j y photoproduction in ultra peripheral au au collisions at s nn 200 gev measured by rhic phenix n.
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J/ y Photoproduction in ultra-peripheral Au+Au collisions at √ s NN =200 GeV measured by RHIC-PHENIX. TAKAHARA, Akihisa for the PHENIX Collaboration (CNS, University of Tokyo and RIKEN). A. Z 1 e. b > 2R. A. v~c. Z 2 e.  Characteristics of Ultra Peripheral Collisions.

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j y photoproduction in ultra peripheral au au collisions at s nn 200 gev measured by rhic phenix

J/y Photoproduction in ultra-peripheral Au+Au collisions at √sNN=200 GeV measured by RHIC-PHENIX

TAKAHARA, Akihisa

for the PHENIX Collaboration

(CNS, University of Tokyo and RIKEN)

characteristics of ultra peripheral collisions

A

Z1e

b > 2R

A

v~c

Z2e

 Characteristics of Ultra Peripheral Collisions

Weizsacker-Williams (EPA):

  • The electromagnetic field is equivalent to a large flux of quasi-real photons, and can be calculated per (Fermi)-Weizsacker-Williams:

b>2R

no nuclear overlap. Possibility to study g- induced reactions

  • Coherence condition:
    • wavelength > nucleus size . Very low photon virtuality

b

RHIC, LHC LA/LAA

RHIC max. photon energies(EPA):3 GeV~g/R

p hysics motivation for upc j y
Physics motivation for UPC J/y

gluon distribution in Nuclei is not same free proton

Theoretical predictions

And 2004 result

RHIC UPC J/y

Q2=2.5GeV2

x~0.01

  • Direct Measurement of gluon distributions at low-x
    • search for Nuclear shadowing
coherent and incoherent distribution
coherent and incoherent distribution
  • Strikman et al calculate that quasi-elastic (incoherent) J/y cross-section comparable to coherent production
  • Incoherent J/y produced from photo-nucleon interaction
    • Much larger t distribution expected for incoherent
  • Both process emit only J/y and neutron. pTdistribution is important to divide them
  • Strikman’s predictions say at central rapidity, coherent process is dominant, but at forward rapidity, incoherent process is dominant at PHENIX

Strikman, Tverskoy, Zhalov, PLB 626 p. 72-79、2005

n incoherent

n incoherent

coherent

coherent

rhic phenix
RHIC-PHENIX
  • Luminosity
    • Au+Au(200GeV) : 2 x 1026 [cm-2s-1]
    • p+p (500GeV): 2 x 1031 [cm-2s-1]
  • 2007-RUN
    • for central
    • AuAu
    • 200GeV
    • ~530/μb
  • Central arm (|y|<0.35)
    • electron (using 2007 data)
  • Forward arm (1.2<|y|<2.2)
    • muon(using 2010 data)
  • 2010-RUN
    • for forward
    • AuAu
    • 200GeV
    • ~800/μb
s ignal from upc j y and its trigger
Signal from UPC J/y and its trigger

n

  • Signal from UPC J/y→ll
    • a lepton pair without any other tracks
    • 50~60% UPC events are associated with nuclear break up
  • PHENIX UPC trigger
    • BBC_VETO(reject nuclear overlap)
    • EMCAl(for central)/Muon track(for forward)
    • ZDC detect at least a neutron

BBC

e+

e-

Coherent UPC

50-60%

PHENIX MB :4kHz

PHENIX ERT2x2(EMcal):8kHz

to reduce trigger rate,

3rd condition was required

BBC(3<|y|<3.9)

BBC is main vertex detector of PHENIX

1stcondition means we can’t use it

upc j y xn measurement at phenix central arm y 0 35 x 1

p

e+

g

e-

UPC J/y+Xnmeasurement at PHENIX Central arm(|y|<0.35,X>1)

+

  • Offline analysis cuts
    • |collision vertex determined from tracks reconstructed in the PAD chambers|< 30cm
    • number of tracks==2
    • North or south BBC charge==0
    • energy deposit of ZDC>30GeV(just confirm there are no noise trigger)
upc j y xn y 0 y n y 0 measurement at phenix forward arm x 1 y 1
UPC J/y+Xn(y>0)Yn(y<0) measurement at PHENIX Forward arm(X>1,Y>1)

2010 run forward UPC mass dist

(North 1.2_<y<2.2)

5 interaction length→

to get vertex information,

both side ZDC fire was required

Clear J/ψ peak

  • Offline analysis cuts
    • |vertex determined from ZDC|<30cm
    • number of tracks==2 (in central and forward)
    • North or south BBC charge==0
    • energy deposit of ZDC>30GeV
      • Just noise cut
    • Both RXNP charge <1000a.u.
real data for dielectron y 0 35
Real data for dielectron(|y|<0.35)

Dimass distribution

Unlike

like

ZDC energy

  • Clear J/y peak
  • Only unlikesign pair (over 2 GeV)
  • Clear Coherent(low pT) peak

dipT distribution

pT(GeV/c)

real data for dimuon 1 2 y 2 2
Real data for dimuon(1.2<|y|<2.2)

North

Unlike

like

South

Unlike

like

  • Clear J/y peak
  • Only unlike sign pair (2 GeV>mass)
  • Coherent(low pT) peak is not so clear
    • But still 0 pT peak

dipTdistribtuion

2.7GeV<dimass<3.5 GeV

North

South

neutron emission for dimuon j y
Neutron emission for dimuon J/y

North ZDC

South ZDC

  • Same side:1300GeV
  • Opp side :700GeV

Dimuon

North

opp side

Multi Photon

excitation

Dimuon

South

Same side

Multi photon excitation

+Nuclear break up by recoiled neutron

  • If there was nuclear over lap, the asymmetry can’t be explained
  • Suggest UPC (incoherent) process
comparison with pp for dielctron
Comparison with pp for dielctron

