The PHENIX Muon Spectrometer and Charmonium Production at RHIC

Outline

Introduction Muon physics goals of PHENIX (emphasis on charmonium) PHENIX Muon Spectrometer design and construction PHENIX Muon Spectrometer performance in Run 2 J/ψ analysis from first PHENIX muon data collected Summary and outlook

Andrew S. Hoover New Mexico State University

Particle Physics at RHIC

The General Idea

We force the fundamental constituents of matter(quarks/gluons) to interact by smashing them together.

Quarks and gluons are confined to exist inside of hadrons and mesons, we can't accelerate them individually. Ions (p,d,Si,Pb,Au) contain protons/neutrons (hadrons) and they are accelerated and collided as a means to get the quarks/gluons to interact.

Ion bunch 1

Ion bunch 2

Collison Quark/GluonInteractions

Acceleration Hadronization (formation of final state particles)

After the collision, we use an array of different detector technologies to record the trajectories of the produced particles in a magnetic field. We can study their mass, energy, momentum, angular distribution, etc... to deduce the circumstances under which they were formed.

The Aftermath of a Collision

Muon Physics at PHENIX

There are a LOT of physics processes being studied at PHENIX.(PHENIX = Progressive High Energy Nucleon Interaction eXperiment) Many of them produce muons in the final state.

Quark Gluon Plasma physics Supression/Enhancement of J/ψ (Debye screening) Melting order of ψ', J/ψ, and Υ (Debye screening) φ production (strangeness enhancement)

Spin physics Open charm production (gluon polarization) W boson production (quark/antiquark polarization) Polarized Drell-Yan (antiquark polarization)

Other Charmonium production (test NRQCD) Charmonium polarization (test NRQCD)

Physics processes producing muons (those in green will be discussed in detail here):

What is Charmonium?

Each of the three processes we will consider in detail involvethe production of charmonium. There are six quarks:

u (up) 2/3 0.004d (down) -1/3 0.007s (strange) -1/3 0.15c (charm) 2/3 1.5b (bottom) -1/3 4.7t (top) 2/3 173

name charge (e) mass (GeV)

And each quark has its own antiquark, which has oppositecharge.

Charmonium is a bound state of a charm and anticharm quark.

Quark/antiquark potential: V ~ + br

“confinement”“asymptotic freedom”

rα (r)s

µ µ+ -

The Charmonium Spectrum

Charmonium Production in pp Collisions

Help us gain a better understanding of QCD. Verify validity of NRQCD factorization method.

Resolve discrepancies between observations and theory (cross section and polarization).

Test various production mechanisms: Color Evaporation Model, Color Singlet singlet and Color Octet contributions to NRQCD.

Baseline measurement leading to search for J/ψ suppression or enhancement in heavy ion collisions.

Color Evaporation Model

Introduced ~1977:

F. Halzen, Phys. Lett. B69, 105 (1977)J.F. Amundson, O.J.P. Eboli, E.M. Gregores, and F. Halzen,

Phys. Lett. B390, 323 (1997)

Color singlet property of ψ is not enforced over short distances,i.e. - color is ignored in the short distance perterbative diagrams.

Observable color singlet state is a result of many soft gluon emissions over large distances.

Cross section for ψ is given by , is the sumof all cross sections for onium states and is computed from pQCD. is the fraction of onium states which materialize as ψ's and isdetermined empirically.

