# Presentation: 620224

## Quarkonium Physics with PHENIX

- Y.Akiba (KEK)
- May 15, 1998
- INT 98 workshop

## Outline

- PHENIX detector at RHIC
- A short summary of its capabilities
- Quarkonium Suppression in A+B collisions
- What factors are involved in J/ production in nucleus-nuclues collisions
- What we can learn about Quarkonium Suppression with PHENIX
- How we can disentangle various factors contribution to the suppression
- Summary

## PHENIX detector at RHIC

- Large general purpose detector made of 11 subsystems
- Good PID capability and high event rate
- To measure as many potential signatures of QGP as possible in Au+Au collisions at RHIC (s = 200 A GeV)
- dN/d, dEt/d, hadrons, photons, electrons, muons
- Strong physics program with polarized protons (s1/2 = 50 - 500 GeV)
- M. Rosati’s talk in this workshop for detail

## PHENIX detector at RHIC

- South muon
-      Arm

- North muon
-      Arm

- West Arm

- East Arm

## Quarkonium measurement with PHENIX

- Central arms (East, West)
- rapidity coverage    -0.35< y < 0.35
- accepatnce               J       4 % of Bee(|y|<0.5) or  0.8% of Bee
-                                  Upsilon:  5 % of Bee(|y|<0.5) or 1.7 % of Bee
- mass resolution        J          20MeV
- 			      Upsilon: 160MeV
- photon measurements by very fine segmented calorimeter
- 			      (E)/E ~ 6%/E1/2  or 8%/E1/2
- Muon arms (North, South)
- rapidity coverage     1.2< y < 2.4 (North)
-                                  1.2< y < 2.2 (South)
- accepatnce                J/         4.3% of Bper arm)
- 			      Upsilon   3.0% of B(per arm)
- mass resolution        J        105 MeV
- 			      Upsilon: 180 MeV
- Global variables (centrality measurement)
- dNch/d	(-2.5 < < 2.5;    (Nch)/Nch better than 10% per 0.2 unit h bin)
- dEtem/d(-0.350.35

## Expected statistics in the second year of RHIC

- Assumption:
- 	Luminosity L of RHIC increases from 10 % (2 x1025 cm2/s) to 100 % (2 x1025 cm2/s) of
- 	design value.
- 	PHENIX runs 30 weeks with full configuration (2 central arms and 2 muon arms)
- 	BllNN(J/) = 400nb is used. (Factor 2 uncertainty)

- Central arms:
- 	central (10%)	min. bias
- J/	~70,000*SAuAu            ~200,000*SAuAu
- (2S)	 ~1,400* SAuAu           ~4,000* SAuAu
- Upsilon 	      ~80* SAuAu              ~250* SAuAu DY(M>4GeV)  ~250                        ~700

- Muon arms:(per arm)
- 	central (10%)	min. bias
- J/	~350,000*SAuAu        ~1000,000*SAuAu
- (2S)	    ~7,000* SAuAu         ~20,000* SAuAu
- Upsilon 	        ~* SAuAu             ~500* SAuAu DY(M>4GeV)     ~1500                     ~4000

- The suppression factor SAuAu  is differnet for J/, (2S), and Upsilon. It is different in central and min. bias events,
- and also different in central arms and muon arms.
- The values of SPbPb measured by NA50 are
-      SPbPb J/(min. bias) = 0.31     (Calculated from data in PLB410,337)
-      SPbPb J/(central)     = 0.27
-      SPbPb S(central)  ~ 0.06     (read from figures on QM97)

