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Anna Phan | USLHC | USA

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Not just B physics!

Today, I’m going to be talking about some lesser known LHCb results. In fact, I’m going to discuss physics that some people thought LHCb couldn’t do, given the detector and software design.

What am I going to be talking about? Electroweak physics. Yes, you read that right, not the heavy quark physics which LHCb was designed and built for, but electroweak physics. In particular, I’m going to discuss some of our new results on Z and W boson cross sections, which will be presented at the DIS workshop in Bonn this week.

But before I go into the results and why they are interesting, let me quickly introduce the Z and W boson, as found at The Particle Zoo. Theorised in the 60s and discovered in the 80s, they are massive elementary particles that mediate the weak force. Z bosons are neutral and decay into a pair of leptons or quarks. W bosons are charged and decay into either a charged lepton and neutrino or two quarks.

At the LHC, Z and W bosons are usually identified by their leptonic decays. The signatures that electrons, muons and tauons leave in the detectors are much easier to find and measure than those left by quarks. In LHCb, we are able to detect Z decays to a pair of electrons, or muons or tauons and W decays into a muon and corresponding muon neutrino. Unfortunately, we aren’t able to cleanly identify W decays to electron or tauons and their corresponding neutrinos.

Above I present a summary of all the Z and W cross sections we have measured so far using data from 2010. On the left are the Z cross sections, given separately for each decay mode, while on the right are the Z and W to muon cross sections and various ratios of them.

If you are used to seeing LHC results, these may look a little strange. Usually the data is shown as black solid points while the theory is shown as coloured bands. Here the data is shown as the coloured bands, while the predictions of various theoretical models are shown as black open points.

Why this confusing presentation you ask? Well, that has to do with why we are trying to measure the Z and W production cross sections in LHCb.

As I’ve mentioned before, LHCb has a unique geometry compared to the other LHC experiments. In particular, with our cone geometry, we cover the forward region of 1.9 < y < 4.9, while ATLAS and CMS cover |y| < 2.5 with their cylindrical geometries. In terms of proton-proton collisions and the production of Z and W bosons, this means we are able to probe a complementary region of phase space. The plot on the right illustrates this, where you can see that LHCb is able to explore the low-\(x\), high \(Q^2\) region inaccessible by other experiments (past and present). This is important as this is the region where there is the highest uncertainty in the theoretical predictions in the Z and W production cross sections. So ideally, we would like to use experiment to constrain the theoretical predictions.
I say ideally, as if you look at our current results, we don’t have the experimental precision to do this. But we will in the future, so be on the look out!

Of course we aren’t the only experiment looking at Z and W production cross sections, ATLAS and CMS are as well, so I feel obliged to show you this plot on the left, which is of the W lepton charge asymmetry as a function of lepton pseudorapidity from ATLAS, CMS and LHCb…

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