Production of two charm quark–antiquark pairs in single-parton scattering within the kt -factorization approach

We present first results for the 2→4 single-parton scattering gg→cc¯cc¯ subprocess for the first time fully within the kt -factorization approach. In this calculation we have used the Kimber–Martin–Ryskin unintegrated gluon distribution which effectively includes some class of higher-order gluon emissions, and an off-shell matrix element squared calculated using recently developed techniques. The results are compared with our earlier result obtained within the collinear-factorization approach. Only slightly larger cross sections are obtained than in the case of the collinear approach. Inclusion of transverse momenta of gluons entering the hard process leads to a much stronger azimuthal decorrelation between cc and c¯c¯ than in the collinear-factorization approach. A comparison to predictions of double parton scattering (DPS) results and the LHCb data strongly suggests that the assumption of two fully independent DPS ( gg→cc¯⊗gg→cc¯ ) may be too approximate

Constraining the double gluon distribution by the single gluon distribution

We show how to consistently construct initial conditions for the QCD evolution equations for double parton distribution functions in the pure gluon case. We use to momentum sum rule for this purpose and a specific form of the known single gluon distribution function in the MSTW parameterization. The resulting double gluon distribution satisfies exactly the momentum sum rule and is parameter free. We also study numerically its evolution with a hard scale and show the approximate factorization into product of two single gluon distributions at small values of x , whereas at large values of x the factorization is always violated in agreement with the sum rule

BCFW recursion for TMD parton scattering

We investigate the application of the BCFW recursion relation to scattering amplitudes with one off-shell particle in a Yang-Mills theory with fermions. We provide a set of conditions of applicability of the BCFW recursion, stressing some important differences with respect to the pure on-shell case. We show how the formulas for Maximally-Helicity-Violating (MHV) configurations with any number of partons, which are well known in the fully on-shell case, are generalized to this kinematic regime. We also derive analytic expressions for all the helicity configurations of the 5-point color-stripped tree-level amplitudes for any of the partons being off the mass shell

BCFW recursion for off-shell gluons

It is shown how tree-level multi-gluon helicity amplitudes with an arbitrary number of off-shell external gluons can be calculated via BCFW recursion. Compact expressions for helicity amplitudes for scattering processes of three and four gluons, with up to three of them off-shell, are presented. Also, maximum-helicity-violating configurations are identified for up to two off-shell gluons, and the expressions for their helicity amplitudes for an arbitrary number of on-shell gluons are derived

The role of finite-size effects on the spectrum of equivalent photons in proton–proton collisions at the LHC

Photon–photon interactions represent an important class of physics processes at the LHC, where quasi-real photons are emitted by both colliding protons. These reactions can result in the exclusive production of a final state X , p+p→p+p+X . When computing such cross sections, it has already been shown that finite size effects of colliding protons are important to consider for a realistic estimate of the cross sections. These first results have been essential in understanding the physics case of heavy-ion collisions in the low invariant mass range, where heavy ions collide to form an exclusive final state like a J/Ψ vector meson. In this paper, our purpose is to present some calculations that are valid also for the exclusive production of high masses final states in proton–proton collisions, like the production of a pair of W bosons or the Higgs boson. Therefore, we propose a complete treatment of the finite size effects of incident protons irrespective of the mass range explored in the collision. Our expectations are shown to be in very good agreement with existing experimental data obtained at the LHC

Search for technipions in exclusive production of diphotons with large invariant masses at the LHC

