Combinatorial Background Research Articles

Medium modifications of light vector mesons in photoproduction reactions at JLab

Theoretical calculations predict the modification of properties of vector mesons, such as a shift in their masses and/or broadening of their widths in dense nuclear matter. These effects can be related to partial restoration of chiral symmetry at high density or temperature. Photoproduction of vector mesons off nuclei were performed at Jefferson Lab using the CEBAF Large Acceptance Spectrometer. The data were taken with a beam of tagged photons with energies up to 4 GeV on various nuclear targets. The properties of the ρ vector mesons were investigated via their rare leptonic decay to e+e−. This decay channel is preferred over hadronic modes in order to eliminate final state interactions in the nuclear matter. The combinatorial background in the mass spectrum was removed by a self-normalizing mixed-event technique. The ρ meson mass distributions were extracted for each of the targets. Statistically significant results regarding medium modification of the ρ meson in the nuclear medium rule out large medium effects. Transparency studies of the ω and ϕ vector mesons allow a determination of their widths in the medium.

Analysis method for jet-like three-particle azimuthal correlations

Jet-like three-particle azimuthal correlations can discriminate various physical scenarios that have been proposed to explain the observed strong modification to two-particle azimuthal correlations. The three-particle correlation analysis is notoriously difficult in heavy-ion collisions due to the large combinatorial backgrounds. We describe the general idea behind the jet-like three-particle azimuthal correlation analysis, with emphasis on the subtraction of the combinatorial backgrounds. We discuss in detail various sources of systematic uncertainties in such an analysis.

Light vector mesons in the nuclear medium

The light vector mesons ($\rho$, $\omega$, and $\phi$) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the $\rho$ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to $e^{+}e^{-}$. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The $\rho$ meson mass spectrum was extracted after the $\omega$ and $\phi$ signals were removed in a nearly model-independent way. Comparisons were made between the $\rho$ mass spectra from the heavy targets ($A > 2$) with the mass spectrum extracted from the deuterium target. With respect to the $\rho$-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion.

Testing gluino spin with three-body decays

We examine the possibility of distinguishing a supersymmetric gluino from a Kaluza-Klein gluon of universal extra dimensions (UED) at the Large Hadron Collider (LHC). We focus on the case when all kinematically allowed tree-level decays of this particle are 3-body decays into two jets and a massive daughter (typically weak gaugino or Kaluza-Klein weak gauge boson). We show that the shapes of the dijet invariant mass distributions differ significantly in the two models, as long as the mass of the decaying particle mA is substantially larger than the mass of the massive daughter mB. We present a simple analysis estimating the number of events needed to distinguish between the two models under idealized conditions. For example, for mA/mB = 10, we find the required number of events to be of order several thousand, which should be available at the LHC within a few years. This conclusion is confirmed by a parton level Monte Carlo study which includes the effects of experimental cuts and the combinatoric background.

THREE PARTICLE CORRELATIONS FROM STAR

Two-particle correlations have shown modification to the away-side shape in central Au + Au collisions relative to pp , d + Au and peripheral Au + Au collisions. Different scenarios can explain this modification including: large angle gluon radiation, jets deflected by transverse flow, path length dependent energy loss, Cerenkov gluon radiation of fast moving particles, and conical flow generated by hydrodynamic Mach-cone shock-waves. Three-particle correlations have the power to distinguish the scenarios with conical emission, conical flow and Cerenkov radiation, from other scenarios. In addition, the dependence of the observed shapes on the pT of the associated particles can be used to distinguish conical emission from a sonic boom (Mach-cone) and from QCD-Čerenkov radiation. We present results from STAR on 3-particle azimuthal correlations for a high pT trigger particle with two softer particles. Results are shown for pp , d + Au and high statistics Au + Au collisions at [Formula: see text]. An important aspect of the analysis is the subtraction of combinatorial backgrounds. Systematic uncertainties due to this subtraction and the flow harmonics v2 and v4 are investigated in detail. The implications of the results for the presence or absence of conical flow from Mach-cones are discussed.

SHORT LIVED RESONANCES IN ALICE

The ability of the ALICE detector to reconstruct the K*(892)0 resonance in p – p and Pb – Pb collisions at LHC energies has been investigated, by means of a detailed study of simulated events. PYTHIA events for p – p collisions and HIJING events for Pb – Pb collisions have been generated and fully reconstructed. The K*(892)0 was identified by its hadronic decay into Kπ through invariant mass analysis. The combinatorial background has been estimated using an event-mixing technique for p – p events and a like-sign technique for the Pb – Pb ones. The role of the particle identification of the decay product is also discussed.

ARE THERE MACH CONES IN HEAVY ION COLLISIONS?

