Low Invariant Mass Research Articles

Visualizations of exclusive central diffraction

The case of the low invariant mass exclusive central diffractive production is considered in the general theoretical framework. It is shown that diffractive patterns (differential cross-sections on variables like transfer momenta squared, the azimuthal angle between final hadrons and their combinations) can serve as a unique tool to explore the picture of the $pp$ interaction and falsify theoretical models. Basic kinematical and dynamical properties of the process are considered in detail. As an example, visualizations of diffractive patterns in the model with three pomerons for processes $p+p\to p+R+p$ and $p+p\to p+\pi^{+}\pi^{-}+p$ are presented.

Phenomenology of photon and dilepton production in relativistic nuclear collisions

We discuss the latest theoretical results on direct photon and dilepton production from relativistic heavy-ion collisions. While the dilepton spectra at low invariant mass show in-medium effects like a collisional broadening of the vector meson spectral functions, the dilepton yield at high invariant masses (above 1.1 GeV) is dominated by QGP contributions for central heavy-ion collisions at relativistic energies. The present status of the photon $v_2$ "puzzle" - a large elliptic flow $v_2$ of the direct photons experimentally observed at RHIC and LHC energies - is also addressed. The role of hadronic and partonic sources for the photon spectra and $v_2$ is considered as well as the possibility to subtract the QGP signal from the experimental observables.

The h → 4ℓ spectrum at low m34: Standard Model vs. light New Physics

We analyze h→4ℓ decays in the kinematical region where the dilepton pair of low invariant mass (m34) is not far from QCD resonances (Ψ and ϒ states). On the one hand, we present precise predictions of the spectrum within the Standard Model, taking into account non-perturbative QCD effects. On the other hand, we illustrate the sensitivity of this spectrum to New Physics models predicting the existence of new exotic light states. In particular, we show that parameter regions of models relevant to explain the (g−2)μ anomaly could be probed in the future by means of h→4ℓ decays.

γγ-PHYSICS AT KLOE/KLOE-2

Photon-photon interactions have been studied at the ϕ-factory DAΦNE with the detector KLOE operating the machine at [Formula: see text] GeV, without tagging of the final leptons. The results about the γγ → η process and the evidence for γγ → π0π0 production at low π0π0 invariant mass are reviewed. The process γγ → π0 will be studied at KLOE-2 running the machine at ϕ-peak thanks to new lepton tagger detectors. In particular, the possibility to measure the two-photon width of π0 and the π0γ*γ transition form factor in the low (space-like) Q2 region is considered.

Dimuon production from in-medium rho decays from QCD sum rules

We compute the dimuon-excess invariant mass distribution at the rho-meson peak in the context of relativistic heavy-ion collisions at SPS energies. The temperature, T, and chemical potential, mu, dependent parameters describing the rho-meson --its width, mass and leptonic decay constant-- are determined from finite energy QCD sum rules. Results show that the rho-meson width increases whereas its mass and leptonic decay constant decrease near the (chemical potential-dependent) critical temperature T_c(mu) for chiral symmetry restoration/quark-gluon deconfinement. As a consequence, starting from T_c(mu), for a short lived cooling the main effect is a broadening of the dimuon distribution. However, when the evolution brings the system to a lower freeze-out temperature, with the rho-meson parameters approaching their vacuum values, the dimuon distribution shows a broad peak centered at the vacuum rho-meson mass. For even lower freeze-out temperatures the peak becomes less prominent since the thermal phase space factor suppresses the distribution for larger values of the invariant dimuon mass, given that the average temperature is smaller. The dimuon distribution exhibits a non-trivial behavior with mu. For small mu values the distribution broadens with increasing mu becoming a bit steeper at low invariant masses for larger values of mu. Our results are in very good agreement with data from the NA60 Collaboration.

