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.


Tienzen said...

Dr. Li xiaojian discusses my “The Final Total TOE” and this LHC 2016 data with Dr. David Gross (Nobel laureate) on August 5, 2016. Photos with Dr. Gross are available here,

andrew said...

Tevatron average mass value for the top quark is mt=174.30 ±0.65GeV/c2.