Diphoton Production Research Articles

VARIOUS MODELS MIMICKING THE SM HIGGS BOSON

The new particle around 125 GeV observed at the Large Hadron Collider (LHC) is almost consistent with the standard model (SM) Higgs boson, except that the diphoton decay mode may be excessive. We summarize a number of possibilities. While at the LHC the dominant production mechanism for the Higgs boson of SM and some other extensions is via the gluon fusion process, the alternative vector-boson fusion (VBF) is more sensitive to electroweak symmetry breaking. Using the well-known dijet-tagging technique to single out the VBF mechanism, we investigate potential of VBF to discriminate a number of models suggested to give an enhanced inclusive diphoton production rate.

Consistent simulation of nonresonant diphoton production at hadron collisions with a custom-made parton shower

We have developed a Monte Carlo event generator for non-resonant diphoton ($\gamma\gamma$) production at hadron collisions in the framework of GR@PPA, which consistently includes additional one-jet production. The jet-matching method developed for initial-state jet production has been extended to the final state in order to regularize the final-state QED divergence in the $qg \rightarrow \gamma\gamma + q$ process. A QCD/QED-mixed parton shower (PS) has been developed to complete the matching. The PS has the capability of enforcing hard-photon radiation, and small-$Q^{2}$ photon radiations that are not covered by the PS are supplemented by using a fragmentation function. The generated events can be passed to general-purpose event generators in order to perform the simulations down to the hadron level. Thus, we can simulate the isolation requirements that must be applied in experiments at the hadron level. The simulation results are in reasonable agreement with the predictions from RESBOS and DIPHOX. The simulated hadron-level events can be further fed to detector simulations in order to investigate the detailed performance of experiments.

Diphoton Production at Hadron Colliders: A Fully Differential QCD Calculation at Next-to-Next-to-Leading Order

We consider direct diphoton production in hadron collisions, and we compute the next-to-next-to-leading order QCD radiative corrections at the fully differential level. Our calculation uses the q(T) subtraction formalism, and it is implemented in a parton-level Monte Carlo program. The program allows the user to apply arbitrary kinematical cuts on the final-state photons and the associated jet activity and to compute the corresponding distributions in the form of bin histograms. We present selected numerical results related to Higgs boson searches at the LHC and corresponding results at the Tevatron.

Next-to-leading-order Monte Carlo simulation of diphoton production in hadronic collisions

We present a method, based on the positive weight next-to-leading-order matching formalism (POWHEG), to simulate photon production processes at next-to-leading-order (NLO). This technique is applied to the simulation of diphoton production in hadron-hadron collisions. The algorithm consistently combines the parton shower and NLO calculation, producing only positive weight events. The simulation includes both the photon fragmentation contribution and a full implementation of the truncated shower required to correctly describe soft emissions in an angular-ordered parton shower.

Recent QCD results from the Tevatron

Recent QCD related results from the CDF and the D0 experiments are presented based on proton anti-proton collision data at sqrt(s)=1.96 TeV, taken in Run II of the Fermilab Tevatron Collider. Measured observables include inclusive photon and diphoton production, vector boson plus jets production, event shape variables, and inclusive multijet production. The measurement results are compared to QCD theory calculations in different approximations. A determination of the strong coupling constant from jet data is presented.

Probing Gluonic Spin-Orbit Correlations in Photon Pair Production

We consider photon pair production in hadronic collisions at large mass and small transverse momentum of the pair, assuming that factorization in terms of transverse-momentum dependent parton distributions applies. The unpolarized cross section is found to have azimuthal angular dependencies that are generated by a gluonic version of the Boer-Mulders function. In addition, the single transversely polarized cross section is sensitive to the gluon Sivers function. We present simple numerical estimates for the Boer-Mulders and Sivers effects in diphoton production at RHIC and find that the process would offer unique opportunities for exploring transverse-momentum dependent gluon distributions.

Hard photon production and matrix-element parton-shower merging

We present a Monte-Carlo approach to prompt-photon production, where photons and QCD partons are treated democratically. The photon fragmentation function is modelled by an interleaved QCD+QED parton shower. This known technique is improved by including higher-order real-emission matrix elements. To this end, we extend a recently proposed algorithm for merging matrix elements and truncated parton showers. We exemplify the quality of the Monte-Carlo predictions by comparing them to measurements of the photon fragmentation function at LEP and to measurements of prompt photon and diphoton production from the Tevatron experiments.

