level (CL) of the best fit is smaller than .1 %. The main reason for this failure is the difficulty to accomodate several large experimental branching ratios of the strange channels. Furthermore, experiment was not able to exclude a large direct CP asymmetry in $\bar {B^0}\toρ^+ π^-$, which is predicted very small by QCD factorisation. Trying a fit with QCD factorisation complemented by a charming-penguin inspired model we reach a best fit which is not excluded by experiment (CL of about 8 %) but is not fully convincing. These negative results must be tempered by the remark that some of the experimental data used are recent and might still evolve significantly.

hep-ph↗
arXiv1999

Inclusive decay of B mesons into $D_s$ or $D^*_s$

We compute the inclusive decay rates $b \to D_s^-(D^{*-}_s) c$ including lowest order QCD corrections on the quark legs, and compare with existing data. Unlike the short distance QCD corrections, that are of higher order, these corrections are of order $α_s$. In the on-shell renormalization scheme and for $α_s(m_b) \cong 0.2$ we find a correction of -10% to the inclusive rate computed using factorization. This gives a total rate $BR(b \to D_s^-(D^{*-}_s)c) \cong 8%$ consistent within $1σ$ with the measured value $BR(B \to D_s^{\pm}X) = (10.0 \pm 2.5)%$. The general formulae given here include the case of vanishing mass for the final quark $b \to D_s^-(D_s^{*-})u$. The radiative correction to this rate is -17%. We show in another place that this process can be useful for the measurement of the CKM matrix element $V_{ub}$. We also give the renormalized vertex at the interesting values $q^2 = 0$ and $q^2 = q_{max}^2$, and compare with existing literature.

hep-ph↗
arXiv1999

Measuring $|V_{ub}|$ with B$\to D_s^+ X_u$ transitions

We propose the determination of the CKM matrix element $|V_{ub}|$ by the measurement of the spectrum of $B \to D_s^+ X_u$, dominated by the spectator quark model mechanism $\bar{b} \to D_s^{(*)+} \bar{u}$. The interest of considering $B \to D_s^+X_u$ versus the semileptonic decay is that more than 50 % of the spectrum for $B \to D_s^+ X_u$ occurs above the kinematical limit for $B \to D_s^+ X_c$, while most of the spectrum $B \to l νX_u$ occurs below the $B \to l νX_c$ one. Furthermore, the measure of the hadronic mass $M_X$ is easier in the presence of an identified $D_s$ than when a $ν$ has been produced. As a consistency check, we point out that the rate $\bar{b} \to D_s^{(*)+} \bar{c}$ (including QCD corrections that we present elsewhere) is consistent with the measured $BR (B \to D_s^{\pm} X)$. Although the hadronic complications may be more severe in the mode that we propose than in the semileptonic inclusive decay, the end of the spectrum in $B \to l νX_u$ is not well understood on theoretical grounds. We argue that, in our case, the excited $D_s^{**}$, decaying into $D K$, do not contribute and, if there is tagging of the $B$ meson, the other mechanisms to produce a $D_s$ of the right sign are presumably small, of $O(10^{-2})$ relative to the spectator amplitude, or can be controlled by kinematical cuts. In the absence of tagging, other hadronic backgrounds deserve careful study. We present a feasability study with the BaBar detector.

hep-ph↗
arXiv1995

Uncertainties on the CP Phase $\bfalpha$ due to Penguin Diagrams

A major problem in the determination of the the CP angle $α$, that should be measured through modes of the type $B_d, \bar{B}_d \to ππ, \cdots ,$ is the uncertainty coming from Penguin diagrams. We consider the different ground state modes $ππ$, $πρ$, $ρρ$, and, assuming the FSI phases to be negligible, we investigate the amount of uncertainty coming from Penguins that can be parametrized by a dilution factor $D$ and an angle shift $Δα$. The parameter $D$ is either 1 or very close to 1 in all these modes, and it can be measured independently, up to a sign ambiguity, by the $t$ dependence. Assuming factorization, we show that $Δα$ is much smaller for the modes $ρπ$ and $ρρ$ than for $ππ$, and we plot their allowed region as a function of $α$ itself. Moreover, we show that most of the modes contribute to the asymmetry with the same sign, and define for their sum an effective $D_{eff}$ and an effective $Δα_{eff}$, an average of $Δα$ for the different modes. It turns out that $D_{eff}$ is of the order of 0.9, $Δα_{eff}$ is between 5 $\%$ and 10 $\%$ and, relative to $ππ$, the statistical gain for the sum is of about a factor 10. Finally, we compute the ratios $Kπ/ππ, \cdots$ that test the strength of the Penguins and depend on the CP angles, as emphasized by Silva and Wolfenstein and by Deandrea et al.

