Combinatorial Background Research Articles

Extracting gluino endpoints with event topology patterns

Abstract In this paper we study the gluino dijet mass edge measurement at the LHC in a realistic situation including both SUSY and combinatorial backgrounds together with effects of initial and final state radiation as well as a finite detector resolution. Three benchmark scenarios are examined in which the dominant SUSY production process and also the decay modes are different. Several new kinematical variables are proposed to minimize the impact of SUSY and combinatorial backgrounds in the measurement. By selecting events with a particular number of jets and leptons, we attempt to measure two distinct gluino dijet mass edges originating from wino $\widetilde{g} \to jj\widetilde{W}$ and bino $\widetilde{g} \to jj\widetilde{B}$ decay modes, separately. We determine the endpoints of distributions of proposed and existing variables and show that those two edges can be disentangled and measured within good accuracy, irrespective of the presence of ISR, FSR, and detector effects.

Opposite power series

In order to analyze the singularities of a power series function P(t) on the boundary of its convergent disc, we introduced the space Ω(P) of opposite power series in the opposite variable s=1/t, where P(t) was, mainly, the growth function (Poincaré series) for a finitely generated group or a monoid Saito (2010) [10]. In the present paper, forgetting about that geometric or combinatorial background, we study the space Ω(P) abstractly for any suitably tame power series P(t)∈C{t}. For the case when Ω(P) is a finite set and P(t) is meromorphic in a neighborhood of the closure of its convergent disc, we show a duality betweenΩ(P)and the highest order poles ofP(t)on the boundary of its convergent disc.

The measurement of Matveev-Muradyan-Tavkhelidze-Drell-Yan processes with SPD detector at NICA. Background studies.

SPD NICA project is under preparation at second interaction point of the NICA collider. The purpose of this experiment is the study of the nucleon spin structure with high intensity polarized light nuclear beams. It is argued that the design of the collider can allow us to reach with proton beams a very high collision energy up to √s ∼ 26 GeV with average luminosity up to 1030 − 1031 cm−2 s−1 . At the same time, the respective number for deuteron collisions is also quite considerable: at a collision energy per nucleon up to √s ∼ 12 GeV, the average luminosity reaches up to 1029 − 1030 cm−2 s−1 . It is of great importance that both proton and deuteron beams can be effectively polarized. The preliminary design of the SPD detector for spin physics studies is based on the requirements imposed by the MMT-DY and J /ψ productions studies. The some sources of background to the MMT-DY process are the combinatorial background from decays (π 0 , η ) and gamma contribution and the decays of charmed-mesons, which are studied now.

RESULTS FROM PHENIX AT RHIC WITH IMPLICATIONS FOR LHC

Results from the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) in nucleus–nucleus and proton–proton collisions at c.m. energy [Formula: see text] are presented in the context of the methods of single and two-particle inclusive reactions which were used in the discovery of hard-scattering in p–p collisions at the CERN ISR in the 1970's. These techniques are used at RHIC in A + A collisions because of the huge combinatoric background from the large particle multiplicity. Topics include J/Ψ suppression, jet quenching in the dense medium (sQGP) as observed with π0 at large transverse momentum, thermal photons, collective flow, two-particle correlations, suppression of heavy quarks at large pT and its possible relation to Higgs searches at the LHC. The differences and similarities of the measurements in p–p and A + A collisions are presented. The two discussion sessions which followed the lectures on which this article is based are included at the end.

Measurement of low mass dielectrons with the HBD upgrade of PHENIX

The PHENIX experiment was upgraded with the Hadron Blind Detector (HBD) for the measurement of low-mass electron pairs. The HBD reduces the combinatorial background by providing additional electron identification in a field-free region close to the vertex and by exploiting the small opening angle of the π0 Dalitz and conversion pairs. The HBD was successfully operated during Run 9 and Run 10 at the RHIC in the measurements of e+e− pairs in p+p and Au+Au collisions, respectively. This paper reports on the present status of the dielectron analysis in Au+Au collisions at = 200 GeV.

Study of the strange resonance Σ(1385) in the decay channel Λ π for simulated pp collisions at √s = 10 TeV and first analysis of data at √s = 900 GeV and √s = 7 TeV with the ALICE Experiment

The strange resonance Σ(1385) proved to be an important tool in the Quark Gluon Plasma (QGP) dynamics studies. The comparison between yields in pp and PbPb collisions can provide an estimate of the time-span between chemical and thermal freeze-out since it approximately determines the net effect of re-scattering and regeneration on the total yield. Tools for the extraction of the Σ(1385) signal in the strongly-decaying Λ π channel in simulated pp collisions at 10 TeV will be presented. The fitting procedure involves different techniques for combinatorial background evaluation. Its implementation and its use to extract the signal, the significances and the yields in different bins of transverse momentum and rapidity, will be presented together with a first analysis of pp collisions data at 900 GeV and 7 TeV in the ALICE Experiment[1].

