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Brian Dorney | USLHC | USA

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Summer Conference Coverage: Lepton Photon 2011

The summer conference season may be winding down, but that doesn’t mean we are quite done yet.  Today was the first day of the Lepton Photon 2011 (LP2011) Conference; which is taking place in Mumbai, India all this week.  The proceedings of LP2011 are available via webcast from CERN (although Mumbai is ~10 hours ahead if you are in the Eastern Standard Timezone).  But if you’re a bit of a night owl and wish to participate in the excitement, then this is the link for the webcast.

The complete schedule for the conference can be found here.

But what was shown today?  Today was a day of Higgs & QCD Physics.  I’ll try to point out some of the highlights of the day in this post.  So let’s get to it.

The Hunt for the Higgs

Today’s update on the CMS Collaboration’s search for the ever elusive Higgs boson made use of ~110-170 trillion proton-proton collisions (1.1-1.7 fb -1); covering eight separate decay channels and a Higgs mass range of 110-600 GeV.   The specific channels studied and the corresponding amount of data used for each are shown in the table at left.  Here l represents a charged lepton and v represents a neutrino.

The CMS Collaboration has not reported a significant excess of events in the 110-600 GeV range at LP2011.  However, the exclusion limits for the Higgs boson mass range were updated from our previously reported values at EPS2011.  By combining the results of the eight analyses mentioned above the CMS Collaboration produced the following plot summarizing the current state of Higgs exclusion (which I have taken from the Official CMS Press Release, Ref. 1; and CMS PAS HIG-11-022, Ref. 2.  Please see the PAS for full analysis details):


Standard Model Higgs boson combined confidence levels showing current exclusion regions, image courtesy of the CMS Collaboration (Ref 1 & 2).


But how do you interpret this plot?  Rather than re-inventing the wheel, I suggest you take a quick look at Aidan‘s nice set of instructions in this post here.

Now then, from the above plot we can see that the Standard Model Higgs boson has been excluded at 95% confidence level (C.L.) in the ranges of 145-216, 226-288 and 310-400 GeV [1,2].  At a lower CL of 90%, the Collaboration has excluded the SM Higgs boson for a mass window of 144-440 GeV [1,2].

These limits shown at LP2011 improve the previous limits shown at EPS2011 (using 1.1 fb-1).  The previous exclusion limits were 149-206 and 300-440 GeV at 95% C.L., or 145-480 GeV at 90% C.L.

While the LP2011 results did not show a Higgs discovery, the CMS Collaboration is removing places for this elusive boson to hide.

QCD Physics

Today’s other talks focused on quantum chromodynamics (QCD).  With the CMS Collaboration’s results shown for a variety of QCD related measurements.

One of the highlights of these results is the measurement of the inclusive jet production cross section.  The measurement was made for a jet transverse momentum over a range of ~20-1100 GeV.  The range in cross-section covers roughly ten orders of magnitude!

Measurement of the inclusive jet cross-section made with the CMS Collaboration, here data are the black points, the theoretical prediction is given by the red line. Image courtesy of the CMS Collaboration (Ref. 3).

In this plot above each of the data series are “binned” by what is known as a jet’s rapidity (denoted by the letter y). Or in this case the absolute value of the jets rapidity.  Rapidity is a measure of where a jet is located in space.

The CMS detector is a giant cylinder, with the collisions taking place in the center of the cylinder.  If I bisect the detector at the center with a plane (perpendicular to the cylinder’s axis), objects with lower rapidities make a small angle with this plane.  Whereas objects with higher rapidities make a large angle with this plane.

As we can see from the above plot, the theoretical prediction of QCD matches the experimental data rather well.

Another highlight of CMS Collaboration’s results shown at LP2011 is the measurement of di-jet production cross-section

Measurement of the dijet production cross-section made with the CMS Collaboration.  Again, data are the black points, the theoretical prediction is given by the red line.  Image courtesy of the CMS Collaboration (Ref. 3).

Here the CMS results shown cover an invariant dijet mass of up to ~4 TeV, that’s over half the CoM collision energy!  Again, the theory is in good agreement with the experimental data!

And the last highlight I’d like to show is the production cross section of isolated photons as recorded by the CMS Detector (this is a conference about leptons and photons after all!).

Measurement of the isolated photon production cross-section made with the CMS Collaboration. Again, data are the black points, the theoretical prediction is given by the red line.  Image courtesy of the CMS Collaboration (Ref. 3).

What happens in isolated photon production is a quark in one proton interacts with a gluon in the other proton.  This interaction is mediated by a quark propogrator (which is a virtual quark).  The outgoing particles are a quark and photon.  Essentially this process is a joining of QCD and QED, an example of the Feynman Diagram for isolated photon production is shown below (with time running vertically):

From the above plot, the theoretical predictions for isolated photon production are, again, in good agreement with the experimental data!

These and other experimental tests of QCD shown at LP2011 (and other conferences) are illustrating that the theory is in good agreement with the data, even at the LHC’s unprecedented energy level.  Some tweaks are still needed, but the theorists really deserve a round of applause.



But I encourage anyone with the time or interest to tune into the live webcast all this week!  Perhaps I’ll be able to provide an update on the other talks/poster sessions in the coming days (If not check out the above links!).

Until Next Time,




[1] CMS Collaboration, “New CMS Higgs Search Results for the Lepton Photon 2011 Conference,” http://cms.web.cern.ch/cms/News/2011/LP11/, August 22nd 2011.

[2] CMS Collaboration, “Combination of Higgs Searches,” CMS Physics Analysis Summary, CMS-PAS-HIG-11-022, http://cdsweb.cern.ch/record/1376643/, August 22nd 2011.

[3] James Pilcher, “QCD Results from Hadron Colliders,” Proceedings of the Lepton Photon 2011 Conference, http://www.ino.tifr.res.in/MaKaC/contributionDisplay.py?contribId=122&sessionId=7&confId=79, August 22nd 2011.


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