## Quarkonium suppression in A+B Collisions

-  Differential cross section or yield of quarkonium in high energy nucleus-nucles collisions can be decomposed as follows:
- Inclusive cross section (for all impact parameters)
-         dAB(y,pt)     = A * B * d(y,pt) * SAB(y,pt,all b)
- Yield per event at impact parameter b
-         dNAB(y,pt,b) = Ncollision(b) * dnNN(y,pt) * SAB(y,pt,b)
-                or
-         dNAB(y,pt,b)  = TAB(b) * d(y,pt) * SAB(y,pt,b)
- with
- 	dAB	cross section in A+B collision
-         dNAB	yield per event in A+B collision
-         A, B	mass number of colliding nuclei
- 	b	impact parameter of the collision
- 	Ncollision # of nucleon-nucleon collisions in the event
- 	TAB(b)	nuclear overlap integral, or
- 		“nucleon-nucleon integrated luminosity per event”
- 	d 	“elementary” nucleon-nucleon cross section
-         dnNN	 yield per event in nucleon-nucleon collision
- 	SAB(y,pt,b) “suppression factor”

- observable

- geometrical factor

- physics

## Quarkonium suppression in A+A Collisions (2)

- Suppression factor SAB(y,pt,b) can be decomposed as
-        SAB = SABinitial * SABfinal
- SABinitial     Initial state suppression
-        In QCD based models, this is caused by “shadowing” of partons due to very high parton density. Therefore, this factor should be a function of x (fractional momentum carried by parton).
- 	SABinitial  = SAini(x1,b)SBini( x2,b)
- SABfinal       Final state suppression
-       This is the survival probability of a quarkonium (or its pre-resonance seed) in the matter formed in the heavy ion collision.
- If one follows the analysis of NA38/NA50, SABfinal is further decomposed as
- 	 SABfinal  = SABabs * SABmedia
- SABabs            Suppression by nuclear absorption
- 	According to NA38/NA50 analysis, this factor corresponds to abs ~ 6 mb and it should be determined by collision geometry only, and it is a function of impact parameter b.
-          SABabs = SABabs (b)
- SABmedium       “medium suppression”, or “anomalous” suppression
- 	This factor is what we are interested in as a probe of dense matter formed in the collision. This is presumably caused by  medium effect. And if it is  a meduim effect,  this factor should be a function of pt of J/Psi (how fast it is escaping from the medium) and energy density of the medium.
- 	 SABmedium  = SABmedium(pt,)

## Quarkonium suppression in A+B Collisions (3)

- In this, perhaps simple minded picture, what we will observe is de-composed as
- 	dAB(y,pt)     = A * B * d(y,pt) * SAIni(x1,b)*SBini(x2,b)*SABabs(all b)*SABmedium(pt,)
- 	dNAB(y,pt,b)  = TAB(b) * d(y,pt) * SAIni(x1,b)*SBini(x2,b)* SABabs(b)*SABmedium(pt,)
- We are interested in the medium effect SABmedium. To measure this factor, we need to know (aside from
- “trivial” geometry factor TAB(b)).
- d
- This is related to the production mechanism of quarkonium in N+N collisions
- SAInit(x,b), SBInit(x,b),
- 	This is a probe to initial stage of collision and shadowing of partons.
- 	TAB(b) * SAIni(x1,b)*SBini(x2,b) is “effective partonic integrated luminosity”(including shadowing).
- SABabs(b)
- 	This is nuclear absorption in ordinary nuclear matter.
- - How well we understand those factors (production mechanism, shadowing, and absorption) ?
- - How well we know about those factors, in particular at RHIC energy (s1/2 = 200 A GeV)?
- - Can we determine them experimentally at RHIC?
- - Can we disentangle those factors?

## Do we understand the basic production mechanism?

- There are many QCD based models of Quarkonium production.
- Color Evaporation model (CEM)
- One of the oldest model.
- Assume certain fraction of cc in between MQQ and 2*MH (Q=c,b H=D,B) becomes quarkonium
- Phenomenologically good description of existing data
- Color Singlet model (CSM)
- quarkonium formed as color singlet state with correct quantum number of final particle.
- Can not explain CDF data at high pt direct J/, (2S) production.
- Color Octet model (COM)
- QQ pair in color octet state “evolve” into quarkonium nonperturbatively.
- Recently very popular and promising model
- But can not explain all data
- 	(non) Polarization of J/ in hadroproduciton
- 	HERA photoproduction data.
- The production mechanism of quarkonium is not yet well understood.
- New data from RHIC energy can help to test and distinguish models.

