Γγ Decay Research Articles

Contributions of electromagnetic and strong anomalies to the η(η′) → γγ decays

We study contributions of electromagnetic and strong axial anomalies in the radiative decays of η and η′. Applying a dispersive approach, we derive the anomaly sum rule for a singlet axial current where both electromagnetic and strong parts of the axial exist. A low energy theorem is generalized for a case of mixing states and is applied to evaluate a subtraction part of the strong anomaly. We found a relatively small contribution of the strong anomaly to the two-photon decay amplitudes of the η and η′ mesons.

Decay of a pseudoscalar into two photons

The existence of new particles whose masses could be found in the region of TeVs are predicted by extensions of the Standard Model, one of these is the Littlest Higgs model, where it is proposed the existence of a new neutral massive pseudoscalar particle. In this context we analyze the ΦP → γγ decay, where γ symbolizes the photon and ΦP is the pseudoscalar particle predicted by this model.

Higgs Boson Physics at CMS

An overview of the standard model Higgs boson analyses and beyond standard model Higgs boson searches will be given. A data sample of proton-proton collisions at s=13 TeV collected with the CMS detector at the LHC is used. The measurements of properties of the Higgs boson in the 4l and γγ decay channels will be presented. The future prospects will be covered.

Studies of the Higgs boson spin and parity using the γγ, ZZ, and WW decay channels with the CMS detector

Studies of the Higgs boson spin and parity are presented using data samples corresponding to the γγ, ZZ, and WW decay channels. The analyses are based on pp collision data collected at centre-of-mass energies of 7 and 8 TeV, corresponding to integrated luminosities of approximately 5 fb-1 and 20 fb-1, respectively. The data are compared to the expectations for the standard model Higgs boson, and for several alternative models.

Charmonia decay constants from the QCD lattice and QCD sum rules

Using lattice QCD and QCD sum rules we compute the lowest state charmonia JPC=0−+(ηc), 1−(J/ψ), and 1+−(hc) decay constants. For calculating the decay constant of J/ψ we use both the vector and tensor currents and compare the results. Lattice QCD results are obtained from the unquenched (Nf=2) simulations using twisted mass QCD at four lattice spacings and taking the continuum limit. In the QCD sum rule calculation we apply the moment sum rules. We also comment the phenomenological implications of calculated charmonia decay constants in ηc→γγ decay, and B→Xcc‾K decays.

Property measurement of the Higgs boson in the γγ final state with the ATLAS detector at the LHC

With pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at center-of-mass energies of 7 TeV and 8 TeV, corresponding to an integrated luminosity of 25 fb-1, an improved measurement of the mass of the Higgs boson is derived from a combined fit to the decay channels H→γγ and H→ZZ⁎→4l with improved energy-scale calibrations for photons, electrons and muons as well as other analysis improvements. It is mH=125.36±0.37(stat)±0.18(syst) GeV. Furthermore, measurement of fiducial and differential cross sections are presented in the H→γγ decay channel using only the 8 TeV data sample with a luminosity of 20.3fb-1. The observed spectra are statistically limited but broadly in line with the theoretical expectations.

The 126 GeV Higgs Boson in a general MSSM model with explicit CP-violation

In this work, we use the results from Higgs searches in the ττ‾ and γγ decay channels at LHC and indirect bounds as BR(B→Xsγ) to constrain the parameter space of a generic MSSM Higgs sector. In particular, we include the latest CMS results to look for additional Higgs states with masses up to 1 TeV. We show that the ττ‾ channel is the best and most accurate weapon in the hunt for new Higgs states beyond the Standard Model. We obtain that recent experimental results rule out additional neutral Higgs bosons in a generic MSSM below 300 GeV for any value of tan β and that, for instance, values of tan β above 30 are only possible for Higgs masses above 600 GeV. ATLAS stored data have the potential to render this bound obsolete in the near future.

