arXiv subjects

Wolfgang Ochs

Publications and source records attributed to Wolfgang Ochs.

At least 19 recordsLinked to original sources

arXiv2026

How to identify the dead cone in the top-quark jet

The gluon emission from an energetic heavy quark is suppressed in the forward direction below the angle $Θ\lesssim m_Q/E$ for a quark of mass $m_Q$ and energy $E$ according to perturbative Quantum Chromodynamics (QCD) (``dead cone"). Another consequence is the suppression of energetic particles in the jet which has been observed already for c- and b-quark jets. The suppression of the forward particles can be explained by an application of the Modified Leading Logarithmic Approximation (MLLA) of perturbative QCD. In this paper we investigate whether this type of analysis can be carried out also for top-quark jets with the much higher heavy quark mass allowing for QCD tests in this new kinematic regime. The new aspect of this analysis is the finite lifetime of the top quark. We consider for simplicity the decay $t\to b\ellν$, where the b-quark radiates gluons as well and partially obscures the dead cone. Guided by the decay amplitude in leading order in $α_s$ for $e^+e^- \to t \bar t$ we propose a method to separate the radiation by the $\widehat{tb}$ dipole in the decay process which is superimposed to the primary radiation from the $\widehat{t \bar t}$ dipole involving the top-quark dead cone effect. The momentum distributions of partons or hadrons are determined for finite decay angles of the b-quark $Θ_b$ and extrapolated into forward direction $Θ_b=0$ where the radiation from the decay process is expected to vanish. This method is successfully tested at the parton level and results obtained for hadrons are compatible with the MLLA relation within an accuracy of around 15\%. Our calculations are carried out with the Pythia 8.3 Monte Carlo Event Generator.

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arXiv2026

The Dead Cone Effect in Heavy-Quark Jets: A Unified Study from Charm and Bottom to Top

We present a unified overview of recent progress in the study of QCD radiation in heavy-quark jets, focusing on the dead-cone effect. Using precision data from LEP at $\sqrt{s}=91.2$~GeV, we demonstrate strong momentum-space suppression in charm and bottom quark jets, supported by Monte Carlo simulations with \textsc{Pythia}8, and provide a quantitative interpretation within the Modified Leading Logarithmic Approximation (MLLA) of perturbative QCD. We then extend the analysis to top-quark jets at $\sqrt{s}=1$~TeV, where finite lifetime effects and decay radiation introduce new conceptual challenges. A new method is presented to isolate the top-quark dead cone by separating production and decay radiation, and it is validated at both parton and hadron level using \textsc{Pythia}8. Together, these results establish a coherent framework for testing QCD radiation dynamics across all three heavy quarks.

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arXiv2023

The dead cone effect in heavy quark jets observed in momentum space and its QCD explanation

The production of a heavy quark is accompanied by gluon bremsstrahlung with angular and momentum spectra predicted by perturbative Quantum Chromo Dynamics (QCD). The radiation off heavy quarks is predicted to be suppressed for large momentum particles, as a consequence of the angular ``dead cone effect''. In this paper, we studied this effect using data from Z boson decays to c- or b-quarks in $e^+e^-$ annihilation. The momentum spectra for charged particles are reconstructed in the momentum fraction variable $x$ or $ξ=\ln(1/x)$ by removing the decays of the heavy hadrons. We find an increasing suppression of particles with rising $x$ down to a fraction of $\lesssim 1/10$ for particles with $x\gtrsim0.2$ in b-quark and $x\gtrsim0.4$ in c-quark jets in comparison to light quark momentum spectra. The sensitivity to the dead cone effect in the present momentum analysis is larger than in the recently presented angular analysis. The suppression for c- and b-quark fragmentation is in good quantitative agreement with the expectations based on perturbative QCD within the Modified Leading Logarithmic Approximation (MLLA) in the central kinematic region. The data also support a two parameter description in the MLLA of these phenomena (``Limiting Spectrum''). The sensitivity of these measurements to the heavy quark mass is investigated.

