A clean signal for a top-like isosinglet fermion at the Large Hadron Collider

We predict a clean signal at the Large Hadron Collider ( $\sqrt{s}=$ 14 TeV) for a scenario where there is a top-like, charge +?2/3 vector-like isosinglet fermion. Such a quark, via mixing with the standard model top, can undergo decays via both flavour-changing Z-boson coupling and flavour-changing Yukawa interactions. We concentrate on the latter channel, and study the situation where, following its pair production, the heavy quark pair gives rise to two tops and two Higgs bosons. We show that when each Higgs decays in the $b\bar{b}$ channel, there can be a rather distinct and background-free signal that can unveil the existence of the vector-like isosinglet quark of this kind.     

Instability of the chiral invariant vacuum due to gluon exchange

It has been shown by Finger, Horn and Mandula in the Tamm-Dancoff approximation that Coulomb exchange induces vacuum instability for α _s bigger than some critical value. We show here in all generality that the critical coupling is lower using the Bogolioubov-Valatin variational method. For Coulomb exchange we find \(\tfrac{4}{3}\alpha _s^{crit} = 1\) instead of \(\tfrac{4}{3}\alpha _s^{crit} = \tfrac{3}{2}\) , and adding transverse gluon exchange with the Breit interaction, \(\tfrac{4}{3}\alpha _s^{crit} = \tfrac{1}{3}\) . It is remarkable that these values of α _s ^crit are not far from the range of perturbative QCD.     

QCD analysis of forward neutron production in DIS

Observing light-by-light scattering at the large hadron collider (LHC) has received quite some attention and it is believed to be a clean and sensitive channel to possible new physics. In this paper, we study the diphoton production at the LHC via the process \({{pp}}\rightarrow {{p}}\gamma \gamma {{p}}\rightarrow {{p}}\gamma \gamma {{p}}\) through graviton exchange in the large extra dimension (LED) model. Typically, when we do the background analysis, we also study the double Pomeron exchange of \(\gamma \gamma \) production. We compare its production in the quark–quark collision mode to the gluon–gluon collision mode and find that contributions from the gluon–gluon collision mode are comparable to the quark–quark one. Our result shows, for extra dimension \(\delta =4\) , with an integrated luminosity \(\mathcal{L} = 200\,\mathrm{fb}^{-1}\) at the 14 TeV LHC, that diphoton production through graviton exchange can probe the LED effects up to the scale \({M}_{S}=5.06 (4.51, 5.11)\,\mathrm{TeV}\) for the forward detector acceptance \(\xi _1 (\xi _2, \xi _3)\) , respectively, where \(0.0015<\xi _1<0.5\) , \(0.1<\xi _2<0.5\) , and \(0.0015<\xi _3<0.15\) .     

Chiral symmetry breaking in the colour dielectric model

Approximating the long-distance gluon dynamics ofSU(3)_colour by colour-dielectric block-spin variables, we obtain an effective QCD theory of a scalar colour-dielectric field and a massive colour-bleached gluon field coupled to light quarks. The massive vector field produces a strong attraction betweenq \(\bar q\) pairs, which leads toq \(\bar q\) condensation when the colour-dielectric field becomes small. We calculate \(\left\langle {\bar \psi \psi } \right\rangle\) and the pion decay constantf _n as a function of the dielectric field expectation value, by evaluating the fermion determinant in a derivative expansion, and integrating out the bosonic variables. We find that the effective quark-gluon coupling,α _s ^eff , including quark effects, is large on the surface of bags, where \(\left\langle {\bar \psi \psi } \right\rangle\) ±0, but decreases inside hadronic bags, where | \(\left\langle {\bar \psi \psi } \right\rangle\) | is decreasing.     

Low-Lying Baryons in Hybrid Quark Model

We study the strong decay processes of the Roper resonance, N*(1440) in the picture of hybrid baryon in which the Roper resonance N*(1440) is interpreted as a state of three quarks and one transverse-electric gluon, q ³ G. A nonrelativistic quark–gluon model is employed, where the dynamics of antiquark–quark–gluon is described in the effective \({^{3}S_{1}}\) vertex in which a quark–antiquark pair is created (destroyed) from (into) a gluon. The wave function of the Roper resonance is properly constructed to take into account the gluon freedom in the nonrelativistic regime. The evaluated strong decay width ratios of N*(1440) are in good agreement with the experimental data.     