Isospin asymmetries inB→(K*,ρ)γ/l+l−andB→Kl+l−in and beyond the standard model

We compute the isospin asymmetries in $B\ensuremath{\rightarrow}({K}^{*},\ensuremath{\rho})\ensuremath{\gamma}$ and $B\ensuremath{\rightarrow}(K,{K}^{*},\ensuremath{\rho}){l}^{+}{l}^{\ensuremath{-}}$ for low lepton pair invariant mass ${q}^{2}$, within the Standard Model (SM) and beyond the SM in a generic dimension six operator basis. Within the SM the $CP$-averaged isospin asymmetries for $B\ensuremath{\rightarrow}(K,{K}^{*},\ensuremath{\rho})ll$, between $1\text{ }\text{ }{\mathrm{GeV}}^{2}\ensuremath{\le}{q}^{2}\ensuremath{\le}4{m}_{c}^{2}$, are predicted to be small (below 1.5%) though with significant cancellation. In the SM the non-$CP$-averaged asymmetries for $B\ensuremath{\rightarrow}\ensuremath{\rho}ll$ deviate by $\ensuremath{\approx}\ifmmode\pm\else\textpm\fi{}5%$ from the $CP$-averaged ones. We provide physical arguments, based on resonances, of why isospin asymmetries have to decrease for large ${q}^{2}$ (towards the endpoint). Two types of isospin violating effects are computed: ultraviolet isospin violation due to differences between operators coupling to up and down quarks, and infrared isospin violation where a photon is emitted from the spectator quark and is hence proportional to the difference between the up- and down-quark charges. These isospin violating processes may be subdivided into weak annihilation (WA), quark loop spectator scattering, and a chromomagnetic contribution. Furthermore we discuss generic selection rules based on parity and angular momentum for the $B\ensuremath{\rightarrow}Kll$ transition as well as specific selection rules valid for WA at leading order in the strong coupling constant. We clarify that the relation between the $K$ and the longitudinal part of the ${K}^{*}$ only holds for leading twist and for left-handed currents. In general the $B\ensuremath{\rightarrow}\ensuremath{\rho}ll$ and $B\ensuremath{\rightarrow}{K}^{*}ll$ isospin asymmetries are structurally different yet the closeness of ${\ensuremath{\alpha}}_{\mathrm{CKM}}$ to $90\ifmmode^\circ\else\textdegree\fi{}$ allows us to construct a (quasi)null test for the SM out of the respective isospin symmetries. We provide and discuss an update on $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{K}^{*0}\ensuremath{\gamma})/\mathcal{B}({B}_{s}\ensuremath{\rightarrow}\ensuremath{\phi}\ensuremath{\gamma})$ which is sensitive to WA.

CP asymmetries in three-body [formula omitted] decays to charged pions and kaons

CP asymmetries have been measured recently by the LHCb collaboration in three-body B+ decays to final states involving charged pions and kaons. Large asymmetries with opposite signs at a level of about 60% have been observed in B±→π±(or K±)π+π− and B±→π±K+K− for restricted regions in the Dalitz plots involving π+π− and K+K− with low invariant mass. U-spin is shown to predict corresponding ΔS=0 and ΔS=1 asymmetries with opposite signs and inversely proportional to their branching ratios, in analogy with a successful relation predicted thirteen years ago between asymmetries in Bs→K−π+ and B0→K+π−. We compare these predictions with the measured integrated asymmetries. Effects of specific resonant or non-resonant partial waves on enhanced asymmetries for low-pair-mass regions of the Dalitz plot are studied in B±→π±π+π−. The closure of low-mass π+π− and K+K− channels involving only ππ↔KK¯ rescattering may explain by CPT approximately equal magnitudes and opposite signs measured in B±→π±π+π− and B±→π±K+K−.

Lepton pair production from viscous QGP

In Israel–Stewart’s second order hydrodynamics, the viscous effect of dilepton emission from a QGP medium is investigated. Dileptons are strongly affected by QGP viscosity. Large invariant mass dileptons, due to their lower velocity, are less affected by viscosity than the low invariant mass dileptons.

Predictions from heavy new physics interpretation of the top forward-backward asymmetry

Abstract We derive generic predictions at hadron colliders from the large forward- backward asymmetry observed at the Tevatron, assuming the latter arises from heavy new physics beyond the Standard Model. We use an effective field theory approach to characterize the associated unknown dynamics. By fitting the Tevatron $ t\overline{t} $ data we derive constraints on the form of the new physics. Furthermore, we show that heavy new physics explaining the Tevatron data generically enhances at high invariant masses both the top pair production cross section and the charge asymmetry at the LHC. This enhancement can be within the sensitivity of the 8TeV run, such that the 2012 LHC data should be able to exclude a large class of models of heavy new physics or provide hints for its presence. The same new physics implies a contribution to the forward-backward asymmetry in bottom pair production at low invariant masses of order a permil at most.

A fast track towards the ‘Higgs’ spin and parity

Abstract The LHC experiments ATLAS and CMS have discovered a new boson that resembles the long-sought Higgs boson: it cannot have spin one, and has couplings to other particles that increase with their masses, but the spin and parity remain to be determined. We show here that the ‘Higgs’ + gauge boson invariant-mass distribution in ‘Higgs’-strahlung events at the Tevatron or the LHC would be very different under the J P =0+ ,0− and 2+ hypotheses. Ouranalysisisbasedonsimulationsoftheexperimental event selections and cuts using PYTHIA and Delphes, and incorporates statistical samples of ‘toy’ experiments. The observation of ‘Higgs’-strahlung at the Tevatron and the expected peaking of backgrounds at low invariant masses suggest that this process could provide a fast-track indicator of the ‘Higgs’ spin and parity.