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arXiv2023

Dead cone effect in charm and bottom quark jets

The evolution of a heavy quark initiated jet is mainly ruled by gluon bremsstrahlung. As a consequence of the dead-cone effect, this radiation is suppressed in the forward direction at angles smaller than that proportional to the heavy quark mass $M_Q$, i.e. $Θ_0=M_Q/E_Q$ at energy $E_Q$ of the primary quark. In this paper, we unveil this effect in charm and bottom quark jets using DELPHI and OPAL data from Z$^0$ boson decays in $e^+e^-$ annihilation at center of mass energy 91.2 GeV. The analysis of the reconstructed heavy quark fragmentation function in momentum space shows the strong suppression of hadrons at high momenta in such events compared to light quark fragmentation by a factor $\lesssim1/10$. The amount of this suppression is well reproduced by perturbative QCD (pQCD) within the Modified Leading Logarithmic Aproximation and the compact scheme of Local Parton Hadron Duality (MLLA-LPHD). As a new result, we obtain an almost perfect agreement between the light quark fragmentation functions expected at $W_0\propto M_Q$ from DELPHI and OPAL data with Pythia8 and shed light on the reasons for the existence of the ultra-soft gluon excess at small momentum fraction in comparison with pQCD predictions.

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arXiv2023

Observation of the dead cone effect in charm and bottom quark jets and its QCD explanation

The production of a heavy quark is accompanied by gluon bremsstrahlung which is suppressed at small angles $Θ\lesssim M_Q/E$ for mass $M_Q$ and high energy $E$ according to perturbative Quantum Chromo Dynamics (QCD) (``dead cone effect''). As particles at small angles typically have large momenta, the heavy quark mass also causes a suppression of high momentum particles. In this paper, we studied this effect in c- and b-quark events using data from Z boson decays in $e^+e^-$ annihilation. The heavy quark fragmentation function for charged particles is reconstructed in the momentum fraction variable $x$ or $ξ=\ln(1/x)$ by removing the decays of the heavy quark hadrons. Indeed, we find an increasing suppression of particles with rising $x$ down to a fraction of $\lesssim 1/10$ for particles with $x\gtrsim0.2$ in b-quark and $x\gtrsim0.4$ in c-quark jets in comparison to light quark fragmentation. The sensitivity to the dead cone effect in the present momentum analysis is considerably increased in comparison to the recently presented angular analysis. This amount of suppression and the differences between c- and b-quark fragmentation are in good quantitative agreement with the expectations based on perturbative QCD within the Modified Leading Logarithmic Approximation (MLLA) in the central kinematic region. The data also support a two parameter description in the MLLA of these phenomena (``Limiting Spectrum''). The sensitivity of these measurements to the heavy quark mass is investigated.

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arXiv2014

The identification of glueballs - further tests

The indirect evidence for gluonic objects in gluon jets is recalled, more accurate tests at the LHC are possible. Estimates of gluonic and quarkonic components of scalar mesons are presented, which could be improved by further studies, in particular of B and B_s decays. The results are consistent with the scalar glueball mixing into f_0(500) and f_0(1500). The study of symmetry relations for 2-body decay rates of charmonium χ_c is suggested as crucial test in the identification of q\bar q multiplets and glueballs. Further related results are discussed in the recent review \cite{Ochs:2013gi}.

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arXiv2013

Spectroscopy with glueballs and the role of f_0(1370)

The existence of glueballs, bound states of gluons, is one of the basic predictions of QCD; the lightest state is expected to be a scalar. The experimental situation, however, is still ambiguous. The existence of f_0(1370) would point to a supernumerous state within the nonet classification of scalars and would therefore provide a hint towards a glueball. In this talk we summarize some arguments in favour and against the existence of f_0(1370) and discuss schemes with and without this state included.