Gluon condensate and the effective gluon mass

Using massive gauge invariant QCD we show explicity how power like corrections to \(\Pi _{\mu v} \left( q \right) = i\int {dx} e^{iq'x} \left\langle {0\left| {j_\mu ^{em} \left( x \right)\bar j_v^{em} \left( 0 \right)} \right|0} \right\rangle \) arise. Using our result for the 1/q ⁴ contribution, a one to one correspondence is made between the gluon condensate and the effective gluon mass. By relating this mass to, \(\langle 0|\frac{{\alpha _s }}{\pi }G_{\mu v}^2 |0\rangle \) a value ofm _gluon=750 MeV is found at ?q ²=10 GeV². In addition, within the context of dimensional regularization, a new technique for evaluating two loop momentum integrals with massive propagators is introduced. This method is a derivative of the Mellin transform technique that was applied to ladder diagrams in the days of Reggeisation.     

Theoretical prediction of cosmological constant Λ in Veneziano ghost theory of QCD

Based on the Veneziano ghost theory of QCD, we predict the cosmological constant ??, which is related to energy density of cosmological vacuum by $ \Lambda = \frac{{8\pi G}} {3}\rho _\Lambda $ . In the Veneziano ghost theory, the vacuum energy density ?? _?? is expressed by absolute value of the product of quark vacuum condensate and quark current mass: $ \rho _\Lambda = \frac{{2N_f H}} {{m_{\eta '} }}c|m_q < 0|:\bar qq:|0 > | $ . We calculate the quark local vacuum condensates ??0|: $ \bar q $ q: |0?? by solving Dyson-Schwinger Equations for a fully dressed confining quark propagator S _f (p) with an effective gluon propagator G _???? ^ab (q). The quark current mass m _q is predicted by use of chiral perturbation theory. Our theoretical result of ??, with the resulting ??0|: 471-4 q: |0?? = ?(235 MeV)³ and light quark current mass m _q ? 3.29?C6.15 MeV, is in a good agreement with the observable of the ?? ?? 10^?52 m^?2 used widely in a great amount of literatures.     

Second order QCD corrections toq\bar q-anihilation into a lepton pair at large transverse momentum

The \(\mathcal{O}{\text{(}}\alpha _{\text{s}}^{\text{2}} )\) correction is presented to \(q\bar q\) -annihilation into a lepton pair at large transverse momentum. I calculate the corresponding hadron cross section difference \(\frac{{d\sigma }}{{d^4 q}}(p\bar p - pp) \to e^ + e^ - + x(q\) is the momentum of the lepton pair system). The correction to this cross section difference is found to be large. This essentially agrees with recently published results by Ellis, Martinelli and Petronzio. Two improtant approximations are used: the invariant mass of the two-gluon-system is put to zero, and only valence-valence quark scattering is considered.     

Contribution of a heavy singlet quark to the mass differences \Delta m_{K^o } , \Delta m_{B_d } , and \Delta m_{B_s } and to the CP-violation parameter ɛ K

It is shown that, within the Standard Model extended by including a singlet quark heavy enough to prevent its direct detection at LHC, the mass differences $ \Delta m_{B_d } $ and $ \Delta m_{B_s } $ and the parameter of CP violation in K-meson oscillations, ? _K , acquire universal corrections of about 5 to 10%. Implicit experimental constraints on the mass of this new quark are discussed.     

Spectroscopy ofQ\bar Qg hybrid mesons

Hybrid mesons composed of a quark, an antiquark, and a gluon are studied in the case of heavy quarks. Their masses are calculated with the potential model which can interpret heavy quarkonium spectroscopy. The ground state of the hybrid mesons \(c\bar cg\) and \(b\bar bg\) is found to be almost spherically symmetric, whereas that of \(t\bar tg\) is two-centered as anH ₂ ⁺ molecule. The \(b\bar bg\left[ {t\bar tg} \right]\) ground state turns out to have a mass below the \(B\bar B\left[ {T\bar T} \right]\) threshold. The excited states contain 0^??, 1^?+ exotic states and 1^?? states which may be examined bye ⁺ e ^? colliders.     

