Saturday, August 13, 2016

A new top quark mass measurement from CMS

A first measurement is presented of the top quark mass using the decay channel t to (W to l nu) (b to J/psi+X to mu+mu- + X), in events selected in proton-proton collisions and recorded with the CMS detector at the LHC at a center-of-mass energy of 8TeV. The data correspond to an integrated luminosity of 19.7 inverse femtobarns, with 666 ttbar and single top quark candidate events containing a reconstructed J/psi candidate decaying into an oppositely-charged muon pair. The mass of the (J/psi+l) system, where l is an electron or a muon from W boson decay, is used to extract a top quark mass of 173.5 +/- 3.0 (stat) +/- 0.9 (syst) GeV
CMS Collaboration, "Measurement of the mass of the top quark in decays with a J/psi meson in pp collisions at 8 TeV" (August 11, 2016).

A combined margin of error of about +/- 3.13 GeV makes the new result almost useless, however, and even in a combined measurement, it would be weighted very minimally as a result of the lack of precision.

Still, the fact that the central result is quote close to the global average in a decay path not previously measured, does help make the existing estimate more robust and supports the integrity of the Standard Model in which the mass is calculated.

Tuesday, August 9, 2016

Prestige Necropolis In North America Better Understood

In Egypt, the pyramids were the ultimate prestige grave sites that shed light on their ancient civilization. Mound 72, discovered by archaeologists in 1967, is what appears to be Cahokia's greatest prestige necropolis.

Cahokia, in modern day Saint Louis, was the capitol of a great copper age, pre-Columbian maize and pumpkin farming civilization of North America which flourished around 1,000 CE. This civilization extended to more or less the entire Mississippi basin and has trade links and cultural influence, at least, that extended as far as the Carolina coast. At its peak, about 75,000 people lived in the capitol, making in a world class city by pre-Bronze Age standards.

Predecessors of the Cahokia civilization may have even been a cultural source for the Mayans, as its predecessor civilizations in Louisiana produced the earliest discovered pyramids in the Americas.

Until now, Mound 72 was believed to have six bodies, all men, presumably kings or heroes. But, further research has determined that there are at least twelve bodies there, including many male-female couples.

This tends to show that there may have been an aristocracy in this civilization in which aristocratic men and women played important roles, until male warrior dominated societies arose in the vicinity of this empire after its collapse. Cahokia's decline began around the 1160s and 1170s during a major New World drought, and eventually collapsed as an urban complex around 1350 CE around the time of the Little Ice Age. The last vestigial remnants of this culture were wiped out when European diseases struck relict communities shortly after Columbus and the conquistadors who followed him made contact with the New World.

The abstract and citation of the source paper are as follows:
The Beaded Burial central to F101 within Cahokia's mound 72Sub1 has been fundamental to some cosmological explanations of the founding of this North American precolumbian polity. The central burial, identified as two males surrounded by retainers, has been interpreted as paradigmatic of a paramount chiefdom, or conversely, as a mythic cosmogram. Recent bioarchaeological reanalysis and two independent osteological studies of F101 and associated burials have identified the presence of male/female pairs, numerous females, and at least one child, suggesting that previous explanations privileging the male Red Horn association should be reexamined. We suggest that 72Sub1 is most likely correlated with ritual practices promoting world creation, renewal, and fertility symbolism.
Thomas E. Emerson, et al., "Paradigms Lost: Reconfiguring Cahokia's Mound 72 Beaded Burial." 81(3) American Antiquity 405 (2016).

Chinese Legendary History Coroborated

China’s historiographical traditions tell of the successful control of a Great Flood leading to the establishment of the Xia dynasty and the beginning of civilization. However, the historicity of the flood and Xia remain controversial. Here, we reconstruct an earthquake-induced landslide dam outburst flood on the Yellow River about 1920 BCE that ranks as one of the largest freshwater floods of the Holocene and could account for the Great Flood. This would place the beginning of Xia at ~1900 BCE, several centuries later than traditionally thought. This date coincides with the major transition from the Neolithic to Bronze Age in the Yellow River valley and supports hypotheses that the primary state-level society of the Erlitou culture is an archaeological manifestation of the Xia dynasty.
Qinglong Wu, "Outburst flood at 1920 BCE supports historicity of China’s Great Flood and the Xia dynasty" Science (05 Aug 2016) Vol. 353, Issue 6299, pp. 579-582 via Dienekes' Anthropology Blog.

Lubos Motl on New Experimental Physics Results

All babies are being killed and embryos are being aborted these days.
- The Reference Frame.

