arXiv subjects

Ahmed Ali

Publications and source records attributed to Ahmed Ali.

At least 91 records · Page 5Linked to original sources

arXiv2012

Transverse Energy-Energy Correlations in Next-to-Leading Order in $α_s$ at the LHC

We compute the transverse energy-energy correlation (EEC) and its asymmetry (AEEC) in next-to-leading order (NLO) in $α_s$ in proton-proton collisions at the LHC with the center-of-mass energy $E_{\rm c.m.}=7$ TeV. We show that the transverse EEC and the AEEC distributions are insensitive to the QCD factorization- and the renormalization-scales, structure functions of the proton, and for a judicious choice of the jet-size, also the underlying minimum bias events. Hence they can be used to precisely test QCD in hadron colliders and determine the strong coupling $α_s$. We illustrate these features by defining the hadron jets using the anti-$k_T$ jet algorithm and an event selection procedure employed in the analysis of jets at the LHC and show the $α_s(M_Z)$-dependence of the transverse EEC and the AEEC in the anticipated range $0.11 \leq α_s(M_Z) \leq 0.13$.

hep-ph↗

arXiv2012

Tetraquark Interpretation of the Charged Bottomonium-like states Z_b^+-(10610) and Z_b^+-(10650) and Implications

We present a tetraquark interpretation of the charged bottomonium-like states Z+-_b(10610) and Z+-_b(10650), observed by the Belle collaboration in the pi+- Upsilon(nS) (n=1,2,3) and pi+- h_b(mP) (m=1,2) invariant mass spectra from the data taken near the peak of the Upsilon(5S). In this framework, the underlying processes involve the production and decays of a vector tetraquark Y_b(10890), e+e- --> Y_b(10890) --> [Z+-_b(10610)pi-+, Z+-_b(10650)pi-+] followed by the decays [Z+-_b (10610), Z+-_b (10650)] --> pi+- Upsilon(nS), pi+- h_b(mP). Combining the contributions from the meson loops and an effective Hamiltonian, we are able to reproduce the observed masses of the Z+-_b(10610) and Z+-_b(10650). The analysis presented here is in agreement with the Belle data and provides crucial tests of the tetraquark hypothesis. We also calculate the corresponding meson loop effects in the charm sector and find them dynamically suppressed. The charged charmonium-like states Z+-_c(3752) and Z+-_c(3882) can be searched for in the decays of the J^{PC}=1^{--} tetraquark state Y(4260) via Y(4260) --> Z+-_c(3752)pi-+ and Y(4260) --> Z+-_c(3882)pi-+, with the subsequent decays (Z+-_c(3752),Z+-_c(3882)) --> (J/psi, h_c)pi+-.

hep-ph↗

arXiv2011

Jets and QCD: A Historical Review of the Discovery of the Quark and Gluon Jets and its Impact on QCD

The observation of quark and gluon jets has played a crucial role in establishing Quantum Chromodynamics [QCD] as the theory of the strong interactions within the Standard Model of particle physics. The jets, narrowly collimated bundles of hadrons, reflect configurations of quarks and gluons at short distances. Thus, by analysing energy and angular distributions of the jets experimentally, the properties of the basic constituents of matter and the strong forces acting between them can be explored. In this review, which is primarily a description of the discovery of the quark and gluon jets and the impact of their observation on Quantum Chromodynamics, we elaborate, in particular, the role of the gluons as the carriers of the strong force. Focusing on these basic points, jets in $e^+ e^-$ collisions will be in the foreground of the discussion and we will concentrate on the theory that was contemporary with the relevant experiments at the electron-positron colliders. In addition we will delineate the role of jets as tools for exploring other particle aspects in $ep$ and $pp/p\bar{p}$ collisions - quark and gluon densities in protons, measurements of the QCD coupling, fundamental 2-2 quark/gluon scattering processes, but also the impact of jet decays of top quarks, and $W^\pm, Z$ bosons on the electroweak sector. The presentation to a large extent is formulated in a non-technical language with the intent to recall the significant steps historically and convey the significance of this field also to communities beyond high energy physics.

