The PHENIX experiment at RHIC

Nuclear Physics A566 (1994) 287c-298~ North-Holland, Amsterdam

NUCLEAR PHYSICS A

PHENIX Experiment at RHIC

Shoji Nagamiya

Department of Physics, Columbia University, W120th St., New York, NY 10027, U. S. A.

1. PHYSICS MOTIVATION AND GOALS

The purpose of my talk is to present an overview of the PHENIX experiment (Pioneering High Energy Nuclear Interaction experiment) at RHIC. Physics issues related to the quark-gluon plasma, in particular, in relation to this experiment, are listed in Table 1.

Two important aspects exist for the quark-gluon plasma. One is “deconfinement”, which is closely related to the Debye screening of the QCD potential. The basic mechanism of J/$ suppression is that, if the J/1c, ra d ius is longer than the screening length, then cz would not be able to find a bound state due to a Debye suppression of the long-range term of the CC potential and, thus, J/+ production is suppressed’). The degree of suppression depends strongly on the relative size difference between the meson radius and the screening length. Since r($‘) = 0.51 fm > r(J/$) = 0.25 fm > r(T) = 0.13 fm, we expect that 4’ must melt first, then J/$, and finally T. A stronger suppression of $J’ than Jill, reported by NA38 at this conference2) is, therefore, very impressive, and this result compels us to study these vector mesons systematically at RHIC. We plan to measure J/?c, in the mid-rapidity region by dielectrons, and J/+, +’ and ‘I at forward angles by dimuons.

The other important element is “chiral symmetry restoration”. Because the mass of the d-meson is close to twice the kaon mass, and because both 4 and K could be distorted in the quark-gluon plasma, it was predicted3) that a change would occur in: (a) the branching ratio between leptonic and hadronic channels, (b) the mass of the 4, and (c) the width of the 4. Here, high-resolution &spectroscopy is required to study these points. We plan to measure &mesons by both electron and hadron channels.

Thermal radiation of a hot gas has been a very topical subject for many years4). There was a confusing period, at least to me, in which it was debated whether or not the radiation was enhanced or suppressed when the phase transition occurred. Recently, a consensus among theorists seems to be that the gluon content is high at an early stage of the quark-gluon plasma and an enhancement must be expected in the region of mr (or, pT for photons) greater th an 2-3 GeV’). We plan to investigate this topic using the photon measurement.

The nature of the phase transition is a very interesting point to study. If the phase transition is first-order, then, an entropy jump would be expected at Tc, because internal degrees of freedom of the constituents increase by a factor of about 12 from the pionic gas to the quark-gluon gas. If one plots T as a function of the energy density (E) of the system, the value of T increases with & in the phase of a pionic gas until it reaches Tc. The system stays at this temperature, even as E increases, until a sufficient energy density

0375-9474/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved.

288c S. Nagamiya I PHENIX experiment at RHIC

1. Deconfinement (Debye Screening of QCD Interactions)

r(T) = 0.13 fm < r(J/+) = 0.29 fm < ~(4’) = 0.56 fm

- JM --t e+e- at y Y 0. * electrons

--f p+pL- at y N 2.

+, T + p+p- at y ? 2. * muons

2. Chiral Symmetry Restoration Mass, Width, Branching: 4 + e + -, K+K- with Am 5 5 MeV. e

-----i electrons + hadrons Baryon Susceptibility: Production of antinuclei. Narrow u meson?

3. Thermal Radiation of Hot Gas Prompt 7, Prompt 7* + e+e-. j photons, electrons, muons

4. Nature of the Phase Transition First Order: Entropy Jump -+ Second rise in < pT >

+ Spectra of x, K, p. ==+ hadrons

Second Order: Fluctuation + N(x’)/N(a+ + r-), d’N/dqd& ==+ hadrons + photons

5. Strangeness and Charm Production Production of K+, K- K” ==+ hadrons

4 + e+e-, K+K- at y’~ “d, 4 + pL+p- at y N 2. D-Meson: ep coincidence. ==-+ electrons, muons

6. Jet Quenching High-pT Jets via Leading Particle Spectra.

7. Space-Time Evolution

a hadrons

HBT Correlations for ~7r and KK. ==+ hadrons

Table 1: Physics issues related to the PHENIX experiment.

is accumulated to allow the system to complete the phase transition into a quark-gluon gas. At this point, the value of T starts to rise again as a function of t7. Since T is closely

related to <pT>, the second rise in <pT> as a function of & is expected6). Such an entropy rise induces the increase of pressure and, thus, induces a hydrody-

namical flow7). The flow effect is stronger for nucleons than for pions due to the mass difference. In order to study the flow effect, combined with the effect of an entropy rise, measurements of “identified” charged hadrons are extremely important. We plan to measure identified hadron spectra in the mid rapidity region.