In other words, beyond the Standard Model physics proposals are being ruled out left and right.

Sabine Hossenfelder at Backreaction likewise bemoans the arrival of the "Nightmare Scenario" in which the LHC is discovering no new physics other than the Higgs boson.  She thinks that there's a moral to the story:
That the LHC hasn’t seen evidence for new physics is to me a clear signal that we’ve been doing something wrong, that our experience from constructing the standard model is no longer a promising direction to continue. We’ve maneuvered ourselves into a dead end by relying on aesthetic guidance to decide which experiments are the most promising. I hope that this latest null result will send a clear message that you can’t trust the judgement of scientists whose future funding depends on their continued optimism.
I agree that particle physics has put too many of its eggs in the same SUSY/string theory basket, and even more on theories that have the same philosophical motivation.

Thursday, August 4, 2016

750 GeV Resonance Gone, SM Repeatedly Confirmed, Higgs Boson Trending Lighter

Lubos Motl has a good summary of the CMS experiment's results based upon the first day of talks at the ICHEP conference in Chicago and some related papers that were released.

Jester's twitter feed provides some charts and analysis generally confirming his conclusions.  So does Matt Strassler's blog.

1. There Is No 750 GeV Bump. The big news is the highly anticipated (and widely rumored) result related to the 750 GeV diphoton bump. A prematurely released CMS paper on the 750 GeV resonance, a bump that has spawned 500 recent papers over roughly the last half year, shows that it has disappeared with the new data. How much new data? Much more than the data set that provide the initial evidence of a bump.
The relevant portion of the data taken by CMS in 2016 is usually given by 12.9 inverse femtobarns of data. Note that this whole 12.9 was taken in the first half of 2016. They never combine the 2015 and 2016 data. They could combine them and increase 12.9 by something like 2.7 that is used in many CMS papers based on the 2015 data.
UPDATE: ATLAS concurs that the 750 GeV bump is not present in the new data (full paper here). See also here.

2. Standard Model Confirmation. The overwhelming share of 39 papers dumped by CMS in connection with the conference perfectly confirm the Standard Model in all but 7 cases (with multiple hypotheses tested in each of most of the papers).  Only one result had a deviation from the Standard Model with more than 2.6 sigma of statistical significance, and that has a 2.84 sigma global significance.  This is in the ballpark for the number of anomalies of this significance that would be expected due to random statistical flukes in a dump of this many results at once.

Only a couple of the anomalies also showed up in previous data sets and at least one of those had an anomaly of declining statistical significance despite the fact that a larger data set that should increase the statistical significance of an anomaly found in prior data that was real by about 2.5 sigma over the previous data set.  So far, there is also no meaningful ATLAS confirmation of the CMS anomalies.

SUSY exclusions and other BSM exclusions exclude more parameter space than they did after the last round of data was analyzed.  Some SUSY exclusions rule out certain sparticals (gluinos) under certain assumptions up to 1.9 TeV.

3. Higgs Boson Mass.  The latest measurement of the Higgs boson mass (based upon four lepton events) by CMS was 124.5 +0.48/-0.46 GeV. This is less than the current global average of about 125.09 +/- 0.24 GeV, which is statistically consistent with the global average but will probably drag down a new global average somewhat, although there is a considerable range of data points that contribute to that global average.

What does the new CMS Higgs boson measurement mean in context?

The current combined estimate of the Higgs boson mass (from the link to that value above) is based upon the following data points:

* ATLAS diphoton mass 126.02 +/- 0.51 GeV
* ATLAS four lepton mass 124.51 +/- 0.52 GeV
* CMS diphoton mass 124.7 +/- 0.34 GeV
* CMS four lepton mass 125.59 +/- 0.45 GeV

So, after this new CMS data point, the new global average should be roughly 124.82 GeV with a pretty similar margin of error, before accounting for any new ATLAS results with its wealth of new data.

The new CMS data point also makes the ATLAS diphoton data point look like an outlier relative to the other three measurements, which suggests that we may expect the combined average is more likely to fall than to rise when ATLAS releases its next Higgs boson diphoton decay based mass measurement, bringing the combined average closer to the theoretically notable value of 124.65 GeV discussed below.

Some of the prior Higgs boson mass measurements at the LHC (by date of publication, some of which were used in the current combined average) include the following:

* ATLAS diphoton mass 125.98 +/- 0.42 +/- 0.28 (June 15, 2014)
* ATLAS four lepton mass 124.51 +0.52 +/- 0.06 (June 15, 2014)
* CMS diphoton number 124.7 +/- 0.31 +/- 0.15 (July 2, 2014)
* CMS four lepton mass 125.6 +/- 0.4 +/- 0.2 (September 10, 2014)

The new CMS four lepton mass measurement is very close to the June 15, 2014 ATLAS four lepton mass measurement.

