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The origins of the Higgs boson
Many questions in particle physics are related to the existence of particle mass. The "Higgs mechanism," consisting of the Higgs field and its corresponding Higgs boson, would give a mass to the elementary particles. By "mass" we mean the inertial mass, which resists when we try to accelerate an object, rather than the gravitational mass, which is sensitive to gravity. In the famous formula of Einstein E = mc 2 the "m" is the inertial mass of the particle. In a sense, this mass is the essential quantity, which defines that there is a particle rather than nothing
In the early 1960s, physicists had a powerful theory of electromagnetic interactions and a descriptive model of weak nuclear interaction. – the force that is at stake in many radioactive decays and in the reactions that make the Sun shine. They had identified deep similarities in the structure of these two interactions, but a unified theory at the deeper level seemed to require that the particles be massless even if the true particles in nature have mass.
In 1964, theorists proposed a solution to this puzzle. . The independent efforts of Robert Brout and François Englert in Brussels, Peter Higgs at the University of Edinburgh and others have resulted in a concrete model known as the Brout-Englert-Higgs mechanism ( BEH). The peculiarity of this mechanism is that it can give mass to elementary particles while retaining the beautiful structure of their original interactions. Importantly, this structure ensures that the theory remains predictive at very high energy. Particles that carry the weak interaction will gain masses by their interaction with the Higgs field, just like particles. The photon, which carries the electromagnetic interaction, would remain massless.
In the history of the universe, particles interacted with the Higgs field barely 10 seconds -12 after the Big Bang. Before this phase transition, all the particles were massless and traveling at the speed of light. After the expansion and cooling of the universe, the particles interacted with the Higgs field and this interaction gave them mass. The BEH mechanism implies that the values of the masses of elementary particles are related to the strength of each particle in the Higgs field. These values are not predicted by current theories. However, once the mass of a particle is measured, its interaction with the Higgs boson can be determined.
The BEH mechanism had several implications: firstly, the weak interaction was mediated by heavy particles, namely the W and Z bosons, which were discovered at CERN in 1983. Second, the new field itself would materialize in another particle. The mass of this particle was unknown, but the researchers knew that it should be less than 1 TeV – a value well beyond the then imaginable limits of accelerators. This particle was later called the Higgs boson and would become the most wanted particle in all particle physics.
The Accelerator, Experiments and the Higgs
The Large Electron-Positron Collider (LEP), which operated at CERN, from 1989 to 2000, was the first accelerator to have significant significance in the potential mass range of the Higgs boson. Although LEP did not find the Higgs boson, it made significant progress in the research, determining that the mass should be greater than 114 GeV.
In 1984, some physicists and engineers at CERN were studying the possibility of installing a proton. Proton accelerator with a very high collision energy of 10-20 TeV in the same tunnel as LEP. This accelerator would probe the entire mass range possible for the Higgs, provided that the luminosity [1] is very high. However, this high brightness would mean that every interesting collision would be accompanied by dozens of background collisions. Given the state of the art detector technology of the time, this seemed like a formidable challenge. CERN has wisely launched a strong R & D program, which has made rapid progress on detectors. The first collaborations became ATLAS, CMS and other LHC experiments
. On the theoretical side, the 90s have seen a lot of progress: physicists are studying the production of the Higgs boson in proton-proton collisions and all its different modes of decay. Since each of these decay modes is strongly dependent on the unknown mass of the Higgs boson, future detectors will need to measure all possible particle types to cover the broad mass range. Each mode of decay has been studied using intensive simulations and important modes of Higgs decay have been among the benchmarks used to design the detector
Meanwhile, at Fermilab National Fermilab (Fermilab) near Chicago, Illinois , the Tevatron collider has a discovery potential for a Higgs boson with a mass of about 160 GeV. Tevatron, the scientific predecessor of the LHC, collided with protons with antiprotons from 1986 to 2011.
In 2008, after a long and intense construction period, the LHC and its detectors were ready for the first beams. On 10 September 2008, the first beam injection into the LHC was a major event at CERN, with the invitation of the international press and the authorities. The machine worked beautifully and we had very high hopes. Unfortunately, ten days later, a problem in the superconducting magnets caused considerable damage to the LHC. A full year was needed for repairs and to install a better protection system. The incident revealed a weak magnets, which limited the collision energy to 7 TeV.
During the restart, we had to make a difficult decision: does it take another year to repair the weaknesses all around the ring? Or should we immediately start and operate the LHC at 7 TeV, even if a factor of three Higgs bosons would be produced? Detailed simulations showed that there was a chance to discover the Higgs boson at reduced energy, especially in the range where the Tevatron competition was the most pressing, so we decided to start immediately at 7 TeV was worth it. [19659003] The LHC restarted in 2010 at 7 TeV with a modest brightness – a brightness that would increase in 2011. The ATLAS Collaboration had made good use of the forced shutdown of 2009 to better understand the detector and prepare the analyzes. In 2010, Higgs experts from experiments and theory created the LHCCHSWG working group of the LHC Higgs [2] which proved to be a valuable forum to accompany the best calculations and discuss the difficult aspects of the production and degradation of Higgs. These results have since been regularly documented in the "LHCHXSWG Yellow Reports", famous in the community.
