1. What ATLAS would like to learn from the community : soft diffraction Emily Nurse (for ATLAS) Not a list of planned analyses, but rather some issues and questions 1

2. Diffraction Emily Nurse Diffraction Why are we interested in soft-diffraction? –Interesting in its own right –Affects inclusive “minbias” measurements : Potential for model dependent unfolding (can be minimised) Can we define our final state to suppress diffraction (ie/ to tune non-diffractive parameters)? –Affects luminosity determination Our philosophy : –Measure physical observables sensitive to diffraction 2

3. Diffraction Emily Nurse ATLAS detectors 3 Minimum Bias Trigger Scintillators LUminosity Cerenkov Integrating Detector Zero Degree Calorimeter 2.12 < |η| < 3.8 5.61 < |η| < 5.93 |η| > 8.3 Inner Detector |η| < 2.5 Calorimeters |η| < 5.0

4. Diffraction Emily Nurse Diffraction in minbias? 4

5. Diffraction Emily Nurse Diffraction suppressed samples? 5 Pythia 6 Pythia 8 Plots from Markus Warsinsky Phojet Where should we cut to suppress diffraction? Depends strongly on the model!

6. Diffraction Emily Nurse Diffraction suppressed samples? 6 Pythia 8 Plots from Markus Warsinsky We start to cut hard into our phase space! Make anti-gap cuts? (needs to be a particle level definition)

7. Diffraction Emily Nurse Luminosity determination 7 Typical forward detector : 2 particles with : 2.5 1 GeV Study by Beate Heinemann for MB&UE workshop Pythia6 vs Pythia8 : ~6% Pythia8 vs Phojet : ~10% Pythia6 vs Phojet : ~17% N.B. σ the same in Pythia6 and Pythia8, but particle spectra different √s= 7 TeV

8. Diffraction Emily Nurse Luminosity determination with MC Ultimately we should constrain with our 7 TeV data What should we use for now? –Should Pythia6 diffractive models be considered “wrong”? –But Pythia6 ND models have received the most “attention” and describe our data better! –Can we combine Pythia8 SD, DD with Pythia6 ND? –Can we make better use of Tevatron/HERA data now? 8 MCs differ by (differential) σ SD, σ DD and particle spectra!

9. Diffraction Emily Nurse Advice we want from this community 1.What is a good particle level definition of a diffraction suppressed / enhanced sample ? 2.What soft diffractive models are on the market for us to use? –So far we have used Phojet, Pythia6, Pythia8 –What inputs/ parameters can we vary / tune 3.Can they already be better constrained from HERA/Tevatron data? –do we need Pythia8 to simulate ep so we could use HERA data to constrain diffraction? 4.What should we measure to help constrain these models? –Differential diffractive cross-sections based on rapidity gaps –Particle distributions (particularly in the forward region) enhancing diffraction through rapidity gaps / low multiplicity cuts 9