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A shot in the dark ? October 6, 2007

Posted by dorigo in personal, physics, science.

Yesterday I was in Bari, where I followed a meeting with some members of the CMS-tracker italian group. At the meeting we discussed a research project aimed at developing new technologies which will be useful for  the upgrade of the detector whose construction our group heavily contributed: the silicon tracker.

The silicon tracker of CMS (of which you can see in the picture the inner layers, called TIB -for Tracker Inner Barrel- being assembled last year) is a fantastic device, and we confidently expect it to deliver what it was designed to, so why are we thinking about upgrading it if it has not seen more than cosmic rays so far ?

Well, experimental physicists specialized in the design of particle experiments have learned to think ahead – ten, fifteen years ahead: that is the time scale of today’s experiments. The Large Hadron Collider (LHC), which will provide proton-proton collisions at 14 TeV to the CMS and ATLAS experiments from next year onwards, cannot be easily upgraded to increase the beam energy it provides, but its instantaneous luminosity can be pushed up by a full order of magnitude by increasing the number of protons and the interaction rate.

Instantaneous luminosity is a quantity basically obtained by multiplying the number of circulating protons in each beam N_1, N_2 by the revolution frequency f, and dividing by the area A of the beams at the interaction point: L = N_1 N_2 f / A. The larger the number of particles in each beam and their crossing rate, and the smaller the area where they interact, the higher is the resulting luminosity. Luminosity is thus directly related to the production rate of a rare process: if we call \sigma the cross section for the process, we have simply that the rate is R = \sigma L.

If LHC works and finds new physics -that is a big if, but let me finish the sentence- an increase of the beam luminosity of LHC is useful in many scenarios: more luminosity means more data, and a larger discovery potential, plus smaller statistical errors on all measurements. Of course, after twenty years spent designing and building the LHC and the experiments, running for only a few years and then selling the pieces as scrap metal is not the best option! Better to think of an upgrade which can extend the lifetime of the experiments and increase their discovery potential, if possible.

While in principle an increase in beam luminosity is a good thing, the detectors that benefit from it must come prepared, because of several issues.

First, a high luminosity causes larger fluences of particles through the detectors,  which thus have to withstand a larger radiation damage. Of course, radiation dose goes inversely with the squared distance from the interaction point, so the closest devices must be the toughest. The silicon microstrip and pixel detectors of CMS are radiation hard, but tolerances are not allowing for an increase by an order of magnitude in doses. 

Second, the 100 interactions occurring every 12.5 nanoseconds in the core of the detectors during SLHC operation will cause a large occupancy of the detector components closest to the interaction region: this imposes constraints on a design capable of maintaining the current tracking performances. One would not like to collect ten times more data if the deal involved a detector performing much worse!

Third, muons – particles which are crucial for several searches of new physics – require a very good momentum resolution at trigger level in order to allow an efficient filtering of the few interesting events among the 80 MHz of interactions.  This is because of the steeply falling probability of finding a true muon as its transverse momentum P_T increases. We are interested in the high-momentum ones (derived from the decay of W and Z bosons, or Higgs bosons too), and so we select them with a cut P_T> X GeV, a threshold dictated by our limited ability to write events to tape.  Now, if there is even a slight chance -because of insufficiently good momentum resolution- that we mistake a low momentum muon for one passing the threshold we are dead, because there are so many more muons with the lower momentum that whatever threshold X we set, we will collect mostly the low momentum ones – filling our tapes with uninteresting stuff.  

The jury will soon be out to decide on a design for the upgrade of the CMS tracker that addresses all these issues. There are quite a few subtleties, many issues involving cost effectiveness, redundancy, triggering capabilities by the tracker alone, and so on. However, what I feel is most urgently needed is a preliminary assessment of the physics the upgrade will address. Nobody’s fault: it is simply impossible to make a really meaningful case for a LHC upgrade right now, if you ask me. That is: an upgrade is certainly a good idea, but which one, well…

Indeed, deciding now on an upgrade of LHC experiment seems to me a shot in the dark. We can only guess whether CMS and ATLAS will discover the Higgs, and whether they will find SUSY. We can argue on the likelihood of new unpredicted discoveries. But what one would need in order to decide whether design A is better or worse than design B, C, D…, in a scenario where LHC runs for a few years with a tenfold increase in luminosity, would be a clear idea of what we would gain in the study of a few flagship signals. And knowing whether SUSY is there or not makes the hell of a difference.

Sure, we could make a physics case with stuff we are almost certain we will find, such as the Higgs. To me that would be more meaningful than trying to sell a better determination of the full spectrum of SUSY particles or large extra dimensions. An example: Would we be able to see exclusive higgs production, and thus measure that particle’s quantum numbers, only with design A ? 

Knowing that would be something… But it looks unimpressive to somebody who has to decide whether to fund a multi-million-dollar project!



