Light Quark Jets Research Articles

Particle flow calorimetry

One of the most important requirements for a detector at the ILC is good jet energy resolution. It is widely believed that the particle flow approach to calorimetry is the key to achieving the ILC goal of a di-jet invariant mass resolution αm/m < ΓZ/mZ. This paper describes the current performance of the PANDORAPFA particle flow algorithm. For simulated light quark jets it shown that a jet energy resolution of better than αE/E ≈ 3.4% can be achieved for jet energies in the range 45 – 250 GeV.

Υ Radiative Decays to Light Quark Jets andColor Octet Mechanism

We study radiative decays of Υ to light quark jets innonrelativistic QCD by taking both the color singlet and coloroctet bb̄ operators into consideration. The cut for quarkjet energy and cut for the angle between two quark jets areintroduced. The sensitivity to the soft and collinearsingularities in the loop integrals are greatly reduced by thesecuts. With the jet energy cut of about 1 GeV, and the jet anglecut of about 36°, the branching ratio forΥ→γqq̄ is found to be 8.2×10−4from color singlet contributions. The color octet contributionscould be much larger than that of color singlet, depending on theestimate of the color octet matrix elements. This process mayprovide a new test for the color octet mechanism in nonrelativistic QCD.

Tagging b jets associated with heavy neutral MSSM Higgs bosons

Since a neural network (NN) approach has been shown to be applicable to the problem of Higgs boson detection at LHC [I. Iashvili, A. Kharchilava, CMS TN-1996/100; M. Mjahed, Nucl. Phys. B 140 (2005) 799], we study the use of NNs in the problem of tagging b jets in pp → b b ¯ H SUSY , H SUSY → τ τ in the Compact Muons Solenoid experiment [F. Hakl, et al., Nucl. Instr. and Meth. A 502 (2003) 489; S. Lehti, CMS NOTE-2001/019; G. Segneri, F. Palla, CMS NOTE-2002/046]. B tagging is an important tool for separating the Higgs events with associated b jets from the Drell–Yan background Z , γ * → τ τ , for which the associated jets are mostly light quark and gluon jets. We teach multi-layer perceptrons (MLPs) available in the object oriented implementation of data analysis framework ROOT [ROOT—An Object Oriented Data Analysis Framework, in: Proceedings of the AIHENP’96 Workshop, Lausanne, September 1996, Nucl. Instr. and Meth. A 389 (1997) 81]. The following learning methods are evaluated: steepest descent algorithm, (BFGS) Broyden–Fletcher–Goldfarb–Shanno algorithm, and variants of conjugate gradients. The ROOT code generation feature of standalone C++ classifiers is utilized. We compare the b tagging performance of MLPs with another ROOT based feed forward NN tool NeuNet [J.P. Ernenwein, NeuNet software for ROOT], which uses a common back-propagation learning method. In addition, we demonstrate the use of the self-organizing map program package (SOM_PAK) and the learning vector quantization program package (LVQ_PAK) [T. Kohonen, et al., SOM_PAK: the self-organizing map program package, Technical Report A31; T. Kohonen, et al., LVQ_PAK: the learning vector quantization program package, Technical Report A30, Laboratory of Computer and Information Science, Helsinki University of Technology, FIN-02150 Espoo, Finland, 1996] in the b tagging problem.

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 \delta_{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 \delta_{b\ell}=4.4\pm0.4. The updated MLLA results on \delta_{b\ell} and \delta_{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.

Production ofπ+,π−,K+,K−,p,andp¯in light(uds),c,andbjets fromZ0decays

We present improved measurements of the differential production rates of stable charged particles in hadronic Z0 decays, and of charged pions, kaons and protons identified over a wide momentum range using the SLD Cherenkov Ring Imaging Detector. In addition to flavor-inclusive Z0 decays, measurements are made for Z0 decays into light (u, d, s), c and b primary flavors, selected using the upgraded Vertex Detector. Large differences between the flavors are observed that are qualitatively consistent with expectations based upon previously measured production and decay properties of heavy hadrons. These results are used to test the predictions of QCD in the Modified Leading Logarithm Approximation, with the ansatz of Local Parton-Hadron Duality, and the predictions of three models of the hadronization process. The light-flavor results provide improved tests of these predictions, as they do not include the contribution of heavy-hadron production and decay; the heavy-flavor results provide complementary model tests. In addition we have compared hadron and antihadron production in light quark (as opposed to antiquark) jets. Differences are observed at high momentum for all three charged hadron species, providing direct probes of leading particle effects, and stringent constraints on models.