hep-ph↗

arXiv2011

Improved sensitivity to charged Higgs searches in Top quark decays $t \to bH^+ \to b (τ^+ν_τ)$ at the LHC using $τ$ polarisation and multivariate tecnniques

We present an analysis with improved sensitivity to the light charged Higgs ($m_{H^+} < m_t-m_b$) searches in the top quark decays $t \to b H^+ \to b (τ^+ν_τ) + ~{\rm c.c.}$ in the $t\bar{t}$ and single $t/\bar{t}$ production processes at the LHC. In the Minimal Supersymmetric Standard Model (MSSM), one anticipates the branching ratio ${\cal B} (H^+ \to τ^+ν_τ)\simeq 1$ over almost the entire allowed $\tan β$ range. Noting that the $τ^+$ arising from the decay $H^+ \to τ^+ν_τ$ are predominantly right-polarized, as opposed to the $τ^+$ from the dominant background $W^+ \to τ^+ν_τ$, which are left-polarized, a number of $H^+/W^+ \to τ^+ν_τ$ discriminators have been proposed and studied in the literature. We consider hadronic decays of the $τ^\pm$, concentrating on the dominant one-prong decay channel $τ^\pm \to ρ^\pm ν_τ$. The energy and $p_T$ of the charged prongs normalised to the corresponding quantities of the $ρ^\pm$ are convenient variables which serve as $τ^\pm$ polariser. We use the distributions in these variables and several other kinematic quantities to train a boosted decision tree (BDT). Using the BDT classifier, and a variant of it called BDTD, which makes use of decorrelated variables, we have calculated the BDT(D)-response functions to estimate the signal efficiency vs. the rejection of the background. We argue that this chain of analysis has a high sensitivity to light charged Higgs searches up to a mass of 150 GeV in the decays $t \to b H^+$ (and charge conjugate) at the LHC. For the case of single top production, we also study the transverse mass of the system determined using Lagrange multipliers.

hep-ph↗

arXiv2011

Theory Overview on Spectroscopy

A theoretical overview of the exotic spectroscopy in the charm and beauty quark sector is presented. These states are unexpected harvest from the $e^+e^-$ and hadron colliders and a permanent abode for the majority of them has yet to be found. We argue that some of these states, in particular the $Y_b(10890)$ and the recently discovered states $Z_b(10610)$ and $Z_b(10650)$, discovered by the Belle collaboration are excellent candidates for tetraquark states $[bq][\bar{b}\bar{q}]$, with $q=u,d$ light quarks. Theoretical analyes of the Belle data carried out in the tetraquark context is reviewed.

hep-ph↗

arXiv2011

Production of the Exotic $1^{--}$ Hadrons $ϕ(2170)$, X(4260) and $Y_b(10890)$ at the LHC and Tevatron via the Drell-Yan Mechanism

We calculate the Drell-Yan production cross sections and differential distributions in the transverse momentum and rapidity of the $J^{PC}=1^{--}$ exotic hadrons $ϕ(2170)$, X(4260) and $Y_b(10890)$ at the hadron colliders LHC and the Tevatron. These hadrons are tetraquark (four-quark) candidates, with a hidden $s\bar{s}$, $c\bar{c}$ and $b\bar{b}$ quark pair, respectively. In deriving the distributions and cross sections, we include the order $α_s$ QCD corrections, resum the large logarithms in the small transverse momentum region in the impact-parameter formalism, and use the state of the art parton distribution functions. Taking into account the data on the production and decays of these vector hadrons from the $e^+e^-$ experiments, we present the production rates for the processes $pp(\bar{p}) \to ϕ(2170)(\to ϕ(1020) π^+π^- \to K^+K^- π^+π^-)+...$, $pp(\bar{p}) \to X(4260)(\to J/ψπ^+π^- \to μ^+μ^-π^+π^-)+...$, and $pp(\bar{p}) \to Y_b(10890)(\to (Υ(1S), Υ(2S), Υ(3S)) π^+π^- \to μ^+μ^-π^+π^-)+...$. Their measurements at the hadron colliders will provide new experimental avenues to explore the underlying dynamics of these hadrons.

