The PHENIX experiment at RHIC has measured a variety of light neutral mesons
($\pi^{0}$, K$_{S}^{0}$, $\eta$, $\omega$, $\eta^{\prime}$, $\phi$) via
multi-particle decay channels over a wide range of transverse momentum. A
review of the recent results on the production rates of light mesons in p+p and
their nuclear modification factors in d+Au, Cu+Cu and Au+Au collisions at
different energies is presented.Comment: 5 pages, 4 figures, talk given at Hard Probes 2008 conference in La
  Toja, Spain. submitted to EPJ 

A. Adare

D. Pal

I. Vitev

J.I. Kapusta

K. Adcox

M. Naglis

S.S. Adler

V. Ryabov

English

arXiv

Springer - Publisher Connector

Eur. Phys. J. C (2009) 61: 835–840
DOI 10.1140/epjc/s10052-009-0879-4
Regular Article - Experimental Physics
Measurement of light mesons at RHIC by the PHENIX
experiment
M. Naglisa
for the PHENIX Collaboration
Weizmann Institute of Science, Rehovot 76100, Israel
Received: 13 September 2008 / Revised: 16 December 2008 / Published online: 5 February 2009
© Springer-Verlag / Società Italiana di Fisica 2009
Abstract The PHENIX experiment at RHIC has measured
a variety of light neutral mesons (π0, K0S , η, ω, η′, φ) via
multi-particle decay channels over a wide range of trans-
verse momentum. A review of the recent results on the pro-
duction rates of light mesons in p+p and their nuclear mod-
ification factors in d + Au, Cu + Cu and Au + Au collisions
at different energies is presented.
1 Introduction
The suppression of high-pT light mesons in ultra-relativistic
heavy ion collisions, compared to expectations from scaled
p+p results, is one of the most interesting findings at RHIC.
Particles with high transverse momentum are believed to be
produced by fragmentation of hard-scattered partons in the
earliest stage of the collision. In nucleus-nucleus collisions,
the scattered hard partons lose their energy while traversing
the subsequently produced hot and dense matter leading to
the phenomenon of jet-quenching. Medium-induced effects
on high-pT particle production can be quantified with the
nuclear modification factor:
RAA(pT ) = d
2NAA/dydpT
(〈Ncoll〉/σ inelpp ) × d2σpp/dydpT
, (1.1)
where d2NAA/dydpT is the differential yield per event in
nucleus-nucleus collisions, 〈Ncoll〉 is the number of binary
nucleon-nucleon collisions averaged over the impact para-
meter range of the corresponding centrality, and σ inelpp and
d2σpp/dydpT are the total and differential cross sections
for inelastic p + p collisions, respectively. In the absence
of medium-induced effects, the yield of high-pT particles
is expected to scale with 〈Ncoll〉, resulting in RAA = 1 at
high-pT . Contrary to the suppression of mesons, baryons
a e-mail: maxim.naglis@weizmann.ac.il
are found to be enhanced in central Au + Au collisions
at RHIC at intermediate transverse momenta (1.5 < pT <
4.5 GeV/c) [1]. The difference in the behavior of mesons
and baryons is still not well understood.
Precise knowledge of meson and baryon yields and their
ratios as a function of transverse momentum and in differ-
ent collision systems, provides a mean to investigate the de-
pendence of hadron formation on particle mass and quark-
flavor composition, and the mechanisms of jet production
and quenching. The PHENIX experiment excels in measur-
ing light neutral mesons via multi-particle decay channels.
This paper presents a review of the light meson data ob-
tained in p+p and heavy-ion collisions at different energies.
2 Experimental setup and data analysis
The PHENIX detector [2] at RHIC (Relativistic Heavy Ion
Collider) has been designed to measure a broad variety of
