Higgs boson : “We have discovered the origin of matter in the Universe”

First pre­dic­ted in 1964 by theo­re­ti­cal phy­si­cists, inclu­ding Peter Higgs and Fran­çois Englert (2013 Nobel lau­reate), to explain why some par­ticles have mass, the Higgs boson was dis­co­ve­red in 2012 at the Large Hadron Col­li­der (LHC). This par­ticle gas pedal loca­ted at CERN on the French-Swiss bor­der pro­du­ced the famous par­ticle as a result of col­li­sions bet­ween pro­tons of very high energy.

During these col­li­sions, the LHC rea­ched a record ener­gy of 7000 GeV, allo­wing par­ticle phy­sics resear­chers to repro­duce the phy­si­cal condi­tions of the first moments of our Uni­verse – a frac­tion of a bil­lionth of a second after the Big Bang – in labo­ra­to­ry condi­tions. They were thus able to glimpse at the first time the moment when the ele­men­ta­ry par­ticles that consti­tute ordi­na­ry mat­ter appea­red in the bud­ding Universe.

Phy­si­cists have known since the 1970s that two of the four fun­da­men­tal forces of nature – weak force and elec­tro­ma­gne­tic force – are clo­se­ly rela­ted. These two forces can be des­cri­bed within the fra­me­work of a single theo­ry, which forms the basis of the Stan­dard Model of par­ticle phy­sics. This ‘uni­fi­ca­tion’ implies that elec­tri­ci­ty, magne­tism, light and radio­ac­ti­vi­ty are all mani­fes­ta­tions of a single under­lying force, known as the elec­tro­weak force.

If the basic equa­tions of the uni­fied theo­ry cor­rect­ly des­cribe the elec­tro­weak force and the par­ticles that car­ry it, name­ly the pho­ton and‘vector bosons’, W and Z, there is a major hitch : all these par­ticles appear to have no mass in cal­cu­la­tions. If the pho­ton is indeed mass­less, the W and Z have a mass almost 100 times that of a pro­ton. To solve this pro­blem, theo­re­ti­cal phy­si­cists Robert Brout, Fran­çois Englert and Peter Higgs pro­po­sed a mecha­nism that gives mass to the W and Z par­ticles when they inter­act with an invi­sible field, cal­led the ‘Higgs field’, which per­meates the entire universe.

Just after the Big Bang, the Higgs field was zero, but when the Uni­verse cooled down and its tem­pe­ra­ture fell below a cer­tain cri­ti­cal tem­pe­ra­ture, the Higgs field spon­ta­neous­ly increa­sed so that any par­ticle inter­ac­ting with it acqui­red a mass. The more a par­ticle inter­acts with this field, the hea­vier it becomes. Par­ticles like the pho­ton that do not inter­act with it remain mass­less while the W and the Z have mass.

This sud­den increase of the Higgs field led to the spon­ta­neous brea­king of elec­tro­weak sym­me­try, with the stri­king conse­quence that the weak inter­ac­tion was sud­den­ly car­ried by the W and the Z. At the same time the pho­ton of zero mass appea­red, as a vehicle of the elec­tro­ma­gne­tic inter­ac­tion. As a result, the weak inter­ac­tion, res­pon­sible for radio­ac­ti­vi­ty, acts only at very short dis­tances, while the elec­tro­ma­gne­tic inter­ac­tion has an infi­nite range. With the appea­rance of the mass of ele­men­ta­ry par­ticles, such as the elec­tron or quarks, and that of the elec­tro­ma­gne­tic inter­ac­tion, which makes it pos­sible to define the elec­tric charge as we know it, the ingre­dients neces­sa­ry for the for­ma­tion of the atoms of ordi­na­ry mat­ter final­ly appear in the Universe.

Like all fun­da­men­tal fields, the Higgs field is asso­cia­ted with a par­ticle – in this case, the Higgs boson. The Higgs boson is the visible mani­fes­ta­tion of the Higgs field, much like a wave on the sur­face of the sea. For many years, howe­ver, there was one major pro­blem : no expe­riment had ever obser­ved the Higgs boson to confirm this theo­ry. On July 4, 2012, the large CMS and ATLAS expe­ri­ments at CERN, both announ­ced the dis­co­ve­ry of a new par­ticle in the mass region around 125 GeV.

The Labo­ra­toire Leprince-Rin­guet (LLR), with the sup­port of CNRS and École Poly­tech­nique, was among the main players of this “dis­co­ve­ry of the cen­tu­ry” – as part of the inter­na­tio­nal col­la­bo­ra­tion, CMS. It has since contri­bu­ted to the pre­cise deter­mi­na­tion of the intrin­sic pro­per­ties of the Higgs boson and its cou­plings to other ele­men­ta­ry par­ticles. It has also been confir­med that it is a ‘sca­lar’ boson, in a kind of its own, because it has no “spin’”: it is nei­ther a mat­ter par­ticle, such as the elec­tron (spin = ½), nor the vehicle of an inter­ac­tion such as the pho­ton (spin = 1). LLR resear­chers have also demons­tra­ted that the Higgs boson couples to other par­ticles of mat­ter with an inten­si­ty pro­por­tio­nal to their mass.

The cur­rent results of the ATLAS and CMS expe­ri­ments indi­cate that the Higgs boson does indeed seem to have all the cha­rac­te­ris­tics of the ele­men­ta­ry par­ticle pre­dic­ted by the spon­ta­neous sym­me­try brea­king mecha­nism at the ori­gin of the mass of par­ticles in the Uni­verse. To bet­ter unders­tand these results, we need to mea­sure how the Higgs boson couples with itself. In prac­tice, this means being able to access the pro­duc­tion of H‑boson pairs, which will only be pos­sible by pushing the per­for­mance of the large pro­ton-pro­ton col­li­der at CERN to the maxi­mum. To achieve this, an intense pro­gram of impro­ve­ments to the col­li­der and the large par­ticle detec­tors is under­way in pre­pa­ra­tion for a new phase of ope­ra­tion of the LHC at very high lumi­no­si­ty, cal­led HL-LHC, from 2027. The lumi­no­si­ty of a col­li­der is a quan­ti­ty pro­por­tio­nal to the num­ber of col­li­sions occur­ring in a time inter­val. The inte­gra­ted lumi­no­si­ty during the HL-LHC phase should allow the ATLAS and CMS expe­ri­ments to increase the num­ber of col­li­sions recor­ded by a fac­tor of at least 10. This increase in sen­si­ti­vi­ty should not only give access to the pro­duc­tion of H‑boson pairs, but also allow to extend the search. We could thus observe, who knows, the direct pro­duc­tion of dark mat­ter (which would consti­tute 27% of the mat­ter of the Uni­verse and which remains an enig­ma for phy­si­cists), or the addi­tio­nal sca­lar bosons pre­dic­ted by various theo­ries going beyond the Stan­dard Model.