The Large Hadron Collider at CERN is running extremely well and its two general-purpose detectors ATLAS and CMS are collecting data at a much faster pace than expected. The LHC successfully concluded its 2011 proton run an the end of last month.
The ILC physics community is preparing for the results expected to emerge from the LHC, and there were many discussions about LHC results at the linear collider workshop held in Granada in September. In the ILC physics session on the first day of the workshop, Keisuke Fujii, associate professor at KEK, made a presentation entitled “Is Higgs enough? Or do we need something clearly beyond the Standard Model?”
“My answer to the question is: it is surely enough and we definitely need the ILC,” said Fujii.
Why would it be enough?
It is generally accepted among the world’s scientists that exciting new discoveries from the LHC are vital to justify the ILC project. “The question here is what discovery would justify the ILC,” he said.
As is stated in a recent CERN press release, proving or disproving the existence of the Higgs boson, which was postulated in the 1960s as part of a mechanism that gives mass to fundamental particles, is among the main goals of the LHC scientific programme.
Many scientists believe that, based on the past experiments and the Standard Model of particle physics, it is highly likely that the Higgs particle is hiding in a low-energy region. “The LHC has not yet discovered any hint of new physics that goes beyond the Standard Model. To put it the other way around, you can say that the probability of finding the Higgs particle in the energy region around 120 gigaelectronvolts (GeV) predicted by the Standard Model is getting higher,” said Fujii.
There are other new particles that scientists are seriously looking for: particles predicted by supersymmetry (often shortened to SUSY). So far, there is no indication of supersymmetric particles in the LHC data. “It is, however, still possible that so-called color-singlet supersymmetric particles such as the chargino and neutralino are in the reach of the 500-GeV ILC, since the currently available LHC data are not yet enough to exclude this possibility. You can also interpret that supersymmetric partners of the Standard Model particles are all rather heavy, and exist in the energy region beyond the reach of the current sensitivity of the LHC,” said Fujii.
If supersymmetric particles are all heavy, the lightest SUSY Higgs would look very much like the Standard Model Higgs, having similar properties including its mass. “You will need to measure the discovered Higgs ultra-precisely to see whether it is a SUSY Higgs or the Standard model one. This is where the ILC comes in,” argues Fujii.
Such a precision measurement can be started at an energy as low as between 230 and 250 GeV, which is enough to produce the Standard Model-like Higgs. “There are many important measurements to be made in this energy range,” Fujii says.
One outstanding example is the absolute measurement of an interaction called ZZH coupling. This can be made by counting the associate productions of the Higgs with the Z boson through the annihilation of a collided electron and positron into a virtual Z particle that subsequently splits into the Higgs and Z. “The observation of the associate production of the Higgs with Z is clear evidence that the produced Higgs is indeed condensed in the vacuum,” Fujii explains.
Under the assumption of the Standard Model, a single Higgs field is responsible for the whole mass of the Z boson. The ILC can check whether the mass of the Z is really entirely coming from the single Higgs field to about 1 percent precision. “If the Higgs here is the only Higgs that gives mass to the Z particle, this mass has to be equal to the known mass of the Z particle. If it is not the case, that means there must be something else condensing in the vacuum, presenting evidence of something beyond the Standard Model.”
With the value of the ZZH coupling determined, the rate of the associate production of the Higgs with the Z boson can be calculated and hence the total number of the produced Higgs bosons can be unambiguously estimated. It is then possible to determine absolute couplings of the Higgs to various matter particles by measuring the branching ratios for the corresponding H decays. “The ILC allows an absolute measurement of the ZZH coupling, even when the Higgs decays into invisible dark matter, which is possible only at the ILC,” Fujii emphasised.
There are many more rich physics opportunities scientists can explore in the reach of the baseline design of the 500-GeV ILC. “The next milestone is the energy region of 350 GeV, where we can produce the top quark,” Fujii said.
The top quark is the heaviest member of the quark family. Since the Higgs is thought to be the process that gives elementary particles their masses, measuring the interaction between those two particles is particularly important.
Another area of physics scientists can explore with the 500-GeV ILC is the ultimate check on the Higgs properties, the measurement of the Higgs self-coupling. “Once we reach 500 GeV, we will be able to produce two Higgs particles, and there we can measure the interaction of the Higgs with itself (self-coupling), which is none other than the force that makes the Higgs field condense in the vacuum”.
Fujii says that he would go so far as to say that we need the ILC even if nothing new, including a Higgs-like object, will be found at the LHC. “If there would be absolutely no hint of a Higgs-like particle or other new particles, that would be a complete contradiction with the success of the Standard Model, which has explained the mechanisms, particles and forces of the universe so well so far. We will need to extend our investigation on what went wrong in every detail by scrutinising the Standard Model particles such as W, Z, and top. The ILC will work very well on this, too.” One major scenario in which there is the case of finding no light Higgs boson is called the strong-interaction mechanism of electroweak symmetry breaking, where the properties of the W, Z and the top quark will look different from those in the Standard Model. These systems will have potential for discovery, one that is not very well appreciated today, Fujii says.
In Granada, the ILC physics community agreed that the 500-GeV ILC will have enough of a physics case to explore whatever finding comes out of the LHC. It also presents the possibility of starting experiments with a shorter, less expensive accelerator.
“But we have not reached consensus on how much energy we should aim for or which direction we should follow for the no-Higgs scenario at the LHC,” said Fujii. There is enough scientific justification for the baseline machine. However, building the next-generation machine involves factors other than science.
The ILC physics community will have a dedicated meeting in February at DESY, Germany, to continue the discussion started in Granada.