Northwest Terascale Research Projects:
W + b quark physics at the LHC
Topics to be discussed
This workshop concerns the production at the LHC of a W or Z boson plus one or more b-quarks. (We can also discuss c-quarks.)
The motivation is that
Here are some more detailed thoughts on these points, meant as starting points for discussion rather than as anything complete or authoritative.
- this is an important signature for new physics signals;
- standard model W+b production is an important background to the new physics signals;
- there is good theory on this;
- in fact, there are NLO theoretical tools and parton shower Monte Carlo tools for this and each has strength and weaknesses that we could try to understand;
- the reliability of the theory depends on how the sought signature is structured; with theorists and experimentalists present we can try to sort out the relative advantages of different experimental strategies;
- there is data from CDF and D0 that could inform our discussions;
- our discussions, in turn, might inform the analysis of the CDF and D0 data.
- [Points 1 and 2] These are more-or-less self-evident. What is not self evident is what strategies are best for separating in the data different mechanisms that result in the production of a W and one or more b-quarks, some of which are standard model processes and some of which might involve beyond the standard model physics. We will have the opportunity to explore these strategies.
- [Points 3 and 4] The leading paper (with multiple predecessors) is
J. M. Campbell, R. K. Ellis, F. Febres Cordero, F. Maltoni, L. Reina,
D. Wackeroth and S. Willenbrock,
``Associated Production of a W Boson and One b Jet,''
This describes a formulation of everything one needs for W + b in the standard model at NLO and an implementation in the package MCFM. This paper leads to questions like the following
This leads to the question of how well parton shower event generators do for this process compared to the NLO calculation. If they disagree, which is more nearly right? A perusal of the MC@NLO website suggests that W+b is not available. However, W+t is. The t-quark is so heavy that the physics is substantially different for W+t compared to W+b, but one may wonder if something can be learned from W+t. Similarly, the paper JHEP07(2008)060 by Alioli, Nason, Oleari and Re discusses matching of NLO to showers with the Powheg method for W production; this method has also been applied to b-quark production. Thus one wonders whether it might be applied to W+b production.
- There is a hard scale Q, say the W mass or transverse momentum, that is much bigger than m_b. What are the issues in making this choice?
- We use the b-quark as a constituent of the proton for this. Is our knowledge of the b-quark parton distribution function adequate?
- Incorporating the b-quark as a constituent of the proton sums logs of Q^2/m_b^2 using the DGLAP evolution equation. However, there are also similar logs for final state evolution of a b-quark as a constituent in a gluon jet. Could one use a fragmentation function approach to sum these logs? Failing that, are there not some advantages to using a parton shower event generator to get this physics right?
- [Point 5] As for the experimental signatures and their relation to the theory, the following thoughts arise.
- Part of the NLO correction has a hard gluon emitting a possibly soft gluon that, in turn, splits into a b-bbar pair. This correction is potentially large if there is not a pretty hard lower cutoff on the PT of the observed b or bbar. What are the experimental implications on this? (This issue does not seem to be mentioned in the CDF paper arXiv:0812.4458 on the experimental results.)
- Would it help to look at angular correlations between the jet center and the b or bbar quark (or quarks) in order to sort out issues about how well the NLO theory is working?
- [Points 6 and 7] There is a 2005 D0 result. The main recent published result is from CDF.
T. Aaltonen et al. [CDF collaboration],
``Measurement of Cross Sections for b Jet Production in
Events with a Z Boson in p anti-p Collisions at
sqrt(s) = 1.96-TeV,''
Also, for Q + b jets:
C. Neu (for the CDF Collaboration),
W ±/Z + Jets and W ± / Z + Heavy Flavor Jets at the Tevatron.,
Proceedings of 43rd Rencontres de Moriond on QCD
and Hadronic Interactions,
La Thuile, Italy, 2008.
We could examine the following questions, among others.
- what is special about W + b vs b + bbar? Could experiments learn lessons from b + b-bar without Ws that help us with the W + b signal?
- What jet algorithms are best for this purpose?
- The Tevatron has triggers for displaced vertices while the LHC experiments do not. How does this affect the LHC possibilities for studies
of b + bbar?
- The CDF W + b results compare pretty well to the NLO MCFM results. On the other hand, Alpgen + Herwig does not appear to do as well. Perhaps this is understandable as a consequence of using NLO versus LO. However, one wonders if comparisons of the results could help us to understand the domain in which one or the other is more reliable. It would be nice also to understand whether MLM matching versus CKKW matching matters. For this purpose, some theoretical insight together with results using Sherpa (with CKKW matching) might be helpful.
We should bear in mind that the experimental cuts that one uses to see (say) W + b are not the same as the cuts that one uses to find a signal for which W + b is a background. We should try to understand the reliability of the theory when cuts are used that treat W + b as background. Can we get information from experiment on this?
Last updated 2 September 2009
Davison E. Soper
Institute of Theoretical Science
University of Oregon
Eugene OR 97403 USA