Scientists at the European Organisation for Nuclear Research - CERN - say they have found signs of – although not yet conclusively discovered – the Higgs boson, an elementary particle which is the missing link in the Standard Model of physics.
The famed particle is the missing link in current theories of physics, used to explain how everything gains its mass. Rumors have been crashing about the scientific community for weeks on these findings.
Fabiola Gianotti, the scientist in charge of the ATLAS experiment at the Large Hadron Collider, said the signal was centred at around 126 – GeV – Giga Electron Volts.
“I think it would be extremely kind of the Higgs boson to be here,” Gianotti told a seminar to discuss the findings ::::
“But it is too early” for final conclusions, Gianotti added.
“More studies and more data are needed. The next few months will be very exciting…I don’t know what the conclusions will be” said Gianotti.
The Higgs boson is, in theory, the particle that gives mass to all other fundamental particles.
While its discovery would cement current knowledge about particles such as electrons and photons, results of work at CERN could also prove it does not exist, which would force physicists to rethink the Standard Model.
Finding the Higgs is the central goal for the $10bn Large Hadron Collider – a 27km (17-mile) circumference accelerator ring of superconducting magnets – designed to re-create the conditions just after the Big Bang in an attempt to answer fundamental questions of science, indeed the Universe itself.
Physicists do not know the mass of the Higgs itself, which has made hunting for it more difficult. They have to look for the particle by systematically searching a range of masses where it is predicted to be.
In 2000, Cern’s LEP particle collider ruled the Higgs out up to a mass of 114 GeV. To search for the Higgs beyond that mass, physicists needed a much more powerful machine, the Large Hadron Collider.
Two detectors Atlas and CMS are looking for signs of it among the billions of collisions that are occurring in each experiment. Hints of the Higgs would look like a little spike or blip in physicists’ graphs.
For more than a week, rumors have been crashing around the web on physics blogs that Atlas and CMS see a Higgs signal at 126 GeV, between the 2.5 and 3.5 sigma level of certainty.
These numbers represent a measure of the likelihood that any blip the scientists see is down to chance, rather than caused by a real physical phenomenon.
The numbers quoted on Tuesday – 126 - GeV – would still not be enough for CERN to make a definitive statement. Three sigma counts as an “observation”, while five sigma is regarded as the threshold for claiming a discovery.
CERN’s Director-General Rolf-Dieter Heuer has said via email that the announcement would not be conclusive.
Such a spike could diminish as more data is gathered. But if Atlas and CMS both see a signal in the same place, there would be an irresistible temptation for physicists to celebrate – behind closed doors.
In public, physicists would be obligated to say that a definitive yes or no would not be available until 2012.
LHC – CERN – ALICE
Published November 9, 2010
Scientists Recreate Big Bang: Physicists working on the ALICE experiment in the Large Hadron Collider at the European Organisation for Nuclear Research – CERN – have started smashing heavy lead ions together at close to the speed of light – in a process recreating the universe as it was 13.7 billion years ago.
The successful collision of lead ions in the accelerator at record energies allows matter to be probed as it would have been in the first moments of the universe’s existence.
Dr David Evans from the University of Birmingham describes the collisions as mini big bangs, creating the highest temperatures and densities ever achieved in an experiment.
Dr Evans says it is generating incredibly hot and dense sub-atomic fireballs with temperatures of over 10 trillion degrees – a million times hotter than the centre of the Sun.
“At these temperatures even protons and neutrons – which make up the nuclei of atoms melt – resulting in a hot dense soup of quarks and gluons known as a Quark-Gluon Plasma,” he said.
By studying this plasma, physicists hope to learn more about the strong nuclear force, one of the four fundamental forces of nature. The others are the weak nuclear force, the electromagnetic force and gravity.
Dr Evans says the strong force not only binds the nuclei of atoms together but is responsible for 98 per cent of their mass.
“I now look forward to studying a tiny piece of what the universe was made of just a millionth of a second after the big bang,” he said.
This new phase of the Large Hadron Collider program comes after seven months of successfully colliding hydrogen proton packets at high energies.
Extreme Conditions: The 10,000 ton ALICE experiment has been specifically designed to study the extreme conditions produced in these lead-ion collisions.
It is one of four main detectors on the giant 27-kilometre underground ring designed to offer up insights about the earliest moments in our universe’s life.
Dr Stephen Myers, the director of accelerators and technology at CERN says the high energy levels involved in lead ion collisions means things could start happening very quickly.
“Lead ions are much more complicated particles than hydrogen protons and so it’s a very exciting time.”
“We’re slamming these ions into each other at over 99.9 per cent of light speed but it’s not the speed, it’s the huge mass and energy levels which is important,” he said.
Until now the main thrust of the Large Hadron Collider has been the search for the Higgs Boson the so called god particle that is thought to generate a Higgs field which would give all other particles their mass.
Myers points out other important experiments are also being carried out, including the search for antimatter, dark matter and supersymmetry.