## Do we know J/ cross section at RHIC energy?

- In QCD based models, beam energy dependence of J/i cross section is determined by gluon density ditribution G(x). In particular, Bd/dy can be approximately described as
-      Bd/dy ~ (xG(x))2
-      x ~ MJ/s1/2  or  x ~ 2Mc/ s1/2
- This is a rather naïve expression, but it seems to explain the existing data reasonably well. Therefore, J/ cross section at RHIC energy can be estimated from low energy data from G(x).
- However, there is a large uncertainty in this estimate because
- (1)  Many existing J/ cross section data have a large uncertainty an different systematics.
- (2)  Although difference among the PDF sets are now relatively small, the prediction strongly depends on the choice where x is sampled and the value of Mc
- (3) J/ cross section measured at ISR seems systematically lower than those of fixed target experiments.  Is this a systemtaics, or failure of the model?
- We need a good measurement of J/ cross section in p+p at RHIC. This will also test the QCD based models.

- J/Psi cross section data is compared with
- gluon distribution (xG(x))2. The normalization
- of the curve is arbitrarily chosen.

## Do we know SABinit at RHIC?

- 	At SPS or FNAL fixed target energies, nuclear dependence are measured as
- 		(pA) = (pp)*A
- 	Drell Yan process:  = 1.00
- 	Charm production:  = 1.00
- 	From those, it is considered that there is little shadowing effect for quarks and gluons in nuclei in x region that is accessed by SPS and FNAL experiments.
- 	Sinit = 1.0
- 	However, at RHIC energies, the x region probed is very small (x~0.02 for J/Y) compared with fixed target experiments, and the parton density expected in RHIC Au+Au collision is very large. Many theorists predicts that there is a gluon shadowing at RHIC, but there are large uncertainties.
- 	NA38/NA50 intermediate mass continuum data suggested that charm production in SPS Pb+Pb collision is enchanced by factor 3. If the observed effect is due to charm enhancement, there is a very strong gluon anti-shadowing in AA collisions. The effect could be much larger at RHIC.
- 	SABinit is perhaps least well known factor in J/ measurement at RHIC energy. It should be evaluated experimentally.

## Do we know SABabs at RHIC?

- A38/NA50 group showed that the nuclear suppression at SPS/FNAL energies can be well described by nuclear absorption N ~ 6 mb.
- Although this is a very good phenomenological description of data, it is fair to say that we do not well understand the nature of this absorption process. For example, it is not clear what is “absorbed”. (Is it J/, or is it “color octet pre-resonance” state?)
- E866 data shows that the nuclear suppression of J/in p+A is a complex process that strongly depends on xF, pT.
- The “absorption cross section” is presumably beam energy independent, but it must be tested experimentally. The energy at RHIC is 5 to 10 times higher in s1/2 than fixed target experiments.
- The absorption hypothesis should be tested experimentally at RHIC energy, and the absorption cross section should be measured/evaluated at RHIC energy.

## What we can learn about Quarkonium Suppression withPHENIX

- Basic production mechanism in p+p
-  NN at RHIC energy
- Test of models of quarkonium production
- Suppression (and enhancement?) mechanisms in A+B collisions
- Sinit or gluon shadowing
- Sabs or nulcear absorption
- extraction of Smedium