Bi-Event Subtraction Technique at hadron colliders

We propose the Bi-Event Subtraction Technique (BEST) as a method of modeling and subtracting large portions of the combinatoric background during reconstruction of particle decay chains at hadron colliders. The combinatoric background arises when it is impossible to know experimentally which observed particles come from the decay chain of interest. The background shape can be modeled by combining observed particles from different collision events and be subtracted away, greatly reducing the overall background. This idea has been demonstrated in various experiments in the past. We generalize it by showing how to apply BEST multiple times in a row to fully reconstruct a cascade decay. We show the power of BEST with two simulated examples of its application towards reconstruction of the top quark and a supersymmetric decay chain at the Large Hadron Collider.

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.

Measurement of ω mesons via radiative decay mode in $\surd s$ = 200 GeV Au+Au collisions at RHIC-PHENIX

We report on ω mesons’ production via radiative decay mode (ω → π0γ, π0 → 2γ) in \(\sqrt s \) = 200 GeV Au+Au collisions at RHIC-PHENIX. The main difficulty of this analysis is the huge combinatorial background inevitable for reconstructing particles from the three- body decay mode. The emphasis is on the feasibility study using both simulation and real data to search the best parameters that improve \(S/\sqrt B \). For this purpose, we examine several sets of energy and momentum cuts for selecting π0 and γ. To evaluate the remaining background, we consider three background sources; correlated/uncorrelated background and Ks0 contribution, then we subtract them. The amount of these contributions is determined by a fit on a mass spectrum. The result suggests that ω meson production is suppressed in central Au+Au collisions at high pT region.

Squark mass measurement in the long-lived stau scenario at the LHC

The long-lived stau scenario is an interesting possibility at the LHC. We study squark mass measurements in this scenario; in particular, we show that left- and right-handed squark masses are both measurable. In SUSY events, multiple jets are expected, which become a source of combinatorial backgrounds. In order to reduce such backgrounds a hemisphere analysis is applied, and we discuss mass measurements of squarks in decay modes of q˜L into qW˜0 or q′W˜± and also q˜R into qB˜.

Measurements of low mass [formula omitted] pairs in p+p and Au+Au collisions with the HBD upgrade of the PHENIX detector

The Hadron Blind Detector (HBD) was developed, installed and successfully operated in the PHENIX detector during RHIC Runs in 2009 (p+p) and 2010 (Au+Au). The HBD is a windowless Cherenkov detector, operated with CF 4 gas in proximity focus configuration. It uses triple GEM elements for signal amplification with CsI photocathode evaporated on top of the first GEM. The purpose of the HBD is to reduce the combinatorial background from the dielectron invariant mass spectrum by recognizing and rejecting e + or e − originating from π 0 Dalitz decays and conversion. This article reports on the in-beam performance of the HBD.

Determination of nonuniversal supergravity models at the Large Hadron Collider

We examine a well motivated non-universal supergravity model where the Higgs boson masses are not unified with the other scalars at the grand unified scale at the LHC. The dark matter content can easily be satisfied in this model by having a larger Higgsino component in the lightest neutralino. Typical final states in such a scenario at the LHC involve W bosons. We develop a bi-event subtraction technique to remove a huge combinatorial background to identify W -> jj decays. This is also a key technique to reconstruct supersymmetric particle masses in order to determine the model parameters. With the model parameters, we find that the dark matter content of the universe can be determined in agreement with existing experimental results.

Measurement of D + and Λ c + production in deep inelastic scattering at HERA

Charm production in deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 120 pb−1. The hadronic decay channels D + → K S 0 π +, Λ c + → pK S 0 and Λ c + → Λπ +, and their charge conjugates, were reconstructed. The presence of a neutral strange hadron in the final state reduces the combinatorial background and extends the measured sensitivity into the low transverse momentum region. The kinematic range is 0 < p T (D +, Λ c +) < 10 GeV, |η(D +, Λ c +)| < 1.6, 1.5 < Q 2 < 1000 GeV2 and 0.02 < y < 0.7. Inclusive and differential cross sections for the production of D + mesons are compared to next-to-leading-order QCD predictions. The fraction of c quarks hadronising into Λ c + baryons is extracted.