The discovery of the Higgs boson
Since Higgs bosons are extremely rare, sophisticated analysis techniques are needed to spot signal events in large backgrounds of other processes. After signal-like events have been identified, powerful statistical methods are used to quantify the significance of the signal. Since statistical fluctuations in the background can also resemble signals, strict statistical requirements are imposed before a new signal is discovered. The meaning is usually indicated by σ, or a number of standard deviations of the normal distribution. In particle physics, a significance of 3σ is called evidence, while 5σ is called observation, corresponding to the probability of a statistical fluctuation of the background less than 1 in a million.
Eager physicists analyzed the data as soon as it happened. In the summer of 2011, there was a slight excess of the decay of the Higgs to two W bosons for a mass of about 140 GeV. Things became more interesting because an excess to a similar mass was also seen in the diphotonic channel. However, as the data set increased, the size of this surplus increased and decreased.
By the end of 2011, ATLAS had collected and analyzed 5 fb -1 data in a data center. mass energy of 7 TeV. After combining all the channels, it was found that the standard Higgs model boson could be excluded for all masses except for a small window around 125 GeV, where an excess of about 3σ was observed, mainly due to the diphoton and four leptons. disintegration channels. The results were presented at a special seminar at CERN on 13 December 2011. Although neither of the two experiments had sufficiently convincing results to claim observation, which was particularly telling was the fact that ATLAS and CMS had excesses at the same mass
. , the LHC energy was increased from 7 to 8 TeV, which increased the cross sections for Higgs boson production. The data arrived quickly: in the summer of 2012, ATLAS had collected 5 fb -1 at 8 TeV, doubling the dataset. As soon as the data arrived, they were analyzed and, of course, the significance of this small bump around 125 GeV increased again. Rumors circulated around CERN when a joint seminar between ATLAS and CMS was announced for 4 July 2012. The seminar seats were so much in demand that only people who were lining up all night could enter the room. The presence of François Englert and Peter Higgs at the seminar further increased the excitement
At the famous seminar, the spokespersons of the ATLAS and CMS Collaborations showed their results consecutively, each finding an excess around 5σ at a mass of 125 GeV. To conclude the session, CERN Director General Rolf Heuer said: "I think we have it."
ATLAS Collaboration celebrated the discovery with champagne and gave each member of the collaboration a t-shirt with the famous parcels. Incidentally, only once they were printed, he discovered that there was a typo in the plot. Regardless, these t-shirts would become collector's items.
ATLAS and CMS published the results in Physics Letters B a few weeks later in an article entitled "Observation of a new model". The Nobel Prize in Physics was awarded to Peter Higgs and François Englert in 2013.
What We've Learned Since Discovery
After the discovery, we started to study the properties of the newly discovered particle to understand if it was not possible. was the standard Higgs Boson model or something else. In fact, we first called it a Higgs-like boson, because we did not want to pretend that it was the Higgs boson until we were certain of it. Mass, the last unknown parameter in the standard model, was one of the first measured parameters and was around 125 GeV (about 130 times larger than the mass of the proton). It turned out that we were very lucky – with this mass, the greatest number of decay modes is possible
In the standard model, the Higgs boson is unique: it has no spin , no electric charge and no strong interaction. Spin and parity were measured by angular correlations between particles for which it has disintegrated. Indeed, these properties were found as expected. At this point, we started to call it "the Higgs boson". Of course, it remains to be seen if this is the only Higgs boson or one of many, such as those predicted by supersymmetry.
The Higgs boson was based on measurements of its disintegration into vector bosons. In the standard model, different couplings determine its interactions with fermions and bosons, so that new physics can affect them differently. Therefore, it is important to measure both. The first direct probe of fermionic couplings was tau particles, which was observed in the combination of the ATLAS and CMS results made at the end of the test 1. During the test 2, the increase from the center of mass energy to 13 TeV and a larger dataset surveyed from other channels. During the past year, evidence has been obtained for the decay of Higgs in the bottom quarks and the production of the Higgs boson with the higher quarks has been observed. This means that the interaction of the Higgs boson with the fermions has been clearly established.
One of the best ways to summarize what we currently know about the interaction of the Higgs boson with other particles of the standard model is to compare the force of interaction. the mass of each particle, as shown in Figure 4. This clearly shows that the strength of interaction depends on the mass of particles: the heavier the particle, the stronger its interaction with the Higgs field. This is one of the main predictions of the BEH mechanism in the standard model.
