# Presentation: 706814

## Exclusive Production of Heavy Quark Mesons in pp and pp collisions

- Photoproduction of heavy vector mesons in pp/pp collisions
- Rates and rapidity distributions
- Interference and pT spectra
- Backgrounds
- Conclusions

- Spencer Klein, LBNL & Joakim Nystrand, Bergen

## Photoproduction

- A photon from the electromagnetic field of one nucleus interacts with the other nucleus, producing a vector meson
- Either nucleus can be photon source
- The photon flux extends up to 0.1 Ep or higher
- These photons can probe low-x gluons

|  | Ecm | kmax (target) | Wgpmax |
| --- | --- | --- | --- |
| RHIC | 500 GeV | 12.5 TeV | 158 GeV |
| Tevatron | 2.0 TeV | 50 TeV | 316 GeV |
| LHC | 14 TeV | 1 PeV! | 4.4 TeV |

|  | Gluon x
Y (y=0) | Gluon x
J/ (y=0) |
| --- | --- | --- |
| RHIC | 0.02 | 0.006 |
| Tevatron | 0.005 | 0.0015 |
| LHC | 0.0007 | 0.0002 |

## Photon Flux

- From Weizsacker-Williams
- b > 2Rp;
- no hadronic interactions
- Clean event signatures
- Take bmin ~ 0.7 fm
- Change to  bmin = 1.0 fm is error estimate
- Sensitive to proton form factor
- Qmin2=k/

- Drees and
- Zeppenfield, 1989

## Vector Meson production ITheory

- g collision at lowest order
- Need another gluon to conserve color
- Refinements
- Relativistic wave functions
- Off diagonal parton dists.
- NLO contributions
- Sensitive to gluon distributions
- Q2=MV2/4

- Ryskin, 1993

## Vector Meson production II Experiment

- ZEUS 

- H1/ZEUS J/

- Wp (GeV)

- Wp (GeV)

- Wp (GeV)

- HERA data
- J/ ~ W0.8
- agreement with theory
- :      ~ W1.7
- limited statistics
- (1), (2), (3) not separated
- Estimate 80% (1)
- Alternately, use ~ W0.8
- So-so match with theory
- Tevatron reaches slightly higher Wp
- Lower gluon x

## Cross Sections, Rates and d/dy

- Photon energy -> rapidity
- Y = 1⁄2 ln (2k/MV)
- The  @ RHIC          (500 GeV, 1031cm-2 s-1)
- 0.4/hr
- difficult
- The  @ the Tevatron (1.8 TeV, 2*1032cm-2s-1)
- 76/hr
- Easy
- J/ rates are high

- RHIC @ 500 GeV (top)

- The Tevatron (bottom)

## Interference

- Either proton can act as emitter or target
- Can’t differentiate between
- How to transform diagram 1 into 2?
- For pp -> parity transform
- For pp -> CP transform
- and J/ are JPC = 1--
- pp: subtract amplitudes
- pp add amplitudes

## pT spectra

- pp & pp are very different!
- <b> ~ few rp
- pTP >> pT
- Different from AA
- Interference is relevant for most of production pT range

## Interference or no interference?

- Interference occurs when we can’t tell which nucleus emitted the photon
- What if scattered proton is detected in a Roman pot?
- Most of the VM pT comes from the target proton, so a measurement of the proton pT can determine where the photon came from.
- But, what about timing?
- What if the J/ or  is detected before the proton hits the Roman pot?
- If detection in a Roman pot destroys the interference, then this can be used to remove the 2-fold ambiguity and study gluons at very low x values

## Backgrounds

- (p(p --> (J/ or pp********(pp --> (J/ or X
- Is the signal visible?
- Cutting on pT < 1 GeV/c eliminates 94% of the hadroproduction.
- Rapidity gaps eliminate the rest.
- At the Tevatron dnch/dy ~ 3.9.
- 3 units of rapidity gap --> exp(-3*3.9) ~ 10-5.
- There are gaps on both sides.
- The 3 units can be split between both sides.
- 3 units of coverage is easy.
- Similar numerology at RHIC; even easier at the LHC.
- Quantum numbers wrong for Pomeron-Pomeron.

## Conclusions

- Photoproduction of heavy vector mesons is substantial in pp and pp colliders.
- The rates are sensitive to the gluon distribution in the proton.
- Production at the two sources can interfere.  The sign of this interference is different for pp and pp collisions.
- The pT spectrum of produced mesons is very different for pp and pp collisions.