1. Plato - October 6, 2007

Very Interesting

2. anomalous cowherd - October 6, 2007

Dear Prof. Dorigo
Congratulations on the CMS silicon tracker. Having had tours of both CMS and ATLAS, I can report that it is really impossible to understand the scope of the accomplishment that construction of these detectors represents, until you have seen them with your own eyes.
Good luck on the commissioning, the running, and the upgrades!
We all eagerly await LHC data.

3. Paolo - October 7, 2007

Very interesting indeed. And many thanks for , of course! Hopefully, I will be able to contribute something to the discussion, but in the meanwhile at least a link I didn’t know about until a few days ago:


that is, this year VIDEO material too! Nima Arkani-Hamed is rather enjoyable also for now experts, IMHO. Highly recommended!

4. Paolo - October 7, 2007

Of course I meant “… many thanks for -Instantaneous luminosity- …”

5. dorigo - October 7, 2007

Thank you all. Indeed, the CMS and ATLAS experiments are among the most complex things ever built by humanity. I am serious…. One only understands it by visiting the experiments, which is a reason for visiting Geneva alone.

Paolo, Nima is a string theorist… I have no doubts he can give entertaining lectures, but I would advise you a very good book: “Not Even Wrong”, by Peter Woit. See the link to his site in my blogroll.


6. anomalous cowherd - October 7, 2007

5. dorigo – October 7, 2007 writes:

“Paolo, Nima is a string theorist…”

I’m not sure that either Nima, or mainstream string theorists, would characterize him as such. He was a Ph.D. student of Berkeley phenomenologist Lawrence Hall [himself a student of Howard Georgi]. He is a gifted and incredibly productive creator of phenomenological models of physics beyond the standard model; in my view he is the most influential phenomenologist of his generation. He has worked on models of: fermion masses and the flavour problem, hypercolour, supercomposite models, large extra dimensions, neutrino mass, supersymmetry, orbifold GUTs, deconstruction, little Higgs, inflation, ghost modifications of gravity, split supersymmetry, and signature driven Monte-Carlo analysis tools for LHC physics [with experimentalist Bruce Knutson, who had already implemented systems like this for the TeVatron experiments. Of his 75 papers, only a handful have had any string theory collaborators. Like his Harvard colleague Lisa Randall, he has frequently adopted the approach of taking ideas that had originated in string theory, and incorporating those ideas in phenomenological models [she too is one of the most influential phenomenologists of her generation].

To get a sense of the difference in interests and style between string theory and beyond the standard model phenomenology and model building, you could do a SPIRES search to compare the publications of the string theory group at Harvard [Strominger and Vafa] to those of the phenomenology group [Arkani-Hamed, Georgi, and Randall]. Or try the same for Stanford [Shenker, Susskind, Kallosh, Kachru and Silverstein, (strings), compared to Dimopoulos, Peskin, Quinn, Wacker, Hewitt, and Rizzo (phenomenology)]. You see some collaboration between members of the groups [which is a very positive thing], but on the whole their approaches and problems are different. This difference is often ignored in popular books whose publishers will put anything on the cover that they think will enhance sales.

P.S. And yes, Nima is an exceptionally charismatic lecturer, which is why he is in so much in demand for physics summer schools.

7. dorigo - October 8, 2007

Hi anomalous,

I will take your word for it – I must admit I was not aware of the breadth of his works. However, Nima’s position on the anthropic landscape of ST is enough for me to warn any student about his views.


8. Paolo - October 9, 2007

Tommaso, is it easy to visit the experiments? I’d like to follow your advice as soon as possible, Geneva isn’t a big deal of a travel for us Italians, anyway…

Otherwise, well, many thanks to AC for the details… what can I add, definitely from the actual lectures neither the landscape neither string theory appear as a dogma. I would say in the lectures there is something for everyone, a lot of “philosophy”, a lot of mathematics, a lot of genuine “physical intuition”. Personally, I had a lot of fun… and his walking incessantly from left to right and way back reminded me a lion in a cage, a LOT of energy

9. anomalous cowherd - October 9, 2007

8. Paolo – October 9, 2007 writes:

” is it easy to visit the experiments? I’d like to follow your advice as soon as possible”

– Go to: http://www.cern.ch
– Click on “For CERN Users”
– Click on “Media and Public Corner”
– Scroll Down
– Click on “Visits Service”
This will give you information on, and booking forms for, visits.

10. Paolo - October 9, 2007


11. Plato - October 9, 2007

Tammaso:However, Nima’s position on the anthropic landscape of ST is enough for me to warn any student about his views.

So is this support for what you think is relevant. I have followed the discussions between Lee Smolin, Jacques Distler, Clifford of Asymptotia and Peter Woit of “Not Event Wrong.”

I wonder if you had “more information,” if this might change your statement above, and conclusion, you may have drawn from seeing another view ? One you might not of seen before?

Is mathematical consistency of value to you when it is developing?

12. dorigo - October 9, 2007

Paolo, it should not be too hard to get a tour of CMS or ATLAS. I have no access to the caverns – I could ask for it but am too lazy to do it, since there is no reason for me to be downstairs. If you decide to go, let me know and I’ll see if I can find a contact for you on site.


