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.