Simulation study of ϕ and ω resonance production and detection in ALICE at LHC

Physical motivations of vector meson ( ϕ and ω ) production study in heavy-ion collisions are discussed using the theoretical predictions and the recent experimental results at SPS and RHIC. A simulation study of ϕ and ω (and J/ ψ for comparison) production through hadronic and leptonic decays in the ALICE detector at LHC is presented for Pb–Pb ans p–p collisions. The possibility to select the resonance signals over the combinatorial backgrounds is demonstrated using a realistic simulation of tracking and particle identification in the ALICE offline framework.

Simulation study on light vector meson decays to electron-positron pairs with the ALICE experiment

Simulation study on ω, ϕ, ρ decays to e−e+ pairs in the ALICE detector for Pb-Pb collisions at LHC energy was performed. The possibility of selecting resonance signals over the combinatorial backgrounds is demonstrated using the realistic simulation tracking and particle identification algorithms of the ALICE offline framework (AliRoot). Results for J/ψ are presented also for comparison.

Measurement of D ± and D 0 production in deep inelastic scattering using a lifetime tag at HERA

The production of D+- and D0 mesons has been measured with the ZEUS detector at HERA using an integrated luminosity of 133.6 pb-1. The measurements cover the kinematic range 5 < Q^2 < 1000 GeV^2, 0.02 < y < 0.7, 1.5 < p_T^D < 15 GeV and eta^D < 1.6. Combinatorial background to the D meson signals is reduced by using the ZEUS microvertex detector to reconstruct displaced secondary vertices. Production cross sections are compared with the predictions of next-to-leading-order QCD which is found to describe the data well. Measurements are extrapolated to the full kinematic phase space in order to obtain the open-charm contribution, F2^ccbar, to the proton structure function, F2.

Scheduling multiprocessor UET tasks of two sizes

In this paper we study task scheduling problems on m identical parallel processors, where each task has unit execution time, and needs either a single processor, or q processors concurrently, and it has a release date and a due date. Under the assumption that the release dates and due dates of the q -processor tasks are agreeable, we describe a polynomial time algorithm for minimising the number of tardy tasks. In addition, we apply this result for minimising the maximum lateness, and the maximum tardiness. We also discuss the combinatorial background of the polynomial time solvability of all these problems under the ‘agreeable’ assumption.

Perspectives for the measurement of D s + mesons via the D s + →K + K − π + channel in the ALICE experiment

Heavy quarks, such as charm, are produced in hard scatterings in the early stages of high energy nucleus-nucleus collisions and are expected to be a powerful tool to investigate the properties of the quark gluon plasma (QGP). The tracking detectors of the ALICE apparatus will allow to track and identify particles in central rapidity range down to low P t . Among D mesons it would be particularly interesting to measure D s yield via an exclusive hadronic decay channel because it could help to disentangle different hadronization mechanisms. The possibility of reconstructing the D s meson through its D s + →K + K − π + decay channel in the central barrel was studied. The problem considered is characterized by the comparatively low yield of the D s mesons against the huge amount of combinatorial background. Different kinematic and topological cuts have been studied in order to increase the signal-to-background ratio and the statistical significance. In addition, D s mesons preferentially decay through intermediate resonant states and this fact can improve the separation of signal from background. Results of cut parameters tuning and values of significance for an analysis performed on simulated data are presented.

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. The light vector mesons (ρ, ω, and ϕ) were photo-produced on 2 H , C , Ti , Fe , and Pb targets at the Thomas Jefferson National Laboratory using the CEBAF Large Acceptance Spectrometer (CLAS). The data were taken with a beam of tagged photons with energies up to 4 GeV. The properties of the ρ vector meson at normal nuclear densities and zero temperature,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. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The ρ meson mass spectrum was extracted after the ω and ϕ signals were removed in a nearly model-independent way. The rho meson mass distributions were extracted for each of the targets. Comparisons were made between the ρ mass spectra from the heavy targets ( A &gt; 2) with the mass spectrum extracted from the deuterium target. With respect to the ρ-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. In this experiment, due to the long lifetimes and momenta greater than 0.8 GeV, the ω and ϕ mesons have a high probability of decaying outside the nucleus in their vacuum state. However, their in-medium widths can be accessed through their absorption inside the nucleus. Preliminary results on the ratios of the nuclear transparencies of the ω and ϕ mesons as a function of the number of target nucleons A, have been obtained and indicate a substantial widening in the medium.

VECTOR MESON MODIFICATION IN NUCLEAR MATTER AT CLAS

Photoproduction of vector mesons off nuclei were performed at Jefferson Lab using the CEBAF Large Acceptance Spectrometer (CLAS). The properties of the ρ vector mesons were investigated via their rare leptonic decay to e+e−. After subtracting the combinatorial background, the ρ meson mass distributions were extracted for each of the targets. We observe no effects on the mass of the ρ meson, some widening in titanium and iron is observed consistent with the collisional broadening.