aa r X i v : . [ nu c l - e x ] A p r ψ ′ to J/ψ
Ratio Measurements in PHENIX at RHIC
Maris´ılvia Donadelli for the PHENIX Collaboration ∗ University of S˜ao Paulo, S˜ao Paulo, Brazil
The ratio of the ψ ′ over the J/ψ production cross section in the dielectron channel hasbeen measured in √ s = 200 GeV p + p collisions with the PHENIX detector at RHIC. Theanalysis is based on fitting of the dielectron invariant mass spectra in the area around the J/ψ and ψ ′ signals in order to extract a ratio ψ ′ over J/ψ of 0.019 ± ± J/ψ from ψ ′ of 8 . ± . I. INTRODUCTION
J/ψ production in hadronic collisions can occur in part through the production of higher excitedresonances, ψ ′ , χ c , which decay into the ground state. For J/ψ it is known experimentally thatabout 40% of hadroproduction rate comes from feed-down of higher mass resonances [1], [2], [3].Measurement of excited charmonium states is of key importance to account for the feed-downcontributions. Thus, this is an aditional test for different production mechanisms of charmonium.
II. MEASUREMENT METHOD
The number of events in the
J/ψ peak, N J/ψ , is given by: N J/ψ = σ ( J/ψ ) · B ( J/ψ → l + l − ) · L · ǫ, (1)i.e. the product of the cross section ( σ ( J/ψ )), the branching ratio into dilepton pairs ( B ), theintegrated luminosity L and the total reconstruction and trigger efficiencies, as well as acceptance( ǫ ). The luminosity is identical for all charmonium states, and therefore cancels in the ratios. Theratio of the ψ ′ and J/ψ cross sections in the e + e − channel, R ψ ′ ( e ) is equal to: R ψ ′ ( e ) = B ′ · σ ′ B · σ = N ψ ′ N J/ψ · ǫǫ ′ (2)where e denotes the leptonic decay channel, σ ( σ ′ ) is the J/ψ ( ψ ′ ) production cross section and B ( B ′ ) is the branching ratio for the e + e − decay into the J/ψ ( ψ ′ ) meson.The ratio N ψ ′ N J/ψ or ’raw ψ ′ J/ψ ratio’ is defined from the fit of the
J/ψ and ψ ′ signals and must becorrected by the efficiency ratio, ǫǫ ′ , where ǫ is defined above and ǫ ′ is the corresponding efficiencyfor detection of ψ ′ mesons. The efficiencies are evaluated by simulation as described in IV. III. EXPERIMENTAL SETUP
Data collected during the 2006 RHIC run with the PHENIX central arms spectrometers whichcover | η | < × π/ ∗ Electronic address: mari@fig.if.usp.br ositrons are reconstructed in the central arms [4] using Drift Chambers (DC) and Pad Chambers(PC), located outside an axial magnetic field, and are identified by hits in the Ring ImagingCherenkov detector (RICH) and by matching the momentum with the energy measured in anElectromagnetic Calorimeter (EmCal).This analysis is based on collision events triggered by minimum activity in the Beam BeamCounters at 3.0 < | η | < IV. ANALYSIS AND DISCUSSION
Two sets of electron identification cuts were used to count
J/ψ and ψ ′ . A first set, named’tight’ with a match between energy and momentum satisfying ( E/p − ≥ -3 standard deviations( σ ), interaction-vertex cut of ±
30 cm. The second set, named ’loose’ with energy-momentumrequirement (E/p-1) ≥ -4 σ , interaction-vertex cut of ±
30 cm and a minimum number of DC hitsused to reconstruct each track of the electron-positron pair.The number of
J/ψ candidates is obtained by counting the unlike sign pairs in the mass window(2.88-3.32) GeV/c and the ψ ′ candidates in the mass window (3.48-3.92) GeV/c . The combina-torial background subtraction is performed as the sum of the like sign pairs. The subtracted plotcontains charmonium resonances and physical background made of correlated ( e + e − ) pairs fromDrell Yan and leptonic decays of charm and bottom. From PYTHIA the ( e + e − ) invariant massdistribution was obtained for all sources of physical background under the J/ψ and ψ ′ as shownin Figure 