The downward trend in the Higgs boson mass revives the possibility that the sum of the squares of the fundamental boson masses is equal to half of the square of the Higgs vacuum expectation value (VEV).  The Higgs boson mass in that scenario would be 124.65 GeV (which is robust to variations within the current margin of error of the W and Z boson masses).  This is consistent within two sigma of the current global average, within one sigma of the latest CMS four lepton based measurement of the Higgs boson mass, and even closer to the likely combined global average once the new CMS result is considered.

It also further disfavors the 2W+Z=2H mass formula, which is already disfavored by 3.7 sigma with the current global average, to the point that it is pretty much conclusively ruled out.

As previously noted at this blog:
There is an argument that the "tree-level" mass of the Higgs boson is 123.114 GeV (half the Higgs vev) but that it is increased by higher order loop corrections that bring it to its experimental value. The "tree-level" estimate of the mass of the W boson is 78.9 GeV. If the percentage increase in mass due to higher order loop corrections for the Higgs boson from the tree level value is the same as the higher order loop corrections of the W boson to the experimental value, then the implied Higgs boson mass value would be 125.43 GeV which is consistent at a 1.4 sigma level with the latest combined mass measurement. No published source actually calculates these higher order loop adjustments, however. While the actual higher order loop calculation is probably of that order of magnitude, it could easily be higher or lower. The claim is plausible, but requires further investigation. If the higher order loop corrections produced a value consistent with 124.65 GeV, that would be remarkable indeed[.] . . . 
This also significantly tightens the expected value of the top mass from the formula that the sum of the square of each of the fundamental particle masses equals the square of the Higgs vacuum expectation value. The uncertainty in the Higgs boson mass had been the second greatest source of uncertainty in that calculation. The best fit for the top quark mass on that basis (using a global fit value of 80.376 GeV for the W boson rather than the PDG value) is 173.73 GeV (173.39 to 174.07 GeV within the plus or minus one sigma band of the current Higgs boson measurement). 
If the the sum of the square of the boson masses equals the sum of the square of the fermion masses the implied top quark mass is 174.03 GeV if pole masses of the quarks are used, and 174.05 GeV if MS masses at typical scales are used.

That compares to the latest top quark mass estimate from ATLAS of 172.99 +/- 0.91 GeV. The latest combined mass estimate of the top quark (excluding the latest top quark mass measurement estimate from ATLAS) is 173.34 +/- 0.76 GeV.
The expected value of the top mass from the formula that the sum of the square of each of the fundamental particle masses equals the square of the Higgs vacuum expectation value, goes up if the Higgs boson mass is reduced.

Other Higgs boson news:

In general, there have been a long string of Higgs boson reports from the LHC tending to show a very tight correspondence between all of the experimentally measured properties of the Higgs boson and the theoretically predicted properties of a Higgs boson of roughly the measured Higgs boson mass. The latest measurements of Higgs boson properties announced today are no exception to this trend.

Strong (3.3 sigma) but not discovery level evidence is found at ATLAS for a Higgs process involving top quark pairs in the frequencies consistent with those predicted by the Standard Model.

Previous experiments have also confirmed that the Higgs boson is spin-0, even parity, and has couplings of the predicted strength all of the now nearly half dozen couplings that have been measured.

Monday, August 1, 2016

Back To Basics About Supersymmetry

The following questions and answers are copied from questions posted and answers I wrote at the Physics Forum (with some significant editing, expanded text, and reformating):


What is the point of sparticles? What will they prove? How will they work? I've read about supersymmetry, but don't really get it. I know it is to unify quantum mechanics and relativity, but how?


Sparticles are particles beyond the Standard Model of particle physics that are necessary for supersymmetry which is a generalization of the Standard Model of particle physics that is attractive for reasons of interest to theoretical physicists.

Standard Model Fundamental Fermions and Bosons

In the Standard Model of particle physics, there are two basic kinds of particles.

