1 Introduction. THE Q W eak EXPERIMENT: A SEARCH FOR NEW PHYSICS AT THE TeV SCALE. W. Deconinck 1, for the Q W eak Collaboration

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1 THE Q W eak EXPERIMENT: A SEARCH FOR NEW PHYSICS AT THE TeV SCALE W. Deconinck 1, for the Q W eak Collaboration (1) College of William & Mary, Williamsburg, VA, USA wdeconinck@wm.edu Abstract The Q W eak experiment, which started in May 2010 and will run until May 2012 in Hall C at Jefferson Lab, aims to determine the weak charge of the proton, Q p W = 1 4 sin2 θ W, to a precision of 4%, and the weak mixing angle sin 2 θ W to a precision of 0.3%. With this precision the experiment will be sensitive to parityviolating new physics at the TeV scale. We access the weak charge by measuring the small parity-violating asymmetry in the elastic scattering of polarized electrons with positive and negative helicity on unpolarized protons in a liquid hydrogen target. Due to the interference of the photon and Z-boson exchange diagrams, this asymmetry is proportional to the weak charge of the proton. To achieve the high precision, we scatter the 150 to 180 µa polarized electron beam on a 35 cm long liquid hydrogen target with 2.5 kw of cryogenic cooling power. The signals of the scattered electrons in eight fused silica detectors are integrated in custom electronics modules. During the first running period, the Q W eak experiment has collected 25% of the total expected data volume. 1 Introduction The Standard Model of particle physics has been very successful in describing a wide range of phenomena in nuclear and particle physics. Proposed four decades ago, the Standard Model combines the strong interaction between quarks and gluons described by Quantum Chromodynamics, and the unified electroweak interaction between fermions by the exchange of photons and weak bosons. However, despite its successes there are compelling experimental and theoretical reasons to search for effects which would require an extension to the current Standard Model: there is no dark matter candidate, the expected mass of the Higgs boson is unnatural, etc. In the Standard Model, quarks do not just possess an electric charge given by their coupling strength under the electromagnetic exchange of photons. They also possess weak charges under the exchange of the weak bosons. An overview of the electromagnetic and weak vector charges is giving in table 1. The weak charge for the proton is given by Q p W = 1 4 sin 2 θ W, with θ W the Weinberg angle describing the mixing of the electromagnetic and weak sectors. The value of sin 2 θ W 1/4 results in a small value for the weak charge of the proton, Q p W 0.07, which makes a measurement of the weak charge of the proton very sensitive to sin 2 θ W. Notice that in contrast to the small weak charge for the proton the weak charge of the neutron is larger, Q n W =

2 Table 1: Electromagnetic and weak vector couplings of the light quarks and nucleons. Particle Electromagnetic charge Weak vector charge u C 1u = sin2 θ W 1 3 d 1 3 2C 1d = sin2 θ W 2 3 p(uud) +1 Q p W = 1 4 sin2 θ W 0.07 n(udd) 0 Q n W = 1 The Q W eak experiment will measure for the first time the weak charge of the proton Q p W. By measuring the weak charge of the proton Q p W and the electroweak mixing angle sin 2 θ W to a high precision, we are sensitive the indirect effects from particles beyond the Standard Model that are out of reach of present accelerators. The prediction for the running of sin 2 θ W with momentum transfer Q 2 is indicated by the solid line in figure 1. Significant deviations from the predicted values would indicate that extensions or modifications to the Standard Model are required [1, 2]. The sensitivity of the weak charge to new physics (leptoquarks, R-parity violating supersymmetry) can be estimated in a model-independent way [4]. Assuming an interaction with coupling constant g and mass scale Λ and effective charges h u V = cos θ h and h d V = sin θ h, we can write the interaction Lagrangian for electron-quark scattering as L = g2 eγ 4Λ 2 µ γ 5 e q hv q qγ µ q. The sensitivity of the Q W eak experiment as a function of the interaction mixing angle θ h is shown as the short-dashed green line in figure 2. The Q W eak experiment will increase the reach beyond the APV(Cs) Standard Model Completed Experiments Future Experiments SLAC E158 Møller [JLab] Qweak [JLab] PV-DIS [JLab] ν-dis Z-pole D0 CDF Figure 1: Running of the electroweak mixing angle sin 2 θ W with momentum tranfer Q 2 as calculated in the Standard Model (blue line) [3]. The available experimental data points are shown in black. Projected data points for the Q W eak and other experiments are shown in red. Figure 2: Model-independent analysis of the sensitivity to physics beyond Standard Model [4]. The long-dashed red curve indicates the limits without results from parity-violating electron scattering experiments. The solid blue curve includes results from parity-violating electron scattering experiments without the Q W eak experiment. The short-dashed green curve shows the projected constraints including the Q W eak experiment, assuming agreement with the Standard Model. 207

