Higgs Discovery - Lisa Randall

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Here is a summary of the key points about the Higgs discovery from the excerpt:

• On July 4, 2012, CERN announced the discovery of a new particle that is likely the long-sought Higgs boson. This was a significant discovery, not just a hint or partial evidence.

• The Higgs boson is important because it is connected to the Higgs mechanism, which gives elementary particles their mass. Understanding particle masses is crucial for the Standard Model of particle physics.

• While most expected a Higgs-like particle, its properties were unknown and not guaranteed to be detectable with current experiments. Finding it confirmed consistency of the Standard Model.

• More data is still needed to fully characterize the new particle and determine if it perfectly matches expectations of a single Higgs boson or hints at new physics.

• The discovery confirms that the initial symmetry of the universe was spontaneously broken as it evolved, allowing particles to acquire mass and build atomic structures.

• It was an exciting moment for physics and generated significant public interest, though some aspects like the Higgs mechanism itself remained bewildering to non-experts.

• The author uses analogies like searching for a friend in a noisy stadium to describe the challenge of making this elusive particle discovery amid quantum background fluctuations.

• The Higgs mechanism is what gives elementary particles their mass. It relies on the existence of the Higgs field, which exists throughout space.

• Particles interact with the Higgs field, acquiring mass as a result. Heavier particles interact more strongly with the field.

• The Higgs boson is a particle associated with the Higgs field. When energy is added to jiggle the Higgs field, it can create Higgs boson particles.

• Finding the Higgs boson provides evidence that the Higgs mechanism operates in nature. But the Higgs boson itself is not essential for particles to have mass - it is the Higgs field that is responsible.

• At the LHC, physicists had to sift through trillions of collision events to find rare Higgs boson production among more common background particles. With more data they were able to identify a clear signal that was distinct from the background.

• This confirmed the existence of a new particle associated with the Higgs field and mass generation, providing strong evidence that the Higgs mechanism is realized in the Standard Model. But further data is still needed to fully understand the Higgs particle’s properties.

• The dark energy associated with empty space contributes to the gravitational influence of the universe. This nonzero energy, known as dark energy, gives empty space its own intrinsic energy density.

• Even without matter, empty space possesses energy according to Einstein’s theory of general relativity. The energy of empty space, including from fields like the Higgs field, has measurable consequences like accelerating the expansion of the universe.

• The Higgs field is akin to a pencil standing on end. It has lowest energy in a nonzero state rather than at zero, breaking electroweak symmetry. This nonzero value generates particle masses.

• The Higgs boson gets its own mass from interacting with the Higgs field, just as other particles do. Its self-interactions could provide clues beyond the Standard Model.

• At the LHC, the Higgs decays almost immediately into heavier Standard Model particles like bottom quarks. Experimenters look for these decay products to identify Higgs production.

• The Higgs decays most to bottom quarks and antiquarks as they are the heaviest particles light enough for the decay to conserve energy and momentum.

• The Higgs boson decays can produce other particles without violating conservation laws, like decaying into a bottom quark and antiquark which have opposite charges and cancel out.

• The dominant decay is expected to be into bottom quarks, but these are hard to detect at the LHC.

• The two decay modes that helped with discovery were into photons and weak gauge bosons.

• Decays into photons and gauge bosons happen through quantum effects involving virtual heavy particles decaying and emitting photons/bosons.

• In late 2011, the ATLAS and CMS experiments saw hints of a new particle decaying to two photons above background expectations, but not strong enough for discovery yet.

• Over 2012, more data was collected to have enough collisions to confirm if the excess was a real signal or statistical fluctuation.

• By summer 2012, both experiments had collected enough data to announce the discovery of a new Higgs-like particle with a mass around 125 GeV.

• The Moriond conference in March discussed possible “faster than light neutrinos” and modest updates on the Higgs boson, but there was no definitive discovery to report at that time. Most did not expect a discovery by March.