In the Standard Model, fundamental fermions are the building blocks of what we crudely in layman's language think about as "matter".  For example, a hydrogen atom is made of three quarks that combine to form a proton with an electron orbiting around it. The six kinds of quarks are fundamental fermions, and these fundamental fermions combine to make protons, neutrons and more exotic composite particles called hadrons which are made of two (mesons) or three (baryons) or more quarks.  Particles made up of quarks are often accompanied by orbiting electrons, or muons (heavy electrons), or taus (really heavy electrons).  When muons and taus and other fundamental particles decay into lighter particles they spew out one of three kinds of neutrinos (which are very light, but non-zero mass particles that interact barely at all except via the weak force and gravity).  Electrons, muons, taus and the three kinds of neutrinos combined are fundamental fermions that are similar in certain ways and are collectively called leptons

In the Standard Model, fundamental bosons are we crudely think of in layman's language as the particles that make up force fields.  Electromagnetic fields are made of bosons called photons. Protons and neutrons and other particles made of quarks (which are fundamental fermions) that are held together by bosons called gluons which carry the strong force.  The weak force is carried by bosons called the W boson (for "weak") and the Z boson (because they needed to give it a name and didn't have any other good ones). Gravity, if it is a field carried by a particle is carried by a hypothetical boson called a graviton.  The Higgs boson carries the "Higgs field" which gives fundamental particles their mass (it isn't clear whether or not the Higgs boson interacts with neutrinos which may get their mass in a different way, how neutrinos get their mass is an unsolved problem in physics).

Supersymmetry Is A Balance Between Fundamental Fermions and Fundamental Bosons

Without getting into all the technical details, supersymmetry (also known as SUSY) is basically about the idea that there are technical reasons that makes it desirable for there to be a fundamental balance between fundamental fermions and fundamental bosons.

The theoretically easiest way to get that balance is to imagine that every fundamental fermion has a new fundamental particle boson counterpart (squarks and sleptons), and that every fundamental boson has a new fundamental fermion counterpart, which have their own special names.* These partner particles are "sparticles."

This then gets jumbled a bit because some of these counterparts have very similar physical properties that cause them to blend into each other and look like different particles (something that happens in the Standard Model as well in the way that the electromagnetic force and weak force are related to each other in very deep ways call electroweak unification), and the theory also requires at least four extra Higgs bosons to work out (a positively charged one, a negatively charged one, an extra heavy one, and one with a different parity - i.e. left handedness v. right handedness than a usual Higgs boson).

More complicated "non-minimal" versions of supersymmetry assume even more new particles.

* It could be that a balance between fundamental fermions and fundamental bosons already exists in the Standard Model in a much more subtle way than the crude and obvious balancing present in supersymmetry theories, which would explain how seemingly "unnatural" aspects of the Standard Model "miraculously" balance out, but so far only the vaguest hints that this might be the case have been worked out by theoretical physicists and only as conjectures and hypotheses, not as proven theories.

Supersymmetry is a GUT and SUGRA is a TOE.

Supersymmetry itself does not unify quantum mechanics and relativity. Instead, it unifies the three forces of the Standard Model (electromagnetism, strong force, weak force) into forms of the same underlying force that is unified at high energies, making it what is known as a Grand Unified Theory (GUT). Supersymmetry also ties in naturally to some mathematical structures known as "groups" in a more elegant way than the Standard Model does (which takes at least three different groups crudely "glued" together to summarize).

If you add quantum gravity to the supersymmetry mix by adding the graviton (a fundamental boson) and a superpartner called a gravitino (a fundamental fermion), you get supergravity also known as SUGRA which is a low energy approximation of a Theory of Everything (TOE), and supergravity, in turn is usually a foundation of string theory.

Why Isn't Supersymmetry Noticeable In Daily Life?

We don't notice any of this in everyday life, or even in high energy physics experiments (if the theory is true) because all of the particles created by supersymmetry except one (which explains dark matter and interacts with other matter no more strongly than neutrinos do) are unstable and decay into ordinary matter before we have time to see it, and also because they only form at all in very high energy situations.

If this sounds familiar, it should. Most of the particles we do know exist decay extremely rapidly into ordinary matter and only form at all in very unusual high energy situations, or are always found confined in composite particles and never seen in isolation, or are neutrinos which are extremely hard to detect because they interact so weakly with everything else.

In daily life, we see mostly protons and neutrons (which are made mostly out of up quarks and down quarks bound together by gluons so tightly that we never see free quarks or free gluons), electrons, and photons.  The force that connections protons and neutrons in the nucleus of an atom is carried mostly by pions which are made up of two up and down quarks bound by gluons which are themselves short lived and travel only short distances before decaying (with the quarks and gluons never visible in isolation).  All other particles in the Standard Model are too ephemeral or ghostlike to notice without high technology instrumentation in carefully constructed lab experiments.  

The vast majority of physics (except radioactivity and high energy physics) can be explained with protons, neutrons, pions, electrons and photons (the first three of which are not actually fundamental), without knowing about the huge menagerie of fundamental particles and composite particles needed to describe the last 0.1% of reality.

