LumiCal background and systematics at CLIC energy

1. LumiCal background and systematics at CLIC energy I. Smiljanić, Vinča Institute of Nuclear Sciences

2. Layout of the Forward Region I. Smiljanić, Vinča Institute of Nuclear Sciences

3. Calorimetry in the Forward Region • LumiCal • Precision luminosity measurement L/L~10-3 (10-4 GigaZ) • BeamCal • Beam diagnostics, veto to SM processes in new particle searches (SUSY)‏ • GamCal • Beam diagnostics, instantaneous luminosity measurement CHALLENGES: High precision, high radiation dose, high occupancy, fast read-out I. Smiljanić, Vinča Institute of Nuclear Sciences

4. Luminosity Calorimeter (Every fourth segment is drawn) Geometry: • rmin = 80 mm • rmax = 195 mm • tungsten thickness = 3.5 mm • silicon thickness = 0.3 mm Segmentation: • 30 layers, 64 radial divisions, 48 azimuthal divisions; • azimuthal cell size -131 mrad; • radial cell size - 0.8 mrad; • z position = 2270 mm I. Smiljanić, Vinča Institute of Nuclear Sciences

5. Luminosity measurement at ILC Integrated luminosity can be determined from the total number of Bhabha events produced in the acceptance region of the luminosity calorimeter and the corresponding theoretical cross-section. Note that Dq is bias of q. I. Smiljanić, Vinča Institute of Nuclear Sciences

6. The aim of study The aim of this work is to test background to signal ratio in LumiCal at CLIC energy. Signal is originating from simulated Bhabha events and background is simulated using WHIZARD and BDK generators. All simulated events will go through full detector simulation using Mokka package. I. Smiljanić, Vinča Institute of Nuclear Sciences

7. Programs • BARBIE (fast LumiCal simulations) – GEANT based FORTRAN routines defining geometry, materials and physical properties; built-in BHLUMI generator for generating Bhabha events; • WHIZARD - designed for the efficient calculation of multi-particle scattering cross sections and simulated event samples. WHIZARD supports the Standard Model (optionally, with anomalous couplings), the MSSM, Little Higgs models, Zprime models, and supports gravitinos and gravitons. • BDK - 4-f event generator. It includes gamma and Z exchange, but not W exchange. Only l+ l- f+ f- final states can be produced. • BHWIDE - event generator for Bhabha scattering at wide angles I. Smiljanić, Vinča Institute of Nuclear Sciences

8. Physics background from 2-photon processes Background electron spectators carry high energy going along the beam pipe, whereas low energetic ffbar pairs are mainly deposited in the LCAL. Simulations will be performed using 4-f (2 gamma) processes generated by BDK and WHIZARD generators. I. Smiljanić, Vinča Institute of Nuclear Sciences

9. How well do we know B/S? • 2-gamma topology is described differently with WHIZARD and BDK • Simulation studies are influenced by statistics • 4-f (2-gamma) cross sections described differently with different generator (WHIZARD vs. BDK) • We do not (always) simulate all processes (i.e. hadronic background)‏ • Different cross-sections (in particular for signal) at 500 GeV and 1 TeV can be taken into account through simple scaling Cross sections for I. Smiljanić, Vinča Institute of Nuclear Sciences

10. Event selection • Bhabha events are identified by 2 electromagnetic cascades carrying the full beam energy, originating from collinear and coplanar Bhabha particles. • Characteristic topology of Bhabha events allows us to establish a set of criteria to distinguish signal from physics background. Criteria intended to use here are: • asymmetric angular cuts (next slide); later stage • relative energy, • No crossing angle. I. Smiljanić, Vinča Institute of Nuclear Sciences

11. Asymmetric cuts Signal and background will be additionally affected by the beam-beam interaction effects. They will modify both initial state, through beamstrahlung, and the final state through electromagnetic deflection, resulting in the total suppression of the Bhabha cross-section (BHSE) of order of 4.4%. In order to reduce this hard-controlled effect (to 1.5%), asymmetric theta cuts are used*. These cuts are applied subsequently to forward and backward sides of the detector, in order to reduce systematics for the IP position and relative position of forward and backward detector. LumiCal angular acceptance for geometry used is 35-87 mrad. Therefore, cuts are set as follows: • cut 1: 39-80 mrad; • cut 2: 35-87 mrad. * Cécile Rimbault (LAL Orsay, France),Impact of beam-beam effects on precision luminosity measurements at the ILC, LCWS 07 I. Smiljanić, Vinča Institute of Nuclear Sciences

12. Asymmetric cuts + relative energy cut This is what I would like to see @ 3 TeV I. Smiljanić, Vinča Institute of Nuclear Sciences

13. Current progress 10 000 ee→eell background events @ 3TeV cms energy generated by BDK now running in Mokka I. Smiljanić, Vinča Institute of Nuclear Sciences

14. Conclusion It will be very interesting to see the background behaviour at the CLIC energy, at 3 TeV cms. Open question at the moment is which program to use to generate background events. Perhaps, after asymmetric cuts applied, this question will become irrelevant, like in cases at lower energy (500 GeV and partly at 1 TeV). One of goals of this study is to check for that. I. Smiljanić, Vinča Institute of Nuclear Sciences

15. Backup slides

17. Asymmetric cuts + relative energy cut I. Smiljanić, ILC ECFA Workshop, Warsaw, 9-12 June 2008

18. More systematics … • Beam-beam interactions • Modification of initial state: Beamstrahlung√s’≤√s, ini≠ 0, Eelec≠ Eposit • Modification of final state: Electromagnetic deflection Bhabha angle reduction (~10-2mrad) + small energy losses Total Bhabha Suppression Effect (BHSE) ~1.5% Luminosity spectrum reconstruction • To control the BHSE from beamstrahlung at the level of 10-2, variations in the rec. lumi spectrum x/x need to be known with the precision of 4.10-3 Beam parameters control • Bunch length z and horizontal size x should be controlled at the 20% level to keep the BHSE from EM deflection at the level of 10-3 QUITE A TASK IN REALISTIC BEAM CONDITIONS…