1. Diffraction and Forward Physics in ATLAS: results and perspectives
ATLAS and its Forward Detectors
Diffractive processes at ATLAS
Elastic and Total Cross Section Measurement
Future perspectives: ALFA and AFP
Program of measurements at very low and low luminosity
Potential for a high luminosity program
Conclusions
Marco Bruschi, INFN Bologna [Italy]
(on behalf of ATLAS collaboration)
ISMD 2014

2. ATLAS and its Forward Detectors
AFP
LUCID
September 10th 2014
TeV (DPMJET)
M. Bruschi, INFN Bologna (Italy)
Example of ATLAS measurement
Rapidity gap cross sections
ALFA elastic ds/dt and stot at sqrt(s) = 7 TeV

3. Classification of diffractive events
September 10th 2014
Diffractive and elastic events: ~ 40% of LHC pp collisions
Traditional measurements use rapidity gap method to separate contributions of inelastic non diffractive production
ATLAS has |h|<4.9 coverage which doesn’t work for “too” small (order of 10 GeV) Mx=sqrt(sx) of diffractive system because Dh=-log(x)
The Indirect measurement of x via energy seen in calorimeter is not very precise due to invisible energy and it works only in a limited region of x
No measurement of t
Proton tagging is the only way for a more detailed probing of diffraction
M. Bruschi, INFN Bologna (Italy)

4. Diffractive processes at ATLAS
September 10th 2014
Forward rapidity gaps defined as larger DhF region on detector edge (h=±4.9) devoid of pT>200 MeV particles
Measured ds/ DhF ~ 1 mb for DhF > 3
Default PHOJET and PYTHIA do not describe the rise of the cross section observed at DhF > 5
Rise interpreted from a triple Pomeron contribution with a Pomeron intercept aP(0) > 1
Slope very sensitive to the precise value of aP(0)
Eur. Phys. J. C72 (2012) 1926
M. Bruschi, INFN Bologna (Italy)

5. Elastic scattering with the ATLAS-ALFA detector
arXiv: [hep-ex]
September 10th 2014
M. Bruschi, INFN Bologna (Italy)

6. Theoretical prediction and Fit results
The theoretical prediction used to fit the elastic data is:
September 10th 2014
Luminosity: ±2.3%
Beam energy: ±0.65%
Main sources of systematic experimental uncertainties for stot (fit input)
M. Bruschi, INFN Bologna (Italy)

7. Comparison with TOTEM
September 10th 2014
M. Bruschi, INFN Bologna (Italy)
Comparison of results using the luminosity-dependent method
The luminosity uncertainty for ATLAS is ±2.3% and for TOTEM ±4%
Luminosity enters with a factor ½ in the error computation

8. The AFP detectors
Horizontal RP
Horizontal RP
September 10th 2014
Purpose:Tag and measure diffractive protons at 210 m (two arms) providing x, t
Precision MASS SPECTROMETER. In case of exclusive production (Double Tag)
M= sqrt(x1x2s)
Detectors (in 2+2 Horizontal RP)
Radiation hard “edgeless” 3D Silicon detectors with ~mrad angular resolution for proton tracks reconstruction (204m,212m)
High performing timing detectors (212m)
(~ 10ps resolution, for proton pile-up background rejection at high mu)
M. Bruschi, INFN Bologna (Italy)
0.3
0.02

9. ALFA and AFP Detectors t=-pT2 and x= DE/E0
In diffraction, coherent interaction of proton is soft  pT ~ 1 GeV
Elastic scattering: x= 0, diffraction: x<0.2
September 10th 2014
MAD-X + MC generator-level
M. Bruschi, INFN Bologna (Italy)

10. AFP Detector System Performance
Results from full simulation of AFP detectors and the whole forward region
Mild degradation of performance due to pile-up
Detector capable of running in pile-up conditions
x acceptance in the range 1.5%-15%
September 10th 2014
M. Bruschi, INFN Bologna (Italy)

