LumiCal background and systematics atCLIC energy

I. Smiljanić, Vinča Institute of Nuclear Sciences

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Layout of the Forward Region

I. Smiljanić, Vinča Institute of Nuclear Sciences

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

Luminosity Calorimeter

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

(Every fourth segment is drawn)

I. Smiljanić, Vinča Institute of Nuclear Sciences

Luminosity measurement at ILCIntegrated 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 is bias of

I. Smiljanić, Vinča Institute of Nuclear Sciences

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

Programs

I. Smiljanić, Vinča Institute of Nuclear Sciences

• 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

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

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

WHIZARD BDK

500 GeV 0.534 nb 4.467 nb

1 TeV 0.239 nb 5.490 nb

3 TeV 0.036 nb 5.902 nb

Cross sections for eeee

I. Smiljanić, Vinča Institute of Nuclear Sciences

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.

beam

jirel E

EEE

2

I. Smiljanić, Vinča Institute of Nuclear Sciences

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*.

Asymmetric cuts

* Cécile Rimbault (LAL Orsay, France), Impact of beam-beam effects on precision luminosity measurements at the ILC, LCWS 07

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.

I. Smiljanić, Vinča Institute of Nuclear Sciences

Asymmetric cuts + relative energy cut

This is what I would like to see @ 3 TeV

I. Smiljanić, Vinča Institute of Nuclear Sciences

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

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

Backup slides

I. Smiljanić, ILC ECFA Workshop, Warsaw, 9-12 June 2008

Asymmetric cuts + relative energy cut

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…