Diphoton production at the Tevatron in the quasi-multi-Regge-kinematic approach

We study the production of prompt diphotons in the central region of rapidity within the framework of the quasi-multi-Regge-kinematics approach applying the hypothesis of quark and gluon Reggeization. We describe accurately and without free parameters the experimental data which were obtained by the CDF Collaboration at the Tevatron Collider. It is shown that the main contribution to studied process is given by the direct fusion of two Reggeized gluons into a photon pair, which is described by the effective Reggeon-Reggeon to particle-particle vertex. The contribution from the annihilation of Reggeized quark-antiquark pair into a diphoton is also considered. At the stage of numerical calculations we use the Kimber-Martin-Ryskin prescription for unintegrated quark and gluon distribution functions, with the Martin-Roberts-Stirling-Thorne collinear parton densities for a proton as input.

Unparticles in diphoton production to next-to-leading order in QCD at the LHC

We compute to next-to-leading order in QCD the tensor unparticle contribution to the diphoton production at the LHC, wherein the unparticle sector is a consequence of (a) scale invariance but not full conformal invariance and (b) conformal invariance. We use the semianalytical two cutoff phase-space slicing method to handle the $\mathcal{O}({\ensuremath{\alpha}}_{s})$ corrections to the $pp\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}X$ and show that our results are insensitive to the soft and collinear cutoffs. In order to avoid the contribution of the photons due to fragmentation, we employ the smooth cone isolation criterion. Significance of the QCD corrections to the diphoton events including unparticles is highlighted.

Diphoton signals in theories with large extra dimensions to NLO QCD at hadron colliders

We present a full next-to-leading order (NLO) QCD corrections to diphoton production at the hadron colliders in both standard model and ADD model. The invariant mass and rapidity distributions of the diphotons are obtained using a semi-analytical two cut-off phase space slicing method which allows for a successful numerical implementation of various kinematical cuts used in the experiments. The fragmentation photons are systematically removed using smooth-cone-isolation cuts on the photons. The NLO QCD corrections not only stabilise the perturbative predictions but also enhance the production cross section significantly.

RS model with a small curvature and two-photon production at the LHC

The p⊥-distribution for the diphoton production at the LHC is calculated in the modified Randall-Sundrum model with a small curvature of the space-time κ, and 5-dimensional Planck scale M5 in the TeV region. The discovery limits on M5 are obtained to be 9.4 TeV and 11.6 TeV for the integrated luminosities 30 fb−1 and 100 fb−1, respectively. These limits do not depend on κ provided it is much smaller than M5. The account of graviton widths is a crucial points of our calculations.

Publisher’s Note: Unparticle physics in diphoton production at the CERN LHC [Phys. Rev. D77, 055013 (2008)

Publisher’s Note: Unparticle physics in diphoton production at the CERN LHC [Phys. Rev. D<b>77</b>, 055013 (2008)

Unparticle physics in diphoton production at the CERN LHC

We have considered the diphoton production in the unparticle physics at the LHC. The contributions of spin-0 and spin-2 unparticles to the diphoton production are studied in the invariant mass and other kinematical distributions, along with their dependencies on the model dependent parameters. It is found that the signal corresponding to the unparticles is significant for moderate values of the couplings.

Search for excited quarks inqq¯→γγat the CERN LHC

If quarks are composite particles, then excited states are expected to play a r\^ole in the Large Hadron Collider phenomena. Concentrating on virtual effects, and using a large part of the compact muon solenoid detection criteria, we present here a realistic examination of their effect in diphoton production at the CERN LHC. For various luminosities, we present the 99% confidence limit (CL) achievable in $\ensuremath{\Lambda}\ensuremath{-}{M}_{q*}$ parameter space where $\ensuremath{\Lambda}$ is the compositeness scale and ${M}_{{q}^{*}}$ the mass of the state. For a ${q}^{*}$ of mass 0.5 TeV, $\ensuremath{\Lambda}\ensuremath{\le}1.55(2.95)$ can be excluded at 99% CL with 30 $(200)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity.