Enhancing the [formula omitted] signal through top-quark spin polarization effects at the LHC

We consider the impact of the tt‾ spin correlations in the Higgs boson production in association with top quark pairs pp→tt‾H and in the corresponding backgrounds. We study the H→bb‾ and H→γγ decay modes. We show that retaining the tt‾ spin-correlation effects could significantly improve the LHC sensitivity to the tt‾H signal, particularly in the H→γγ channel.

Constraints on non-Standard Model Higgs boson interactions in an effective Lagrangian using differential cross sections measured in the H → γγ decay channel at [formula omitted] with the ATLAS detector

The strength and tensor structure of the Higgs boson's interactions are investigated using an effective Lagrangian, which introduces additional CP-even and CP-odd interactions that lead to changes in the kinematic properties of the Higgs boson and associated jet spectra with respect to the Standard Model. The parameters of the effective Lagrangian are probed using a fit to five differential cross sections previously measured by the ATLAS experiment in the H→γγ decay channel with an integrated luminosity of 20.3 fb−1 at s=8 TeV. In order to perform a simultaneous fit to the five distributions, the statistical correlations between them are determined by re-analysing the H→γγ candidate events in the proton–proton collision data. No significant deviations from the Standard Model predictions are observed and limits on the effective Lagrangian parameters are derived. The statistical correlations are made publicly available to allow for future analysis of theories with non-Standard Model interactions.

Performance of photon reconstruction and identification with the CMS detector in proton-proton collisions at √s = 8 TeV

A description is provided of the performance of the CMS detector for photon reconstruction and identification in proton-proton collisions at a centre-of-mass energy of 8 TeV at the CERN LHC. Details are given on the reconstruction of photons from energy deposits in the electromagnetic calorimeter (ECAL) and the extraction of photon energy estimates. The reconstruction of electron tracks from photons that convert to electrons in the CMS tracker is also described, as is the optimization of the photon energy reconstruction and its accurate modelling in simulation, in the analysis of the Higgs boson decay into two photons. In the barrel section of the ECAL, an energy resolution of about 1% is achieved for unconverted or late-converting photons from H→γγ decays. Different photon identification methods are discussed and their corresponding selection efficiencies in data are compared with those found in simulated events.

Results and prospects on hadronic cross section and γγ physics at KLOE/KLOE-2

The KLOE experiment was the first to exploit Initial State Radiation (ISR) processes to obtain the e+e−→π+π−(γ) cross section below 1 GeV. Three measurements have been published with small angle photon in the initial state: normalized using luminosity from Bhabha in 2005 and 2008 and using ππγ/μμγ ratio in 2014. An independent measurement with the large angle photon normalized to Bhabha has been published in 2011. These measurements are in good agreement, showing a good consistency among different analyses. A preliminary Gounaris-Sakurai fit of the pion form factor of the combined KLOE measurements is presented. We also discuss the γγ physics program of KLOE-2 experiment, made possible after the recent KLOE upgrade with new small angle tagging detectors along the DAFNE beam line. These detectors allow to detect the final leptons coming from process e+e−→e+e−γ⁎γ⁎ giving the possibility to measure the π0→γγ decay and γ⁎γ→π0 transition form factor. These measurements provide constraints to the models used to calculate the hadronic light-by-light contribution to the muon anomaly.

ChPT test at NA48 and NA62 experiment at CERN

The new published measurements of the K±→π±γγ decay with minimum bias data samples collected by the two experiments NA48/2 and NA62 are discussed. The results include a model independent decay rate measurement and fits to Chiral Perturbation Theory (ChPT) description. The model independent branching ratio in the kinematic range z=(mγγ/mK)2>0.2, obtained by combining the NA48/2 and NA62 results, is BMI=(0.965±0.63)×10−6. The combined branching ratio in the full kinematic range assuming a ChPT description is BChPT=(1.003±0.056)×10−6. The uncertainties are dominated by the statistical errors. The data support the ChPT prediction for a cusp in the di-photon invariant mass spectrum at the two pion threshold and are in agreement with both O(p4) and O(p6) ChPT predictions.