## Study of basic production mechanism in p+p

-  Bd/dydpt in p+p at 200 GeV and 500 GeV (and possibly at other energies)
-  new data of inclusive quarkonium production at much higher energy than previously available
- measurement from pt=0 to pt ~ 5-10 GeV
- new data in low pt region. No high energy collider data so far
- s1/2 dependence of cross section ---> test of QCD based models
- measurement rapidity distribution---> another test of QCD based models
- measurement of J/ families and Upsilon families
- does the same mechanism is responsible for the production?
- The rapidity distribution and s1/2 dependence is consistent with the same gluon density?
- production ratios J/:(2S), Y(1S):Y(2S):Y(3S)
- measurement of c
- reconstruction of c  from J/
- spin asymmetry of production cross section
- can be a powerful tool to distinguish models
- many theoretical papers on this subject
- basic input to the measurement of suppression factors in Au+Au collisions

## Example: Measurement of c -> J/ + 

-  J/ in p+p and p+A can be measured in PHENIX in central arms.
- Separation of c1 and c2 is difficult because of mass resolution. More study is required.
- Use fit to extract yields of two resonances?
- Use photon conversion for better resolution? (event rate).
- Expected yield:
- about 6% of J/->e+e- (if  feed down fraction of J/is 30%)
- 100-150 per day of p+p running at design luminosity of RHIC
- Spin asymmetry measurement could be a good test of models (D. Kharzeev)
-  b could be reconstructed ? (Rate, S/B)

- An estimate of S/B of  measuement in p+p in
- PEHNIX acceptance. It is assumed that 30% of
- J/is from  feed down.

## Study of J/suppression in Au+Au

- High statistic measurement of J/
- Comparable to NA50
- Detailed centrality dependence of suppression factor is possible
- Measurement from pt = 0 to high pt ~ 4-5 GeV/c
- Comparison of J/ measurements in central arms (y=0) and muon arms(1.2<y<2.4)
- Unique features in PHENIX
- Very useful to decompose contribution factors in the suppression (see next)
- Measurement J/ yield relative to other processes
- J/: continuum (charm, Drell Yan)
- J/:  single lepton (charm)
- J/single photon
- etc
-        Those ratios can be used to determine TAB(b), SABinit

## Measurement in central and muon arms

- Pion rapidity distribution from VNI at RHIC energy
- (from nucl-th/9705041)

- center of e arm

- center of muon arm

- PHENIX can measure J/suppression factor in
-      e-arms          -0.35<y<0.35
-      muon arms    1.2 <y <2.4
- The rapidity density of pions at these rapdity are predicted as
-       rapidity                   0            1.25     1.75       2.25
-       dN/dy(HIJING)     1500      1250     1050       830
-       dN/dy(VNI)           2150      1250       850       600
-                                     e- arms            muon arms
- Rapdity density in e-arms and mu-arms can be factor 2 to 3 different!
- We can simultaneously measure J/Psi suppression in those very different pion rapidity density --- different energy density of the medium created in Au+Au collisions
- We can compare J/ yield in muon arms and electron arms in the same centrality class. In this comparison, most of the common factors will cancel out, and ratio of SAuAumedium can be extracted. For example, SAuAuabs should be the same in e-arm and in muon arm if absorption cross section of J/ or its precursor is energy independent.
- If SAuAumedium is a function of energy density of the medium, it should scale as a function of dN/dy. This can be tested experimentally by comparing the SAuAumedium in e-arm and SAuAumedium in muon arm in different centrality classes where dN/dy of (less central) e-arm and more (central)  is the same.

## Measurement of J/ and lepton pair continuum

- At RHIC energy, lepton pair continuum in 4-8 GeV region is likely dominated by double leptonic decay of charm anti-charm paris.
- The yield of charm decay pair is ~10 time higher than that of Drell-Yan in M>4 GeV/c. In higher massregion, b quark decay should have a significant contribution.
- This lepton pair continuum can be also used as a meaurement of charm and bottom production---a measurement of SAuAuinit
- The event rate of the lepton continuum above 4 GeV is expected as 5-10K per year (min. bias events) in central arms, and 4-5 times larger in muon arms. Although it is not so copious as single leptons, it has a sufficient statistics to do measurement in several centrality bins.
- Extraction of Drell Yan signal in the lepton pair continuum would be difficult unless charm contribution is sufficiently suppressed by energy loss effect as suggeted by Shryak.