Dimass distribution

Unlike

like

  • Even pp(ncoll=1 limit),
  • J/y events are associated with additional tracks
  • At pT ~2.5 GeV/c, unlike/like~2
  • UPC and PP J/ypT distribution is different

dipT distribution

Unlike

Like

~1GeV/c peak

Number of tracks in central arm

UPC(without central track cut)

PP

Normalized at 2

comparison with pp for dimuon
Comparison with pp for dimuon

Dimass distribution

Unlike

like

  • Even pp(ncoll=1 limit),
  • J/y events are associated with additional tracks
  • At pT ~2.5 GeV/c, unlike/like~2
  • UPC and PP J/ypT distribution is different
  • Central multiplicity distribution suggest little most peripheral contamination
    • About 3 J/y per arm.

dipT distribution

Unlike

like

Number of tracks in central arm(not muon arms)

UPC(without central track cut)

PP(0track/non 0tracks~30%)

Normalized at 0

contaminatio n form diffractive process
Contamination form diffractive process
  • Diffractive J/y should have just 2 tracks
    • UPC like events !
  • Typically, diffractive collisions /all pp~30%
    • 10k J/y was generated by PYTHIA (pp 200GeV,msel2(minimum byas))
      • 0/10k J/y was generated by diffractive process
          • Can be neglected
background sources for dielectron y 0 35
Background sources for dielectron(|y|<0.35)

Rapidity distribution

STARLIGHT simulation

for gg->dilepton

  • In central(|y|<0.35) region, γγ->dielectron is main background source
    • “Simulated continuum curve +Gaussian” fit
  • J/ygaus+trig,detecterAccxeff(mass)x exp
  • Exp slope was fixed by simulation
background sources for dimuon y 0 35
Background sources for dimuon(|y|<0.35)
  • HERA:gp->J/y measurement
    • y(2s)/J/y=7%
    • Eur. Phys. J. C 24, 345–360 (2002)
  • gg->dimuon can be neglected this rapidity region
  • Expected most peripheral contamination
  • doesn’t have enough statistics to explain all background

North dipT <0.5GeV/c

J/y

y(2s)

Total background

  • Background by UPC process are suggested
  • UPC ccbar
  • gAu->dipion->dimuon
integrated cross section j y xn central 2004 2007
Integrated cross sectionJ/y+XnCentral2004&2007

sys errors

Acc(over all) 5%

simulation 12%

Lumi 4%

ERT 0.2%

Njpsi1.4%

BBC1.6%

2004+2007

2004 PHENIX

76  31 (stat) 15 (syst) b

J. Nystrand, Nucl. Phys. A 752(2005)470c; A.J. Baltz, S.R. Klein, J. Nystrand, PRL 89(2002)012301; S.R. Klein, J. Nystrand, Phys. Rev. C 60(1999)014903

M. Strikman, M. Tverskoy and M. Zhalov, Phys. Lett. B 626 72 (2005)

V. P. Goncalves and M. V. T. Machado, arXiv:0706.2810 (2007).

Yu. P. Ivanov, B. Z. Kopeliovich and I. Schmidt, arXiv:0706.1532 (2007).

2007 PHENIX

j y xn invariant yield @ 0 35 y 0 35
J/y+Xn Invariant Yield@(-0.35<y<0.35)
  • theoretical
  • calculations for Coherent(@y=0)
  • noshadowing 113μb
  • DS10 μb
  • EKS 83μb
  • Kopeliovich GBW 61μb
  • Kopeliovich KST 54μb
  • EPS 53μb
  • Strikmanimpulse 40 μb
  • Strikmanglauber 30μb
  • We can see both coherent and incoherent distribution
  • 46.7 ±13μb for pT < 0.4GeV(upper limit of coherent)
    • compatible with calculations including strong suppression of gluons at low x
j y xn y 0 yn y 0
J/y+Xn(y>0)Yn(y<0)
  • The pTdistributions at forward rapidity shows that incoherent process is very visible at forward(can’t see coherent peak)
  • There are no theoretical predictions with XnYn condition
summary and outlook
Summary and Outlook

Summary

  • PHENIX measured J/ ψphoto-production yield and its pTdependence in a broad rapidity region.
    • characteristics of UPC signals is obviously different from pp
  • - J/ψ +Xn result at mid-rapidity is consistent with calculations suggesting strong gluon shadowing
  • - important contribution from incoherent processes in J/ψ+Xn(y<0)Yn(y>0) at forward rapidity, looking forward for calculations for this exclusive process

Outlook

  • New vertex detectors will help in the further study of UPC events at RHIC.
gg dimuon distribution
gg->dimuon distribution
  • gg->dimuon distribution isvery sharp in this region
  • Because of edge of detector, Acceptance x efficiency is very low at Y=1.2
detail of fitting
Detail of fitting
  • J/ygaus+trig,detecterAccxeff(mass)x exp
  • Exps lope was fixed by simulation
detail of fitting1
Detail of fitting

North dipT <0.5GeV/c

J/y

y(2s)

Total background

  • Divide into pT bins
    • Due to hadron suppresser
  • Fitting function
  • Acceff(dimass)x
  • Gaus1(J/y)
  • Gaus2(J/y tail)
  • Gaus3(y(2s))
  • +exp(background)
  • Shape of gaus 1&2 was fixed to pp data
  • Gaus3/Gaus(1+2) is fixed to 7%