Measurement of the Higgs boson mass with the ATLAS detector

A summary of the latest results on the combined measurement of the Higgs boson mass in the H → ZZ* → 4l and the H → γγ decay channels with the ATLAS detector is presented. The analysis uses 25 fb−1 of pp collision data recorded by the ATLAS detector at the CERN Large Hadron Collider at centre-of-mass energies of 7TeV and 8 TeV during 2011 and 2012. The combined measured value of the Higgs boson mass is mH = 125.36 ± 0.37 (stat) ± 0.18 (syst) GeV.

Measurement of the Higgs boson mass with the ATLAS detector

A summary of the latest results on the combined measurement of the Higgs boson mass in the H → ZZ * → 4l and the H → γγ decay channels with the ATLAS detector is presented. The analysis uses 25 fb−1 of pp collision data recorded by the ATLAS detector at the CERN Large Hadron Collider at centre-of-mass energies of 7TeV and 8 TeV during 2011 and 2012. The combined measured value of the Higgs boson mass is mH = 125.36 ± 0.37 (stat) ± 0.18 (syst) GeV.

On Z → γγ decay and cancellation of axial anomaly in Z → γγ transition amplitude for massive fermions

$Z \to \gamma \gamma$ decay amplitude is considered and proven to be zero due to properties of polarization vectors and Bose statistics. Triangular diagrams for a pseudoscalar $\to \gamma \gamma$ and $Z \to \gamma \gamma$ processes with massive fermions in the loop are explicitely calculated. In the Standard Model axial anomaly vanishes in the sum of these diagrams as Z boson is mixed with one of the Goldstone bosons.

Lattice QCD and QCD sum rule determination of the decay constants of [formula omitted], [formula omitted] and [formula omitted] states

We compute the decay constants of the lowest cc¯-states with quantum numbers JPC=0−+ (ηc), 1−− (J/ψ), and 1+− (hc) by using lattice QCD and QCD sum rules. We consider the coupling of J/ψ to both the vector and tensor currents. Lattice QCD results are obtained from the unquenched (Nf=2) simulations using twisted mass QCD at four lattice spacings, allowing us to take the continuum limit. On the QCD sum rule side we use the moment sum rules. The results are then used to discuss the rate of ηc→γγ decay, and to comment on the factorization in B→Xcc¯K decays, with Xcc¯ being either ηc or J/ψ.

Kaons at CERN: status and prospects

The NA48/2 and NA62 experiments at CERN exploit the decay in flight technique to study rare kaon decays and the underlying physics. Two selected topics will be presented in this paper. The first is a stringent test of the Chiral Perturbation Theory, obtained by new measurements of the K± → π±γγ decay performed by the two experiments. The second one is the measurement of the Branching Ratio of the very rare decay K+ → π+vv̄ at 10% level, that will provide a test of the Standard Model and possible extensions beyond it.

ηandη′Physics at MAMI

The Crystal Ball/TAPS setup at MAMI is ideally suited to detect neutral and electromagnetic decays of the η and η′ mesons. In this contribution recent results on η and η′ decays from the Crystal Ball/TAPS experiment at MAMI are presented. These are the accurate measruement of η′ photoproduction cross-sections in the threshold region, the determination of the timelike electromagnetic transition form factors, and a test of low-energy QCD with the η → π 0 γγ decay. Finally, further perspectives are discussed.

On Z →γγ Decay and Cancellation of Axial Anomaly in Z →γγ Transition Amplitude for Massive Fermions

On Z →γγ Decay and Cancellation of Axial Anomaly in Z →γγ Transition Amplitude for Massive Fermions

Kaons at CERN: recent results and prospects

The NA48/2 and NA62 experiments at CERN study rare kaon decays using the decay in flight technique. In this paper two selected topics will be presented: new measurements performed by the two experiments of the K ± → π ± γγ decay which rep- resents a stringent test of the Chiral Perturbation Theory and the measurement of the Branching Ratio of the very rare decay K + → π + νν at 10% level, that will provide a test of the Standard Model and possible extensions beyond it.