ρ-MESON SPECTRAL FUNCTION IN HOT NUCLEAR MATTER

The ρ-meson spectral function in hot nuclear matter is studied by taking into account the pion and the nucleon loops within the quantum hadrodynamics (QHD) model as well as using an effective chiral SU(3) model. The effects of density and temperature on the spectral function of the ρ-meson are studied assuming the ρ-meson to be with a finite momentum. These investigations are performed using both the mean field approximation (MFA) and the relativistic Hartree (RHA) approximation. The inclusion of the nucleon loop is observed to considerably change the ρ-meson spectral function. Due to a larger mass drop of ρ-meson in the RHA, it is seen that the spectral function shifts towards the low invariant mass region, whereas in the MFA the spectral function is seen to be almost centered around the nominal ρ-pole, but develops a second peak due to the opening of the Nh-channel. Within both the Walecka and the chiral SU(3) models, it is observed that the ρ-meson spectral function has a strong dependence on the nucleon-ρ meson tensor coupling.

γγ physics at KLOE/KLOE-2

Two-photon physics from e+e− collision is studied at DAΦNE. The processes e+e− → e+e− X, with X being either the η meson or π0π0 have been studied with beams colliding at GeV, below the ϕ resonance peak and without tagging of the outgoing e+e−. Preliminary results are presented on the observation of the γγ → η process, with both η → π+π−π0 and η → π0π0π0 channels, and the evidence for γγ → π0π0 production at low π0π0 invariant mass. The process e+e− → e+e−π0 will be studied at KLOE-2 running the machine at ϕ peak thanks to new lepton tagger detectors. In particular, the possibility to measure the width Γγγ → π0 and the π0γγ* form factor, F(Q2), at low invariant masses of the virtual photon (in the space-like region) is considered. The feasibility of these measurements is estimated on the basis of a Monte Carlo simulation, whose results are presented here.

On the possibility to measure the π 0→γγ decay width and the γ ∗ γ→π 0 transition form factor with the KLOE-2 experiment

A possibility of KLOE-2 experiment to measure the width Gamma(pi0 to gamma gamma) and the (pi0 gamma gamma*) form factor F(Q^2) at low invariant masses of the virtual photon in the space-like region is considered. This measurement is an important test of the strong interaction dynamics at low energies. The feasibility is estimated on the basis of a Monte-Carlo simulation. The expected accuracy for Gamma(pi0 to gamma gamma) is at a per cent level, which is better than the current experimental world average and theory. The form factor will be measured for the first time at Q^2 less or equal 0.1 GeV^2 in the space-like region. The impact of these measurements on the accuracy of the pion-exchange contribution to the hadronic light-by-light scattering part of the anomalous magnetic moment of the muon is also discussed.

Dilepton excess from local parity breaking in baryon matter

We propose that local parity breaking induced by a large-scale fluctuation of topological charge at large temperatures and/or condensation of pseudoscalar mesons in the isotriplet channel for large baryon densities may be responsible for the substantial dilepton excess that is found for low invariant masses and moderate values of pT in central heavy ions collisions. This insofar unexplained enhancement could be understood by a combination of two effects leading to an excess of both e+e− and μ+μ− pairs with respect to theoretical predictions based on conventional hadronic processes: (a) a modification of the dispersion relation of photons and vector mesons propagating in such a nuclear medium due to local parity breaking; (b) the appearance of new decay channels, forbidden by parity conservation in QCD in the usual vacuum. Possible signatures of this effect and perspectives for its detection are discussed.

Soft collinear effective theory, QCD, and Wilson lines

Soft Collinear Effective Theory (SCET) is an effective field theory which describes the interactions of low invariant mass jets which are highly boosted with respect to one another. In the standard formulation of SCET, the effective Lagrangian for collinear fields is expanded in inverse powers of the energy. At leading order this leads to manifest decoupling of soft and collinear degrees of freedom; however, subleading terms in the effective Lagrangian violate this manifest decoupling. In this paper we point out that the collinear expansion in the SCET Lagrangian is unnecessary, and that the SCET Lagrangian may instead be written as multiple decoupled copies of QCD. The interactions between the sectors in full QCD are reproduced in the effective theory by an external current consisting of QCD fields coupled to Wilson lines. We illustrate this picture with two examples: dijet production and B->X_s + gamma.

