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A new idea to search for Higgs bosons at LHC February 14, 2007

Posted by dorigo in internet, news, physics, science.

The Higgs boson, a particle predicted thirty years ago to explain a feature of the Standard Model of electroweak interactions, is the hottest topic in particle physics these days. All attempts at finding its decay signal have been unsuccessful, and past and present experiments have so far been able to only set upper limits on its production rate -and through the predicted relationship between production rate and mass, lower limits to the latter.

LEP II, the electron-positron collider dismantled in 2001, saw a hint of a possible Higgs decay signal at 115 GeV (the black points on the rightmost bins in the plot on the left, which are there compared to a simulated prediction for the signal appearance in red), but in the end could only set a lower limit of 114.4 GeV to the mass of the Higgs “at 95% confidence level” – a physics jargon to mean that the inequality is unlikely to be proven wrong although not 100% certain. On the other hand, indirect information coming from precise measurements of other particle properties, such as top quark and W boson masses, suggests that M(H) is not much larger than that: the most recent fits to all electroweak observables indicate that M(H)<166 GeV or so, again at 95% CL.

In the predicted mass range, one of the most frequent modes of decay of the Higgs boson is to a pair of b-quarks, each of which fragments producing a hadronic jet – a collimated stream of particles that can be measured in modern-day detectors. It is on this decay that the 2-TeV proton-antiproton Tevatron collider experiments, CDF and D0, pin their hopes to discover the Higgs before the start of the competing, higher energy machine being constructed at the CERN laboratories, the LHC – a 14 TeV proton-proton collider. But the Tevatron might not be able to produce enough of the sought particles to claim discovery.

If the Tevatron falls short of seeing the Higgs,The 7-fold increase in energy of the LHC collisions will allow a higher production rate for the signal, but also a huge increase in background processes which are hard to tell apart. For that reason, CMS and ATLAS will be much more likely to see the Higgs decay to pairs of W or pairs of Z bosons, a cleaner signature.

Discovering the Higgs boson is one of the major objectives of the LHC. However, physicists look further. Once found, the Higgs will demand to be studied in detail, to verify whether the properties it displays obey the predictions of the Standard Model.

One of these properties is the so-called “coupling” to heavy fermions such as the b-quark. You can measure the coupling by determining the decay rate to b-quark pairs, but at the LHC this is problematic, due to the already mentioned backgrounds from QCD strong interactions, producing the same final state – pairs of b-quarks.

A new paper by Fabio Maltoni (a former colleague Summer Student of mine at Fermilab, back in 1992) and colleagues is hot in the arXiv. I was pointed at the paper by a talk by Barbara Mele, a theorist who gave an invited seminar during the CMS-Italy workshop I am attending in Naples yesterday and today.

In the paper, the authors show that by searching for the associated production of a Higgs boson and a high-energy photon the nasty QCD background gets strongly inhibited, because most QCD processes yielding b-quark jets are unable to radiate a photon off the gluonic initial state. The Higgs coupling to b-quarks could thus be measured with good accuracy given the expected luminosity of the first few years of LHC running.

I have very well known this suppression effect for a long time – indeed, I have several times advocated that the search for Z boson decays to pairs of b-quark jets at the LHC could be performed by seeking for an additional photon. The Z boson is a well-known particle, and its detection allows to calibrate the energy measurement of b-quark jets. Finding the associated signal of a Z->bb decay and a photon would be doubly interesting now, since it now becomes a prerequisite to finding the much more elusive production of associated Higgs-gamma events.


1. trevercrow - February 14, 2007

would you be willing to teach me what you know this week. im sure you could fit it in your schedule if you use imaginary time.

much respect,

p.s. what is the earliest form of chess?

2. dorigo - February 14, 2007

Chess was born in India a thousand years ago, the game was slightly different though. You can find that information on wikipedia.


3. Stefano - February 15, 2007

I would also expect Higgs particle to be found at CERN.
But what if it isn’t? Actually, what if Higgs boson does not exist at all? In the end, there is no experimental evidence for it, so the Higgs mechanism could be eventually disproved.
I am not aware of alternatives for mass generation in the SM; or is there any? Despite a dusty degree, I am not a physicist, so I could well ignore some important theory.

4. Arun - February 15, 2007

Doesn’t this look like a rook?


5. trevercrow - February 15, 2007


6. Arun - February 16, 2007

Excavated from Lothal, one of the cities of the Indus valley civilization. Likely from one of the ancestors chess.


Sorry for going entirely off topic, but couldn’t resist.

7. trevercrow - February 16, 2007

tommorrow, i will be purchasing that ancient version of chess at barnes and nobles. would anyone like to play thru emails. there is energy in those pieces not obtained thru the wire. this is not to take away from the inherent energy of our word exchanges. b/t/w who is ya’lls favorite writer on our beloved subject?


8. Jester - February 16, 2007

Hi Tommaso (i’m scared like hell i could it get it wrong again😉
Do you know how precisely can the LHC measure the h b \bar b coupling? Is it something like 10% or rather 100% accuracy ? What about other couplings? Any chance to measure h t \bar t ?

9. Arun - February 17, 2007

In the paper, the authors show that by searching for the associated production of a Higgs boson and a high-energy photon the nasty QCD background gets strongly inhibited, because most QCD processes yielding b-quark jets are unable to radiate a photon off the gluonic initial state.

Does such a discovery result from a systematic search through possible signatures, or does physical intuition play a role? e.g., you stare at the feynman graphs, and do some back-of-the-envelope and are reasonably certain of the results before you do a comprehensive calculation?

10. dorigo - February 17, 2007

Arun, thank you for the links to ancient chess pieces… I had never seen them. Nor had I known about Lothal, really a wonderful archaeological site.


11. dorigo - February 17, 2007

Trevercrow, I think Aaron Nimzowitsch was a great chess writer among the classics. More recently, Kasparov has shown to be inferior to noone in writing about chess, with his wonderful 5-volume opus about chess champions of the past.


12. dorigo - February 17, 2007

Hi Jester,

yours is a good question… I need to study the issue for a moment. I will post about it later today.


13. dorigo - February 17, 2007


about the issue of photons allowing the signal to noise to increase, it is something you know must be true from the outset, but then you need to run a Monte Carlo simulation of the relevant processes to verify just how much your physical intuition about production rates, angular distribution, and final states agrees with the theoretical calculations folded with your detector response. But yes, it is physical intuition. Usually it works🙂


14. trevercrow - February 18, 2007

there is no way that one can go off topic once one realizes that everythimg is connected.

thanks dorigo,
you are a wise soul.

15. dorigo - February 18, 2007

🙂 UW trevercrow.
I concur with you that playing chess on a real board with a real opponent to look in the eyes is quite a different thing from playing over the internet… But the fact is, there are things one can do and things that are too expensive. For me, over-the-board chess is almost totally off-limits…

If you wish to play a game, I am on the Internet Cless Club, as “tonno”. But we need to arrange a date and time for that.

16. trevercrow - February 19, 2007


17. Probing the Hbb vertex at the LHC « A Quantum Diaries Survivor - February 21, 2007

[…] by dorigo in physics, Blogroll, politics, science, news, internet. trackback In a comment to a recent post about non-mainstream searches for the Higgs boson at the LHC, Jester asked me with what precision […]

18. trevercrow - February 28, 2007

i have been away but am back now and ready to learn all that ya’ll have to teach

19. dorigo - March 1, 2007

Great trevercrow, I will try not to disappoint you. But this week I have little to report, being on vacation… Will be back on business soon though!

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