Pair production of charged Higgs bosonsin association with bottom quark pairs at the Large Hadron Collider

We study the process $gg\to b\bar b H^ + H^-$ at large $\tan\beta$ , where it represents the dominant production mode of charged Higgs boson pairs in a Type II 2-Higgs doublet model, including the minimal supersymmetric standard model. The ability to select this signal would in principle enable the measurements of some triple-Higgs couplings, which in turn would help understanding the structure of the extended Higgs sector. We outline a selection procedure that should aid in disentangling the Higgs signal from the main irreducible background. This exploits a signature made up by “four b-quark jets, two light-quark jets, a $\tau$ -lepton and missing energy”. While, for $\tan\beta\gtrsim30$ and over a significant $M_{H^\pm}$ range above the top mass, a small signal emerges already at the Large Hadron Collider after 100 fb-1, ten times as much luminosity would be needed to perform accurate measurements of Higgs parameters in the above final state, rendering this channel a primary candidate to benefit from the so-called “Super” Large Hadron Collider option, for which a tenfold increase in instantaneous luminosity is currently being considered.

Energy loss of quarks in deconfined matter at RHIC: photon-tagged jets, single electron and dilepton spectra from open charm

We report a first attempt (i) to derive constraints on the energy loss of charm quarks in a deconfined medium from the recent PHENIX single-electron transverse momentum spectra and (ii) to estimate the resulting suppression of dileptons from correlated semileptonic decays of open charmed mesons. The momentum imbalance of photon-tagged light-quark jets is also considered.

Production of π±, K±, p and $\bar p$ in Quark, Antiquark and Gluon Jets

We present measurements of identified charged hadron production over a wide momentum range using the SLD Cherenkov Ring Imaging Detector. In addition to studying flavor-inclusive Z0 decays, we compare particle production in decays into light, c and b flavors and compare production in gluon jets with that in light quark jets, where the jet flavors are selected using precision vertex information.

Detecting heavy charged Higgs bosons at the LHC with triple b-tagging

We investigate the charged Higgs boson signal at the LHC using its dominant production and decay modes with triple b-tagging, i.e. tH −→t t ̄ b→b b ̄ bW +W − , followed by leptonic decay of one W and hadronic decay of the other. We consider the continuum background from the associated production of t t ̄ with a b- or a light quark or gluon jet, which can be mis-tagged as b-jet. We reconstruct the top quark masses to identify the 3rd b-jet accompanying the t t ̄ pair, and use its p T distribution to distinguish the signal from the background. Combining this with the reconstruction of the H ± mass gives a viable signature over two interesting regions of the parameter space – i.e. tan β∼1 and ∼ m t / m b .

Diffractive light quark jet and gluon jet production at hadron colliders in the two-gluon exchange model

Massless diquark jet and gluon jet production at large transverse momentum in the coherent diffractive processes at hadron colliders are studied in the two-gluon exchange parametrization of the Pomeron model. We use the helicity amplitude method to calculate the cross section formulas. We find that for the light quark jet and gluon jet production the diffractive process is related to the differential off-diagonal gluon distribution function in the proton. We estimate the production rate for these processes at the Fermilab Tevatron by approximating the off-diagonal gluon distribution function by the usual diagonal gluon distribution in the proton. We also use the helicity amplitude method to calculate the diffractive charm jet production at hadron colliders, by which we reproduce the leading logarithmic approximation result of this process we previously calculated.

Correlation studies in quark jets using combinants

A brief summary of the basic properties of combinants is provided. Their behavior is analysed using the Opal multiplicity data for light quark jets in restricted rapidity bins. For | Δy|≤1 the combinants do not exclude clustering effects with Poisson superposition. For | Δy|≤2 this pattern of correlations is ruled out by the sign-changing oscillations of combinants, as well as other models which predict oscillations alternating in sign. The future experimental possibilities concerning combinants are discussed.

