25 March 2013

1. Questions from the CLIC accelerator team(D. Schulte, LCD “monthly” 25 Feb. 2013)-> a first attempt to answers 25 March 2013

2. Energy stages • How many energy stages (in terms of CLIC construction) / which ones ? • For each energy stage, which energies would you want to operate at (eg. build for 700, operate at 375 GeV, etc) • Answers come in two parts: • [A] considering what we know today (i.e. the Higgs) • [B] all other possible scenarios (new particles found at LHC, etc.) 25 March 2013

3. Energy stages • How many energy stages (in terms of CLIC construction) / which ones ? • For each energy stage, which energies would you want to operate at (eg. build for 700, operate at 375 GeV, etc) 25 March 2013

4. Energy stages • How many energy stages (in terms of CLIC construction) / which ones ? • For each energy stage, which energies would you want to operate at • (eg. build for 700, operate at 375 GeV, etc) • Answers come in two parts: • [A] considering what we know today (i.e. the Higgs): • Stage 1: construct 375 GeV operate 375 GeV • (plus the tt(bar) scan 344-353 GeV • - 10 steps with 10 fb-1 each) • Stage 2: construct 1.4 TeV operate 1.4 TeV • (reason: ttH somewhat less b.g. at 1.4 vs. 1 TeV, • triple Higgs clearly better at 1.4 TeV) • Stage 3: construct 3 TeV operate 3 TeV • (reason: for physics, go to highest possible energy; • [B] all other possible scenarios (new particles found at LHC, etc.) • -> we have no answer for now

5. Luminosity • For each of the stages above, what should be the integrated luminosity in the 1% peak / in the total luminosity spectrum (which one matters most?) • What should be the criterion against which to optimise the CLIC accelerator? Same methods as for CDR Vol. 3 ? • For this bullet, we would like to first clarify “what is the question” • For the lumi in the total luminosity spectrum, we assume: • 375 GeV 500 fb-1 • 1.4 TeV 1.5 ab-1 • 3 TeV 2 ab-1 • From our present knowledge, based on the analyses with the luminosity spectra provided by the machine, we would always put the highest weight on total integrated luminosity – if more luminosity can be shifted to the peak for constant integrated luminosity, that would of course be welcome…

6. Beamstrahlung • Do we prefer a different Luminosity Spectrum w.r.t. CDR ? • More Lumi in the peak? Less gg -> hadrons ? • Even when performing the tt(bar) threshold scan, the key issue is total luminosity – Higgs physics data will be collected at the same time, and this has priority. • In our analyses so far, we have taken the gg-hadrons as “given”, and have managed to get good physics results in spite of this background. In one study (Fig. 12.9 of the CDR) one can see the effect of doubling the number of gg-hadrons events. • Present knowledge • -> higher total luminosity has priority over lower rate of gg-hadrons • . • In our analyses so far, we have taken the gg-hadrons as “given”, and have managed to get good physics results in spite of this background. In one study (Fig. 12.9 of the CDR) one can see the effect of doubling the number of gg-hadrons events. • Present knowledge -> higher total luminosity has priority over lowering the gg-hadrons

7. Beamstrahlung • Do we prefer a different Luminosity Spectrum w.r.t. CDR ? • More Lumi in the peak? Less gg -> hadrons ? • As one can see in a recent paper (http://arxiv.org/abs/1303.3758), different luminosity spectra will not give very different results on the top mass from a threshold scan: • CLIC error on top mass 34 MeV • ILC 27 MeV • Comment from Frank Simon: • (a) Knowledge of the width of the peak in the lumi-spectrum is important, we want this to better than 20% ; • (b) Knowledge of the total luminosity to 1% is important • The figure (next page) shows the comparison of CLIC and ILC case, for 350 GeVc.m. operation(from LCD-Note-2012-013).

8. CLIC ILC

9. Other points • Do we need to consider more than one scenario (e.g. for the intermediate energy stages) depending on hypothetical findings (or not) at LHC ? • Intermediate stages / other intermediate energies would not be motivated from physics as far as we know today, but rather from accelerator considerations. • For example, the second stage CLIC energy could be higher than 1.4 TeV if there would be a klystron-based first stage (this investment could be used to go to > 1.4 TeV) • Do we need to consider a scenario without and another scenario with an ILC in Japan? • We do not think so – if there is an ILC in Japan, the first stage of CLIC would probably be dropped, and one would propose to go straight to the 1.4 TeV (or equivalent, see above)

10. Polarisation • Although not a major parameter for the re-baselining studies of CLIC, would be good if we could specify what we expect/assume in terms of electron and positron polarisation • Some CLIC studies have looked into the issue / need for polarisation: • HHH at 1.4 TeV and 3 TeV (see annex to CLIC contribution to European Strategy Update, EDMS doc. No. 1235623) - PRELIMINARY: • improvement on cross section vs. unpolarised case: • 20% if e- polarisation 80% • 30% if e-pol. 80%AND e+pol. 30% • Smuons (see LCD-Note-2012-012): • Analysis done for unpolarized case; • Table given for cross sections for polarisation • Z’ (see LCD-Note-2012-009): • Analysis done for 80% e- polarisation, 0% e+ polarisation

11. Polarisation (cont.) • Chargino/neutralino(see CDR Vol. 2 Fig. 1.19)

12. Polarisation (cont.) • ttHat 1 TeV (see SiD DBD) • (from Philipp Roloff) • accuracy on top Yukawa coupling yt • 1 ab-1unpolarised or 80/20 mixed -> Dyt/ yt = 4.5% • 1 ab-1 e- 80% e+ 20% -> Dyt/ yt = 4.0% • see also message from M. Thomson, attached in Indico • Summary: 80% electron polarisation is a “must”; • 20 to 30% positron polarisation would be helpful