hep-ph↗

arXiv2011

Tetraquark-based analysis and predictions of the cross sections and distributions for the processes e^+ e^- --> Upsilon(1S) (pi^+ pi^-, K^+ K^-, eta pi^0) near Upsilon(5S)

We calculate the cross sections and final state distributions for the processes e^+ e^- --> Upsilon(1S) (pi^+ pi^-, K^+ K^-, eta pi^0) near the Upsilon(5S) resonance based on the tetraquark hypothesis. This framework is used to analyse the data on the Upsilon(1S) pi^+ pi^- and Upsilon(1S) K^+ K^- final states [K.F. Chen et al. (Belle Collaboration), Phys. Rev. Lett. 100, 112001 (2008); I. Adachi et al. (Belle Collaboration), arXiv:0808.2445], yielding good fits. Dimeson invariant mass spectra in these processes are shown to be dominated by the corresponding light scalar and tensor states. The resulting correlations among the cross sections are worked out. We also predict sigma(e^+ e^- --> Upsilon(1S) K^+ K^-)/sigma(e^+ e^- --> Upsilon(1S) K^0 Kbar^0) = 1/4. These features provide crucial tests of the tetraquark framework and can be searched for in the currently available and forthcoming data from the B factories.

hep-ph↗

arXiv2011

Tetraquark interpretation of $e^+ e^- \to Υπ^+π^-$ Belle data and $e^+ e^- \to b \bar{b}$ BaBar data

We summarize the main features of the spectroscopy, production and decays of the $J^{PC}=1^{--}$ tetraquarks in the $b\bar{b}$ sector, concentrating on the lowest state called $Y_b(10890)$. The tetraquark framework is used to analyze the BaBar data on the $e^+ e^- \to b\bar{b}$ cross section ($R_b$ energy scan) between $\sqrt{s}= 10.54$ and 11.20 GeV and the Belle data on the processes $e^+e^- \to Υ(1S) π^+π^-,Υ(2S) π^+π^-$ near the peak of the $Υ(5S)$ resonance. The BaBar $R_b$ energy scan is consistent with an additional state at a mass of 10.90 GeV and a width of about 28 MeV, in broad agreement with the state $Y_b(10890)$ GeV seen by Belle in the exclusive final states. We argue that the decay widths and the dipion invariant mass distributions measured by Belle are naturally explained by the tetraquark interpretation of $Y_b(10890)$.