We focus on exclusive production of neutral technipion <math altimg="si1.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mover accent="true"><mrow><mi>π</mi></mrow><mrow><mo stretchy="false">˜</mo></mrow></mover></mrow><mrow><mn>0</mn></mrow></msup></math> in pp collisions at the LHC, i.e. on <math altimg="si2.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mi>p</mi><mi>p</mi><mo stretchy="false">→</mo><mi>p</mi><mi>p</mi><msup><mrow><mover accent="true"><mrow><mi>π</mi></mrow><mrow><mo stretchy="false">˜</mo></mrow></mover></mrow><mrow><mn>0</mn></mrow></msup></math> reaction. The dependence of the cross section on parameters of recently proposed vector-like Technicolor model is studied. Characteristic features of the differential distributions are discussed. For not too large technipion masses the diphoton decay channel has the dominant branching fraction. This is also the main reason for an enhanced production of neutral technipions in γγ -fusion reaction. We discuss potential backgrounds of the QCD and QED origin to the <math altimg="si3.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mi>p</mi><mi>p</mi><mo stretchy="false">→</mo><mi>p</mi><mi>p</mi><mo stretchy="false">(</mo><msup><mrow><mover accent="true"><mrow><mi>π</mi></mrow><mrow><mo stretchy="false">˜</mo></mrow></mover></mrow><mrow><mn>0</mn></mrow></msup><mo stretchy="false">→</mo><mi>γ</mi><mi>γ</mi><mo stretchy="false">)</mo></math> process at large invariant γγ masses. We conclude that compared to inclusive case the signal-to-background ratio in the considered exclusive reaction is very favorable which thereby could serve as a good probe for Technicolor dynamics searches at the LHC

Search for light vector boson production in e+e−→μ+μ−γ interactions with the KLOE experiment

We have searched for a light vector boson U , the possible carrier of a “dark force”, with the KLOE detector at the DAΦNE e+e− collider, motivated by astrophysical evidence for the presence of dark matter in the Universe. Using e+e− collisions collected with an integrated luminosity of 239.3 pb −1 , we look for a dimuon mass peak in the reaction e+e−→μ+μ−γ , corresponding to the decay U→μ+μ− . We find no evidence for a U vector boson signal. We set a 90% CL upper limit for the mixing parameter squared between the photon and the U boson of 1.6×10−5 to 8.6×10−7 for the mass region 520<mU<980 MeV

Measurement of the absolute branching ratio of the K+→π+π−π+(γ) decay with the KLOE detector

The absolute branching ratio of the K+→π+π−π+(γ) decay, inclusive of final-state radiation, has been measured using ∼17 million tagged K+ mesons collected with the KLOE detector at DAΦNE, the Frascati ϕ -factory. The result is: BR(K+→π+π−π+(γ))=0.05565±0.00031stat±0.00025syst a factor ≃ 5 more precise with respect to the previous result. This work completes the program of precision measurements of the dominant kaon branching ratios at KLOE

Study of the Dalitz decay ϕ→ηe+e− with the KLOE detector

We have studied the vector to pseudoscalar conversion decay ϕ→ηe+e− , with η→π0π0π0 , with the KLOE detector at DA Φ NE. The data set of 1.7 fb−1 of e+e− collisions at s∼Mϕ contains a clear conversion decay signal of ∼31,000 events from which we measured a value of BR(ϕ→ηe+e−)=(1.075±0.007±0.038)×10−4 . The same sample is used to determine the transition form factor by a fit to the e+e− invariant mass spectrum, obtaining bϕη=(1.28±0.10−0.08+0.09) GeV−2 , that improves by a factor of five the precision of the previous measurement and is in good agreement with VMD expectations

Parton distribution functions at LO, NLO and NNLO with correlated uncertainties between orders

Sets of parton distribution functions (PDFs) of the proton are reported for the leading (LO), next-to-leading (NLO) and next-to-next-to-leading-order (NNLO) QCD calculations. The parton distribution functions are determined with the HERAFitter program using the data from the HERA experiments and preserving correlations between uncertainties for the LO, NLO and NNLO PDF sets. The sets are used to study cross-section ratios and their uncertainties when calculated at different orders in QCD. A reduction of the overall theoretical uncertainty is observed if correlations between the PDF sets are taken into account for the ratio of WW di-boson to Z boson production cross sections at the LHC