From: James Wells Subject: Re: 400 GeV or 500 GeV? Date: January 25, 2012 1:24:15 PM GMT+01:00 To: Mark Thomson Cc: Lucie Linssen Greetings from Germany where I am giving seminars this week (will be back at CERN on Monday). It's too bad that there is not a semi-definitive study of how well HHZ can be done. If it could be done at energy of peak cross-section (maybe 550 GeV, or so, including beamstrahlung?), that would be extremely powerful. Is there anything that can be done "quickly" with the only goal to have a better idea if it could be done in principle? (My guess is no, but had to ask...) Best, James -- James Wells, CERN, +41 22 767 2449 On Tue, Jan 24, 2012 at 9:09 PM, Mark Thomson wrote: Hi Lucie, There isn't a strong physics arguments for 400 compared to 500 GeV and I would not argue strongly for 500 over 400 GeV.  I would expect the Higgs BR precision to be very similar. Higgs coupling at 500 GeV will be very hard (although we shouldn't give up all hope). The only real loss is in new physics reach were the potential for low lying sleptons/gauginos are not ruled out - but again this is not a strong argument. If the machine are pushing for 400 GeV then that's fine, provided it is 400 GeV i.e. sufficiently above the top threshold to allow some safety factor if thing don't work out well. If 400 GeV means 360 GeV, I would have some concerns. When we hopefully get to build CLIC, what goes into CDR volume 3 is unlikely to carry much weight.  So at this stage, it is really a political/strategic decision as to what best sells the machine. I would be slightly wary of coming in with a 400 GeV machine that costs more than the 500 GeV ILC, but I haven't seen the current cost estimates. If the cost considerations argue strongly for 400 GeV then I don't think we should push against this. As a slight aside, I don't think the 250 GeV machine is really relevant to the CLIC discussion. Assuming the Higgs is discovered, there will be a strong physics case for a 250 GeV machine built as soon as possible, and the current excitement is that there is a sense that this might be the way to get the ILC built on a reasonably short timescale. It is less clear to me that such an option will seem all that appealing when there is an opportunity to build CLIC. I hope that helps a bit. cheers, Mark p.s. I have just booked another trip to CERN to coincide with the next WG6 meeting and will be around on the 14th, 15th and most of the 16th Feb. On 24 Jan 2012, at 18:46, Lucie Linssen wrote: Dear Mark, I am sorry, but it seems that we are still iterating on the 400 GeV or 500 GeV choice. Yesterday James and myself were invited to a meeting with Steinar Stapnes, Daniel Schulte and Philippe Lebrun. Philippe and Steinar were expressing some difficulties in understanding our 500 GeV choice. Philippe is heavily involved in the CLIC accelerator costing, so he looks from that end. They now hear that everyone wants a very low energy e+e- machine, so our 500 GeV seemed to go in an opposite direction. We agreed, however, that 250 GeV was not a good choice for an initial CLIC stage, as it would force CLIC to propose two low-E stages in order to cover some Higgs and top physics. The discussion then concentrated on the choice between 400 GeV and 500 GeV. The 400 GeV choice can be explained easily with very simple physics arguments. The 500 GeV choice is more tricky to explain. The stretch in scope for real new physics is not very large, so the explanation has to come from the Higgs measurement potential. ====== Following your recommendation, I browsed a bit through the January 2011 ILC workshop at SLAC: http://ilcagenda.linearcollider.org/conferenceOtherViews.py?view=cdsagenda&confId=4612 Higgs mass measurement (Z recoil measurement): Better precision (factor ~3 better for similar luminosity) on the Higgs mass at 250 GeV ILC than at 350 GeV ILC, despite a significantly better signal/noise ratio at 350 GeV compared to 250 GeV. The measurement of the cross section for the recoil process is also a bit better at 250 GeV than at 350 GeV. Higgs branching ratio to bbar abd ccbar For the same luminosity the result of sigma(ccbar)/sigma(bbar) is 25 % better at 350 GeV than at 250 GeV. This is mainly due to a better separation between signal and background. This improvement comes in particular from the Z=>nunu and the Z=>qq decays, while the Z=>ll has a slight advantage at 250 GeV. (The flavour tagging quality is similar at both energies. The background from top at 350 GeV does not have an impact on the result.) This confirms the argument that Mark was giving in favour of a higher energy for the branching ratio measurement. (Sorry, I did not mean to check on you.) ====== In case we want to defend our 500 GeV preference, we probably need to have an additional argument. In my view, the only possible candidates are the Higgs coupling measurements using ZHH and the ttH. This is also indicated in Keisuke Fujii's talk here: http://ilcagenda.linearcollider.org/materialDisplay.py?contribId=243&sessionId=25&materialId=slides&confId=5134 However, both channels really seem very marginal at 500 GeV. The question is: are these channles really sufficiently convincing at 500 GeV, or should we accept 400 GeV for the first stage? In case we want to retain the 500 GeV option for the first stage, it seems to me that we necessarily have to include some study of ZHH and the ttH at that energy. Please give your opinion (again!), Regards,    Lucie Prof. Mark Thomson E-mail: thomson@hep.phy.cam.ac.uk Phone: +44-1223-765122 (Cavendish)             : +44-1223-762332 (Emmanuel College)             : +44-7512-250090 (Mobile) Fax     : +44-1223-353920 http://www.hep.phy.cam.ac.uk/~thomson/