3 1 TeV scale at 95% confidence. 2 Parity-Violating Electron Scattering The Q W eak experiment uses the technique of parity-violating electron scattering to access the weak charge of the proton. A longitudinally polarized electron beam with quickly alternating helicity strikes a cryogenic liquid hydrogen target. Elastically scattered electrons are detected in quartz Čerenkov detectors, and custom-built data acquisition electronics measures the integrated photomultiplier current generated by the rapid succession of electron pulses. A helicity asymmetry can be constructed from the integrated detector response in consecutive helicity intervals. Although the total scattering cross section from protons or electrons in hydrogen is dominated by the parity-conserving exchange of photons, independent of the incoming electron helicity, a small parity-violating asymmetry is introduced by the interference of the photon exchange and Z-boson exchange diagrams. Based on the propagators of the interactions the size of the parity-violating asymmetry can be easily estimated as A P V = σ R σ L Q2, (1) σ R + σ L MZ 2 where Q 2 is the squared momentum transfer, and M Z = 91.2 GeV is the mass of the Z-boson. At momentum transfers common at Jefferson Lab (Q 2 values up to a few GeV), the small asymmetry is usually expressed in parts per million (ppm) or parts per billion (ppb). For elastic scattering the parity-violating asymmetry on the proton can more accurately be written as A P V (p) = G [ F Q 2 ɛg γ 4πα E GZ E + τgγ M GZ M (1 4 sin2 θ W )ɛ G γ ] M GZ A 2 ɛ(g γ E )2 + τ(g γ, (2) M )2 with G F the Fermi coupling constant, Q 2 the squared fourmomentum transfer, and α the fine structure constant. The dimensionless kinematic factors τ = Q 2 /4M 2, ɛ = (1 + 2(1 + τ) tan 2 θ/2) 1, and ɛ = τ(1 + τ)(1 ɛ2 ) combine with the electric and magnetic form factors G γ E, GZ E, Gγ M, GZ M under γ and Z exchange and the electron axial form factor G e A to complete the expression. In the forward-angle limit θ 0, where the momentum transfer Q 2 is small, the expression for the Figure 3: Normalized parity-violating asymmetry A p P V = Q p W + Q2 B(Q 2 ) measured by other parity-violating experiments on a proton target and extrapolated to the forward-angle limit [4]. The extrapolation to Q 2 = 0 of the fit to the data represents the weak charge of the proton. The prediction of the Standard Model is shown with the red star. 208

4 Table 2: Summary of projected uncertainties on the asymmetry A P V for the Q W eak experiment. Source of uncertainty δa P V /A P V δq p W /Qp W Statistical uncertainty 2.1% 3.2 Hadronic structure N/A 1.5% Beam polarization 1.0% 1.5% Absolute value of Q 2 0.5% 1.0% Inelastic ep scattering 0.5% 0.7% First order beam properties 0.5% 0.7% Systematic uncertainty 1.3% 2.5% Total uncertainty 2.5% 4.1% asymmetry simplifies to A P V (p) Q2 0 G F Q 2 4πα [ Q p W 2 + Q2 B(Q 2 ) ]. (3) In this expression the function B(Q 2 ) contains corrections due to the hadronic substructure of the proton. The normalized parity-violating asymmetry A p P V = Qp W + Q2 B(Q 2 ) measured by other parity-violating experiments and extrapolated to the forward-angle limit is shown in figure 3. By extrapolating the expression for the asymmetry to Q 2 = 0, we obtain the weak charge of the proton. 3 Experimental Apparatus The Q W eak experiment is located in Hall C at the Thomas Jefferson National Accelerator Facility or Jefferson Lab. The experiment started in May 2010 and will collect data until May 2012, with a six month long break between May 2011 and November 2011 separating the experiment in two data taking phases. The experiments consists of three major components: the longitudinally polarized electron beam, the liquid hydrogen target, and the spectrometer and detector system. A summary of the projected systematic uncertainties in the experiment is presented in table 2. Polarized Electron Beam Polarized electrons are generated in a strained GaAs superlattice photocathode. Left or right circularly polarized laser strikes the surface of the GaAs crystal and electrons of the corresponding helicity are photo-emitted and pre-accelerated in the injector. Beam currents after the injector of 180 µa and beam polarizations exceeding 85% are routinely achieved. The circular polarization of the laser light is determined by the polarity of the high voltage on a Pockels cell. The polarity is changed at a rate of 960 Hz, corresponding to a settling time of 70 µ and an integration time of 971 µs. To reduce the sensitivity to low frequency noise components and drifts in the beam parameters we do not simply toggle the helicity states (e.g ) but we use a pseudo-random sequence of quartets of the form + + or ++. Electronic cross-talk between the helicity signal and detector 209