“It’s all about finding new physics, that’s why we built the Large Hadron Collider,” Myers said.
The Higgs boson – the God particle – in popular media) is a hypothetical massive elementary particle that is predicted to exist by the Standard Model of particle physics. The Higgs boson is an integral part of the theoretical Higgs mechanism. If shown to exist, it would help explain why other particles can have mass. It is the only predicted elementary particle that has not yet been observed in particle physics experiments. Theories that do not need the Higgs boson also exist and would be considered if the existence of the Higgs Boson were ruled out. They are described as Higgsless models.
If shown to exist, the Higgs mechanism would also explain why the W and Z bosons, which mediate weak interactions, are massive whereas the related photon, which mediates electromagnetism, is massless. The Higgs boson is expected to be in a class of particles known as scalar bosons. (Bosons are particles with integer spin, and scalar bosons have spin 0.)
Experiments attempting to find the particle are currently being performed using the Large Hadron Collider at CERN, and were performed at Fermilab’s Tevatron until its closure in late 2011. Some theories suggest that any mechanism capable of generating the masses of elementary particles must be visible at energies below 1.4 TeV; therefore, the LHC is expected to be able to provide experimental evidence of the existence or non-existence of the Higgs boson.
On 12 December 2011, the ATLAS collaboration at the LHC found that a Higgs mass in the range from 145 to 206 GeV was excluded at the 95% confidence level. On 13 December 2011, experimental results were announced from the ATLAS and CMS experiments, suggesting that if the Higgs Boson exists, it is probably limited to a range of 115–130 GeV at the 3.6 sigma level (ATLAS) or 117–127 GeV at the 2.6 sigma level (CMS), and indicating possible scope for a 124 GeV (CMS) or 125-126 GeV (ATLAS) Higgs. As of 13 December 2011, a joint estimate is not available.
Theory Origin: The Higgs mechanism is a process by which vector bosons can get a mass. It was proposed in 1964 independently and almost simultaneously by three groups of physicists: François Englert and Robert Brout; by Peter Higgs (inspired by ideas of Philip Anderson); and by Gerald Guralnik, C. R. Hagen, and Tom Kibble.
The three papers written on this discovery were each recognized as milestone papers during Physical Review Letters’s 50th anniversary celebration. While each of these famous papers took similar approaches, the contributions and differences between the 1964 PRL symmetry breaking papers are noteworthy. These six physicists were also awarded the 2010 J. J. Sakurai Prize for Theoretical Particle Physics for this work.
The 1964 PRL papers by Higgs and by Guralnik, Hagen, and Kibble (GHK) both displayed equations for the field that would eventually become known as the Higgs boson. In the paper by Higgs the boson is massive, and in a closing sentence Higgs writes that “an essential feature” of the theory “is the prediction of incomplete multiplets of scalar and vector bosons”. In the model described in the GHK paper the boson is massless and decoupled from the massive states. In recent reviews of the topic, Guralnik states that in the GHK model the boson is massless only in a lowest-order approximation, but it is not subject to any constraint and it acquires mass at higher orders. Additionally, he states that the GHK paper was the only one to show that there are no massless Nambu-Goldstone bosons in the model and to give a complete analysis of the general Higgs mechanism. Following the publication of the 1964 PRL papers, the properties of the model were further discussed by Guralnik in 1965 and by Higgs in 1966.
Steven Weinberg and Abdus Salam were the first to apply the Higgs mechanism to the electroweak symmetry breaking. The Higgs mechanism not only explains how the electroweak vector bosons get a mass, but predicts the ratio between the W boson and Z boson masses as well as their couplings with each other and with the Standard Model quarks and leptons. Many of these predictions have been verified by precise measurements performed at the LEP and the SLC colliders, thus confirming that the Higgs mechanism takes place in nature.
The Higgs boson’s existence is not a strictly necessary consequence of the Higgs mechanism: the Higgs boson exists in some but not all theories which use the Higgs mechanism. For example, the Higgs boson exists in the Standard Model and the Minimal Supersymmetric Standard Model yet is not expected to exist in Higgsless models, such as Technicolor. A goal of the LHC and Tevatron experiments is to distinguish among these models and determine if the Higgs boson exists or not.
Theoretical Overview: The Higgs boson particle is the quantum of the theoretical Higgs field. In empty space, the Higgs field has an amplitude different from zero; i.e. a non-zero vacuum expectation value. The existence of this non-zero vacuum expectation plays a fundamental role; it gives mass to every elementary particle that couples to the Higgs field, including the Higgs boson itself. The acquisition of a non-zero vacuum expectation value spontaneously breaks electroweak gauge symmetry. This is the Higgs mechanism, which is the simplest process capable of giving mass to the gauge bosons while remaining compatible with gauge theories. This field is analogous to a pool of molasses that “sticks” to the otherwise massless fundamental particles that travel through the field, converting them into particles with mass that form (for example) the components of atoms.