σonium

ρψ

σoniumρψσψ =

CEM Predictions for Hadroproduction of ψ

� 0.5 is determined from photoproduction data (some theoretical uncertainty due to choice of scales and PDFs). Higher order QCD corrections are determined by fitting the hadroproduction cross section of DD (open) pairs with a global K factor.

and the K factor are then used to predict the cross section for J/ψ (bound)

ρψ

ρψ

_

From Amundson et al, Phys. Lett. B390, 323 (1997)

PHENIX at200 GeV

CEM and the Tevatron p DistributionT

K factor = 2.2

From Amundson et al, Phys. Lett. B390, 323 (1997)

NRQCD Factorization MethodColor singlet and color octet contributions to NRQCD theory are more recent ideas for charmonium production. Color is included. But first, a brief overview of NRQCD factorization and scaling:

p + p ψ + X

σψ =i,j 0

1

dx dx f (x ) f (x ) σ(pp ψ)1 2 i

p 1 2^

σ(pp ψ)^ =n

M �

nψ

n

Perturbative short distance coefficients.Calculable with pQCD and have expansions in α . Non perturbative long distance

matrix elements. Describe the hadronizationprocess, have expansions in the heavy quark velocity v, and are determined by fits to data. The dependence on v is determined by the “velocity scaling rules”.

s

Separation of the amplitude into shortdistance coefficients and long distancematrix elements is the NRQCD “ factorization method ”.

jp

Scaling of the Matrix Elements

The color singlet state S is the dominant Fock state for charmonium.Other charmonium states can be reached by some number of electric andmagnetic transistions. Electric transition: ∆L = 1, ∆S = 0; Magnetictransition: ∆L = 0, ∆S = 1.

For each electric transistion, the matrix element scales like v .

For each magnetic transition, the matrix element scales like v .

In general, a matrix element scales like v , where L isthe angular momentum, E is the number of electric transistions, andM is the number of magnetic transistions.

Usually the velocity scaling of the matrix elements is listed with respectto the base state S . That is, S ~ v , but will usually be listed as S ~ v .

31(1)

+-

+-

2

4

3+2L+2E+4M

3

33

1

11(1)

(1)(1) 3

0

The Color Singlet Contributions

Assumes only heavy quark pairs formed in the dominantFock state S will form observable charmonium.3

1(1)

Diagrams from Kramer, Prog. Part. Nucl. Phys. 47, 141 (2001)

The Color Octet Contributions

Charmonium in a color octet state can form observable charmoniumvia emission of soft gluons.

Experimental Verification of NRQCD

The four matrix elements S , S , S , and P are determined by fits to the Tevatron p data.

T

333 1

11 0 J

(8)(8)(8)(1)

Tevatron p distributionT

From Beneke and Rothstein, Phys. Rev. D54 2005 (1996)

Once the matrix elements are obtained, we can use them topredict other observables like total cross sections.

PHENIX at 200 GeV

Experimental Verification of NRQCD

Charmonium Polarization

Can distinguish between different production models, and study relative importance of COM matrix elements.

Define θ as the angle between the lepton 3-momentum in the ψ rest frame and the ψ 3-momentum in the lab frame. The polarization is measured by the angular distribution:

dσ (ψ

� �

)+ -

d cosθ1 + λcos θ2

-1 < λ < +1 λ = +1 for transverse polarization (J = 1) λ = -1 for longitudinal polarization (J = 0)z

z +-

Color Evaporation Model: any polarization is lost due to multiple gluon emissions, the model predicts λ=0.

Color Singlet Model: see M. Vanttinen, et al., Phys. Rev. D51, 3332 (1995)

λ ~ 0.25

�

Charmonium Polarization

At large p , direct J/ψ and ψ' cross sections should be dominated bygluon fragmentation into color octet charmonium. The process is suppressed by v w.r.t. color singlet, but picks up two powers of α which overcompensate.

When p >> 2m , intermediate heavy quark pairs are transversly polarized.NRQCD spin symmetry preserves the transverse polarization in the final state.Thus, at low p ψ should be unpolarized, at high p ψ should be transverselypolarized.

ψ should be nearly 100% transversely polarized. J/ψ is contaminated by feeddown from higher states ψ and χ . This reduces, but does not eliminate, the transverse polarization at high p .

T

4s

T c

TT

NRQCD with color octet contributions:

The current best model.