Supersymmetry just adds more exotic, ephemeral fundamental particles and particles that are very hard to detect (a dark matter candidate called a WIMP) to the mix for relative obscure theoretical reasons set forth in the next section.

Why Supersymmetry?

Supersymmetry is attractive as a theory for many reasons, some of which are now obsolete:

(1) It provides natural candidates for dark matter particles of a variety called "WIMPS".

(2) It makes the constants of the Standard Model such as the Higgs boson mass seem more "natural".

(3) It makes it much easier to do math that sheds light on how particles interact at very high energy, because the balance between fermions and bosons makes lots of terms in calculations that would otherwise have to be calculated cancel out.

(4) It unified the three fundamental forces into one master force at high energies called the GUT scale.

(5) It provides a way to explain where the matter in the universe came from that are unexplained in the Standard Model.

(6) It sheds some light on the kind of reasons that Standard Model constants might have the values they do although not particular clear guidance.

(7) Supersymmetry is so mathematically similar to the Standard Model of particle physics, it is easy to tweak properties of particular versions of supersymmetry like particle masses in such a way that it predicts essentially the same things as the Standard Model down to the limits of experimental error. So it is hard to reject outright.

(8) Before we knew the mass of the Higgs boson, lots of Standard Model predictions in high energy situations were nonsense answers where the likelihood of all possible events didn't add up to 100% if Higgs boson mass is not just right, but this doesn't happen in supersymmetry.  This is less of a big deal than it used to be because the mass of the recently discovered Higgs boson is "just right" and prevents the Standard Model from becoming pathological mathematically at high energies in the way that it would if the Higgs boson where much heavier or much lighter than it is in reality. 

(9) Supersymmetry is also a very natural low energy approximation of string theory.  Many versions of string theory require, for mathematical reasons, that fundamental fermions and fundamental bosons have counterparts for each other for reasons related to the way a fundamental superstring in that theory can vibrate.

Theoretical physicists are very reluctant to abandon supersymmetry because that would mean giving up hope that their best shot at a good theory of quantum gravity through string theory as explained below. So they'd have to start over from scratch trying to merge quantum mechanics and general relativity. 

Why String Theory?

The Standard Model and general relativity are mathematically incompatible with each other. The reasons that the Standard Model (i.e. quantum mechanics) and general relativity are incompatible are quite mathematical and technical but include, for example, the fact that point particles which are assumed in quantum mechanics would instantly turn into black holes in general relativity.

The Standard Model and supersymmetry are both fully compatible with special relativity, however.

Scientists from Einstein onward have been trying very hard to unify gravity and other forces of nature ever since general relativity and quantum mechanics were conceived in the early 1900s. So far, no one has even come close to succeeding.

A potential connection to string theory is attractive because string theory offers a reasonable hope that it could provide a mathematically consistent way to create a theory of quantum gravity that could be consistent with the rest of quantum mechanics which is called the Standard Model. 

String theory is pretty much the only game in town that creates a potential theory of quantum gravity with particle based force fields like those used in the rest of quantum mechanics so it is very tempting to find a way to connect what we know to it.

There is another approach to quantum gravity that involves applying quantum mechanical concepts to the nature of space-time itself, which includes approaches known as Loop Quantum Gravity (LQG), rather than using the force field carried by particles approach of string theory, but that is a story for another day that doesn't involve supersymmetry.

Why Not Supersymmetry?

The obvious problem with supersymmetry is that nobody has ever seen any of the supersymmetric particles, either because they are not real, or because the new particles are simply too heavy to see in colliders, or because they are otherwise not visible due to something called "R-parity" (a property that basically keeps sparticles and regular matter separated).

(1) If superpartners exist, the LHC has determined that they are much heavier than they were expected to be.  At some point, if they are not found at low enough masses, they would be so heavy that they would lead to predictions that are contrary to experimental evidence.

(2) "Naturalness" which is an important reason for supersymmetry is being questioned as a useful theoretical concept.

(3) The evidence of force unification that should have showed up by now has not appeared.  

(4) And, the LUX experiment has pretty much ruled out the kinds of WIMP dark matter particles that supersymmetry predicted.

Why Not String Theory?

String theory has all of the problems of SUSY and SUGRA and also lots of problems of its own. Basically, there are thousands or millions or more versions of string theory (called "vacua") and nobody knows which version is remotely close to our reality.

Where Does This Leave Us?

The Standard Model, in contrast, has no obvious generalizations that could for example be used as a basis for a version of string theory. So figuring out how to meld it with quantum gravity is even more difficult.