Mysteries of the Standard Model

Transcription

1 Mysteries of the Standard Model Methods of exploring the nature of the Electroweak force at the Compact Muon Solenoid [CMS] experiment. Final REU presentation given by Kellen McGee in partial fulfillment of the requirements of the Wayne State University 2011 REU program. Work completed in conjunction with Kristina Krylova, The State University of New York at Buffalo. Advisors: Dr. Robert Harr, Wayne State University & Kalanand Mishra, Fermi National Accelerator Laboratory.

2 A Brief Review Since the experimental confirmation of the existence of quarks in the 1970s, we ve (more or less) felt we have a good understanding of the fundamental particles and forces of nature. The Standard Model breaks matter down into two basic types of fundamental particle: the Fermion and the Boson. Fermions = Leptons (e -....) and Quarks Bosons = Force mediation particles In the SM, bosons are responsible for mediating the Strong (gluon), Weak (W ±, Z 0 ) and Electromagnetic (photon) forces. The framework of the SM calls for massless bosons, and the massless gluon and photon conform well to this expectation. However, the W ± and Z 0 bosons have been found to weigh 80 and 90 GeV/c 2 respectively. Guiding Question: Um What?

3 Spotlight on the W ± and Z 0 The existence of the W and Z was predicted in 1979 and confirmed at CERN in Involved in the nuclear weak interaction, the weak force operates between electrons and neutrinos. Weak force bosons turn quarks of one flavor (u,d,c,s,t,b) into another, making ß-decay and p -> n processes possible (Solar fusion). W + and W - carry electrical charge. This introduces the possibility of self-interaction if charge is conserved. Weinberg, Salam, and Glashow (1979) also proposed a method by which the Weak and Electromagnetic interactions could be unified: Electroweak Unification The discovery of W ± and Z 0 bosons provide the necessary pieces by which to unify these interactions. However, their nonzero mass required an explanation, which took the form of spontaneous symmetry breaking.

4 Electroweak Unification Between to seconds after the Big Bang, T universe 100GeV, or 1.16x10 15 K. Above this energy, the weak and electromagnetic forces merge. They are understood as different aspects of the same force. Yet, at lower energies, they look different: EM: Strength = 1/137 SF, range = infinite. Wk: Strength = 10-6 SF, range = m. Theory suggests, Under certain conditions a force of large strength can have the appearance of a force of small strength if the particle that carries the force is very massive. WSG, 1979 Theoretical calculations show that force carrier particles of 80 and 90 GeV are required to account for the difference in strength. Sound familiar? In this context, W ± and Z 0 should have mass and do. But, where does this mass come from?

5 At or above 100 GeV, EM and Wk forces appear identical. However, as the universe cooled, the mass of the W ± and Z 0 spontaneously broke EWK symmetry. After the temperature of the universe cooled below the mass of the Wk bosons, the EM and Wk forces began to behave differently. Spontaneous Symmetry Breaking Mass derives from these bosons ability to interact with the proposed Higgs field.

6 If we knew what we were doing, it wouldn t be called research... Methods and Motivation for the Investigation of W ± and Z 0 Why and how does this symmetry breaking occur? These questions, among others, make the particles at the heart of the issue, W ± and Z 0 very interesting to study. Since they carry charge, these bosons can interact with themselves, Triple Gauge Couplings. Studying these interactions directly tests our understanding of the electroweak force. Though not a direct search for a Higgs boson, this line of inquiry also directly tests the accuracy of the Standard Model. If the Higgs is not found, this method of study could be the next big thing. Goal: Predict how W ± and Z 0 interact with each other, and learn why/how EWK symmetry breaking occurs. Method: Study triple gauge couplings.

7 Triple Gauge LHC TGCs produce pairs of bosons, which are identified by their decay products in the detectors. Each type of decay provides research options. We chose the lepton-neutrino-quark-quark channel, where quarks appear as reconstructed jets [lnujj]. Background, Non-diboson events that produce decay products that fake a diboson event, is a problem inherent in any channel. To find TGC events, the first step is to understand and remove the background noise.

8 Lessons I learned in high-energy particle physics: 1. Now you see it. 2. Now you don t. 3. Now you see it only if you want to. Why lnujj? Studies have been conducted before in different channels that are easier to detect and have less background. This channel is beset with noise from W+Jets events. Jets are hard to see, and hard to model. Existing simulations (Monte Carlo [MC] programs) do not handle jets very accurately. But, there are far more of these kind of events than those in any previously studied channel. With more data, it is more likely we will see something anomalous? Next Steps: Learning how to model these events with MC, distinguish WW and WZ from W+Jets, and subtract the background from the diboson signal.

9 Number of Events / 9.1 GeV W + W -, W ± Z 0, and W ± +Jets WW WJ WZ Number of Events / 0.0 GeV WW 2nd Jet WJ 2nd Jet WZ 2nd Jet Number of Events / 0.2 GeV WW WJ WZ P T Dijet System pt/dijet Mass "! Number of Events / 9.1 GeV WW WJ WZ Number of Events / 0.0 GeV WW 2nd Jet WJ 2nd Jet WZ 2nd Jet Number of Events / 0.2 GeV WW WJ WZ P T Dijet System pt/dijet Mass "!

10 Ongoing research Anomalous Triple Gauge Couplings The physical expression [Lagrangian] for WWV interactions tells us a few things about problems the SM. a) As collision energies increase, the probabilities of WW production become greater than 1. b) Actually, as CM->infinity, terms in the Lagrangian diverge. c) There are specific WWV interactions that are not allowed in the SM, but are mathematically present in the Lagrangian. These are the anomalous triple gauge couplings. HOWEVER, these problems are solved within the SM. Divergences miraculously cancel themselves out, and by virtue of the Higgs field, probabilities are kept below 1. We think it s funny that things just happen to cancel out so well. We also want to make sure we really don t see things we shouldn t, i.e., atgcs. Whether we find something or find nothing (set a limit), we still advance our knowledge of the bounds of the Standard Model. Even if the Higgs is found, this could prove to be a useful crosscheck of the Higgs mechanism, and reveal further aspects of the Weak force, and by extension, EWk Symmetry Breaking.