Friday, October 13, 2017

Back To Basics In High Energy Physics UPDATED

Usually, the headlines in physics go to people who are proposing or discovering "new Physics", but an important part of the high energy physics enterprise is constantly pushing for more precise measurements of quantities involved in plain old Standard Model physics that has been settled science for decades. Today's research results are mostly of that variety:

Top Quark Measurements:

* The DZero collaboration at the now completed Tevatron experiment released another review of its old data regarding the mass of the top quark. Bottom line:
The most precise single measurement at the Tevatron of mt = 174.98 ± 0.58(stat+JES) ± 0.49(syst) GeV is performed by the D0 Collaboration in the `+jets channel, corresponding to a relative precision of 0.43%. The combination with all other measurements from the D0 experiment results in mt = 174.95±0.40(stat)±0.63(syst) GeV. The Tevatron combination yields mt = 174.30±0.35(stat)± 0.54(syst) GeV, which corresponds to a relative precision of 0.37%.
The directly measured top quark mass according to the Particle Data Group is as follows, based upon a combined CMS measurement, a combined ATLAS measurement and a combined Tevatron measurement:

OUR AVERAGE  Error includes scale factor of 1.6.
ATLScombination of ATLAS
CMScombination of CMS
TEVATevatron combination

The top quark mass measured indirectly via cross-section measurements per PDG is as follows:

 ± 1
D0 , +jets channels
CMSe + μ + T + 0j
ATLS+T+5j (2b-tag)
ATLSpp at s = 7, 8 TeV

"An extended Koide's rule estimate of the top quark mass using only the electron and muon masses as inputs, predicted a top quark mass of 173.263947 ± 0.000006 GeV", via this prior blog post, which is very close to the best fit measured value.

As previously noted in a December 16, 2016 blog post at this blog:
If the the sum of the square of the boson masses equals the sum of the square of the fermion masses equals one half of the Higgs vacuum expectation value, 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. . . . 
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 vaccum expectation value (a less stringent condition because the fermion and boson masses don't have to balance), given the global average Higgs boson mass measurement (and using a global fit value of 80.376 GeV for the W boson rather than the PDG value) is 173.73 GeV. The top quark mass can be a little lighter in this scenario because the global average measured value of the Higgs boson mass is a bit heavier than under the more stringent condition.
Both of the predictions from these phenomenological hypotheses are a bit heavier than the currently measured top quark masses as the LHC measurements have turned out to be smaller than the Tevatron measurements which are quite close to these expected values. But, the scatter in the various measurements is still broad enough that these values cannot be ruled out.

UPDATE October 18, 2017: A new review of ATLAS top quark mass measurement is available, but most, if not all are already incorporated in the PDG global weighted average:
Results of top-quark mass measurements in the di-lepton and the all-jets top-antitop decay channels with the ATLAS detector are presented. The measurements are obtained using proton-proton collisions at a centre-of-mass energy s = 8 TeV at the CERN Large Hadron Collider. The data set used corresponds to an integrated luminosity of 20.2 fb1. The top-quark mass in the di-lepton channel is measured to be 172.99 ± 0.41 (stat.) ± 0.74 (syst.) GeV. In the all-jets analysis, the top-quark mass is measured to be 173.72 ± 0.55 (stat.) ± 1.01 (syst.) GeV. In addition, the top-quark pole mass is determined from inclusive cross-section measurements in the top-antitop di-lepton decay channel with the ATLAS detector. The measurements are obtained using data at s = 7 TeV and s = 8 TeV corresponding to an integrated luminosity of 4.6 fb1 and 20.2 fb1 respectively. The top-quark pole mass is measured to be 172.9+2.52.6GeV.
Teresa Barillari, "Top-quark mass and top-quark pole mass measurements with the ATLAS detector"(October 16, 2017). END UPDATE.