13. dorigo - October 9, 2007

Hi Plato,

I think there are scientific theories, and one has a chance to draw one’s own conclusion on those based on proven and unproven fact and experimental hints; and then, there is scientific method, on which one has an opinion regardless of any input. I think that “solving” the problem of foldings of the extra dimensions by calling it a “feature” of the theory, is not science.


14. Paolo - October 9, 2007

Tommaso, thanks a lot again about the visit. To tell you the truth now I’m a bit confused, because never considered visiting before and now, all of a sudden, I have only to fill a form, too good to be true! Probably, I will try to make a reservation for next spring, ideally I would find something else to do in Switzerland (e.g., the famous kunstmuseum in Basel?) and spend overall ~1 week. Since you are so kind, I will probably contact you privately later for some hints (about lodging too, probably)… In the meanwhile, if you think there is something I should know immediately (like, no-no times of the year, for some reason), I’m all ears… Ciao, Paolo.

15. anomalous cowherd - October 10, 2007

14. Paolo – October 9, 2007

If you want to visit ATLAS or CMS you should plan your visit soon! Once the experiments close up for beam commissioning there will be no more visits to them. [You will be able to do CERN visits, but you won’t be able to see ATLAS or CMS during the visit].

Check with Prof. Dorigo that my information is accurate, but that is what I had been told by my experimental colleagues.


16. Plato - October 10, 2007

Tammasobased on proven and unproven fact and experimental hints; and then, there is scientific method, on which one has an opinion regardless of any input.

Then there is a division amongst your colleques? Ones who have scientiifc theories and those that practise scientific method?

17. Plato - October 11, 2007

Jacques Distler:

It’s not a matter of liking or disliking the alternatives. It’s a matter of whether they are viable candidates. As I’ve tried to explain, they are not.

That doesn’t mean I don’t think they’re worth thinking about. There’s a long and noble history of “wrong” ideas having useful (and, sometimes, unexpected) spinoffs. The same may well turn out to be true of String Theory, which continues to have a rich set of spinoffs for Quantum Field Theory and for Mathematics but which could, ultimately, turn out to be wrong.

Quote is direct link.

If one did not extend the methods of “genus figures” then mathematically the issue would have been dead, and Lee Smolin’s point would have been right. But he was shown to be wrong in his new book, and Peter Woit had to have recognize this?

That’s a simple question of Peter Woit in face of the link Tammaso you supplied on this matter. I have nothing to gain other then be corrected on the insights that people point too, to help me decide what is where and what is the state of things.

As you have corrected on dual blackholes.

Sometimes recognizing “the differences” you have pointed out can hurt others who continue to research by stating explicitly that the individual character could have been flaw in any scientist, by association?

That’s all I have right now

18. dorigo - October 11, 2007


as always I have some trouble decyphering you. Can you try writing simpler sentences, like explaining what are “genus figures”, what is Smolin’s point, where he was shown wrong, why peter should know, when did I discuss dual blackholes… My memory is at fault, help me answer you meaningfully!

I even find it hard to understand your last sentence. I apologize, it must be too late – I think I’ll just go to sleep 🙂

Sorry for being useless,

19. Plato - October 13, 2007


That is my difficulty in being clear. You have every right to ask for it.

1.What are “genus figures”

We can also consider donuts with more handles attached. The number of handles in a donut is its most important topological information. It is called the genus.

2.What is Smolin’s point?

His view on Finiteness of String theory. and the development to two loop genus figure. Saying, that this is all that has been done.

The Trouble With Physics,” by Lee Smolin, Index page 382, Mandelstam, Stanley, and string theory finiteness, pages 117,187, 278-79, 280, 281, 367n14,15

3.Why Peter should know?

In regards to any arguments Peter Woit has on string theory, this particular development is explicitly clear in the mathematical progress.

Jacques Distler :

This is false. The proof of finiteness, to all orders, is in quite solid shape. Explicit formulæ are currently known only up to 3-loop order, and the methods used to write down those formulæ clearly don’t generalize beyond 3 loops.

4.When did I discuss dual blackholes?


Plato, the paper you link to appears a bit outdated, I think. The RHIC has called off early claims of being able of producing black holes, IIRC.

20. Plato - October 13, 2007


Mouse over the quotes. They are directly linked to their source. Click on them.

21. dorigo - October 14, 2007

Hi Plato,

since you are by now a regular here, you should learn to spell my name right 😉 Tommaso.

Anyway, thank you for the information – things look a whole lot clearer to me now! Ok, genus figures I think I should have known. I have read about them, and indeed in English. But some “new words” stick less than others…
As for dual black holes, ok… I did not “discuss” them but you are right, I mentioned that.
As for Jacques Distler vs Peter Woit, I take the latter 🙂 Distler is a bit too partisan. I have a bet out against him actually, on finding SUSY at LHC.


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