1. Ivan Hollins 08/05/06 The University of Birmingham
Direct Photon Studies
Ivan Hollins 08/05/06
The University of Birmingham

2. Overview 1) Recap on what I’ve been doing 2) My result at LO c.f. NLO
3) Background rejection
4) Things I don’t understand…

3. 1) Expected Event Rates 1033cm-2s-1
Compton
Annihilation
~ 1 month low lumi = 1 fb-1

4. 1) Event Overview 3 main reasons to look at  + jets
Pp ->  + X measurement -> has been mail focus recently
Use the  to probe the gluon component of the proton
Jet Calibration through Pt balance of the events
Transverse momentum probed by the photon is given by…..
If partons have equal momentum system back-2-back in lab frame ,  = 0 , xT = x
If one parton has much great momentum -> system gets boosted
Since gluons typically carry much less momentum than a quark then system gets boosted in the direction of the incoming quark
So high  events will probably contain high x from the quark and low x from the gluon…

5. 50 GeV 150 GeV 350 GeV  distributions for events with  and jet
PT > 50, 150, 350 GeV.
Each plot shows xa and xb associated with an event twice. Once for jet and once for .
Events typically combine one high x and one low x parton.
At increasing energy events become narrower in 
Sensitivity to high x events comes at high energies and high .
350 GeV

6. 2) Differences I saw between pdf sets
Plots are for photon distributions only
~700k events in each ~ 100fb-1 at 330 GeV
Plots look only at the shape, no comparison made to absolute numbers
 - distributions for MRST2004 and Cteq61. PT > 330 GeV  PT
At ~350 GeV,  = 0, xT~0.05
Is this caused by
High x uncertainly in the quark
Mid x uncertainty in the gluon
A v large uncertainly in the high x gluon?
A poor LO cal?

7. 2) Work by Kumar et al. (Physics Review D 67)
Pt~20-60GeV
GeV
Looking at photon distributions and sensitivity to the gluon at low (20-60GeV) and high ( GeV) Pt
Uses a NLO calculations –Vogelsang
Shows sensitivity greatest at low pt! – not much a high Pt
These results seem to contradict that I found!
Could it be a LO v NLO difference?

8. 3) Background : S / B all from QCD samples
QCD Background, have falling Pt
20 = Gen 15, filter 17, Cut 20 GeV
50 = Gen 45, filter 50, Cut 60 GeV
150 = Gen 100, filter 115, Cut 130 GeV
350 = Gen 300, filter 350, Cut 370 GeV
Look at number of true photon v fakes from QCD background
Calculate signal / background
Signal = direct photons + bremsstrahlung photons
Done with various isolation criteria, % in a DeltaR=0.4 Cone…

9. S / B with Efficiencies Single photon Pt=20
QCD is on a failing Pt Spectrum

10. Background composition
Cone R=0.2
Demand I find 80%
Looking back into the truth to categorise what caused the background…
Pt~20 GeV
With increasing Pt background decreases
The background from pi0 also decreases, but background from multiple particles increases as calorimeter can’t separate them
Pt~130 GeV

11. 4) Things I don’t understand… or only half understand
What experimentally needs to be measured to do a gluon pdf fit?
How is this fit then made?
To show the process is feasible what should I be concentrating on?
Do NLO generators exist? If so where?
How do / can I look at NLO calculations with different pdfs?

12. Conclusions and comments
Something looks strange with my LO work, would be nice to go to NLO if possible and confirm
I am uncertainty as to the exact experimental process I’d go though to measure the gluon component of the proton -> as a result unsure what to focus on next
Good S/B for isolated photons at all energies
S/B increases with increasing energy
Will be subject to uncertainties of fragmentation function of the generator
Need to think about exp suitable Isolation cuts -> these need to be compatible with NLO cal if a measurement of the xsection (and gluon pdf?) is to be done/

13. Backup Slides

14. Separation using EM Calorimeters
Separation done using
Had Cal
EM2
EM1 (strips)
Use Had Cal to reject against jets with hadronic components
Use EM2 to reject against broad jets
Should just leave narrow jets with little hadronic activity.
Mainly consisting of ,  and  decaying into 2 photons
Use fine  granularity of EM1 to reject against these g p0

15. Unconverted True photon v Fakes – all  bins
HadCal
EM2
EM1 (strips)
Different shapes of photons and jets clearly seen
However some plots have broad spectra with tails…