A summary of “Chamonix 2011 LHC Performance Workshop”

Chamonix 2011 LHC Performance Workshop started last Monday, 24 January 2011. Next friday, 28 January, the final decision on the operation and performance of the LHC at CERN will be taking. The slides of the presentations, the majority only of technical interest, can be downloaded here. For another summary of Chamonix you could read Philip Gibbs, “Chamonix conference considers LHC running parameters,” viXra log, January 25, 2011.

Massimiliano Ferro-Luzzi (CERN), “LHC Operation – as viewed from the Experiments,” summarizes how the LHC experiments have perceived the 2010 run, including proposals for improvements during 2011. Out of ~6600 h of operation, only 1074 h of stable beams (p: 851 h , Pb: 223 h); 147 fills with stable beams (110 proton ones and 37 ion ones); the rest of the fills were for special activities, disturbing the activities of the Experiments. They complain that 45 /pb collisions at 7 TeV c.m. have been collected in order to assure that a 8 TeV c.m. physics start in 2011 is secure; maybe such verification could have speed up. They also claim that in 2011 the LHC operation must go up quickly to a peak luminosity of 2×10³²/cm²/s and then gradually increase to 10³³/cm²/s. Experiments also claim for very much much flexibility, since the 6 experiments at LHC have widely different scopes. Ferro-Luzzi knows how to make many enemies: Could they have reached 100 /pb of data instead of only 45 /pb if LHC operation has not been driven by machine protection? The Experiments proposal is to reach 300 bunches per beam as fast as possible. Current proposal is 3 weeks of commissioning plus 2 weeks of ramp up to 300 bunches (50-100-150-200-250-300). In summary, 2010 has been terrific and demonstrated the excellence of the LHC and of the people who built/commissioned/operated it, but 2011 could be the year of discovery. The Experiments claim for at least 5 /fb of data. But a hypothetical incident caused by an electrical arc during the 2011/12 operation (similar to that of september 2008) could seriously impact the LHC physics program: Corresponding risks must be carefully assessed.

Mike Lamont (CERN), “Operational challenges (Feed forward from Evian LHC operation workshop),” summarizes the outcomes of the Evian workshop, 7-9 December 2010; Mike Lamont (CERN), “Operational consequences of running at a higher energies,” examined the possibility of  collisions with 7 TeV, 8 TeV and 9 TeV c.m., concluding that 10 TeV is unsecure, but 8 TeV is not; see also Stefan Roesler, “What are the consequences of delaying the shutdown from 2012 to 2013 for Radiation Protection?” In summary, Lamont opinion is that during 2011 and 2012 the LHC must operate at 8 TeV c.m (4 TeV for each proton beam); during 2011 with a bunch spacing of 75 ns, to be reduced during 2012 down to 50 ns; the maximum number of proton bunches to be reached in 2011 and 2012 will be 936 and 1400, resp.; the number of protons per bunch approximately of 1.2×10¹¹; the resulting peak luminosity will be 6×10³²/cm²/s during 2011 and 2×10³³/cm²/s during 2012. After 200 days of physics operation, the estimated integrated luminosity will be 1-3 /fb in 2011 and  ~5 /fb in 2012. These values are either optimistic or pesimistic, depending on each one perspective.

Bill Murray, “How much Physics benefits from running at higher energies?,” compares the LHC discovery potentials for the main physics objectives as a function of center of mass energy (7 TeV, 8 TeV, and 10 TeV) and integrated luminosity. The star of LHC physics is Higgs discovery; in fact, the cross-sections for  gluon fusion has at least a factor of 10 advantage over quark fusion at Tevatron. Moreover, collisions with 8 TeV result in a 30% gain in Higgs searches with respect to those with 7 TeV. The greater advantage of the LHC over the Tevatron is for a low mass Higgs (below 120 GeV/c²) and for a very high mass one (above 200 GeV/c²). The hardest point is the lowest mass (around 115 GeV/c²). Among the five decay channels to be considered in such a case, the most promising at the LHC is H→γγ, a rare decay which cannot be studied at Tevatron, but for which ATLAS it is well suited; the decay H→WW (preferably WW→lνlν) is also a good option at the LHC, since the backgrounds to Higgs decays into WW and γγ are very small in pp collisions as compared with pp collisions in Fermilab; another interesting channel for a low mass Higgs at the LHC is H→ττ. For the search of a very high mass Higgs, the LHC prefer the decay H→ZZ (concretely ZZ→llll, ZZ→llνν, and ZZ→llbb) and H→bb; also, but more difficult decays for the analysis, are the productions ttH, WH, and ZH.

The Higgs discovery potential at the CMS experiment is very similar to that of ATLAS, as shown in the figure above. The most promising Higgs decays are very similar, so it is possible to combine the data from both experiments to duplicate the effective integrated luminosity. Hence, at the end of 2011 is reasonable to expect about 5/fb of data for Higgs searches (whose analysis will be published in 2012), and at the end of 2012 about 10 /fb of data (to be published in 2013).

Sensitivity of Higgs search with 5/fb at 8 TeV gives 3σ for masses on the interval 114 to > 500 GeV/c². As seen in the figure above, from a preliminary analysis for the ATLAS experiment using Montecarlo simulation, a 5σ discovery for a Higgs with very low mass (about 115 GeV/c²) will require more than 12 /fb, i.e., it is beyond the integrated luminosity expected at the end of 2012 run of LHC. The combination ATLAS+CMS, including optimized analysis techniques to improve the sensitivity, will result in a 5 σ discovery at the end of 2012 if 10 /fb collisions with 8 TeV c.m. are reached in each experiment. If only 5 /fb are recorded, then a 5 σ discovery at the end of 2012 is possible if the Higgs mass is in the interval 120-500 GeV/c². In the worst case, if only 2 /fb for experiment is recorded, a 5 σ discovery at the end of 2012 is limited to a Higgs mass in the interval 130-200 GeV/c².

Obviously, the discovery potential of the LHC with 8 TeV c.m. until the end of 2012 also includes SUSY particles, W’/Z’ bosons, fourth generation, etc. Next friday we will learn the ultimate fate of the LHC during next two years.

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