In the Standard Model, the Higgs field consists of two neutral and two charged component fields. Both of the charged components and one of the neutral fields are Goldstone bosons, which act as the longitudinal third-polarization components of the massive W+, W–, and Z bosons. The quantum of the remaining neutral component corresponds to the massive Higgs boson. Since the Higgs field is a scalar field, the Higgs boson has no spin, hence no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even.
The Standard Model does not predict the mass of the Higgs boson. If that mass is between 115 and 180 GeV/c2, then the Standard Model can be valid at energy scales all the way up to the Planck scale (1016 TeV). Many theorists expect new physics beyond the Standard Model to emerge at the TeV-scale, based on unsatisfactory properties of the Standard Model. The highest possible mass scale allowed for the Higgs boson (or some other electroweak symmetry breaking mechanism) is 1.4 TeV; beyond this point, the Standard Model becomes inconsistent without such a mechanism, because unitarity is violated in certain scattering processes. There are over a hundred theoretical Higgs-mass predictions.
Extensions to the Standard Model including supersymmetry (SUSY) predict the existence of families of Higgs bosons, rather than the one Higgs particle of the Standard Model. Among the SUSY models, in the Minimal Supersymmetric Standard Model (MSSM) the Higgs mechanism yields the smallest number of Higgs bosons: there are two Higgs doublets, leading to the existence of a quintet of scalar particles, two CP-even neutral Higgs bosons h and H, a CP-odd neutral Higgs boson A, and two charged Higgs particles H±. Many supersymmetric models predict that the lightest Higgs boson will have a mass only slightly above the current experimental limits, at around 120 GeV/c2 or less.
Experimental Search: As of November 2011, the Higgs boson has yet to be confirmed experimentally, despite large efforts invested in accelerator experiments at CERN and Fermilab.
Prior to the year 2000, the data gathered at the LEP collider at CERN allowed an experimental lower bound to be set for the mass of the Standard Model Higgs boson of 114.4 GeV/c2 at the 95% confidence level. The same experiment has produced a small number of events that could be interpreted as resulting from Higgs bosons with a mass just above this cut off — around 115 GeV—but the number of events was insufficient to draw definite conclusions. The LEP was shut down in 2000 due to construction of its successor, the LHC, which is expected to be able to confirm or reject the existence of the Higgs boson. Full operational mode was delayed until mid-November 2009, because of a serious fault discovered with a number of magnets during the calibration and startup phase.
At the Fermilab Tevatron, there were ongoing experiments searching for the Higgs boson. As of July 2010, combined data from CDF and DØ experiments at the Tevatron were sufficient to exclude the Higgs boson in the range 158 GeV/c2 – 175 GeV/c2 at the 95% confidence level. Preliminary results as of July 2011 have since extended the excluded region to the range 156 GeV/c2 – 177 GeV/c2 at the 90% confidence level. Data collection and analysis in search of Higgs are intensifying since 30 March 2010 when the LHC began operating at 3.5 TeV. Preliminary results from the ATLAS and CMS experiments at the LHC as of July 2011 exclude a Standard Model Higgs boson in the mass range 155 GeV/c2 – 190 GeV/c2 and 149 GeV/c2 – 206 GeV/c2, respectively, at the 95% confidence level. All of the above confidence intervals were derived using the CLs method.
It may be possible to estimate the mass of the Higgs boson indirectly. In the Standard Model, the Higgs boson has a number of indirect effects; most notably, Higgs loops result in tiny corrections to masses of W and Z bosons. Precision measurements of electroweak parameters, such as the Fermi constant and masses of W/Z bosons, can be used to constrain the mass of the Higgs. As of 2006, measurements of electroweak observables allowed the exclusion of a Standard Model Higgs boson having a mass greater than 285 GeV/c2 at 95% CL, and estimated its mass to be 129+74
−49 GeV/c2 (the central value corresponding to approximately 138 proton masses). As of August 2009, the Standard Model Higgs boson is excluded by electroweak measurements above 186 GeV at the 95% confidence level. These indirect constraints make the assumption that the Standard Model is correct. It may still be possible to discover a Higgs boson above 186 GeV if it is accompanied by other particles between the Standard Model and GUT scales.
In a 2009 preprint, it was suggested that the Higgs boson might not only interact with the above-mentioned particles of the Standard model of particle physics, but also with the mysterious weakly interacting massive particles (or WIMPS) that may form dark matter, and which play an important role in recent astrophysics.
