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Posts Tagged ‘dijet’

The LHC turns back on this year for Run II. What might we see day 1?

The highest-p_T jet event collected by the end of September 2012 (Event 37979867, Run 208781): the two central high-p_T jets have an invariant mass of 4.47 TeV, and the highest-p_T jet has a p_T of 2.34 TeV, and the subleading jet has a p_T of 2.10 TeV. The missing E_T and Sum E_T for this event are respectively 115 GeV and 4.97 TeV. Only tracks with p_T> 0.7 GeV are displayed. The event was collected on August 17th, 2012. Image and caption credit: ATLAS

The highest-p_T jet event collected by the end of September 2012 (Event 37979867, Run 208781): the two central high-p_T jets have an invariant mass of 4.47 TeV; the highest-p_T jet has a p_T of 2.34 TeV, and the subleading jet has a p_T of 2.10 TeV. The missing E_T and Sum E_T for this event are respectively 115 GeV and 4.97 TeV. Only tracks with p_T> 0.7 GeV are displayed. The event was collected on August 17th, 2012. Image and caption credit: ATLAS

In seven weeks CERN’s Large Hadron Collider (LHC), the largest and most energetic particle accelerator in history, is scheduled to turn back on. The LHC has been shutdown since December 2012 in order for experimentalists to repair and upgrade the different detector experiments as well as the collider itself. When recommissioning starts, the proton beams will be over 60% more energetic than before and probe a regime of physics we have yet to explore directly. With this in mind, today’s post is about a type of new physics that, if it exists, we can potentially see in the first days of LHC Run II: excited quarks.

Excited Quarks and Composite Quarks

Excited quarks are interesting little beasts and are analogous to excited atoms in atomic physics. When light (a photon) is shined onto an atom, electrons orbiting the nucleus will become energized and are pushed into higher, metastable orbits. This is called an excited atom.

excited_atom_NASA

After some estimable and often measurable period of time, an electron will radiate light (photon) and drop down to its original orbit. When this happens, we say that an excited atom has relaxed to its ground state.

emission_atom_NASA

In analogy, if quarks were bound states of something smaller, i.e., if they were composite particles, then we can pump energy into a quark, excite it, and then watch the excited quark relax back into its ground state.

Feynman diagram representing heavy excited quark (q*) production from quark (q)-gluon (g) scattering in proton collisions.

Feynman diagram representing heavy excited quark (q*) production from quark (q)-gluon (g) scattering in proton collisions.

Observing an excited quark would tell us that the quark model may not be the whole story after all. Presently, the quark model is the best description of protons and neutrons, and it certainly works very, very well, but this does not have to be the case. Nature may have something special in store for us. However, this is not why I think excited quarks are so odd and interesting. What is not obvious is that excited quarks, if they exist, could show up immediately after turning the LHC back on.

Early Dijet Discoveries at LHC Run II

Excited quarks participate in the strong nuclear force (QCD) just like ordinary quarks, which means they can absorb and radiate gluons with equal strength. This is key because protons at the LHC are just brimming with highly energetic quarks and gluons. Of particles in a proton carrying a small-to-medium fraction of the proton’s total energy, gluons are the most commonly found particle in a proton (red g curve below). Of those particles carrying a large fraction of the proton’s energy, the up and down quarks are the most common particles (blue u and green d curves below). Excited quarks, if they exist, are readily produced because their ingredients are the most commonly found particles in the proton.