If the phase transition is second-order, then, “fluctuation” measurements could prove important, as pointed out by Wilczek at this conference’). We plan to measure n’/(x++r-) from the detection of both photons and hadrons.

Table 1 lists other important physics issues, such as strangeness and charm enhancementg), jet quenching”), and space-time evolution . 11) I would like to emphasize that simultaneous

S. Nagamiya I PHENIX experiment at RHIC 289c

measurements of dielectrons, dimuons, photons, and identified hadrons are important and necessary to pin down the formation of quark-gluon plasma at RHIC, because while all the theoretical predictions listed in Table 1 are very attractive, they all contain many weak points and ambiguities.

We plan to measure all the variables listed in Table 1 as a function of a reasonably well-defined experimental quantity which is proportional to the energy density. Because the energy density is given by”)

1 dET E=qdy’ (1)

the geometry of the collision (namely, RI) will be fixed first by multiplicity measurements. A wide coverage for the multiplicity measurement is very important, because a possible fluctuation effect must be smeared out for the determination of the collision geometry. By using an electromagnetic calorimeter or a subset of the multiplicity detector, the quantity of dSr/dy will be measured to probe the local energy density at that rapidity, including the effect of the fluctuation.

2. PHENIX DETECTOR AND EXAMPLES OF ITS PERFORMANCE

The PHENIX detector is an axial-filed spectrometer in the midrapidity plus a piston- magnet muon spectrometer at forward angles. The detector is shown in Fig. 1.

TlME EXPANSION CHAMBER-.., r-EM CALORIMETER a.., WON TRACKER N IO

Figure 1: A three-dimensional view of the PHENIX detector.

290~ S. Nagamiya / PHENIX eqeriment at RHIC

In the central arm, the magnetic field is applied along the direction of the beam. Two arms are prepared, 1-sr each. Most of the tracking chambers are installed outside the magnetic field. In addition, various particle identification devices are installed, such as a

ring-imaging Cherenkov detector (RICH), a time expansion chamber (TEC) to measure dE/dx, a time-of-flight (TOF), and an electromagnetic calorimeter (EM Cal). The open- ing angle between the centers of the two arms is 135”. This angle was selected to attain the most uniform pT acceptance for dielectrons in the l-2 GeV mass region. In the muon arm, the magnetic field is applied perpendicular to the beam. Muon tracking (pT) and

identification ($D) d evices are installed. A silicon multiplicity-vertex detector (MVD) and two sets of phototube arrays, called the beam-beam counter, are installed close to the vertex point.

Performance of the PHENIX Baseline detector is summarized in Table 2. Contrary to STAR, our approach is to attain very good particle identification for a limited number

of particles. A typical multiplicity of charged particles in the detector is 300-500. The detector was designed, however, to allow us to handle up to twice this multiplicity.

Electrons rr/e < 10v4 at p 5 4 GeV/c

(2 sr) l RICH for < 4 GeV/c l TEC (dE/dx) for < 2 GeV/c

l EM Cal for > 1 GeV/c

Photons pr > 1 GeV/c for 0.5 sr with 1OK blocks of Pb-glass.

(2 sr) pT > l-l.5 GeV/c for 1.5 sr with 18K blocks of Pb-scintillator

Hadrons < 2.5 GeV/c ?r-K for 0.36 sr

(0.36 sr) l 10K elements of TOF with cr < 100 ps.

Muons* n/p < a few x lop4 at p > 2.5 GeV/c

(1 sr) l Central yoke as a first absorber, < 7 layers of pID.

Global d’N/dndd for -2.7 < 7,~ < 2.7

* Muon magnet and coils will be included in the baseline; instrumentation is deferred.

Table 2: Performance of the PHENIX baseline detector.

2.1. Dielectron measurements

Three detectors are used for electron identification: RICH, TEC (for dE/dx), and EM Cal. Initial test results for a subset of the actual RICH detector, using pion and

electron beams at KEK, clearly demonstrates that a Cherenkov photon ring is observed by a photomultiplier read-out method and it shows that the pion rejection of lo-” is possible for single pions and electrons. In the high-multiplicity environment, the pion rejection power will be reduced to the lop3 level.