5 signals is avoided by reporting the actual helicity state with a delay of eight quartets. To remove the effects of helicity correlated beam properties, the circular polarization of the laser is inverted every eight hours by inserting or removing a half-wave plate, and every week by performing a Wien rotation of the electron polarization in the injector. The beam transport line from the polarized electron source and injector to the liquid hydrogen target in the experimental hall is instrumented with beam intensity and beam position monitors. The measurements of several beam position monitors are combined to determine the beam position and beam direction at the target itself. The beam position monitors in dispersive regions are used to measure the beam energy. Liquid Hydrogen Target The target consists of a 35 cm long aluminum cell filled with liquid hydrogen (LH2). To maintain a temperature of 20 K while operating with electron beam currents up to 180 µa a cryogenic cooling system requires 2500 W of cooling power, provided by liquid helium coolant at 4 K and 15 K in a heat exchanger with three layers of coils. An important design criterion for the target was to minimize pressure and temperature fluctuations. This was achieved by extensive computational fluid dynamics simulations to determine the optimal shape of the target and the flow velocity transverse to the beam. During short intervals without beam a high-power heater in the LH2 loop compensates for the lost beam heating to stabilize the loop and to prevent the LH2 from freezing. Observed pressure variations are substantially slower than the helicity reversal rate. Spectrometer and Detector System. The spectrometer and detector system are shown schematically in figure 4. Electrons scatter off the protons in the liquid hydrogen target, and the scattered electrons in the angular region of interest pass through the octagonally symmetric holes of the three collimators. The toroidal magnet coils bend the scattered electrons outwards, as indicated by the envelope in the top octant. Behind the 8 m tall shield wall all eight Figure 4: Schematic view of the Q W eak experiment in Hall C at Jefferson Lab. octants have quartz bar Čerenkov detectors with custom-built integrating data acquisition electronics. Two opposite octants are instrumented with tilted particle tracking detectors, horizontal drift chambers before and vertical drift chambers after the toroidal magnet. The horizontal and vertical drift chamber packages can be rotated independently for measurements of the momentum transfer Q 2 in each octant. For both types of drift chambers there is one pair of octants can be reached by both positive and negative rotations, providing us with redundancy that aids in determining systematic effects. 210

6 Polarimetry Two beam polarimetry techniques are used to reach the required 1% systematic uncertainty on the measurement of the electron beam polarization. In the existing Møller polarimeter the electron beam is scattered from the polarized outer-shell electrons in an iron foil that has been magnetized to saturation in a large external magnetic field. The Møller measurements are invasive to the Q W eak experiment and have to be conducted at beam currents below 10 µa to avoid depolarization due to foil heating. There is therefore an uncertainty associated with extrapolating to the experimental conditions. To measure the beam polarization non-invasively and continuously, a new Compton polarimeter was commissioned for the Q W eak experiment. The polarized electron beam is collided nearly head-on with a high-intensity circularly polarized laser beam in a low-gain Fabry-Pérot cavity in the center of a magnetic chicane. The scattered photons (with energies up to 50 MeV) are detected in a PbWO 4 calorimeter. The scattered electrons are bent away from the primary beam by the dipole field of the chicane and their separation, measured in four diamond strip detector planes, is used to deduce their momentum. An asymmetry in the cross section for left and right circularly polarized laser light is proportional to the polarization of the electron beam. Analysis The measured asymmetry A meas of the signal yields Y in the quartz bars is related to the physical asymmetry A P V by A meas = Y + Y Y + + Y = P e (1 f)a P V + fa bkg + A false, (4) where P e is the electron polarization, f is the dilution factor determined by the fraction of background over signal plus background, A bkg is the background asymmetry, and A false is the false asymmetry due to helicity-correlated beam properties. The integrated yields in the detectors depend on the beam intensity, beam energy, bean position, and beam direction. During data taking we measure the beam intensity and feed this information back to the high voltage of the Pockels cell in the polarized electron source to reduce the helicity-correlated beam intensity asymmetry. Because these beam properties X i are generally correlated with the helicity, they result in a false asymmetry. Using the measured helicity-correlated differences X i we can correct the measured asymmetry for this false asymmetry given knowledge of the correlation sensitivities α i : A false = α i X i. (5) i Using natural beam motion the helicity-correlated beam properties are correlated, and the sensitivities require a diagonalization of their correlation matrix. We also use an active beam modulation system that drives the beam energy and the horizontal and vertical beam position and direction separately, largely removing the internal correlations between the beam properties. Backgrounds The two largest sources of background events are the aluminum walls of the target cell and inelastic scattering off the protons. Because the experiment integrates the detector response for all tracks, it is impossible to remove these background events individually and their collective effect has to be corrected for. 211

