# Presentation: 524003

## Measuring Parton Densities withUltra-Peripheral Collisions in ALICE

- Photonuclear Interactions
- Measuring Structure Functions
- 	Vector Mesons
- 	Open Charm/Bottom
- 	Other Channels
- Experimental Feasibility in ALICE

- Spencer Klein, LBNL

## A word from your sponsor:Why use UPCs to study parton distributions?

- Several reactions are directly sensitive to gluon distributions
- Directly probe ‘new phases of matter’ like colored glass condensates.
- Understand initial state for central collisions
- ~ 10X  photon energies than even HERA
- Much lower x than fixed target experiments
- Smaller (or at least different) systematics than pA collisions

## Photonuclear Interations

- One nucleus emits a photon
- Weizsacker-Williams flux
- The photon fluctuates to a qq pair
- The pair interacts with the other nucleus
- If b>2R --> No accompanying hadronic interactions
- Possible Reactions
- Elastic qq Scattering  --> Vector Meson Production
- , J/, ’, (1s), (2S),...
- For heavy Quark mesons, production is sensitive to parton densities
- If qq = cc or bb  --> Heavy quark photoproduction
- Diject production

## Vector Meson Production

- Quark-nucleus elastic scattering
- Coherent enhancement to                                            cross section
- Light vector mesons (,,*...)
- Soft (optical model) Pomeron
- Heavy vector mesons (J/, ’, ...)
- Probe short distance scales
- Scattering may be described via 2-gluon exchange
- Sensitive to gluon distribution at
- x = MV/2mp exp(± y)
- Q2 ~ MV2

## The effect of shadowing

- (VM) ~ |g(x,Q2)|2
- Shadowing reduces cross section
- Factor of 5 for one standard shadowing model
- Rapidity maps into x
- y = ln(2xmp/mV)
- x= mV/2mp exp(y)
- A colored glass condensate could have a larger effect

- No shadowing

- FGS shadowing

- Frankfurt, Strikman
- & Zhalov, 2001

- y

- d/dy, mb

## The 2-fold ambiguity

- Which nucleus emitted the photon?
- X or k ~ mV/2mp exp(y)
- Photon fluxes, cross sections for two different photon energies (directions) are different
- 2 Solutions
- Stick to mid-rapidity
- For J/ at y=0, x= 6*10-4
- Compare two reactions
- Pb + Pb --> Pb + Pb + J/Y
- Pb + Pb --> Pb* + Pb* + J/Y
- Different reactions have different photon flux ratios
- System of 2 linear equations

- Or...

## J/ -->  3.2 Hz --> 3M/ 106 s
~ 300,000 ’ and 17,000 (1S)
36,000 J/ --> e+e- with |y|<1 (ALICE central)
~360 ’ and ~100 (1s)
27,000 J/ --> e+e- with 2.5 <y< 4 (ALICE  spect.)
~ 270 ’ and ~75 (1s)
Rates are higher with lighter ions (higher LAA)

- ~ 300,000 ’ and 17,000 (1S)
- 36,000 J/ --> e+e- with |y|<1 (ALICE central)
- ~360 ’ and ~100 (1s)
- 27,000 J/ --> e+e- with 2.5 <y< 4 (ALICE  spect.)
- ~ 270 ’ and ~75 (1s)
- Rates are higher with lighter ions (higher LAA)

**Rates (w/o shadowing)**

## AA vs. pA vs. pp

- Exclusive J/ production can be studied in AA, p/dA and pp collisions
- Rates are high with all species
- With it’s distinctive signature, signal to noise ratio should be high for all species
- In p/dA collisions the photon usually comes from the nucleus, and strikes the proton/deuteron.
- Avoids two-fold ambiguity for proton targets
- Measure parton distributions in protons and nuclei
- Compare reactions --> low-systematics measurement of shadowing

## Reconstruction

- Exactly 2 tracks in event
- pT < 150 MeV/c
- leptons are nearly back-to-back
- PID as leptons
- 1 e in PHOS + 1 e in EMCAL?
- 2 muons in spectrometer

- PHENIX, QM2004

## Triggering

- For UPCs, Level 0 cannot use multiplicity trigger
- Need L0 from muon spectrometer (present?) or calorimetry or TRD
- L1 from and/or muon spectrometer & calorimetery/TRD
- L2 due to dileptons (+ low multiplicity?)
- N.b. Many ‘central collision’ analyses will face the same problem with pp collisions

## Open Charm/Bottom

- Occurs via gluon exchange
- Well described in QCD (& tested @ HERA)
- Sensitive to gluon distribution
- x and Q2 depend on final state configuration (M(QQ)...)
- Wider range of Q2 than vector meson production
- High rates
- (charm) ~ 1.8 barns ~ 25% of hadronic
- S(bottom) ~ 700 b ~ 1/10,000 of hadronic
- (without shadowing)
- One nucleus breaks up

- Au

- V

- 

- qq

- Au

- Nuclear debris

## Charm kinematic distributions

**Charm kinematic distributions**

- Solid – Total
- Dashed – Direct/2
- Dot-Dashed – Resolved
- Photon
- Contribution
- 2 sets of curves are with EKS98 (small) and w/o shadowing

- SK, Joakim Nystrand &
- Ramona Vogt, 2002

## Charm Reconstruction

- cc reconstruction
- Well-understood techniques
- Direct Reconstruction of charm
- Should be feasible in ALICE
- Probably can’t reconstruct both c and cbar
- Semi-leptonic decay
- Should be able to detect leptons from dual semi-leptonic decay

## Separating Photoproduction and Hadroproduction

- Photoproduction of charm is very favorable
- P(--> cc) ~ 4/10
- Cross Sections are large
- (charm photoproduction) ~ 1.8 barns
- For lead at the LHC
- No shadowing
- (charm hadroproduction) ~ 240 barns

- SK, Joakim Nystrand &
- Ramona Vogt, 2002

## Rejecting Hadroproduction

- Single nucleon-single hadronic interactions
- Eliminate more central reactions with multiplicity cut
- 1n1n ~ 700 mb
- 1n1n, with charm ~ 5.5 mb
- b>2R with charm ~ 1100 mb
- One nucleus remains intact
- Assume <10% chance
- Particle-free rapidity gap around photon emitter
- P~ exp(-y dN/dy)
- dNch/dy ~ 4.4 at midrapidity for pp the LHC
- 40% lower at large |y|
- For y= 2 units, P ~ 0.005
- remaining, with charm  ~ 3 b

- SK, Joakim Nystrand &
- Ramona Vogt, 2002

## Triggering

- Direct Charm reconstruction
- Trigger is tough
- Semileptonic Decays
- Use leptons, as with vector mesons
- May get some usage out of multiplicity detectors

## Other channels

- Dijets
- Large rates
- (ET> 20 GeV) ~ 1/minute
- Separating photoproduction and hadroproduction seems difficult
- Photon + jet (Compton scattering)
-  (1% of dijet rate)

- Ramona Vogt, 2004

## Other UPC studies in ALICE

- Photoproduction of W+W-
- Study WW vertex
- --> Higgs?
- Rate is low, but this is important in establishing the nature of the Higgs
- Vector Meson Spectroscopy
- If trigger problem can be solved
- e+e- pair production
- A test of strong field QED
- Enormous rates (13,000 barns in SPD1)
- A trigger is probably not needed

## Conclusions

- UPC photoproduction can be used to measure the parton distributions in ions and protons, and test colored glass condensate models of nature.
- Both vector meson and open charm can be studied with ALICE.
- The analyses seem rather straightforward.
- Attention to the trigger will be required to collect the data required for these analyses.
- Many other UPC studies are possible