Various reports of potential evidence for the existence of the Higgs boson have appeared in recent years but to date none has provided convincing evidence. In April 2011, there were suggestions in the media that evidence for the Higgs boson might have been discovered at the LHC in Geneva, Switzerland but these had been debunked by mid May. In regard to these rumors Jon Butterworth, a member of the High Energy Physics group on the Atlas experiment, stated they were not a hoax, but were based on unofficial, unreviewed results. The LHC detected possible signs of the particle, which were reported in July 2011, the ATLAS Note concluding: “In the low mass range (c 120−140 GeV) an excess of events with a significance of approximately 2.8 sigma above the background expectation is observed” and the BBC reporting that “interesting particle events at a mass of between 140 and 145 GeV” were found. These findings were repeated shortly thereafter by researchers at the Tevatron with a spokesman stating that: “There are some intriguing things going on around a mass of 140GeV.”
On 22 August it was reported that the anomalous results had become insignificant on the inclusion of more data from ATLAS and CMS and that the non-existence of the particle had been confirmed by LHC collisions to 95% certainty between 145–466 GeV (except for a few small islands around 250 GeV). A combined analysis of ATLAS and CMS data, published in November 2011, further narrowed the window for the allowed values of the Higgs boson mass to 114-141 GeV. On 12 December 2011, the ATLAS collaboration found that a Higgs mass in the range from 145 to 206 GeV was excluded at the 95% confidence level.
At almost the same time, on 13 December 2011, the results published by ATLAS excluded all masses outside the range of 115–130 GeV, whereas the results published by the CMS team narrowed the range to 117–127 GeV. The ATLAS result corresponds to a 125–126 GeV Higgs of at most 3.6 standard deviations, and the CMS team reported a 124-GeV signal at a statistical level of at most 2.6 standard deviations. As of 13 December 2011, a joint estimate is not available yet.
The God Particle: The Higgs boson is often referred to as “the God particle” by the media, after the title of Leon Lederman’s book, The God Particle: If the Universe Is the Answer, What Is the Question? Lederman initially wanted to call it the “goddamn particle,” but his editor would not let him. While use of this term may have contributed to increased media interest in particle physics and the Large Hadron Collider, many scientists dislike it, since it overstates the particle’s importance, not least since its discovery would still leave unanswered questions about the unification of QCD, the electroweak interaction and gravity, and the ultimate origin of the universe. A renaming competition conducted by the science correspondent for the British Guardian newspaper chose the name “the champagne bottle boson” as the best from among their submissions: “The bottom of a champagne bottle is in the shape of the Higgs potential and is often used as an illustration in physics lectures. So it’s not an embarrassingly grandiose name, it is memorable, and it has some physics connection too.
Higgsless Model: In particle physics, a Higgsless model is a model that does not involve the Higgs boson or in which the Higgs field is not dynamic. Such models must employ a different mechanism of mass generation, electroweak symmetry breaking and unitarity.
In the years since the Higgs mechanism was first described, there have been several alternatives proposed. All of the alternative mechanisms use strongly interacting dynamics to produce a vacuum expectation value that breaks electroweak symmetry. A partial list of these alternative mechanisms includes:
- Technicolor models break electroweak symmetry through new gauge interactions, which were originally modeled on quantum chromodynamics.
- Extra-dimensional Higgsless models use the fifth component of the gauge fields to play the role of the Higgs fields. It is possible to produce electroweak symmetry breaking by imposing certain boundary conditions on the extra dimensional fields, increasing theunitarity breakdown scale up to the energy scale of the extra dimension. Through the AdS/QCD correspondence this model can be related to technicolor models and to “UnHiggs” models in which the Higgs field is of unparticle nature.
- Models of composite W and Z vector bosons.
- Top quark condensate.
- “Unitary Weyl gauge”. If one adds a suitable gravitational term to the standard model action with gravitational coupling, the theory becomes locally scale invariant (i.e. Weyl invariant) in the unitary gauge for the local SU(2). Weyl transformations act multiplicatively on the Higgs field, so one can fix the Weyl gauge by requiring the Higgs scalar to be a constant.
- Asymptotically safe weak interactions based on some nonlinear sigma models.
- “Regular Charge Monopole Theory” by Eliyahu Comay.
- Preon and models inspired by preons such as Ribbon model of Standard Model particles by Sundance Bilson-Thompson, based in braid theory and compatible with loop quantum gravity and similar theories. This model not only explains mass but leads to an interpretation of electric charge as a topological quantity (twists carried on the individual ribbons) and color charge as modes of twisting.
- Symmetry breaking driven by non-equilibrium dynamics of quantum fields above the electroweak scale .
- Unparticle physics and the unhiggs . These are models that posit that the Higgs sector and higgs boson are scaling invariant, also known as unparticle physics.
- In theory of superfluid vacuum masses of elementary particles can arise as a result of interaction with the physical vacuum, similarly to the gap generation mechanism in superconductors.
- UV-Completion by Classicalization, in which the unitarization of the WW scattering happens by creation of classical configurations.
- This upper bound for the Higgs boson mass is a prediction within the minimal Standard Model assuming that it remains a consistent theory up to the Planck scale. In extensions of the SM, this bound can be loosened or, in the case of supersymmetry theories, lowered. The lower bound which results from direct experimental exclusion by LEP is valid for most extensions of the SM, but can be circumvented in special cases. [PDF]
- The masses of composite particles such as the proton and neutron would only be partly due to the Higgs mechanism, and are already understood as a consequence of the strong interaction.