We are not only testing to verify that the properties of the Higgs boson are consistent with those predicted by the standard model – we are looking specifically for properties that would provide evidence for a new physics. For example, constraining the rate of Higgs boson disintegration into invisible or unobserved particles imposes strict limits on the existence of new particles whose masses are smaller than those of the Higgs boson. We are also looking for disintegrations of forbidden particle combinations in the standard model. So far, none of this research has found anything unexpected, but that does not mean we will stop watching soon!
Outlook
2018 is the last year that ATLAS will take data as part of the LHC's Run 2. During this series, proton-proton collisions of 13 TeV produced about 30 times more Higgs bosons than those used in the discovery of the Higgs boson in 2012. As a result, more and more results were obtained to further investigate the Higgs boson.
Over the next few years, the analysis of the big data set of the 2 series will not only be the opportunity to reach a new level of precision. previous measurements, but also to study new methods to probe the predictions of the standard model and test the presence of new physics as independently of the model as possible. This new level of accuracy will rely on achieving a deeper level of understanding of the detector's performance, as well as the simulations and algorithms used to identify the particles passing through it. This also poses new challenges for theorists to follow the improvement of experimental accuracy.
In the longer term, another big step in performance will be brought by the high-brightness LHC (HL-LHC), which is expected to come into service in 2024 The HL-LHC will increase the number of collisions one Another factor of 10. Among other measures, this will open the possibility of studying a very particular property of the Higgs boson: that it couples to itself. The events produced by this coupling involve two Higgs bosons in the final state, but they are extremely rare. Thus, they can only be studied in a very large number of collisions and by using sophisticated analysis techniques. To match the increased performance of the LHC, the ATLAS and CMS detectors will undergo complete updates in the years leading up to the HL-LHC.
More generally, the discovery of the Higgs boson with a mass of 125 GeV particle physics to build. Many questions remain on the ground, most of which are related to the Higgs sector. For example:
- A popular theory beyond the standard model is "supersymmetry", which has interesting features for solving current problems, such as the nature of dark matter. The minimal version of supersymmetry predicts that the mass of the Higgs boson should be less than 120-130 GeV, depending on other parameters. Is it a coincidence that the observed value lies exactly at this critical value, thus allowing this supersymmetric model to be marginally still?
- Several models have recently been proposed where the only connection of dark matter with ordinary matter would be the Higgs boson.
- Stability of the Universe: The value of 125 GeV is almost at the critical limit between a stable universe and a metastable universe. A metastable system has another basic state, in which it can disintegrate at any moment because of the quantum tunnel [3] Is this also a coincidence?
- The phase transition: the details of this transition can play a role in the process. leads our universe to be entirely matter and not contain any anti-matter. Current calculations with the standard Higgs model boson are not consistent with observed material-antimatter asymmetry. Is it a call to new physics or only incomplete calculations?
- Are the masses of fermions all related to the Higgs boson field? If so, why is there such a large hierarchy between masses of fermions extending from fractions of electrons-volts for mysterious neutrinos to very heavy top quark, with a mass of the order of hundreds of billions of electron volts? ] From what we have learned about it, the Higgs boson seems to play a very special role in nature … Can it show us the way to answer other questions?
About the Authors
Heather Gray is an experimental physicist at the Lawrence Berkeley National Lab, USA. She is a member of the ATLAS experiment at CERN's Large Hadron Collider, to which she has contributed in many ways, including measuring the interactions of the Higgs boson with the quarks. Bruno Mansoulié is a scientist at CEA-IRFU, Saclay, France. He has worked both as a theoretical and experimental physicist and is a founding member of ATLAS where he has, among other things, conducted combined analyzes of the Higgs boson and led the Higgs working group. Both appreciate the communication of particle physics to non-specialists.
[1] Brightness is the machine parameter that determines the number of events per second for a given physical process. The higher the brightness, the more events per second
[2] The cross-section is a measure of the likelihood that this process will occur during a particular event. proton-proton collision. Processes with larger sections occur more often than processes with small cross sections.
[3] Fortunately, even though we are a bit on the metastable side, the lifetime for disintegration is extremely long compared to the current age of the universe …
lecture
- ATLAS Collaboration, Observation of an excess of events in the search for the standard Higgs boson in the gamma-gamma channel with the ATLAS detector, July 2012
- ATLAS Collaboration, Observation of an excess of events in the search for the standard Higgs model boson in the H-> ZZ (*) -> 4l channel with the ATLAS detector, July 2012
- ATLAS Collaboration, Observation of a new particle in the search for the standard model Higgs boson with the ATLAS detector at the LHC, Phys. Lett. B 716 (2012) 1-29
- ATLAS Collaboration, a particle conforming to the Higgs boson observed with the ATLAS detector at the Large Hadron Collider, Science Vol. 338, no. 6114 p1576-158
- ATLAS Collaboration, Higgs Bismon Production and Decay Rates Measurements and Constraints on Couplings from a Combined ATLAS and CMS Analysis of LHC 7 and 8 LHC Collision Data TeV, JHEP 08 (2016) 045 [19659046] George Redlinger and Paul de Jong, broken symmetry: search for supersymmetry at the LHC
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