How many inverse femtobarns at 7 TeV to find Higgs?

[petergreat]

If the Higgs mass is around 115 GeV, how many inverse femtobarns at 7 TeV are needed to find the Higgs?

 

[Bill_K]

A lot. As I'm sure you know, a Higgs with a mass of 115 GeV is the hardest to detect, and everyone hopes it is somewhere else! With a mass of 115 GeV,and assuming by "find" you mean a solid 5-sigma discovery, the lowest estimate I've seen is 20/fb. By that time, the LHC will have undergone the long shutdown of 2012 and hopefully will be running at near max power, 6 TeV per beam.

 

[Vanadium 50]

A very large number. Perhaps 30-50.

 

[petergreat]

Can I find a graphical plot showing the required integrated luminosity (at 7 TeV) against the Higgs mass, for various confidence levels?

 

[petergreat]

I've found such a graph on Tommaso Dorigo's blog:
http://www.science20.com/quantum_diaries_survivor/lhc_will_run_7_tev_2011_and_2012-75788
The two graphs are at the bottom of the page. According to the graphs, if we are optimistic and assume the LHC gathers 5/fb at 7 TeV this year, and another 5/fb at 8 TeV next year, then even a 115 GeV SM Higgs can be "discovered" at above 3 sigmas.

 

[petergreat]

Dorigo also says that the graph is for CMS only. Combining data with ATLAS typically turns 3 sigma into 4 sigma. On the graph, 5/fb at 7 TeV gives a 2.7 sigma significance for a 115 GeV Higgs. Currently CERN expects to gather 2-4 /fb for this year alone, and more optimistic estimates are abound. So by combining data from both detectors, it's likely that even a 115-GeV Higgs will show up at above 3 sigma before shutdown at the end of 2012.

 

[Vanadium 50]

Petergreat, I am beginning to worry that your university is not properly teaching you how to find information from primary sources. There are things you ask about and should be able to find directly on the experiments' web sites, but you are looking at people's blogs - and for that matter you're looking here. PF is many things, but we are not a primary source.

You need to know how to use the PDG, Spires, HEPData, arXiv and the major experiments' web sites.

Now, let's take a look at your analysis. CMS shows a 1.2 sigma expected significance for a 115 GeV Higgs with 1 fb-1. The standard for discovery is 5, so CMS needs roughly (5/1.2)^2 = 17+ fb. ATLAS is reporting a sensitivity less than CMS, requiring a little less than 2x as much data as CMS for the same significance. On the other hand, ATLAS is the only group that has made public their level of background. Also remember that something like 80-90% of the data delivered ends up used by the experiments in the final plots. Hence 30-50. 30 would be lucky, 50 would be unlucky, the truth is probably in between.

As for what's delivered, the best week has the LHC averaging 20 pb-1/day now. Assume it runs twice this well for 18 more months and you get 11 fb-1. So it doesn't really matter if we are talking 17, 30 or 50.

 

[petergreat]

Thanks for your advice. Actually I had a look at PDG's 2010 review on Higgs search, where the graphs are showing discovery potential at 14 TeV rather than 7 TeV, then I turned to Tommaso's blog... I understand that 5 sigma is the gold standard for discovery, however I was a little impatient and figured out that 3 to 4 sigma is more or less guaranteed in 2012, which is short of a discovery, but IMHO enough reasons for excitement and enough to tell us the approximate mass of the Higgs if the signal is genuine.

As for what's delivered, the best week has the LHC averaging 20 pb-1/day now. Assume it runs twice this well for 18 more months and you get 11 fb-1. So it doesn't really matter if we are talking 17, 30 or 50.

According to the LHC website, on 1 May the luminosity was increased to 8.4e32 /cm^2/s, which corresponds to roughly 70/pb per day. Is this to be sustained? Where can I find up-to-date information on the latest weakly average?

 

[Bill_K]

The official LHC luminosity data is here: http://lpc.web.cern.ch/lpc/lumiplots.htm . Expecially note the link at the bottom of this page where the exact numbers are tabulated.

 

[Vanadium 50]

According to the LHC website, on 1 May the luminosity was increased to 8.4e32 /cm^2/s, which corresponds to roughly 70/pb per day.

Not even close!

You can't get the expected luminosity for a year by taking the best instantaneous luminosity, and multiplying that by the number of seconds in a year. First, the luminosity falls, and second the fraction of time the machine is colliding is nowhere near 100%.

 

[jason12345]

According to the LHC website, on 1 May the luminosity was increased to 8.4e32 /cm^2/s, which corresponds to roughly 70/pb per day. Is this to be sustained? Where can I find up-to-date information on the latest weakly average?

At most, they run the LHC for 18 hours for a good fill where currently it can collect 40/pb. Potentially they could double this to 80/pb per day over the next few weeks, but let's say they manage potentially 50/pb per day. There is a Hubner factor which gives the fraction of time available for collecting data which has been estimated at 0.3 based upon runs during 2010. So with 125 days of running, this gives the total integrated luminosity as 0.3*50*125 = 1.9/fb.

The hope was to collect 1/fb during 2011 based a Hubner factor of 0.2 since they felt 0.3 was too optimistic and it looks as if they should easily do this since they have already collected over 300/pb.

They're in the middle of a technical stop which finishes on Thursday, and you can see how things are progressing at:
http://lhcdashboard.web.cern.ch/lhcdashboard/

and clicking on the four links under Simple LHC Lumi Calculator
http://lpc.web.cern.ch/lpc/

 

[Bill_K]

I've found such a graph on Tommaso Dorigo's blog:... On the graph, 5/fb at 7 TeV gives a 2.7 sigma significance for a 115 GeV Higgs.

I stand by my original statement: 20/fb.

What is apparently the same graph, marked "CMS Preliminary: Oct 2010", appears in the Chamonix 2011 conference paper by Bill Murray. Apparently the same, EXCEPT - that there it contains an additional curve: 10/fb at 7 TeV. And even that curve falls well short of 5 sigma. Moreover, what is clear from comparing the 1/fb, 2/fb, 5/fb and 10/fb curves is that they are equally spaced. (As they should be.) And to get to 5 sigma at a Higgs mass of 115 GeV, you'll need to at least double the luminosity yet again, to 20/fb.

 

[petergreat]

Thanks for all the information. So Higgs still may be quite a few years away. Kind of boring, but definitely good news for people who like to keep playing with all sorts of speculative models.