Production ofπ+,K+,K0,K*0,φ,pandΛ0in hadronicZ0decays

We have measured the differential production cross sections as a function of scaled momentum x_p=2p/E_cm of the identified hadron species pi+, K+, K0, K*0, phi, p, Lambda0, and of the corresponding antihadron species in inclusive hadronic Z0 decays, as well as separately for Z0 decays into light (u, d, s), c and b flavors. Clear flavor dependences are observed, consistent with expectations based upon previously measured production and decay properties of heavy hadrons. These results were used to test the QCD predictions of Gribov and Lipatov, the predictions of QCD in the Modified Leading Logarithm Approximation with the ansatz of Local Parton-Hadron Duality, and the predictions of three fragmentation models. Ratios of production of different hadron species were also measured as a function of x_p and were used to study the suppression of strange meson, strange and non-strange baryon, and vector meson production in the jet fragmentation process. The light-flavor results provide improved tests of the above predictions, as they remove the contribution of heavy hadron production and decay from that of the rest of the fragmentation process. In addition we have compared hadron and antihadron production as a function of x_p in light quark (as opposed to antiquark) jets. Differences are observed at high x_p, providing direct evidence that higher-momentum hadrons are more likely to contain a primary quark or antiquark. The differences for pseudoscalar and vector kaons provide new measurements of strangeness suppression for high-x_p fragmentation products.

Multiplicity distributions of gluon and quark jets and a test of QCD analytic calculations

Gluon jets are identified in e +e − hadronic annihilation events by tagging two quark jets in the same hemisphere of an event. The gluon jet is defined inclusively as all the particles in the opposite hemisphere. Gluon hets defined in this manner have a close correspondence to gluon jets as they are defined for analytic calculations, and are almost independent of a jet finding algorithm. The mean and first few higher moments of the gluon jet charged particle multiplicity distribution are compared to the analogous results found for light quark (uds) jets, also defined inclusively. Large differences are observed between the mean, skew and curtosis values of the gluon and quark jets, but not between their dispersions. The cumulant factorial moments of the distributions are also measured, and are used to test the predictions of QCD analytic calculations. A calculation which includes next-to-next-to-leading order corrections and energy conservation is observed to provide a much improved description of the separated gluon and quark jet cumulant moments compared to a next-to-leading order calculation without energy conservation. There is good quantitative agreement between the data and calculations for the ratios of the cumulant moments between gluon and quark jets. The data sample used is the LEP-1 sample of the OPAL experiment at LEP.

Multiplicity distributions of gluon and quark jets and tests of QCD analytic predictions

Gluon jets are identified in ${\rm e^+e^-}$ hadronic annihilation events by tagging two quark jets in the same hemisphere of an event. The gluon jet is defined inclusively as all the particles in the opposite hemisphere. Gluon jets defined in this manner have a close correspondence to gluon jets as they are defined for analytic calculations, and are almost independent of a jet finding algorithm. The charged particle multiplicity distribution of the gluon jets is presented, and is analyzed for its mean, dispersion, skew, and curtosis values, and for its factorial and cumulant moments. The results are compared to the analogous results found for a sample of light quark (uds) jets, also defined inclusively. We observe differences between the mean, skew and curtosis values of gluon and quark jets, but not between their dispersions. The cumulant moment results are compared to the predictions of QCD analytic calculations. A calculation which includes next-to-next-to-leading order corrections and energy conservation is observed to provide a much improved description of the data compared to a next-to-leading order calculation without energy conservation. There is agreement between the data and calculations for the ratios of the cumulant moments between gluon and quark jets.

B-tagging at LHC

The review of the recent progress in the area of b-tagging studies at LHC is presented. Vertex b-tagging will be the major tool for the search for low mass Higgs both in the Standard Model scheme and in its supersymmetric extensions at LHC experiments. The simulation results indicate that the rejection of gluon and light quark jets of R q g-jet = 50 with b-tagging efficiency ϵ b-jet = 50% can be achieved at LHC experiments.

Test of QCD analytic predictions for the multiplicity ratio between gluon and quark jets

Gluon jets with about 39 GeV energy are identified in hadronic Z 0 decays by tagging two jets in the same hemisphere of an event as quark jets. Identifying the gluon jet to be all the particles observed in the hemisphere opposite to that containing the two tagged jets yields an inclusive gluon jet definition corresponding to that used in analytic calculations, allowing the first direct test of those calculations. In particular, this jet definition yields results which are only weakly dependent on a jet finding algorithm. We find r ch. =1.552±0.0041 ( stat) ±0.061 ( syst.) for the ratio of the mean charged particle multiplicity in gluon jets to that in light quark uds jets, where the uds jets are identified using an inclusive jet definition similar to that used for the gluon jets. Our result is in general agreement with the prediction of a recent analytic calculation which incorporates energy conservation into the parton shower branching processes, but is considerably smaller than analytic predictions which do not incorporate energy conservation.

