24 Hours of the LHC vs 24 Years of the Tevatron

In the last 24 hours the Large Hadron Collider has delivered a record 43/pb worth of collision data, equivalent to the entire proton physics run during 2010. Almost all the published results from the LHC so far are based on that amount of data, (with the single exception of one report needed to squash a recent rumour)   This provides a convenient opportunity to compare 24 hours of data from the LHC with 24 years from the Tevatron.

The Tevatron which began running in 1987 has of course made some significant discoveries including the top quark and some real hints of physics beyond the standard model. It has also excluded the Higgs over a wide mass range. The LHC has so far made no new discoveries but with its 43/pb of 2010 it has surpassed the Tevatron in searches for heavy quarks, supersymmetry and other exotic physics, placing limits well beyond the reach of the Tevatron. This superiority at heavier mass scales is due to its higher proton energy. In many ways, 24 hours of LHC data is more interesting than 24 years worth of Tevatron data. Luckily we are not forced to make the difficult choice between the two possibilities, we have had both.

There was some controversy when it was announced that the Tevatron would stop running this year and it was a decision based on lack of government funding rather than the wishes of the Fermilab directors. By time the summer conferences of 2011 have passed CDF and D0 will have presented their last results from the Tevatron worth listening to and the LHC will have surpassed them in all channels. The Tevatron will cease its work of colliding hadrons and Fermilab will concentrate on its admirable neutrino physics program and the ambitious plan to design and build Project X, a high intensity proton accelerator that could lead one day to a future muon collider and leap frog to the next scale of physics.

To help appreciate the formidable success of the LHC it is worth looking back to its status one year ago and the predictions for its future running. They were summarized in this table presented at ICHEP 2010.

As you can see from the first line, they planned to reach luminosities of 188/μb/s during 2010 and run at that level during 2011 to collect 1/fb before the long shutdown of 2012. They intended to do this using a squeeze of 2m, and 796 bunches with 75ns spacing. This plan changed quickly in the Autumn of 2010 when they found that the running of the LHC was much better than expected.

They thought they would only be able to get bunch intensities of up to 80 billion protons as shown on this table. In fact they quickly reached nominal intensities of 115 billions protons with no difficulty. This year they have pushed the limit higher to 170 billion, well over twice what they thought was possible before interactions between bunches would take their toll, leading to instabilities. The emittance of the beams was also looking good. They had predicted an emittance figure of 3.75 μm, a measure of the distribution of particle momentums away from the ideal in each bunch. Recently they have dropped the emittance as low as 1.5 μm in development tests.

Given this unanticipated performance they adapted the plans to suit. Last year they used a squeeze of just 3.5m and a larger bunch spacing of 150ns. With these less ambitious parameters they were still able to exceed target luminosities reaching 210/μb/s in 2010. This left them open to the possibility of much better luminosities for 2011. In fact they have gone beyond the original plan on all fronts using an even tighter squeeze of 1.5m and 50ns spacing to allow up to 1380 bunches in each ring. The result is luminosities already peaking at 1100/μb/s and likely to end at 10 times higher than those expected when the above slide was prepared a year ago.

With the extra data it became worth extending the early run of the LHC into 2012, with the long shutdown now scheduled for 2013. By then it is likely that the LHC will have produced enough data to discover the Higgs if it is there, whatever mass it has. There is always the possibility that nature has it in store for us to see nothing because it is not there, but as Lubos Motl put it “if nothing “new” is found, it will be a bizarre situation in which “new physics” has to exist but its goal is to suppress the number of interesting and qualitatively different things you can see.”

The early discovery of the Higgs will mean that the LHC has achieved its primary goal. With the pressure off, the engineers and physicists of the collider teams will be able to concentrate on the upgrades for the long shutdown that itself is likely to be extended to include further enhancements. The LHC will return towards the end of 2014 ready to explore even higher energies with even higher luminosities. If nature is kind there will be much to learn.