hep-ph↗

arXiv2010

Prospects of measuring the CKM matrix element $|V_{ts}|$ at the LHC

We study the prospects of measuring the CKM matrix element $\vert V_{ts}\vert$ at the LHC with the top quarks produced in the processes $p p \to t\bar{t}X$ and $p p \to t/\bar{t} X$, and the subsequent decays $t \to W^+s$ and $\bar{t} \to W^- \bar{s}$. We insist on tagging the $W^\pm$ leptonically, $W^\pm \to \ell^\pm ν_\ell$ ($\ell =e, μ, τ$), and analyse the anticipated jet profiles in the signal process $t \to W s$ and the dominant background from the decay $t \to W b$. To that end, we analyse the $V0$ ($K^0$ and $Λ$) distributions in the $s$- and $b$-quark jets concentrating on the energy and transverse momentum distributions of these particles. The $V0$s emanating from the $t \to W b$ branch have displaced decay vertexes from the interaction point due to the weak decays $b \to c \to s$ and the $b$-quark jets are rich in charged leptons. Hence, the absence of secondary vertexes and of the energetic charged leptons in the jet provide additional ($b$-jet vs. $s$-jet) discrimination in top quark decays. These distributions are used to train a boosted decision tree (BDT). Using the BDT classifier, and a variant of it called BDTD, which makes use of decorrelated variables, we calculate the BDT(D)-response functions corresponding to the signal ($t \to W s$) and background ($t \to W b$). Detailed simulations undertaken by us with the Monte Carlo generator PYTHIA are used to estimate the background rejection versus signal efficiency for three representative LHC energies $\sqrt{s}=7$ TeV, 10 TeV and 14 TeV. We argue that a benchmark with 10\% signal ($t \to W s $) efficiency and a background ($t \to W b$) rejection by a factor $10^3$ (required due to the anticipated value of the ratio $\vert V_{ts}\vert^2/\vert V_{tb} \vert^2 \simeq 1.6 \times 10^{-3}$) can be achieved at the LHC@14 TeV with an integrated luminosity of 10 fb$^{-1}$.

hep-ph↗

arXiv2010

Tetraquark interpretation of the BELLE data on the anomalous $Υ(1S) π^+π^-$ and $Υ(2S) π^+π^-$ production near the $Υ(5S)$ resonance

We analyze the Belle data [K. F. Chen {\it et al.} (Belle Collaboration), Phys.\ Rev.\ Lett.\ {\bf 100}, 112001 (2008); I. Adachi {\it et al.} (Belle Collaboration), arXiv:0808.2445] on the processes $e^+ e^- \to Υ(1S)\; π^+π^-, Υ(2S)\; π^+π^-$ near the peak of the $Υ(5S)$ resonance, which are found to be anomalously large in rates compared to similar dipion transitions between the lower $Υ$ resonances. Assuming these final states arise from the production and decays of the $J^{PC}=1^{--}$ state $Y_b(10890)$, which we interpret as a bound (diquark-antidiquark) tetraquark state $[bq][\bar{b}\bar{q}]$, a dynamical model for the decays $Y_b \to Υ(1S)\; π^+π^-, Υ(2S)\; π^+π^-$ is presented. Depending on the phase space, these decays receive significant contributions from the scalar $0^{++}$ states, $f_0(600)$ and $f_0(980)$, and from the $2^{++}$ $q\bar{q}$-meson $f_2(1270)$. Our model provides excellent fits for the decay distributions, supporting $Y_b$ as a tetraquark state.

hep-ph↗

arXiv2010

A case for hidden $b\bar{b}$ tetraquarks based on $e^+e^- \to b\bar{b}$ cross section between $\sqrt{s}=10.54$ and 11.20 GeV

We study the spectroscopy and dominant decays of the bottomonium-like tetraquarks (bound diquarks-antidiquarks), focusing on the lowest lying P-wave $[bq][\bar{b}\bar{q}]$ states $Y_{[bq]}$ (with $q=u,d$), having $J^{PC}=1^{--}$. To search for them, we analyse the BABAR data \cite{:2008hx} obtained during an energy scan of the $e ^+ e^- \to b \bar{b}$ cross section in the range of $\sqrt{s}=10.54$ to 11.20 GeV. We find that these data are consistent with the presence of an additional $b \bar{b}$ state $Y_{[bq]}$ with a mass of 10.90 GeV and a width of about 30 MeV apart from the $Υ(5S)$ and $Υ(6S)$ resonances. A closeup of the energy region around the $Y_{[bq]}$-mass may resolve this state in terms of the two mass eigenstates, $Y_{[b,l]}$ and $Y_{[b,h]}$, with a mass difference, estimated as about 6 MeV. We tentatively identify the state $Y_{[bq]}(10900)$ from the $R_b$-scan with the state $Y_b(10890)$ observed by BELLE \cite{Abe:2007tk} in the process $e^+e^- \to Y_b(10890) \to Υ(1S, 2S) π^+ π^-$ due to their proximity in masses and decay widths.