A comparison of b and uds quark jets to gluon jets

Symmetric three-jet events are selected from hadronic Z0 decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momentum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets, which are produced with the same energy of about 24 GeV and under the same conditions. We observe b jets and gluon jets to have similar properties as measured by the angular distribution of particle energy around the jet directions and by the fragmentation functions. In contrast, gluon jets are found to be significantly broader and to have a markedly softer fragmentation function than uds jets. For thek ⊥ jet finder withy cut=0.02, we find\(\begin{gathered} \frac{{\left\langle {n^{ch.} } \right\rangle gluon}}{{\left\langle {n^{ch.} } \right\rangle b quark}} = 1.089 \pm 0.024(stat.) \pm 0.024(syst.) \hfill \\ \frac{{\left\langle {n^{ch.} } \right\rangle gluon}}{{\left\langle {n^{ch.} } \right\rangle uds quark}} = 1.390 \pm 0.038(stat.) \pm 0.032(syst.) \hfill \\ \end{gathered} \) as the ratios of the mean charged particle multiplicity in the gluon jets compared to the b and uds jets. Results are also reported using the cone jet finder.

A comparison of b and uds quark jets to gluon jets

Symmetric three-jet events are selected from hadronic Z0 decays such that the two lower energy jets are each produced at an angle of about 150° with respect to the highest energy jet. In some cases, a displaced secondary vertex is reconstructed in one of the two lower energy jets, which permits the other lower energy jet to be identified as a gluon jet through anti-tagging. In other cases, the highest energy jet is tagged as a b jet or as a light quark (uds) jet using secondary vertex or track impact parameter and momentum information. Comparing the two lower energy jets of the events with a tag in the highest energy jet to the anti-tagged gluon jets yields a direct comparison of b, uds and gluon jets, which are produced with the same energy of about 24 GeV and under the same conditions. We observe b jets and gluon jets to have similar properties as measured by the angular distribution of particle energy around the jet directions and by the fragmentation functions. In contrast, gluon jets are found to be significantly broader and to have a markedly softer fragmentation function than uds jets. For thek ⊥ jet finder withy cut=0.02, we find $$\begin{gathered} \frac{{\left\langle {n^{ch.} } \right\rangle gluon}}{{\left\langle {n^{ch.} } \right\rangle b quark}} = 1.089 \pm 0.024(stat.) \pm 0.024(syst.) \hfill \\ \frac{{\left\langle {n^{ch.} } \right\rangle gluon}}{{\left\langle {n^{ch.} } \right\rangle uds quark}} = 1.390 \pm 0.038(stat.) \pm 0.032(syst.) \hfill \\ \end{gathered} $$ as the ratios of the mean charged particle multiplicity in the gluon jets compared to the b and uds jets. Results are also reported using the cone jet finder.

A search for jet handedness in hadronic Z0 decays

We have searched for signatures of polarization of partons in hadronic jets from Z 0 → qq¯ decays using the ‘jet handedness’ method. We find no evidence for jet handedness in a sample of light quark jets. We set an upper limit of 9.1% on magnitude of the analyzing power of this technique at the 95% C.L. We have studied an alternative definition of jet handedness and found no signal.

Diffractive production of dijets at HERA

We investigate the production of a quark-antiquark pair in diffractive photon-proton scattering, approximating soft pomeron exchange by the exchange of two nonperturbative gluons. In deep inelastic scattering at HERA, events with two jets and the scattered proton in the final state are predicted to be observable, with an important contribution from charm production. For photoproduction of light quark jets with high transverse momentum we find that both exchanged gluons must have a large invariant mass, so that the cross section is very small, whereas for charm quarks it is quite appreciable. From our calculation we also extract the quark structure function of the pomeron for the scaling variablez no too close to 0 or 1, finding a strong flavour dependence and a behaviour somewhat harder thanz(1−z) for light quarks.