signals from heavy ion collisions. PHENIX is capable of
measuring electrons, photons and hadrons in a large dy-
namic range. The tracking of charged particles and mea-
surements of their momentum are accomplished by the high-
resolution multi-wire proportional Drift Chambers (DC) and
the first layer of Pad Chambers (PC1). The typical value of
the momentum resolution is σ(pT )/pT ≈ 1.0%pT ⊕ 1.1%.
The energy of photons and electrons is measured by highly
segmented lead-glass (PbGl) and lead-scintillator (PbSc)
Electro Magnetic Calorimeters (EMCal) which have an en-
ergy resolution of σPbGl(E)/E ≈ 5.9%/
√
E ⊕ 0.8% and
σPbSc(E)/E ≈ 8.1%/
√
E ⊕ 2.1%, respectively. Charged
hadron identification is based on the particle mass calculated
from the momentum and the time-of-flight information de-
rived from the TOF or EMCal subsystems and the Beam
Beam Counters (BBC’s). The TOF subsystem with a time
resolution σ  120 ps and the PbSc part of the EMCal with
σ  500 ps allow to achieve reliable pion-kaon separation
836 Eur. Phys. J. C (2009) 61: 835–840
in the pT ranges 0.3–2.5 GeV/c and 0.3–1.0 GeV/c, respec-
tively. The Zero Degree Calorimeters (ZDC’s) and BBC’s
are dedicated subsystems that determine the collision ver-
tex and event centrality and also provide the minimum bias
(MB) interaction trigger. The results of data presented in
this paper are based on data samples accumulated during
p + p, d + Au, Cu + Cu and Au + Au collisions at √sNN =
200 GeV, p+p and Cu+Cu collisions at √sNN = 62.4 GeV,
and Cu + Cu collisions at √sNN = 22.4 GeV. The MB trig-
ger used for p + p, d + Au and Cu + Cu collisions requires a
coincidence between the BBC’s with at least one hit in each
BBC arm. For Au + Au collisions the MB trigger requires
a coincidence between the BBC’s and ZDC’s with at least
two hits in each BBC arm, at least one neutron detected in
each ZDC arm. Besides, PHENIX utilizes a hardware high-
pT photon/electron trigger (ERT) based on the analog sum
of the signals of adjacent 4 × 4 EMCal towers in coinci-
dence with the MB trigger condition. All events used in the
analysis are required to have the collision vertex position
along the beam axis within 30 cm of the geometrical center
of PHENIX.
A summary of the decay channels studied and data sam-
ples analyzed is given in Table 2.1. The procedure used to
reconstruct the invariant mass spectra of π0, K0S , η, η
′
, ω
mesons is as follows. All photon clusters measured in the
EMCal in a given event are paired to form the invariant mass
distribution. Pairs with invariant mass within two standard
deviations from the π0 (η) mass are considered as π0 (η)
candidates. Raw π0 (η) yields are determined by subtract-
ing the yield of combinatorial pairs estimated by pairing
photon clusters from different events. Then, the selected π0
(η) candidates are combined between themselves, or with
other photon clusters, or with charged tracks, according to
the decay modes listed in Table 2.1. The invariant mass
Table 2.1 Summary of analyzed decay channels and data samples. In Au+Au (p+p) collisions, the measurements of the φ meson were performed
using two different analysis techniques referred to in text as “two kaons PID” and “no PID” (“one kaon PID” and “no PID”), respectively
Data sample √sNN (GeV)
∫
Ldt Trigger Decay mode pT (GeV/c) Reference
Run-2 Au + Au 200 24 μb−1 MB η → γ γ 2.25–9.5 [6]
Run-3 p + p 200 0.35 pb−1 ERT K0S → π0π0 2.5–6.5 [7]
MB η → γ γ 2.75–11.0 [6]
ERT η → π0π+π− 3.0–8.0 [6]
ERT ω → π0π+π− 2.75–9.25 [5]
ERT ω → π0γ 2.5–6.5 [5]
Run-3 d + Au 200 2.74 nb−1 ERT K0S → π0π0 3.5–8.5 [7]