We present results from STAR on 3-particle azimuthal correlations for a 3 < pT < 4 GeV /c trigger particle with two softer 1 ≤ pT ≤ 2 GeV /c particles. Results are shown for pp , d + Au and high statistics Au + Au collisions at [Formula: see text]. We observe a 3-particle correlation in central Au + Au collisions which may indicate the presence of conical emission. In addition, the dependence of the observed signal angular position on the pT of the associated particles can be used to distinguish conical flow from simple QCD-Čerenkov radiation. An important aspect of the analysis is the subtraction of combinatorial backgrounds. Systematic uncertainties due to this subtraction and the flow harmonics v2 and v4 are investigated in detail.

Construction and expected performance of the hadron blind detector for the PHENIX experiment at RHIC

A new hadron blind detector (HBD) for electron identification in high density hadron environment has been installed in the PHENIX experiment at RHIC in the fall of 2006. The HBD will identify low momentum electron–positron pairs to reduce the combinatorial background in the e+e− mass spectrum, mainly in the region below 1 GeV/c2. The HBD is a windowless proximity-focusing Cherenkov detector with a radiator length of 50 cm, a CsI photocathode and three layers of gas electron multipliers (GEM). Pure CF4 is used as a radiator and a detector gas. This proceeding describes the construction details and the expected performance of the HBD.

Combinatorics of transformations from standard to non-standard bases in Brauer algebras

Transformation coefficients between standard bases for irreducible representations of the Brauer centralizer algebra and split bases adapted to the subalgebra (f1 + f2 = f) are considered. After providing the suitable combinatorial background, based on the definition of the i-coupling relation on nodes of the subduction grid, we introduce a generalized version of the subduction graph which extends the one given in Chilla (2006 J. Phys. A: Math. Gen. 39 7657) for symmetric groups. Thus, we can describe the structure of the subduction system arising from the linear method and give an outline of the form of the solution space. An ordering relation on the grid is also given and then, as in the case of symmetric groups, the choices of the phases and of the free factors governing the multiplicity separations are discussed.

Comparison of 3-Particle Correlation Methods

Three-particle azimuthal correlations can be used to explore the away-side structure of jets in heavy ion collisions. The combinatorial background in such measurements has angular structure similar to the signal. This paper discusses two different techniques for subtracting the backgrounds. A simple simulation model is used for illustration. A brief review of the current experimental results is also given.

A hadron blind detector for the PHENIX experiment at RHIC

A hadron blind detector (HBD) is to be installed in the PHENIX experiment for Run7, starting in fall 2006. The HBD is a threshold Cerenkov detector designed to measure electrons in a field free region surrounding the collision vertex. The HBD’s primary purpose is to tag background electrons originating from photon conversion and Dalitz decays. These background electrons can subsequently be excluded from a dilepton analysis, thereby reducing the combinatorial background by up to a factor of 100. The detector is realized by a proximity-focus windowless Cerenkov detector operating with pure CF4 and read out by a reflective photocathode consisting of a stack of gas electron multipliers (GEM), the first of which is coated with 300 nm of CsI. The avalanche charge from the GEM stack is collected on a PCB with pads having similar size to the Cerenkov blob. Dalitz and photon conversions are tagged by having twice the amplitude of a single blob. An excellent Quantum Efficiency of the photocathode is thus crucial to the success of the device. Outstanding results have been accomplished using a vacuum evaporation facility on loan from INFN Rome. We will give an overview of the experimental methods to identify blobs and techniques developed for photocathode production.

Direct observation of dijets in central Au+Au collisions with STAR

We present first results of azimuthal ( Δ ϕ ) correlations of high transverse momentum ( p T ) hadrons from the high-statistics Au+Au dataset collected at RHIC in 2004. Previous correlations measurements observed the disappearance of back-to-back ( Δ ϕ ∼ π ) correlations in central Au+Au collisions, supporting the emerging picture of parton energy loss in dense QCD matter. In the present analysis, increasing the p T thresholds reveals a narrow back-to-back dijet peak which emerges above the decreasing combinatoric background. We study the possible medium modification of the dijet fragmentation function and compare to models, to place constraints on the nature and magnitude of in-medium energy loss.

The ATLAS B-Physics Trigger

The b-quark production cross-section at the LHC is many orders of magnitude higher than at b-factories. However high event rates makes it necessary to have a very selective trigger system. In addition, the high particle-multiplicity in b-quark events, combined with pile-up of an average of 4.6 interactions per bunch crossing at a luminosity of 2 × 10 33 c m −2 s −1 , gives rise to a large combinatorial background, which must be rejected at the trigger level. This article gives an overview of the ATLAS Trigger system with an emphasis on the B-physics selections.

Confirmation of the doubly charmed baryon [formula omitted](3520) via its decay to [formula omitted

We observe a signal for the doubly charmed baryon Ξcc+ in the decay mode Ξcc+→pD+K− to complement the previous reported decay Ξcc+→Λc+K−π+ in data from SELEX, the charm hadroproduction experiment at Fermilab. In this new decay mode we observe an excess of 5.62 events over a combinatoric background estimated by event mixing to be 1.38±0.13 events. The mixed background has Gaussian statistics, giving a signal significance of 4.8σ. The Poisson probability that a background fluctuation can produce the apparent signal is less than 6.4×10−4. The observed mass of this state is 3518±3 MeV/c2, consistent with the published result. Averaging the two results gives a mass of 3518.7±1.7 MeV/c2. The observation of this new weak decay mode confirms the previous SELEX suggestion that this state is a double charm baryon. The relative branching ratio for these two modes is 0.36±0.21.

