Γγ Width Research Articles

The KLOE-2 high energy taggers

The precision measurement of the π0 → γγ width allows to gain insights into the low-energy QCD dynamics. A way to achieve the precision needed (1%) in order to test theory predictions is to study the π0 production through γγ fusion in the e+e− → e+e−γ*γ* → e+e−π0 reaction. The KLOE-2 experiment, currently running at the DAΦNE facility in Frascati, aims to perform this measurement. For this reason, new detectors, which allow to tag final state leptons, have been installed along the DAΦNE beam line in order to reduce the background coming from ϕ-meson decays. The High Energy Tagger (HET) detector measures the deviation of leptons from their main orbit by determining their position and timing. The HET detectors are placed in roman pots just at the exit of the DAΦNE dipole magnets, 11 m away from the IP, both on positron and electron sides. The HET sensitive area is made up of a set of 28 plastic scintillators. A dedicated DAQ electronic board, based on a Xilinx Virtex-5 FPGA, has been developed for this detector. It provides a MultiHit TDC with a time resolution of 550(1) ps and the possibility to clearly identify the correct bunch crossing (ΔTbunch ∼ 2.7 ns). The most relevant features of the KLOE-2 tagging system operation as time performance, stability and the techniques used to determine the time overlap between the KLOE and HET asynchronous DAQs will be presented.

Partially composite Higgs in supersymmetry

We propose a framework for natural breaking of electroweak symmetry in supersymmetric models, where elementary Higgs fields are semi-perturbatively coupled to a strong superconformal sector. The Higgs VEVs break conformal symmetry in the strong sector at the TeV scale, and the strong sector in turn gives important contributions to the Higgs potential, giving rise to a kind of Higgs bootstrap. A Higgs with mass 125 GeV can be accommodated without any fine tuning. A Higgsino mass of order the Higgs mass is also dynamically generated in these models. The masses in the strong sector generically violate custodial symmetry, and a good precision electroweak fit requires tuning of order ~ 10%. The strong sector has an approximately supersymmetric spectrum of hadrons at the TeV scale that can be observed by looking for a peak in the WZ invariant mass distribution, as well as final states containing multiple W, Z, and Higgs bosons. The models also generically predict large corrections (either enhancement or suppression) to the h → γγ width.

σ and [formula omitted] substructures from [formula omitted], [formula omitted] radiative and [formula omitted] semi-leptonic decays

Using an improved “analytic K-matrix model”, we reconsider the extraction of the σ≡f0(600) and f0(980)γγ widths from γγ→ππ scatterings data of Crystal Ball and Belle. Our main results are summarized in Tables 3 and 4. The averaged σ “direct width” to γγ is 0.16(3) keV which confirms a previous result of Mennessier et al. (2007, 2008) [1] and which does neither favour a large four-quark/molecule nor a pure q¯q components. The “direct width” of the f0(980) of 0.28(2) keV is much larger than the four-quark expectation but can be compatible with a s¯s or a gluonium component. We also found that the rescattering part of each amplitude is relatively large indicating an important contribution of the meson loops in the determinations of the σ and f0(980)γγ total widths. This is mainly due to the large couplings of the σ and f0(980) to ππ and/or K¯K, which can also be due to a light scalar gluonium with large OZI violating couplings but not necessary to a four-quark or molecule state. Our average results for the total (direct + rescattering) γγ widths: Γσtot=3.08(82) keV, Γf0tot=0.16(1) keV are comparable with the ones from dispersion relations and PDG values. Using the parameters from QCD spectral sum rules, we complete our analysis by showing that the production rates of unmixed scalar gluonia σB(1) and G(1.5–1.6) agree with the data from J/ψ, ϕ radiative and Ds semi-leptonic decays.

A global fit to determine the pseudoscalar mixing angle and the gluonium content of the η' meson

We update the values of the η-η' mixing angle and of the η' gluonium content by fitting our measurement R = BR( → η'γ)/BR( → ηγ) together with several vector meson radiative decays to pseudoscalars (V → Pγ), pseudoscalar mesons radiative decays to vectors (P → Vγ) and the η' → γγ, π0 → γγ widths. From the fit we extract a gluonium fraction of ZG2 = 0.12±0.04, the pseudoscalar mixing angle ψP = (40.4±0.6)° and the −ω mixing angle ψV = (3.32±0.09)°. ZG2 and ψP are fairly consistent with those previously published. We also evaluate the impact on the η' gluonium content determination of future experimental improvements of the η' branching ratios and decay width.

