# Presentation: 626451

## David Silvermyr, ORNL
for the PHENIX collaboration

- for the PHENIX collaboration

*La Londe-les-Maures**, September 2008*

***‘******Photo-production’ of J/******Ψ****** & high mass e******+******e******-****** pairs******in Ultra-Peripheral Au+Au Collisions******at ******√******s******NN****** = 200 GeV in PHENIX***

## Overview

*Physics intro**:  **** , ****A collisions in UltraPeripheral A+A*
*Experimental aspects**: Ultraperipheral A+A collisions*
*    **(signatures, background, triggers, detectors, analysis cuts, ...)*
*Results**: Quarkonia and continuum ****production in UPC **@** RHIC (PHENIX)*

## Electromagnetic Field of a Relativistic 

**Charged Particle**

- Fermi 1924: *The effect of the electromagnetic field of a relativistic *
*particle  is equivalent to a flux of photons with a continous energy *
*spectrum.* (hep-th/0205086)

- Quantum Mechanical derivation 1935 by Weizsäcker,Williams.   
*Weizsäcker-Williams method*

## Ultra-peripheral collisions

**Ultra-peripheral collisions**

- We study “collisions” with b > 2R..

## A physics in UPC A+A collisions

- Typical diagrams for  and A collisions:
- Main interest of -induced collisions in UPC A+A collisions
- _Precision QCD_: Low bkgd & simpler initial state than nuclear A+A colls.
- _Measurements_: Dilepton pairs, hard photo-production (Quarkonia, jets,                                  heavy-Q), ...
- _Physics topics_: QED in strong regime (Zem~1), nuclear GA(x,Q2) function, small-x physics, QQbar dynamics in cold nuclear matter, ccbar (bbar) spectroscopy,. ..

## xGA(x,Q2) via diffractive QQ -production (UPC)

- Large uncertainties in *xG*(x,Q2) for x<10-2 !

**xG****A****(x,Q****2****) via diffractive QQ ********-production (UPC)**

- +A->VM+A (VM=J/) sensitive to gluon density squared:

- Strikman, Frankurt, Guzey, et al.

**_**

- ~

- Kinematical (*x*,Q2) domains
-     covered experimentally:

- Armesto, J.Phys.G32:R367 (2006)

- Ratio of gluon densities in Pb to p

- D. Silvermyr, PHENIX

## ExistingA measurements @ RHIC

- Measured processes in A+A UPC collisions:
- STAR (published):
-     (1) Coherent production: ******+A********A*+ ******+-)
-     (2) Dielectron continuum at low minv : ******A*) + e****+****e****–**
**PHENIX **(preliminary):
-     (1) Coherent J/production: ******+A********A*+ **J/******e****+****e****–**)
-     (2) Dielectron continuum at high minv : ******A*) + e****+****e****–**

- 

## Coherence

- Many scattering centra

- Total scattering amplitude:

*Photon flux: ****Z**2** (EPA; Fermi, WW)*

## contribute at e.g. SPS or CEBAF energies)  

- 			W  2 CM (hc/R)

- Two-photon interactions (and any coherent process) will be significant only at high energies:

- For Au/Pb
- 			CM		W [GeV]
- BNL AGS		    3		0.1
- CERN SPS		    9		0.5
- RHIC			100		6
- LHC		          2940	        160

- RHIC is the first heavy-ion accelerator where significant
- particle production can occur in ultra-peripheral collisions/
- coherent interactions.
- (cutoff not sharp though; and incoherent processes could
- contribute at e.g. SPS or CEBAF energies)

## A model [STARLight] predicts cross sections, rapidity and pT 

- distributions of e.g. vector mesons.
- 				For Au+Au 200 GeV at RHIC:

- ———————————
-            [mb]  (req. Xn)
- ———————————
- 	590   (170)
- 	  59   ( 17)
- 	  39   ( 13)
- J/	0.29  (0.16)
- ———————————

- [Baltz, Klein, Nystrand: PRC 60(1999)014903,
- PRL 89(2002)012301]
- Cross sections in the 0.3-600 mb range!
- Requiring (Xn) neutron coinc. lowers  by
- factor 1.8 – 3.5, but useful for trigger.
- Photonuclear part dominates over +
- The pT distribution determined by the
- nuclear Form Factor, pT  1/R