Low mass dilepton production at RHIC energies

Recent results on low mass dilepton measurements from the PHENIX experiment are reported. Invariant mass spectra of \(\phi\rightarrow e^ + e^-\) are measured for the first time in Au-Au collisions at \(\sqrt{s_{NN}} = 200\) GeV in Run2. In d-Au collisions, the yields and M T slopes of both \(\phi\rightarrow e^ + e^-\) and \(\phi \rightarrow K^ + K^-\) are measured. Both results are consistent with each other within errors. In the future, a Hadron Blind Detector will be installed in PHENIX which will enhance our capabilities of rejecting external photon conversions and Dalitz pairs, that will result in a significant reduction of the large combinatorial background.

Dilepton analysis in the HADES spectrometer for C+C at 2 A GeV

The high-acceptance dielectron spectrometer (HADES) has become operational at GSI Darmstadt. The primary physics programme is to study in-medium changes of light vector mesons via their e+e− decays. The methods of dilepton signal reconstruction in the HADES spectrometer and preliminary dilepton spectra for C+C reactions at 2 A GeV are presented. In the signal reconstruction, particularly important is the reduction of the huge combinatorial background arising from e+e− combinations of leptons from gamma conversion in materials with other leptons in the collision. The purity of the dilepton signal is determined by using GEANT simulations with the full HADES geometry and a realistic detector response.

DILEPTON ANALYSIS IN THE HADES SPECTROMETER FOR 12C+12C AT 2 AGEV

The identification of the light mesons in the HADES spectrometer is based on an invariant mass reconstruction of their decay into e+e- pairs. In the dilepton (e+e-) signal reconstruction, particularly important is the reduction of a huge combinatorial background arising from wrong combinations of e+ and e- to unlike sign pairs. Methods of the dilepton signal and combinatorial background reconstruction in the HADES spectrometer will be presented.

Observation of a Narrow Charm-Strange MesonDsJ+(2632)→Ds+ηandD0K+

We report the first observation of a charm-strange meson D(+)(sJ)(2632) at a mass of 2632.5+/-1.7 MeV/c(2) in data from SELEX, the charm hadro-production experiment E781 at Fermilab. This state is seen in two decay modes, D(+)(s)eta and D0K+. In the D(+)(s)eta decay mode we observe a peak with 101 events over a combinatoric background of 54.9 events at a mass of 2635.4+/-3.3 MeV/c(2). There is a corresponding peak of 21 events over a background of 6.9 at 2631.5+/-2.0 MeV/c(2) in the decay mode D0K+. The decay width of this state is <17 MeV/c(2) at 90% confidence level. The relative branching ratio Gamma(D0K+)/Gamma(D(+)(s)eta) is 0.14+/-0.06. The mechanism that keeps this state narrow is unclear. Its decay pattern is also unusual, being dominated by the D(+)(s)eta decay mode.

Inclusive production of D + , D 0, D s + and D * + mesonsin deep inelastic scattering at HERA

Inclusive production cross sections are measured in deep inelastic scattering at HERA for meson states composed of a charm quark and a light antiquark or the charge conjugate. The measurements cover the kinematic region of photon virtuality 2 < Q^2 < 100 GeV^2, inelasticity 0.05 < y < 0.7, D meson transverse momenta p_t(D) > 2.5 GeV and pseudorapidity |eta(D)| < 1.5. The identification of the D-meson decays and the reduction of the combinatorial background profit from the reconstruction of displaced secondary vertices by means of the H1 silicon vertex detector. The production of charmed mesons containing the light quarks u, d and s is found to be compatible with a description in which the hard scattering is followed by a factorisable and universal hadronisation process.

The ATLAS discovery potential for a heavy charged Higgs boson in gg → tbH± with H± → tb

The feasibility of detecting a heavy charged Higgs boson, m(H^{+-})>m(t)+m(b), decaying in the H^{+-}->tb channel is studied with the fast simulation of the ATLAS detector. We study the gg->H^{+-}tb production process at the LHC which together with the aforementioned decay channel leads to four b-quarks in the final state. The whole production and decay chain reads gg->H^{+-}tb->t\bar{t}b\bar{b}->b\bar{b}b\bar{b}l\nu\bar{q}q'. Combinatorial background is a major difficulty in this multi-jet environment but can be overcome by employing multivariate techniques in the event reconstruction. Requiring four b-tagged jets in the event helps to effectively suppress the Standard Model backgrounds but leads to no significant improvement in the discovery potential compared to analyses requiring only three b-tagged jets. This study indicates that charged Higgs bosons can be discovered at the LHC up to high masses (m(H^{+-})>400 GeV) in the case of large tan(beta).