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arXiv2013

The Status of Glueballs

Calculations within QCD (lattice and sum rules) find the lightest glueball to be a scalar and with mass in the range of about 1000-1700 MeV. Several phenomenological investigations are discussed which aim at the identification of the scalar meson nonets of lowest mass and the super-numerous states if any. Results on the flavour structure of the light scalars f0(500), f0(980) and f0(1500) are presented; the evidence for f0(1370) is scrutinized. A significant surplus of leading clusters of neutral charge in gluon jets is found at LEP in comparison with MCs, possibly a direct signal for glueball production; further studies with more energetic jets at LHC are suggested. As a powerful tool in the identification of the scalar nonets or other multiplets, along with signals from glueballs we propose the exploration of symmetry relations for decay rates of C=+1 heavy quark states like chi-c or chi-b. Results from chi-c decays are discussed; they are not in support of a tetra-quark substructure of f0(980). A minimal scenario for scalar quarkonium - glueball spectroscopy is presented.

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arXiv2011

Glueballs from gluon jets at the LHC

The existence of glueballs within QCD is uncontroversial but their experimental verification is still in doubt. We discuss the new possibilities for a search of glueballs as the leading object in gluon jets at the LHC. We summarize previous results from LEP which demonstrate a significant excess rate of electrically neutral leading clusters in comparison with MC models.

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arXiv2010

Universal properties of particle production in the soft limit p_T -> 0

The momentum spectra of particles in different high energy processes, such as e^+e^- annihilation, pp and nuclear collisions in the limit p,p_T -> 0 exhibit similar properties because of the dominant role of coherent soft gluon bremsstrahlung. We observe the following general features: the inclusive particle density approaches a limiting behaviour and becomes independent of primary collision energy; furthermore, it becomes proportional to the QCD colour factors C_A,C_F which appear in the Born term for the respective minimal partonic processes. In this limit, nuclear collisions reach with good accuracy participant (``wounded nucleon'') scaling. Particle ratios in the low momentum region display a universal behaviour. Future measurements at the LHC will provide crucial tests for the contributions from additional incoherent multi-component processes.

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arXiv2010

Limiting soft particle production and QCD

We present some basic elements of the treatment of particle multiplicities in jets from high energy collisions within perturbative QCD. Then we discuss the universal features of the inclusive particle spectrum for the limiting case of momentum p\to 0 (or p_T\to 0) as expected from soft QCD gluon bremsstrahlung. The energy independence of the invariant particle density in this limit I_0=E dN/d^3p|_{p\to 0} is predicted as well as the dependence of this quantity on the the colour factors characteristic of the underlying partonic processes. These properties are first recalled from e+e- collisions and then extended to pp and nuclear collisions according to Ref. 1. Present data support these predictions. It will be interesting to see whether new incoherent contributions show up in the new energy regime of LHC.

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arXiv2010

Limiting soft particle emission in e+e-, hadronic and nuclear collisions

In e+e- collisions the particle spectra at low momenta reflect the properties of the underlying "soft" QCD gluon bremsstrahlung: the particle density, in the limit p\to 0, becomes independent of the incoming energy \sqrt{s} and directly proportional to the colour factors C_A,C_F for primary gluons or quarks respectively. We find that experimental data from the pp and nuclear reactions reveal the same behaviour: in the limit p_T\to 0 the invariant particle spectra become independent of the collision energy, and their intensities in e+e-, pp and nuclear reactions are compatible with the expected colour factors C_F: C_A: (N_{part}/2) C_A for N_{part} nucleons, participating in the interaction. Coherent soft gluon bremsstrahlung is, therefore, suggested to be the dominant QCD mechanism for the soft particle production in all these reactions. These "soft" particles probe the very early stage of hadron formation in the collision. Future measurements at the LHC will provide crucial tests on the contributions from possible incoherent multi-component processes.

