Beauty Physics at HERA-B

Symmetries play a fundamental role in particle physics. Gauge theories are based on gauge symmetries. They describe interactions between particles and allow unification of different forces. Symmetry invariance implies conservation of a quantum number. It is the case of U(1) in quantum electrodynamics (QED) and SU(3)_c in quantum chromodynamics (QCD) which lead to conservation of electric charge and colour charge, respectively. Symmetries are also a mean of classifying particles. The Gellman-Nishijima formula $Q = I_3 + \frac{B+S}{2}$, which relates the quantum numbers Q (electric charge), I₃ (third component of isospin), B (baryon number) and S (strangeness), played a key role in hadron spectroscopy. The SU(2) isospin symmetry could be extended to SU(3) -flavour and later to SU(3) -colour which lead to the formulation of QCD, the theory of strong interactions. Weak interactions, which are based on SU(2) -weak isospin, are special in the sense that they violate most symmetries. Non-conservation of strangeness, charm and bottom quantum numbers leads to quark flavour mixing and to oscillations in neutral K , D and B meson systems. Parity, P , charge conjugation, C and time reversal, T , which are also violated in weak interactions, are of special interest. CP violation, which is established in the kaon system even if the effects are small, might play an important role in sheding light on the mysterious origin of matter-antimatter asymmetry in the universe. The problems of (i) particle masses - which can be generated by the so-called Higgs mechanism, currently the weakest point of the standard model (SM) - (ii) CP -violation and (iii) the number of fermion generations are closely related. CP violation could only be accomodated in the standard model with at least 3 fermion generations. As a member of the third quark family the b -quark, unlike the t -quark, has a relatively long lifetime. Hadrons containing a b quark (b -hadrons) can hence be identified, reconstructed and studied. CP violation is expected to be large in the b -system because processes involved are of first order.

HERA-B at DESY/Hamburg is one of the experiments that will be able to observe CP violation in the B meson system and to constrain the parameters of the quark mass mixing matrix, the Cabbibo-Kobayashi-Maskawa (CKM) matrix, allowing a test of unitarity, i.e of the hypothesis of three-quark-generations. Equivalently, one measures three angles $\alpha$, $\beta$ and $\gamma$. If one angle is non-zero it would mean that CP symmetry is violated. Unitarity requires that the 3 angles form a triangle, i.e. $\alpha +\beta + \gamma = \pi$.

Bottom production at HERA-B proceeds via gluon-gluon fusion: $p+N \to\ gg+X \to\ b\overline{b}+ X$. A few times 10⁸ $b\overline{b}$ pairs are expected per year, making it possible to study various exclusive decay channels of b mesons and baryons. The $c\overline{c}$ production cross section is a factor 1000 higher and allows detailed studies of D mesons. $D^o-\overline{D^o}$ oscillations, CP violation in the D system and rare D decays can be addressed.

The HERA-B detector is an experimental challenge as it is based on modern technology developed for LHC. The data acquisition, online monitoring and trigger systems have to cope with high rates comparable to the LHC environment. HERA-B is, hence, a necessary input to future hadronic experiments, like ATLAS. The software tools, tagging and analysis techniques will help prepare for LHC physics. Last but not least, the B physics programme at LHC will be a natural continuation of the HERA-B efforts.

As the detector has been commissioned and data taking is about to start, we would like to suggest the following topics:

• Study of the reaction $B^o_d (\overline{B^o_d}) \to\ J/\Psi K^o_s$ , $J/\Psi\to\ l^+l^-$ ($l=e,\mu$) , $K^o_s\to\ \pi^+ \pi^-$. The goal is to measure the asymmetry between B^o_d and $\overline{B^o_d}$ decays, which is directly proportional to $\sin{2\beta}$. Large CP violation asymmetries are expected.
• Study of the reaction $B^o_d \to\ J/\Psi K^{\ast o}$ , $J/\Psi\to\ l^+l^-$ , $K^{\ast o}\to\ K^+ \pi^-$. This channel does not present any asymmetry and allows good control and normalisation of the above reaction, as the kinematics are similar.
• Study of the reaction $B^o_s (\overline{B^o_s}) \to\ J/\Psi \Phi$ , $J/\Psi\to\ l^+l^-$ ($l=e,\mu$) , $\Phi\to\ K^+ K^-$. The goal is to measure the asymmetry between B^o_s and $\overline{B^o_s}$ decays. Very small CP asymmetries are expected in the standard model, such that any large measured numbers would mean new physics, beyond the SM.
• Search for the rare decays: $B^o_d \to\ l^+l^- K^{\ast o}$, $B^o_s \to\ l^+l^- \Phi$. Current experimental upper limits are close to the theoretical predictions, such that signals are expected at HERA-B. These flavour changing neutral currents (FCNC) are suppressed in the standard mode, and are sensitive to physics beyond the SM, such as supersymmetry. Lepton Family Violating decays like $B^o_{d,s} \to\ e^{\pm}\mu^{\mp} K^{\ast o} (\Phi)$ are strictly forbidden in the SM and any observation would mean onset of new physics.
• The study of the decays $B \to\ \pi\pi, K\pi, KK$ are interesting because (i) they would allow the determination of the angles $\alpha$ and $\gamma$ and (ii) the interpretation of the data requires more theoretical inputs. This study and the above one are well suited to a theoretician who would like to do some phenomenology.

As the HERA-B experiment is located at DESY in Hamburg, it is possible for candidates to travel in order to know the experiment and take advantage of the local expertise.

For any further information, please contact Torleiv Buran or Farid Ould-Saada.