For the TEC in the 1 GeV/c region, the value of dE/dx in a gas is approximately 50% higher for electrons than for pions. This feature is used for electron identification. Test results show a clear separation between electrons and pions, and we confirmed that this method allows an additional 106’ level pion rejection up to 2 GeV/c. The EM Cal further

helps pion rejection by at least another one order of magnitude. Our design criterion of the pion rejection below 10W4 can thus be attained.

S. Nagamiya I PHENIX experiment at RHIC 29152

Simulation of the Full Dielectron Moss Spectrum

10 31.7 Million Events

RICH + EMCol ID ’

R. Vogt Meson Rates

1

0 0.4 0.8 1.2 1.6 2 2.4 2 2% 3.2 3.6 Invariant Mass (GeV/c’

Figure 2: Dielectron spectrum in central Au + Au collisions for &day runs at RHIC.

We also paid special attention to achieve a high mass resolution in the tracking system.

Our Monte Carlo calculations have shown that the mass resolution of approximately 4

MeV for &mesons can be achieved for the e + - e channel and less than 1 MeV for the

K+K- channel. Shown in Fig. 2 is a full Monte Carlo result of the expected e’e- spectra for &day

runs at RHIC, including scatterings of surrounding materials. Very clean peaks for w- and q+-mesons are seen. The signs-t~background ratio in the w-4 region is l/2. The

peak for the J/+ is much cleaner. The background is primarily from Dalitz pairs and combinatorials. Our Baseline

detector configuration does not include a Dalitz rejector near the vertex; however, a partial Dalitz rejection is possible by applying mass and momentum cuts. The signal-to- background ratio could go up to l/l in the w-4 region. A full Dalitz rejector is included in the upgrade option, which would suppress the background by two orders of magnitude.

292~ S. Nagamiya / PHENIX experiment at RHIC

2.2. Photon measurements

In order to measure direct photons, the first step is to learn the background associated with x0 and 7. Invariant mass spectra for 27 have the following general tendency: At

low-pr the background is high, and as pr increases the peak is more pronounced. Thus, the yield ambiguity increases as pr decreases.

If K’ and 71 peaks are found in the 2y spectrum, the next step is to estimate the differential cross sections of x0 and n and, then, to evaluate the spectrum of single photons by using these x0 and T. This spectrum is shown in Fig. 3b. On the other hand, the

actual photon spectrum recorded on the detector is given in Fig. 3a. If the reconstruction

procedure is perfect, Fig. 3a must be identical to Fig 3b. In fact, these two seem identical. However, if one takes the ratio between the two spectra, Figs. 3a and 3b, the ratio

deviates from 1 at pr < 1 GeV/c, as seen in Fig. 3c. This deviation would originate

for two reasons: (a) The signal-to-noise ratio of r ’ becomes worse as pT decreases and,

therefore, the yield ambiguity of single photons from K’ increases. (b) The opening angle of 27 from rr” increases as pr decreases, so that the contribution from R”S produced in

Reconstruction of pion decay photon spectrum in EMCAL

b) Reconstructed y

w e 0 0.5 2 0 0.5 1 1.5 2

Photon energy (G&J) Photon energy (GeV)

0.8 - Original/reconstructed (normalized at 1 GeV)

0.6 -

0.4 -

I c) Ratio of Single y (left) to Reconstructed 7 (right) I 0.2 -

0 “~“‘~““““““““““‘1”“‘1”11 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.6 2

Photon energy (CM)

Figure 3: Measured single photons as compared with the reconstructed single photons from K’ and n for two days runs at RHIC for central Au + Au collisions. An electromag- netic calorimeter resolution of 7% at 1 GeV is assumed.

S. Nagamiya I PHENIX experiment at RHIC 793c

the rapidity region outside the detector coverage becomes stronger as pT decreases. Our conclusion is that the direct photon measurement is possible at pT > 1 GeV/c,

if an enhancement of the direct photon is well above a few percent of the single photons from A’.

2.3. Charged hadron measurements

A recent test of a prototype time-of-flight (TOF) counter (1.2~ 1.2 x 100 cm3) showed that the time resolution of d = 88 ps is possible, even if the light guide was bent by 180”. With this resolution, a x-K separation up to 2.4 GeV/c is possible.