Three-loop relation of quark\overline {MS} and pole masses

We calculate, exactly, the next-to-leading correction to the relation between the \(\overline {MS} \) quark mass, \(\bar m\) , and the scheme-independent pole mass,M, and obtain $$\begin{gathered} \frac{M}{{\bar m(M)}} \approx 1 + \frac{4}{3}\frac{{\bar \alpha _s (M)}}{\pi } + \left[ {16.11 - 1.04\sum\limits_{i = 1}^{N_F - 1} {(1 - M_i /M)} } \right] \hfill \\ \cdot \left( {\frac{{\bar \alpha _s (M)}}{\pi }} \right)^2 + 0(\bar \alpha _s^3 (M)), \hfill \\ \end{gathered} $$ as an accurate approximation forN _F?1 light quarks of massesM _i <M. Combining this new result with known three-loop results for \(\overline {MS} \) coupling constant and mass renormalization, we relate the pole mass to the \(\overline {MS} \) mass, \(\bar m\) (μ), renormalized at arbitrary μ. The dominant next-to-leading correction comes from the finite part of on-shell two-loop mass renormalization, evaluated using integration by parts and checked by gauge invariance and infrared finiteness. Numerical results are given for charm and bottom \(\overline {MS} \) masses at μ=1 GeV. The next-to-leading corrections are comparable to the leading corrections.     

B^0 - \bar B^0 mixing within and beyond the standard model

The standard model with three fermion families is found to be compatible with the Argus observation of a relatively large amount of \(B_{^d }^0 - \bar B_d^0 \) mixing provided that the top quark is rather heavy. From an analysis of the existing information on mixing angles, and of the constraints imposed by the kaon system, in particular by the ε parameter, we conclude that almost certainlym _t <45 GeV and probablym _t >90 GeV. In view of the Argus result the standard model unambiguously leads to the prediction of a nearly maximal amount of \(B_{^s }^0 - \bar B_s^0 \) mixing. Apart from the rather obvious case of a fourth family of quarks, most “minimal” extensions of the standard model preserve the prediction of a large \(B_{^s }^0 - \bar B_s^0 \) mixing. We discuss \(B^0 - \bar B^0 \) mixing in minimal supersymmetric models compatible with the UA1 lower bounds on gluino and squark masses. Contributions from charged Higgs exchange in the box diagrams are also discussed. While supersymmetry (although marginally) and charged Higgses can lead to an appreciable enhancement of \(B^0 - \bar B^0 \) mixing, minimal left-right symmetric models actually predict a modest suppression of the effect with respect to the standard model.     

Charm Production in Antiproton-Proton Annihilation

We study the production of charmed mesons (D) and baryons (?? _c ) in antiproton-proton ${(\bar{p}p)}$ annihilation close to their respective production thresholds. The elementary charm production process is described by either baryon/meson exchange or by quark/gluon dynamics. Effects of the interactions in the initial and final states are taken into account rigorously. The calculations are performed in close analogy to our earlier study on ${\bar{p}p \to \bar{\Lambda} \Lambda}$ and ${\bar{p} p \to \bar{K} K}$ by connecting the processes via SU(4) flavor symmetry. Our predictions for the ${\bar{\Lambda}_c \Lambda_c}$ production cross section are in the order of 1 to 7 mb, i.e. a factor of around 10?C70 smaller than the corresponding cross sections for ${\bar{\Lambda} \Lambda}$ However, they are 100 to 1000 times larger than predictions of other model calculations in the literature. On the other hand, the resulting cross sections for ${\bar{D} D}$ production are found to be in the order of 10^?2 ?C 10^?1 ??b and they turned out to be comparable to those obtained in other studies.     

Two-jet production inp\bar p collisions

The production of two quark/gluon jets in \(p\bar p\) collisions is analyzed. We determine the cross section dependence on the transverse momentum, the rapidities, and on the two-jet invariant mass, and we evaluate the sensitivity of the results on the theoretical input assumptions and the experimental cuts. We extrapolate these results into the TeV energy region.     

Form factors of η c in light-front quark model

We study the form factors of the η _c meson in the light-front quark model. We explicitly show that the transition form factor of η _c →γ ^? γ as a function of the momentum transfer is consistent with the experimental data by the BaBar collaboration, while the decay constant of η _c is found to be $f_{\eta_{c}}=230.5^{+52.2}_{-61.0}$ and $303.6^{+115.2}_{-116.4}~\mathrm{MeV}$ for $\eta_{c}\sim c\bar{c}$ by using two η _c →γγ decay widths of 5.3±0.5 and 7.2±2.1 keV, given by Particle Data Group and Lattice QCD calculation, respectively.     

