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

ATLAS and its Forward Detectors AFP LUCID September 10th 2014 p-p @14 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

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)

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)

Elastic scattering with the ATLAS-ALFA detector arXiv:1408.5778 [hep-ex] September 10th 2014 M. Bruschi, INFN Bologna (Italy)

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)

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

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

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)

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)

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)

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

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

High lumi program Exclusive Jets γ September 10th 2014 QUARTIC Gauge Couplings – testing BSM models Reaching limits predicted by string theory and grand unification models (10-14-10-13 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)

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)

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)

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

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

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

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

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

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