Finally you may ask how did the LHC manage to get to such an advanced position ahead of schedule compared to the many years it took the Tevatron. The answer is of course that the LHC has built on lessons learnt from the Tevatron and other predecessor accelerators such as LEP, HERA and KEK. There is only one international scientific community with a common goal of understanding nature. The physicists who work on these machines stay in the game for many years and move from one project to the next crossing borders without any regrets and bringing their expertise with them. The LHC uses magnets built at Fermilab and many of its systems such as the cryogenics are direct descendants of the pioneering systems built for the there. The Tevatron may be yesterdays gadget but the LHC could not have been created without the knowledge it brought.

Update: Here are some of the people from Fermilab celebrating 11/fb (from facebook today)

Higgs exclusion plots presented so far have only used about 8/fb, so there is always some hope for a last minute revelation before they are eclipsed by the LHC, good luck!

8 Responses to 24 Hours of the LHC vs 24 Years of the Tevatron

  1. [...] alcanzar en 2010 los 210/μb/s de luminosidad. Nos lo cuenta, como no, Philip Gibbs, “24 Hours of the LHC vs 24 Years of the Tevatron,” viXra log, May 24, 2011. Más información actualizada sobre las noticias en el LHC en [...]

  2. Luboš Motl says:

    A detailed text and a wise message.

  3. Rafael de Lima says:

    1) Project X will not be a muon collider, but a high intensity proton accelerator http://projectx.fnal.gov/ . The proposed FNAL Muon Collider is still in feasibility R&D.

    2) As you said, the LHC is already ahead of Tevatron on the high mass end of the spectrum. But for lighter particles, due to the smaller background, the later still has more sensitivity today. For example, if there is a light SM Higgs, it’s much more likely to be found at Fermilab than at CERN, at least for the next one or two years. After that, of course, the LHC will be unreachable.

    • Philip Gibbs says:

      Rafael, It may be more accurate to say that project X is a high intensity proton accelerator, but its ultimate role will be as the front end for a muon collider :)

      The LHC will have enough data to beat the Tevatron on Higgs searches in a few weeks and I predict that they will present wider exclusions in time for EPS HEP 2011 in July.

      Although the Tevatron may be theoretically better at searching for the Higgs at 115 GeV, this is a rather moot point since it will not now run long enough to explore that limit. The LHC on the other hand has a chance to observe a Higgs at 115 GeV in 2011. Its reach is looking much better than it did even just a couple of months ago.

      • Rafael de Lima says:

        The Muon collider project is still in a very early stage, maybe they find out they can’t actually do it but, nevertheless, Project X will have it’s place.

        Luminosity for Higgs, at this point, is not that of a deal, since heavy analysis, which takes a lot of time, has to be done. For example, people at Fermilab experiments are still analysing data that was taken long ago, even already presented as the last limits, and finding out they can improve their analysis.
        So, probably for next year’s summer, DZero and CDF might have statistics to exclude a good chunk of the SM light Higgs.

        Bottom line, luminosity is nothing without a long, dark period of analysis and re-analysis…

      • Philip Gibbs says:

        Point taken about project X and I modified the main text accordingly

        Apart from higher energy, luminosity, beam lifetimes etc, the LHC have beaten the Tevatron in one other important metric. They get their results out much quicker. Initial results have been presented within weeks of data collection and anything of significance from 2010 data was published within a few months. Some of the latest important results from CDF and D0 such as the forward-backward asymmetry have only used about half the data now collected. Presumably they could turn that 3.2 sigma effect into a hard discovery with the data they have been sitting on for months if not years. Perhaps the LHC collaborations will be more careful when they have more positive results to announce but they do seem to have a significantly different attitude to publishing quick results. It would be sad if the Tevatron found something interesting in their data long after the LHC has announced its discovery with newer data.

  4. Bill K says:

    “… [smaller background at lower energy]…”

    When you consider the background, it’s a strange business in any case, looking for weakly interacting particles by colliding strongly interacting particles.

    You’re right in that lower energy means smaller background. But at least today we are running at “only” 3.5 TeV, which from that consideration is better than 7. If it was really necessary they could dial it back some. No one is suggesting that, of course. They did do proton collisions for a while at 1.38 TeV, if you recall, at the request of ALICE.

    But I guess LHC’s philosophy is, even if higher energy means a larger haystack, you might find something else besides needles!


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