• Discovery came sooner than expected because CERN engineers boosted the particle accelerator’s intensity and experimenters advanced analysis techniques.

• By December 2012, most expected the announced Higgs signal would be confirmed, though some theorists hoped it would be disproved due to interesting theoretical implications.

• François Englert, one of the six original theorists who developed the Higgs mechanism, was in attendance, so talks referred to the “scalar particle” rather than “Higgs boson.”

• Englert and Peter Higgs both took winding paths to physics. Englert’s story highlighted serendipitous opportunities that led him to crucial collaborations.

• Between March and July, engineers and experimenters worked hard while results fluctuated. The 5 sigma discovery thresholds were only met in the last days before announcement.

• Both CMS and ATLAS experiments reported 5 sigma evidence of a discovery, pointing to a particle connected to the Higgs mechanism. Its properties provided opportunities to study multiple decay modes.

• Scientists at the LHC have discovered a new particle that has properties consistent with the predicted Higgs boson. However, more data is needed to confirm if it is definitively the Higgs boson or part of a more complex theory.

• Further measurements of how the particle decays into different particles like bottom quarks, photons, etc. will help determine if its properties match the standard Higgs boson predictions precisely or show deviations indicating new physics.

• The Tevatron accelerator at Fermilab provided some early hints of a Higgs-like particle but did not have enough data for a clear discovery. The LHC has more powerful collisions needed for detailed investigation.

• Continued running of the LHC in the coming months and years will allow collection of more collision data, permitting more precise measurements of the new particle’s properties and searches for additional new particles or interactions beyond the standard model.

• Whatever is found, it will help guide the path forward for particle physics as more fundamental questions about mass and the hierarchy problem remain open. Both experimental and theoretical work will continue to build upon this discovery.

• Theorists will continue pursuing solutions to the hierarchy problem of why the Higgs boson mass is so much lower than the Planck scale.

• Researchers like the author will analyze how recently discovered Higgs boson mass fits with existing particle theories like supersymmetry. Previous theories may need modification to reconcile with new LHC data.

• The author’s investigations focused partly on supersymmetry. If true, each known particle has a heavier supersymmetric partner with the same charges but different spin. The LHC energy should be sufficient to produce these superpartners if light enough.

• However, current limits are challenging standard supersymmetry models. The measured Higgs boson mass also stretches simpler supersymmetry models. This raises questions if theorists have been on the wrong track or if previous models were too simplistic.

• The author and collaborators stumbled upon a possible explanation - a version of supersymmetry where some superpartners have large masses while others do not. This difference could explain LHC search results so far and allow for a higher Higgs boson mass. Their model unexpectedly fit the initial Higgs boson mass measurement.

• In general, modified theories will require different search strategies than standard models at the LHC, to ensure all possible signatures are explored in the enormous data. The LHC will also search for dark matter and new heavier particles.

• The Higgs mechanism is an elegant theoretical idea that allows elementary particles like quarks and leptons to acquire mass, without which all particles would have to be massless.

• Without the Higgs mechanism, theories with massive particles would make incorrect predictions at high energies. The Higgs mechanism elegantly solves this issue.

• It relies on the phenomenon of spontaneous symmetry breaking, where a symmetry exists in the laws of physics but is not preserved by the actual state of the system.

• Examples of spontaneous symmetry breaking include choosing between left and right glasses at a circular table, or a pencil spontaneously falling in a particular direction after balancing on its tip.

• The weak force is also subject to spontaneous symmetry breaking. Its short range suggested its gauge bosons must have mass, but original theories only worked for massless particles.

• The Higgs mechanism acknowledges both the required internal symmetry of the weak force, and the necessity of this symmetry being spontaneously broken, in order to explain massive particles consistently at all energies.

• Gauge bosons can have three possible polarizations if they are massive, but only two if they are massless. This poses a problem for theories of massive gauge bosons.

• Theories without an internal symmetry make incorrect high-energy predictions due to an extra polarization of massive gauge bosons. But this extra polarization is real and needed for low-energy interactions.

