Quest for omega mesons by their radiative decay mode in √s=200 GeV A+

A collisions at RHIC-PHENIX

Quest for omega mesons by their radiative decay mode in √s=200 GeV A+

A collisions at RHIC-PHENIX~Why is it “Quest”?~~Why is it “Quest”?~

Simulation Study

Simulation Study

Real Data AnalysisReal Data Analysis

Misaki Ouchida Hiroshima University ouchida@hepl.hiroshima-u.ac.jp for the PHENIX collaboration

Misaki Ouchida Hiroshima University ouchida@hepl.hiroshima-u.ac.jp for the PHENIX collaboration

γ

γ

γ

0π γ

γη

γ γγγ

ProbeProbeIntroductionIntroduction

Experimental SetupExperimental Setup

Efficiency

calculation

Efficiency

calculation

Improvement of significanceImprovement of significance

ResultsResults

Mixed Event Trial

Mixed Event Trial

Outlook

Outlook

The goal of the relativistic heavy ion physics is to explore nature’s most basic ingredients and phenomena.The state of the nuclear matter is described by the Quantum Chromo Dynamics(QCD).

・ The chiral symmetry is spontaneously broken <qq>~ 250MeV‥‥ 3.・ At high temperatures or high densities <qq>~0, i.e. chiral symmetry is restored in this region.‥‥This means, there is a transition of the system that constitute mass when it’s going to the extreme condition that high energy heavy ion experiments make.

Low-mass vector mesons should provide an unique tool to observe in-medium modifications of its properties (e.g. mass and/or width).

The typical fireball, the hot and dense matter created by the heavy ion collision is said to have ~10fm/c lifetime. So then many vector mesons have a chance of decaying inside the medium.

A promising approach to investigate in-medium modifications of the ω meson is to study radiative decay mode,ωπ0γ being blessed with following essential advantages. 　

・ clean way to investigate the properties (electromagnetic coupling mode)・ large branching ratio (about 3 orders of magnitude larger than e+e-)・ no ρ-contribution

Pick ω mesons (mass:782MeV, lifetime:23fm/c)

The Relativistic Heavy Ion Collider (RHIC) is proven to have an enough energy for researching the chiral phase transition leading to the expected mass modification.

The data set is Au+Au collisions at C.M.S collisions energy per nucleon pairs of 200GeV taken at PHENIX from the year of 2003 to 2004.

The number of events used from the data: 1.06×109

The measurement of ω mesons in the heavy ion collisions is really the “Quest”. About 9% of ω are going to π0 and γ, then 98.8% of those π0 is going to 2γ. That is, we have to reconstruct 3γ from huge number of γ measured in EMCal; approximately 300 γ are measured per one collisions. The formula of the invariant mass is,

The question is, “does it really possible to search out ω mesons in the high multiple collisions that produce huge combinatorial background?”.To find the feasibility, we execute the simulation in advance then calculate an acceptance and a multiplicity dependence.Furthermore, we search the best parameters such as momentum and energy for selecting π0 and γ using both simulation and real data.

We generated single ω event using an event generators which is detailed by Monte Carlo codes. To simulate the PHENIX detector, the PHENIX implementation of the GEANT based simulation and event particle tracking software system, “PISA” was used.

Efficiency = Single Particle Correction(SPC): correction due to geometrical acceptance+ Multiplicity Dependence Correction(MDC)

To evaluate MDC, we used the technique called “embedding”; embedding of the simulated particles into a real event. The main issue of this analysis is the huge

combinatorial background inevitable for reconstructing particles from the three-body decay mode. So then an improvement of the peak significance, S/√B is a vital importance.We considered following parameters that should have a great influence on the significance.

・ Transverse Momentum Cut (pT) of π0

・ Photon Energy Cut (except photons from π0 candidate)・ Width of π0 invariant Mass

We succeeded in improving the significance up to 4, implying that it is possible to measure ω in real data.

γπ 0 ηγ

Counts

2/ cGeV

ω 0π

γγγ

MinBias8.5<pT<9.5[GeV/c]

After introducing above best cuts, reconstructed ω invariant mass spectra as a function of ω transverse momentum are shown.

We first reconstruct 2γ and set as “π0 candidate” by selecting the invariant mass within 1.25σ or 1.5σ(see the table above), but there’s a probability that the uncorrelated 2γ satisfies the criteria of π0 candidate. The shape of reconstructed invariant mass shows the behavior of such combination.

The main background we want to exclude is coming from the correlation between the one of π0 candidate and third photon.

We introduced “Event Mixing” to create such correlation.

Finally, ω mesons come to appear more than before the subtraction.

From the results, we can see ω mesons plateau and it shows not inconsistent with the simulated ω mesons, however, it is hard to evaluate the values such as yield, peak and width. The more fully subtraction of background additional to the event mixing is in the progress. We are going to push forward simulation study and see the other particle’s contribution, such as Ks π0π0. This particle contribution study will allow to extract the background in detail and help to approach the low momentum region where the in-medium modification mainly dominate.

PHENIX DetectorPHENIX Detector

Invariant mass spectrum of simulated ω embedding to the real data

Demonstration showing how it works in the simulation data

determined cuts for each parameters

ω

after cut optimization

before

main background

The peak around 0.2~0.4GeV/c2 is due to the combination of π0

γ(π02γ) failed to reconstruct ω. The around 0.5~0.7GeV/c2 is due to the combination of ηγ(η2γ).

Mixed Event as, π0 candidate is selected from different events(having no correlation) and chose third photon as the same event with one of the candidate.

subtraction

comparison between the foreground(black lines) and background using mixed event(red lines), and the subtracted spectra. Green lines denote the expected ω mass calculated by simulation.

(where E is the measure energy of γ 　 and p is combined momentum for each coordinates)