- G.S. Guralnik, C.R. Hagen and T.W.B. Kibble (1964). “Global Conservation Laws and Massless Particles”. Physical Review Letters 13 (20): 585. Bibcode 1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585.
- G.S. Guralnik (2009). “The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles”. International Journal of Modern Physics A 24 (14): 2601–2627. arXiv:0907.3466. Bibcode 2009IJMPA..24.2601G.doi:10.1142/S0217751X09045431.
- Guralnik, G S; Hagen, C R and Kibble, T W B (1967). Broken Symmetries and the Goldstone Theorem. Advances in Physics, vol. 2
- F. Englert and R. Brout (1964). “Broken Symmetry and the Mass of Gauge Vector Mesons”. Physical Review Letters 13 (9): 321. Bibcode 1964PhRvL..13..321E. doi: 10.1103/PhysRevLett.13.321.
- P. Higgs (1964). “Broken Symmetries, Massless Particles and Gauge Fields”. Physics Letters 12 (2): 132. Bibcode 1964PhL….12..132H.doi:10.1016/0031-9163(64)91136-9.
- P. Higgs (1964). “Broken Symmetries and the Masses of Gauge Bosons”. Physical Review Letters 13 (16): 508. Bibcode 1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508.
- P. Higgs (1966). “Spontaneous Symmetry Breakdown without Massless Bosons”. Physical Review 145 (4): 1156. Bibcode 1966PhRv..145.1156H. doi:10.1103/PhysRev.145.1156.
- Y. Nambu and G. Jona-Lasinio (1961). “Dynamical Model of Elementary Particles Based on an Analogy with Superconductivity”. Physical Review 122: 345–358. Bibcode 1961PhRv..122..345N. doi:10.1103/PhysRev.122.345.
- J. Goldstone, A. Salam and S. Weinberg (1962). “Broken Symmetries”. Physical Review 127 (3): 965. Bibcode 1962PhRv..127..965G.doi:10.1103/PhysRev.127.965.
- P.W. Anderson (1963). “Plasmons, Gauge Invariance, and Mass”. Physical Review 130: 439. Bibcode 1963PhRv..130..439A.doi:10.1103/PhysRev.130.439.
- A. Klein and B.W. Lee (1964). “Does Spontaneous Breakdown of Symmetry Imply Zero-Mass Particles?”. Physical Review Letters 12 (10): 266. Bibcode 1964PhRvL..12..266K. doi:10.1103/PhysRevLett.12.266.
- W. Gilbert (1964). “Broken Symmetries and Massless Particles”. Physical Review Letters 12 (25): 713. Bibcode 1964PhRvL..12..713G.doi:10.1103/PhysRevLett.12.713.
- Hunting the Higgs boson at C.M.S. Experiment, at CERN
- The Higgs boson” by the CERN exploratorium.
- Particle Data Group: Review of searches for Higgs bosons.
Transcrip of Live Chat:
Robert M. Roser
Rob Roser has served as a particle physicist at the Fermilab National Accelerator Laboratory inBatavia,Illinois, since 1997. His research focuses on one of the central mysteries in physics: the origin of electroweak symmetry breaking and the mechanism by which fundamental particles are endowed with mass. Among other efforts, his work has engaged him in the hunt for the Higgs boson. Roser is a spokesperson for the collision detector at the Fermilab experiment, a collaboration of nearly 600 high energy physicists, and has published more than 500 peer-reviewed scholarly articles. He is a fellow of the American Physical Society.
Gordon Kane focuses on ways to test, extend, and strengthen the very successful Standard Model of Particle Physics in his research. He has particularly emphasized the supersymmetric extension of the standard model, along with methods for discovering and understanding the Higgs sector.
|The Higgs Boson at Last?|
Thursday December 15, 2011
Welcome everyone to today’s ScienceLive discussion about the discovery, or not, of the Higgs Boson. I’m Daniel Clery from Science magazine’s news team and with me today are Rob Roser of the Fermi National Accelerator Laboratory and Gordon Kane of the Universityof Michigan, Ann Arbor.
The two detector teams at CERN revealed their latest results earlier this week and they weren’t quite as certain as everyone had hoped. The Atlas detector team seemed to see something at a mass of 126 GeV and the CMS detector team also saw signs at around the same mass, but it picked signals at other masses too. So, have they found the Higgs or not?
We have learned a lot about the Higgs in the last few days– we know where it is not… it is not above 128 GeV. However, it can still exist within a narrow region of 114 to 128 GeV. More data is required before we know whether we have found it or not — it is a very exciting time to be a physicist
YES. an experimenter from one experiment can’t say that, but theorists see that two different experiments both saw a signal at about the same mass, and also saw additional channels, so it’s a discovery!
|Comment From Jose|
On how this discovery could help our society?
