Today at 21:10 hours European Time the Large Hadron Collider passed an important milestone when it reached 1/fb of integrated luminosity delivered to each of the large experiments CMS and ATLAS. The third major proton experiment LHCb which limits its luminosity has around 0.35/fb. These figures include the 47/pb delivered in 2010, but after another one or two good runs the total for 2011 alone will also surpass the one inverse femtobarn milestone.
Update 14-Jun-2011: With another good run today the total delivered for CMS passed the 1/fb mark for 2011 data alone at about 20:25.
This is an impressive achievement for the worlds most powerful particle accelerator at CERN which had originally expected to collect this amount of data only by the end of 2011. Better than expected performance now means that it records about 30/pb worth of collision data each day on average. With about 120 days of proton physics left this year, the beam operations team can expect to deliver at least 4.4/fb this year if they continue at this rate.
Potential luminosity increases
There is still some potential to increase this figure if they can continue to increase the number of protons circulating in the rings. The current quantity of 1092 proton bunches circulating in each direction will shortly be increased to 1236 and then finally 1380 once they overcome difficulties with power to the RF systems. This will increase the luminsity by 25%. Another goal will be to increase the efficiency by keeping the collider in Stable Beams for longer. Recent figures show that this state is only achieved for around 40% of the time due to a variety of technical issues. As these are sorted the figure may go up to 60% or even higher to give 50% more data. If this can be done quickly it would bring the expected total for 2011 up to around 7/fb. At 1380 bunches the rings are full to capacity with current bunch spacing so further improvements this year are only possible if the bunch intensity can also be increased, but this is not yet planned. 5/fb to 7/fb is already a spectacular number to aim for and they may not want to put these numbers in danger by taking such risks.
Expected conference announcements
The amounts of useful data recorded by the experiments is typically 90% to 95% of the amounts delivered. In a few days these figures will also pass 1/fb and this should be in time for the next big particle physics conference EPS-HEP2011 at the end of July. At last weeks PLHC2011 conference we already saw a few results using 200/pb, one fifth of the current standing. However, there are many standard searches for which we have still only seen plots using the 40/pb collected last year. For example the classic dimuon resonance curve has not yet been shown in updated form for either ATLAS or CMS. This could be because it was too dull to show. The dimuon signal is very clean but it is not expected to be the first place where new physics will appear. On the other hand, it may not have been shown for the opposite reason. If it had an inconclusive signal of a new resonance then they would surely want to wait for more data before showing it.
When will the Higgs be seen?
To get a better impression of just how significant the quantities of data now being collected are, it is useful to look at the projected Higgs Limits as shown in this figure.
With 1/fb of data there will either be a signal or an exclusion for the Higgs boson above 130 GeV. If it is excluded then it will be known to lie between the 115 GeV limit previously set by LEP and a new limit of 130 GeV. This is highly significant because the standard model on its own predicts that the vacuum would be unstable if the Higgs has a mass less than 135 GeV. New particles such as those predicted by supersymmetry would be needed to restore stability. In other words, this exclusion expected from 1/fb would be indirect evidence of physics beyond the standard model at the electro-weak scale. The EPS-HEP conference is likely to be a historic event where they will either describe the first signals for the Higgs Boson or the first good evidence for new BSM physics. If you want to attend today is the last day to register at the standard fee or 350 Euro!
If EPS-HEP is an anti-climax, the next big particle physics conference is Lepton-Photon 2011 during the last week of August. Another femtobarn of data will be available for analysis in time for that. There are smaller workshops and seminar opportunities going on all the time so a new discovery can come at any moment, but the physicists do like to time their results for these big events.
By the end of the year the situation will be even more dramatic. About 5/fb should be available, enough to exclude the Higgs boson at all masses, or more likely to discover it. If it is indeed a light Higgs there is a good chance that some other new particles will be discovered too.
What if it does not show up?
The calculations that are used to calculate the exclusion limits are themselves based on assumptions that the Higgs will decay according to the predictions of the standard model. If the standard model is ruled out by not seeing the Higgs then these calculations will themselves be invalid. For example, a possibility is that there are heavy unknown particles into which the Higgs could decay. If these new particles produce jets that are hidden in the sea of background QCD it may be much harder to detect it. Another even weirder possibility is that the Higgs boson just isn’t there. If nature is devious enough we could still see no new particles this year.