T

c''

Charmonium Polarization Measurement

Fermilab CDF experiment (Affolder, et. al., Phys. Rev. Lett. 85, 2886 (2000))

Shows agreement at moderate p , but disagreement at large p .T T

The RHIC Accelerator Complex

Located at Brookhaven National Laboratory on Long Island, NY.

The ring is 2.4 miles in circumference.

RHIC accelerates any ion species from proton to Au at 0.99999c.

Beam storage lifetime is about 10 hours (on a good day).

The RHIC Accelerator Complex

PHENIX

The PHENIX ExperimentOne of the four experiments at RHIC. A large detector anda large collaboration of ~400 physicists, ~50 institutions, ~12 countries.

Muon Spectrometer Arms: LANL (P-25 and NIS-4), NMSU, UNM, UT, Japan, France, Korea, ORNL, BNL.

The PHENIX Experiment

The PHENIX Experiment

Rapidity and φ coverage

y = ln( )12

E - pz

E + pz

Magnetic field of the PHENIX magnets.(muon magnets ~ kG)

The PHENIX Muon Arms

muon trackingchambers

muonidentifier

Two arms of tracking chambersfollowed by a muon identifier.

collision point

The PHENIX Muon Arms

South muon arm under construction (Fall 2000)

The PHENIX Muon ArmsSouth muon arm under construction and some members of our team

gas volume etched strip cathode planes

anode wires

copper cladFR4 panels

cathode signal readout connectors

1 cm wide cathode strips are etched onto a copper clad FR4 panel.

6 cathode planes and 3 anode planesper each chamber.

Station 2 is constructed from thin Mylar foils instead of FR4 to minimize multiple scattering.

Muon Tracker Cathode Strip ChambersConstruction is that of a conventional wire chamber: anode wiressandwiched between cathode strips.

Muon Tracker Cathode Strip Chambers

Station 1 chambers Station 2 chamber

Muon Tracker Front End Electronics

A competely assembled Front End Electronics Module

The PHENIX Muon Identifier

Six layers of steel interleaved withIarocci tube detectors (proportionaltube counters).

Seperates muons from pions.π/µ rejection of ~10

Provides a trigger for the muonarms.

-4

Muon Spectrometer Design Requirements

Separate J/ψ (3.1 GeV) from ψ' (3.7 GeV) Separate φ (1.0 GeV) from ρ + ω (0.77 GeV + 0.78 GeV) Separate Υ(1S) (9.5 GeV) from Υ(2S+3S) (10.0 GeV + 10.4 GeV)

Determination of particle charge to ~50 GeV (for W physics)

Perform in both pp and Au-Au collisions, which have different chamber occupancies and event rates.

MuTr chamber spatial resolution 100 µm MuTr electronics signal to noise ratio 100:1 (0.8 fC RMS noise,

or about 3000 electrons)

Muon Spectrometer Performance

The south muon arm took data for the first time during RHIC Run 2, in 2001.

Did the detector work? (yes) Did we meet our design goals? (yes)

All data shown here was taken with the South muon arm duringRHIC Run 2.

The north arm was added for RHIC Run 3, which is happeningright now.

Tracking Chamber Resolution

Our goal was to build a system of cathode strip tracking chambers and readout electronics capable of 100 µm spatial resolution.

This is studied by fitting a track using hits from 5 of the 6 cathode planes ina station, and computing the distance from the track to the hit on the 6th plane.

xxx

xxx residual

cathodeplane 1

cathodeplane 6

Residual (cm)

130 µm chamber +projection error

Tracking Chamber Resolution

Prior to installation, one station 2 octant was tested using cosmic ray tracks. The measured residual distribution givesa result consistent with our goal of 100 µm spatial resolution.