The gVσγ coupling constants in hadron electrodynamics

Recent measurements of the branching ratios for the decays ω→ π 0 π 0 γ and ρ 0→ π 0 π 0 γ lead to coupling constants of the Vσγ interaction ( V is a vector meson) one order of magnitude smaller than previously assumed to describe the threshold cross section of γ+ p→ p+ ρ 0 and the HERMES effect. The new g 2 Vσγ couplings are in contradiction with the predictions of the QCD sum rules, but are in agreement with vector meson dominance estimation of the σ→ γγ width.

E835 results on χc states of charmonium

The experiment E835 at FNAL has done an extensive set of studies on charmonium triplet P states. Precision measurements on mass and width of the χ c0 resonance, not studied by the predecessor experiment E760, are reported. Results on both partial widths and higher multipoles of the radiative transitions χ c → J/ ψγ are given, together with the γγ widths of χ c0 and χ c2 . The first clear evidence of charmonium from p p annihilations in hadronic decay modes is described, and preliminary results on BR(χ c0 → π oπ o, ηη) ∗ BR(χ c0 → p p ) are given.

Search for the glueball candidates f0(1500) and fJ(1710) in γγ collisions

Data taken with the ALEPH detector at LEP1 have been used to search for γγ production of the glueball candidates f 0(1500) and f J (1710) via their decay to π + π −. No signal is observed and upper limits to the product of γγ width and π + π − branching ratio of the f 0(1500) and the f J (1710) have been measured to be Γ(γγ→f 0(1500))· B R(f 0(1500)→π +π −)<0.31 keV and Γ(γγ→f J(1710))· B R(f J(1710)→π +π −)<0.55 keV at 95% confidence level.

Three-jet final states and measuring the gamma gamma width of the Higgs boson at a photon linear collider.

The identification of the intermediate-mass Higgs process $\gamma\gamma\to H \to b \bar b$ will be one of the most important goals of a future photon linear collider. Potentially important backgrounds from the continuum $\gamma\gamma \to c \bar c, b \bar b$ leading-order processes can be suppressed by a factor $m_q^2/s$ by using polarized photon beams in the $J_z=0$ initial-state configuration. We show that the same $m_q^2/s$ suppressions do not necessarily apply to the radiative processes $\gamma\gamma \to c \bar c g, b \bar b g$. These processes can mimic the two-jet topology of the Higgs signal when two of the three partons are collinear, or when one of the partons is soft or directed down the beam pipe. We calculate the contribution of these processes to the two-jet background in the $J_z=0$ channel. We investigate the effect of imposing additional event shape, jet width and secondary vertex cuts on both signal and background, and show that with reasonable detector capabilities it should be possible to reduce the background to a manageable level.

On the 2S meson nonets

A scheme is proposed to identify the first radial excitations of the well established pseudoscalar and vector meson nonets. The states nominated all lie in the mass rangem=1.2–1.5 GeV; and each finds some occurrence in experimental literature, although much confirmation is needed. Identification is aided by a simple model that depends heavily onm2 differences among the states. This model gives a good fit to the ground (1S) states; with one exception, no surprises are found on extension to 2S systems. The exception is the comparative absence till now of observed γγ widths for the 2S pseudoscalars in the predicted range of 2 5 keV. Further investigation is desirable but faces severe difficulties with experimental statistics.

Measurement of γγ widths of charmonium states

Using the CLEO detector at the Cornell Electron Storage Ring, we have searched for two-photon production of charmonium in four-track final states. We have measured Γ γγ ( η c )=5.9 +2.1 −1.8±1.9 keV, and we have obtained 95% CL upper limits of Γ γγ ( X c0 )<6.2 keV and Γ γγ ( X c2 )<1.0 keV.

Gluonia enhancements of the Higgs boson hadronic widths

We use scalar gluonia dominance of the anomalous trace of the energy-momentum tensor and some results from QCD low-energy theorems and spectral sum rules in order to analyze the hadronic and γγ widths of a light Higgs boson of the standard model with a mass below the charm threshold. Large enhancements of the ππ and ηη′ modes occur respectively on the top of the σ(0.9) and G(1.6) poles, while the γγ width can be important around the G mass. Consequently, the Higgs into μ + μ − branching ratio, which is about 10 −2 on these gluonia poles, is much smaller nthan naively expected. Values of these various widths versus the Higgs boson mass are given. A search for the Higgs boson from the study of the ππ and η′η invariant masses could be feasible.