Collider phenomenology of unparticle physics

Low-energy phenomenology of the unparticle physics associated with an exact scale invariant sector possessing a nontrivial infrared fixed point at a higher energy scale is explored for both electron-positron and hadronic colliders. Feynman rules for a spin 0, 1, or 2 unparticle coupled to a variety of standard model gauge invariant operators that are relevant to many low-energy processes involving either real emissions of unparticles or their virtual propagator effects are presented. Missing energy and/or recoil mass distributions of the unparticle in the associated production of the unparticle together with a photon or Z boson at LEP2 and ILC as well as in Z decay into an unparticle plus a fermion-antifermion pair are studied. In addition, monojet production with missing energy from the unparticle at hadronic collisions are explored. The complex phase in the unparticle propagator that can give rise to interesting interference effects between an unparticle-exchange diagram and the standard model amplitudes are studied in details for the Drell-Yan process as well as muon pair and diphoton production in electron-positron annihilation. These energy and/or recoil mass distributions (with the exception in hadron colliders) and interference effects are found sensitively depending not only on the scaling dimension but also onmore » the spin of the unparticle. For the spin-2 unparticle, its physical effects are found to resemble that of a tower of Kaluza-Klein gravitons, which strongly indicates that the underlying unparticle physics may have root in a higher dimensional theory. A connection between unparticle physics and theories of large extra dimensions is speculated. Experimental constraints on the unparticle scale are deduced from the LEP2 data on monophoton production and from the 4-fermion contact interactions.« less

Calculation of prompt diphoton production cross sections at Fermilab Tevatron and CERN LHC energies

A fully differential calculation in perturbative quantum chromodynamics is presented for the production of massive photon pairs at hadron colliders. All next-to-leading order perturbative contributions from quark-antiquark, gluon-(anti)quark, and gluon-gluon subprocesses are included, as well as all-orders resummation of initial-state gluon radiation valid at next-to-next-to-leading logarithmic accuracy. The region of phase space is specified in which the calculation is most reliable. Good agreement is demonstrated with data from the Fermilab Tevatron, and predictions are made for more detailed tests with CDF and DO data. Predictions are shown for distributions of diphoton pairs produced at the energy of the Large Hadron Collider (LHC). Distributions of the diphoton pairs from the decay of a Higgs boson are contrasted with those produced from QCD processes at the LHC, showing that enhanced sensitivity to the signal can be obtained with judicious selection of events.

New CDF results on diffraction

We report new diffraction results obtained by the CDF collaboration in proton-antiproton collisions at the Fermilab Tevatron collider at <FONT FACE=Symbol>Ö</FONT>s = 1.96 TeV. The first experimental evidence of exclusive dijet and diphoton production is presented. The exclusive results are discussed in context of the exclusive Higgs production at LHC. We also present the measurement of the Q² and t dependence of the diffractive structure function.

Diphoton production from quark–gluon plasma and hadronic matter

The rate of diphoton production is calculated from a quark–gluon plasma and compared with the rate from hadronic matter. Background diphotons are identified and quantified, including effects from hadronic form factors. Robust thermometry and spectroscopy are shown to be possible owing to the structure of the expected invariant mass spectrum. Charmonium could also be probed with diphotons.

All-orders resummation for diphoton production at hadron colliders

We present a QCD calculation of the transverse momentum distribution of photon pairs produced at hadron colliders, including all-orders soft-gluon resummation valid at next-to-next-to-leading logarithmic accuracy. We specify the region of phase space in which the calculation is most reliable, compare our results with data from the Fermilab Tevatron, and make predictions for the Large Hadron Collider. The uncertainty of predictions for production of diphotons from fragmentation of final-state quarks is examined.

Measurement of the Cross Section for Prompt Diphoton Production inpp¯Collisions ats=1.96 TeV

We report a measurement of the rate of prompt diphoton production in $p\bar{p}$ collisions at $\sqrt{s}=1.96 ~\hbox{TeV}$ using a data sample of 207 pb$^{-1}$ collected with the upgraded Collider Detector at Fermilab (CDF II). The background from non-prompt sources is determined using a statistical method based on differences in the electromagnetic showers. The cross section is measured as a function of the diphoton mass, the transverse momentum of the diphoton system, and the azimuthal angle between the two photons and is found to be consistent with perturbative QCD predictions.