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.