11. Very low luminosity
September 10th 2014
ATLAS and CMS agree within systematic uncertainties
(hadron |h|<4.7 vs. |h|<4.9: 5%
Diff. model for unfolding: 10%)
CMS sytematically above ATLAS
Pythia8 predicts SD~DD
Proton tagging could shed light on 1) and 2)
More generally, the possibility to tag diffractive protons with AFP will improve the quality of the interpretation
Underlying event
MPI
All the other soft QCD measurements performed using only the central detector
M. Bruschi, INFN Bologna (Italy)

12. Low/Medium luminosity
September 10th 2014
And a S
“Standalone” MC simulations
M. Bruschi, INFN Bologna (Italy)

13. Running Scenario at low-m
September 10th 2014
M. Bruschi, INFN Bologna (Italy)
Program approved by ATLAS (pending resources)
No or loose (~50 ps) timing needed from AFP

14. High lumi program Exclusive Jetsγ
September 10th 2014
QUARTIC Gauge Couplings – testing BSM models
Reaching limits predicted by string theory and grand unification models ( for gggg)
Exc. Jets – verification of QCD production models, unintegrated gluon PDFs
Program will be discussed in ATLAS when data on beam background available
gggg requires moderate timing, the other final states ≤ 10 ps
Exclusive Jets
M. Bruschi, INFN Bologna (Italy)

15. Status on timing
The low-medium luminosity program who has been conditionally approved by ATLAS (pending resources found), does not need an accurate timing (actually, only trigger is necessary)
The high luminosity program (still not confirmed) requires instead a very performing timing (~ 10 ps or even better) to reduce pile-up background
The LQBAR solution is the one with the most advanced R&D and can guarantee the needed timing performances
In parallel, there is a very interesting R&D effort (in collaboration with CMS/TOTEM) on Silicon LGAD/Diamond detectors readout by a very performing module developed by the Saclay group (SAMPIC, ~4 ps intrinsic time resolution)
September 10th 2014
M. Bruschi, INFN Bologna (Italy)

16. Conclusions
The ATLAS experiment produced and is producing important results for diffractive physics
This will provide a valuable input
Tune Monte Carlo generators
Mechanism of diffractive processes
Mechanism of hadronization and confinement
Search for new QCD dynamics
The quality of these studies will improve when the detector will be upgraded with AFP adding the capability to add diffracted proton tagging
ALFA will continue its program of total cross section measurements at the new LHC energy and at higher b* (so reaching more inside the CNI region) and also contributing to complement the AFP acceptance in the high b * - low luminosity physics program
The hard diffractive program of AFP at low b * - high luminosity seems very interesting but it will be approved only when data on the beam background in normal running conditions will be available
In any case, in order to have AFP definitively running, RESOURCES MUST BE FOUND!
September 10th 2014
M. Bruschi, INFN Bologna (Italy)

17. Backup
September 10th 2014
M. Bruschi, INFN Bologna (Italy)

18. Importance of ToF
Significance >100 for 0.1<m<1 also with no ToF
Factor 10 pile-up reduction in AFP for m~50 for ToF with Dt~10 ps
Conclusion: High performance ToF needed mainly for the high-lumi program (and not for the approved program in RUN2)
AFP trigger is instead needed for RUN2: but in this case also a system based on scintillators or Silicon/Diamond will be sufficient to fully exploit the approved program
September 10th 2014
M. Bruschi, INFN Bologna (Italy)
0.3
0.02

19. AFP Full Simulation September 10th 2014
M. Bruschi, INFN Bologna (Italy)

20. Full Simulation results
September 10th 2014
M. Bruschi, INFN Bologna (Italy)

21. The total cross section
September 10th 2014
M. Bruschi, INFN Bologna (Italy)

22. Tracking+Timing in one RP
AFP Detector System
Tracking+Timing in one RP
September 10th 2014
M. Bruschi, INFN Bologna (Italy)