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arXiv2010

No indication of f0(1370) in pi pi phase shift analyses

The scalar meson f_0(1370) - indicated in particular in the low energy p\bar p \to 3 body reactions - is a crucial element in certain schemes of the scalar meson spectroscopy including glueballs. The most definitive results can be obtained from elastic and inelastic pi pi phase shift analyses using the constraints from unitarity where the discrete ambiguities can be identified and resolved. We reconsider the phase shift analyses for pi^+ pi^- \to pi+ π-, pi^0 pi^0, K \bar K, eta eta. While a clear resonance signal for f_0(1500) in the resp. Argand diagrams is seen in all channels above a large ``background'' from f_0(600) there is no clear signal of a second resonance ``f_0(1370)'' in this mass range in any reaction, at the level of \sim 10% branching ratio into pi-pi.

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arXiv2008

Searching for the Scalar Glueball

Existence of gluonic resonances is among the early expectations of QCD. Today, QCD calculations predict the lightest glueball to be a scalar state with mass within a range of about 900-1700 MeV but there is no consensus about its experimental evidence. In a re-analysis of the phase shifts for pi pi scattering up to 1800 MeV where such states should show up we find the broad resonance f_0(600)/σcontributing to the full mass range and the narrow f_0(980) and f_0(1500) but no evidence for f_0(1370). Phenomenological arguments for the broad state to be a glueball are recalled. It is argued that the large radiative width of f_0(600)/σreported recently is not in contradiction to this hypothesis but is mainly due to pi pi rescattering. The small ``direct'' radiative component is consistent with QCD sum rule predictions for the light glueball.

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arXiv2007

The glueball among the light scalar mesons

The lightest gluonic meson is expected with J^{PC}=0^{++}, calculations in full QCD point towards a mass of around 1 GeV. The interpretation of the scalar meson spectrum is hindered as some states are rather broad. In a largely model-independent analysis of pi+ pi- \to pi+ pi-, pi0 pi0 scattering in the region 600-1800 MeV a unique solution for the isoscalar S-wave is obtained. The resonances f_0(980), f_0(1500) and the broad f_0(600) or ``sigma'' are clearly identified whereas f_0(1370) is not seen at the level B(f_0(1370)\to pi pi)\gtrsim 10%. Arguments for the broad state to be a glueball are recalled. We see no contradiction with the reported large B(sigma \to gamma gamma) and propose some further experimental tests.

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arXiv2006

Scalar mesons: in search of the lightest glueball

According to the QCD expectations the lightest glueball should be a scalar particle (J^{PC}=0^{++}). Different scenarios have been considered for a classification of these states but -- despite considerable progress in recent years -- the experimental basis for various parameters is still rather weak. We present a new analysis of the elastic and charge exchange pi pi scattering between 1000 and 1800 MeV. A unique solution is selected which shows clear evidence for f_0(1500) and a broad state (σor ``f_0(1000)''), but there is no evidence for f_0(1370) at a level of \gtrsim 10% branching ratio to pi pi. Arguments in favour of the broad state to be a glueball are summarized.

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arXiv2005

Scalar Mesons in B-decays

We summarize some persistent problems in scalar spectroscopy and discuss what could be learned here from charmless B-decays. Recent experimental results are discussed in comparison with theoretical expectations: a simple model based on penguin dominance leads to various symmetry relations in good agreement with recent data; a factorisation approach yields absolute predictions of rates. For more details, see Ref. 1.

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arXiv2005

Multiplicity Difference between Heavy and Light Quark Jets Revisited

The perturbative QCD approach to multiparticle production predicts a characteristic suppression of particle multiplicity in a heavy quark jet as compared to a light quark jet. In the Modified Leading Logarithmic Approximation (MLLA) the multiplicity difference δ_{Q\ell} between heavy and light quark jets is derived in terms of a few other experimentally measured quantities. The earlier prediction for b-quarks needs revision in the light of new experimental results and the improvement in the understanding of the experimental data. We now find δ_{b\ell}=4.4\pm0.4. The updated MLLA results on δ_{b\ell} and δ_{c\ell} are compared with the present data from e^+e^- annihilation. Their expected energy independence is confirmed within the energy range between 29 and 200 GeV; the absolute values are now in better agreement with experiment than in the previous analysis, and the remaining difference can be attributed largely to next-to-MLLA contributions, an important subset of which are identified and evaluated.

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