7 Figure 5: Uncorrected and blinded raw experimental asymmetries, averaged over all eight main detector octants, for a selection of slugs (periods of constant insertable half-wave plate settings). Each slug corresponds to approximately eight hours of data taking. Slugs with the insertable half-wave plate inserted are shown as blue disks, slugs with the insertable half-wave plate out are show as red squares. The corresponding blue and red lines show the average asymmetry for this subset of slugs. All asymmetries are blinded by an unknown additional asymmetry to avoid biases during the analysis. Neutrons have a larger weak charge than protons (Q n W = 1 versus Qp W 0.07). The parity-violating asymmetry for aluminum is therefore significantly larger than the asymmetry for liquid hydrogen. The approximately 100 µm thin target cell windows lead to a correction to the asymmetry of approximately 20%. To reduce the uncertainty present in this correction, a substantial amount of data has been collected on aluminum dummy targets to measure the parity-violating asymmetry in aluminum. 4 Preliminary Results 4.1 Integrating Mode Data The width of the measured parity-violating asymmetry distribution at a beam current of 165 µa is 236 ppb. This value is in agreement with the expectation from counting statistics (215 ppm), when taking into account detector resolution, current normalization, and target density fluctuations. The measured asymmetry is expected to change sign when the helicity is reversed at the polarized electron source by inserting a half-wave plate in the laser path. Each period during which this half-wave plate remains in the same state is called a slug. In figure 5 the average asymmetry for a series of slugs is shown, and the expected sign change for alternating half-wave plate states is clearly visible. The asymmetries shown in figure 5 are uncorrected and blinded asymmetries, and therefore not amenable to interpretation in terms of Q p W. 212

8 Figure 6: Projection of the electron tracks reconstructed in the vertical drift chambers to the main detector quartz bars. The figure corresponds to one 2 m 18 cm large quartz bar. 4.2 Event Mode Data During dedicated tracking runs at low currents from 50 na to a few µa the tracking detectors were commissioned, and are being used to measure the distribution of the momentum transfer Q 2. The horizontal drift chambers are functioning well, and the shape and mean value of the scattering angle distribution is in agreement with predictions from Monte Carlo simulations. The corresponding mean momentum transfer of tracks reaching the main detector is also consistent with simulations. The performance of the vertical drift chamber has been excellent. As a qualitative demonstration of their performance, figure 6 shows the projection of the electron tracks reconstructed in the vertical drift chambers to the surface of the main detector quartz bars. The characteristic mustache shape of the event distribution is in agreement with the predictions from Monte Carlo simulations. 5 Conclusion The Q W eak experiment has successfully completed the first phase of data taking in May 2011, and accumulated approximately 25% of the total data volume necessary to achieve the 4% uncertainty on the weak charge of the proton. The second phase of the experiment will start in November 2011 and continue until May Acknowledgements The author would like to thank the organizers for the invitation to present these results at the workshop. References [1] J. Erler, M. J. Ramsey-Musolf, Phys. Rev. D72 (2005) ; arxiv:hep-ph/ [2] M. J. Ramsey-Musolf, S. Su, Phys. Rept. 456, 1 8 (2008); arxiv:hep-ph/ [3] W. Bentz, I. C. Cloet, J. T. Londergan, A. W. Thomas, Phys. Lett. B693 (2010) [4] R. D. Young, R. D. Carlini, A. W. Thomas, J. Roche, Phys. Rept. 99, (2007); arxiv: [hep-ex]. 213