## Peak at low pT 
		coherent interaction

- Peak at low pT 
- 		coherent interaction

- Topology Trigger	 AuAu  AuAu 0

- 200GeV

- Preliminary

- STAR Result

- Cross-sections consistent with expectations from STARLight
- [PRL 89(2002)272302 – result at 130 GeV;
- also see e+e- low Minv continuum result (52 pairs): PRC **70** (2004) 031902(R)]

## A collisions: experimental signatures

- Central rapidities:
-     (1) Low multiplicities: N <~ 10
-     (2) Low total transverse momentum (“coherence condition”):
-          pT < √2 ħ/RA or pT~minv /~ 30 – 50 MeV
-     (3) Zero net charge: even # of charged tracks of opposite signs.
- (Very) Forward rapidities:
-     (4) Large probability of multiple e.m. interactions
-          (3 exch.): Mutual Coulomb excitation (GDR)
-          leading to A* dissociation via (forward) neutron
-          (Xn) emission: P~30-50% (J/).

- Note: Coulomb-dissoc. probab. factorizes
-           in UPC cross-section calculations

*  **A + A -> A + A + **** *** -> ***A + A + X  (X = J/**** *

-  [All here also valid for: *A + A  -> A + A+ ****  -> A + A + X* ]

## PHENIX (bird’s eye view)

- _Strengths and weaknesses_:
- + Designed for lepton and photon detection,
- high rate and rare triggers.
-  Limited acceptance
- [2x90 deg. in phi,  ||<0.35 used here]

- _Goal [Run4 (2004)]:_
- Via electron channel, look
- for heavier vector meson (J/)
- and continuum at higher Minv.

- beam-beam counters (BBC)

- zero-degree calorimeter (ZDC)

- beam pipe

- electromagnetic calorimeter (EMCal)

- Drift Chamber, Pad Chamber, RICH,..

**Central**
** ****Arm**

**Muon **
** ****Arm**

## +A ->J/Ψ+A : UPC trigger example

-  L1 UltraPeripheral Trigger:
-  Veto on coincident BBC (|y| ~3-4):
-   [avoid periph. nuclear, beam-gas colls.]
-  Neutron(s) in at least one ZDC (E>30 GeV)
-   [sensitive to Au* Coulomb dissociation]
-  Large energy (E > 0.8 GeV) cluster in EMCal:
-   [e+e- decay from J/]
- Events collected (~0.4% of MinBias (BBC) trigger)
***       ***

- PHENIX Run-4 (2004) AuAu UPC trigger:
- Sensitive to Au Au* + J/(e+e-)

## PHENIX UPC analysis cuts

- Global cuts:
-  Std. vtx. cut: | zvtx | < 30 cm
-  Multiplicity(tracks)<15  [removes non-UPC events]
- Loose PID e ± cuts (compared to std. AuAu-nuclear analysis):
-   RICH: n0>=2  [# of photo-tubes within nominal ring radius]
-   Track-EMCal matching (plus no dead tower within 2x2).
-   E1 > 1 GeV || E2 > 1 GeV   [offline high-pT trigger threshold]
-  Pair cuts:
-       arm1 != arm2   [back-to-back di-electrons from J/ ~at rest]
- Residual background subtraction:
-       minv[unlike-sign ee pairs] – minv[like-sign ee pairs]

## +A ->J/Ψ+A : possible background sources (I)

-  “Non-physical”:
-      (1)  Cosmic rays: no ZDC, no good vtx.
-      (2)  Beam-gas: no good vtx., large multiplicity, asymmetric dN/dy

-   A + A -> A + A +   -> A + A + J/

- Trigger
- level

## +A ->J/Ψ+A : possible background sources (II)

- Physical processes (possibly affecting final signal):
-      (3)  Peripheral nuclear A+A: “large” multiplicity, large pT (~2 GeV/c)
-      (4)  Hadronic diffractive (Pomeron-Pomeron, rapidity gap evt.):
-             forward proton emission, larger pT: pT() < pT(PP), like-sign pairs.
-             Hard-diffractive J/ production.
-      (5)  Incoherent UPC +n -> n+J/: pT() < pT(P), wider & asymm. dN/dy,
-            ≥2 neutrons (induced nuclear break-up) w/ same direction as J/.
-      (6)  Other coherent UPC processes:
-  -> e+e- (Important !) , A -> jet(s)+A (lower cross-sections) ?