1. The resulting sum of all the three components was fit with a power law.To count the number of J/ψ s and ψ ′ s different fit procedures were applied for each one of theelectron identification cuts. Figure 2 shows a double gaussian shape for the J/ψ peak, a singlegaussian for ψ ′ . The background was fit with a power law, the J/ψ - ψ ′ mass difference was fixed withthe Particle Data Group value (0.589 GeV/ c ) and both J/ψ and ψ ′ widths varied independently.For this fit, loose electron identification cuts set was applied. FIG. 1: e + e − invariant mass distribution for allsources of physical background under J/ψ and ψ ′ . / ndf c ] [GeV/c - e + Mass e2 2.5 3 3.5 4 4.5 5 C oun t s / M e V / c / ndf c PHENIX PRELIMINARY 0.2 % – – = 1.9 ee fi y ee fi ’ y FIG. 2: Invariant mass distribution e + e − in theregion of the J/ψ and ψ ′ peaks for triggered dielec-tron events. Acceptance and reconstruction efficiency were calculated by generating
J/ψ and ψ ′ withPYTHIA event generator and checking the response of PHENIX detector with a GEANT basedMonte Carlo. In order to generate J/ψ , all
J/ψ decays were turned off except the dielectron one.To account for direct and indirect production (through χ and ψ ′ decays) the subprocesses reportedon the left of Table I were used. In order to generate ψ ′ particle id=100443 was requested, insteadof 443 and all ψ ′ decay modes were turned off except the dielectron one plus the following on theright of Table I.The average transverse momentum is about 1.7 GeV for J/ψ and 1.9 GeV for ψ ′ as shown inFigures 3 and 4. The electron average transverse momentum is about 1.5 GeV for electrons coming → g + g → J/ Ψ + g g + g → χ c + g g + g → χ c + g g + g → χ c + g g + g → χ c g + g → χ c g + g → J/ Ψ + γ [b] 96 Semihard QCD 2 → g + g → ψ ′ + g g + g → ψ ′ + γ TABLE I:
J/ψ and ψ ′ subprocesses in PYTHIA event generator. J/Psi PT (Gev)0 1 2 3 4 5 6 7 8 9 10 E ve n t s J/Psi with |y|<0.5 h1Entries 243465Mean 1.656RMS 0.8816
J/Psi with |y|<0.5
FIG. 3: Transverse momentum distribution for all
J/ψ with rapidity − . < y < . Psiprime PT (Gev)0 1 2 3 4 5 6 7 8 9 10 E ve n t s Psiprime with |y|<0.5 h1Entries 525130Mean 1.872RMS 0.9845
Psiprime with |y|<0.5
FIG. 4: Transverse momentum distribution for all ψ ′ with rapidity − . < y < . from J/ψ decays and 1.8 GeV for electrons coming ψ ′ . J/ψ and ψ ′ with rapidity − . < y < . × reconstruction efficiency for J/ψ and ψ ′ are shown in Figures 5 and 6 respectively,both for loose electron identification cuts. The integrated acceptance × reconstruction efficiencyis 2.48 for ψ ′ and 2.41 for J/ψ .Integrating over the momentum distribution predicted by PYTHIA we find that the ratio of the ψ ′ acceptance × reconstruction efficiency to the J/ψ is ǫ ′ /ǫ = 1 . ± . V. SUMMARY
FIG. 5: The acceptance × reconstruction efficiencyfor J/ψ for loose electron id cuts. FIG. 6: The acceptance × reconstruction efficiencyfor ψ ′ for loose electron id cuts. (GeV) NN S10 ( % ) y J / s ll y J / / B ’ ys ll’ y B p+A p+p NRQCD Theory FIG. 7: ψ ′ over J/ψ ratio as a function of the en-ergy in the center of mass frame measured in variousexperiments.
The ψ ′ over J/ψ ratio is quoted as obtainedwith loose electron identification cuts and thefit double gaussian (for
J/ψ ), single gaussian(for ψ ′ ) and power law (for the physical back-ground) · ǫ ′ /ǫ (acceptance × reconstruction ef-ficiency): 0.019 ± √
12 is ± . J/ψ from ψ ′ is 8 . ± .
5% in good agreement with thecalculation provided by [5], which is 8 ± Experiment target Energy GeV Result
NA51 [6] p 29.1 1.6 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± TABLE II: ψ ′ over J/ψ ratio for different energy regimes and collision species.
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