“ATLAS and CMS collaborations (the two largest experiments of the Large Hadron Collider or LHC) have managed to exclude the data collected in 2011 Higgs masses in the standard model above about 127 GeV, which represents a breakthrough in this search “, explains to SINC Juan Alcaraz, principal investigator of CIEMAT in the CMS.

Cintíficos experiments ATLAS and CMS were presented today at a seminar at CERN, the status of your search for the Higgs boson predicted by the Standard Model of Particle Physics.Their results are based on the analysis of a data amount considerably higher than the results presented at the conference last summer, enough to make significant progress in the search for the Higgs boson, but not to make a strong statement on the existence of this elusive particle.

“In the mass range 114-127 GeV, both collaborations are slight excess, particularly on channel two-photon decay and mass in the 124-126 GeV, but the amount of data collected to date is insufficient to to determine if this really is the Higgs particle or simple statistical fluctuations somewhat higher than expected, “says Alcaraz.

The researcher contextualizes the advance: “These fluctuations are the order of about 2-3 standard deviations, and the common practice in the case of discovery of new particles dictates the observation of at least 5 standard deviations, which would ensure that it is a too stable. It is noteworthy that there is little difference between the capabilities of ATLAS and CMS regarding the detection of the Higgs is concerned. The statistical fluctuations in this quest are now more important than small differences in the characteristics of each experiment. “

The main conclusion

For now, the main conclusion is that if the Higgs boson mass range is likely between 116 and 130 GeV, according to the ATLAS experiment, and between 115 and 127 GeV, according to CMS. Both experiments have been indications in the same mass region, but not strong enough to be considered a discovery.

The Higgs boson, if any, has a very short duration and is broken in many ways. His discovery was based on observing particles which decays more than the Higgs itself. Both ATLAS and CMS have analyzed several decay channels, and excesses have small low-mass region where the presence of the Higgs boson has not yet been excluded.

Separately, none of these excesses is more significant statistically than rolling a die and draw two consecutive six. Interestingly, there are several independent measures that point to the region between 124 and 126 GeV. It is too early to say if ATLAS and CMS have discovered the Higgs boson, but these recent results are generating great interest in the scientific community of particle physics.

“We have restricted the region most likely mass for the Higgs boson of 116 to 130 GeV, and in recent weeks have begun to see a fascinating excess of events in the mass range around 125 GeV,” said the spokesman of the experiment Fabiola Gianotti ATLAS. “This excess can be due to a fluctuation, but it could be something more interesting. At this point we can not conclude anything. We need to study more data. Given the exceptional performance of the LHC this year, we will not have to wait long to have sufficient data and hope to resolve this puzzle in 2012. “

“We can not exclude the presence of the Higgs boson in the Standard Model between 115 and 127 GeV due to a modest excess of events in this mass region that appears, quite consistently, in five different decay channels,” said CMS spokesman, Guido Tonelli. “The excess is most compatible with a Higgs boson of the Standard Model in the vicinity of 124 GeV and below, but the statistical significance is not large enough to say anything conclusive. What we see today is consistent with both a background fluctuation or the presence of the boson. More refined analysis and additional data contributed by the LHC in 2012 will definitely respond. “

Solve the mystery in 2012

In the coming months, both experiments will further refine their analysis ahead of the winter conference of particle physics due in March. However, a definitive statement on whether or not the Higgs boson require more data, and is unlikely to occur until the end of next year.

“The next step is to analyze the data that the LHC will provide in 2012, between 10 and 20 femtobarn reverse, ie, two to four times the current data, and this new sample should help confirm or disprove the existence of this excess, “he adds Alcaraz. “In the case confirmed the existence of a signal, the next step would be to determine more precisely if the number of events is consistent with that predicted by the Standard Model, what is its mass and other properties (its spin is it really zero? ). But if it is confirmed that this is a simple statistical fluctuation, the search would continue until the lowest possible mass (114 GeV). “

Failure to observe the Higgs boson, the search would continue in other models. In many of the popular models that extend it is expected not only one Higgs field, but at least two, with a possible reduction in the number of expected events. The Higgs couplings could also have various other particles, so that the channels seen as more likely may not be true.

Even under the assumption of a complete absence of signal in the 114-600 GeV mass range, it is essential the presence of something with similar properties to the Higgs boson. His absence at low mass is then manifested in the LHC as a detectable increase in the production of pairs of bosons at high collision energies and possibly give rise to new structures in the mass spectrum in the region of the thousands of GeV.