MuTr Electronics Noise Levels (Station 1)

strip ID

AD

C c

ount

s

Our goal was 1.5 ADC counts (1 ADC count = 2 mV)

MuTr Electronics Signal/Noise RatioOur goal was S:N = 100:1. With the typical cluster charge from a signal event at 150 ADC counts, and RMS noise at 1.5 ADC counts, we have achieved it.

preamplifier chip saturation

Muon Data from RHIC Run 2Proton collison data has been reconstructed. Au collision data is in progress. We collected data for about 150 nb integrated luminosity.-1

interaction point

beamline

muon identifier muon

trackerstation 3 muon

trackerstation 2 muon

trackerstation 1

Muon Track FindingMuon track candidates are found by connecting compatible hits in the MuId and the three MuTr tracking stations. The momentum (and thus the mass) is determined by the amount of bend in the B field.

Muon Tracks vs. Simulation

There is good agreementbetween the simulation andthe real data. For example,p and x distributions.FT

An accurate simulation iscritical for determining the detector acceptance, efficiency,and performance.

Observation of the J/ψ Peak

The J/ψ particle should appear as a peak in the mass spectrum of unlike sign muon pairs at 3.1 GeV.

There are multiple background sources, at lower mass it ismostly from decays of π and K .

+-

example of typical dimuon mass spectrum

+-

(π, K decays)

Observation of the J/ψ Peak

The background for the +- muon pairs will be constructedfrom the ++ and - - pairs.

N = 2RA N N++ --+-

back

R is a factor to correct for charge correlation effects at low multiplicity.A is a factor to correct for any charge asymmetry in the detector.

The shape of the background is determined from the like sign pairs.

The constructed background is then normalized to the +- spectrum with an R factor.

Because we have relatively low statistics this is acceptable. With higher statistics, we would need to determine R from simulation.

Observation of the J/ψ PeakWe collected the very first data ever with the south arm of themuon spectrometer in 2001-02. We collected a modest sampleof J/ψ's.

J/ψ peak

Observation of the J/ψ Peak

The background subtracted unlike sign dimuon mass spectrum:The J/ψ peak is evident, and occurs at 3.1 GeV where we expected.

N = 41.7 + 6.5J/ψ -

The J/ψ Cross Section

The cross section for J/ψ production is given by

σ = J/ψ

NJ/ψ

�

That is, the total number of J/ψ produced for an integratedluminosity

�

.

N =J/ψ

NJ/ψdet

(Aε)µµ Br(J/ψ µµ)

�

= Nmin bias

εtrig σpptotal

σ = 4.6 + 0.7 (stat) + 1.2 (sys) µb- -J/ψ

The J/ψ Cross Section (version 2)In a parallel analysis, the muon data is divided into two binsof rapidity and combined with an electron point from the PHENIX central arms. The rapidity distribution is then fitted and integrated to get the total cross section.

σ = 3.8 + 0.6(stat) + 1.3(sys) µb--J/ψ

The J/ψ Cross Section

Comparison with the Color Evaporation ModelTheoretical calculations by Amundson, et al., Phys. Lett. B390, 323 (1997)

The J/ψ Cross Section

Comparison with the NRQCD model, includingcolor octet diagrams:

Theoretical calculations by H. Sato and N. Saito

The J/ψ Polarization

To measure the angular distribution requires a good understandingof the detector acceptance.

A simulation is used to find the shape of the acceptance for J/ψ withvarious polarizations.

longitudinalunpolarizedtransverse

The J/ψ PolarizationDefine two regions: 0 < |cosθ| < 0.31 and 0.31< |cosθ| < 0.8

region 1

region 2

A relation exists between the number of J/ψ in each region and the polarization.

The acceptance function for arbitrary polarization is

f(x) = (1+bx +cx + dx )(1+λx )2 4 6 2

The J/ψ Polarization

Then, the number of J/ψ in each of the two regions is related:

NJ/ψ1

NJ/ψ2

f(x)dx0

0.31

f(x)dx0.31

0.8=

Using the returned fit parameters for a,b,c,d and performing theintegration, one gets:

λ = 0.274r - 0.0391

0.00122 - 0.00600 r

NJ/ψ1

NJ/ψ2

r =, where

The J/ψ Polarization

Now we need to find the number of J/ψ in each of the two regions 0 < |cosθ| < 0.31 and 0.31< |cosθ| < 0.8, compute theratio, and find the polarization.