It is difficult to know how a Higgs discovery will influence society — if we look back at the discovery of the electron 100 or so years ago — I doubt one could have projected iphones and computers… What we do know is that the tools we are using to search for it are of great benefit — from superconducting magnets, high speed computing, world wide web, precision electronics and on and on
help society — in two big ways. first as we understand our universe and our place in it better it affects our lives and how we view ourselves. second looking at the frontier necessarily lads to “spinoffs” such as Rob mentioned that more than pay for all the research costs of all research. the best example is WWW which was specifically develooped at CERN for particle physics
The Higgs boson can decay into many different final states — that is states which experimenters can observe in their detectors. How it decays depends on two things — its mass and how it was produced — namely, proton proton or proton/antipron collisions. The LHC is looking for it decaying into two photons. The Tevatron is looking for it in the similar mass region by looking for two b-quarks. these are complimentary approaches and thus both machines remain very relavent
Reader Andrew asked:
1) What is the discussed higgs decay channel exactly? 2) Why has it been overlooked at the Tevatron?
If the Higgs mechanism generates the masses of fundamental particles, does this mechanism also generate the mass of the Higgs boson itself?
the interactionss with the higggs field generate the masses of quarks, leptons, W and Z bosons, and the higgs itself. the theory is clear on how to calculate that.
Could Fermilab have taken us to this point?
The Tevatron collider has been searching for the Higgs for the better part of a decade. We have shown that we have both the detectors and analysis techniques to find it. The tricky part is that this particle is rarely produced — at the tevatron, we only make of order 3/week in each detector. So one needs to run for a long time in order to acquire sufficient sample to say something meaningful. The tevatron experiments will release their next set of results in 3 months — which will be able to provide strong evidence (if a higgs exists) or exclude it if it does not — stay tuned
for something that gives rise to a pervasive force in the universe, the higgs certainly seems shy. why is it so difficult to pin down and identify? or, are we just not really sure where, or how to look for it? or maybe both?
the higgs field does pervade the universe. the way we see it is by detecting the quanta of the field (like photons are quanta of the electromagnetic f ield), and it is hard to produce and detect the quanta for technical reasons, for example the beams of protons contain up and down quarks that are light so have small mass and the strength of their coupling to the higgs boson is proportional to the mass. similarly detecting the higgs boson is for purely technicala reasons hard. i can elaborate if needed
Finding an excess in the data is the first step in finding the Higgs — the next step is to find enough of them that we can say beyond a shadow of a doubt that what we are seeing is a signal and not just a background fluctuation. This will require more data than what is currently available at either the LHC or Tevatron. If we assume it is found, the next step is to measure its various properties to see if it behaves the way our theoretical understanding tells us it should. Finding it is then the beginning of a quest, not the end!
you can tell it is a higgs boson from several tests. one is that is has to have spin zero which can be seen from decay angular disstributions. another test is that it interacts proportional to mass as a higgs must, and that can be seen from its relative decays to different final states of quarks and leptons. another is that it hass to be seen in WW or ZZ to break t he electroweak symmetry and the few events reported in ZZ show that
Forgive me if my question is naïve, but how does it work that there are two teams looking for the same thing using the same equipment?
At both the LHC and Tevatron, protons travel around in circles at the speed of light. The machine optics manipulate those beams as they travel around to collide inside each of the detectors. So two experiments on the same ring can collect similar data — but each are looking at their own unique collisions — what they share is just the accelerator to deliver the protons
What are the alternatives (besides the Higgs) to explain the signals that the experiments saw?
For any discovery people will propose alternative explanations, and that is good since testing them leadss to better tests and underestandings. In this case it is very hard to make alternatives, and i guess it will be widely accepted as settled soon
I dislike the use of the term “god particle” by the media and the religious cults trying to make a religious war for how we attempt to explain in the best way we can our origins… What are you thoughts on this also…
Joseph — i understand what you are saying — I am sure Leon Lederman (book author) did not title his book — rather the publisher did to improve sales. At this point — I am happy people are paying attention to the science we are doing — and want to support its continuation. Our search is a quest for knowledge
Is it really a discovery like Gordy said? I mean, the golden standard is 5 sigma, and definitely latest results do not satisfy it! Could it be that those results fade away, what do you think about it?
The 5 sigma is a criteria people have chosen. i think as soon as the data are combined from two detectors, which is entirely legitimate, then the signal will indeed be over 5 sigma.
If we find higgs, what is next?
Assuming we found a higgs like object, the next step is to then make lots of them and investigate all of its properties to see if it behaves as expected. This will take a decade at least before we get everything mapped out completely and we really understand what we have found. What we learn in that pursuit will then lead us naturally to next steps
How will the confirmation of the Higgs Mechanism help quantum gravity move forward? Will it rule in or rule out certain approaches to quantum gravity
finding the higgs will have only an indirect on quantum gravity. the higgs will help significantly to lead us to a unified theory, presumably a string theory, that will in turn help unify iwth gravity and quantum gravity
Why is the Higgs boson so difficult to detect? Is it possible that it only exists under certain circumstances?