The first interesting subject is to measure single particle spectra of identified hadrons, as discussed in Section 1. Charged hadron measurements further allow us to study 4 + K+K-. An expected K+K- invariant mass spectrum to be obtained by l-day runs at RHIC is shown in Fig. 4.

8000 -

7000 -

6000 -

5000 -

4000 -

3000 -

2000 -

1000 -

0”

E

4

Figure 4: K+K- invariant mass spectrum in central Au + Au collisions.

Two boson correlations are another subject to be studied. Although I do not show the spectrum here, we can measure these correlations for both ETA and KK to determine the source radius up to 20 fm. For a source radius larger than 10 fm, the detector bin size of relative momentum has to be corrected rather carefully in particular for KK correlations, because the resolution of relative momentum, Aq, is proportional to -ym and, thus, Aq(r7r) < Aq(KK).

The decay kinematics of 4 + K+K- together with the measurement of two-boson correlations determine the solid angle coverage (0.36 sr) of the TOF wall in the Baseline detector configuration.

294c S. Nagamiya / PHENIX experiment at RHIC

2.4. Dimuon measurements

Muon identification will be performed by a standard technology of streamer tubes, sandwiched by several layers of iron (Fe) absorbers. Contrary to pp collision experiments,

it is not easy to track muons before the first absorber, because the multiplicity is too high in heavy-ion collisions. We therefore use the central magnet yoke as the first absorber and the tracking starts after this absorber.

The mass resolution is, therefore, not very good; it is about 100 MeV for the J/+. Also, for identifiable single muons the minimum energy (about 2.5 GeV) is relatively high. In the presence of such a large energy cut-off for single muons, we are forced to install the muon arm at forward angles to cover a small pT region.

A clear advantage at forward angles is a gain in the yield. Although the muon arm covers only 1 sr, which is half the solid angle of the electron arm, the value of AyAd is 2.4~ rad for the muon arm, while it is 0.7x rad for the electron arm. Because the pair yield is proportional to [AyAd]“, the yield of J/$.J is, for example, about a factor of 10

higher in the muon arm than in the electron arm. For one year of RHIC running, we

expect to have 5.7K of $J’ and 1.2K of ‘I for 10% central collisions at the luminosity of 2

x 102s. An expected mass spectrum for one month of running at 2 x 10z6 luminosity is shown

in Fig. 5. The peak to background ratio for $’ is not impressive in the raw dimuon mass

MUON ARM MASS DISTRIBUTION

105 i/I/( IO**8 CENTRAL Au-Au EVENTS

charm + D-Y

upsilon

MUON PAIR MASS (GeV)

Figure 5: Expected dimuon mass spectrum.

S. Nagamiya I PHENIX experiment at RHIC 79%

spectrum, whereas, if the subtraction of like-sign muon pairs as well as the kinematical

cut were applied, the T,!J’ peak is much more pronounced. For ‘I the peak to background

ratio is very good.

2.5. Electron-muon coincidence

Once both the electron and muon arms are installed, the measurement of the ep coincidence is possible. The main leptonic decay mode of the D-meson is an emission of an electron or muon, like the 0 decay of the neutron. Therefore, the unlike-sign ep pairs subtracted by the like-sign ep pairs are primarily from DB, where the unlike-sign pairs

are mainly from combinatorial background pairs’““4).

Figure 6 shows a Monte Carlo result of the mass spectrum for ep pairs. Backgrounds include A -+ e+e-T and T -+ pv, ey. At m,, grater than 3-4 GeV, the charm contribution is dominant. If the charm production is enhanced, one may probe DB at a lower mass region.

MUON - ELECTRON COINCIDENCE EVENTS

1 O*+t3 CENTRAL Au-Au EVENT:

__ lllr-l I

5 6 7 6 9 10

PAIR MASS (GeV)

Figure 6: Expected ep mass spectrum.

2.6. multiplicity and vertex

We plan to install both a silicon multiplicity-vertex detector (Si MVD) and the two sets of quartz Cherenkov timing counters (beam-beam counters).

Tests for the beam-beam counters showed 35 ps time resolution in the magnetic field of 3 kG. These counters are used as a time reference of the TOF counters. Also, they are used for the on-line vertex determination at the level of & = 2 cm.