The key for the Standard Model Higgs

The Standard Model is the theory that physicists use to describe the behavior of fundamental particles and forces acting between them. Describe the ordinary matter of which we are made us and all observable. However, it describes 96% of the universe, which is invisible. One of the main goals of the LHC research program is to go beyond this model and the Higgs boson could be the key.

This boson confirm this theory, first introduced in the sixties, but may take other forms related to theories that go beyond. A Higgs boson within the Standard Model may continue to point to new physics through subtleties in behavior that arise only after studying a large number of decays of this particle.

But if you were out, the absence of the Higgs boson in the Standard Model would indicate the presence of vastly new physics in the energy range that is designed for the LHC, 14 TeV (tera) to be achieved after 2014. Whether ATLAS and CMS in the coming months show that the Higgs boson in the Standard Model or not, the LHC program is paving the way for new physics.

“Whatever then no doubt that the LHC will be a great great first step in understanding this concept so natural yet so dark: the mass,” concludes Juan Alcaraz.

The mysteries of the Higgs boson

The Higgs boson is the only particle not yet discovered the so-called standard model. It is a very special particle, essential in the Standard Model to explain the unification of electromagnetic and weak forces, as well as the existence of particles with mass, as observed in the universe around us. Your search has focused LHC in the mass region between 114 and 600 giga-electron volts (GeV), where 1 GeV about the mass of a proton. The mass of

114 GeV corresponds to the lower limit set by LEP experiments, the Large Hadron Collider at CERN electron-positron, who served in the past the tunnel that now occupies the LHC. The mass of 600 GeV corresponds to the limit beyond which it is considered that the Higgs can not be treated as a particle itself, having an excessively high probability of decay and the mass from which appear difficult to interpret in the standard model. The integrated luminosity have collected ATLAS and CMS experiments, around 5 femtobarn reverse, it is sufficient for an efficient search for the Higgs boson in the mass range quoted. (To get an idea, a reverse femtobarn represents something close to 300 million events analyzed in a detector like CMS.)

The Higgs is an unstable particle, which immediately decays into other particles in the same proton collision point.Depending on the mass hypothesis studied decay channels are more suitable than others. In the mass range 130-140 GeV above, the most effective channels correspond to pair the search for the Higgs decay into pairs of W or Z bosons with subsequent decay into four leptons or two leptons and two quarks.

In the low mass range, 114-130 GeV, the most promising channel is the Higgs decay into a pair of photons, although an unlikely channel is much less affected by other background processes. Note that these tests look for signs of dozens of events over a total of millions and, even after applying strict selection criteria, it ends up looking a little too much on a background of hundreds or thousands of events.

Spanish participation

Since the implementation of the ATLAS detector, which involved 3,000 scientists from 174 institutions from 38 countries, researchers at the Institute of Corpuscular Physics (IFIC), Joint Center for Scientific Research Council (CSIC) and University of Valencia, the Institut d’Altes Energies Physics (IFAE), a consortium between the Generalitat de Catalunya and the Universitat Autònoma de Barcelona, ​​Institute of Microelectronics of Barcelona (IMB-CNM-CSIC) and the Autonomous University of Madrid (UAM), actively participate in operation and maintenance of detectors, with a strong presence on the alignment and calibration activities.

Within the extensive research program of the LHC, the ATLAS Spanish groups involved in a number of lines of research in the analysis of the data, which cover many of the most interesting topics of the program priorities of the LHC. In particular, the search for Higgs boson in the Standard Model, the groups have studied different final states resulting from the decay of the Higgs particle into two photons, two leptons taus and two Z bosons, or W

For its part, CMS, attended by 3,000 scientists from 172 institutes in 40 countries, experimental groups are present at the Institute of Physics of Cantabria (IFCA), CSIC-University joint center of Cantabria, the University of Oviedo, the Center for Energy Research , Environment and Technology (CIEMAT) and the Autonomous University of Madrid (UAM), involved in the search for the Higgs boson. Stresses the important participation of researchers from the University of Oviedo and the IFCA in the analysis channel of the Higgs boson decay into bosons WW, and CIEMAT researchers in the decay channel ZZ bosons, both very relevant to this search and have been key to exclude its mass is between 127 and 600 GeV.

The participation of the Spanish research groups at the LHC has the support of the Ministry of Education and Science through the National Programme for Particle Physics and National Center for Particle Physics, Astroparticle and Nuclear ( CPAN ), Consolider-Ingenio 2010 whose main objectives are the promotion and scientific coordination of the Spanish participation in international projects, development of joint R & D and training and incorporating new groups of researchers and technicians. The CPAN aims to consolidate these actions by setting up a center in permanent network, similar to those in other countries around us.

Source: CERN / CPAN