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Home > Physics > A Quantum Diaries Survivor New CMS Results
By Tommaso Dorigo | October 27th 2014 06:50 AM | 4 comments | Print | E-mail | Track Comments Tommaso Dorigo
The Large Hadron Collider at the CERN laboratories in Geneva is currently in shutdown, finalizing the upgrades that will allow it to restart next year at the  centre-of-mass energy of 13 TeV - over 60% more than the last 8 TeV run. ATLAS and CMS have collected no more proton-proton collisions since almost two years ago; yet the collaborations are as busy as ever producing physics results from the analysis of the 2012 data.

Rather than focusing on any single result, below I give some highlights of the most recent publications by CMS. Another post will discuss ATLAS results in a few days.

One interesting and competitive new measurement -although one produced with only 2011 data taken at 7 TeV- is the one of the strong coupling constant, alpha_s. Alpha_s is in fact not a constant, but a number which depends on the energy at which quantum chromodynamical processes take place. It dictates the strength of the interaction between quarks and gluons, and its determination is not an easy task. The recent preprint by CMS discusses the measurement of parton distribution functions of the proton from the measurement of inclusive jet cross sections, and then uses the same data to determine the strong coupling constant at high values of energy - well, rather at high values of four-momentum transver, Q, which is the quantity on which the coupling directly depends. A host of past measurements are shown in the graph above (including others from CMS), together with the latest CMS data points (the black dots at high Q on the right). The measurement extracted from the data, which is the one obtained at the highest value of four-momentum transfer, is alpha_s(M_Z) = 0.1185+-0.0019 +0.0053-0.0024, where the first uncertainty is statistical and the second asymmetric one is due to theoretical uncertainties; the value is quoted at the scale equivalent to the mass of the Z boson, as is customary. The yellow band corresponds to the theoretical predictions given the CMS result.

Another recent CMS preprint presents results of the search for the rare Higgs boson decays into muon pairs and electron pairs. While the former will eventually be seen by the LHC as we collect more data, the latter will probably never be observed directly. This is due to the fact that the Higgs boson "couples" to electrons and muons in manner proportional to the mass of these particles - and the muon is 200 times heavier than the electron. Below you can see the mass distribution of pairs of muons in one of the search topologies investigated by CMS. The signal template shown at the bottom of the upper panel near 125 GeV is the one expected if the Higgs decayed 20 times as frequently to muon pairs as the standard model predicts. The lower panel shows the deviations of the data from the background model. From the absence of a signal in the data an upper limit on the branching fraction is obtained at 0.0016 - meaning that less than one in 600 Higgs bosons decay to muon pairs.

Finally, in a preprint submitted four days ago CMS reports on measurements of vector boson scattering by electroweak processes. The signature sought is one of same-sign dileptons accompanied by two jets with a large rapidity separation (Δη>2.5) and invariant mass above 500 GeV. What this means is that the two bosons are produced together with two "forward jets" resulting from the hadronization of the quarks that originally emitted the interacting bosons. Focusing on same-sign lepton pairs allows to remove much of the backgrounds. As you can see in the graph above, the data (black points) beautifully follow the prediction of standard model (red histogram), while anomalous couplings would enhance the number of lepton pairs of large invariant mass (blue and pink histograms). The cross section of vector-boson scattering yielding same-sign WWjj pairs is measured by CMS at σ(WWjj)=4.0+2.4-2.0(stat.) +1.1-1.0(syst) femtobarns. From the agreement with SM predictions, limits are set on those possible anomalous couplings.

A Quantum Diaries Survivor




Hi Tommaso,
there is some study following CMS results. It was reported by Michael Schmitt:
My opinion: This is only a wish.

Erestarchos (not verified) | 10/28/14 | 02:54 AM Ah, but you are talking about the CMS edge, which is an analysis published several monthsago. Sure, there are phenomenologists who have taken that signal seriously. I share your
hunch - there is nothing there.


Tommaso Dorigo | 10/28/14 | 07:31 AM On a somewhat different subject, I was reading this:

Syracuse physicists closer to understanding balance of matter, antimatter in universe

“Results from D0 and CDF showed that the matter-antimatter oscillations of the Bs meson deviated from the Standard Model of Physics, but the uncertainties of their results were too high to make any solid conclusions,” Stone says.

He and Zhang had no choice but to devise a technique allowing for more precise measurements of Bs mesons. Their new result shows that the difference in oscillations between the Bs and anti-Bs meson is just as the Standard Model has predicted.

Stone says the new measurement dramatically restricts the realms where new physics could be hiding, forcing physicists to expand their searches into other areas. “Everyone knows there is new physics. We just need to perform more sensitive analyses to sniff it out,” he adds.

Any comments on this?

I am reminded of the time, many years ago, when I walked the easy way up Ben Nevis, height 1,344 m (4,409 ft).  One would see what looked like the top, only for it to reveal a further slope, and again, and again.  It seems that the new physics is always just around the corner.

As I was walking up the stair,
I met a man who wasn’t there
He wasn’t there the other day
I wish that man would go away.

Robert H. Olley / Quondam Physics Department / University of Reading / England
Robert H Olley | 10/29/14 | 07:17 AM what can I comment... I have made up my mind in 2006 when I observed that
the exceptional agreement overall of the SM with precision measurements
(especially in the B sector, when back then I was impressed during, I remember,
a talk I heard at QCGP in the Azores) could only be taken to imply that if there is
new physics is far away from our reach.
I would love to be wrong, but my hunch is that we're not going to be so lucky to
see NP appear in front of us in the next decade...


Tommaso Dorigo | 10/29/14 | 07:48 AM