MB η → γ γ 2.25–11.0 [6]
ERT η → π0π+π− 5.0–8.0 [6]
ERT ω → π0π+π− 3.5–9.0 [5]
ERT ω → π0γ 3.0–7.0 [5]
MB φ → K+K− 1.45–5.1 [9]
Run-4 Au + Au 200 241 μb−1 MB π0 → γ γ 1.25–19.0 [15]
MB ω → π0γ 4.0–9.0 [7]
MB φ → K+K− 0.8–4.0 [8]
MB φ → K+K− 2.45–7.0 –
Run-5 p + p 200 3.78 pb−1 MB/ERT π0 → γ γ 0.625–19.0 [14]
ERT K0S → π0π0 2.25–10.0 [7]
ERT ω → π0π+π− 2.25–13.0 [7]
ERT ω → π0γ 2.5–11.0 [7]
ERT η′ → ηπ+π− 3.25–10.75 –
MB φ → K+K− 0.9–4.5 –
MB φ → K+K− 1.3–7.0 –
Run-5 Cu + Cu 200 3.06 nb−1 MB/ERT π0 → γ γ 1.25.0–11.0 [12]
62.4 0.19 nb−1 MB π0 → γ γ 1.25–7.5 [12]
22.4 2.7 μb−1 MB π0 → γ γ 1.25–4.75 [12]
Run-6 p + p 62.4 0.1 pb−1 MB/ERT π0 → γ γ 0.625–6.75 [12]
Eur. Phys. J. C (2009) 61: 835–840 837
spectra of the φ meson are constructed using three differ-
ent techniques. The first does not require track identifica-
tion in the final state and assumes that all tracks are kaons
(“no PID”). The second requires identification of only one
kaon in the TOF subsystem (“one kaon PID”). In the third
technique both kaons are identified in the TOF or EMCal
subsystems (“two kaons PID”). Kaon candidates with op-
posite charge are combined to form the invariant mass dis-
tribution. Mass distributions obtained by the procedures de-
scribed above contain both the signal and an inherent com-
binatorial background. Meson raw yields were extracted by
simultaneously fitting the signal and background, or by in-
tegrating the spectra in the vicinity of the meson masses af-
ter subtraction of the combinatorial background, estimated
using an event-mixing technique. Representative examples
of the K+K− invariant mass distributions obtained using
“no PID”, “one kaon PID” techniques in p+p collisions and
using “no PID” technique in 0–10% most central Au + Au
collisions, and an example of the ηπ+π− invariant mass dis-
tribution are shown in Fig. 2.1. Solid lines are fits to a sum
of a Gaussian convoluted Breit-Wigner function to account
Fig. 2.1 Invariant mass distributions of the φ mesons reconstructed in
the K+K− decay channel using “no PID” (top left), “one kaon PID”
(top right) techniques in p + p collisions and using “no PID” (bot-
tom left) technique in 0–10% most central Au+Au collisions. Invariant
mass distribution of the η′ mesons reconstructed in the ηπ+π− decay
channel (bottom right) in p + p collisions
for the φ signal, or a pure Gaussian for the η′ signal, and
a polynomial background function. Corrections to the raw
yields for the limited detector acceptance and resolution, re-
construction and trigger efficiency, multiplicity effects and
various analysis cuts are determined from the full single-
particle Monte Carlo simulation and analysis of the data.
3 Results
3.1 Particle spectra
The study of inclusive particle production in nucleon-
nucleon collisions is a rich source of information on the
fragmentation properties of partons in perturbative QCD and
hadronization mechanisms. The PHENIX experiment has
measured a variety of mesons in p + p collisions at √s =
200 GeV. These data in comparison to existing leading-
and next-to-leading order perturbative QCD calculation can
be used to constrain the model parameters. A compilation
of published and preliminary results on meson production
in p + p collisions is presented in Fig. 3.1. The different
symbols for a given meson correspond to different decay
channels or different analysis techniques. The third set of
points from the bottom represents the first measurement of
η′ → ηπ+π− at RHIC at high-pT . The solid lines shown in
the figure are just to guide the eye.