## PHENIX UPC Preliminary Results

## AuAu UPC preliminary results (I): dN/dminv, dN/dpT ee pairs

**AuAu UPC preliminary results (I): ****dN/dm****inv****, dN/dp****T ****ee pairs**

-  dN/dminv ,dN/dpT distributions after QA, global-, single- & pair- cuts
-    for unlike-sign (red) and like-sign (yellow) pairs:

- Very small wrong-sign background (located in “non-coherent”
- high pT region) well reproduced by MC.
- Can be removed by tighter eID (E/p) cuts that were however
- not used in preliminary analysis shown here.

**Trigger **
**  ****threshold**
**effects **
**important**

- Clear coherent peak at
- low pT for unlike pairs

- ~60 “surviving”
-   pairs

## AuAu UPC preliminary results (II): dN/dminv e+e-  pairs

**AuAu UPC preliminary results (II): ****dN/dm****inv**** e****+****e****-****  pairs**

- dN/dminv after e+e- continuum subtraction

- J/Ψ peak & width in good agreement
- w/ theoretical input + full MC resp.+reco

- Shape of e+e- continuum in good
- agreement w/ theoretical input
- + full-MC resp.+ reco

- Peak ~ 3.10 GeV/c2; Width ~ 130 MeV/c2

- NJ/Ψ= 10 ± 3 (stat)
-              ± 3 (syst)

- dN/dminv (backgd subtracted) & with
-     2 fits of expected e+e- continuum shape
-     (normalized at mee = 1.8 – 2.2 GeV/c2)

## Monte Carlo: dN/dminv  J/Ψ & e+e- continuum

**Monte Carlo: dN/dm****inv  ****J/****Ψ**** & e****+****e****-**** continuum**

- Good agreement with expected signals from “Starlight” MC

## Continuum Cross sections

- Total, Au+Au  Au+Au+e+e- :  32000 b
- (Alscher-Hencken-Trautmann-Baur, PRA 55(1997)396)
- With minv cut:
- 				Total		Xn-fragmentation
- Minv > 1.5 GeV		1.4 mb	0.61 mb
- Minv > 3.0 GeV		45 μb		25 μb
- Minv > 6.0 GeV		450 nb	300 nb
- Xn-fragmentation = Coulomb break-up of one or both nuclei.

## AuAu UPC preliminary results (III): dN/dpT e+e- pairs

**AuAu UPC preliminary results (III): ****dN/dp****T**** e****+****e****-**** pairs**

- dNee/dpT peaked at low
-     pT~90 MeV/c as expected
-     for coherent production.
-    Good detailed agreement
-     w/ theoretical expectations [*]

- [*] Nuclear form factor fit from
-     J.Nystrand, nucl-th/0112055

## AuAu UPC preliminary results (IV): J/Ψ cross-section

**AuAu UPC preliminary results (IV): ****J/****Ψ c****ross-section**

-  Measured J/Ψ yield at y=0
-     consistent w/ theoret. calcs.
-  Syst. uncertainty: coherent      e+e- continuum under J/Ψ
-     (*work in progress*).
-  Reduction of stat. errors need         larger luminosity.
-  Current uncertainties preclude
-     yet detailed study of crucial                model ingredients:
-     GA(x,Q2), (J/Ψ absorption)

- dσJ/Ψ/dy|y=0 = 1/BR × 1/(Acc| y=0.·ε) × 1/εtrig × 1/Lint × NJ/Ψ /Δy =
-                   = 1/(5.9%)×1/(5.7%·56.4%)×1/(90%)×1/120 μb-1×(10 ± 3± 3) =
-                   = 48. ± 16. (stat) ± 18. (syst) μb

- [10] Starlight: S.R. Klein, J.Nystrand PRC 60(1999)014903, NPA 752(2005)470.
- [11] M. Strikman, M. Tverskoy, M. Zhalov, PLB 626(2005)72.
- [12] V. P. Goncalves and M. V. T. Machado,arXiv:0706.2810 (2007).
- [13] Yu. P. Ivanov, B. Z. Kopeliovich and I. Schmidt, arXiv:0706.1532 (2007).

