Image courtesy of Brookhaven National Laboratory

PHENIX– looking at hot and cold nuclear matter and resurrecting itself again and again

The Relativistic Heavy Ion Collider, RHIC, atof the most fundamental building blocks of theBrookhaven National Laboratory on Long Island,

visible world today, protons and neutrons wasNY, was built to study the conditions in an early

not understood well and particularly the spin

universe shortly after the Big Bang. In thatcomposition between quarks, gluons and theirepoch the temperature of the universe was so hot,orbital angular momenta were at odds with earlier

that quarks and gluons, mediators of the strong

nucleon models. RHIC was also set to understandinteraction, were not yet condensated into hadrons

the role of gluon spins and sea quarks. The spinsuch as protons and neutrons. This phase of theprogram was realized at the initiative and with

universe is known as the quark-gluon plasma (QGP).

substantial support of RIKEN and greatly improved

Its existence was predicted and RHIC was set toour understanding of cold nuclear matter anddiscover it which it eventually did.

brought in turn new surprises.

At the same time also the internal structure

PHENIX– looking at hot and cold nuclear matterand resurrecting itself again and again

Image courtesy of Brookhaven National Laboratory

Figure 1: Aerial view of the RHIC
accelerator complex including preaccelerators and locations of currently
operating detectors.

While at the beginning of RHIC one typical collider

photons were not suppressed as they are only

detector was already approved to be built, several

interacting electromagnetically. These features areother proposals for more dedicated detectors weresummarized in Figure 2.

initially rejected out of funding constraints. As aOne of the main surprises was that this QGP does

result these rejected proposals worked together

not behave like a gas but more like a perfect liquid

and, like the fabled bird, rose from the ashes being

with a very small viscosity. With the help of lightappropriately named PHENIX (though the official

emitted inside the plasma it was even possible toname stands for Pioneering High Energy Nuclear

roughly determine the temperature of the QGPInteraction eXperiment). PHENIX’s strong points[2] within models which led to the 2011 Nishina

were the rather precise central
detectors, especially electromagneticcalorimetry, forward muon detectors
and high rate data taking capabilities
at the expense of coverage. Currently
about 400 collaborators from 78
institutions (including 11 Japanese) in
14 countries are participating in thePHENIX experiment.
With this initial detector configuration,
it was possible to both find the QGP[1] and learn about its properties.
One telltale sign of the QGP was thesuppression of hadrons and jets at
higher energies due to the additional
interactions within the hot, dense QGPmedium. In contrast, high energetic

Figure 2: Summary of cross section ratios between most central AuAu collisions
and proton proton collisions. All hadrons seem to be suppressed at hightransverse momenta as expected when traversing the Quark-gluon plasma.

in the proton spin. Figure 4 displays the current
knowledge of the total gluon spin contribution as
a function of the minimum Bjorken variable x.
To extend our knowledge to even lower x, more
forward measurements are ongoing at RHIC while
eventually an electron-ion collider will reach thelowest fractional momenta similar to HERA in theunpolarized case.
To evaluate the contribution of sea quarks theparity violation of the weak interaction was used inproton-proton collisions to select their contribution
in real W boson production. Their contributions
appear to be nonzero and not symmetric between
up- and down-type flavors.
Figure 3: Photon cross sections in AuAu and proton
proton collisions. An enhancement indicates thethermal radiation of the Quark-gluon plasma and allows
estimating its temperature.

Furthermore, many transverse spin effects,
originally expected to be low-energy phenomena,continue to surprise to the highest collisions energies

Memorial Prize [3]. The low-energetic photon spectra

available at RHIC with the RHIC experiments

are shown in Figure 3 for Au-Au collisions at various

slowly unraveling its origins.

centralities in comparison to “cold” proton-proton
collisions and display an additional contribution due

1.5

to the thermal radiation within the QGP.

For the study of the spin structure of the nucleon

RHIC projection

DSSV 2014

EIC projection

1

data ≤ 2015¬√s = 78 GeVwith 90% C.L. bandmin1

part of this upgrade was performed by RIKEN.

∫xdx ∆g

QGP properties using heavy quarks which required
the installation of a silicon vertex tracker. A large

DIS + SIDIS

90% C.L. constraint

In recent years the focus moved towards studying

proton spin

0.5

for a long time spin double spin asymmetries inpolarized proton-proton collisions were small,ruling out extreme gluon spin contributions to02

Q = 10 GeV

the spin of the nucleon as suggested by earlymodels (i.e., contributing several units of ħ to bemostly compensated by opposite orbital angularmomentum). With the high statistics data setsin the years 2009 [4] at a collision energy of200GeV and 2012/2013 [5] for energies of 510GeV
it is now established that the gluon spin plays asubstantial and potentially even dominant role [6]-0.5-51010-4

210-3

10-2

10-1

xmin

1

Figure 4: Running integral of the gluon spin contribution tothe proton spin from non RHIC data (light blue), including
published RHIC data, expected improvements and an eventual
electron ion collider (in successively darker shades of blue).
The RHIC data first showed a rather substantial contribution. [7]PHENIX– looking at hot and cold nuclear matterand resurrecting itself again and again

As the name suggests,
PHENIX has re-emerged
already once and it isbound to do the sameo n c e m o r e r i g h t n o w.
Th e H ea v y I on p hysics
interest at RHIC moves
towards jets, and heavy
quark-antiquark states toprovide complementarymeasurements to theLHC heavy ion program.
Also the study of the spin
structure is interested tolook at forward rapidities
which allows to accesstr a n s v er s e s pin effect sFigure 5: GEANT4 Rendering of the sPHENIX detector including forward upgrades currently
under consideration in a 500 GeV proton-proton collision event.

unique to RHIC at higher
momentum fractions while also looking at lower
momentum fractions to pin down the gluon spin inregions not covered yet.
As such, PHENIX is currently re-inventing itself as

References:[1] K. Adcox et al.[PHENIX Collaboration], “Formation of dense
partonic matter in relativistic nucleus-nucleus collisions at RHIC:
Experimental evaluation by the PHENIX collaboration”, Nucl.
Phys. A757, 184 (2005)

sPHENIX and forward sPHENIX concentrating on[2] A. Adare et al. [PHENIX Collaboration], ”Enhanced production

jet detection at central and forward rapidities withof direct photons in Au+Au collisions at √sNN=200 GeV andelectromagnetic and hadronic calorimetry, tracking
and the former BABAR 1.5 Tesla solenoid magnet
being already transferred to BNL. These mostly
new systems could then even re-rise yet again in
the case the electron ion collider (EIC) is built atBNL as a zero-day EIC detector currently dubbed asePHENIX.

implications for the initial temperature”, Phys. Rev. Lett. 104,132301 (2010)[3] www.nishina-m.or.jp/NishinaMemorialPrize-E.html[4] A. Adare et al. [PHENIX Collaboration], “Inclusive doublehelicity asymmetries in neutral-pion and eta-meson production in
p+p collisions at √s=200 GeV”, Phys. Rev. D91, 012007, (2014)[5] A. Adare et al. [PHENIX Collaboration], ”Inclusive cross section
and double-helicity asymmetry for π0 production at midrapidity in
p+p collisions at √s=510 GeV,” Phys. Rev. D93, 011501 (2016)[6] D. de Florian, R. Sassot, M. Stratmann and W. Vogelsang,“Evidence for polarization of gluons in the proton”, Phys. Rev.
Lett. 113 , 012001 (2014)[7] E. Aschenauer, R. Sassot, M. Stratmann, "Unveiling the Proton
Spin Decomposition at a Future Electron-Ion Collider" Phys.Rev.
D92 (2015) 094030