Coherent versus incoherent quark fragmentation picture

We study the helicity density matrix of vector mesons produced ine ⁺ e ^? interactions at high energies through the two step process \(e^ + e^ - \to q\bar q \to VX\) . Whereas in the usual incoherent fragmentation picture in which each quark decays independently, ρ(V) is predicted to be diagonal, we find that final state interactions (coherent fragmentation) give rise to a non-vanishing ρ_1-1(V). Various corrections to the standard picture such as, e.g., gluon bremsstrahlung, quark masses, transverse momenta, are shown to be small. Therefore, any significant non-zero value of ρ_1-1(V) found experimentally can be considered as a clear measurement of coherence effects.     

Higgs production in gluon fusion at next-to-next-to-leading order QCD for finite top mass

The inclusive Higgs production cross section from gluon fusion is calculated through NNLO QCD, including its top quark mass dependence. This is achieved through a matching of the 1/M _t expansion of the partonic cross sections to the exact large- $\hat{s}$ limits which are derived from k _T -factorization. The accuracy of this procedure is estimated to be better than 1% for the hadronic cross section. The final result is shown to be within 1% of the commonly used effective theory approach, thus confirming earlier findings.     

On strange quark scalar condensate in the nucleon

A well known difficulty with a large value of the σ term in πN scattering is analysed from positions of the QCD sum rules approach. The matrix element \(\left\langle {p\left| {\bar ss} \right|p} \right\rangle\) is related to flavour singlet correlation function of two quark condensates at zero momentum. The splittings \(\left\langle {p\left| {\bar uu - \bar ss} \right|p} \right\rangle\) and \(\left\langle {p\left| {\bar dd - \bar ss} \right|p} \right\rangle\) are calculated and turn to be in agreement withSU ₃ relations.     

Large N approach to Kaon decays and mixing 28 years later: \Delta I=1/2 rule, \hat{B}_K,and \Delta M_K

We review and update our results for $K\rightarrow \pi \pi $ decays and $K^0$ – $\bar{K}^0$ mixing obtained by us in the 1980s within an analytic approximate approach based on the dual representation of QCD as a theory of weakly interacting mesons for large $N$ , where $N$ is the number of colors. In our analytic approach the Standard Model dynamics behind the enhancement of $\hbox {Re}A_0$ and suppression of $\hbox {Re}A_2$ , the so-called $\Delta I=1/2$ rule for $K\rightarrow \pi \pi $ decays, has a simple structure: the usual octet enhancement through the long but slow quark–gluon renormalization group evolution down to the scales $\mathcal{O}(1\, {\hbox { GeV}})$ is continued as a short but fast meson evolution down to zero momentum scales at which the factorization of hadronic matrix elements is at work. The inclusion of lowest-lying vector meson contributions in addition to the pseudoscalar ones and of Wilson coefficients in a momentum scheme improves significantly the matching between quark–gluon and meson evolutions. In particular, the anomalous dimension matrix governing the meson evolution exhibits the structure of the known anomalous dimension matrix in the quark–gluon evolution. While this physical picture did not yet emerge from lattice simulations, the recent results on $\hbox {Re}A_2$ and $\hbox {Re}A_0$ from the RBC-UKQCD collaboration give support for its correctness. In particular, the signs of the two main contractions found numerically by these authors follow uniquely from our analytic approach. Though the current–current operators dominate the $\Delta I=1/2$ rule, working with matching scales $\mathcal{O}(1 \, {\hbox { GeV}})$ we find that the presence of QCD-penguin operator $Q_6$ is required to obtain satisfactory result for $\hbox {Re}A_0$ . At NLO in $1/N$ we obtain $R=\hbox {Re}A_0/\hbox {Re}A_2= 16.0\pm 1.5$ which amounts to an order of magnitude enhancement over the strict large $N$ limit value $\sqrt{2}$ . We also update our results for the parameter $\hat{B}_K$ , finding $\hat{B}_K=0.73\pm 0.02$ . The smallness of $1/N$ corrections to the large $N$ value $\hat{B}_K=3/4$ results within our approach from an approximate cancelation between pseudoscalar and vector meson one-loop contributions. We also summarize the status of $\Delta M_K$ in this approach.