• The Higgs mechanism resolves this by introducing a Higgs field that takes on a nonzero value in vacuum. This spontaneously breaks the internal symmetry at low energies but not high energies.

• Two Higgs fields (Higgs1 and Higgs2) experience the weak force. When one of the Higgs fields takes a nonzero value in vacuum, it coats spacetime with weak charge even without particles.

• This nonzero Higgs field value is the origin of particle masses. It hides the internal symmetry at low energies so massive gauge bosons can have three polarizations, while preserving the symmetry at high energies to eliminate bad predictions.

So in summary, the Higgs mechanism introduces a Higgs field that acquires a vacuum value, spontaneously breaking the symmetry at low energies and reconciling the requirements for a theory of massive gauge bosons.

• The Higgs field permeates all of space, giving it a weak charge. This weak charge acts like a “fog” or “paint” that blocks the weak gauge bosons from traveling long distances.

• At short distances, there is very little weak charge, so particles can travel freely. But at long distances, particles encounter more weak charge, blocking their movement.

• This means weak gauge bosons can only travel very short distances (about 0.00001 picometers), giving them an effective mass like other massive particles. Their movement is “interrupted” by the weak charge in the vacuum.

• The density of weak charge in the vacuum corresponds to charges separated by 0.00001 picometers. This gives the weak gauge bosons (W and Z bosons) their measured masses of around 100 GeV.

• Quarks and leptons also acquire mass through their interaction with the Higgs field and vacuum weak charge over long distances. Without the Higgs field, they would be massless.

• When one of the two Higgs fields takes a non-zero value, it spontaneously breaks the weak force symmetry that treats the two Higgs fields as equivalent. This is necessary to give particles associated with the weak force (weak gauge bosons, quarks, leptons) their masses.

• In the 1960s, physicists developed the electroweak theory which unified the weak and electromagnetic forces. This theory proposed that at very high energies in the early universe, there were four weak gauge bosons, not including the photon.

• The photon as we know it today is actually a mixture of two of the original four gauge bosons. It became massless because unlike the other bosons, it is not obstructed by the Higgs field that permeates the vacuum. Only the photon can communicate long-range electromagnetic forces without interference from the vacuum.

• The electroweak theory successfully explains the origin and properties of the photon. Previously, physicists thought they fully understood the photon, but it was only through the more complex electroweak theory that its true nature was revealed.

• A key part of the electroweak theory is the Higgs mechanism, which gives particles mass through interactions with the Higgs field. Finding the hypothesized Higgs boson particle would confirm this mechanism, but its unexpectedly low predicted mass poses theoretical problems for physicists.

• Ultimately, experiments at the Large Hadron Collider aim to discover new particles like the Higgs boson that can help explain how electroweak symmetry breaking occurs and particles acquire their masses as predicted by the Standard Model.

• Without a Higgs mechanism, elementary particles in the Standard Model would not be allowed to have mass. Their masses would be a mystery according to E=mc^2.

• Symmetries play an important role in theories of fundamental forces by filtering out “unphysical” particles that don’t behave as expected. This ensures theoretical predictions are sensible.

• For massless force carriers like photons, symmetries solve the problem of bad high-energy behaviors from unphysical particles.

• However, weak force carriers are known to have mass. Giving them mass introduces an additional physical mode of oscillation that symmetries would filter out, causing issues.

• Fermions also cannot have mass without breaking symmetries, as left-handed and right-handed fermions experience different weak forces. Mass would connect them.

• The Higgs mechanism provides a solution by introducing a Higgs field that permeates the vacuum and carries weak charge. Interactions with this field can give particles mass without causing problems.

• Heavier particles interact more with the Higgs field, lighter ones less. The photon remains massless as it does not interact with the weak charge of the Higgs field.

So in summary, the Higgs mechanism is needed to explain how elementary particles can have the masses observed while respecting the underlying symmetries of force-carrying fields in the Standard Model.