## Incoherent J/ production

- How to separate incoherent from the coherent?
- Via t distribution , t=pT2

- Study yield as a function of cuts in t,
- and compare with cut expectations.
- Eg. a la
- Tight cut:
- N1 = 0.9*C + 0.05*I [below]
- N2 = 0.1*C + 0.95*I [above]
- Looser cut:
- N3 = 0.95*C + 0.3*I [below]
- N4 = 0.05*C + 0.7*I [above]

## Looking Forward

- Goal: Publish the Run4 (2004) results
- [turn preliminary into final results a.s.a.p.]
- Increased luminosity in later runs should allow for a more significant result.
*[Side note: Longer term future at RHIC includes eRHIC, whose program is being defined now – input from this community is most welcomed.]*
*And then there is also the LHC..*

## J/ Excitation Function

*Changes from  RHIC**** LHC: *
- ρ: 	RHIC 590mb  → LHC 5200mb	factor 9
- J/ψ	RHIC 0.3 mb  → LHC 32mb	    	factor 100!

- J/ Excitation Function

**AuAu @ RHIC**

**PbPb @ LHC**

## Summary

- UPC A+A collisions generate high-energy  beams for “non-QGP”   studies: , A physics
- Physics topics in UPC quarkonia photo-production:
- Nuclear GA(x,Q2) at small-x  [Gluon saturation, CGC, ...], QQbar propagation in                  cold nuclear matter, QQbar spectroscopy: -> 0+-2++ states, ...
- Lessons from RHIC (STAR and PHENIX):
- Efficient trigger w/ forward neutron tagging (A* dissoc.) + high-pT at y=0
- Physics signal accessible w/ relative “simple” cuts & analysis
- Good theoretical description of **J/****Ψ** (pQCD) & high-mass e+e- (QED)
- Large source of syst. uncertainty for J/: coherent -> e+e- physics background
- Run-7 (2007): expected x3 stat. improvement for more significant result
- Prospects for LHC:
- Unexplored kinematic regime (max. energies ever, small-x, A -> , ...)
- Expected rates orders of magnitude higher than at RHIC; triggering
- is a key issue.

## What is PHENIX? 

**600+ Collaborators**
**600+ MByte/s peak**
**600+ TByte/run (year)**** **

**Notes:**
Here’s also our collaboration list; 14 countries and about 600 participants. So, we are a pretty large collaboration..
Some numbers that I like to remember are that we are more than 500 collaborators, write more than 500 MByte/s of data as our peak rate, and more than 500 TByte in a Run/year.

## Backup slides

## Electromagnetic interactions in heavy-ion interactions vs. in e+e– and ep (eA)

**Electromagnetic interactions in heavy-ion interactions vs. in e****+****e****–**** and ep (eA)**

-  Directional symmetry. Both beams (nuclei) can act as photon emitter or target.
-  Away from y=0, the different photon emitter/target combinations give different contributions.
-  Strong fields lead to high probability for emission of multiple photons.

## xG(x,Q2) via diffractive QQ -production (HERA)

- pVM+p (VM=J/) sensitive to gluon density squared:

- J/

- J/

- Ryskin et al. ZPC 76 (1997)231

- Fleischmann,Teubner
- DIS07

- Discriminates
- different Ansätze
- of xG(x,Q2)

**_**

- perturbative QCD calculations available:

- D. Silvermyr, PHENIX

## PHENIX UPC- key measurement detectors 

-  DC + PCs: Full central-arm
-       charged tracking (e± momentum).
-  RICH + EMCal: e± identification
-       in central rapidity, and
-       triggering (ERT)
-  ZDC: Forward neutron detection
-       (Au* dissociation):

## No tagging of the nuclei

- 		  0.175 / ( · A4/3)
- At RHIC
- 	Au 	A=197		     1  rad
- 	Si	A=28	 	   17  rad
- At LHC
- 	Pb	A=208		   0.05 rad
- 	Ar	A=40		   0.3   rad
- Not possible to tag the outgoing nuclei. Might be possible with protons.
- Experimental method: Rapidity gaps, reconstruct the entire event, signal of coherence from low pT.

**No tagging of the nuclei**

## Intermission: ‘Normal’ J/psi distr.

- Results from regular p+p interactions shown for comparison..

## <pT2> vs Collision Energy

- PHENIX <pT2 > measurements compared to measurements at other energies.
- As a function of collision energy approx.  a linear dependence on the ln(√s).
*- Significantly larger p**T **values than for coherent interactions.*

## J/ Cross Section vs Rapidity

- J/ Cross Section vs Rapidity

- The statistics available are large enough to allow eleven rapidity bins!
- p+p data now limited by systematic error not statistics
- The data slightly favor a flat distribution over the rapidity range |y|<1.5
- But!
- Remember the systematic errors on the mid and forward rapidity points are independent   a narrower distribution is not excluded.
*Even with this very good stat., the shape is not unambiguosly defined.. *