hep-ph↗

arXiv2009

Radiatively corrected lepton energy distributions in top quark decays $t \to bW^+ \to b(\ell^+ ν_\ell)$ and $t \to bH^+ \to b (τ^+ ν_τ)$ and single charged prong energy distributions from subsequent $τ^+$ decays

We calculate the QED and QCD radiative corrections to the charged lepton energy distributions in the dominant semileptonic decays of the top quark $t \to b W^+ \to b(\ell^+ ν_\ell)$ $(\ell=e, μ, τ)$ in the standard model(SM), and for the decay $t \to b H^+ \to b(τ^+ ν_τ)$ in an extension of the SM having a charged Higgs boson $H^\pm$ with $m_{H^\pm} < m_t -m_b$. The QCD corrections are calculated in the leading and next-to-leading logarithmic approximations, but the QED corrections are considered in the leading logarithmic approximation only. These corrections are numerically important for precisely testing the universality of the charged current weak interactions in $t$-quark decays. As the $τ^+$ leptons arising from the decays $W^+ \to τ^+ ν_τ$ and $H^+\to τ^+ ν_τ$ are predominantly left- and right-polarised, respectively, influencing the energy distributions of the decay products in the subsequent decays of the $τ^+$, we work out the effect of the radiative corrections on such distributions in the dominant (one-charged prong) decay channels $ τ^+ \to π^+ \barν_τ, ρ^+ \barν_τ, a_1^+ \barν_τ$ and $\ell^+ ν_\ell \barν_τ$. The inclusive $π^+$ energy spectra in the decay chains $t \to b(W^+,H^+) \to b (τ^+ ν_τ) \to b (π^+ \barν_τν_τ+X)$ are calculated, which can help in searching for the induced $H^\pm$ effects at the Tevatron and the LHC.

hep-ph↗

arXiv2009

Heavy flavours: working group summary

The talks presented in the working group ``Heavy flavours'' of the DIS-2009 workshop are summarised. New and recently updated results from theory, proton antiproton and heavy ion colliders, as well from HERA and $e^+e^-$ colliders are discussed.

hep-ph↗

arXiv2009

Anatomy of the pQCD Approach to the Baryonic Decays $Λ_b \to pπ, p K$

We calculate the CP-averaged branching ratios and CP-violating asymmetries for the two-body charmless hadronic decays $Λ_b \to p π, pK$ in the perturbative QCD (pQCD) approach to lowest order in $α_s$. The baryon distribution amplitudes involved in the factorization formulae are considered to the leading twist accuracy and the distribution amplitudes of the proton are expanded to the next-to-leading conformal spin (i.e., "P" -waves), the moments of which are determined from QCD sum rules. Our work shows that the contributions from the factorizable diagrams in $Λ_b \to p π, pK$ decays are much smaller compared to the non-factorizable diagrams in the conventional pQCD approach. We argue that this reflects the estimates of the $Λ_b \to p$ transition form factors in the $k_T$ factorization approach, which are found typically an order of magnitude smaller than those estimated in the light-cone sum rules and in the non-relativistic quark model. As an alternative, we adopt a hybrid pQCD approach, in which we compute the factorizable contributions with the $Λ_b \to p$ form factors taken from the light cone QCD sum rules. The non-factorizable diagrams are evaluated utilizing the conventional pQCD formalism which is free from the endpoint singularities. The predictions worked out here are confronted with the recently available data from the CDF collaboration on the branching ratios and the direct CP asymmetries for the decays $ Λ_b \to p π$, and $Λ_b \to p K$. The asymmetry parameter $α$ relevant for the anisotropic angular distribution of the emitted proton in the polarized $Λ_b$ baryon decays is also calculated for the two decay modes.