The φ meson invariant pT spectra measured in p + p,
d + Au and Au + Au collisions at √sNN = 200 GeV are
Fig. 3.1 Compilation of PHENIX results on meson production in p+p
collisions at
√
s = 200 GeV. The solid lines are just to guide the eye.
Charged kaon and pion data are taken from [3], J/ data are taken
from [4]
838 Eur. Phys. J. C (2009) 61: 835–840
Fig. 3.2 Invariant pT spectra of
φ meson measured in p + p,
d + Au and Au + Au collisions
at
√
sNN = 200 GeV
Table 3.1 Results of the fits to a constant for η/π0, K0S/π0 and ω/π0 ratios in p + p and d + Au collisions at
√
sNN = 200 GeV
System η/π0 K0S/π0 ω/π0
p + p 0.48 ± 0.02(stat)±0.02(syst) 0.45 ± 0.01(stat)±0.05(syst) 0.81 ± 0.03(stat)±0.07(syst)
d + Au 0.47 ± 0.02(stat)±0.02(syst) 0.58 ± 0.06(stat)±0.05(syst) 0.94 ± 0.08(stat)±0.12(syst)
shown in Fig. 3.2. As compared to the previously reported
Au + Au results [8] obtained with the “two kaons PID” ap-
proach, the new “no PID” analysis extends the range of the
measurements to higher pT up to 7 GeV/c. In turn, the new
“one kaon PID” analysis of p + p data extends the range
of the “no PID” measurements reported in [9] to lower pT
down to 0.9 GeV/c. In spite of the very different sources of
systematic uncertainties, the results of the “no PID” analy-
sis are in good agreement with the results of “one kaon PID”
and “two kaons PID” in p + p and Au + Au collisions, re-
spectively. The combined p + p results of the “one kaon
PID” and “no PID” analyses constitute a new p + p ref-
erence for φ meson, which supersedes the one used previ-
ously [8] having both a wider pT range and smaller error
bars.
The details of the K0S , η and ω measurements in p + p,
d + Au and Au + Au collisions at √sNN = 200 GeV
can be found elsewhere [5–7]. Good agreement was ob-
tained in the measurements of the same meson via two dif-
ferent decay channels, ω → π0π+π− or ω → π0γ and
η → π0π+π− or η → γ γ . Analyses of MB and ERT data
samples collected in Run-3 and Run-5 gave consistent re-
sults.
3.2 Particle ratios
Non-identical particle ratios measured at high-pT are use-
ful to assess the particle composition of jet fragmentation.
In p + p collisions, meson-to-meson ratios (η/π0, K/π0,
η′/π0, ω/π0, φ/π0, φ/K) derived using a parameteriza-
tion of the π0 invariant pT spectra [10] exhibit a universal
flat behavior above pT = 2.5 GeV/c [9] indicating that the
measured transverse momentum spectra for all light mesons
have the same slope at high-pT . Similarly to p + p re-
sults, η/π0, K0S/π
0 and ω/π0 ratios in d + Au collisions
are also found to be flat in the same pT range. Results
of the fits to a constant performed in the pT region above
2.5 GeV/c for η/π0, K0S/π0 and ω/π0 ratios are presented
in Table 3.1. As one can see, the results obtained in p + p
and d + Au collisions are consistent within errors for the
same ratio, indicating that the production of light mesons
at pT > 2.5 GeV/c is not affected by cold nuclear mat-
ter.
3.3 Nuclear modification factors
Using the new p + p reference, φ mesons’s RAA has been
derived in central Au + Au collisions in the pT range of
2.45–7 GeV/c. There is an ongoing work to extend this pT
range towards low-pT . Figure 3.3 shows a comparison of
the RAA for φ mesons, other mesons (π0, η, K, ω), pro-
tons and direct photons. Neutral pions and η mesons fol-
low the same suppression pattern over the entire pT range.
The RAA of φ mesons exhibits less suppression than η
and π0 in the pT range of 2.45 < pT < 4.5 GeV/c. At
Eur. Phys. J. C (2009) 61: 835–840 839
Fig. 3.3 Nuclear modification factor RAA in central Au + Au colli-