## Measurement of J/ and single lepton (charm, bottom)

- It is expected that charm quarks are copiously produced in
- Au+Au collisions at RHIC. Charm production can be detected as high pt single leptons (e, )
- They can be used as a probe of initial state of collison and we can measure of Gluon shadowing from charm production.
- SAuAuinit can be measured from single lepton.
- Also
- SAuAuinit can be canceled out by measuring the ratio J/ and single lepton.

## Systematic studies in light ions and p+A

- RHIC is a deciated machine for study of heavy ion collisions, and it can provide collisions of p+p, p+A and A+A other than Au+Au. The nucleon-nucleon luminosities are approximately the same for any of those beam configuration. (For point-like process, the yield per day is approximately the same.)
- Systematic studies in p+p, p+A, and A+A (A<Au) will provide measurements of SABinit, SABabs
- Charm production in p+A --> gluon shadowing --> SABinit
- high pt photons in p+A -->  gluon shadowing -->  SABinit
- J/,(2S) production in p+A and light ions  --> SABabs
- Runs with lower beam energies are also possible for the study of  s1/2 dependence.
- Energy dependence of SABabs.
- Does Smedium scale as function of Nch/d?

## Can we separate out and measure SABmedium by J/ measurement with PHENIX?

- 	dAB(y,pt)     = A * B * d(y,pt) * SABInit(y)*SABabs(all b)*SABmedium(pt,)
- 	dNAB(y,pt,b)  = TAB(b) * d(y,pt) * SABInit(y)*SABabs(b)*SABmedium(pt,)
- d(y,pt):
- 	- direct measurement in p+p --> J/
- 	- canceled out in “ratio” of central arms and muon arms measurement
- TAB(b)
- 	measurement of Drell Yan, high pt photons.
- SABInit(y):
- 	measurement single lepton (charm, bottom production ---> shadowing)
- 	measurement of lepton mass contiuum (charm, bottom production --> shadowing)
- 	SABabs(b):
- 	measurement of very peripheral collision
- 	measurement of p+A collisions and light ion collisions
- 	comparison of central arms and muon arms
- SABmedium(pt,)
- 	comparison with central arm and muon arm measurement will test if this is a function of  or not
- 	With a large number of redundant measurements  that are possible with PHENIX, we can separate the medium effect SABmedium(pt,) and measure it in wide range of .

## Summary

- Quarkonium production in nucleus -nucleus collision is a complex process that involves many factors
- Each factors should be well understood and measured to separate out the medium suppression factor S and to use it as a probe of dense matter formed in nuclear collison.
- PHENIX is well suited to attack this problem with
- wide rapidity coverage  (y~0 by electron and y = 1.2-2.4 by muon)
- high statistic measurement (100K - 1M J/per year)
- ability to measure electron, muon and photons simultaneously in a single trigger condition
- ability to measure same physics quantity in different observable channels
- ability to measure in variety of beam combination (p+p, p+A, and A+A).
- From many measurements J/Psi and other quarkonium in wide range of rapidity, pt, and centrality, and by combining those measurement with other measurements like single leptons or single photons, PHENIX will be able to  separate out the medium effect SABmedium(pt,) and measure it in wide range of .

## We want predictions at RHIC energies

- RHIC will be completed in 1999, and PHENIX will start taking data in 1.5 years.
- We may have some data on J/ in first year (where luminosity is low), and will have a good data of J/ in central and muon systems in the second year (2001).
- Although this is a INT/_**RHIC**__ _workshop, I heard little predicitons on J/at RHIC energies.
- We are interested in
- Predictions on the factors in J/ suppression
- Their rapidity dependences
- Their dN/d dependences
- Their consequences on other observables
- etc. etc.
**We can test those predictions with PHENIX**