296~ S. Nagamiya / PHENIX experiment at RHIC

The Si MVD is used to determine the event multip~city and the determination of the vertex position. With this device the vertex can be determined within 100-200 pm.

Figure 7 shows a simulation result of how well the multiplicity can be measured with

Si MVD. Input multiplicity distribution is shown by the histogram, and the data points with error bars indicate the reconstructed distributions by Si MVD. Both agree very well, even for one event. A multip~city fluctuation in rapidity can, thus, be probed.

Figure 7: Capability of multiplicity measurements. The left figure is for 125 events and the right figure is for one event.

2.7. Upgrade options

We have three upgrade options: (a) a transition radiation detector (TRD) to expand the capability of electron identification beyond pT > 4 GeV/c, (b) a high-resolution elec- tromagnetic calorimeter, most likely with CsI or BaFz crystals, to extend the capability

of direct photon measurements down to pi z 0.5 GeV/ c, and (c) a hadron-blind tracking

detector (HBD), 1 c ose to the vertex point to reject Da&z-pair electrons and, hence, to re- duce dielectron continuum backgrounds. Among these three, the R&D work was finished for the first two items, whereas R&D efforts are currently in progress for HBD.

3. COLLABORATION AND MANAGEMENT

Institutions participating in the PHENIX experiment are listed in Table 3. A total of 326 scientists from 43 institutions in 10 countries are involved in PHENIX. About 130 physicists are from the US, as well as large teams are from China, Japan and Russia. I

also would like to point out that there is a very important contribution from Sweden, the host country of this conference. The present management team is listed also in Table 3.

Institutional participation in the detector construction is truly international. For

example, the Pad chamber is constructed by the collaboration of McGill (Canada), Lund (Sweden), IHEP-Beijing (China) and BNL (US). Most of the elements in PHENIX are constructed by the international collaboration.

4. SUMMARY

PHENIX is an approved experiment at BNL, and is currently preparing for the final cost and schedule review to be held in the fall of 1993 for Phase 1 construction. Before this review the construction of the magnet has already started.

We measure electrons, photons, hadrons, and muons as a function of a reasonably well-defined variable. Physics highlights are:

Brasil: U. Sao Pa010

Canada: McGiIl U.

China: CIAE

IHEP Inst. Mod. Phys. Peking U.

Germany: U. Miinster Individual

India: BARC, Bombay

Japan: Hiroshima U.

INS, U. Tokyo KEK Kyoto U. Nat. Inst. Rad. Sci.

U. Tokyo U. Tsukuba Individual

Korea: Chung-ang U. Korea U. Seoul Nat. U. Soong-Sil U.

3

8

13

8 8 4

4 1

3

4 5 7

2 1 5

16 1

1

4 6 1

Russia:

IHEP-Protvino 28 INR-Moscow 7 ITEP-Moscow 8

JINR-Dubna 16 Kurchatov Inst. 8

PNPI-St. Petersburg 14 Individual 2

Sweden: Lund U. 9

U. S. A.: U. Alabama 4 BNL 20 UC-Riverside 5 Columbia U. 13 Florida State 2 Georgia State 3

Idaho NEL 4

Iowa State /Ames Lab. 3 LLNL 9 LANL 14

Louisiana State 4 MIT 2 SUNY-Stony Brook 12 ORNL 11 U. Tennessee 4

Vanderbilt U. 3 Yale U. 9 Individual 2

Total 326

Spokesperson: S. Nagamiya (Coiumbia) nevis::nag Project Director: S. Aronson (BNL) bnlcl6::aronsons Deputy Project Director: G, Young (ORNL) orphOl::young Project Engineer: L. Paffrath (BNL) bnldag::paffrath

Table 3: Institutional participation and the management team in the PHENIX Collabo- ration.

298~ S. Nagamiya 1 PHENIX experiment at RHIC

l Systematic study of J/$, $J’, and T via e+e- and p+pL- channels to study Debye screening and deconfinement ,

l High resolution &meson spectroscopy via e+e- and K+K- channels to study chiral symmetry restoration,

l Photon spectroscopy to study thermal radiation from a hot gas, and

l Identified charged hadron measurements to study the nature of phase transition and the space-time evolution of the collision.

PHENIX is a large international collaboration which includes the US, Brasil, Canada,

China, Germany, India, Japan, Korea, Russia, and Sweden.

I acknowledge Walter Kehoe (Assistant for the Project Management) for his proof- reading of the manuscript.

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