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