The J/ψ Polarization

0 < |cosθ| < 0.31

0.31 < |cosθ| < 0.8

N = 26J/ψ

N = 18J/ψ

r = 26 + 5.118 + 4.2

= 1.4 + 0.44---

λ = 0.11 + 1.6 (stat.)

It appears the polarization issmall (as expected); however,the error is too large to drawconclusions.

Conclusions The Muon Spectrometer can study the production of charmonium via its

cross section and polarization. These studies will test the theory of NRQCD and its factorization method.

The PHENIX Muon Spectrometer was completed for RHIC Run 2 in 2001-02. Its performance was consistent with expectations for the initiation of a detector of this size and complexity.

The total production cross section for J/ψ in pp collisions was measured and is consistent with the Color Evaporation Model and also the NRQCD factorization method.

An attempt was made to measure the J/ψ polarization, which can discriminate between CEM and NRQCD, but the available statistics will not allow any conclusions to be drawn.

The future looks promising. The north muon arm was added for Run 3, doubling the acceptance. We expect increases in the integrated luminosities delivered to PHENIX, and proton collisions at = 500 GeV.s

The Quark Gluon Plasma

In a QGP, the color force is screened and the quarks and gluons are no longer confined to bound states.

The deconfinement allows for new physics processes to occur. We cannot observe these directly, but we can observe the final state products produced in these interactions.

Nucleon Spin Structure

Original theory deduced that since a proton is made ofthree quarks (uud), then the spin of the proton should be the sum of the spins of the three quarks.

Experiments (EMC, SLAC, SMC) have shown that only about 1/3 of the nucleon spin comes from the quarks.

Where is the rest? Theories: gluon spin, sea quark spin, orbital angular momentum

12_ 1

2_= ∆Σ + ∆G + L

proton spinquark spin~ 0.2 - 0.3

gluon spinorbital angularmomentum

Nucleon Spin Structure

To study the nucleon spin, we need polarized beams. Thismeans that the spin of the nucleons in the colliding beams isoriented in a specific direction.

In collisions of polarized beams, conservation of helicity and angularmomentum produce asymmetries in the production cross sections:

A =LLσ + σ − σ − σ++ -- + +- -

σ + σ + σ + σ++ -- + +- -

++ : beams have parallel spin, positive helicity

-- : beams have parallel spin, negative helicity+ : beams have anti-parallel spin, mixed helicity-

~∆GG

∆qq,

gluon spin

quark spin

Previous low p experiments:T

E537: λ = -0.115 (pN), 0.028 (πN) CIP: λ = ~0E672/706: λ = -0.01 E866: λ ~0.1E771: λ = -0.09

-

Muon Tracker Front End Electronics

Collects and digitizes the charge on the cathode strips.

charge from cathode stripsinput here.

Preamplifier chip:converts charge tovoltage pulse.

Analog memory unit andanalog to digital convertor.Digitizes the analog pulse.

00110010101...Digitized data outputto PHENIX DAQ.

Cathode Readout Card (CROC)

Muon Tracker Front End Electronics

Controller Card

FPGA: the brains of the operation. Controls the AMU/ADCs and sends data to PHENIX DAQ.

ARCNet: local LAN connection for configuration data.

Data to/from CROC andPHENIX DAQ.

Backplane

Interaction point

station 1station 2station 3

Muon Tracker South Arm

Muon Tracker Acceptance/Active AreaThe electronics components are completely isolated with no access duringthe RHIC run.

Muon Spectrometer Momentum Resolution

North performs slightlybetter at largemomentum because itis longer in z direction.

South Arm

North Arm