There are several challenges for finding a Higgs — first, they are not produced that often inside our detectors so we need to collect a lot of data to find it. Second — their decay products mimic many standard model (mundane) physics processes and so you have to really understand these processes to tease out the signal. Some people say finding a higgs is like looking for a needle in a haystack. Its much harder than that because a needle looks different — its like looking for a particular piece of hay in that haystack
I know it might be hard to answer here. But why does there have to be a Higgs boson – can’t there just be a Higgs field? I don’t really understand either – but, all I’ve read is that the Higgs field gives the mechanism for particles having mass.
relativistic quantum field theory is a very well tested framework. in it eveery field has to have quanta.
When you say that different channels have been discovered, what does that mean? Are there unexpected decay channels that have been observed?
the Higgs can decay (convert) into a variety of different final states — such as two W bosons, two photons, two b quarks just to name a few. How it decays depends on the mass of the higgs. Since physicists are not patient — we look at ALL of hte decay channels and sum them together to try to find this object. Before we would be willing to say we have discovered it — we would want to see strong evidence in several different final states that are consistent with each other
I thought the Tevatron was shut down: data still being interpreted, but no new data. Yes? No?
yes bob, the tevatron stopped collecting data in September 2011. However, we still have about 25% new data that has not been analyzed since we last updated in July 2010. We are working on this now and expect to make this result public (with the full data set) in march of 2012 — so you won’t have long to wait…
is the search for the Higgs boson in anyway related to trying to figure out what dark matter in astronomy? or are these two totally unrelated topics?
the search for the higgs boson itself is not directly related to the dark matter searches, but there are major indirect connections. for example the probability of a dark matter particle scattering on a nucleus in a detector depends on the higgs boson mass so now that can be calculated better. there are also connections in the theory that relates them
Are there Higgs particles surrounding me and everything else? How can this be if the Higgs particle immediately decays?
all particles pop in and out of the vacuum for short times that are allowed by the uncertainty principle, including the higgs particle. not only around us but in us. they are typically absorbed back into the vacuum before decaying, but not always.
But still there is a chance that the Higgs won’t be there, right? What would happen then? Are there some falsifable theories that could step in?
Right — we still do not know beyond a shadow of a doubt whether the Higgs particle exits and thus confirming this explanation as do why particles have the masses that they have. This is not our only theory — we have other theories — bringing this chapter to conclusion is important — and if its not there, we will pursue others until we get it…
This is a very exciting time for physics, what is the next move and will it only be possible through the acceleration capabilites at CERN. What role will Fermilab have in this
CERN is now the owner of what we call the energy frontier — and thus it will be driving searches for new physics for quite some time. Fermilab is redefining itself — its next step will be to build a high flux accelerator to look for rare neutrino processes as well as other physics — the so called intensity frontier. We know that the Standard Model is broken when it comes to neutrino’s — so we best investigate them and try to understand what is going on. The Higgs investigation could one day come back to theUnited Statesif we decide to build a lepton collider here.
This is a very good question. The Standard Model higgs boson is really a kind of benchmark because quantum corrections to its m ass make it an unrealistic object. So most people expect what is found to be the higgs boson of supersymmetry. it is surprising, and a clue to how to embed the SM in a larger theory, that the observed one looks so much like the SM one. i and colleagues last summer showed that string theories do predict that the supersymmetric higgs boson should look very much like the SM one, including having this mass, so we are extra excited by this discovery
So you found a particle at 126 Gev, did you learn anything about the properties of this particle that either confirm or contradict the theory
at the moment, one experiment has shown weak evidence for a higgs like object at 126. However the statistics are still sufficiently limited — what you want to measure properties is a strong signal in the two photon mode, and a complimentary peak in the tau tau and b-bbar mode at 126 as well. Then one can start measuring relative rates, etc to start to really understand what kind of object we are seeing. This is going to take years, not months
Some theories predict more types of Higgs particle. Have you detected any signs of those?
yes, supersymmetric and other kinds of theories do predict more higgs bosons. The string theories i mentioned earlier predict the others exist but are too heavy to observe at LHC with it’s limited energy (or even with an eventual 14 TeV energy)
Are physicists surprised they are finding signs of the Higgs at the Energies they are?
With what we know about the standard model of particle physics, theorists can make predictions as to what the best value of mass the Higgs should be at. Based on current indirect measurements — the prediction is right around 126. So, no — people are not surprised by this value — its perhaps the most likely place for it to be.
Rob Roser said, “… at the tevatron, we only make of order 3/week in each detector.” How many, order of magnitude, at CERN?