hep-ph↗
arXiv1994

The $B \to K^\ast ψ$ Polarization Puzzle

We point out that current estimates of form factors fail to explain the non-leptonic decays $B \to ψK(K^{\ast})$ and that the combination of data on the semi-leptonic decays $D \to K(K^{\ast})\ell ν$ and on the non-leptonic decays $B \to ψK(K^{\ast})$ (in particular recent po\-la\-ri\-za\-tion data) severely constrain the form (normalization and $q^2$ dependence) of the heavy-to-light meson form factors, if we assume the factorization hypothesis for the latter. From a simultaneous fit to \bpsi and \dk data we find that strict heavy quark limit scaling laws applied to the form factors do not hold when going from $D$ to $B$ and must have large corrections that make softer the dependence on the masses. This is in contrast with the matrix elements themselves which are found to need smaller $1/m_Q$ corrections to the asymptotic heavy quark scaling laws. We also find that $A_1(q^2)$ should increase slower with \qq than $A_2, V, f_+$. We propose a simple parametrization of these corrections based on a quark model or on an extension of the \hhs laws to the \hl case, complemented with an approximately constant $A_1(q^2)$. This model may be viewed as assuming a precocious validity of strict heavy quark scaling laws for the current matrix elements. Although this model reproduces qualitatively the wanted features for mass and $q^2$ dependence, and thus reduces the discrepancy with data, it is insufficient to reach a full agreement with the experiment. In our opinion the puzzle is still there.

hep-ph↗
arXiv1994

Critical Analysis of Theoretical Estimates for $B$ to Light Meson Form Factors and the $B \to ψK(K^{\ast})$ Data

We point out that current estimates of form factors fail to explain the non-leptonic decays $B \to ψK(K^{\ast})$ and that the combination of data on the semi-leptonic decays $D \to K(K^{\ast})\ell ν$ and on the non-leptonic decays $B \to ψK(K^{\ast})$ (in particular recent po\-la\-ri\-za\-tion data) severely constrain the form (normalization and $q^2$ dependence) of the heavy-to-light meson form factors, if we assume the factorization hypothesis for the latter. From a simultaneous fit to \bpsi and \dk data we find that strict heavy quark limit scaling laws do not hold when going from $D$ to $B$ and must have large corrections that make softer the dependence on the masses. We find that $A_1(q^2)$ should increase slower with \qq than $A_2, V, f_+$. We propose a simple parametrization of these corrections based on a quark model or on an extension of the \hhs laws to the \hl case, complemented with an approximately constant $A_1(q^2)$. We analyze in the light of these data and theoretical input various theoretical approaches (lattice calculations, QCD sum rules, quark models) and point out the origin of the difficulties encountered by most of these schemes. In particular we check the compatibility of several quark models with the heavy quark scaling relations.

hep-ph↗
arXiv1994

Determination of the CP Violating Phase $γ$ by a Sum Over Common Decay Modes to $B_s$ and $\bar{B}_s$

To help the difficult determination of the angle $γ$ of the unitarity triangle, Aleksan, Dunietz and Kayser have proposed the modes of the type $K^-D^+_s$, common to $B_s$ and $\bar{B}_s$. We point out that it is possible to gain in statistics by a sum over all modes with ground state mesons in the final state, i.e. $K^-D^+_s$, $K^{*-}D_+^s$, $K^-D^{*+}_s$, $K^{*-}D^{*+}_s$. The delicate point is the relative phase of these different contributions to the dilution factor $D$ of the time-dependent asymmetry. Each contribution to $D$ is proportional to a product $F^{cb}$ $F^{ub}$ $f_{D_s}$ $f_K$ where $F$ denotes form factors and $f$ decay constants. Within a definite phase convention, lattice calculations do not show any change in sign when extrapolating to light quarks the form factors and decay constants. Then, we can show that all modes contribute constructively to the dilution factor, except the $P$-wave $K^{*-}D^{*+}_s$, which is small. Quark model arguments based on wave function overlaps also confirm this stability in sign. By summing over all these modes we find a gain of a factor 6 in statistics relatively to $K^-D^+_s$. The dilution factor for the sum $D_{tot}$ is remarkably stable for theoretical schemes that are not in very strong conflict with data on $B \to ψK(K^*)$ or extrapolated from semileptonic charm form factors, giving $D_{tot} \geq 0.6$, always close to $D(K^- D^+_s)$.

hep-ph↗
arXiv1994

B To Light Meson Form Factors

The heavy to light form factors in $B$ decays are discussed. Critical discussion of theoretical approaches is made with special emphasis on their failure to describe the $B \to K^{(\ast )}ψ$ data.

hep-ph↗
arXiv1993

In Pursuit of Gamma

After reviewing techniques for extracting clean information on CP-violating phase angles from $B$ decays, we explain the rules for finding decay modes that can probe the phase angle $γ$ of the unitarity triangle. We identify the more promising of these ``$γ$ modes,'' estimate their branching ratios, and examine the degree to which they are theoretically clean. We then show that when the quark mixing matrix is not approximated as usual, but is treated exactly, none of the ``$γ$ modes'' actually measures $γ$. Rather, each of them measures $γ$ plus some correction. In all modes, the correction is small enough to be disregarded in first-generation experiments, but in some of them, it may be large enough to be observed in second-generation experiments. Our treatment of the $γ$ modes calls attention to the fact that when the quark mixing matrix is treated exactly, there are six unitarity triangles, rather than just one triangle. However, only four of the angles in these six triangles are independent. Examining the role played by these four angles, we discover that, in principle at least, measurements of nothing but CP-violating asymmetries in $B$ decays are sufficient to determine the entire quark mixing matrix.

hep-ph↗