sions as a function of pT for π0, (K+ + K−)/2, η, ω, (p + p¯)/2, φ and
direct γ . Direct γ data are taken from [11]
higher pT (>5 GeV/c) the amount of suppression of φ and
ω could be similar to that of π0 and η mesons. A more
conclusive statement is not possible due to the large er-
rors of the measurements. The similarity between the sup-
pression patterns of different mesons at high-pT favors the
production of these mesons via jet fragmentation outside
the hot and dense medium created in the collision. It is
not clear whether the RAA of charged kaons follow or not
the trend of φ mesons, since the present measurements
have no overlap in pT . Interestingly, that RAA of protons
which are known to be enhanced in the intermediate pT
range [1], reach a value below unity at pT ≈ 4.2 GeV/c.
Further investigations are required for understanding the in-
fluence of quark flavor composition on the suppression pat-
tern.
3.4 Onset of jet quenching
In contrast to RHIC results at √sNN = 130 GeV and
200 GeV, no suppression of high-pT hadrons has been
observed at SPS energies. This implies that there must
be an intermediate value of collision energy at which the
onset of high-pT suppression happens. The results of a
beam energy scan performed at RHIC with Cu + Cu col-
lisions at √sNN = 22.4 GeV, 62.4 GeV and 200 GeV,
shown in Fig. 3.4, indicate that parton energy loss starts
to dominate over Cronin enhancement at a center of mass
energy somewhere between 22.4 and 62.4 GeV per nu-
cleon pair [12]. Numerical calculations based on a par-
ton energy loss model [13] describe well the suppression
of π0 mesons above pT = 3 GeV/c at 62.4 GeV and
200 GeV. Measurements at 22.4 GeV favor the calcula-
tions carried out without taking into account parton energy
loss.
Fig. 3.4 (Color online) RAA of π0 as a function of transfer momentum
in central Cu + Cu collisions at √sNN = 22.4 GeV, 62.4 GeV and
200 GeV shown in black, blue and red, respectively. The shaded bands
represent parton energy loss calculations [12]
4 Summary and outlook
The PHENIX experiment at RHIC has performed a set of
measurements of light mesons (π0, K0S , η, ω, η′, φ) via
multi-particle decay channels at intermediate and high pT
in different collision systems. A first measurement of η′ →
ηπ+π− at RHIC performed by PHENIX at high-pT is re-
ported here. It would be very interesting to measure the pro-
duction rate of η′ in Au + Au collisions in view of pre-
dictions of a possible enhancement as its mass decreases
in the hot and dense matter [16]. In p + p and d + Au
pT -dependent non-identical meson-to-meson ratios exhibit
a universal flat behavior above pT = 2.5 GeV/c demonstrat-
ing that pT spectra of all measured light mesons have the
same slope in this pT range. Cold nuclear matter does not af-
fect the production rates of light mesons at pT > 2.5 GeV/c
as seen from the matching of meson-to-meson ratios mea-
sured in p + p and d + Au. Using the new p + p reference,
φ meson’s RAA has been derived in central Au + Au colli-
sions. At intermediate-pT φ meson is less suppressed than
π0 and η and it exhibits similar suppression level to those of
π0 and η at high-pT . Parton energy loss calculations repro-
duce well the π0 meson suppression for pT > 3 GeV/c in
Cu + Cu collisions at √sNN = 62.4 GeV and 200 GeV. The
onset of jet quenching is between 22.4 GeV and 62.4 GeV.
Acknowledgements The author acknowledges support by the Is-
rael Science Foundation, the MINERVA Foundation and the Nella and
Leon Benoziyo Center of High Energy Physics Research.
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