• The Higgs mechanism explains why some elementary particles like photons have zero mass, while others like the W and Z bosons have non-zero mass.

• It does this by positing a Higgs field that permeates the vacuum. This field breaks the electroweak symmetry of the weak force at very high energies.

• Particles that do not interact with the Higgs field, like photons, remain massless. Those that do interact acquire mass proportional to the density of the Higgs field.

• The Higgs mechanism associates electroweak symmetry breaking with a specific energy scale, set by the distribution of charges in the Higgs field.

• At high energies above this scale, particles behave as if massless and electroweak symmetry is restored. Below this scale, particles appear massive due to interactions with the Higgs field.

• This allows the theory to make sensible predictions at both low and high energies, resolving issues that would arise from particles with fixed masses.

• Spontaneous symmetry breaking is the mechanism by which the Higgs field acquires a non-zero value in the vacuum, breaking electroweak symmetry.

• Evidence for the Higgs mechanism would be observation of the Higgs boson particle, which results from excitations in the Higgs field. Experiments at the LHC seek to discover this missing particle.

• Fluctuations in the Higgs field can produce Higgs bosons, just as fluctuations in the electromagnetic field produce photons.

• The Higgs field gives mass to elementary particles through the Higgs mechanism. Evidence that it exists is that particles have mass.

• Discovering the Higgs boson would confirm the Higgs mechanism as the origin of particle masses.

• While called the “God particle” by the media, the Higgs boson is just a theoretical particle and not something mystical.

• Theoretical reasons like avoiding problems at high energies strongly suggest something like the Higgs boson must exist to save the Standard Model.

• Precision electroweak data also favors a relatively light Higgs boson below around 140 GeV.

• The Higgs boson is hard to detect because it interacts most strongly with heavy particles like the top quark, but colliders like the LHC produce collisions of light particles.

• Higgs bosons are primarily produced through interactions involving virtual heavy particles like the top quark or weak bosons.

• Once produced, the Higgs boson immediately decays into the particles it interacts with most, like bottom quarks, making its decays hard to identify.

• The Higgs mechanism gives mass to particles that interact with the Higgs field. For a particle to be produced from Higgs boson decay, it must weigh less than half the Higgs boson mass to conserve energy.

• A relatively heavy Higgs boson (heavier than twice the W boson mass but less than twice the top quark mass) would decay primarily into W and Z bosons. This would make Higgs discovery simple since experiments know how to identify Ws and Zs.

• A light Higgs boson, as expected, would decay primarily into bottom quarks. This is difficult to observe due to large backgrounds from other quark/gluon production. Alternatives like tau leptons or photon pairs are being searched for despite lower rates.

• Experiments aim to fully characterize the Higgs boson through both production and decay to test the Standard Model and search for new physics beyond it. The nature of the Higgs sector, including possible additional particles, could provide insights.

• Finding the Higgs boson is important but not the end goal - it is hoped the LHC will shed light on the hierarchy problem of why mass scales differ so greatly in fundamental particle interactions. New physics may be needed to explain this.

• Gravitational interactions depend inversely on the Planck mass, which is extremely large. This explains why gravity is much weaker than the other fundamental forces.

• Quantum field theory predicts that the weak interaction mass scale and Planck mass scale should be similar, but they differ by 16 orders of magnitude. This is known as the hierarchy problem.

• Virtual particles in quantum loop corrections should contribute to the Higgs boson mass and drive it up to the Planck scale. But the observed Higgs mass is much lower, requiring an unnatural cancellation of contributions to 16 decimal places.

• No simple extension to the Standard Model satisfactorily solves the hierarchy problem. Supersymmetry and other theories propose new particles and symmetries that could stabilize the Higgs mass naturaly at the weak scale.

• Solving the hierarchy problem is a major goal of the LHC experiments, as it may reveal new physics beyond the Standard Model needed to adequately address this discrepancy between theoretical predictions and observations.