* The ATLAS and CMS experiments at the Large Hadron Collider (LHC) have released a summary of their findings regarding top quark properties. Unsurprisingly, they are consistent with Standard Model expectations in all respects and strictly limit anomalies in the transition from the top quark to the bottom quark via the weak force as the accuracy of the measurement of this quantity improves. Several measurements were made and two of the easiest to summarize results were as follows:

W Boson Helicity In Top Quark Decays
The W-boson helicity fractions (left-handed, right-handed and longitudinal) are defined as FL,R,0 = ΓL,R,0/ΓTotal, where ΓL,R,0 are the partial decay widths in left-handed, right-handed, and longitudinal helicity states respectively, with ΓTotal being the total decay width. The SM next-tonext-to-leading order (NNLO) calculations [4] including the electroweak effects predict the values of FL = 0.311±0.005, FR = 0.0017±0.0001 and F0 = 0.687±0.005, for a top quark mass of 172.8±1.3 GeV. 
The crude average of the CMS and ATLAS measurements (which do be exactly right should be error weighted and should include the error bars of the combined result) are as follows:

FL=0.311 (v. predicted value 0.311), FR=0.006 (v. predicted value 0.0017), and F0=0.695 (v. predicted value 0.687).

The results for FL and F0 are within the combined theoretical uncertainty of the Standard Model prediction even before considering experimental uncertainty.

The result for FR is within the experimental uncertainty of Standard Model prediction at the two sigma level. Also, keep in mind that FL, FR and F0 are not independent of each other and involve just two degrees of freedom, not three, but the crude average measurements above don't globally fit the data to reflect that fact.

Combining the ATLAS and CMS results properly, on an error weighted basis and doing a global, would give a better result, because the measurement that is closer to the predicted value has a significantly lower margin of error and because the experimental measurements of FR and F0 combined would each be reduced proportionately by a little bit in a global fit since FL+FR+F0 estimated independently slightly exceed 1.0 which must be true as a consequence of the way those variables are defined. The deviation if it was all done properly would be a bit more than one sigma.

Top Quark Decay Width

The Standard Model prediction for the top quark decay width (which as explained in a previous post at this blog is good global measure of deviations from the Standard Model) is ΓSM=1.324 GeV for mt=172.5 GeV.  The CMS measurement of the decay width of the top quark is Γt = 1.36 ± 0.02(stat)+0.14 −0.11 (syst) GeV, which agrees with the Standard Model prediction at the 0.3 sigma level.

The CMS boundary on the top mass decay width is tighter than the most recent ATLAS measurement of:
Γt = 1.76 ± 0.33 (stat.) +0.79 −0.68 (syst.) GeV = 1.76+0.86 −0.76 GeV.
The Charm Quark Mass

A previous determination of the charm quark mass at a 3 GeV energy scale was updated based upon late breaking new experimental data. Bottom line: "Our final result for the MS charm-quark mass reads mc(3 GeV) = 0.993 ± 0.008 GeV and mc(mc) = 1.279 ± 0.008 GeV."

Higgs Boson Measurements

The CMS experiment at the LHC has released its latest experimental data on decays of Higgs bosons to W bosons, to tau lepton pairs, and to muon pairs. It finds that:
the combined observed limit for 7 and 8 TeV [at the 95% confidence level] is 7.4, while the background-only expected limit is 6.5 +2.8 −1.9 . This corresponds to an observed upper limit on B(H → µµ) of 0.0016, assuming the SM cross section. The best fit value of the signal strength for a Higgs boson mass of 125 GeV is 0.8 +3.5 −3.4.
Given the large margin of error, the biggest surprise is that it is so close to the Standard Model expectation (0.059 sigma from the expected value in the Standard Model), which suggests that either the error in this measurement is probably greatly overestimated, or that CMS just got lucky, or both.


None of the results are particularly surprising, but greater precision in these measurements of fundamental Standard Model physics quantities makes all other high energy physics work more accurate and broadly constrains all forms of beyond the Standard Model physics.

1 comment:

andrew said...

Latest CMS measurement (decays with one lepton) 172.25 +/- 0.62 +/- 0.08.

Not far from previous LHC measurements although on the low side. Quite a bit lighter than the Tevatron average.