hep-ph↗

arXiv2009

Search for Gauge Extensions of the MSSM at the LHC

The extensions of the minimal supersymmetric model (MSSM), driving mainly from the need to solve the μproblem, involve novel matter species and gauge groups. These extended MSSM models can be searched for at the LHC via the effects of the gauge and Higgs bosons or their fermionic partners. Traditionally, the focus has been on the study of the extra forces induced by the new gauge and Higgs bosons present in such models. An alternative way of studying such effects is through the superpartners of matter species and the gauge forces. We thus consider a $U(1)^\prime$ gauge extension of the MSSM, and perform an extensive study of the signatures of the model through the production and decays of the scalar quarks and gluino, which are expected to be produced copiously at the LHC. After a detailed study of the distinctive features of such models with regard to the signatures at the LHC, we carry out a detailed Monte Carlo analysis of the signals from the process pp-> n leptons + m jets + EMT, and compare the resulting distributions with those predicted by the MSSM. Our results show that the searches for the extra gauge interactions in the supersymmetric framework can proceed not only through the forces mediated by the gauge and Higgs bosons but also through the superpartner forces mediated by the gauge and Higgs fermions. Analysis of the events induced by the squark/gluino decays presented here is complementary to the direct Z' searches at the LHC.

hep-ph↗

arXiv2008

Towards $B \to V γ$ Decays at NNLO in SCET

We compute NNLO (${\cal O}(α_s^2)$) corrections to the hard-scattering kernels entering the QCD factorization formula for $B\to Vγ$ decays, where $V$ is a light vector meson. We give complete NNLO results for the dipole operators $Q_7$ and $Q_8$, and partial results for $Q_1$ valid in the large-$β_0$ limit and neglecting the NNLO correction from hard spectator scattering. Large perturbative logarithms in the hard-scattering kernels are identified and resummed using soft-collinear effective theory. We use our results to estimate the branching fractions for $B\to K^*γ$ and $B_s\to ϕγ$ decays at NNLO and compare them with the current experimental data.

hep-ph↗

arXiv2007

Theoretical Interest in B-Meson Physics at the B factories, Tevatron and the LHC

We review the salient features of $B$-meson physics, with particular emphasis on the measurements carried out at the $B$-factories and Tevatron, theoretical progress in understanding these measurements in the context of the standard model, and anticipation at the LHC. Topics discussed specifically are the current status of the Cabibbo-Kobayashi-Maskawa matrix, the CP-violating phases, rare radiative and semileptonic decays, and some selected non-leptonic two-body decays of the $B$ mesons.

hep-ph↗

arXiv2007

Charmless non-leptonic B_s decays to PP, PV and VV final states in the pQCD approach

We calculate the CP-averaged branching ratios and CP-violating asymmetries of a number of two-body charmless hadronic decays $\bar{B_s^0} \to PP, PV, VV$ in the perturbative QCD (pQCD) approach to leading order in $α_s$ (here $P$ and $V$ denote light pseudoscalar and vector mesons, respectively). The mixing-induced CP violation parameters are also calculated for these decays. We also predict the polarization fractions of $B_s \to VV$ decays and find that the transverse polarizations are enhanced in some penguin dominated decays such as $\bar{B_s^0} \to K ^{*}\bar{K^{*}}$, $K ^{*}ρ$. Some of the predictions worked out here can already be confronted with the recently available data from the CDF collaboration on the branching ratios for the decays $ \bar{B_s^0} \to K^+π^-$, $ \bar{B_s^0} \to K^+K^-$ and the CP-asymmetry in the decay $ \bar{B_s^0} \to K^+π^-$, and are found to be in agreement within the current errors. A large number of predictions for the branching ratios, CP-asymmetries and vector-meson polarizations in $\bar{B_s^0}$ decays, presented in this paper and compared with the already existing results in other theoretical frameworks, will be put to stringent experimental tests in forthcoming experiments at Fermilab, LHC and Super B-factories.

hep-ph↗