Words

Distributions of partons in a proton. The x-axis represents the fraction of the proton’s energy a parton has (x=1 means that the parton has 100% of the proton’s energy). The y-axis represents the likelihood of observing a parton. The left (right) plot corresponds to low (high) energy collisions. Credit: MSTW

When an excited quark decays, it will split back into quark and gluon pair. These two particles will be very energetic (each will have energy equal to half the mass of the excited quark due to energy conservation), will be back-to-back (by linear momentum conservation), and will each form jets (hadronization in QCD). Such collisions are called “dijet” events (pronounced: die-jet) and look like this

Words. Credit: CMS

Display for the event with the highest dijet mass (5.15 TeV) observed in CMS data. Image and caption credit: CMS

Although gluons and quarks in the Standard Model can mimic the signal, one can add up the energies of the two jets (which would equal the excited quark’s mass due to energy conservation) and expect to see a bump in the data centered about the mass of the excited quark. Unfortunately, the data (below) do not show such a bump, indicating that excited quarks with masses below a couple TeV do not exist.

cms_dijet_spectrum

Inclusive dijet mass spectrum from wide jets (points) compared to a fit (solid curve) and to predictions including detector simulation of multijet events and signal resonances. The predicted multijet shape (QCD MC) has been scaled to the data. The vertical error bars are statistical only and the horizontal error bars are the bin widths. For comparison, the signal distributions for a W resonance of mass 1.9 CMS.TeV and an excited quark of mass 3.6 CMS.TeV are shown. The bin-by-bin fit residuals scaled to the statistical uncertainty of the data are shown at the bottom and compared with the expected signal contributions. Image and caption credit: CMS

However, this does not mean that excited quarks do not or cannot exist at higher masses. If they do, and if their masses are within the energy reach of the LHC, then excited quarks are very much something we might see in just a few months from now.

Happy Colliding,

Richard Ruiz (@BraveLittleMuon)

Appreciation to Ms. Frost and her awesome physics classes at Whitney M. Young High School in Chicago, Illinois for motivating this post. Good luck on your AP exams!

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Blois Blois Blois!

Tuesday, June 7th, 2011

(2011/06/10: Updated to include D0 result!)

At the Blois conference last week there were plenty of interesting results and updates from experts across the world. The work spanned a whole range of fields from particle physics to cosmology, and I think the most interesting result was the update on the CDF dijet anomaly. It’s worth taking a look at the program for the conference if you want a nice cross section of the status of experiment and theory for various topics.

Giovanni Punzi presented a [pdf]talk on the search for the Higgs boson. As we would expect, we see the usual exclusion plots, with LEP at the low mass end, Tevatron making inroads on the region around 165GeV and the LHC starting to close in across the whole range (especially in the WW mode). Of course we’re recording data so rapidly at the moment that the results in this talk are not really representative of our current sensitivity, and the conferences will be playing catch-up for a while!

In his talk, Punzi showed the updated dijet mass spectrum from CDF. This was blogged about by Michael and Flip a while back. At the time there was a tantalizing bump in a spectrum, with a 3.2σ excess, suggesting that there could be a new resonance around 150Gev! 3.2σ is exciting, but it’s not enough to justify calling this a discovery. We usually ask for 5σ for that, as well as a whole host of other tests to make sure there hasn’t been some error in the analysis or some non-trivial hardware effect:

CDF anomaly at 4.3fb^-1

CDF anomaly at 4.3fb^-1

The big question on everyone’s mind at the time was “Is this just a fluctuation?” Well the answer seems to be “No!”, and Punzi shows us why:

CDF anomaly at 7.3fb^-1

CDF anomaly at 7.3fb^-1

Adding another 3.0fb-1 of data (about 70% more than before) gives an excess of nearly 5σ, and taken on its own has an excess of 2.85σ. Either there is a serious systematic problem at CDF, or this is a new effect! If this indeed new physics, D0 should see something similar, but so far we haven’t heard anything from them. There are efforts at D0 and at LHC to see if we can recreate this peak, so right now all we can do is wait for more results to come in. No doubt you’ll hear about more updates on this blog, so keep reading. There are already quite a few papers on the arXiv about this effect and if you have any more information please leave it in the comments!

Update: D0 see no evidence of any anomaly in the dijet mass spectrum! Source.

D0 result

D0 result. The black points show the data. The dashed histogram shows the CDF anomaly. The red and blue histograms show expected contributions from the Standard Model.

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