Discovery of the Higgs Boson Andrei Kryjevski Physics, NDSU 1 SRLS NDSU 1/29/2013

Why is the Higgs boson called the "God particle”? 2 The nickname for the elusive particle was created by Nobel Prize winning physicist Leon Lederman – reportedly against his will, as Lederman has said he wanted to call it the “Goddamn Particle” because “nobody could find the thing” for ~45 years (CNN, BBC). “God particle” is a nickname Physicists generally don’t use, but, apparently, mass media do. I am unaware of any real connection with any religion.

Standard Model (SM) of Particle Physics 3 SM agrees extremely well with the experiments. But the nature of mass generation mechanism not confirmed. Quarks and leptons (mostly u, d and e) constitute ordinary matter, interact by exchanges of the fundamental force carriers which hold them together to form matter: protons, neutrons, nuclei, atoms, molecules, etc. E.g., a proton made of 2 u and 1 d is bound together by the gluon exchanges –strong interactions. Proton + electron = hydrogen atom. Proton and electron are bound by the electromagnetic interactions.

Bosons vs. Fermions, Quarks vs. Anti-quarks There are two fundamental types of particles in quantum mechanics – bosons and fermions. Quarks and leptons are fermions, force carriers (gluon, photon, W, Z) are bosons. Two fermions cannot occupy the same state (sit on top of each other). Two bosons can occupy the same state (sit on top of each other). Also, there are particles and anti-particles (quarks - anti quarks, electron – positron, etc.). Particles constitute matter, anti-particles – antimatter. Everything we can see is made of matter. 4

Unification of the Electroweak Interactions In the SM two of the four fundamental forces – the weak force and the electromagnetic force – can be described using the same mathematical formalism. Electromagnetic phenomena driven by photon-matter interactions and some types of radioactivity driven by Ws and Z are all due to a single underlying force called the electroweak force. To work, all the force carriers (photon, Ws and Z) have to be massless, which contradicts the experiments – only photons are massless. Physicists Anderson-Englert-Brout-Higgs-Guralnik– Hagen-Kibble came up with a solution called today the Higgs mechanism: Ws and Z appear massive due to interactions with the Higgs boson field permeating everything. 5

Physicist Peter Higgs 6

Why do material things weigh? 7 In the SM elementary particles that constitute matter (quarks, leptons) also appear massive due to their interactions with the Higgs boson field permeating everything. Indirectly, this picture has been supported by the unquestionable success of SM in explaining experimental results, e.g., massive W and Z bosons observed in But to prove that this is, indeed, the mass generation mechanism realized in nature one needs to confirm the existence of the Higgs boson particle, the excitation of the Higgs field.

Interactions with the Higgs boson field 8 The "Higgs field" is a room full of people talking to each other. Mass is a measure of the resistance to motion.

Interactions with the Higgs boson field 9 A celebrity walks into the room and attracts admirers, interacts strongly with them - signing autographs and stopping to chat.

Interactions with the Higgs boson field 10 As the celebrity is surrounded by fans, he/she finds it harder to move across the room - in this analogy, this person acquires mass due to the "field" of fans, with each fan acting like a single Higgs boson.

Interactions with the Higgs boson field 11 If a less popular person enters the room, only a small crowd of fans gathers. It is easier for him/her to move across the room - by analogy, their interaction with the boson field is lower, and so the 2 nd person has a lower mass.

Large Hadron Collider (LHC) Inside is LHC, the most powerful particle accelerator ever built, where 328 feet underground in a 17-mile tunnel, beams of protons (p) traveling at nearly light speeds (~ miles/hour) are smashed head- on. The debris consist of various highly energetic sub-atomic particles created in these collisions which are then detected by the particle detectors. By deciphering readings of these detectors properties of the subatomic particles created in these collisions can be studied. In particular, one expects that Higgs particles will be created, and then decay quickly (~ sec). From the nature of the decay products one infers Higgs’ existence. 13

Inside the Large Hadron Collider (LHC) 14 The p beams circle the tunnel about 40 million times a second. During each crossing, 20 pairs of protons will collide on average. Thus each detector must inspect of order a billion collisions per second.

Visualization of a p-p collision 15

Higgs production modes In the collision protons are broken into quarks and gluons which then may create heavy q/anti-q pair which annihilates into a Higgs pairs of W, Z bosons which then fuse into a Higgs which then decays into a particle/antiparticle pair, which, in turn, may decay into something else, etc. The most useful modes are the Higgs decays into two photons, or two Zs. 16

Background Checks One of the greatest challenges in the Higgs identification quest is the backgrounds. Most of the time Higgs decay products seen by the detectors are indistinguishable from the decay products of other known particles. The ZZ and di-photon modes are the easiest to use since relatively few particles can decay into them – low background. 17

Observation of Higgs-like particle Thanks to the previous searches the possible Higgs mass range has been reduced (LEP at CERN, Tevatron at the Fermi Lab), but before LHC there were not enough data (events) to identify the Higgs boson conclusively. On 07/04/2012 LHC scientists announced discovery of a new particle with mass of 125 GeV (~130 heavier than a Hydrogen atom) and properties consistent with those expected of the Higgs boson. LHC is equipped by 2 separate sets of Higgs detectors (CMS and ATLAS) operating completely independently. The public announcement was preceded by a number of checks. The two collaborations came to the same conclusion. There is 1 chance in 3.5 million that the observed signal is just a statistical fluctuation. Currently, it is being checked whether the Higgs-like particle observed is, indeed, the Higgs boson. The current status is that it is “very, very likely”. The official announcement will be made later this year (possibly March). 18

SM does not include quantum gravity 5/6 of the mass of the universe is made of the dark matter (that can’t be seen in telescopes) Need to go beyond SM. Higgs, neutrino masses are among the very few clues. LHC costs $10bn. What are we buying? For the last few centuries exploring the outer frontier of our knowledge of nature has been pushing modern technology to its limits, often yielding new technology of great practical importance. For example, 1. At LHC collision data analysis pushes the limits of the real time computing. 2. Data sharing method used at CERN contributed to the development of World Wide Web. 3. More generally, without discovery of electron and development of quantum mechanics significant part of today’s technology would not exists.

D. Lincoln, “The Higgs Boson: Is the End in Sight?”, The Physics Teacher, 50 (2012) CNN, BBC news articles, CERN website The Higgs Boson, Part I The Higgs Boson, Part II: What is Mass? Higgs Boson Part III: How to Discover a Particle John Ellis, a theoretical physicist, answers the question "What is the Higgs boson?" LHC description