Hadron production in two-photon collisions

The contributions of two-photon physics to meson spectroscopy are first of all the measured γγ widths in Table 1. Further results and conclusions can be summarized as follows: •• Pseudoscalars: the pseudoscalar mixing angle is φp = (−22.2 ±1.2)°; non-qq admixtures, like gluonium, are not needed; no other light 0− states, like radial excitations, are observed; the measured γγ width of the ηc is consistent with the charmonium model.•• Tesor Mesons: Well measured, well understood multiplet; mixing angle ≈ 28°.•• Axial vectors: φ1(1285) and φ1(1420) could be members of the same nonet with mixing angle 45–57°; an exotic explanation is not required by the γγ data.•• Scalar Mesons: the measured γγ width of α0(980) and φ0(975) are smaller than expected for qq states, but consistent with four-quark or KK molecule models; no other I=0 state has been observed.•• D-wave qq(2−+): γγ → π2(1680) → π0π0π0 observed, Γγγ = (1.4±0.3) keV.•• Glueballs: Non-observation of η(1440), φ2(1720) in γγ rreactions supports glueball hypotheses.•• Vector meson pairs: All channels involving ϱ,ω,φ,K∗ are studied; contributions from four-quark states not yet clear; no other good explanation for γγ → ϱϱ near threshold.

Photon photon interactions

Recent results in γγ physics are presented. They include a number of new measurements of the γγ widths of members of the J PC = 0 −+ and 2 ++ SU(3) nonets, a search for 0 -+ radial excitations, studies of low mass ππ production and new results in the measurements of exclusive β c and inclusive D∗ ± production. Formation of spin=1 resonances in tagged γγ∗ scattering is described as well as new results in vector meson pair production. Finally new results on the measurement of the photon structure function F 2 γ are reported.

Search for two-photon production of resonances decaying intoK $$\bar K$$ andK $$\bar K$$ ?

An analysis of the production ofKS0KS0 andK±Ks0π∓ by two quasi-real photons is presented. The cross section forγγ→K0\(\overline {K^0 } \), which is given for the γγ invariant mass range fromK\(\bar K\) threshold to 2.5 GeV, is dominated by thef′(1525) resonance and an enhancement near theK\(\bar K\) threshold. Upper limits on the product of the two-photon width times the branching ratio intoK\(\bar K\) pairs are given forΘ(1700),h(2030), and ξ(2220). For exclusive two-photon production ofK±Ks0π∓ no significant signal was observed. Upper limits are given on the cross section ofγγ→K+\(\overline {K^0 } \)π− orK−K0π+ between 1.4 and 3.2 GeV and on the product of the γγ width times the branching ratio into theK\(\bar K\)π final states for theηc(2980) and the ι(1440), yieldingΓ(γγ)→i(1440))·BR(i(1440)→K\(\bar K\)π<2.2 keV at 95% C.L.

Meson pair production in γγ scattering and implications for scalar mesons

We describe a simple analytic unitary model which provides with a parametrization of low energy (from threshold up to 1.4GeV) two photon production of ππ andK\(\bar K\) pairs in term of a few parameters related to the γγ width of the scalar and tensor resonances. It can be useful for analysing data because of the large continuum found in γγπ+π− production besides thef0 (1,270) peak in recent experiments. We show that the present data already provides a strong evidence for the existence of a broad low mass scalar state, the old σ or e (600) with a γγ width of 5 to 9KeV, and that it will be difficult to get more information on the controversialS* parameters from ππ pair production only. In thef0 range we explain the downwards shift of the π+π− peak but we expect the π0π0 peak around 1.27–1.29 GeV which could raise the question either of systematic errors in mass calibration of crystal Ball experiment or of the precise value of thef0 mass.

Two-photon annihilation into pion pair

The first experimental study of the interaction γγ → π + π t- has been carried out in the region of ϵ(660) through the reaction e +e − → e +e − π + π −. Based on two observed events, the γγ width of ϵ(660) is obtained to be Γ ϵγγ = 9.6 −8.0 +12.3 keV. Applied on this, theory of P.C.D.C. anomaly predicts R ∞ = 5.8 −3.5 +3.2 for the asymptotic value of σ(e +e − → hadrons)/ σ(e +e − → μ + μ −).