PROJECTION OF A MEASUREMENT OF THE NONPHOTONIC ELECTRON v2 IN Au + Au COLLISIONS AT STAR

We present a method of measuring the elliptic flow (v2) of nonphotonic electrons from heavy flavor quark decays using the STAR detector at RHIC in Au + Au collisions at [Formula: see text]. Inclusive electrons are identified in the pT range 0.5–4 GeV /c at mid-rapidity -1 < η < 0 by the STAR time projection chamber (TPC) and time-of-flight detector. Photonic e+e- pairs with characteristic low invariant mass are reconstructed in the TPC and their v2 is measured using the standard event plane method by subtracting the combinatorial background in the dN/dΔϕ distributions in each pT bin. The nonphotonic electron v2 and its propagated error can be derived from the inclusive and photonic electron v2. We estimate the necessary statistics and errors of nonphotonic electron v2 for a more recent RHIC run.

A new method for resolving combinatorial ambiguities at hadron colliders

We present a new method for resolving combinatorial ambiguities that arise in multi-particle decay chains at hadron colliders where the assignment of visible particles to the different decay chains has ambiguities. Our method, based on selection cuts favoring high transverse momentum and low invariant mass pairings, is shown to be significantly superior to the more traditional hemisphere method for a large class of decay chains, producing an increase in signal retention of up to a factor of 2. This new method can thus greatly reduce the combinatorial ambiguities of decay chain assignments.

A COMPARATIVE STUDY ON B →K*ℓ+ ℓ- AND $B \to K_0^\ast (1430) \ell^+ \ell^-$ DECAYS IN THE SUPERSYMMETRIC MODELS

In this paper, we compare the branching ratio and rate difference of electron channel to muon channel of [Formula: see text] and B →K*ℓ+ℓ- decays, where [Formula: see text] is the p-wave scalar meson, in the supersymmetric models. MSSM with R parity is considered since considerable deviation from the standard model predictions can be obtained in B→Xs ℓ-ℓ+. Taking CQ1 and CQ2 about one which is consistent with the B→K*μ+μ- rate at low dileptonic invariant mass region (1 ≤ q2 ≤ 6 GeV 2). It is found that, firstly, the [Formula: see text] decay is measurable at LHC, secondly, in comparison with B →K*ℓ+ ℓ- decay a greater deviation in the [Formula: see text] decay can be seen. Measurement of these observables for the semileptonic rare [Formula: see text], in particular, at low q2 region can give valuable information about the nature of interactions within Standard Model or beyond.

Low-mass dilepton rate from the deconfined phase

We discuss low-mass dilepton rates ($\ensuremath{\leqslant}1$ GeV) from the deconfined phase of QCD using both perturbative and nonperturbative models and compare them with those from lattice gauge theory and in-medium hadron gas. Our analysis suggests that the rate at very low invariant mass ($M\ensuremath{\leqslant}200$ MeV) using the nonperturbative gluon condensate in a semiempirical way within the Green function approach dominates over the Born rate, independent of any uncertainty associated with the choice of the strong coupling in perturbation theory. On the other hand, the rate from $\ensuremath{\rho}$-$q$ interaction in the deconfined phase is important at $200$ MeV $\ensuremath{\leqslant}M\ensuremath{\leqslant}$ $1$ GeV as it is almost of same order as the Born rate as well as the in-medium hadron gas rate. Also, the higher order perturbative rate, leaving aside its various uncertainties, from the hard-thermal-loop approximation becomes reliable at $M\ensuremath{\geqslant}200$ MeV and also becomes comparable with the Born rate and the lattice rate for $M\ensuremath{\geqslant}500$ MeV, constraining on the broad resonance structures in the dilepton rate at large invariant mass. We also discuss the lattice constraints on the low-mass dilepton rate. Furthermore, we discuss a realistic way to advocate the quark-hadron duality hypothesis based on the dilepton rates from quark-gluon plasma and hadron gas.

Probing dense nuclear matter with electromagnetic and rare hadronic probes

We present preliminary results on strangeness and dilepton production in Ar+KCl reactions at 1.76 AGeV. For the first time an ω signal could be extracted in heavy-ion collisions in this energy region. Furthermore we find a strong enhancement of yield in the low invariant lepton pair mass region over an elementary reference suggesting the onset of non trivial effects of the medium. This hint is supported by a nearly complete vanishing of the OZI suppression of the ϕ to ω ratio which is characteristic for elementary reactions.