At CERN, they collide protons together at an energy 3.5 times h igher than at the TEvatron. So they are making a Higgs in each detector at about the rate of one per hour!
Does the existence of a 125 Gev Higgs give any support to supersymmetry?
Yes. first, for a long time it has been known that the lightest higgs boson of supersymmetry should be lighter than about 135 GeV (actually closer to 140 GeV but people make assumptions), so this is consistent. Then the supersymmetric string theories as i mentioned do predict the 125 number and it is a supersymmetric lightest higgs boson
How long could it take to find enough data to declare it exists (or not)?
The LHC is performing amazingly well — delivering 5x more luminosity this year than promised. Expectations for next year based on this years performance is that the experiments should get 4X more data than what they have now. That will be sufficient to either discover a Standard Model Higgs or rule it out. Thus we don’t have long to wait. If the Higgs is not a standard model object but rather say some super symmetric object, then all bets are off and it may take much more data!
Every thing moves across the bosons field, some particles interact with them, others not, sounds like´s the ancient idea of Ether, but more elaborated, what do you think about?
it only sounds like ether if you don’t look closely. for the higgs case there is a quantitative mathematical theory that tells just how the particles interact with the higgs field, which particles do and how strongly, etc. Also, the higgs field carries some quantum numbers and is not like a neutral vacuum
For Roser –how many standard deviations of excess should be seen in order for CMS and ATLAS to declare discovery? I assume this has been discussed. And how do/should experimentalists react when a theorist like Kane openly says “discovery”?
Particle physicsts have a convention that a discovery is 5 sigma — that is the probablillty that a background process could fluctuate to mimic the signal be less than 1 in 2 million. At the moment, the LHC experiments are seeing something at the level of 2 sigma — 1 in a hundred. They have to collect much more data before they get sufficient statistical precision to get 5 sigma
For the benefit of those of us who are not highly fluent with the Standard Model, could you elaborate a little on exactly what was detected?
in this case — the two lhc experiments were performing a search in their data for events which contain exactly two photons. In that set of events, they measure the mass of that system to see if they see a peak — a peak indicates that some heavy object is decaying into the two photons. The weak peaks you see is the first hints there might be something there
What are the implications of the discovery of the presence or absence of the Higgs boson on the neccessity of multiple universes. Isn’t that a predictionof the Standard Model?
The discovery of a higgs boson does not have direct consequences for multiple universes, but indirectly it gets us closer to understanding our own string vacuum. i expect that as we understand our string vacuum it will help us figure out how such theories work, and help us make sense of the arguments for multiple universes in the traditional way physics works
from 1-10 how sure you are , that the higgs is found?
For me — I am a 1. What they showed very convincingly is that its not above 128 GeV. However I am not yet even willing to bet your house its at 126 let alone mine. I need to see more evidence than what is available. That will come in 2012.
Gordy, just curious, have you or Acharya ever discussed the G2-MSSM scenario with Ed Witten and if yes, what does he think about it?
yes, both Bobby Acharya and I have had some discussions withWitten, and he has been mildly encouraging.
Since CMS sees two of them, and appears to exclude the area where ATLAS’ signal peaks, is it possible that there is nothing there? Or perhaps does this mean the Higgs is more complex than we assume?
all of the above. CMS see’s a signal at 124 and ATLAS at 126. That sounds close — but these detectors are amazing. Its not that close. If one overlays the two peaks — they do not lie on top of each other. They should if its there. Its possible one experiment has its energy scale off but I doubt it — these experiments know what they are doing. Thats not to say its not there — but one could have caught a background event fluctuating which dragged the mass up or down. We will have to wait and see…
Why is it legitimate to combine them when presumably it would not be legitimate if they were performed at different places?
High energy physics llikes to have two detectors operating at once for two reasons — one each cross checks the other. Furthermore, they are each making independent measurements. So, one can combine the two data sets and effectively double the luminosity — much easier than running twice as long. Now one has to take into account correlations — to this combo should be left to experts only
The Standard Model strongly implies the quarks and leptons are the last level of taking things apart, it will not go on forever. First, the SM theory is a full relativistic quantum field theory so it makes sense to have the quarks and leptons be the fundamental particles, the theory does not break down at high energies. Second, at very short distancess the strenghts of the forces come together if the quarks and leptons are the fundamental ones, but would not if they were not. one can list more such reasons. of course we expect the electron to be a line of energy in a string theory, not a point, but it is stil an electron.
|Comment From Jim Reiser|
I thought I heard a scientist say that it would actually be more exciting if the Higgs was NOT found. If so, why?
IT is always more fun to find something totally unexpected — changes our paradigm dramatically and gives us a very new way to look at the Universe.
actually i disagree with that, i think confirming a good theory is the best of all, since it leads us on toward deeper and deeper understanding
I’m going to have to wind this up now. Our hour is finished. Many thanks to our guests, Rob and Gordy, and to everyone who sent in questions. There were hundreds and I wish we could have answered more.